Colloq 2 Immunology Lectures - Aurora Killi PDF

Aurora Killi Infection, bacterial pathogenicity factors, & interaction of micro- and macroorganisms Immunology lecture 1 Infection Types of symbiosis Symbiosis = the interaction between micro- and macroorganisms (the interaction between bacteria and human). There are several types of symbiosis M...

Aurora Killi Infection, bacterial pathogenicity factors, & interaction of micro- and macroorganisms Immunology lecture 1 Infection Types of symbiosis Symbiosis = the interaction between micro- and macroorganisms (the interaction between bacteria and human). There are several types of symbiosis Mutualism Definition: An association between two organisms where both organisms benefit from the interaction Example: E. coli living in human GI tract E. coli is a part of the normal microbiota The human gets some benefits from the presence of E. coli o Breakdown of nutrients o Providing enzymes o Protection of invasion of pathogenic microorganisms There will be a congruence for place of attachment, nutrients, oxygen, so E. coli will try to survive and kill the pathogenic invader E. coli also benefit from the interaction because it gets a place to live, grow, replicate etc. Commensalism Definition: An association between two organisms in which one benefits and the other derived neither benefit nor harm Example: transitory microbiota of the skin Microorganism will actually get some nutrients from the sweat glands, but since it is transitory, the human will not benefit nor be harmed Parasitism Definition: An association between two organisms in which one species gets benefits from the interaction while the other organisms is harmed (when we speak about infectious diseases caused by microorganisms, we practically often speak about parasitism) Types of parasites Hemi-parasite o Can survive both inside and outside the macroorganism o Only one part of the life cycle is provided inside of the microorganisms o Free parasite that can live both in outer environment and human organism Obligate parasite o Host organism is needed for parasite´s life cycle, it can survive outside a host for a very short period of time o Example: Trichomonas vaginalis which cause urogenital tract infection Facultative intracellular parasite o Can live both inside and outside of macroorganism´s cells o Host cell is not mandatory for survival Obligate intracellular parasite o Can only live inside macroorganism´s cells o Example: viruses  Aurora Killi Definition of infection Infection = an invasion of causative agent into human´s tissues, replication of it, and reaction of host organism to the infectious agent and toxins produced by it o When we speak about infection, we mean both (1) the invasion of a causative invasion and (2) the reaction of the host organism to the infectious agent and the substances produced by it (toxins, enzymes) o It is a combined process that describes both the activity of a causative agent inside of the human body and also our immunity system reaction to the presence of the causative agent inside our organism o If the immunity system is not strong able to fight the pathogen, infectious disease will develop Infectious disease = develops as a result of the infection o Infection of M. tuberculosis → tuberculosis o Infection of N. gonorrhoeae → gonorrhea o Infection of S. pneumoniae → pneumonia Description of infectious process There is a typical way how the infection process goes on Port = infectious agent´s site of entry 1. Infectious agent and exit. If human is the future host, Virus, bacteria, parasite there are six ports To start the infection process we need an infectious 1. Respiratory tract agent 2. Conjunctiva The causative agent is usually found in some kind of 3. Urogenital tract reservoirs 4. Gastrointestinal tract (oral cavity) 2. Reservoir 5. Transplacental (from mother to Soil, water, food, human, animal baby during pregnancy) Sources of infection agents 6. Skin (usually damaged skin, fungi Place where we can find the pathogenic agent can break through healthy skin) 3. Port of exit Sneezing, coughing etc. The pathogenic agent needs a way out of the reservoir to infect the person 4. Transmission Aerogenic, blood transmission, direct contact, vector transmission etc. Transmission from one person to another 5. Port of entry Skin, mucous membranes To cause the disease there has to be an entry point for the infectious agent If we want to describe this way how microorganisms get out of the reservoir and inside another macroorganisms we use the term port 6. Susceptible host Immunosuppression Since they have found a port of entry, we now speak about infection and whether the host will be susceptible to the infection Not each microorganisms that gets inside the human body is able to cause infection Infection is only caused in susceptible host  Aurora Killi After the last step the cycle repeats, meaning that the susceptible host now becomes the reservoir that is holding the infection agent, and the infectious agent find a way out of the host to be transmitted to another person and so on. Types of infection Exogenous infection Caused by microorganisms which enters human body from the outer environment The cycle described above describes exogenous infection development Endogenous infection Caused by microorganisms located in human body and is a part of normal microbiota Can be due to o Bacterial translocation from one place to another (E. coli from GI tract → urethra causing urinary tract infections) o Immunosuppression o Decrease of immune system function Sources and transmission Reservoir is needed (some place, object, or subject that will have the pathogenic agent) and a susceptible host There are several ways how these infections can be transmitted Transmission type Explanation Reservoir Susceptible host Direct contact or Infection develops in case of interaction Human Human horizontal transmission between two people In that case we mainly speak about STDs (syphilis, etc.) Vector borne From human to human → called Human Human transmission anthroponosis Animal From animal to human → called anthropozoonosis There is something in-between the source (reservoir) and the susceptible host. This is called the vector There are different vectors o Anopheles mosquito → malaria o Tics → lyme disease o Blood → hepatitis B and C, HIV Indirect contact These sources are pretty often Water Human contaminated by already infected human or Food animal Soil Food → food poisoning (S. aureus) Air Water → cholera, hepatitis A Soil → skin infection (C. tetani) Air → respiratory tract infections (Influenzae, Coronavirus) Vertical transmission 1. Transplacental transmission Mother Child o We mainly speak about transplacental transmission in translocation of microorganisms from mother´s blood  Aurora Killi stream to baby´s blood stream via the placenta o Examples: Hepatitis C and B, HIV, toxoplasma, etc. 2. Transmission during delivery o We can also speak about vertical transmission during delivery o Example: if mother has Chlamydia trachomatis in her genital system and gives vaginal delivery in which the baby gets in contact with the mother´s genital tract microbiota, it will be infected with Cl. trachomatis as well Factors affecting the infection process There are three main factors (1) Environment, (2) host, and (3) microorganisms If all three factors predispose for the development and initiation of the infection process, most likely the infection will result in an infectious disease 1. The microorganism that is strong enough to cause the disease, fight the host barriers Infectious disease 2. The host must be susceptible to infection and has some decrease of develops immune system reaction 3. The environment must be overall predisposing for infection All three factors are needed to get the infectious disease There are other factors inside each of the three blocks that will predispose to the development of infection and infectious disease Factors inside host Factors in environment 1. Species immunity 1. Temperature Cannot be changed or influenced Each bacteria have its own range of 2. Physiologic conditions of organism (can be temperature where it can grow, multiply, changed, except for age) and be active → optimal temperature Age → more predisposed to infections. There If the environmental temperature is not are infections that are typical for different optimal for the causative agent, it will ages. For example, childhood infections such most probably not survive and cause a chicken pox and measles disease Diet → healthy food provides us with all 2. Climate, humidity nutrients necessary for our development and If these factors are which bacteria cannot for functions of our immune system. Immune tolerate, bacteria will die and cannot system is highly dependent on different cause infections micro- and macroelements and the balance 3. Sunlight of these elements If this factors are which bacteria cannot Endocrine system → balanced hormonal tolerate, bacteria will die and cannot system is a key to proper functioning of cause infections immune reactions and immune defenses. In 4. Seasonality puberty, people are more likely to get 5. Social conditions different fungal skin infections due to If person lives in a cold, humid disbalance of endocrine system and the environment the person is more excess amount of hormones predisposed for different respiratory tract infections  Aurora Killi Nervous system → constant stresses and sleep deprivation Immune system (1) Environmental impact on the development of the infection process Seasonality o Winter → is usually flu and influenza season because viruses transmit more quickly in this environment, people are more predisposed for infections during cold times of the year o Spring → chickenpox is mainly transmitted during spring and also during increased contact between children when they go to kindergarten and play outside o Summer → late spring and summer are typically lyme disease season or tick-borne encephalitis season o Autumn → autumn and early winter is a period of respiratory virus infection Climate changes o If it gets hotter in some areas, there is huge probability that some microorganisms will not be able to survive Deforestation Urbanization o Deforestation and urbanization might stop replication of some vectors Human migration o Humans can be the ones that transmit infections from one region to another o Can happen if the region from where the human is coming has a very high prevalence of the infectious disease and these humans are traveling to somewhere this disease is rare there is huge probability that the disease will spread in that society which has not faced this disease and has not immune memory and immunity against it Population density o If the density of the population is high, the causative agent can spread more quickly Fecal contamination, agriculture, and soil changes o If water is contaminated, we get higher risk of different GI tract infection o If there are soil changes and some bacteria can then multiply there more than before the microorganism can live there and replicate and cause infections more commonly (2) Macroorganisms impact on the development of the infection process Macroorganisms is also very important factor in the development of infection process and initiation. There are many factors that can predispose us to infectious disease or on the other hand can help us to stop the spread of the microorganisms. There are two main types of immune responses Innate immunity → is with the person from the birth. Person gets this immunity when he/she is born, and is transmitted from mother to baby Adaptive immunity → created during our life when we meet different pathogens and agents and learn how to react against them Immunity (cannot be changed) o Problems with innate immunity → macrophage dysfunction o Our innate immune response will decrease  Aurora Killi o Macrophages provide phagocytosis, can activate inflammation, and serve as a bridge between innate and adaptive immunity, activate new adaptive processes o Person will be more predisposed to the infection o Problems with adaptive immunity → cellular and humoral deficiencies (e.g., lymphopenia) o Lymphopenia so the decrease of lymphocytes that are the main cells that try to protect us from foreign invaders from the pathogenic bacteria. If there is decrease in number of those cell, the immune response will not be complete and effective Infection predisposing factors o Unbalanced diet o Stress o Slows down the production of immune cells o Sleep deprivation o Smoking o Affects ciliary clearance in the respiratory tract, if there is no clearance, everything that gets inside the respiratory tract can travel down from the upper airways to the lower airways and cause infection o Alcoholism o Toxic for some immune cells o Broad-spectrum antibacterial therapy o Lack of physical activity (3) Microorganism´s impact on the development of the infection process This is when we start to talk about bacterial pathogenicity factors, because bacterial pathogenicity factors are the ones that help bacteria to induce infection and infectious disease The more pathogenicity factors the bacteria has, the more pathogenic it is (not all bacteria have the same amount of pathogenicity factors). There are several groups of factors Adhesion factors Attachment is provided by surface factors (capsule, pili, cell wall elements) o Pili → has adhesions on the tip that bind glycoprotein receptors on our cells and make a strong connection to the bacteria can stay on the surface o Capsule → can attach to our cells and different cell wall elements such as membrane teichoic acids, wall teichoic acids, lipopolysaccharides When bacterium starts the infection and infection process, it needs a place to attach. Adhesion factors are the ones that help bacteria attach to our skin, mucous membranes etc. Invasion factors Invasion is provided by different enzymes After the bacteria has attached, it invades to go deeper in the tissues  Aurora Killi Enzyme Function Bacterium Colagenase Breaks down tissue collagen Clostridium spp. Coagulase Forms clot at the site of entry which S. aureus will protect the bacteria from outer factors Elastase Breaks down membranes. Creates Pseudomonas ports for bacteria to get deeper. aeruginosa Hyaluronidase Hydrolyses hyaluronic acid. Our Streptococcus spp. connective tissue is very rich in Staphylococcus spp. hyaluronic acid. Clostridium spp. Lecithinase Breaks down phosphatidylcholine in Staphylococcus spp. membranes. Clostridium spp. Streptokinase Breaks down fibrin clots. Fibrin is a Streptococcus spp. part of immune reaction, and it surrounds the site of infection and stop the pathogenic agent, but this enzyme can break down this fibrin wall and invade deeper. Evasion factors Avoidance of complement system, phagocytes, destruction of antibodies. Provided by surface factors (capsule, pili, antigens) After invasion, bacteria need to escape other defense mechanisms of our immune system o Complement system → bacterium needs to avoid complement system which is a system of proteins that gets activated and as a result they create a pore inside of the microorganisms. Through this pore, fluid is able to get inside the bacterium and the bacterium will die due to lysis. o Phagocytes → bacterium also needs to avoid phagocytes, so they are not phagocytosed o Destruction of antibodies → antibodies try to bind bacterium and neutralize them, thus the bacterium needs to destruct these antibodies The bacterium escapes these defense mechanisms by evasion factors. The main evasion factors are surface factors such as capsule, pili, and antigens o Capsule → the capsule hides all possible places where macrophages can bind (antigens or glycoproteins) and catch the bacterium. The capsule makes it more difficult for the macrophage to attach to the bacterium and more difficult to provide phagocytosis. Enzyme Functions Leucocidins Lytic leukocyte agent. Provides lysis of cells to they will The bacterium can not be able to provide their functions synthesize different Hemolysin Creates pores in the cell, including macrophages enzymes with Proteases Break down antibodies so they cannot bind the antiphagocytic bacterial cell and neutralize it activity Polysaccharide capsule Surface factors Here are different Gr+ cocci examples of bacteria Streptococcus pneumoniae Streptococcus pyogenes having either capsule M protein or surface factors Staphylococcus aureus A protein Gr- cocci Neisseria meningitidis Gr+ rods Bacillus anthracis Gr- rods Haemophilus influenzae Escherichia coli Pili  Aurora Killi Toxigenicity Bacterial ability to synthesize exotoxins or have endotoxins This is another way how the bacterium can harm the macroorganism and induce development of infectious disease Exotoxin Endotoxin Species Few Gr+ and Gr- bacteria Majority of Gr- bacteria and Listeria spp. Structure Proteins Lipopolysaccharides Encoding genes Plasmids, bacteriophages Bacterial genes Toxicity High Low Antigenicity High Low Heat resistance Heat labile Heat stable Example Cholera, tetanus, botulism Sepsis, meningococcemia Exotoxins are excreted from the bacteria o Bacterium that can synthesize exotoxins can regulate the synthesis and excretion of it o The ability to synthesize endotoxin is obtained via plasmids and bacteriophages that inject DNA into the bacteria and bacteria get more pathogenic. o Example: C. diphtheriae does not have the plasmid coding activity to synthesize exotoxin. It gets ability to cause diphtheriae only if it obtains a plasmid that contains the ability to synthesize endotoxin o Cause different symptoms and can cause specific diseases Endotoxins are inside the bacteria o The bacterium itself cannot regulate the excretion of it because it is a part of the bacterial cell wall o Endotoxins are only released during cell lysis o When the bacterial cell is destroyed, lipopolysaccharides are released in the human body and cause symptoms o Symptoms caused by endotoxins are non-specific and are very similar in each case Mechanism of endotoxin action Endotoxin work very similarly in each and every case. Endotoxin will most commonly activate Kupfer cells (and other macrophages) o This will induce synthesis of interleukin-1 (IL-1) and tumor necrosis factor o Both of IL-1 and TNF are proinflammatory cytokines which means that they are able to induce inflammation and fever since they will change the temperature set point in hypothalamus. Neutrophile leukocytes o Causing increases kinin concentration and other vasoactive substances causing vasodilation o Vasodilation is also a part of inflammation B lymphocytes o This will lead to antibody synthesis Complement system  Aurora Killi o Contains proteins o Complement system has three ways of activation, but endotoxin activation usually causes activation of alternative pathway o During the alternative pathway there are different substances secreted and activated which usually also help to increase inflammation Important bacterial exotoxins, mechanism of action, caused diseases Bacterium Toxin function Disease Gr+ rods Corynebacterium diphtheriae Inactivates protein synthesis Diphtheria Clostridium tetani Inhibits glycine release Tetanus Clostridium botulinum Inhibit acetylcholine release Botulism Clostridium difficile Activates GTFases in enterocytes Pseudomembranous colitis Clostridium perfringens Superantigen Gas gangrene Gr+ cocci Staphylococcus aureus Superantigen. Protease which disrupts Toxic shock syndrome, food desmoglein in desmosomes poisoning, scalded skin syndrome Streptococcus pyogenes Superantigen Scarlet fever Gr- rods Escherichia coli Stimulates adenyl cyclase and guanyl Watery diarrhea and cyclase. Verotoxin (enterocyte cytotoxin) bloody diarrhea Vibrio cholerae Stimulates adenyl cyclase Cholera Bordetella pertussis Stimulates adenyl cyclase. Inhibits Whooping cough chemokine receptors  Aurora Killi Pathogenesis and pathogenicity factors 1. Adhesion Bacterium needs to attach Pili, capsule, LPS, teichoic acids 2. Colonization After attachment, it needs to colonize the area IgA proteases Immunoglobulin A it an antibody usually secreted in different ports of entry such as oral cavity, mothers breast milk etc. It is a protein that tries to eliminate invader right at the port of entry. If the bacterium has the enzyme protease and is able to break down this immunoglobulin, it will be able to colonize without any problems 3. Invasion One colonization is done, some bacterium will try to invade tissues. This is done by enzymes Enzymes increase the ability to invade deeper layers of tissues and of course it can also colonize the area once it has invaded it There are some bacteria that will stay at the site of entry and not invade further (e.g., Clostridium tetani, Corynebacterium diphtheriae) 4. Evasion During all of these processes the bacterium needs to evade our defense mechanisms Evasion factors stops our immune system from killing them Enzymes, capsule, pili 5. Toxigenicity Endotoxin, endotoxin Once bacterium has evaded all of the host defenses, it can start other processes such as toxigenicity process. There are several ways the bacterium can to it Stay at the site of entry o It can stay at the site of entry and excrete exotoxin o This is what happens in case of Clostridium tetani o Exotoxin is released in the blood stream and reaches other sites of the body Enter the blood stream o Bacteremia is the presence of bacteria in the blood stream o Excrete toxins while being in the blood stream o Sepsis is when our body reacts to massively, it is a hyperbolic reaction to the presence of bacteria, and the reaction itself damages our organism 6. Contagiousness Once the bacterium has provided all the previous steps, it needs to infect another person and find its port of exit This is when we speak about contagiousness Contagiousness of microorganisms R0 → represents (on average) the number of people that a single infected person can be expected to transmit that disease to Number of people that the infected person 0 will spread the infection to. The greater the number, the greater the contagiousness Stages of infectious disease Once the bacterium has invaded our organism, started the infection, and if our immunity is not able to fight the bacterium, the disease will develop  Aurora Killi 1. Invasion of microorganism Once the bacteria or another microorganism invades the human body, the incubation period starts 2. Incubation period Incubation period is the time period between bacterial invasion and the first symptoms Symptoms do not appear directly after the bacterial invasion. There is an incubation period in- between when the bacterium is already inside the body and undergoes its pathogenesis, but no symptoms appear Incubation period differs in different diseases o Influenzae → 2-4 days o Chicken pox → 21 days 3. Prodrome After incubation period, the first symptoms appear Prodrome involves non-specific symptoms These symptoms are cold and flu-like symptoms o Headache o Runny nose o Cough o Febrile temperature Prodrome symptoms usually do not help us in diagnostics because they are too unspecific Prodrome duration is different o Influenzae → 2 hours o Other → 2 days 4. Acute period Include specific symptoms Example: “Strawberry tongue” in case of scarlet fever caused by Streptococcus pyogenus Symptoms in the acute period cannot be very pathognomonic, meaning that the symptoms will narrow the diagnosis but will not precisely show us what the possible causative agent is Example: jaundice. If we see the patient that has jaundice, we most likely understand that there might be some problem with the liver and bilirubin conjugation, so we have narrowed our field of diagnostics but still there can be some infectious reasons to it such as hepatitis A, B, and C. Acute period shows more specific symptoms than in case of prodrome 5. Outcome Either the person o Gets better and healthy o Exitus lethalis (death) Becomes chronic carrier Prevalence of infectious diseases Sporadic infections Infections that appear spontaneously and irregularly Most commonly they are not tied together, meaning that the infection in one place appear independently of an infection another place One population got infected with rod virus in one area, and simultaneously some people got infected in another area, then we suppose that these infections are not tied together but rather spontaneous and independent from each other  Aurora Killi Endemic infections Infections that are typical for exact geographic regions and population of that regions Hepatitis E virus in India, China, central Asia, and some districts in Africa When we travel to these region, we assume that we might face the infection typical there For example, malaria We can undergo some precaution measured such as getting vaccinated Epidemic infections Wide spread of infectious disease in society at specific time period and region If majority of a society gets infected with an infectious disease and spread it between one another, we understand that an epidemic has started This is the distribution prevalence of an infectious disease in a big amount of people in a specific region (it can include countries, geographic regions) Example: influenzae Pandemic infections Spread of an infectious disease in large regions, such as several continents or worldwide Example: SARS-Cov-2 which causes covid Common signs of infection process Source (there will always be a source from where the person got the infection) Presence of infectious agent Transmission, port of entry Stages of infectious disease (if it is pathogenic enough) Immune response  Aurora Killi Innate immunity Immunology lecture 2 Immunity and immunology Immunity → is a series of physiological defense reaction against any agent with foreign genetic elements and capability of activating the immune system. If we talk about any agent we mean microbes, viruses, and other pathogens Immune system → is a large regulatory system composed of o Tissues o Cells o Specialized molecules (protect us from any pathogenic microbes and viruses that enter our body) Immune system → interacts with the endocrine, nervous, and hematopoietic systems Immunology → is the study of the immune system The immune system defends the host against pathogens. It uses different recognition systems to effectively eliminate the invading pathogen or its products Immunity is composed of two parts o Natural, innate, non-adaptive immunity o Acquired, adaptive immunity Immune response → a response generated against a potential pathogen First line of defense Second line of defense Innate immunity Adaptive immunity The first line of defense is known as the innate The second line of defense is known as the immunity, which is adaptive immunity, which Non-specific to the invading pathogen Is specific for the pathogen Quick, it is rapidly mobilized at the initial site of Confers protective immunity to reinfection infection (portals of entry) with that pathogen (immunologic memory) Lack immunologic memory (does not Can specifically recognize and destroy the remember what pathogens it has been in pathogen because lymphocytes produce contact with before) specific antibodies, i.e., proteins produced in response to a particular pathogen. The substance that induces the production of antibodies it called an antigen Specific response for a specific type of invader Innate immunity Adaptive immunity Characteristics Rapid, immediate response Slow response Antigen non-specific Highly antigen specific No memory, not long-lasting protection Induces memory, responds rapidly and vigorously to second antigen exposure Immunologic components Natural barriers to infection → skin, mucous Cells → T lymphocytes-cell mediated, B- membranes lymphocytes-antibody mediates, APCs Cells → phagocytes, NK cells, innate lymphoid Mediators → secreted molecules (cytokines, cells chemokines, complement) Mediators → complement defensins, cytokines, sensors 1  Aurora Killi Summary The innate immune response is effective and critical in eliminating most pathogens If this initial mechanism fails, the adaptive immune response is activated, confronting the specific pathogen, and establishing immunity against it Both systems interact and collaborate to achieve the final goal of destroying the pathogen Innate immunity Provides an immediate response to a pathogens Does not confer long lasting protective immunity Innate immunity It is a non-specific defense system, which includes Components of innate immunity Intact skin o Barrier surfaces to infectious agents (skin and Mucous membranes and their mucous membranes) secretions o Inflammatory response Normal microbiota o Innate immune cells Phagocytes (neutrophils, eosinophils, o Complement system dendritic cells, macrophages) o Cytokines, NK cells etc. Inflammation Innate immunity works within the first 12 hours Fever After this time, the process is switched to adaptive Antimicrobial substances released immunity which reaches its effective peak from the 7th from cells day (2nd week), but of course it starts working from the day after infection Barrier function of innate immunity Barriers that protect from infections Mechanical barriers o Can mechanically and structurally protect from pathogens Chemical barriers o They can release different chemicals Biological barriers Epithelial barrier Mechanical o Epithelia at the portals of entry of microbes serves as a physical barrier to infection due to the tight junctions between epithelial cells o Microbes cannot enter the intracellular space due to the tight junctions o Marked flow of air and fluids across the surface of epithelial cells protects the surface from microbial colonization o Epithelium harbor intraepithelial lymphocytes that kill microbes and infected cells Chemical → produces a number of antimicrobial peptides that kill microbes o Fatty acids on the skin o Enzymes → for example, lysozyme that dissolves some bacterial cell walls of Gr+ bacteria o Antimicrobial peptides (AMPs) directly kill bacteria o E.g., defensins which is a peptide located in the GI and lower respiratory tract. It creates holes in the bacterial cell wall and disrupts the bacterial membrane o Acidic pH in the stomach, in sweat, and sebaceous secretions Microbiological o Normal microbiota are commensals 2  Aurora Killi o They compete with pathogenic microorganisms for nutrients, adhesion to host epithelium, and produce antimicrobial compounds o Prevent pathogen colonization o Help train the immune system o Aid with digestion (other function of microbiome, not actually included in innate immunity) o Aid with the vitamin production (other function of microbiome, not actually included in innate immunity) Epithelia throughout the body serve as a barrier, and are present in the Skin Few microorganisms can Airways of respiratory tract penetrate body surfaces Gastrointestinal tract Genitourinary tract Skin barrier The barrier surface that keeps our insides in and outside out Mechanical barrier o Skin is the largest organ (2 m2) o It has a massive surface area, and it keeps the body closed off to pathogens o Viruses cannot infect the dead cornified cells (stratum corneum is the outermost layer that contains dead cells) Skin microbiome Chemical barrier Corynebacterium Staphylococcus o pH 5.5 is quite low Propionibacterium o Release other substances such as o Urea Skin microbiome has strategies to prevent o Fatty acids colonization of pathogens (e.g., S. aureus) o Lactates etc. Production of antibiotics proteases Biological barrier (?) Production of antimicrobial o Skin barrier defense includes a unique population of substances (AMPs) skin dendritic cells or Langerhans cells o Langerhans cells extend their dendrites in stratum corneum o They mobilize innate immunity and adaptive immunity o They are tolerant to the microbiome of the skin, meaning that they do not bind or affect normal microbiome cells that must be present in our skin microbiome o These cells transmit information into deeper parts (lymph nodes) and activate adaptive immune system o Works as a bridge between the two protective systems Mucosal epithelium: respiratory tract Surface area of 100 m2 Adult person inhales about 10 000 L air/day Contains protective elements called mucus Mucus is a major component of the mucosal epithelium It is a complex mixture of o Mucins o Proteins 3  Aurora Killi o Proteases o Protease inhibitors Mechanism of action o Limits bacterial adhesion to cell surfaces o Once trapped in the mucus, bacteria are removed by ciliary clearance o Airway epithelial cells also produce defensins Nasal microbiota resembles that of the skin (Corynebacterium, Staphylococcus, Propionibacterium) Microbiota begins to diversify in the upper respiratory tract which is a major initial reservoir of pathogens that can infect the lower respiratory tract Upper respiratory tract microbiome is important for preventing colonization of pathogens that could spread to the lower respiratory tract Airway epithelial cells o Produce cytokines and chemokines to recruit immune cells to the site of infection o Alveolar macrophages carry out phagocytosis Mucosal epithelium: gastrointestinal tract Mouth o The immune defense in the gastrointestinal tract begins in the mouth o Saliva contains defensins and antimicrobial substances e.g., lysozyme Stomach o Surviving microbes pass through the stomach, which produces o Gastric acid o Digestive enzymes such as proteases and lipases that break down food and destroy many microbes Intestines o Most microbes do not survive transit through the stomach, however some move to the intestines o The intestines are lined by intestinal epithelium that covers a large surface area and is optimized for nutrient absorption and protects the body from getting infected through o Peristalsis → a unidirectional movements of intestinal content that prevents microbes from attaching and colonizing o A layer of mucus → coats the epithelium and prevent colonization o Gut microbiome → establishes ecological niches that help to prevent intestinal colonization by pathogens Epithelial lining o Regenerates every 4-5 days o Dead cells slough off and are expelled from the body o Lamina propria is full of immune cells like Peyer´s patches o Peyer´s patches contain M-cells that can transport antigens from the intestinal lumen to immune cells o Contains antigen presenting cells such as dendritic cells to active T-lymphocytes in Peyer´s patches o T-lymphocytes belongs to adaptive immunity, meaning that this is the bridge between the innate and adaptive immunity Recognition of microbes by the innate immune system The innate immune system recognizes molecular structures that are produced by microbial pathogens The microbial substances that stimulate innate immunity are called pathogen associated molecular patters (PAMPs), e.g., o Lipopolysaccharides in Gr- bacteria 4  Aurora Killi o Lipoteichoic acids in Gr+ bacteria o Complex lipids and carbohydrates synthesized by microbes o Nucleic acids of replicating viruses The innate immune system detects the presence of infection, not the specific pathogen It triggers host defense regardless of the particular species of microbe The innate immune system recognizes only about 1000 products of microbes and damaged cells. In contrast, the adaptive immune system can recognize millions of different microbial and environmental antigens The innate immune system also recognizes endogenous molecules that are released from damaged or dying cells called damage-associated molecular patterns (DAMPs), e.g., damage caused by o Chemical toxins o Burns o Trauma Pattern recognition receptors There are several types of cellular receptors present in different location in cells These receptors are called pattern recognition receptors because they recognize PAMPs and DAMPs Receptors are expressed both extra- and intracellularly. These receptors are found on: o Phagocytes (macrophages and neutrophils) o Dendritic cells o Epithelial cells o Present in phagocytic vesicles Intracellularly o Present in the cytosol When pattern recognition receptors (PPR) bind to PAMPs and DAMPs, they activate signal transduction pathway that promote antimicrobial and proinflammatory functions of the cells The innate immune system does not react against normal, healthy cells and tissues When a pathogen enters the skin, it is confronted by macrophages and dendritic cells expressing the widest variety and greatest number of PRRs This is in the line with the fundamental role of phagocytes – detecting microbes and damaged cells and ingesting them for destruction PRRs are linked to intracellular signal transduction pathways that activate various cellular responses They elicit inflammation and subsequent activation of the adaptive immunity Examples of PRRs Toll-like receptors Toll-like receptors are a type of pattern recognition receptors expressed by (TLR) many cell types They recognize products of wide variety of microbes and damaged cells TLRs are important part of the body´s non-specific resistance as they ensure signalling and activation of the specific immune response Found on the surface of different cells o Macrophages o Dendritic cells o Neutrophils o Epithelial cells of mucosal membranes o In intracellular structures 10 human TLRs have been identified Each receptor is involved in the recognition of a unique set of microbial patterns, however, TLRs do not recognize healthy mammalian cells Examples of TLRs and their function TLR2 → recognizes lipoteichoic acids expressed by Gr+ bacteria 5  Aurora Killi TLR1, TLR6 → recognize multiple diacyl peptides (mycoplasma) TLR3, TLR7, TLR8 → recognize products of viral replication. These are intracellular receptors TLR4 → specific for Gr- lipopolysaccharides TLR5 → recognizes bacterial flagellin TLR10 → remain an orphan receptor Pathway of signalling and activation 1. Receptor binds the recognized structure and transmit the signal intracellularly 2. Recruitment of adapter proteins 3. Recruitment and activation of protein kinases 4. Activation of transcription factors 5. Gene transcription 6. Expression inflammatory cytokines, chemokines, endothelial adhesion molecules, costimulatory molecules, antiviral cytokines Macrophage surface Also called scavenger receptors receptors These are expressed on the surface of macrophages Bind microorganisms and facilitate their endocytosis Cellular components and phagocytosis Phagocytes are cells whose primary function is to ingest and destroy microbes The functional responses of phagocytes in the host defense follow a set of sequential steps 1. Recruitment of the cells to the sites of infection 2. Recognition of and activation by microbes 3. Ingestion of the microbes by phagocytosis 4. Destruction of ingested microbe Any antigen (microorganism) that enters the body through the lymphatics, lungs, or bloodstream is engulfed by phagocytic cells Phagocytes are the cells responsible for the uptake and removal of the foreign antigen and they are present in o Blood o Lymphoid tissue o Liver o Spleen o Lungs and other tissues Phagocytes include o Mononuclear phagocytes → monocytes and macrophages Neutrophils Macrophages o Granulocytes → neutrophils, eosinophils, basophils Fast response Slower response o Dendritic cells Lifespan short (1- Lifespan long Although blood neutrophils and monocytes are actively 2 days) (days, weeks, phagocytic, they are significantly different in various years) ways Neutrophils Neutrophils are the most abundant population of circulating white blood cells Granulocytes (leukocytes) and the principal cell type in acute inflammatory reactions The nucleus is segmented into 3-5 connected lobules thus they are called polymorphonuclear leukocytes 6  Aurora Killi Cytoplasm contains two types of membrane bound granules. The majority of these granules (called specific granules) are filled with enzymes o Lysozyme o Collagenase o Elastase Neutrophils are important phagocytic cells that destroy pathogens, especially opsonized microbes, within intracellular vesicles An adult human produces 1 * 1011 neutrophils/day Production of neutrophils is stimulated by granulocyte colony-stimulating factor (G-CSF) Each neutrophil circulates in the blood for a few hours up to 5 days before dying Neutrophils may migrate to the site of infection rapidly after the entry of the microbe After entering tissues, neutrophils function only 1-2 days and most of them subsequently die Eosinophils & Two other types of circulating leukocytes with cytoplasmic granules (azurophilic basophils due to their staining properties) Granulocytes They are less abundant Store granules containing enzymes and toxic proteins that can be released upon activation of these cells Basophils number in tissues are low, their importance in host defense is uncertain Eosinophils are present in mucosal linings of the respiratory, GI, and genitourinary tracts. Their numbers can increase in the setting of inflammation Monocytes & Monocytes are small leukocytes that circulate in the blood and mature into macrophages macrophages that can be found in almost all tissues Mononuclear o Kupffer cells in the liver phagocytes o Microglial cells in the nervous tissue Macrophages are critical for the immune response and they o Engulf and kill pathogens o Process and present antigens (antigen present cells). They display fragments of protein antigen to T-Lymphocytes, activate them and produce a variety of molecules (cytokines) Monocytes and macrophages express class II major histocompatibility complex (MHC) Dendritic cells Dendritic cells are tissue-resident and found in mucosal epithelium and lymphoid tissues They are circulating cells that detect the presence of microbes and initiate innate immune defense reactions Dendritic cells capture microbial proteins that enter through epithelial barriers, take them to lymph nodes for display to T cells to initiate adaptive immunity They produce regulatory cytokines (IFN-α) Phagocytosis Phagocytosis is a multistep process whereby a phagocytic cell recognizes a pathogen, ingests it, and then destroys the engulfed organism Phagocytosis is an active, energy-dependent process of engulfment of large particles (> 0.5 um) 7  Aurora Killi First step o Begins once a pathogen enters the blood or tissue resulting in the migration of the phagocytic cell to that site o Chemotaxis is cell migration towards the site of infection and is dependent on the release of chemoattractant signals (IL-8) Second step o Attraction of binding o Microbes bind to phagocyte receptors (TLRs, lectin receptors, C3b receptors) which a non-specific cell surface receptors o This process is markedly enhanced by bacterial opsonization o Opsonin’s are plasma proteins which promote attraction of binding o Antibodies (e.g., IgG) o C3b Third step o Ingestion o Phagocytes engulf the pathogen by extending pseudopodia around it, internalizing it into an endocytic vesicle called phagosome o Phagosome-lysosome fusion accumulates most of the microbicidal mechanisms o Enzymes (proteolytic, hydrolytic etc.) o Lysozyme o Acidic reaction (pH 3.5-4.0) Fourth step o Digestion or killing o There are several antimicrobial mechanisms used by phagocytes to eliminate the pathogen o Microbicidal molecules in phagolysosomes o Antimicrobial peptides that participate in pathogen killing (elastase-derived peptides in macrophages, defensins in neutrophils) o Toxic oxygen-derived products o Superoxide O2- o Hydrogen peroxide H2O2 o Singlet oxygen O2 o Toxic nitrogen oxides and nitric oxides Outcome of phagocytosis Complete phagocytosis o Microbes are killed o Undigested microbial products may be released in the surrounding environment, presented to T- Lymphocyte o 1 Neu 25 microbes o 1 M/ph 100 microbes Incomplete phagocytosis o Microbes remain intracellularly o M. gonorrhoeae o N. meningitidis o M. tuberculosis Microbes are engulfed and released o Staphylococcus 8  Aurora Killi Antigens & antibodies Immunology lecture 4 Antigens Antigens are antibody generating agents Antigens have two main properties o Immunogenicity → an antigen (AG) is a substance that can induce antibody (AB) or T-lymphocyte production o Antigenicity → an antigen can react with an antibody or T cell Antigens produce immune response, and immune response can be humoral with production of antibodies, or cellular with production of T-lymphocytes, this property is called immunogenicity Produced antibodies and T-lymphocytes can then react with the antigen and that property is called antigenicity. Features that determine immunogenicity 1. Recognition of foreignness Self-molecules are not immunogenic Immunogenic molecules must be recognized as foreign or “non- self” 2. Size The most potent immunogens are usually large, complex molecules Some small molecules called haptens become immunogenic only when linked to a carrier protein 3. Chemical and structural complexity E.g., amino acid homopolymers are less immunogenic than heteropolymers that contain two or more different amino acids 4. Genetic constitution of the host due to differences in MHC alleles Some people have quite strong immunity and they do not become infected very often Another group of people might be very susceptible to different causative agents This is determined by genetic constitution 5. Dosage, route, timing of antigen administration Immunogenicity of a substance (vaccine) can be enhanced by combining it with an adjuvant Antigen activity can be enhanced in a vaccine by combining it with an adjuvant Properties of antigens Genetic properties Autologous → self-antigen Homologous, allogenic → antigen of two individuals of same species Heterologous → foreign antigen Immunologic properties Immunogenicity is the ability to provoke an immune response Antigenicity is the ability to react with an antibody o An antibody binds to only a portion of the macromolecule called epitope o The presence of multiple identical epitopes on the surface of antigens is referred to as polyvalency or multivalency 1  Aurora Killi Specificity → antigens are unique, thus causing production of unique antibodies Thymus dependent antigens (proteins) o Induce an immune response o Induce immunological memory (this is the basis of vaccination) Thymus independent antigens (haptens) o React with antibody o No induction of immunologic memory Intrinsic properties of protein antigens that influence immunogenicity Parameter Increased immunogenicity Decreased immunogenicity Size Large Small (MW < 2500) Composition Complex Simple Similarity to self-protein Multiple differences Few differences Interaction with host MHC Effective Ineffective Microbial antigens O somatic antigens o Lipopolysaccharides from bacterial cell wall o These are endotoxins o Thermostable 100 °C for 1-2 hours H flagellar protein antigen o Thermolabile 56 °C for 30 minutes K capsular antigen o Surface antigen o Located on surface of capsulated bacteria o Mixed structure Antigenic heterogeneity The surface structures of microorganisms have considerable antigenic heterogeneity These antigens are often used as a part of serologic classification systems for bacteria, e.g., o The classification of 2000 different salmonella is based principally on the types of O and H antigens o There are more than 150 E. coli O types and more than 100 E. coli K types The antigenic type of bacteria may be a marker for virulence o V. cholerae O antigen type 1 and O antigen type 139 produce the cholera toxin o Only some group of A streptococcal M protein types are associated with a high incidence of poststreptococcal glomerulonephritis Major histocompatibility complex (MHC) The human MHC molecules were first discovered in 1954, as the result of attempts to use skin grafts from donor to repairs badly burned pilots and bomb victims during WWII The genes which control the compatibility and rejection of tissue grafts were described It is understood that MHC molecules bind peptide antigens and present them to T cells for MHC is responsible for T-cell antigen recognition (TCR T-cell receptors) TCR is different from antibodies o Antibody molecules bind antigens directly o TCR only recognizes peptide antigens present to them by an MHC molecule on the surface of the APC 2  Aurora Killi Human MHC Human MHC is called the human leukocyte antigen (HLA) complex MHC is a cluster of closely associated genes in humans found on chromosome 6 Among the many important genes in the human MHC are those that encode o Classes I, II, and III o MHC proteins MHC class I proteins are encoded by o HLA-A gene o HLA-B gene o HLA-C gene MHC class II proteins are encoded by HLA-D region This locus controls immune responsiveness and and consist of three main families o HLA-DP different allelic forms of these genes confer differences in ability of an individual immune o HLA-DQ o HLA-DR response MHC class III locus encodes complement proteins and cytokines MCH Class II molecule The molecule is composed of 2 transmembrane glycoprotein chains α and ß Each chain has two domains o α1 and α2 o ß1 and ß2 Between the two domains there is a peptide-binding cleft, which can bind large peptides composed of about 10-20 amino acids The MHC class II proteins have a rather restricted tissue distribution and are expressed on the surface of o Macrophages o Dendritic cells Antigen presenting cells o B cells The MHC class II molecules are synthesized in the endoplasmic reticulum (ER), and they proceed through the Golgi apparatus and are transported to an endosome Antigen processing and presentation: MHC Class II 1. Proteins from exogenous antigens, such as bacteria, are internalized by APC such as dendritic cells or macrophages 2. These proteins undergo denaturation or partial proteolysis in the endocytic vesicles with the antigen presenting cells 3. While in the endosomal compartment, these peptide fragments fuse with the MHC class II molecules 4. Bound peptides are transported and expressed on the surface of the APC as the peptide-MHC complex 5. MHC class II molecules present peptide molecules to CD4+ T cells In other words 1. Microbes are engulfed by macrophages, and they are destroyed by different mechanisms. During the phagocytosis process the MHC class II molecules will be present in these endosomes 3  Aurora Killi 2. When the microbe is destroyed in the phagocytosis process, peptides bind the MHC class II molecule by the peptide binding cleft 3. The bound peptides are transported with the vesicle to the surface of the cell, and they are expressed on the surface of antigen presenting cells as the peptide-MHC complex. The cell that receives this information or MHC class II molecules, present these peptides to CD 4 T cells MCH Class I molecule Consists of a heterodimer of a membrane-spanning α chain non-covalently associated with ß2- microglobulin which does not span the membrane The α chain folds into three domains o α1 o α2 o α3 The folding of the α1 and α2 domains creates a long peptide binding cleft which can bind about 9 amino acids The gene of ß2-microglobulin is located on chromosome 15 MHC class I molecules are expressed by nearly all nucleated cells in the body (except in the retina and brain) MCH class I molecules are present in the endoplasmic reticulum and never reach vesicles, so they remain in the ER Antigen processing and presentation: MHC Class I 1. The interaction of antigens within a virus- infected cell (intracellular) 2. During viral replication process they reach ER and there are MHC class I molecules present 3. The viral molecules bind peptide binding cleft of the MHC class I molecule 4. MCH class I-peptide antigen complex is transported to the cell surface for display and recognition by CD8+ cytotoxic T-Ly 4  Aurora Killi Antibodies Produced by B cells and is a part of humoral immune reaction of adaptive immunity Antibody function Antibodies are immunoglobins that react specifically with the antigen that stimulated their production AB make up about 20% of plasma proteins Antibodies that are generated in response to a single complex antigen are heterogenous (formed by many different clones of cells) In humoral immune response, antibody-producing plasma cells (differentiated B cells) produce immunoglobins Antibody structure is composed of four chains o The green parts are shorter and called light-chains o The pink chains are longer and called heavy chains Antibody structure An antibody molecule has a Y shape and consists of 4 polypeptide chains All immunoglobulin molecules are composed of o Light polypeptide chains o Heavy polypeptide chains The terms light and heavy refer to their molecular weight o Light chains have a molecules weight of 25 000 o Heavy chains hace a molecular weight of 50 000 Each immunoglobin molecule consist of two identical light chains and two identical heavy chains linked by disulfide bridges In the center of the antibody molecules there are double disulfide bonds called the hinge region o The hinge region provides flexibility so it can reach antigens o Promotes functional activity The light and heavy chains are subdivided into o Variable regions → involved in AG binding (Fab fragment) o Constant regions → responsible for C´ fixation, attachment to various cells, e.g., macrophages (Fc fragment) The regions are composed of repeating segments folded in a three- dimensional configuration called domains o L chains are composed of o One variable domain VL o One constant domain CL o H chains are composed of o One variable domain VH o Three constant domains CH1-3 Within the variable regions of both light and heavy chains are subregions consisting of extremely variable amino acid sequences called hypervariable regions that form antigen-binding site The hypervariable regions form the area of the Ig molecule complementary in structure to the antigenic epitope This area is known as the complementarity-determining region (CDR) 5  Aurora Killi Light chains can be either of two variants depending on the amino acid difference in their constant regions No functional difference has been o λ lambda found between antibodies having o κ kappa λ or κ light chains Heavy chains are distinct for each of the five immunoglobins classes and are designated o γ gamma → IgG (monomeric) o µ micro → IgM (pentameric) o α alpha → IgA (di- and trimeric) o Δ delta → IgD (monomeric) o ε epsilon → IgE (monomeric) Heavy chains determine the functional activity of the immunoglobulin Ig molecule Functional activity of Ig Monomeric molecules are monovalent because they can bind one antigen or two antigens (maximum) Pentameric molecules are polyvalent and can bind 10 antigens Valency refers to how many antigens the immunoglobulin molecule can bind The hinge region makes the molecule very flexible so the Ig can reach the antigens Protective functions of antibodies Neutralization o Antibodies do not destroy the antigen o They rather bind to the antigen to prevent toxins from binding to surface receptors of antigens o By this binding the antibody neutralizes the antigen o The antigen-antibody complexes are ingested by macrophages (phagocytosis) Opsonization o If bacteria are covered by opsonin’s (immunoglobulins G or C3) they enhance phagocytosis process and macrophages ingest them very actively Complement activation o Antibodies bind to antigen on bacteria forming the Ag-Ab complex that activates the complement by its classical pathway o That leads to the last stages of the complement activation which is C5-C9 and pore formation causing cell lysis Immunoglobulins Five classes of immunoglobulins Properties IgG IgM IgA IgD IgE Structure Heavy chains γ µ α δ ε Nr. of antigen binding sites 2 10 4 2 3 Molecules 150 000 900 000 385 000 180 000 200 000 weight (Daltons) % of total antibody in 80% 6% 13% < 1% < 1% serum 6  Aurora Killi Crosses placenta Yes No No No No Fixes Yes Yes No No No complement Fc binds to Phagocytes - - - Mast cells and basophils Functions Neutralization Neutralization Neutralization B-cell receptor Activation of Agglutination Agglutination and trapping of basophils and Complement Complement pathogens in mast cells activation activation mucus against parasites Opsonization Monomer form and allergens Antibody- serves as B-cell dependent cell- receptor mediated cytotoxicity Immunoglobin G (IgG) IgG is the major class of Ig present in the serum Percentage of total antibody in serum is 80% of the total Ig pool There are 4 subclasses → Ig1-4 Each subtype contains a distinct, yet related heavy chain o γ1, γ2, γ3, and γ4 The only Ig class to cross the placenta and is therefore, the most abundant Ig in newborns o At the moment of birth, newborns have 100% of adult human level of IgG o They have crossed the placenta and reaches the fetus, thus it is in such high concentration o This is called passive immunity, meaning that it is passively transported from mother to fetus Mediates opsonization of antigen Complement fixation and activation by classical pathway Immunoglobulin M (IgM) First immunoglobin produced in response to an antigen o IgG however are produced after a few days The neonate (fetus) produces its own IgM IgM is secreted as a pentamer with one molecule of a J chain (joining chain) The pentamer has a molecular weight of 900 000, and a total of 10 identical antigen-binding sites It is the most efficient immunoglobin in o Agglutination o Complement fixation o Defense against bacteria and viruses as it has 10 antigen binding sites Immunoglobulin A (IgA) The major immunoglobin responsible for mucosal immunity The levels of IgA in serum are low, constituting only 10-15% of the total serum Ig In contrast, IgA is the predominate class of Ig found in extravascular secretions o Milk o Saliva o Tears o Secretions of respiratory, intestinal, and genital tracts Mainly produced by plasma cells located in glands and mucous membranes 7  Aurora Killi IgA if the first line of defense against bacteria and viruses in the portals of entry Properties of IgA molecules are different depending on where the IgA is located o In serum o Secreted as a monomer that resembles IgG o In mucous secretions o Secreted as a dimer and referred to as secretory IgA o A secretory component is incorporated into the secretory IgA when it is transported through an epithelial cell o Two IgA subclasses are distinguished: IgA1 and IgA2 Formation of secretory IgA (s-IgA) 1. IgA dimers are secreted into the intestinal lamina propria by plasma cells and bind to the poly-Ig receptor on the surface of epithelial cell 2. The secretory IgA-receptor complex is endocytosed and transported across the cell 3. These transport vesicles fuse with the plasma membrane at the luminal surface, releasing IgA dimers with bound secretory component derived from the cleavage of the receptor 4. The secretory IgA is produced from proteolytic enzymes in the lumen by the presence of this secretory component Immunoglobulin E (IgE) Present in serum in very low quantities The Fc region of the IgE binds to its high-affinity receptor on the surface of Fc region is on the o Mast cells bottom of the antibody o Basophils molecule o Eosinophils The bound IgE acts as a receptor for the specific antigen that stimulated its production The resulting antigen-antibody complex triggers allergic responses of the immediate (anaphylactic) type through the release of inflammatory mediators such as histamine Immunoglobulin D (IgD) IgD is the only present in trace amounts However, it is the major surface-bound immunoglobulin on mature B lymphocytes These B-cells contain IgD and IgM at a ratio of 3:1 At the present time, the functions of IgD are unclear 8  Aurora Killi T lymphocytes Immunology lecture 5 Lymphoid organs Immune response is realized by lymphoid system Lymphoid organs weigh about 1.5-2 kg Make up about 2% of the total body weight Lymphoid organs are organized by tissues characterized by very large numbers of lymphocytes interacting with a non- lymphoid stroma Primary lymphoid organs Secondary lymphoid organs The main primary lymphoid organs are The main secondary lymphoid organs are o Bone marrow o Lymph nodes o Thymus o Spleen Lymphocytes are generated about 109 Ly/day o Mucosal-associated lymphoid tissues Antigen independent proliferation (tonsils, Peyer´s patches) Regulation of immune response Adaptive immune responses are initiated Antigen binding Antigen dependent proliferation Cells of adaptive immune response 1. T lymphocytes CD4 + Thelpers CD8 + Tcyt CD are clusters of differentiation 2. B lymphocytes 3. APC Lymphocytes Functions 1012 cells 1. Bind antigens and become o In blood → 30-40% of total amount activated o In lymph → 90% 2. Differentiation into effector Properties of lymphocytes cells or working cells o Mobility 3. Proliferation o Sensitivity to cells transforming factors (mitogens) 4. Immunological memory o Life cycle (days/months) 5. Production of lymphokines T lymphocytes T lymphocytes migrate at a very early stage to the T cells migrate precursors migrate to the thymus thymus to mature. They derive from the o In the thymus, the receptor gene rearrangement bone marrow stem cells, whose progeny and maturation of T cells occur migrate from the bone marrow to the The crucial different in T and B lymphocytes is how to thymus, where the development of T recognize an antigen cells take place. Mature T cells leave the o B cells can recognize an antigen directly because thymus and recirculate from the they have such receptors on their surface bloodstream through secondary o T lymphocytes are MHC restricted and recognize lymphoid tissues such as lymph nodes, foreign antigens only in the form of peptide spleen, or Peyer´s patches, where they fragments bound to molecules encoded by the MHC may encounter antigen. 1  Aurora Killi Development of T lymphocytes The development of T cells can be divided into two types 1. Antigen independent → each individual´s T cells are able to recognize foreign antigenic peptides 2. Antigen dependent → receptor binds antigen peptide (we will look at this one) Mechanism of development 1. The primary lymphoid organs, the bone marrow and thymus, have stem cells and T lymphocyte mature 2. Naive T and B lymphocytes then will reach the secondary lymphoid organs → spleen and lymph nodes 3. These lymphocytes are ready to bind an antigen, to recognize and antigen (Naive T and B cells are lymphocytes before contact with an antigen) 4. An antigen (microbe) then enters our body, and it is present in the skin and portals of entry, and is ready to bind to the naive cells 5. In the secondary organs (spleen and lymph nodes) the naïve T and B cells are activated cells and becomes effector (working) cells 6. They are now ready to destroy the antigens, which happens in the peripheral tissue Differentiation of T lymphocytes in the thymus 1. Pro-thymocyte The first cell recognized in thymus in the subcapsular region is pro-thymocyte It has surface receptors and differentiation antigen The surface receptor is CD2+ and it remains on the surface of T-lymphocyte during maturation Pro-thymocyte migrate deeper in thymus to the medulla 2. Pre-thymocyte Pro-thymocyte become pre-thymocyte The pre-thymocyte has two other clusters of differentiation on its surface → CD4+ and CD8+ CD2+ remains from the pro-thymocyte so the pre-thymocyte has a total of three clusters of differentiation on its surface (CD2+, CD4+, CD8+) Such a thymocyte is called double-positive thymocyte Why is it called double positive and not triple positive when there are three cell clusters? o Pre-thymocytes contain two absolutely different clusters of differentiation The main difference between o CD4 cells are helpers CD4 and CD8 T cells is that the CD4 T cells are the helper T cells, o CD8 cells are cytotoxic and kill the target cells which assist other blood cells to o Helpers and killers cannot exist together further on produce an immune response, o In this stage however, they are both present and therefore whereas the CD8 T cells are the we focus on these two clusters cytotoxic T cells that induce cell o Thus, these cells are given the name double-positive, even death either by lysis or apoptosis though there are three clusters 2  Aurora Killi 3. Immature CD3+4+8+ double-positive thymocytes Not mature since thymocytes with CD 4+ and CD8+ cannot exist Due to somatic gene recombination, T cell receptors (TCR) are synthesized on the surface of T cells T cell receptors are very important and are necessary for antigen recognition Structure of T cell receptor (TCR) o Located on the surface of T-lymphocyte o These receptors are always in association with the CD 3 cluster of differentiation o Composed of 8 polypeptide chains o 2 chains → disulfide-bonded chains of the T-cell receptor that recognizes antigen peptide o 6 chains → collectively called CD3. Four of the chains are located externally, while two internally, however all six are a part of CD3 complex CD3 externally receives information about an antigen from the T cell receptor and then further transmit the signals intracellularly in T lymphocyte to two other CD3 molecules that are located internally 4. Immature single positive thymocyte 95% of the double positive thymocytes die by apoptosis in the thymus Remaining 5% mature and become single positive and are exported from the thymus to periphery o Either CD4+ and CD8- o Or CD4- and CD8+ TCR:CD3 remains either way (also in the next stages) The clonal selection and deletion 5. Naive mature T-cells Two subclasses of T lymphocytes reach the periphery o ThCD4+ have surface molecules to bind class II MHC + peptide (Th = helper) o TcCD8+ bind class I MHC + peptide (Tc = cytotoxin) When single positive cells reach the periphery they become naive mature T-cells (either CD4 or CD8). In the periphery, naive mature T-cells proliferate and differentiate Surface molecules on T lymphocytes TCR receptors → always in association with CD3 for recognition of an antigen and transmission of the information intracellularly CD4 or CD8 → either one is found on the surface, and they are different in structure. Important in signal transduction CD28 → necessary for transmission of co-stimulating signals Integrin → important for adhesion Stages of Stem cell Pro-T Pre-T Double Single Naive mature maturation positive positive T cell TCR DNA, Unrecombined Unrecombined Recombined ß Recombined Recombined Recombined ß, RNA (germline) (germline) chain gene ß, α chain ß, α chain α chain gene DNA DNA gene gene ß chain mRNA α and ß chain α chain mRNA α chain mRNA mRNA TCR None None Pre-T Membrane αß Membrane Membrane αß expression receptor TCR αß TCR TCR Surface c-kit+ c-kit+ c-kit + CD4 + CD8+ + CD4 + CD8 or CD4 + CD8- or + - + markers CD44+ CD44+ CD44 - TCR/CDRlo CD4- + CD8+ CD4- + CD8+ CD25- CD25+ CD25+ CR/CDRhi TCR/CDRhi Anatomic Bone marrow Thymus Periphery site 3  Aurora Killi Response None None None Positive and Activation to antigen negative (proliferation selection and differentiation) Clonal selection and deletion Single progenitor cell gives rise to many lymphocytes, each which a different specificity seen in different colors Removal of potentially self-reactive immature lymphocytes by clonal deletion o Self-reactive means that these lymphocytes are reacting with self-antigens o Antigens of kidneys, liver, and bones (etc.) o They are removed by clonal deletion Pool of mature naive lymphocytes o Only three cells remain as mature naive lymphocytes o They are called naive lymphocytes before reacting with an antigen Proliferation and differentiation of active specific lymphocytes to form a clone of effector cells o By binding they proliferate and produce a pool of effector cells o This pool is a clone of the effector cell (working cells) o All these cells are ready to bind and eliminate an antigen Mechanism of CD4 and CD8 in antigen presentation CD8+ CD4+ Intracellular infection (virus) Extracellular infection (bacteria) Viral particles in cytosol bind as peptides MHC Ingulfed in phagocytosis by vesicle derived class I which is present in the endoplasmic antigen reticulum Peptide in the vesicle binds to MHC class II and They present the peptides on the surface on they are presented to the CD4+ cell the cell to CD8+ cell The receptor receiving this information is The receptor receiving this information is TCR/CD3 and the T cell become active TCR/CD3 Activation of naive CD4 T cell T helper cells can recognize MCH Class II + peptide on the surface of dendritic cells and antigen presenting cells o T helpers do not recognize the pure antigen, only antigen fixed on the surface of MHC T helper cells requires 3 signals to become activated 1. First activation signal o MHC Class II + peptide on surface of antigen presenting cell → The first signal is provided by binds CD4 and TCR/CD3 interaction of TCR with o Then the CD3 molecule transmits 1 activation signal st antigenic peptide presented by 2. Second activation signal class II MHC molecules on the o B7 present on antigen presenting cells binds CD28, which antigen presenting cell transmits costimulatory signal or 2nd activation signal The second signal 3. Third activation signal (costimulatory) is mediated by binding of B7 on the o Activated M/ph, DC → IL-1, IL-2, IL-6 macrophage to the CD28 on T o 3rd activation signal is cytokine signal cells o Cytokines must be sent from antigen presenting cell to T The third signal is cytokine helper cell signal sent from APC to T o These cytokines are interleukin 1, 2, and 6. Now the T cell is helper cell completely activated because it has received a total of three signals 4  Aurora Killi Results of T-helper activation Proper activation of the T helper cell promotes production of IL-2 and increases expression of IL-2 receptors on the T helper cell surface, enhancing the cells own ability to bind and maintain activation by IL-2 Binding of IL-2 induces T-cell proliferation, so effector and memory T cells are generated o Effector cells → produce cytokines IL-4 and IL-5 (Bly stimulating factor) o Memory cells → Activated T cells express CD40L on the surface, which will interact with the CD40 molecule on the antigen presenting cell and B lymphocyte, resulting in mutual activation of the T cell and APC Functions of T effector cells The different types of T-helper cells are defined by o The cytokines they produce o And thus, the immune response they induce Proliferating CD4 T cells can become one of five main categories of effector T cells o TH1 cells o Th2 cells o Th17 cells o Tfh cells (follicular helper) o T reg cells Th0 are uncommitted to a specific response and their activation initiates a generic response by producing cytokines that promote Ly growth and activate DC to expand the immune response Subclasses of T-helper cells T-helpers Explanation T-helper 17 Mature from T-helper 0 CD4 effector cell Initial antibact