Chapters 13-15: Viruses, Viroids and Prions PDF

Chapter 13 Viruses, viroids and prions Copyright © The McGraw-Hill Companies, Inc. Permission required for reproductio...

Chapter 13 Viruses, viroids and prions Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1 Viruses: Obligate Intracellular Parasites  Viruses are simply genetic information: composed of DNA or RNA contained within protective protein coat Inert particles: no metabolism, no replication Viral genome hijacks host cell’s replication machinery in order to replicate Infectious agents, but not alive General Characteristics of Viruses  Viruses are very small.  They cannot be seen with an ordinary light microscope.  An electron microscope is needed to see them General Characteristics of Viruses  Virion (virus particle) is composed of nucleic acid surrounded by a protein coat Protein coat is called the capsid: it protects the nucleic acid Composed of identical subunits called capsomeres Enveloped viruses have lipid bilayer envelope Naked viruses lack envelope; more resistant to disinfectants General Characteristics of Viruses General Characteristics of Viruses  Viral genome is either DNA or RNA, never both Useful for classification (i.e., DNA or RNA viruses) Genome is linear or circular, double- or single-stranded Affects replication strategy  Viruses have protein components for attachment Phages have tail fibers Many animal viruses have spikes Tail fibers and spikes allow virions to attach to specific receptor sites E.g. flu virus has H spikes that allow it to attach to respiratory epithelium General Characteristics of Viruses  Three shapes: Icosahedral (Polyhedral) Helical Complex General Characteristics of Viruses  Virus families end in suffix -viridae Names follow no consistent pattern Some indicate appearance (e.g., Coronaviridae from corona, meaning “crown”) Others named for geographic area from which first isolated (e.g., Bunyaviridae from Bunyamwera in Uganda, Africa)  Genus ends in -virus (e.g., Enterovirus)  Species name often name of disease E.g., poliovirus causes poliomyelitis Viruses commonly referred to only by species name General Characteristics of Viruses  Viruses often categorized informally.  For example, unrelated viruses sharing routes of infection are often grouped Oral-fecal route: enteric viruses Respiratory route: respiratory viruses Zoonotic viruses cause zoonoses (animal to human) Arboviruses (from arthropod borne) are spread by arthropods - Cause important diseases: dengue fever, West Nile encephalitis Bacteriophages  There are three general types of bacteriophages, classified based on relationship with host Lytic phages (kill host) Temperate phages (do not kill host) Bacteriophages - Lytic Phage Infections Lytic or virulent phages exit host after replicating Host cell is lysed during the exit This is a productive infection: new particles are formed T4 phage Five step process Attachment Genome entry Synthesis Assembly Release Bacteriophages - Lytic Phage Infections Bacteriophages - Lytic Phage Infections Attachment Phage attaches to receptors on the bacterial surface Genome entry Viral lysozyme degrades cell wall Tail contracts, injects genome through cell wall and membrane Synthesis of proteins and genome occurs Early proteins are translated within minutes; nuclease degrades host DNA; protein modifies host’s RNA polymerase to not recognize its own promoters Late proteins are then made, and include structural proteins (capsid, tail); produced toward the end of the cycle Bacteriophages - Lytic Phage Infections Assembly (maturation) Some components spontaneously assemble, while others require protein scaffolds Release Cell lyses and releases new phages Burst size (number of particles released from an infected cell) of T4 is ~200 Bacteriophages - Temperate Phage Infections These phages have the option of the lytic life cycle or the incorporation of their DNA into host cell genome Incorporation of viral DNA into the host genome is called a lysogenic infection Infected cell is referred to as a lysogen Lambda (λ) phage is a model for the lysogenic life cycle Bacteriophages - Temperate Phage Infections Bacteriophages - Temperate Phage Infections Lambda (λ) phage: has a linear chromosome but after injection of the DNA into the host cell, the phage chromosome circularizes The resulting circular molecule either directs lytic infection or integrates into E. coli chromosome If integration occurs, the phage enzyme integrase inserts the viral DNA at specific site in the host chromosome Integrated phage DNA termed prophage It replicates with host chromosome Can be excised by phage-encoded enzyme, resulting in a lytic infection. A repressor protein usually prevents this, maintaining a lysogenic state Bacteriophages - Temperate Phage Infections A lysogen is immune to superinfection (infection by same phage) Repressor maintaining integrated prophage also binds to operator on incoming phage DNA and prevents gene expression: immunity to superinfection It may undergo lysogenic conversion Change in phenotype of lysogen from presence of prophage – the bacterial cell acquires a new characteristic form the phage gene E.g., toxins encoded by phage genes; only strains carrying prophage produce the toxins Bacteriophages Roles of Bacteriophages in Horizontal Gene Transfer  Generalized Transduction Results from packaging error during phage assembly Some phages degrade host chromosome; fragments of the host chromosome can be mistakenly packaged into phage head These phages with host chromosome in them are called generalized transducing particles These phages cannot replicate Following release, can bind to new host, inject DNA DNA may integrate via homologous recombination, replacing host DNA Any gene from donor cell can be transferred (which is why the transduction is generalized) Generalized Transduction Roles of Bacteriophages in Horizontal Gene Transfer  Specialized Transduction Results when an excision mistake is made during transition from lysogenic to lytic cycle of temperate phage A short piece of bacterial DNA next to the viral prophage is removed when the viral DNA is excised; a piece of phage DNA remains in the host chromosome Excised DNA is incorporated into new phage heads; defective particles are released These particles can bind to new host, inject DNA Bacterial DNA may integrate via homologous recombination Only bacterial genes next to integrated phage DNA are transferred Specialized Transduction Bacterial Defenses Against Phages  Restriction-Modification Systems: protect bacteria by degrading phage DNA using enzymes Restriction enzymes recognize and cut up short nucleotide sequences (i.e., incoming phage DNA) Bacteria have different REs; hundreds of Res exist Modification enzymes usually protect host DNA from REs by methylating sequences that are normally recognized by restriction enzymes. Restriction enzymes now do not recognize Bacterial Defenses Against Phages Methods Used to Study Bacteriophages  Viruses multiply only inside living cells (obligate intracellular parasites) Must cultivate suitable host cells to grow viruses Plants and bacteria easier to grow than animal cells Plaque assays are used to quantitate phage particles in samples: sewage, seawater or soil Soft agar inoculated with bacterial host and specimen, poured over surface of agar in Petri dish Bacterial lawn forms Zones of clearing form where bacterial lysis has occurred. These are called plaques Counting plaque forming units (PFU) gives titer Methods Used to Study Bacteriophages Animal Virus Replication  Five-step infection cycle: attachment, penetration and uncoating, synthesis, assembly and release Attachment: Viruses bind to receptors (usually glycoproteins on plasma membrane of host cell) Normal function of these receptors is unrelated to viral infection Specific receptors are required; this limits range of virus E.g., dogs do not contract measles from humans Animal Virus Replication Penetration: by fusion (enveloped viruses) or endocytosis (naked viruses) Uncoating: Because the whole virus enters the whole cell, the capsid is still intact. This must be degraded by proteases, in a step called uncoating Animal Virus Replication Animal Virus Replication Synthesis Expression of viral genes to produce viral structural and catalytic genes (e.g., capsid proteins, enzymes required for replication) Synthesis of multiple copies of genome Most DNA viruses multiply in nucleus Enter through nuclear pores following penetration Three general replication strategies depending on type of genome of virus – DNA viruses – RNA viruses – Reverse transcribing viruses Animal Virus Replication Replication of RNA viruses Majority single-stranded; replicate in cytoplasm Requires virally-encoded RNA polymerase (called replicase), which lacks proofreading ability; this leads to errors during replication Allows antigenic drift (small changes in viral proteins) – E.g., influenza viruses Animal Virus Replication Replication of reverse-transcribing viruses Retroviruses have ss (+) RNA genome (e.g., HIV) Encode reverse transcriptase: makes DNA from RNA Reverse transcriptase synthesizes single DNA strand Complementary strand then synthesized; dsDNA is integrated into host cell chromosome Integrated virus (prophage) can direct productive infection or remain latent Cannot be eliminated by host cell Animal Virus Replication Assembly Protein capsid forms; genome, enzymes packaged into capsid Takes place in nucleus or in organelles of cytoplasm Release Enveloped viruses released mostly via budding Viral protein spikes insert into host cell membrane; matrix proteins accumulate; Naked viruses released when host cell dies, often by apoptosis initiated by virus or host Animal Virus Replication Categories of Animal Virus Infections  Acute Infections  Rapid onset  Short duration Categories of Animal Virus Infections  Persistent Infections Continue for years or lifetime May or may not have symptoms Some viruses exhibit both acute and persistent phases (e.g., HIV) Persistent infections are chronic or latent Chronic infections: continuous production of low levels of virus particles (example: Hepatitis B) Latent infections: viral genome (provirus) remains silent in host cell but can be reactivated (example: chickenpox) Provirus integrated into host chromosome or replicates separately, much like a plasmid Cannot be eliminated Can later be reactivated Categories of Animal Virus Infections Categories of Animal Virus Infections Viruses and Human Tumors  A tumor is caused by abnormal cell or tissue growth  Cancerous or malignant tumors can metastasize (spread); benign tumors do not  Control of cell growth involves proto-oncogenes (that stimulate cell growth) and tumor suppressor genes (that inhibit cell division)  Mutations in these genes may cause abnormal and/or uncontrolled growth (usually multiple changes at different sites required)  Some viruses have oncogenes that are similar to host proto-oncogenes; these can interfere with host control mechanisms, inducing tumors Cultivating and Quantitating Animal Viruses  Viruses must be grown in an appropriate host Historically done by inoculating live animals Embryonated (fertilized) chicken eggs later used Cell culture or tissue culture now commonly used Can process animal tissues to obtain primary cultures Problem is that cells divide only limited number of times Tumor cells therefore often used, because these multiply indefinitely Cultivating and Quantitating Animal Viruses Other Infectious Agents: Prions  Prions are proteinaceous infectious agents Composed solely of protein; no nucleic acids Linked to slow, fatal human diseases and animal diseases Usually transmissible only within species Example: Mad cow disease in England killed >170 people Other Infectious Agents: Prions Prion proteins accumulate in neural tissue Neurons die Tissues develop holes Brain function deteriorates Characteristic appearance gives rise to general term for all prion diseases: transmissible spongiform encephalopathies Other Infectious Agents: Prions Cells produce normal form of prion protein PrPC (prion protein, cellular) Proteases readily destroy these proteins Abnormal prion proteins sometimes arise or are contracted by some means e.g from surgical equipment, food etc PrPSC (prion protein, scrapie) Resistant to proteases; become insoluble and aggregate Unusually resistant to heat, chemical treatments Abnormal prion proteins are infectious. It has been hypothesized that PrPSC converts PrPC folding to PrPSC Other Infectious Agents: Prions  Chapter 14 The Innate Immune Response Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1 Immune System: Innate and Adaptive Immunity  To microbes, the human body is a nutrient-rich environment  Surprisingly however, blood, muscles, bones, and organs are generally sterile  This is partly because skin and mucous membranes prevent microbe entry into the body  Also, sensor systems detect invaders, and make a response  There are two types of immunity:  Innate immunity is routine protection. Although considered non-specific, it involves pattern recognition of specific molecules (example: bacterial cell walls) Adaptive immunity develops throughout life and is more specialized: antigens cause immune response, and the body then system produces antibodies to bind to antigens, targeting them for destruction Overview of the Innate Defenses  First-line defenses are barriers blocking entry  If invaders breach the first line, sensor systems detect them and send out signals  Innate defenses work to destroy invaders First-Line Defenses  First line defenses, also known as the non-specific immune system comprises the cells and mechanisms that defend the host from infection by other organisms in a non-specific manner. They include: Anatomical and physical barriers (skin, mucous membranes, antimicrobial substances, normal microbiota) Second Line Defenses Blood cells of the immune system Sensor systems (pattern recognition receptors and the complement system) Phagocytosis Inflammation Fever First-Line Defenses: Physical Barriers  Skin Difficult for microbes to penetrate Dermis: tightly woven fibrous connective tissue Epidermis: many layers of epithelial cells Outermost are dead, filled with keratin (repels water, maintains dry environment) Continually slough off along with any attached microbes First-Line Defenses: Physical Barriers  Mucous Membranes Digestive, respiratory, genitourinary tracts are all lined by mucous membranes These are constantly bathed in secretions (mucous) Peristalsis of intestines, and the mucociliary escalator of the respiratory tract remove microbes trapped in secretions First-Line Defenses: Physical Barriers  Antimicrobial Substances Protect skin and mucous membranes. Include: Salt from perspiration Lysozyme degrades peptidoglycan Peroxidase enzymes break down hydrogen peroxide to form ROS that kill bacteria Lactoferrin binds iron Defensins form pores in microbial membranes First-Line Defenses – Normal Microbiota  Normal Microbiota Competitive exclusion of pathogens Normal microbiota cover attachment sites of competing bacteria, consume available nutrients etc Production of toxic compounds Propionibacterium degrade lipids, produce fatty acids that reduce pH E. coli may synthesize colicins (toxic proteins) in intestinal tract Disruption of normal microbiota (e.g., antibiotic use) can predispose person to infections Clostridium difficile in intestine Second-Line Defenses - Cells of the Immune System Blood cells originate from hematopoietic stem cells Found in bone marrow Three general categories Red blood cells (erythrocytes) carry O2 Platelets (from megakaryocytes) involved in clotting White blood cells (leukocytes) important in host defenses – Four types: granulocytes, mononuclear phagocytes, dendritic cells, lymphocytes The Cells of the Immune System The Cells of the Immune System  Granulocytes Contain cytoplasmic granules Neutrophils engulf and destroy bacteria, other material by phagocytosis Basophils involved in allergic reactions, inflammation – Mast cells similar; found in tissues Eosinophils fight parasitic worms – Also involved in allergic reactions The Cells of the Immune System  Mononuclear Phagocytes Includes monocytes (circulate in blood) and cell types that develop as they leave blood stream Macrophages and dendritic cells differentiate from monocytes. Often named after location where found in body The Cells of the Immune System  Dendritic Cells Sentinel cells, function as “scouts” Engulf material in tissues, bring it to cells of adaptive immune system for “inspection”  Lymphocytes Responsible for adaptive immunity B cells, T cells highly specific in recognition of antigen Found in lymph nodes and lymphatic tissues Natural killer (NK) cells lack specificity Cell Communication  Communication between blood cells allows a coordinated response to microbial invasion Surface receptors serve as “eyes” and “ears” of cell These are proteins that usually span the membrane, connect outside of cell to inside Binding to specific ligand (compound) induces response Cytokines are “voices” of cell Produced by one cell, diffuse to others, bind to appropriate cytokine receptors to induce changes such as growth, differentiation, movement, cell death Adhesion molecules allow cells to stick to other cells E.g., endothelial cells can adhere to phagocytic cells in the blood, allowing the phagocytes to exit bloodstream Cell Communication Cytokines Chemokines: chemotaxis of immune cells Colony-stimulating factors (CSFs): multiplication and differentiation of leukocytes Interferons (IFNs): control of viral infections, regulation of inflammatory response Interleukins (ILs): produced by leukocytes; important in innate and adaptive immunity Tumor necrosis factor (TNF): inflammation, apoptosis Pattern Recognition Receptors (PRRs)  Pattern recognition receptors (PRRs) allow the body’s cells to detect signs of microbial invasion.  Three types: Toll-like receptors NOD-like receptors RIG-like receptors Pattern Recognition Receptors (PRRs) Toll-like receptors (TLRs) are anchored in membranes of sentinel cells (e.g., macrophages, dendritic cells and cells lining sterile body sites) When TLR detects a compound, it sends a signal to the nucleus Signal induces gene expression, causing a response of some kind (inflammatory response, antiviral response) Pattern Recognition Receptors (PRRs) NOD-like receptors (NLRs) found in cytoplasm Detect bacterial components indicating cell has been invaded; some detect cell damage Initiate a series of events to protect host Sometimes damage the host cell Pattern Recognition Receptors (PRRs) RIG-like receptors (RLRs) found in cytoplasm Detect viral RNA indicating viral infection. Infected cell produces interferons as a response Interferons diffuse to neighboring uninfected cells, causing them to express inactive antiviral proteins (iAVPs) AVPs are activated by viral dsRNA. They degrade host mRNA, stop protein synthesis, and cause the host cell to undergo apoptosis Pattern Recognition Receptors (PRRs) The Complement System  Assists activities of adaptive immune system, hence the name “complement” Complement proteins are inactive proteins circulating in blood and bathing tissues Proteins named in order discovered: C1 through C9 Become activated when split into fragments e.g. C3 is split to C3a and C3b C proteins become activated in response to signals indicating microbial invasion Activation of one protein leads to cascade of activation Activated forms of complement protein have specialized functions in helping host destroy and remove invader The Complement System Activated by three different pathways Alternative pathway triggered when C3b binds to foreign cell surfaces Lectin pathway: pattern recognition molecules (mannose-binding lectins, or MBLs) bind to mannose of microbial cells, interact with complement system components Classical pathway: activated by antibodies bound to antigen, which interact with complement system The Complement System  Activation causes three major outcomes Opsonization: C3b binds to bacterial cells and foreign particles, allows phagocytes to engulf more easily Inflammatory Response: C5a attracts phagocytes to area; C3a and C5a increase permeability of blood vessels, induce mast cells to release cytokines Lysis of Foreign Cells: membrane attack complexes (MACs) formed by proteins C5b, C6, C7, C8, and C9 molecules assembling in cell membranes of Gram negative bacteria The Complement System Phagocytosis  Phagocytes engulf and digest material and pathogens Chemotaxis: phagocytes are recruited by chemoattractants (products of microorganisms, phospholipids from injured host cells, chemokines, C5a) Recognition and Attachment: Phagocytes bind to pathogen – Direct (receptors bind mannose) and indirect (binding to opsonins) binding occurs Engulfment: pseudopods surround the pathogen, then fuse to form a phagosome Phagocytosis Phagosome Maturation and Phagolysosome Formation: Phagosome fuses with lysosone pH is lowered and lysosomes deliver enzymes Destruction and Digestion: toxic ROS produced; pH decreases; enzymes degrade; defensins damage membrane of invader; Exocytosis: Phagolysosome fuses with plasma membrane, and expels any undigested material Phagocytosis The Inflammatory Response  Tissue damage results in inflammation Inflammation contains the site of damage, localizes the response, eliminates invader, and restores tissue function Inflammation results in swelling, redness, heat, pain, sometimes loss of function Triggers cause host cells to release inflammatory mediators (cytokines, histamine, bradykinin) If blood vessels are damaged, two enzymatic cascades activated; these lead to coagulation and increased blood vessel permeability The Inflammatory Response  Inflammatory process involves cascade of events Dilation of small blood vessels Greater blood flow (heat, redness); slower flow rate Leakage of fluids (swelling, pain) Migration of leukocytes from bloodstream to tissues Endothelial cells “grab” phagocytes, slow them down Phagocytes squeeze out of vessel (diapedesis) into tissue Clotting factors wall off site of infection Dead neutrophils and tissue debris accumulate as pus Acute inflammation is short term; macrophages clean up damage by ingesting dead cells and debris If acute fails, chronic inflammation results; macrophages, giant cells accumulate, and granulomas form The Inflammatory Response The Inflammatory Response  Damaging Effects of Inflammation Process is like a fire sprinkler system: prevents spread, but damages building Enzymes and toxic compounds from phagocytic cells are released, damage tissues If limited (e.g., cut on finger) then damage minimal If in delicate system (e.g., membranes surrounding brain, spinal cord) then can be severe, even life-threatening Fever  Fever is important host defense mechanism Strong indicator of infectious disease, especially bacterial Temperature-regulation center in brain normally holds at 37°C but raises during infection in response to pyrogens (fever-inducing substances) Endogenous pyrogens - cytokines produced by macrophages following detection of microbial products Exogenous pyrogens produced by microbes Increased temperature slows bacterial growth, allows more time for host defenses Moderate temperature rise increases rates of host enzymes Enhances inflammatory response, phagocytic killing by leukocytes, multiplication of lymphocytes, release of attractants for neutrophils, production of interferons and antibodies, release of leukocytes from bone marrow  Chapter 15 Adaptive Immune System Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1 Adaptive Immunity Develops Throughout Life  Adaptive immunity develops and matures throughout your life It is the most effective means to eliminate invader  It takes a week or more to build effective immunity following first exposure to pathogen Innate immunity must protect during this time. In some cases, person may not survive long enough for adaptive immunity to develop  Adaptive immunity has memory - this gives a stronger response to re-exposure to a pathogen. Vaccination relies upon this ability  Adaptive immunity also has molecular specificity – protection against one pathogen does not protect against others  It is also tolerant - must distinguish between “healthy self” cells and “dangerous” cells such as pathogens, cancer cells Strategy of the Adaptive Immune Response  The adaptive immune response depends on the lymphocytes B cells T cells  First response to an antigen is the primary response  Adaptive immune system then “remembers” mechanism that proved effective against that specific antigen  If encountered again, a stronger secondary response results Strategy of the Adaptive Immune Response  Two basic strategies exist for fighting foreign materials Humoral immunity works to eliminate extracellular antigens (bacteria, toxins, and viruses in the bloodstream and tissue fluids) Depends on B lymphocytes Cell-mediated immunity (CMI) or cellular immunity deals with antigens residing within a host cell (E.g., virus infecting cell) Depends on T lymphocytes Both are powerful and can damage the body’s own tissues if misdirected, so the system is tightly regulated Overview Strategy of the Adaptive Immune Response  Overview of Humoral Immunity Humoral immunity involves the B lymphocytes (B cells) In humans, B cells develop in bone marrow The presence of extracellular antigens cause B cells to proliferate (divide) and differentiate (mature) into plasma cells and memory cells Plasma cells produce Y-shaped proteins called antibodies – Antibodies bind to antigens with a high degree of specificity – Many different antibodies are needed for the wide array of antigens that exist Memory B cells respond to antigens encountered again – Very quick response B cells Strategy of the Adaptive Immune Response  Overview of Humoral Immunity Antibodies protect both directly and indirectly Directly: binding to antigen, preventing its attachment to the host cell Indirectly: “tagging” antigen like a flag, so that other cells of the immune system know to eliminate it B cells have receptors on their membrane. These are B- cell receptors (BCRs), which are membrane-bound versions of a B-cell’s specific antibody. When an antigen binds the BCR, the B cell is activated to form plasma and memory cells Usually needs assistance from helper T cell Strategy of the Adaptive Immune Response  Overview of Cell-Mediated Immunity Cellular immunity depends on T lymphocytes, or T cells, which mature in the thymus Two subsets of T cells help eliminate antigens Cytotoxic T cells and helper T cells Both have multiple surface copies of T-cell receptor (TCR) – TCR are similar to BCR, but do not recognize free antigen - the antigen must be presented by body’s own cells A third subset of T cells is the regulatory T cells The role of Treg cells is to prevent immune system from mounting a response against “self” molecules – Failure to do so would lead to autoimmune disease Strategy of the Adaptive Immune Response  Overview of Cell-Mediated Immunity Like B cells, helper T cells and cytotoxic T cells must be activated before they can multiply Dendritic cells are responsible for T-cell activation Once activated, T cell proliferates and differentiates – Forms effector helper T cells (TH cells) or effector cytotoxic T cells (TC cells) – Also forms cytotoxic memory cells and helper memory cells TC cells respond to intracellular antigens, induce cell to undergo apoptosis (e.g., virally infected cell) TH cells help coordinate humoral and cell-mediated immunity: activate B cells, macrophages; produce cytokines to direct and support T cells Strategy of the Adaptive Immune Response  Lymphocyte Receptors B cells and T cells have membrane-bound receptors These function to recognize specific antigens BCR is the specific antibody that the B cell is programmed to make TCR recognize antigen presented by body’s own cells The Nature of Antigens  The word Antigen comes from antibody generator An antigen is a molecule that reacts specifically with an antibody, a B-cell receptor, or a T-cell receptor An antigen that elicits immune response is called an immunogen There is an enormous variety of antigens (e.g., microbes, pollen). Two general categories Most are T-dependent antigens: activate B cell, but activation requires confirmation from TH cell T-independent antigens: activate B cells without TH cell help; include lipopolysaccharide (LPS) and molecules with repeating subunits (for example some carbohydrates) The Nature of Antigens  Response to antigens varies depending on type of antigen Proteins generally cause strong response; lipids cause a weak one. Small molecules are usually not antigenic  Epitopes (antigenic determinants) on the antigen trigger the immune response Epitopes are regions of macromolecules E.g., 10 or so amino acids; three-dimensional shapes A bacterial cell has many different epitopes The Nature of Antigens The Nature of Antibodies  Structure and properties of antibodies are critical for function Antibodies also called immunoglobulins They are Y-shaped proteins with two general parts Two identical arms (Fab regions) bind antigen Stem (Fc region) The Nature of Antibodies All antibodies have same basic Y-shape called an antibody monomer. This monomer consists of: Two copies of polypeptide called the heavy chains and two copies of polypeptides called light chains, held together by disulfide bridges Variable region at ends of Fab regions accounts for antibody specificity The antigen binds to the antigen-binding site of the variable region like a key in a lock Constant region includes Fc and lower part of two Fab regions – the constant region amino acid sequence is identical among classes of antibodies Allows immune system components to recognize otherwise diverse antibody molecules The Nature of Antibodies  Immunoglobulin (Ig) Classes Five major classes: IgM, IgG, IgA, IgD, IgE Have same basic monomeric structure, but some form multimers of basic monomeric structure Each class has different constant region of heavy chain, but all antibodies within a class have identical constant regions Each class has distinct functions and properties The Nature of Antibodies  IgM 5–13% of circulating antibodies First class produced during primary response Main class produced in response to some T-independent antigens Pentamer Five monomeric subunits give 10 antigen-binding sites Aggregates very effectively Large size prevents crossing from bloodstream to tissues Primary role therefore in bloodstream infections Most efficient class in activating complement system IgM production begins at birth although an infected fetus can make IgM The Nature of Antibodies  IgG 80–85% of total serum immunoglobulin Provides longest-term protection: half-life is 21 days Generally first and most abundant circulating class produced during secondary immune response Transported across placenta to fetus’s bloodstream Maternal IgG protects fetus and newborn The Nature of Antibodies  IgA Monomeric form is 10–13% of serum antibodies Most IgA is a dimer IgA is most abundant immunoglobulin class produced Secreted form important in mucosal immunity Gastrointestinal, genitourinary, and respiratory tracts Also in secretions including saliva, tears, breast milk The Nature of Antibodies  IgD Less than 1% of serum immunoglobulins Involved with development and maturation of antibody response Function in blood not clearly defined The Nature of Antibodies  IgE Barely detectable in normal blood Bound to basophils and mast cells Allows these cells to detect, respond to antigens E.g., antigen binds to two adjacent IgE molecules carried by mast cell, cell releases histamine and other inflammatory mediators Basophils and mast cells also release chemicals when IgE binds to normally harmless foods, dusts and pollens, causing allergic reactions of coughing, sneezing, and swelling Outcomes of antigen-antibody binding Outcomes of antigen-antibody binding  Neutralization Prevents toxins and viruses from attaching to cell surface, thereby preventing damage  Opsonization Antibodies bound to Fc region on bacteria makes phagocytosis easier and more efficient  Complement system activation Activation of complement leads to opsonization, inflammation and formation of MACs and cell lysis Outcomes of antigen-antibody binding  Immobilization and prevention of adherence Binding of antibodies to flagella and pili prevents bacteria from entering host cells and form attaching to surfaces  Agglutination (cross-linking) Antibodies can bind to different bacteria simultaneously, causing clumping and making phagocytosis more efficient  Antibody-dependent cellular cytotoxicity Antibodies bind to large pathogen; NK cells bind to Fc portion of antibody; attached NK cell then secretes enzymes and other molecules that cause cell lysis Clonal Selection and Expansion of Lymphocytes  Clonal Selection Theory Billions of different B cells are found in the body Each cell interacts with only a single epitope of an antigen (by means of the BCRs) A single pathogen has millions of epitopes Thus we have billions of B cells Only those B cells that recognize their specific epitope respond to the antigen and become activated This is selection Activation leads to formation of plasma cells, which produce antibodies in the blood A large clone of identical B cells is produced, all making the same antibody against the pathogen Clonal Selection and Expansion of Lymphocytes  Clonal Selection Theory – proliferation of lymphocytes Applies to both B cells and T cells Population of B cells and T cells is generated In the case of B cells, clonal selection leads to formation of large clone of plasma cells making antibodies against pathogen. Also forms a clone of memory cells In the case of T cells, clonal selection leads to formation of large clone of effector T cells (TH and TC). Also form a clone of memory cells – TH cells secrete cytokines that help in activation of B cells and macrophages – TC cells secrete “death packages” that lead to death of infected host cells and cancer cells B Lymphocytes and the Antibody Response  B-cell activation by T-dependent antigens B-cell receptor binds to antigen Antigen is internalized via endocytosis and degraded into peptide fragments Fragments are delivered to MHC class II molecules at the B cell surface for inspection by TH cells: this is antigen presentation If a T cell has a receptor for that fragment, it binds the fragment, and then produces cytokines that activate that B If no TH cells can recognize the fragment, the B cell may become unresponsive to future exposure of antigen – Results in tolerance to antigen B Lymphocytes and the Antibody Response B Lymphocytes and the Antibody Response  Characteristics of Primary Response Takes 10–14 days for substantial antibody accumulation Person may be sick, possibly seriously so, although immune system is actively responding Additional exposure to antigen leads to much faster secondary response B Lymphocytes and the Antibody Response  Characteristics of Primary Response (continued…) Some B cells differentiate to form plasma cells Plasma cells generate antibodies These cells undergo apoptosis after a few days Activated B cells continue proliferating and differentiating in presence of antigen, so antibody titer steadily increases B Lymphocytes and the Antibody Response Class Switching: All plasma cells initially secrete IgM TH cells can induce some activated B cells to become plasma cells that secrete other antibody classes (usually IgG) B Lymphocytes and the Antibody Response  Characteristics of Secondary Response Significantly faster and more effective than primary Pathogens are usually eliminated by this response before causing harm Memory B cells responsible Antibodies produced by these cells bind antigen effectively When activated, some quickly become plasma cells, producing large amounts of antibodies B Lymphocytes and the Antibody Response  The Response to T-Independent Antigens Some antigens can lead to B cell activation without the help of TH cells Relatively few such antigens exist, but medically important Molecules with many identical evenly spaced epitopes (e.g., LPS, polysaccharide capsules) are bound by groups of B-cell receptors Leads to activation Not very immunogenic in young children T Lymphocytes: Antigen Recognition and Response  T cells play different role from B cells Never produce antibodies Effector T cells directly interact with target cells Cause distinct changes in target cells T Lymphocytes: Antigen Recognition and Response  General Characteristics of T Cells (continued…) Two types of MHC molecules MHC class I present endogenous antigens – Produced by all nucleated cells MHC class II present exogenous antigens – Produced by antigen-presenting cells (dendritic cells, B cells, macrophages) T cell recognizes peptide-MHC complex T Lymphocytes: Antigen Recognition and Response  General Characteristics of T Cells (continued…) Cytotoxic T cells recognize antigen presented on MHC class I molecules TC cells respond to endogenous antigens Helper T cells recognize antigen presented on MHC class II molecules TH cells respond to exogenous antigens Cluster of differentiation (CD) receptors are found on both TH and TC cells TH cells have CD4 proteins – CD4 is receptor for HIV, which infects TH cells TC cells have CD8 proteins T Lymphocytes: Antigen Recognition and Response T Lymphocytes: Antigen Recognition and Response  Effector Functions of TC (CD8) Cells TC cells induce apoptosis in infected “self” cells and cancer cells They recognize infected or cancerous cells from normal cells by the MHCI molecules All cells routinely present internal proteins on MHCI molecules at the cell membrane If only normal proteins are presented, TC cells don’t recognize them But if some pathogen or cancer proteins are also presented, TC cells recognize them and bind to the cell TC induces apoptosis in the infected/cancer by releasing proteases and cytotoxins (perforin) T Lymphocytes: Antigen Recognition and Response Lymphocytes: Antigen Recognition and Response  Effector Functions of TH (CD4) Cells TH cells coordinate the immune response When a TH cell recognizes a peptide presented by a B cell or a macrophage, they release cytokines to activate those cells They control the activities of the activated B cells and macrophages They also control the activities of other T cells Lymphocytes: Antigen Recognition and Response  Role of TH Cells in B Cell Activation This was described earlier in the chapter TH recognize antigen presented on MHCII from antigen- presenting cells (APCs) and activates that cell with cytokines T Lymphocytes: Antigen Recognition and Response  Role of TH Cells in Macrophage Activation TH cells can recognize peptides presented by a macrophage on MHCII molecules They activate the macrophage (by releasing cytokines) to increase their power Activated macrophages have increased size and metabolism, and a greater number of lysosomes; they produce toxic compounds Natural Killer (NK) Cells  Natural Killer cells induce apoptosis in “self” cells NK cells recognize host cells with foreign proteins in membrane bound by antibodies by a process called antibody-dependent cellular cytotoxicity (ADCC) NK cells have FC receptors for IgG molecules, so can bind the FC part of antibodies NK cells bind to the FC receptor, then deliver perforin- and protease-containing granules to cell, initiating apoptosis Also recognize and destroy stressed host cells lacking MHCI Some viruses interfere with antigen presentation and prevent host cells from displaying MHCI NK recognize these cells and destroys them Natural Killer (NK) Cells