Innate Immune System Lecture Notes PDF

Foundations in Pharmacology PHRM2005 Dr. Ricky R Lareu Innate Immune System Key Concepts Divisions of the Immune system Lymph system Primary and secondary lymphoid organs Primary and secondary (1st and 2nd line) defence systems Phagocytosis PRRs and PAMPs Toll-like receptors Complement & Acute phase...

Foundations in Pharmacology PHRM2005 Dr. Ricky R Lareu Innate Immune System Key Concepts Divisions of the Immune system Lymph system Primary and secondary lymphoid organs Primary and secondary (1st and 2nd line) defence systems Phagocytosis PRRs and PAMPs Toll-like receptors Complement & Acute phase proteins Cytokines & Interferons Acute inflammatory response Phagocytic killing mechanisms Reading Mim’s Medical Microbiology, Ch 9, pages 67-81 Learning Outcomes Understand the overall scheme and division of the Immune system. Define the function of primary lymphoid organs Explain the role the lymphatic system plays in immune surveillance and list the major lymphoid organs Be able to describe primary defence mechanisms (1st line) – biochemical and physical barriers Describe the function and steps involved in phagocytosis Describe the principle of pathogen recognition by the innate immune S. Describe mechanisms of pattern recognition using Toll-like receptors as an example Explain the general activation mechanisms for the complement system and describe the various antimicrobial consequences Explain the actions of various soluble factors including acute phase proteins and interferons Describe mechanisms of intracellular killing by phagocytic cells Describe the general features of the acute inflammatory response Divisions of the Immune System 4 Structure of Immune System Immune system provides places (niches) for immune cell generation (hematopoiesis), maturation, residence and enable interaction between immune cells and foreign/deleterious agents. Primary lymphoid organs: Organs: bone marrow and thymus Function: generation and maturation of immune cells Secondary lymphoid organs: Organs/tissues: spleen, lymph nodes, Peyer’s patches, MALT (mucous-associated lymphoid tissue) distributed throughout body provide niches for interaction between immune cells, their proliferation and to produce products e.g. antibodies and identification of microbes, inactivation, killing and removal 5 The Lymph System The lymph system – network of vessels draining excess body fluid from organs back to heart, immune surveillance and highways for immune cells System of blind-ended vessels collecting lymph – extracellular fluid that has not returned to capillary network Lymph eventually returns to blood through thoracic duct near heart Provides a network for the immune system to monitor tissue Lymphatic vessels connect at lymph nodes, uniting into fewer and larger vessels Lymph travels through several to many lymph nodes 6 1° Lymphoid Organ – Bone Marrow Hematopoiesis – generation of formed elements i.e. cells Red blood cells (erythrocytes) White blood cells (leukocytes) Granular leukocytes: neutrophils, eosinophils, basophils Agranular leukocytes ▪ T cells, B cells (lymphocytes), Natural Killer cells ▪ Monocytes → Macrophages Platelets (Thrombocytes) – involved in clotting 7 Major Cell of the Immune System Innate immunity Rapid response Adaptive immunity Slower response 8 Primary Defence Mechanisms (1st line defence) Defences against entry into the body Disease – 1st pathogen must overcome biochemical and physical barriers Keratinised skin – lining external surfaces of body many layers of impermeable dead cells sweat and sebaceous secretions – lactic acid and fatty acids, low pH Mucus membranes – lining inner surfaces of body cavities mucus stops attachment of microorganisms, adhesive nature traps foreign particles mechanical removal – coughing, sneezing, cilia (to stomach) Body fluids and secretions Flushing action – frequent flushing by tears, saliva and urine stop microbial growth on epithelial surfaces 10 Defences against entry into the body cont. Microcidal factors – directly damage and kill microorganisms ▪ acid in gastric juices, spermine and zinc in sperm, lactoperoxidase in milk, lysozyme in tears, nasal secretions and saliva Norma body flora (or microbiota) – host provides environment and nutrients without harm (i.e. commensals) suppress growth of potentially pathogenic microorganisms by: ▪ already occupying growth surface ▪ effectively competing for nutrients ▪ producing inhospitable environment including acid and bactericidins like colicins 11 Defences Against Entry into the Body Summary 12 When organisms penetrate 1st line → 2nd line Innate IS – Nonspecific defences Adaptive IS – Specific defences 13 Secondary Defence Mechanisms (2nd line defence) 2nd line of defence is part of the Innate IS Major mechanisms of defence: ▪ Phagocytosis ▪ Soluble chemical factors 14 Phagocytic Cells of the Innate IS Generated in bone marrow from myeloid progenitor Two lineages: ▪ Monocytes-derived macrophages – large macrophages ▪ Polymorphonuclear granulocyte – small cell e.g. neutrophil Figure 9.4 Phagocytic cells. (A) Blood monocyte which gives rise to macrophage (B) polymorphonuclear neutrophil, both derived from bone marrow stem cells 15 Macrophages Bone marrow → blood monocytes → macrophages Macrophages in tissues – called the mononuclear phagocytic system Long-lived (months-years), contain mitochondria for energy production Produce many anti-microbial secretory products, over 80 Associated with basement membrane of small blood vessels ▪ concentrated in lung, liver, spleen and lymph node Remove by phagocytosis: ▪ foreign material e.g. bacteria in lymph nodes ▪ spent and damaged host cells e.g. red blood cells in spleen 16 Sites of Resident Macrophages 17 Polymorphonuclear cells Produced in bone marrow and released into blood - Nucleus is lobulated ▪ Granulocytes – neutrophil, eosinophil, basophil ▪ Agranulocyte – monocyte Neutrophil Eosinophil Basophil Monocyte Additional information in tutorials 18 Neutrophils Most abundant leukocyte circulating in blood, 60-70% Phagocytic: bacteria, immune complexes with antigens pale staining Circulate in blood and first at infection site, called pus cells short-lived, ~4 days no mitochondria – energy from glycogen stores → glycolysis enables function under anaerobic conditions, at inflammatory site dark blue Basophils Least abundant leukocyte, 0.5-1% Release cytokines to produce inflammation e.g. histamine Also produce heparin which stops clotting Leave circulating blood and turn into mast cells Phagocytosis – Sequence of Events A) Phagocyte attaches to microorganism with cell surface receptors that recognise foreign proteins on the microorganism B) Once attachment happens the microorganism is taken into the phagocyte by pseudopodia extending around it C) Once inside, various vesicles (i.e. lysosomes) fuse with phagosome that contain many different chemicals in an acidic environment. D) These chemicals are released into phagosome and the microbe is killed by a battery of microcidal mechanisms and the remnants are released into solution again. 20 Phagocytosis Figure 9.10 from MMM, p71 How do immune cells of the innate system recognise foreign cells/ pathogens? Videos on Phagocytosis Live https://www.youtube.com/watch?v=T5W6VpKPt1Y https://www.youtube.com/watch?v=UKwwvIaR9aI 21 Pathogen Recognition Pathogen and macrophage must come into contact → through specific receptors Innate immune system - use germline-encoded receptors for the recognition of microbial pathogens – has relatively few receptors The germline are the gametes (sperm or egg) for reproduction, relatively invariant Adaptive immune system – (only in vertebrates) uses receptors generated by somatic mechanisms (gene rearrangements) during the development of each individual immune cell (has many, different types of receptors, 1013) ▪ Cells that are not in the germline are called somatic cells. This refers to all of the cells of body apart from the gametes 22 Pathogen Recognition cont. The Innate immune system (IS) detects molecular patterns found in other organisms (e.g. bacteria, viruses, fungi): ▪ not present in the host; detection sets up a response that can mark and/or kill ▪ limitation – parasitic microbes (pathogens) have a much shorter generation time than higher animals → variants can arise to evade recognition or response. The Acquired IS learns the molecular “self” and anything else is potentially a target for response: ▪ ability to respond to new molecular determinants is on a time-scale similar to the generation time of microbes. 23 Pathogen-Associated Molecular Patterns Because microbes evolve rapidly, innate immunity must focus: ▪ on broadly expressed molecules characteristic of microbes ▪ target molecules tend to be structural elements that are common to broad classes of microbes and are unlikely to change Pathogen-associated molecular patterns (PAMPs) – the targets recognised on pathogens : to avoid damage to “self” cells and tissues, they are absolutely distinct from host proteins/molecules ▪ none of these compounds are produced in the host organism they are essential for the physiology and survival of the microbes → do not change therefore, the ability of the Innate IS to discriminate between self and non-self is virtually perfect 24 Pattern Recognition Receptors Pattern recognition receptors (PRR) – used by phagocytes to detect PAMPS Various types – both extracellular (on surface of cell membrane) and intracellular (in phagosome) Direct detection through receptor a) Toll-like receptors (TLRs) b) Carbohydrate receptors c) Scavenger receptors Through opsonisation intermediate – Soluble factors a) Complement b) C reactive protein c) Collectins 25 PRR Categories [Mannose binding lectins (MBL)] C o m p l e m e n t 26 Toll-like Receptors Found on the surface of lymphocytes, neutrophils, basophils, macrophages and dendritic cells Play a very important role in Innate IS for recognising bacteria, viruses, fungi and protozoa 11 different TLRs and are usually specific to one type of PAMP Interaction between TLR-PAMP activate secretion of cytokines that activate both Innate and Adaptive IS (cytokines – small messenger molecules) Identify ‘foreignness’ for phagocytosis and destruction 27 List of Toll-like Receptors cont. 28 Soluble Factors 29 Complement System 30 proteins, major called C1-C9, secreted by liver, inactive state, circulate in blood Enhance (or complements) the ability antibodies and phagocytic cells to bind to and eliminate pathogens and can also directly kill foreign cells Activation results in a rapid amplification of the signal and responses There are various ways complement can be activated: a) Classical pathway – antigen-antibody complexes (Adaptive IS) b) Alternative pathway – select molecules on bacterial cell wall (Innate IS) c) Acute phase proteins or Lectin pathway – CRP and MBL (Innate IS) 30 The Complement Cascade 31 Activities of the Complement Components Activation of central C3 protein results in the orchestration of an integrated antimicrobial defence strategy: C3b coat microbes in immediate area and results in enhanced phagocytosis (i.e. opsinization) C3a/C5a cause mast cells to degranulate and release proinflammatory cytokines to initiate an acute inflammatory response: increased vascular permeability, chemotactic factors These attract other polymorphs from blood into tissue e.g. eosinophils, basophils, mast cells C5b activates other downstream complement components at site of C3b/C5b attachment, membrane attach complex – creates channels across membrane, resulting in cell lysis 32 Activities of the Complement Components cont. 33 Acute Phase Proteins Increase dramatically in concentration in response to infection, inflammation and tissue injury They are an “early alarm” mediators, They recognise and bind to PAMPS Examples: ▪ C-reactive protein ▪ Mannose binding lectin (a type of Collectin) They also recruit complement 34 Complement & Acute Phase Protein Mechanism 35 Cytokines Cytokines are small protein secreted by many cells (inc. immune cells) with diverse activities ▪ secreted by lymphocytes and antigen presenting cells, fibroblasts, endothelial cells, monocytes, hepatocytes, and kidney cells Many are called interleukins and used by the immune cells to control processes and communicate Functions: ▪ Stimulate/inhibit many cell functions, such as cell growth and differentiation ▪ stimulate proliferation of progenitor blood cells in bone M. ▪ regulate immune cell activity ▪ role in driving hypersensitivity reactions and inflammation ▪ imbalances in cytokine production can lead to diseases 36 Interferons (IFN) Interferons are antimicrobial cytokines Named because observed to ‘Interfere’ with viral replication Three types: IFN-α, IFN- and IFN- Produced by a variety of immune and non-immune cells: Bcells, monocytes, macrophages, fibroblast, epithelial cells among others, natural killer (NK) cells Protect uninfected cells from viruses Also induced by tumours and protect against other intracellular parasites inc. bacteria and protozoa 37 IFNs Mode of Action – Early Defence 38 Acute Inflammatory Response Normal body response to local injury, irritation, microbial infection or toxin Result in a series of events Physiological level: redness, heat, swelling (oedema), pain Cellular level: ▪ Increased capillary diameter (vasodilation), increases blood flow ▪ Increased capillary permeability, escape of plasma proteins e.g. antibodies, complement, fibrinogen ▪ Escape of leukocytes and accumulation at site Burton’s Microbiology for Health Sciences 10th Ed. 39 Sequence of Inflammatory Events Burton’s Microbiology for Health Sciences 10th Ed. 40 Phagocytosis Killing Mechanisms Once pathogen is phagocytosed, fusion of lysosomes result in phagosome – secretion of many antimicrobial proteins and chemicals to kill and break-down Oxidative mechanisms e.g. macrophages and PMN Production of reactive oxygen intermediates (ROI) ▪ Oxidative burst, oxygen is required – under aerobic conditions only Enzymatic reactions progressively reduce oxygen to water producing ROI in the process e.g. superoxide ions, hydrogen peroxide and free hydroxide radicals 41 Phagocytosis Killing Mechanisms cont. Non-oxidative mechanisms – macrophages and PMN Can function under anaerobic conditions e.g. Clostridia species causing gas gangrene, deep abscess ▪ Lysosomes fuse with phagosome and release contents Cathespin G – enzyme, damages microbial surfaces Defensins – insert into microbial membrane and produce channel, bacterial cytoplasm lost to environment and allows entry of other antimicrobial agents Cationic proteins – bind or penetrate cell membranes and have various bactericidal actions Lysozyme – enzyme, digests proteoglycans of cell wall Final step: biochemical breakdown of killed organism – acid hydrolases – enzymes e.g. proteases, DNases 42 Summary The Lymph system provides a network of vessels and filters (nodes) for immune surveillance of tissues Immune cells are generated in bone marrow and mature in bone marrow and the thymus (primary lymphoid organs) The 1st line of immune defence consists of physical and biochemical-chemical barriers The 2nd line of defence consists of phagocytic cells and soluble factors The Innate immune system uses PRR to detect foreign material (including pathogens) e.g. Toll-like receptors PAMPs are unique molecules only present on microbes Cytokines are soluble proteins secreted by cells (including immune cells) to communicate and stimulate activities e.g. proliferation of progenitor cells IFNs are soluble molecules that interfere with pathogen infection and replication e.g. viruses The acute inflammatory response is a normal response to cell injury, increasing blood flow and capillary permeability Phagocytic cells have various chemical and biochemical killing agents including oxidative and non-oxidative mechanisms Additional Recourses The following links are videos on the Innate Immune System from the ‘Crash Course’ group. They are very entertaining and informative. Use them as a revision source after going through the lectures and text book material. Innate immunity https://www.youtube.com/watch?v=GIJK3dwCWCw Lymphatic system https://www.youtube.com/watch?annotation_id=annotation_1508290589&featur e=iv&index=44&list=PL8dPuuaLjXtOAKed_MxxWBNaPno5h3Zs8&src_vid=GIJK3dw CWCw&v=I7orwMgTQ5I 45