Immunology and Infectious Diseases

5372 Immunology and Infectious Diseases Lect. 1: Review of Immunology Lect. 2: Immunological Assays Lect. 3. part 1: TSE diseases Dr. Karin Peterson 3309 Vet. Med. Bldg. kpeterson@vetmed.lsu.edu 578-9681 Immune System • Defend host against foreign substances • Surveillance against abnormal self • Limit destruction to host Images in presentation from: Kuby, Immunology 6th edition DeFranco, Immunity Tizard, Veterinary Immunology Despair.com Infection and Immunity Balance infection immunity Disease = Bolus of infection x virulence immunity Veterinary Species • Immune system highly conserved throughout evolution – High degree of homology between cells and molecules in different species – Understanding of the immune system extrapolated from other species • Human and mouse • Rat • Cattle, sheep, pigs • Dogs, cats and horses • Exotics and wildlife Requirements for protective immune response:  be able to detect foreign object prevent host from being killed by foreign object limit replication of foreign object in host limit damage to host by foreign object and immune system Types of Infectious Agents • 5 groups – 1. Viruses – 2. Bacteria – 3. Fungi – 4. Protozoans – 5. Helminths Differences in infectious agents • Intracellular or Extracellular pathogen – Does pathogen require cell to replicate? • Virus or intracellular bacteria versus fungi or worm • Mechanism of Pathogen to evade immune response • Toxins produced • Effect on immune system • Life cycle of infectious agent • Site of infection / replication First line of protection Immune response can be divided into two major parts: Innate and adaptive Overview – Innate immunity • First line of defense – Immediate, always available • Not pathogen/antigen specific • Does not require previous exposure • Does not increase in magnitude with each exposure Overview – Adaptive immunity • Takes time to develop • Pathogen/antigen-specific • Shows memory • Major Components – Antibody – Cytotoxic T cells (CD8) – Helper T cells (CD4) Cells of the immune system Granule releasing cells: Basophils, mast cells, eosinophils • Protection of epithelial surfaces • Allergic responses, parasitic infections • Densely stained cytoplasmic granules • Basophils: basic blue dye • Eosinophils: stain with eosin • Do not phagocytose microorganisms • Release contents of granules to exterior upon activation • Histomines: released by basophils, mast cells • cationic proteins: released by eosinophils and basophils to damage helminthic pathogens Phagocytic cells and killer cells Response to infection Phagocytes • Phagocytes = eating cells – Neutrophils (PMNs) are present in the highest numbers in blood – Macrophages (“big eaters”) in the tissues encounter the pathogen first • Secrete cytokines ---> inflammation, systemic responses Phagocytosis • Attachment •Pseudopod extension •Phagosome formation •Granule fusion •Phagolysosome formation Pathogen Destruction • Pathogen is killed by enzyme digestion, toxic oxygen molecules, and defensins • Some pathogens resist killing and live in phagosome or escape to cytosol (Mycobacterium tuberculosis, Listeria monocytogenes) Phagocytosis • Phagocyte must bind pathogen to begin phagocytosis • Microbes with capsules are difficult to bind • Phagocytosis is easier if the pathogen is coated with antibodies or complement (opsonins) The Complement System Discovered as a heat-labile antibacterial substance in immune serum kills bacteria non immune (normal) serum…………… no immune serum………………………….. yes heated* immune serum…………………. no heated immune serum plus non-immune serum…………………. yes Conclusion: Two components are needed for bacterial inactivation: a heat-stable immune component (antibody) and a heat-labile non immune component (complement). (there is something in normal serum that complements immune serum) *560C for 30 minutes or 600C for 5 minutes Complement Mechanisms of complement Adaptive Immune Response • Antigen Specific: Immune response specific to component of pathogen Antigens • Generally proteins (not always) • Can be foreign or self (autoimmunity) • Examples of antigens – Envelope protein from FIV – Bacterial proteins • One lymphocyte or antibody molecule interacts with only a small portion of the antigen = epitope • Free antigen – Taken up by antigen presenting cells (APCs) or bind to B cells • From infected cell – Presented by MHC class I molecules – MHC class I on all nucleated cells Cells that recognize antigen T helper Cell: CD4 cell Provides help to: Cytoxic T cells B cells Cytotoxic T Cell: CD8 cell Recognizes Antigen Kills Infected cells B Cell: Antibody producing cell Produce Antibodies that Recognize antigens Humoral immunity Cell-mediated immunity (CMI) B cells • B cells are continuously generated in the bone marrow – Undergo selection process to remove auto-reactive cells • (heavy or light chain reactive with stromal cells) are removed via apoptosis • Naïve B cells migrate to the periphery (spleen/lymph nodes) – Express surface IgM, low level secreted IgM • Contact with antigen in the presence of activated CD4 Th cells – stimulation, proliferation, maturation • Memory B cells • Does not produce large amounts of Antibody (Ab) • Antibody response can be modified • Plasma cell produces • Secrete large amounts of antibodies (IgG) Antibodies • Four polypeptides chains – 2 identical heavy (H) – 2 identical light (L) • H and L chains combine to form antigen binding site (amino-terminal ends) • H chains combine to form effector functional end (carboxy-terminal ends) – Binds to Fc receptors on cells – Activates complement Humoral Immunity (antibody) • Neutralization • Antibody blocks protein binding or function • Opsonization • Increased phagocytosis via FcR on cells – FcR: expressed on many innate immune cells, binds Fc portion of antibody • Increased phagocytosis via C3 receptors on cells • Direct lysis of microbe/abnormal cell • Classical complement pathway • Antibody dependent cell-mediated cytotoxicity (ADCC) • FcR on NK cells or macrophages bind antibody and arm activated killing mechanisms when antigen is bound • Activation of other effector cells • Mast cells, eosinophils via FcR Recognition - B cell receptor • Antigen recognized by the B cell receptor (BCR) • membrane immunoglobulin • B cell receptor binds unprocessed antigen – Soluble – Cell associated • Two types of responses – T independent – T dependent Immunoglobulin isotypes • IgD– found as a surface molecule on immature B lymphocytes • IgM – first isotype produced following activation – five basic units linked together by joining (j)-chain – (=10 Ag binding sites) – efficient at agglutination, – binds complement (classical pathway) – inefficient diffusion into tissue. • IgG - Second isotypoe produce – major serum antibody that diffuses readily into tissue; – binds complement (classical pathway) – can cross placenta in some species – Multiple subclasses in most mammalian species each with specific functional characteristics Antibody Level IgG IgM 7 14 21 28 Days Anamnestic Response: Response produced faster at higher levels following second exposure Adjuvant: A substance that augments the immune response -Fruends adjuvent, LPS, Alum Antibody Level With Adjuvant Ig G IgM 7 14 Weeks 21 28 No Adjuvant 7 14 Weeks 21 28 IgA • IgA – most important in secretions, often found as a monomer in blood (varies by species); and dimer in secretions where it is protected by the secretory piece; does not bind complement. IgE • IgE - most important in responses to parasites and in immediate hypersensitivity (type 1); • found as a monomer • binds FcR on surface of mast cells, basophils and eosinophils; Immune Response To Helminth infection Regulation of Ig subclass Cytokines • Cytokines – soluble mediators of the immune system that are released by one cell and have an effect the same cell (autocrine), a neighboring cell (paracrine) or a distant cell (endocrine). – low molecular weight proteins that bind specific cell receptors and initiate intracellular signaling pathways – Produced transiently – High activity – Most often work in networks of multiple cells and cytokines CD4 helper T cells • Antigen specific • Provides help – B cells: • Cytokines (class switching) • Cell surface markers – Cytotoxic CD8 T cells • Cytokines (IL-2, IFNg) • Regulate Immune Response Develop from naïve cell to: – Th1 : Viral and intracellular pathogens – Th2 : Extracellular bacteria and parasites CD8 Cytotoxic T Cells • Recognizes epitope – Bound to MHC class I molecules – All nucleated cells express MHC class I Target cell CD8 • forms tight junctions with target cell • movement of lytic granules to contact site • Contents of granules released ‘Lethal hit’ delivered – Perforin forms pores in membrane – Granzymes enter cells through pores – trigger apoptosis (DNA fragmentation). • CD8 T cell releases to target additional cells Recognition of antigenic epitope • CD4 Helper T cells – Exogenous pathway • Extracellular protein • CD8 Cytotoxic T cells – Endogenous pathway • Intracellular protein – Epitope presented by MHC class II – Class II only on antigen presenting cells (APCs) • Dendritic cells • Macrophages • B cells – Epitope presented by MHC class I – Class I on all nucleated cells – Primary stimulation by APC • Does not kill cell – Generally does not kill cell – Recognizes infected cell • Kills infected cell Antigen processing Recognition by CD4 Th and CD8 Tc cells CD8 CD4 T cell stimulation • MHC – Multigenic • Multiple copies • Second signal for initial stimulation – T cells require 2 signals • TCR • Costimulation – Polymorhic • many alleles – Lack of second signal • No response to antigen – Same species • Different response to pathogen – Costimulation provided by • CD4 Th cell: Activated antigen presenting cell (APC) • CD8: APC + IL-2 CD4 help to Cytoxic T cell CD8 Cytotoxic T cell Self Antigens • Process to eliminate T cells and B cells that recognize self – Central tolerance • Deletion of selfrecognizing T cells and B cells – Peripheral tolerance • Danger theory • Lack of second signal to T cells Mechanisms that break tolerance • Release of sequestered antigen – Eye – Testis – Brain • Molecular mimicry – Self epitope similar to pathogen epitope • Induction of Tolerance – Neonatal exposure Autoimmunity Hypersensitivity to antigens IgE soluble Ag Mast cell degranulation Atopic dermatitis IgM or IgG cellular Ag ADCC immune complex T cell mediated soluble Ag soluble or cellular Ag Immune complex complement activation Systemic lupus erythromatosis activated Th1 cells Acquired immune hemolytic anemia Rheumatoid arthritis Contact dermatitis ADCC: antibody dependent Cell mediated cytotoxicity Immune response • Complex • Response dependent on: – Insult (type of pathogen) – Tissue (local response) – Previous exposure • Balance – Removal of pathogen – Damage to self • break-down of tolerance to self antigens