Immunology Part II: Adaptive Immunity

1. Immunology Part II:Adaptive Immunity Lecture #18Bio3124

2. Overview of Specific (Adaptive) Immunity three major functions recognize nonself Discriminates btw self and nonself, antigenic determinants respond to nonself: specific, diverse (>1011 antigens) effector response eliminates or renders foreign material harmless anamnestic response upon second encounter with same pathogen immune system mounts a faster and more intense response remember non-self: memory cells

3. Antigens and Haptens • Antigens: elicit immune response • Whole cells, large and complex molecules • Epitopes:antigenic determinant sites on antigens to which antibody binds; • Proteins-> linear vs conformational • Valence • number of epitopes on an antigen • determines number of antibodies to bind antigen Haptens • small organic molecules • antigenic when bound to larger carrier molecule • e.g., penicillin binds serum proteins among allergic patients

4. Types of specific immunity (adaptive) • humoral immunity • also called antibody-mediated immunity • based on antibody activity and B-cells • cellular immunity • also called cell-mediated immunity (CMI) • based on action of specific kinds of T lymphocytes

5. Cell Mediated Adaptive Immune Response (CMI)

6. Recognition of Foreignness • distinguishing between self and non-self • allows for selective destruction of invading pathogens without destruction of host tissues • MHC molecules: • APCs present foreign antigens to T cells • T-cells mediate launching either cell mediated immune response (CMI) or humoral response (antibody) • MHC-I:presents antigens of endogenous origin eg. viral infection; all nucleated cells • MHC-II: antigens of exogenous origineg bacteria/toxins entering APC; include macrophages, dendritic cells, B-cells

7. MHC-I and MHC-II • MHC-I: • α-chain:45 kD, α1, α2 and α3 domains • β2 microglobulin:12 kD, associated with α3 • α1 and α2:antigen binding pocket • MHC-II • α-chain and β-chain • Form large antigen binding pocket for peptide presentation by APCs

8. Antigen processing and presentation • MHC-I: Bind foreign peptides originating in cytoplasm (endogenous antigens) eg. viral proteins • Viral proteins breakdown to peptides • pumped into ER via LMP and TAP • α and β chains synthesis on ER • α and β join together in ER lumen and pick up viral antigenic peptides • Pass through golgi→ cell surface → antigen presentation LMP: low molecular mass polypeptide component of proteosome TAP: transporter of antigen peptides

9. Antigen processing and presentation • MHC-II: Bind foreign peptides originating outside cytoplasm (exogenous antigens) eg. bacteria • Enter APCs via endocytosis • Breakdown in endosomes by proteolytic degradation • In the ER: • MHC-II α and β chains synthesis • Associated with invariant chain, fits in Ag binding cleft temporarily, all MHC-II • Pass through Golgi, invariant chain degraded • Endocytic vesicles fuse with golgi vesicles • Fitting antigenic determinant picked up by MHC-II • Carried and presented on cell surface

10. T- Cell Biology • major players in cell-mediated immune response • have major role in B-cell activation • produce regulatory cytokines; Cytotoxic function • recognize and bind antigens using specific TCR • Specific binding to antigen presented by antigen-presenting cells (APCs) using MHC-I or MHC-II • CD3 complex activation & signaling and proliferation, cytokine production T-cell receptor (TCR)

11. Types of T-cells 1. T-helper (TH) cells (CD4+ T Cells) • TH0 • undifferentiated precursors of TH1 and TH2 cells • carry CD4+ that is MHC-II co-receptor • activated by antigens presented on MHC-II • TH1 cells • produce specific set of cytokines (IL-2, TNFs, IFN) • involved in cellular immunity • responsible for CTLs and macrophage activation • TH2 • produce cytokines (IL4, 5, 6, 10, 13) • involved in humoral immunity (B-cell activation) • defend against helminth parasites 2. Cytotoxic T Lymphocytes (CTLs) • CD8+ T cells that have been activated by antigen presented on MHC-1 molecules of nucleated cells

12. MHC-I and Cell Mediated Immune Response • On the scene: virus infected nucleated cells • MHC-I: viral antigen presented on MHC-I • CTL response: • activated by binding to antigen presented on MHC-I • Differentiate • Targets:other infected cells • Kill target cells by • perforin and granzyme • Inducing apoptosis

13. Helper memory cells produced APC is not targeted MHC-II and Cell Mediated Immune Response • CTL response via MHC-II • Can also be activated through TH1 helper T-cells and MHC-II • TH1 activation: two signals required • 1st signal: TH1 binds by CD4 to antigen presented by MHC-II on infected APC • 2nd signal: B7 and CD28 interaction • Triggers IL-2, IFN and TNF production • Activate Naïve cytotoxic T cells • attacks other infected cells presenting same Ag on MHC-I • Cytolysis or apoptosis ensues

14. Humoral Adaptive Immune Response

15. B-Cell Biology • immunologic roles • proliferate and differentiate into plasma cells • Produce antibodies and memory cells • act as APC • BCR: B-cell receptor (monomeric IgM) • accompanied by Igα and Igβ • BCR binds to its specific antigen • Capping: BCR clustering through Igα and Igβ • Signaling to nucleus -> DNA recombination • differentiate into plasma cells -> secrete antibodies Capping

16. B-cell Ag specificity & clonal selection theory • Billions of different B cells • Antibodies attached to membrane • Each cell binds different antigens • Clonal Selection • Cells that bind antigen replicate • Differentiate to plasma cells • Secrete antibodies • Some differentiate to memory cells • Second antigen exposure • Memory cells replicate, differentiate

17. B-Cell Activation • needs antigen binding (triggering) • leads to proliferation and differentiation into plasma cells • cytokines produced by helper T cells can act on B cells and assist in growth and differentiation • typically antigen-specific • two mechanisms for antigen-specific activation • T- dependent • T- independent

18. T dependent B-Cell Activation Triggered by bacteria, hapten-carrier, proteins requires two signals • antigen-BCR interaction • T cell cytokines Three cells involved: • macrophages • T-helper cells • B-cells specific for antigen Mechanism: • Naïve TH0 cells bind Ag/MHC-II on APC • Activated to TH2 cells • TH2 cells bind B-cells displaying same Ag/MHC-II • Produce cytokines IL4,5,6 • B-cells proliferate and differentiate • antibody producing plasma cells • Form memory cells

19. T-independent B cell activation • T-independent antigens • polymeric antigens with large number of identical epitopes (e.g., bacterial lipopolysaccharides) • mediate capping and activation • less effective than T-dependent B cell activation • antibodies produced have a low affinity for antigen • no memory B cells formed

20. Antibodies • immunoglobulins (Ig) • glycoprotein made by activated B cells (plasma cells) • serve as antigen receptor (BCR) on B cell surface • found in blood serum, tissue fluids, and mucosal surfaces of vertebrate animals • an antibody can recognize and bind antigen that caused its production

21. Immunoglobulin Structure • same basic structure • four polypeptide chains • two identical heavy chains • two identical light chains • heavy and light chains connected to each other by disulfide bonds • both chains contain two different regions • Constant (C) regions (CL and CH) • Variable (V) regions (VL and VH) • Isotypes:IgM, IgG, IgA, IgD, IgE

22. Immunoglobulin Function • Fab binds antigen • marks antigen for immunological attack • activates nonspecific defense mechanisms that can destroy antigen • e.g., opsonization • Fc mediates binding to: • host tissue • various cells of immune system • first component of complement system

23. Effector and anamnestic response • Effector response: first exposure, IgM or IgD followed by isotype switch to IgG • Memory cells stored • Anamnestic response • Some IgM • Lots of IgG • Rapid • intensive response

24. Consequences of Antigen-Antibody Binding

25. Classical Complement activation • Antibodies coat antigenic particles • FC portions stick out • Fc bound by C1 complex • C1 cleaves C4, C2 • C4b2a is a convertase (C3 convertase) • C3  C3a and C3b • C3bC5 form C5 convertase • Cleaves C5 to C5b and C5a • C5b recruits C6, 7, 8 and C9 • Form membrane attack complex • Cytoplasmic leakage-> cell dies C3 convertase C5 convertase

26. Classical Complement activation

27. Resistance to Viral Infections • antibodies neutralize viruses • antibodies enhance phagocytosis of viruses by opsonization • interferons cause death of infected cells • NK cells stimulated by interferons and antibodies • Cytotoxic T lymphocytes (CTLs) destroy virus-infected cells

28. Resistance to Bacterial Infections • Opsonization: certain antibodies and complement proteins act as opsonins • IgM and IgG agglutinate bacteria • Classical complement activation: antibodies trigger formation of membrane attack complex • certain complement proteins attract phagocytic cells • antitoxin antibodies neutralize bacterial exotoxins • cytokines attract and stimulate macrophages • cytokines stimulate T cells and increase population of cells involved in cell-mediated response

29. Summary Animation: CMI and Humoral responses