1. Lecture 4 Activation of Adaptive Immunity

2. Overview In order to initiate an immune response antigen must be recognized. In order to initiate an immune response antigen must be recognized. Antigen recognition depends on detection of antigen by special receptors. Antigen recognition depends on detection of antigen by special receptors. Antigen recognition depends on cellular cooperation. Antigen recognition depends on cellular cooperation. Cellular cooperation is controlled by recognition of MHC-encoded receptors. Cellular cooperation is controlled by recognition of MHC-encoded receptors.

3. Clonal Selection Theory (F. Macfarlane Burnet) Pre-existence of of many different potential antibody producing cells Pre-existence of of many different potential antibody producing cells Each cell displays surface receptors for specific antigens Each cell displays surface receptors for specific antigens Antigen encounter selects cells Antigen encounter selects cells

4. Postulates of the Clonal Selection Hypothesis Each lymphocyte bears a single type of receptor of a unique specificity Each lymphocyte bears a single type of receptor of a unique specificity Interaction between a foreign molecule and a lymphocyte receptor capable of binding that molecule with high affinity leads to lymphocyte activation Interaction between a foreign molecule and a lymphocyte receptor capable of binding that molecule with high affinity leads to lymphocyte activation The differentiated effector cells derived from an activated lymphocyte will bear receptors of identical specificity to those of the parental cell from which that lymphocyte was derived The differentiated effector cells derived from an activated lymphocyte will bear receptors of identical specificity to those of the parental cell from which that lymphocyte was derived Lymphocytes bearing receptors specific for self molecules are deleted at an early stage in lymphocyte development and are therefore absent from the repertoire Lymphocytes bearing receptors specific for self molecules are deleted at an early stage in lymphocyte development and are therefore absent from the repertoire

5. First a Word About “Cluster of Differentiation/Designation” (CD) Antigens What are they? What are they? Differentiation antigens Differentiation antigens Expressed by cells at distinct stages of differentiation Expressed by cells at distinct stages of differentiation Expressed by cells having different functions Expressed by cells having different functions How are they detected? How are they detected?

6. Fluorescence-Activated Cell Sorting (FACS)

7. Major Lymphocyte Populations B lymphocytes and plasma cells B lymphocytes and plasma cells T lymphocytes T lymphocytes Helper/Inducer (CD4) Helper/Inducer (CD4) Suppressor (CD8) Suppressor (CD8)

8. T Cell Recognition of Antigen Recognize antigen peptide fragments bound to specialize cell surface molecules on antigen-presenting cells (APC). Recognize antigen peptide fragments bound to specialize cell surface molecules on antigen-presenting cells (APC). Molecules are encoded by major histocompatibility complex Molecules are encoded by major histocompatibility complex Peptides are displayed to T cells as peptide:MHC complexes Peptides are displayed to T cells as peptide:MHC complexes T cell antigen receptors recognize peptide:MHC complexes T cell antigen receptors recognize peptide:MHC complexes Each MHC molecule can bind numerous different peptides Each MHC molecule can bind numerous different peptides Two classes of MHC molecules Two classes of MHC molecules

9. Major Histocompatibility Complex (MHC) Gene Products Class I Class I Antigen is usually endogenous (e.g. viral proteins). Antigen is usually endogenous (e.g. viral proteins). CD8 + cytotoxic T lymphocytes (CTLs) recognize antigen in association with class I MHC gene product on APC. CD8 + cytotoxic T lymphocytes (CTLs) recognize antigen in association with class I MHC gene product on APC. Class II Molecules Class II Molecules Antigen is usually extracellular. Antigen is usually extracellular. CD4 + Helper T Lymphocytes recognize antigen in association with class II MHC gene product on APC. CD4 + Helper T Lymphocytes recognize antigen in association with class II MHC gene product on APC.

10. Cellular Cooperation and Antigen Recognition APC ExtracellularAntigen CD4 + Helper T Lymphocyte Class II MHC- associatedantigen +

11. B Cell Antigen Recognition Cell surface immunoglobulin receptor or B-cell receptor (IgM and IgD) Cell surface immunoglobulin receptor or B-cell receptor (IgM and IgD) Antigen contact initiates B-cell activation, clonal expansion, maturation to plasma cell Antigen contact initiates B-cell activation, clonal expansion, maturation to plasma cell Antigen receptor is identical to immunoglobulin that will ultimately be produced Antigen receptor is identical to immunoglobulin that will ultimately be produced

12. Antigen Presenting Cells Macrophages/monocytes Macrophages/monocytes Dendritic cells (e.g., Langerhan’s cells) Dendritic cells (e.g., Langerhan’s cells) B cells B cells

13. Properties of Antigen- Presenting Cells

14. Cellular Cooperation T H B antigen antigen Antibody secretion by plasma cells Plasma Cells Antigen presenting cell Antigen presentation to T and B cells by APC T cells elaborate cytokines to facilitate B cell proliferation and maturation

15. Adjuvants Freund's Complete Adjuvant (Water-in-oil emulsion) Freund's Complete Adjuvant (Water-in-oil emulsion) mineral oil mineral oil emulsifying agent emulsifying agent microbial preparation (eg. heat-killed extract of Mycobacterium tuberculosis) microbial preparation (eg. heat-killed extract of Mycobacterium tuberculosis) aqueous phase containing antigen aqueous phase containing antigen Aluminum Hydroxide Gel Aluminum Hydroxide Gel Microbial Adjuvants Microbial Adjuvants C. parvum C. parvum BCG BCG Peptides and Synthetic Polymers Peptides and Synthetic Polymers

16. Effector Mechanisms Mechanisms that are used by the immune system to eliminate pathogens (or other substances) from the body Mechanisms that are used by the immune system to eliminate pathogens (or other substances) from the body Cellular effector mechanisms Cellular effector mechanisms Activated T cells Activated T cells Natural killer cells Natural killer cells Humoral effector mechanisms (antibody) Humoral effector mechanisms (antibody) Neutralization Neutralization Opsonization Opsonization Complement activation Complement activation Antibody-dependent cell-mediated cytotoxicity (ADCC) Antibody-dependent cell-mediated cytotoxicity (ADCC)

17. Effector Activity Against Pathogens

18. Clonal Expansion Following Antigen Exposure Virgin lymphocyte pool PRIMARY RESPONSE SECONDARY RESPONSE effector cells memory cell pool effector cells memory cell pool

19. The Bottom Line In order to initiate an immune response antigen must be recognized. In order to initiate an immune response antigen must be recognized. Antigen recognition depends on detection of antigen by special receptors. Antigen recognition depends on detection of antigen by special receptors. Antigen recognition depends on cellular cooperation. Antigen recognition depends on cellular cooperation. Cellular cooperation is controlled by recognition of MHC-encoded receptors. Cellular cooperation is controlled by recognition of MHC-encoded receptors. Antigen “drives” the process resulting in “effector” cells and “memory” cells. Antigen “drives” the process resulting in “effector” cells and “memory” cells.