Aims

1. Aims • Humoral immune response. • primary and secondary antibody responses. • T cell-dependent and T cell-independent antigens and the antibody response to each. • Effector functions of antibodies including neutralization and ADCC. • Readings: Abbas & Lichtman, Chapter 7 & 8

2. Humoral Response • Antigen recognition phase • Naïve B cells express membrane bound IgM and _IgD_. Abbas & Lichtman’s Basic Immunology 7-1

3. Humoral Response • Activation phase • T cell independent • T cell dependent. Abbas & Lichtman’s Basic Immunology 7-1

4. Humoral Response • Activation phase • Clonal expansion • Differentiation (Ab secretion, Class switch, Affinity maturation, Memory cells) Abbas & Lichtman’s Basic Immunology 7-1

5. Primary vs. Secondary Antibody Responses • Primary response • Lag of 5-10 days following infection for antibody peak. • Smaller response of mostly lower affinity IgM. Abbas & Lichtman’s Basic Immunology 7-2A

6. Primary vs. Secondary Antibody Responses • Secondary response • Lag of only 1-3 days following repeat infection for antibody peak. • Due to presence of Memory B cells. • Larger response of mostly higher affinity IgG. Abbas & Lichtman’s Basic Immunology 7-2A

7. B-Cell Activation • T cell-dependent (Protein antigens) • Utilize helper T cells to stimulate naïve B cell. • T cell-independent (Polysaccharide, lipid, and other nonprotein antigens) • Does not utilize helper T cells to stimulate naïve B cell.

8. B-Cell Activation • Initiated when naïve antigen -specific B cells in the peripheral lymphoid organs recognize antigens with their membrane bound Ig. (IgM and IgD) • Specifically in the follicle of lymph nodes and _spleen_. • Requires second signal.

9. T-Cell Independent B-Cell Activation • Primary signal • Antibody clustering (cross-linking of 2 or more antibodies) • Easier for polyvalent antigens with multiple identical epitopes (polysaccharides). Naïve B cell

10. T-Cell Independent B-Cell Activation • Primary signal • As a result polyvalent antigens cause a stronger primary signal response. • These antigens do not induce a strong overall B cell response because they usually do not elicit T cell help since they are not proteins. ?????????????? Naïve B cell

11. Consequences of Primary Signal • Mitosis • Increased expression of B7 costimulators. • Increased expression of cytokine receptors. • Reduced expression of chemokine receptors. • Results in B cell wandering out of the follicle. • Secretion of low levels of IgM Adapted from Abbas & Lichtman’s Basic Immunology 7-5

12. T-Cell Independent Response • Little heavy chain class switch. • thus, B cell makes mostly _IgM . • Little affinity maturation. • thus weaker antibody affinity.

13. T-Cell Dependent B-Cell Activation • Antigen recognition by B cell. • Must crosslink 2 or more BCRs. • APC present MHC/ peptide antigens to the TH cells. • B cell interaction with Th cells. • Thus, B cells and T cells recognize and are activated by different epitopes (forms) of the same antigen. Abbas & Lichtman’s Basic Immunology 7-6

15. Antigen Presentation by B-Cell • B cell • Recognizes native protein and activates. • Processes native protein and presents peptides in MHC class II molecules. • Expresses B7 co-stimulator. • T cell • Recognizes peptide/class II MHC with TCR. • Binds B7 with CD28. Adapted from Abbas & Lichtman’s Basic Immunology 7-7

16. B-Cell Present Antigen to Helper T-Cell • BCR binds protein antigen. • Protein antigen gets internalized and degraded. • Antigen peptides combine with MHC class II molecules and get expressed on the B cell surface. Adapted from Abbas & Lichtman’s Basic Immunology 7-8

17. Helper T-Cell is Activated • T helper cell binds MHC/peptide with TCR and B7 with CD28 • This activates T cell and it expresses CD40 ligand and secretes cytokines. Adapted from Abbas & Lichtman’s Basic Immunology 7-8

18. T Helper Cell-Mediated B-Cell Activation • CD40 on B cell binds to CD40 ligand. • Cytokines stimulate B cell. • B cell activates. • Proliferation • Differentiation • Class switch • affinity maturation Adapted from Abbas & Lichtman’s Basic Immunology 7-8

19. Affinity Maturation In germinal centers of lymphoid follicles. Increased antibody affinity. Due to somatic hypermutation of Ig gene and positive selection of high-affinity Ig producing B cells by dendritic cells. Abbas & Lichtman’s Basic Immunology 7-12

21. Memory B-Cells • A fraction of activated B cells that have undergone class switch and affinity maturation become memory B cells. • Memory B cells do not secrete antibody. • Memory B cells circulate in the blood looking for antigen to bind to their BCR. • Memory B cells survive for months to years.

22. B-Cell Death • After B cells differentiate into antibody secreting cells and memory cells most die by _apoptosis_.

23. Anatomy of Humoral Immune Response Recognizes antigen in follicle. Interacts with Th cell at the edge of the follicle. Differentiates and undergoes class switch mainly outside follicle. Affinity maturation occurs in the germinal center. Memory cells develop in the germinal center. Naive B Cell Abbas & Lichtman’s Basic Immunology 7-13

25. Feedback Regulation • Regulation of antibody production. • B cells have Fc receptors that block BCR signaling when they are bound by antibody. • Thus when there is a lot of antibody around the B cell membrane the BCR shuts down and antibody production decreases. Abbas & Lichtman’s Basic Immunology 7-15

27. Effector Functions of Antibodies • Neutralization • Opsonization • ADCC • Complement activation Adapted from Abbas & Lichtman’s Basic Immunology 8-1

28. Neutralization Prevents microbes from infecting cells by binding them first. Prevents toxin from poisoning cells by binding them first. Abbas & Lichtman’s Basic Immunology 8-2

30. Antibody-Dependent Cellular Cytotoxicity (ADCC) • IgG bind to target cell (Fab). • NK cells have low affinity receptors for IgG Fc (CD 16). • NK cells release granules. Adapted from Abbas & Lichtman’s Basic Immunology 8-4

31. ADCC • IgE bind to Helminth. • Eosinophils cells have high affinity receptors for IgE Fc. • Eosinophils release granules killing the parasite. Abbas & Lichtman’s Basic Immunology 8-4

32. Evasion Mechanisms by Microbes • Mutation of antigenic surface molecules. • Inhibit complement. • Resist phagocytosis. Abbas & Lichtman’s Basic Immunology 8-10

33. Cytokines Promote B cell Proliferation and Differentiation • IL-2 • Secreted by _T-cells__. • Induces B cells to proliferate. • Induces B cells to make antibody. • IL-1 and IL-4 • Stimulates the production of IL-2 receptor. • IL-4 • Promotes cell division. • Promotes class switch from IgM to _IgE . • IL-4, IL-10, and IFNg • Promote differentiation into antibody forming cells.

34. Next Time • Immunologic Tolerance • Autoimmunity • Readings: Robbins, Chapter 5

35. Objectives • Describe the humoral immune response. • Compare and contrast the primary and secondary antibody responses. • B-cell activation, Memory B cells • Compare and contrast T-dependent and T-independent antigens and the antibody response to each. • Describe how B cells present antigen to T cells. • Describe effector functions of antibodies including neutralization and ADCC.