Aims Humoral immune response primary and secondary antibody

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

Humoral Response • Antigen recognition phase – Naïve B cells express membrane bound Ig. M and _Ig. D_. Abbas & Lichtman’s Basic Immunology 7 -1

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

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

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

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 Ig. G. Abbas & Lichtman’s Basic Immunology 7 -2 A

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.

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

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

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

Consequences of Primary Signal • Mitosis • Increased expression of B 7 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 Ig. M Adapted from Abbas & Lichtman’s Basic Immunology 7 -5

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

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

Antigen Presentation by B-Cell • B cell – Recognizes native protein and activates. – Processes native protein and presents peptides in MHC class II molecules. – Expresses B 7 costimulator. • T cell – Recognizes peptide/class II MHC with TCR. – Binds B 7 with CD 28. Adapted from Abbas & Lichtman’s Basic Immunology 7 -7

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

Helper T-Cell is Activated • T helper cell binds MHC/peptide with TCR and B 7 with CD 28 • This activates T cell and it expresses CD 40 ligand secretes cytokines. Adapted from Abbas & Lichtman’s Basic Immunology 7 -8

T Helper Cell-Mediated B-Cell Activation • CD 40 on B cell binds to CD 40 ligand. • Cytokines stimulate B cell. • B cell activates. – Proliferation – Differentiation • Class switch • affinity maturation Adapted from Abbas & Lichtman’s Basic Immunology 7 -8

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

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.

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

Anatomy of Humoral Immune Response Naive B Cell • 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. Abbas & Lichtman’s Basic Immunology 7 -13

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

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

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

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

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

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

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 Ig. M to _Ig. E. • IL-4, IL-10, and IFNg – Promote differentiation into antibody forming cells.

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

Objectives 1. Describe the humoral immune response. 2. Compare and contrast the primary and secondary antibody responses. 1. B-cell activation, Memory B cells 3. Compare and contrast T-dependent and Tindependent antigens and the antibody response to each. 4. Describe how B cells present antigen to T cells. 5. Describe effector functions of antibodies including neutralization and ADCC.