Chapter 7: Humoral Immune Responses

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Humoral response phases

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1. Ag recognition
2. Activation of B lymphocytes
3. Clonal expansion
4. Differentiation:
-Ab secretion-effector cells
-Class switching
-Affinity maturation
-Memory B cell

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T-dependant antibody responses

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Require helper (CD4) T cells for B cell activation
MHC class II molecules
Protein antigens processed by APC
Many classes of antibodies can be produced

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T-independent antibody responses

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Non-protein antigens like lipids, carbohydrates, etc.
IgM antibodies are the majority produced

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Subsets of B cells:

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Follicular B cells
Marginal zone B cells and B-1 B cells

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Follicular B cells

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T-dependant responses to protein Ag
Make class-switched, high affinity Ab
give rise to long lived plasma cells

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Marginal zone B cells and B-1 B cells

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T-independent Ab responses
Ag receptors of limited diversity
primarily 1gM response

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The secondary response to an Ag

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The secondary response to an Ag produces more Ab than during the primary response
-With protein Ag, secondary responses show increased heavy chain class switching and increased affinity maturation due to an increase in the number of helper T cells

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Activation of B cells need:

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B lymphocytes are initially stimulated by antigen but there must also be co-receptor molecule activation and in the case of protein antigens, signals from helper T lymphocytes. All this is required for sufficient activation of naïveB lymphocytes.

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ITAMS

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Immunoreceptor Tyrosine-based Activation Motifs found in cytoplasmic tails of membrane proteins

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Kinases

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Enzymes that add phosphate groups to certain amino acids of proteins. Many kinases are specific for tyrosine and are involved in signal transduction

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BCR

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B Cell Receptor complex consists of the naïve B membrane receptors (IgD or IgM) and the two proteins Igalpha and Igbeta. Analogous to the TCR

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B lymphocyte activation, step 1

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Binding and cross-linking of antibody molecules on mature naïve B cells (BCR) by repetitive epitopes on Ag

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Step 2

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Clustering of BCR allows tyrosine kinases to phosphorylate ITAMS on the tails of Igalpha and Igbeta

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Step 3

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The doubly phosphorylated
Igbeta chain binds other TK allowing further phosphorylation reactions and signaling

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Step 4

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The co-receptor molecule CR2 binds to C3d on the
antigen. This is co-ligation of the BCR and the
co-receptor

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The complement system

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-Functions as an effector mechanism of host defenses second signal for B cell activation
- A collection of plasma proteins
--Activated by microbes
--Activated by antibodies attached to microbes
-Microbe becomes coated with C3d, a breakdown product of C3, the most abundant complement protein

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Co-receptor proteins

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CR2
CD19
CD81

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CR2

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CR2 binds to complement components (C3d) on the pathogen

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CD19

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CD19 acts as signaling chain of the receptor

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CD81

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CD81 unknown function. Is the surface receptor for Hepatitis C Virus (HCV)

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Final outcome

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The final outcome of all of this is translation of proteins that control B cellproliferation and differentiation.

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Consequences of antibody-mediated B cell activation

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Proliferation resulting in Ag-specific clones
Produce more IgM, may be in secreted form
-Early phase of humoral immune response

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Consequences of antibody-mediated B cell activation: For T-dependant responses

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Migration towards T cell-rich zones of the lymphoid organs (away from B cell follicles)
Enhanced ability to interact with helper T cells
- increased expression of B7 co-stimulators
- increased expression of cytokine receptors

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Helper T lymphocyte functions in response to protein antigens

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-Activation and migration of helper T cells
-Presentation of Ag by B cells to helper T cells
-Mechanisms of helper T cell-mediated B cell activation
-Heavy chain (Isotype) class switching
-Affinity Maturation

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Activation and migration of helper T cells

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Both types of cells (T and B) migrate to the edges of the lymphoid follicles, responding to production of chemokines and an increase in expression of receptors for them

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Presentation of Ag to helper T cells

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B lymphocytes are very efficient APC for the Ag they specifically recognize
-Protein Ag is endocytosed
-Protein Ag is presented with MHC II molecules and co-stimulators like B7 that are necessary for T cell activation
-CD4+ T cells recognize (the same Ag) and bind presented Ag

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Mechanisms of helper T cell-mediated B cell activation

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-Helper T cells activate B cells by expressing CD40L (CD40 ligand) and secreting cytokines
-CD40L binds to CD40 on B lymphocytes
-Physical contact needed between cells for further activation
-T cell cytokines bind to B cell cytokine receptors
-B cell proliferation and differentiation (antibody production/secretion) can then occur

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Heavy chain class (isotype) switching

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-Provides plasticity in the humoral immune response
- Allows for different Ab responses dependent on the infecting microbe
-Initiated by CD40L-mediated signals
--After binding CD40 of B cell
-Stimulated by different cytokines from
helper T cells
--IFNgamma, IL-4, TGF-beta, etc.

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IgG

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IgG is best for opsonization: coating bacteria with Ab to be phagocytosed

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IgE

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IgE coats helminths (worms) which are eliminated by eosinophils that bind IgE

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IgA

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IgA is able to be transported across epithelia
IgA is the first defense against intestinal microbes due to its ability to bind to mucus of epithelial tissues

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IgM

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IgM is a major factor in complement activation
B cells do not need helper T cell stimulation

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IFNgamma

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- stimulates switching to IgG
Is the major cytokine of Th1 cells
Enhanced opsonization, phagocytosis

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IL-4

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- stimulates switching to IgE
Is the major cytokine of Th2 cells
Helminth infections

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TGF-beta

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- stimulates switching to IgA
Transforming growth factor
Secreted through mucosal epithelia

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X-linked hyper-IgM syndrome

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-Caused by inactivating mutations in the CD40L gene, on the x chromosome
-Almost all serum Ab is IgM due to defective heavy chain class switching
-Patients have defective cell-mediated immunity against intracellular microbes
-This is a rare, genetic immune deficiency disorder

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Heavy chain class switching is the result of DNA rearrangement

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IgM-producing B cells have a Cμ as the first constant region in the Ig heavy chain gene
VDJ RNA is spliced to the Cμ RNA to give rise to the heavy chain of IgM
Switch recombination occurs when the VDJ is spliced to be adjacent to another C region (γ or ε), giving rise to a new heavy chain class

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Affinity Maturation

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-The process by which the affinity of Ab produced in response to a protein Ag increases with prolonged or repeated exposure to that Ag
-Occurs in the germinal centers of lymphoid follicles
- increased ability of Ab to bind to Ag
-Due to point mutations in the V regions, especially the hypervariable regions of Ab (somatic hypermutation)

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Selection of high-affinity B cells

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-Germinal B cells die if they do not recognize displayed Ag on follicular dendritic cells
-Low Ag concentration means that only B cells with high affinity for that Ag will be selected
-Selected B cells leave germinal centers and secrete their high affinity Ab that enter blood
--Some go to bone marrow and live for years
--1/2 of an adult’s Ab are produced by these cells

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Within germinal centers

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Within germinal centers, B cells proliferate rapidly so that one cell may produce about 5000 progeny within a week

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Memory B cells

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Do not secrete Ab (on surface)
Progeny of high affinity Ab products
Circulate in blood and survive for months or years
Ready to respond rapidly if antigen is reintroduced

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Where it all happens...

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-B cells become activated in primary follicles
-T cells leave the T cell rich areas of lymphoid organs, B cells leave the B cell rich areas
-They meet and interact in intermediate zones
-Here is where B cells differentiate, secrete Ab and undergo heavy chain class switching
-Germinal centers are where affinity maturation and development of memory cells takes place

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Antibody responses to T-independent antigens

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-Non-protein AG does not need helper T cell stimulation to elicit an antibody response
--No MHC molecules involved at all
-Very little is known about the Ab response
-Polysaccharides and lipids contain multivalent arrays of the same epitope
--Much cross-linking of B cell Ag receptors
--This alone may activate B cells (proliferation and differentiation)

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Antibody Feedback

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A mechanism for shutting off antibody production
-IgG binds to available Ag and forms immune complexes
-B cells may bind the Ag part of these complexes
-Fc tail of IgG may bind to Fc receptor on B cell
-Negative signals are generated to shut off antibody production, terminating B cell responses
- this means that sufficient IgG has been produced