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The cytokine milieu is critical for determining the outcome of an immune response following B cell–T cell interaction. Recent work has shown that CD4+_{T cells in}

both mouse and human can be divided into different subsets, depending on their cytokine profile (Fig. 8.9): • TH1 cells – CD4+ T cells that produce IL-2 and

interferon-γ (IFNγ), but not IL-4, are designated TH1

and are chiefly responsible for delayed-type hyper- sensitivity responses, but can also help B cells produce IgG2a (mouse), but not much IgG1 or IgE;

• TH2 cells – CD4+ T cells that produce IL-4, IL-5,

IL-10, and IL-13, but not IL-2 or IFNγ, are designated TH2 and are very efficient helper cells for production

of antibody, especially IgG1 and IgE;

• TH0 cells – many CD4+T cells, especially in humans,

have cytokine profiles intermediate between TH1 cells

and TH2 cells, and are known as ‘TH0’ cells. These

cells are capable of producing both the TH1 cytokine

IFNγ and the TH2 cytokine IL-4. However, classic

TH1 and TH2 cells have been well documented in

humans, especially in diseased tissues.

During B cell–T cell interaction, T cells can secrete a number of cytokines that have a powerful effect on B cells (see Fig. 8.9). IL-2, for example, is an inducer of prolif- eration for B cells.

The specific cytokines produced by TH2 cells also

affect B cells. These cytokines include IL-4, IL-5, IL-6, IL-10, and IL-13:

• IL-4 (previously known as B cell activating or differ- entiation factor-1) acts on B cells to induce activation and differentiation, and acts on T cells as a growth factor and promotes differentiation of TH2 cells,

thus reinforcing the antibody response – excess IL-4 plays a part in allergic disease, causing production of IgE;

• IL-5 in humans is chiefly a growth and activation factor for eosinophils and is responsible for the eosinophilia of parasitic disease (in the mouse it also acts on B cells to induce growth and differentiation);

• IL-6 is produced by many cells including T cells, macrophages, B cells, fibroblasts, and endothelial cells, and acts on most cells, but is particularly important in inducing B cells to differentiate into AFCs – IL-6 is considered to be an important growth factor for multiple myeloma, a malignancy of plasma cells; • IL-10 acts as a growth and differentiation factor for

B cells in addition to modulating cytokine production by TH1 cells;

• IL-13, which shares a receptor component and signal- ing pathways with IL-4, acts on B cells to produce IgE. Other cytokines such as IL-7, originally isolated from a stromal cell line as a factor supporting pre-B cell growth, have been shown to be indispensable for B cell development.

Q. Give three different ways in which IL-4 can reinforce the TH2-type immune response.

A. It promotes differentiation of TH0 cells into TH2 cells. It acts

on B cells to promote their division, differentiation, and anti- body synthesis. It also acts on endothelium and tissue cells to promote the synthesis of chemokines that selectively attract

TH2 cells.

Cytokines and B cell development

Fig. 8.9 B cell development is influenced by cytokines from T cells and APCs, and by direct interactions with TH2 cells. IL-4 is most important in promoting division, and and a variety of cytokines including IL-4, IL- 5, IL-6, IL-10, and IFNγ influence

development into antibody-forming cells (AFCs), and affect the isotype of antibody that will be produced.

TH0 TH1 TH2 IL-4 IL-1 IL-13 IL-2 IL-4 IL-4 IL-5 IL-6 IL-10

activation division differentiation

IL-12, IFN

IFN

B _B

APC

Cytokines can also influence antibody affinity. Antibody affinity to most TD antigens increases during an immune response, and a similar effect can be produced by certain immunization protocols. For example, high- affinity antibody subpopulations are potentiated after immunization with antigen and IFNγ (Fig. 8.10).

A number of adjuvants are capable of enhancing levels of antibody, but few have this characteristic of also poten- tiating affinity. As affinity markedly influences the bio- logical effectiveness of antibodies, IFNγ may be an important adjuvant for use in vaccines.

Q. Why do adjuvants not generally enhance antibody affinity, though they do enhance antibody titers? – think of when and how adjuvants act.

A. Most adjuvants act as depots for antigen or have components that promote antigen presentation to T cells. They do not act directly on B cells.

In addition to the effects of cytokines on B cell prolif- eration and differentiation, cytokines are capable of influencing the class switch from IgM to other immuno- globulin classes (see below).

CYTOKINE SECRETION FROM CD4+_{T CELLS CONTROLS B CELL PROLIFERATION AND DIFFERENTIATION}

Cytokine influence on antibody affinity

Fig. 8.10 Mice were immunized either with antigen alone (Ag) or with antigen plus 30 000 units of IFNγ (Ag + IFNγ). The affinity of the antibodies was measured either early or late after immunization. Mice receiving IFNγ showed more high- affinity antibody (darker bars) in both the early and late bleeds than mice that received antigen alone. (Adapted from Holland GP, Holland N, Steward MW. Clin Exp Immunol

1990;82:221–226) Ag 50 25 50 25 % of total antibody increasing affinity early test bleed

late test bleed

Ag Ag IFN

Ag IFN % of total

antibody

increasing affinity

increasing affinity increasing affinity

Cytokine receptors aid in B cell growth and differentiation

Receptors for the many growth and differentiation factors required to drive the B cells through their early stages of development are expressed at various stages of B cell differentiation. Receptors for IL-7, IL-3, and low molecular weight B cell growth factor are important in the initial stages of B cell differentiation whereas other receptors are more important in the later stages (Fig. 8.11).

B cell–T cell interaction may either activate or inactivate (anergize)

The above description of B cell–T-cell interaction suggests that the only possible outcome is activation of the B cell. However, this is not the case.

APC–T cell interaction may yield two diametrically opposing results, namely activation or inactivation (clonal

anergy, see Fig. 7.18).

In the same way, B cells frequently become anergic. This is an important process because affinity maturation of B cells (see below) during the immune response as a result of rapid mutation in the genes encoding the antibody variable regions could easily result in high- affinity autoantibodies.

Clonal anergy and other forms of tolerance in the periphery are important for silencing these potentially damaging clones. However, the molecular details of this process are still unclear. Moreover, the respective roles of IgM and IgD, the two cell surface receptors for antigen on B cells, are also not understood in terms of activation or inactivation. Both IgM and IgD appear to be capable of transmitting signals for both functions.

Following activation, antigen-specific B cells can follow either of two separate developmental pathways:

• the first pathway involves proliferation and differ- entiation into AFCs in the lymph nodes or in the periarteriolar lymphoid sheath of the spleen – these AFCs function to rapidly clear antigen, but the great majority of these cells die via apoptosis within 2 weeks, so it is unlikely that these AFCs are responsible for long-term antibody production;

• in the second pathway some members of the expanded B cell population migrate into adjacent follicles to form germinal centers before differentiating into memory B cells.

The mechanism that determines which path a B cell takes is unknown. However, it is likely that the decision can be influenced by the nature of the naive B cells initially recruited into the response, the affinity and specificity of the BCR, the type of antigen driving the response, and the levels of T cell help.

Q. Which type of APC is specifically located in the germinal center?

A. FDCs (see Fig. 7.2), and these turn out to be very important in driving B cell development.

B CELL AFFINITY MATURATION TAKES