T cell-B cell conjugates are formed when trinitrophenyl-specific B cells are exposed to trinitrophenyl-ovalbumin and ovalbumin-specific T hybridoma cells. The proportion of conjugates was increased two- to threefold when antigen-pulsed trinitrophenyl-specific B cells, but not T cells, were pre-exposed to interleukin 4. Antigen-specific B cells pretreated with antigen and interleukin 4 and cultured in the presence of specific T helper cells also produced a larger proportion of antibody-secreting cells as compared to cells pretreated with antigen alone. The interleukin 4-induced enhancement of T/B conjugate formation occurred over a wide range of antigen concentrations, was dependent on the concentration of interleukin 4, and was inhibited by the monoclonal anti-interleukin 4 antibody, 11B11. The importance of Ia antigens in the enhancement of conjugate formation and generation of antibody-secreting cells is suggested by a) the fact that the interleukin 4-mediated increase in the density of Ia antigens on the antigen-specific B cells correlated with their enhanced ability to form T/B conjugates, b) the kinetics of the interleukin 4-mediated increase in conjugate formation and surface Ia expression were similar, c) 10- to 20-fold higher concentrations of anti-I-A antibody were required to inhibit T/B conjugate formation by 50% with interleukin 4-treated antigen-specific B cells compared with untreated antigen-specific B cells, and d) interferon-gamma, which inhibits the interleukin 4-mediated increase in Ia antigens, inhibited the interleukin 4-induced enhancement of T/B conjugate formation. These results indicate that the interleukin 4-induced increase in the expression of Ia antigens on B cells plays an important role in the enhancement of T/B cell interactions and the subsequent differentiation of antigen-specific B cells into antibody-secreting cells.

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