In this report we have extended our previous studies on interleukin 4 (IL-4) [previously termed B-cell stimulatory factor-1 (BSF-1)]. Our results demonstrate that 8 hr of exposure to IL-4 is sufficient to induce maximal expression of Ia antigens. This increase in expression of Ia antigens on resting B cells is due to the direct action of IL-4 on the B cells since adding or removing adherent cells or utilizing low density cultures of B cells at 50-100/culture had no effect on the IL-4-mediated increase in Ia. Monoclonal anti-IL-4 antibody completely abrogated the Ia-inducing activity of IL-4. A variety of other purified lymphokines including interleukin 2 (IL-2), interleukin 1 (IL-1), and a source of either B-cell differentiation factor for IgM (BCDFμ), or B-cell growth factor II (BCGF II), did not alter the expression of Ia antigens on resting B cells. However, interferon-γ can partially inhibit the IL-4-mediated induction of Ia.
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