The recently cloned human interleukin 13 (IL-13) is a novel cytokine expressed in activated T cells that has been shown to inhibit inflammatory cytokine production by lipopolysaccharide-activated monocytes. The protein encoded by the IL-13 cDNA is the human homologue of a mouse Th2-product called P600. Here, we show that IL-13 acts at different stages of the B cell maturation pathway: (a) it enhances the expression of CD23/Fc epsilon RII and class II MHC antigens on resting B cells; (b) it stimulates B cell proliferation in combination with anti-Ig and anti-CD40 antibodies; and (c) it induces IgE synthesis. Thus, the spectrum of the biological activities of IL-13 on B cells largely overlaps that previously ascribed to IL-4. The present observations suggest that IL-13 may be an important factor, in addition to IL-4, in the development of allergic diseases.

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