Jacalin, an IgA-binding lectin, inhibits differentiation of human B cells by both a direct effect and by activating T-suppressor cells.

Jacalin, a lectin extracted from the seeds of Artocarpus intergifolia (jackfruit), has been reported to bind specifically to IgA while inducing B-cell polyclonal immunoglobulin secretion. We confirmed that jacalin only binds to IgA and not to IgG or IgM and extended these findings by showing that it does not bind to IgE. Addition of jacalin to either unfractioned peripheral blood lymphocytes or purified B cells failed to induce immunoglobulin synthesis; indeed immunoglobulin production was diminished in the presence of jacalin. We found that jacalin directly inhibited the induction of immunoglobulin synthesis from B cells in the presence of T-cell replacing factor. Cell lines making IgG, IgM, and IgA were inhibited by jacalin. Furthermore, T cells incubated with jacalin also inhibited immunoglobulin production by stimulated B cells. Under these conditions jacalin was found to be a potent mitogen for T cells but to induce little or no activation of B cells. Jacalin appears to be a potent T-cell mitogen which can induce suppressor T cells for Ig production. It also has a direct inhibitory effect on B-cell Ig production.