Soluble, but not immobilized, anti-IgM antibody inhibits post-activation events leading to T-cell-dependent B-cell differentiation.

The potential for surface immunoglobulin-binding ligands to modify B-cell differentiation responses induced by activated T cells has been investigated. Activated T cells in human splenic mononuclear cells cultured on anti-CD3-coated plates induced B cells to produce large amounts of IgM and IgG. In this experimental system, cross-linking of B-cell antigen receptors by soluble, bivalent monoclonal or polyclonal anti-IgM antibodies completely inhibited IgM production, and greatly diminished IgG production, in a dose-dependent manner. Similar results were obtained using a F(ab')2 fragment of a goat anti-IgM antibody. Inhibition of B-cell differentiation by bivalent cross-linking reagents did not require the presence of antigen-presenting cells (APC), as comparable results were obtained in co-cultures of purified T and B cells. In contrast, enhanced immunoglobulin secretion was seen when surface IgM was cross-linked using anti-IgM antibody immobilized on the culture plate. Interestingly, activated T cells induced similar levels of expression on B cells of the activation antigens CD23, CD25 and CD71, and of class II molecules, irrespective of any treatment with soluble or immobilized anti-IgM antibody. This indicates that soluble anti-IgM specifically inhibits B-cell differentiation without altering initial events of T-cell-dependent B-cell activation.