Low-affinity IgE receptor (CD23) function on mouse B cells: role in IgE-dependent antigen focusing.

B-cell surface immunoglobulin very efficiently focuses specific protein antigens for presentation to T cells. We have demonstrated a similar role in antigen focusing for the low-affinity Fc epsilon receptor (Fc epsilon RII) on mouse B lymphocytes. B cells treated with an IgE monoclonal antibody to 2,4,6-trinitrophenyl (TNP) (IgE-B cells) were 100-fold more effective than were untreated B cells in presenting low concentrations of TNP-antigen to T cells. Blocking the binding of IgE to Fc epsilon RII on IgE-B cells with a monoclonal antibody to Fc epsilon RII completely eliminated this increased effectiveness. Preformed complexes of IgE anti-TNP and TNP-antigen were more effectively presented (approximately 100-fold) than TNP-antigen in the presence of nonspecific IgE. In contrast, complexes of IgG1 anti-TNP and TNP-antigen, capable of binding to Fc gamma receptors on B cells, were presented less effectively than TNP-antigen in the presence of nonspecific IgG1. Antigens focused by means of Fc epsilon RII or by means of B-cell surface immunoglobulin receptors were presented at comparably low concentrations. For several reasons, Fc epsilon RII on B lymphocytes seems to be particularly effective in efficiently focusing IgE-antigen complexes.