In vitro synthesis of IgE by human lymphocytes. II. Enhancement of the spontaneous IgE synthesis by IgE-binding factors secreted by RPMI 8866 lymphoblastoid B cells

Abstract

RPMI 8866 lymphoblastoid cells, known to express surface Fc epsilon R, were tested for their ability to regulate the in vitro synthesis of human IgE. Cell-free supernatants (CFS) of RPMI 8866 cells enhanced in a dose-dependent fashion the spontaneous IgE synthesis by B cells of allergic individuals. For maximum activity the CFS had to be added during the first 3 days of culture. CFS did not significantly alter the spontaneous synthesis of IgM or IgG, but they suppressed IgA synthesis both in B cell cultures and in pokeweed mitogen-stimulated peripheral blood mononuclear cells cultures. Cyclosporin A did not suppress either the spontaneous Ig production by B cells nor the IgE-potentiating activity of CFS. The enhancing activity of CFS was related to its content in IgE binding factors (IgE-BFs); these factors were detected by their ability to inhibit the rosetting of RPMI 8866 cells with IgE-coated erythrocytes (E-IgE). Both the IgE-BFs and the IgE-potentiating activity of the supernatants of RPMI 8866 cell cultures could be removed by absorption with IgE-Sepharose, from which they could subsequently be eluted with glycine-HCl buffer. IgE-BFs were identified as glycoproteins on the basis of their sensitivity to trypsin and to neuraminidase. By filtration of the RPMI 8866 cell supernatants through a Sephadex G75 column, IgE-binding activity was found to be associated with two fractions with molecular sizes in the range of 10,000-15,000 and 30,000-40,000. The IgA-suppressing activity of the RPMI 8866 culture filtrates could be absorbed with sIgA-Sepharose from which it was subsequently recovered by elution with glycine-HCl buffer. Most unexpectedly, sIgA-Sepharose also removed IgE-BFs and IgE-potentiating activity from the RPMI 8866 supernatants; both could be recovered by subsequent elution from sIgA-Sepharose with gycline-HCl buffer. These data are provisionally interpreted as indicating that the IgE-BFs secreted by RPMI 8866 cells had affinity for both IgE and sIgA and that they exerted a reciprocal effect on the in vitro synthesis of IgE and IgA.