Modulation of polyclonally activated human peripheral B cells by aggregated IgG and by IgG-binding factors: differential effect on IgG subclass synthesis.

The regulation of IgG subclass production by polyclonally activated human B cells was investigated by using two systems previously shown to selectively suppress the generation of IgG-containing cells (CC) but not that of IgMCC or IgACC. The first one involved a brief exposure of peripheral blood mononuclear cells (PBMNC) to heat-aggregated human IgG (Agg-IgG) followed by repeated washings and culture with untreated autologous PBMNC. The second one was achieved by addition of human IgG-binding factor(s) (IgGBF) prepared by affinity chromatography from supernatants of unstimulated PBMNC. Pokeweed mitogen (PWM) and Nocardia opaca delipidated cell mitogen (NDCM) were used as polyclonal B cell activators. The latter can induce the terminal differentiation of peripheral B lymphocytes into plasma cells in the absence of helper T cells. After 6 days of culture, the number of cells containing IgM, IgG, or IgA was determined by direct immunofluorescence, and that of cells containing IgG1, IgG2, IgG3, or IgG4 was determined by indirect immunofluorescence with the use of subclass-specific monoclonal antibodies. After stimulation with PWM, exposure of PBMNC to Agg-IgG resulted in a selective diminution of the number of IgG4CC. With NDCM-stimulated cultures the same procedure induced a selective suppression of the generation of IgG2CC and IgG4CC. Conversely, the addition of IgGBF at the third day of culture was found to induce a 30 to 40% decrease in the number of cells containing each of the four IgG subclasses. Because of their differential pattern of IgG subclass suppression, Agg-IgG and IgGBF are likely to trigger distinct regulatory pathways.