IgG2b-dependent down regulation of the LPS-induced PFC-response and its blockade by Fc gamma 2bR protein

Fc gamma receptor (Fc gamma R)-dependent immunoregulation by murine heat-aggregated (HAgg) IgG subclasses on the bacterial lipopolysaccharide (LPS)-induced plaque forming cell (PFC) response to trinitrophenylated sheep red blood cell (TNP-SRBC) antigen and the competitive effect by Fc gamma 2bR-protein on the down regulation by HAgg-IgG2b were studied in murine T-cell-deprived spleen cell cultures. HAgg-IgG1 and HAgg-IgG3 enhanced the PFC response, but HAgg-IgG2b strongly suppressed the LPS-induced PFC response. HAgg-IgG1 could not compete with the suppressive effect of HAgg-IgG2b. The HAgg-IgG2b seemed to act on both macrophages (M phi) and B-cells, because the cell cultures that had been reconstituted with HAgg-IgG2b-pretreated M phi and untreated B-cells and vice versa showed poor PFC responses. The suppression induced by HAgg-IgG2b on the LPS-induced PFC response in the T-cell-deprived cultures was abolished by the addition of phospholipase C (PLC)-treated Fc gamma 2bR protein at the early stage of the culture. The mechanisms by which HAgg-IgG2b suppress the LPS-induced PFC response and PLC-treated Fc gamma 2bR protein restores this response were discussed.