IgG recruiting component (GRC): B cell-derived signal for IgG antibody synthesis.

The B cell-derived soluble factor that has been described as an IgG-recruiting component (GRC) was investigated to: a) ascertain whether it is governed by genetic constraints, b) determine what triggers its synthesis, and c) identify its cellular target. GRC has been shown to be unrestricted by histocompatibility barriers since it enhanced IgG antibody production in mice of diverse genetic backgrounds. Further, we report that eliminating IgG-bearing cells from B cells to be immunized in vitro allows T cells-replacing factor (TRF) to increase the number of IgM but not IgG PFC. Thus, TRF appears to act on IgM-bearing cells by expanding the IgM PFC number. Adding GRC 48 hr after the addition of TRF to such IgG-depleted cells caused expression of IgG PFC. Hence, B cells lacking IgG but possessing IgM surface immunoglobulins appear to be those that are acted upon by GRC. These data indicate that in whole splenic cell populations, GRC is derived from IgG-bearing B cells that are stimulated by antigen and a component in TRF.