The effects of IgM rheumatoid factor on EAM and EAG rosette formation with Fc receptor-bearing lymphoid cells.

IgM rheumatoid factors (IgM-RF) and IgM from normal donors (IgM-C) were tested for their effects on rosette formation of Fc receptor (FcR)-bearing lymphoid cells with IgM- or IgG-coated bovine red blood cells (EAM or EAG). All cell populations tested bearing FcR for IgG (FcRG) showed reduced numbers of EAG rosettes formed in the presence of IgM-RF; IgM-C had no effect on EAG rosette formation with FcRG-bearing cells. T cells or L cells bearing FcRM showed reduced numbers of EAM rosettes in the presence of any IgM preparation. In addition, IgM-RF facilitated EAG-rosette formation with TM cells. These studies confirm that native IgM can bind to FcRM, blocking EAM rosettes, and that IgM-RF can modulate the binding of an IgG-containing immune complex to FcR-bearing cells.