Anti-immunoglobulin stimulation of murine lymphocytes. II. Identification of cell surface target molecules and requirements for cross-linkage.

Splenic B cells frliferate in vitro with soluble anti-immunoglobulin (Ig) reagents. To investigate whether the integrity of the anti-Ig molecule is necessary for stimulation and to determine whether cross-linkage of cell surface Ig is required, experiments were done by using F (ab')2 fragments and Fab monomers prepared from anti-MIgM serum. To determine whether antibodies directed against heavy chains would induce cell proliferation, class-specific antisera were prepared and tested. The results showed that cell proliferation was induced by F (ab')2 fragments but not by Fab monomers. In addition, cell proliferation was obtained with monospecific antiserum directed against mu heavy chains but not with antisera directed against alpha- or gamma- chains. Thus cross-linkage of mu heavy chains on the B cell surface is required for soluble anti-Ig-induced proliferation. Further experiments were done to investigate the nature of the age-associated response by comparing membrane immunoglobulin density and class on spleen cells from old and young (2 to 3 months) mice; no differences in surface immunoglobulins were found which would explain the age-associated response to anti-Ig reagents.