A monoclonal antibody (MAb), KuN241, recognized an antigen present on all monocytes, polymorphonuclear leukocytes (PMNs), B cells, and a subpopulation of T cells. Cell surface expression pattern and immunoprecipitation studies indicated the molecule recognized by KuN241 to be complement receptor 1 (CR1). This was confirmed by sequential immunoprecipitation using E11, a CR1- specific monoclonal, and by immunoprecipitation analysis of a truncated transfection product of CR1. In vitro activation of PBLs for 7 days with pokeweed mitogen (PWM) abrogated surface expression of CR1 on B cells. Overnight culture with phorbol 12-myristate 13-acetate (PMA) downregulated the expression of CR1 detected by KuN241 both on PMNs and monocytes. Addition of MAb KuN241 to purified PMN and monocytes resulted in a transient increase in intracellular calcium ([Ca2+](i)). This was blocked by the Fab fragment of MAb KuFc79 directed against the Fcγ receptor. Further, Fab fragment of KuN241 cross-linked with F(ab')2 goat anti-mouse Ig failed to increase [Ca2+](i) levels. Taken together, these results suggested a novel mechanism for transmembrane signaling events by dual binding of KuN241, the Fab region to CR1 and the Fc region to FcγRII.
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