gp 140, the C3d/EBV receptor (CR2), is a specific marker of human B lymphocytes. Very recent data suggest that CR2 is a membrane site involved in early B cell activation. These properties of CR2 led us to analyze the molecular events associated with gp 140. We analyzed whether in some conditions of B lymphocyte activation, CR2 could be phosphorylated. We have found that when highly enriched peripheral B cells were cultured for 48 h with anti‐μ Ab and/or SAC, in order to provide an optimal activating signal, phosphorylation of the CR2 was induced.

The C3d receptor is a specific marker of B lymphocytes. Recently we have shown that C3d receptor activity is carried by a gp140 membrane antigen. A polyclonal antibody has been prepared by immunizing a rabbit with highly purified gp140 molecule isolated from membranes of the human B lymphoblastoid cell line Raji and its high specificity was previously demonstrated. We tested the effect of this antibody to the C3d receptor on the B cell proliferative response. Purified B cells from human blood were induced to proliferate by a B cell growth factor (BCGF)‐containing partially purified supernatant from activated T cells. The anti‐C3d receptor F(ab′) 2 enhanced the BCGF‐dependent B cell proliferation. This effect was dose dependent, was observed in the presence of different concentrations of BCGF and did not correspond to a change in the time course of the response. The anti‐C3d receptor F(ab′) 2 had no mitogenic effect in the absence of T cell supernatant. In contrast the undigested anti‐C3d receptor IgG suppressed the BCGF‐dependent B cell proliferation. These results emphasize the potentialities of anti‐gpl40 F(ab′) 2 to explore the involvement of the C3d receptor in the regulation of B cell response to T cell products.