IGD EXPRESSION ON B-CELLS IS MORE EFFICIENT THAN IGM BUT BOTH RECEPTORS ARE FUNCTIONALLY EQUIVALENT IN UP-REGULATION CD80 CD86 COSTIMULATORY MOLECULES/

The expression and function of IgM and IgD antigen receptors were stud ied in a series of anti-hen egg lysozyme (HEL) immunoglobulin (Ig)-tra nsgenic mice expressing either IgM alone, IgD alone, or both IgM and I gD. B cell surface expression of IgD was found to be more efficient th an that of IgM. Thus antigen receptor density on IgD(+), IgM(-) B cell s was twofold higher than on IgM(+), IgD(-) cells despite the presence of sevenfold lower levels of membrane heavy chain mRNA, and coexpress ion of IgD with IgM led to almost complete inhibition of surface IgM. In addition, less extensive down-regulation of IgD occurred following exposure to antigen ia vitro. When regulation of CD80/CD86 co-stimulat ory molecules by surface Ig was examined, up-regulation of the former was initiated at lower antigen concentrations on IgM(-), IgD(+) compar ed to IgM(+), IgD(-) B cells. On correcting for antigen receptor densi ty, however, induction of CD80/CD86 by IgM and IgD was comparable. Tak en together, these results reinforced the functional similarity of IgM and IgD antigen receptors while at the same time revealing difference s in expression which may explain their simultaneous presence on matur e B cells.