CD28 FUNCTIONS AS AN ADHESION MOLECULE AND IS INVOLVED IN THE REGULATION OF HUMAN IGE SYNTHESIS

Activated T cells induce IgE switching in B cells via a combination of lymphokines and direct T:B cell contact. As CD28-deficient mice have reduced basal levels of IgG1 and 1gG2a and diminished Ig class switchi ng, we investigated whether the CD28/B7.1 (CD80) ligand pairing might also be involved in human IgE regulation. Go-incubation of an allergen -specific, human T cell clone with tonsillar B cells caused a marked u p-regulation of CD28 expression, whereas, in contrast, CD45 RE express ion was unaffected. To test whether blocking the CD28:B7.1 interaction affected IgE synthesis, a dialyzed anti-CD28 monoclonal antibody (mAb ) was added to cultures containing tonsillar B cells, pre-activated T cell clones and interleukin-4. Anti-CD28 treatment caused a reproducib le, dose-dependent inhibition of IgE, but not IgG synthesis that was a ccompanied by a visible decrease in cell aggregate formation. Converse ly, an anti-B7.1 mAb had no effect in this system. The effect of block ing CD28-ligand interactions on lymphocyte adhesion was formally asses sed on human T cell clones and B cell lines using dual intracellular s taining and flow cytometry. Go-incubation with an anti-CD28 mAb, but n ot control IgG or anti-B7.1 mAb, resulted in a marked impairment of co njugate formation that correlated well with T cell surface expression of CD28. Using this system we found that an anti-CTLA-4 mAb but not an anti-B7.2 mAb inhibited T:B cell conjugate formation. Lastly, in addi tion to a direct effect of anti-CD28 mAb on conjugate formation, 14-da y culture of T and B cells in the presence of anti-CD28 caused a marke d decrease of ICAM-1 (CD54) expression on aggregated lymphocytes. In c ontrast, LFA-1 (CD18) expression was unaffected. We, therefore, conclu de that the T cell co-stimulatory molecule CD28 is involved in the reg ulation of IgE synthesis in vitro. CD28 may act to a limited extent as an adhesion molecule, though apparently not by pairing with B7.1 or B 7.2. It is more likely that ligation of CD28 under certain conditions modulates the expression of other T and B cell surface molecules.