Differential expression of the B cell-restricted molecule CD22 on neonatalB lymphocytes depending upon antigen stimulation

Newborns respond poorly to certain antigens and produce mainly IgM antibodi es. By flow cytometry we analyzed on neonatal and adult B cells the express ion of CD22, a B cell receptor (BCR)-associated membrane molecule, known as negative modulator of BCR signaling. After T cell-independent (TI-)stimula tion with anti-mu F(ab'), fragments we found a dramatic decrease in the per centage of neonatal CD22(+) B cells and CD22 mean fluorescence intensity (M FI) shift, whereas adult B cells remained unaffected, Survival and prolifer ation rates of neonatal B cells were higher compared to adult B cells where as the degrees of apoptosis and necrosis were comparable. Surprisingly, aft er stimulation with lower doses of anti-CI apoptosis as well as proliferati on increased significantly in contrast to adult B cells. T cell-dependent ( TD)-stimulation with anti-CD40 monoclonal antibody and IL-4 resulted in a d ramatic increase in the percentage of CD22(+) neonatal B cells in contrast to unaffected adult B cells. CD22 MFI shifts showed no significant changes, respectively. The survival rate was higher for adult B cells, whereas apop tosis and cell death were comparable. These results suggest that TI antigen s lower the neonatal BCR signaling threshold via down-regulation of CD22, r esulting in hyperresponsive B cells apt to premature apoptosis. On the othe r hand; up-regulation of CD22 after TD stimulation may allow increased inhi biting influence of CD22 on neonatal ECR signaling, impairing B cell activa tion and differentiation.