We have previously described the APO-1 (CD95) cell surface molecule, a novel member of the nerve growth factor/TNF receptor superfamily, identical with the Fas Ag. Triggering of APO-1-induced apoptosis in APO-1+, apoptosis sensitive cells. The data in this study demonstrate that human peripheral blood B cells acquire sensitivity to APO-1-mediated apoptosis on PWM activation. To also study APO-1-mediated apoptosis in an Ag-specific human B cell response, we reconstituted severe combined immunodeficient (SCID) mice i.p. with human PBMC (SCID-hu mice). SCID-hu mice were then injected i.p. with soluble, adjuvant-free tetanus toxoid or diphtheria toxoid, respectively, directly after transfer of the PBMC (day 1) and received an Ag-booster injection on day 14. Two weeks after PBMC transfer human Abs were detected and shown to be Ag specific. SCID-hu mice co-injected with monoclonal anti-APO-1 Ab (IgG3,k) showed significantly suppressed anti-tetanus toxoid or anti-diphtheria toxoid titers, respectively. Sensitivity to anti-APO-1-mediated suppression was only observed in Ag-activated cells from day 0 to day 6 after Ag immunization. Suppression of Ab titers correlated with a decrease of Ag-specific Ig-secreting cells. Re-injection of syngeneic T cells and Ag did not reverse anti-APO-1-induced suppression. These data show a direct suppressive effect of anti-APO-1 on Ag-activated, "specific" B cells. Thus, APO-1-mediated apoptosis in Ag-activated B cells may contribute to the regulation of the humoral immune response.