B cells induce apoptosis via a novel mechanism in fibroblasts infected with mouse hepatitis virus.

B cells from nonimmune mice mediate the cytolysis of fibroblasts infected with the coronavirus, mouse hepatitis virus (MHV), strain A59. In this investigation, we report that splenic B cells and a B cell hybridoma induced the fragmentation of MHV-infected target cell DNA into a nucleosomal ladder pattern, characteristic of apoptosis. To determine the mechanism by which B cells mediated this killing event, we used criteria previously established for the killing of target cells by cytotoxic T lymphocytes (CTLs) and compared this B-cell-mediated killing to lymphocytic choriomeningitis virus (LCMV)-specific CTL killing of LCMV-infected target cells. Unlike CTL-mediated cytotoxicity, B cells efficiently lysed and induced the fragmentation of the DNA in their target cells in the presence of EGTA, arguing against a Ca(2+)-dependent granule exocytosis model for killing. In addition, paraformaldehyde-fixed B cells were able to kill MHV-infected targets. We were unable to detect TNF-alpha-associated cytotoxicity via bioassay with nonimmune effector B cells against the TNF-sensitive cell line, LM, or the TNF-alpha-resistant subline, L929.w, infected with MHV. Serine esterase inhibitors (benzamidine hydrochloride and N alpha-p-tosyl-L-arginine methyl ester) blocked CTL-induced 51Cr release and DNA fragmentation. In contrast, the inhibitors did not block the B-cell-induced 51Cr release, but did cause an inhibition in the fragmentation of the DNA of the target cell. These data indicate that B cells are capable of inducing the lysis and DNA fragmentation of MHV-infected target cells similar to CTL-induced apoptosis. However, we show that the mechanism(s) by which these processes are induced by B cells is distinct from CTL-mediated cytotoxicity.