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 infec ted with the coronavirus, mouse hepatitis virus (MHV), strain A59. In this investigation, we report that splenic B cells and a B cell hybrid oma induced the fragmentation of MHV-infected target cell DNA into a n ucleosomal ladder pattern, characteristic of apoptosis. To determine t he mechanism by which B cells mediated this killing event, we used cri teria previously established for the killing of target cells by cytoto xic 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 effi ciently lysed and induced the fragmentation of the DNA in their target cells in the presence of EGTA, arguing against a Ca2+-dependent granu le exocytosis model for killing. In addition, paraformaldehyde-fixed B cells were able to kill MHV-infected targets. We were unable to detec t TNF-alpha-associated cytotoxicity via bioassay with nonimmune effect or B cells against the TNF-sensitive cell line, LM, or the TNF-alpha-r esistant subline, L929.w, infected with MHV. Serine esterase inhibitor s (benzamidine hydrochloride and Nalpha-p-tosyl-L-arginine methyl este r) blocked CTL-induced Cr-51 release and DNA fragmentation. In contras t, the inhibitors did not block the B-cell-induced Cr-51 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 lys is and DNA fragmentation of MHV-infected target cells similar to CTL-i nduced apoptosis. However, we show that the mechanism(s) by which thes e processes are induced by B cells is distinct from CTL-mediated cytot oxicity.