Murine sera cytotoxicity toward human B cells and their effect on human mixed lymphocyte reactions.

Seven murine anti-H-2 and three nonimmune mouse sera were tested for cytotoxicity toward B and T lymphocytes from a panel of human donors. One group of sera, including two anti-H-2.33 sera, exhibited cytotoxicity directed exclusively toward human B but not T cells from all donors. Absorption studies on human lymphoblastoid cell lines (LCLs) of B or T cell origin corroborated these findings. Some nonimmune sera showed similar characteristics, indicating that the observed reactions were not attributable to cross-reactivities between mouse H-2K or D specificities and human antigens coded by the HLA-A, B, or C locus. Another set of mouse sera (anti-H-2.28b and anti-H-2.31) was highly cytotoxic to both B and T cells of some donors but not of others, suggesting that activity in these sera may arise from cross-reactions between mouse and human specifities. A third set of anti-H-2 as well as normal mouse sera showed only background cytotoxicity when tested on human cells. The ability of the B cell cytotoxic mouse sera to block the human mixed lymphocyte culture reaction (MLR) was compared to that of appropriate human alloantisera with exclusive B cell activity or a rabbit serum raised against human B cells. None of the mouse sera resulted in a significant reduction in the human MLR, whereas the human alloantisera as well as the rabbit antiserum caused a significant amount of blocking at several dilutions beyond their highest cytolytic titer.