Costimulatory molecules are active in the human xenoreactive T-cell response but not in natural killer-mediated cytotoxicity

Background. T-cell costimulatory blocking agents inhibit allospecific T-cel l responses in vitro and prevent allograft rejection in vivo. Costimulatory requirements for discordant xenospecific cellular responses remain undefin ed. We have evaluated costimulatory molecule expression by porcine endothel ial cells (PEC) after interaction with human cells and tested agents known to inhibit allospecific responses for their ability to inhibit xenospecific responses in vitro. Methods. Human-specific agents were screened for their ability to bind porc ine costimulatory molecules by FACS. Up-regulation of B7 molecules on PEC w as evaluated by FACS after exposure to human cells or supernatants. The eff ect of human and/or porcine costimulatory blockade was tested in xeno-mixed lymphocyte reactions (XMLRs) and in natural killer (NR) cell cytotoxicity assays. Results. B7 expression was induced on PEC after exposure to human T and NK cells or T cell-conditioned medium. The human XMLR was attenuated by human CTLA4-Ig and anti-human CD154 (hu5C8), and the combination was synergistic. Anti-human CD80 and CD86 antibodies alone had minor effects in the XMLR, b ut in combination with hu5C8 were as effective as human CTLA4-Ig plus hu5C8 . Anti-hCD80 and hCD86 antibodies that did not cross-react with porcine CD8 0 or CD86 were as effective in blocking the MLR as those that did cross-rea ct, indicating that the predominant costimulation in vitro was derived fi o m the responding cells. None of the agents affected the xeno-NK response. Conclusions. We conclude that the costimulation-modulating agents block hum an anti-porcine T-cell responses in vitro predominantly through interruptio n of costimulation derived from responding cells. They have no effect on NK cell-mediated cytotoxicity.