Kidney allograft tolerance in primates without chronic immunosuppression--the role of veto cells.

Posttransplant infusion of specific donor bone marrow cells into ATG-treated recipients promotes long survival of allografts in the absence of immunosuppressive drug therapy. DBMC infusion also inhibits antidonor CTL activation in allograft recipients, a trend analogous to the veto phenomenon. The present studies investigated a hypothesis that veto activity in DBMC is involved in the induction of donor-specific unresponsiveness in rhesus monkey kidney transplant recipients given ATG and DBM. Normal monkey BMC were fractionated into subpopulations by depletion with mAbs and immunomagnetic beads. The unfractionated BMC and BMC subsets were tested for veto activity in in vitro MLR-induced CTL and for in vivo tolerance-promoting activity in ATG-treated monkey kidney transplant recipients. In MLR-induced CTL assays, BMC specifically suppressed CTL activity to PBL from the BMC donor. The suppression was mediated by a small population of BMC that expressed a CD2+, CD8+, CD16+, DR-, CD3-, CD38- phenotype. Results of in vivo studies showed a striking correlation with the in vitro results. ATG-treated recipients given either DR- DBMC or DR- CD3- DBMC infusions had significantly prolonged graft survival and a 40-50% incidence of long survivors over 150 days (P less than 0.001 vs. ATG controls). In contrast, in recipients given CD2- DBMC or DR- CD16- DBMC infusions, the tolerance-promoting effect of DBMC was absent, and there were no long-term survivors. The results also showed an association between long survival and suppressed in vivo antidonor CTL activity. Thus acute rejection and in vivo and in vitro antidonor CTL responses were suppressed by a minor subpopulation of DBMC with similar phenotypic markers. We suggest that a veto mechanism may control the induction phase of allograft tolerance in this model, providing a critical period of CTL silence to allow development of host immunoregulatory mechanisms necessary for maintaining graft tolerance.