Recipient-Specific Tolerance after HLA-Mismatched Umbilical Cord Blood Stem Cell Transplantation

Background: Lower incidence and severity of acute graft versus host disease (GVHD) has been observed in leukemia patients receiving HLA-mismatched umbilical cord (UCB) transplants. However, despite the increased use of UCB in stem cell transplantation, the mechanisms underlying these favorable outcomes are not well delineated.Methods: We analyzed antigen specific lymphocyte responses after transplant to determine whether the decreased allogeneic responsiveness of UCB lymphocytes is attributable to pan-unresponsiveness, lymphocyte repressive or recipient-specific tolerance.Results: Circulating lymphocytes collected early (3 months) after UCB transplant demonstrate a less naive phenotype compared with that in the infused graft. Additionally, after transplant, circulating peripheral blood UCB-derived lymphocytes produced normal levels of interferon-[gamma] and proliferated normally when stimulated with mitogen or third party alloantigen. In contrast, when stimulated with recipient antigen, circulating lymphocytes emerging posttransplant did not proliferate nor produce interferon-[gamma]. Moreover, analysis of interleukin-4 production revealed a Th2 response to recipient antigens. These data indicate early induction of immune tolerance of naive UCB graft lymphocytes with skewing of transplant recipient-specific immune response towards Th2 cytokine profile.Conclusions: UCB graft lymphocyte immune naivety and observed early tolerance induction may contribute to the observed favorable GVHD incidence, despite infusion of HLA mismatch grafts in the unrelated allogeneic setting