Impaired Alloantigen Presenting Activity of Cord Blood Nucleated Cells.

Transplantation of HLA-mismatched cord blood (CB) nucleated cells has limited risk of severe acute graft-versus-host disease and graft rejection. This may depend on naïve T cells not yet exposed to many antigens and on immature antigen-presenting cells (APC) not delivering appropriate signals to allogeneic T cells. In order to test the APC activity of human circulating CB cells in-vitro, we initially used irradiated CB mononuclear cells (MNC) or immunomagnetically selected CD34+ cells, CD133+ cells, or CD14+ monocytes to stimulate the proliferative response of incompatible blood T cells in mixed leukocyte culture (MLC). CB MNC failed to induce allogeneic T cell proliferation, while CD34+ and CD133+ progenitors or CD14+ monocytes induced potent T cell alloresponses. Nevertheless, since allogeneic T cell response was not restored after depletion of CD3+ cells in the CB, nor the add-back of irradiated CB MNC to CD34+ or CD14+ stimulators inhibited allo-T cells, a direct suppressive effect of CB MNC was excluded. Allogeneic peripheral blood cytotoxic T-lymphocyte (CTL) responses were not induced after 7 days of stimulation with irradiated CB MNC, although after 4 weekly rechallenges with CB MNC, on average a 23% lysis of antigen-specific CB PHA-blasts was observed at the highest effector:target ratio (50:1). To test the tolerogenic potential of CB MNC, T cells initially exposed to CB MNC were rechallenged in secondary MLC with CB MNC, or CD34+ cells, or monocyte-derived dendritic cells (Mo-DC) generated in liquid culture with GM-CSF and IL-4. Allogeneic T cells were still unresponsive upon rechallenge with CB MNC, but proliferated upon 3 days of restimulation with CD34+ cells or Mo-DC from the same CB. Surprisingly, the supernatant of these latter MLCs did inhibit completely a 3^rd party MLC. Instead, the supernatant of blood T cells that had been activated by CB CD34+ cells or Mo-DC both in primary and secondary MLC did not. These results show an impaired allo-APC activity of CB MNC but not CB CD34+ cells, and suggest that T cells releasing immunosuppressive cytokines may be activated by CB MNC and then expanded by a second more potent stimulation with professional APC. This hypothesis could explain the sustained engraftment of HLA-mismatched CB stem cell transplants in humans. Based on these results, the in-vivo or ex-vivo downregulation of T cell alloreactivity induced by CB MNC will be tested in experimental models of stem cell, as well as solid organ transplantation.