Suppressive Efficacy and Proliferative Capacity of Human Regulatory T Cells in Allogeneic and Xenogeneic Responses

Background. An understanding of the mechanisms that suppress the human anti-pig cellular response is key for xenotransplantation. We have compared the ability of human regulatory T cells (Tregs) to suppress xenogeneic and allogeneic responses in vitro. Methods. Human peripheral blood mononuclear cells (PBMC), CD4(+)T cells, or CD4(+)CD25(-)T cells were stimulated with irradiated human or wild type (WT) or alpha 1,3-galactosyltransferase gene-knockout (GT-KO) pig PBMC in the presence or absence of human CD4(+)CD25(high)Tregs. In separate experiments, 5- (and 6)-carboxyfluorescein diacetate succinimidyl ester-labeled human CD4(+)T cells were stimulated with human or pig PBMC. The expansion and precursor frequencies of allo- and xeno-reacitve Tregs were assessed by labeling with FoxP3 mAb and flow cytometric analysis. Results. The responses of human PBMC, CD4(+)T cells, and CD4(+)CD25(-)T cells to pig PBMC were stronger than to human PBMC (P<0.05). Human anti-WT responses were weaker than anti-WT responses (P<0.05). Human CD4(+)CD25(high) Tregs suppressed proliferation of CD4(+)CD25(-)T cells to both human and pig PBMC stimulator cells with the same efficiency. Alloreactive CD4(+)CD25(+)FoxP3(high) responder T cells proliferated more than their xenore-active counterparts (P<0.05), although xenoreactive CD4(+)CD25(+)T cells proliferated more than alloreactive cells (P<0.05). There was no difference in precursor frequency between allo- and xeno-reactive CD4(+)CD25(+)FoxP3(high) cells. Conclusions. Human T-cell responses to pig cells are stronger than to allogeneic cells. The human response to GT-KO PBMC is weaker than to WT PBMC. Although human Tregs can suppress both responses, expansion of CD4(+)CD25(+)FoxP3(high) cells against pig PBMC is weaker than against human PBMC. More human Tregs may be required to suppress the stronger xenogeneic response.