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

Abstract

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 α1,3-galactosyltransferase gene-knockout (GT-KO) pig PBMC in the presence or absence of human CD4⁺CD25^highTregs. 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 (PP+CD25^highTregs 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 xenoreactive counterparts (P+CD25⁺T cells proliferated more than alloreactive cells (P+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.