In vitro expanded human CD4+CD25+ regulatory T cells are potent suppressors of T-cell-mediated xenogeneic responses

Background. Cellular rejection of xenografts is predominantly mediated by CD4(+) T cells. Little is known of the effectiveness of CD4(+)CD25(+) T regulatory (Treg) cells at suppressing this strong T-cell mediated immune response. In this study, we evaluated the activity of fresh Treg cells and expanded Treg cells to suppress the xeno immune response in vitro. Methods. Human Treg cells were preferentially expanded by CD3/CD28 expand beads, interleukin (IL)-2, and rapamycin. Human CD4(+)CD25(-) T cells were stimulated with irradiated porcine peripheral blood mononuclear cells in the presence or absence of fresh or expanded human Treg cells for 5 days before proliferation assay. In a separate experiment, the porcine xenoantigen-stimulated CD4(+)CD25(-) T cells were separated from Treg cells by transwells and assessed for cytotoxicity of porcine peripheral blood mononuclear cells target cells. Cytokine-producing cells and cytokine release in the cocultures were examined by enzyme-linked immunosorbent spot and enzyme-linked immonosorbent assay, respectively. Results. Human Treg were expanded up to 3500-fold after 14 days in culture. The addition of fresh Treg suppressed the T-cell mediated xenoimmune response. Compared with fresh Treg cells, expanded Treg cells were more Potent at suppressing CD4(+)CD25(-) T-cell-mediated antiporcine xenogeneic responses. This suppression required cell contact. However, the enhanced suppression by expanded Treg cells was associated with increased secretion of IL-4 and IL-10 when compared with their nonexpanded Treg counterparts. Conclusion. This study shows that expanded human Treg cells were capable of suppressing antiporcine xenogeneic responses in vitro and involve both contact dependent and cytokine mediated mechanisms.