Introduction: Both human regulatory T cells (Treg) and regulatory macrophages (Mreg) are shown to be important in regulation of immune responses and have potential as a promising cell therapy for immunomodulation in both allo- and xeno-transplantation. However, which of these two types of regulatory cells are superior to the other in suppression of a particular immune response are yet to be shown. In this study we compared the suppressive capacity of human Treg and Mreg in different immune environments in vitro, thereby providing information to select an appropriate type of regulatory cells for suppression of a particular immune response when required.

Materials and Methods: Human PBMC were used for isolation of CD4+CD25+CD127lo Treg and CD14+ monocytes, respectively. Treg were polyclonally expanded with anti-CD3/CD28 beads, IL-2 and rapamycin and Mreg were induced from CD14+ monocytes with macrophage colony-stimulating factor (MCSF) and IFN-γ. Mix lymphocyte reaction (MLR) were performed to assess suppressive capacity of both regulatory cells in different immune responses using anti-CD3/CD28 beads and irradiated human and porcine PBMC as polyclonal, allogeneic and xenogeneic stimulators, respectively, human PBMC and CD4+CD25- T cells as responder cells and autologous Treg and Mreg as suppressor cells. Viability of regulatory cells and responder cells in MLR cultures was analyzed using annexin v apoptosis detection kit.

Results: Both Treg and Mreg demonstrated great capacity to suppress proliferation of PBMC and CD4+ T cells in response to polyclonal, allogeneic and xenogeneic stimulation. Despite no noticeable difference in their potency in suppression of either polyclonal or allogeneic response, Treg exhibited stronger suppression of proliferating xenoreactive responder PBMC, CD4+ and CD8+ T cells at ratios of 1:4 through to 1:16 of Mreg or Treg: responder cells when compared to that shown by Mreg. More Mreg than Treg were found undergoing apoptosis detected in the all three MLR settings.

Discussion: Compared to polyclonally expended human Treg, Mreg were less efficient in suppression of the xenogeneic response, which however, may not be correlated with their reduced viability in all MLR settings as they demonstrated similar suppressive potency to that Treg showed in the polyclonal and allogeneic responses.

Conclusion: Human Treg and Mreg have similar capacity to suppress either polyclonal or allogeneic response. However Treg are superior to Mregwhen required for suppression of the xenogeneic response. This different suppressive capacity between Treg and Mreg needs be further tested in vivo.