Inhibition of γδ T Cell Proliferation by CD4⁺CD25⁺FOXP3⁺ Regulatory T Cells (Treg).

CD4⁺CD25⁺FOXP3⁺ regulatory T cells (Treg) are a unique population of T cells that maintain immune tolerance by suppressing self-reactive cells. Treg also contribute to the establishment of a dominant tolerance during infections and after allogeneic transplantation and suppress immune response to tumors. Increasing evidence supports the existence of elevated numbers of Treg in both solid tumors and hematological malignancies. In this study we show that Treg may also suppress other arms of an effective immune response. In vitro, purified human CD4⁺CD25⁺FOXP3⁺ Treg (by immunomagnetic selection) directly inhibit phosphoantigen (BrHPP)-mediated proliferation of Vγ9Vδ2 T cells, the major γδ T lymphocyte subset in humans. Importantly, suppression of γδ T cell proliferation by Treg was maintained when Treg where separated from γδ T cells by Transwells, suggesting that the inhibitory function of Treg on γδ T cell proliferation is not cell-contact independent and rather soluble factors produced by Treg contribute to this suppressive effect. However, Treg do neither influence the expression of activation markers (CD69, CD25) nor the production of IFN-γ by Vγ9Vδ2 T cells stimulated with phosphoantigen indicating that not all effector functions of Vγ9Vδ2 T cells are suppressed by Treg. As we have recently reported, phosphoantigen-mediated γδ T cell proliferation is frequently suppressed in cancer patients. This observation prompted us to address the role of Treg in controlling γδ T cell proliferation in cancer patients. An inverse correlation between Treg frequencies (i.e. the ratio between Treg and Vγ9Vδ2 T cells) in peripheral blood and phosphoantigen-mediated γδ T cell proliferation was found (mean Treg/Vγ9Vδ2 T cell ratio in cancer patients with maintained phosphoantigen-mediated γδ T cell proliferation (n=14): 4.06; mean Treg/Vγ9Vδ2 T cell ratio in cancer patients without phosphoantigen-mediated γδ T cell proliferation (n=55): 33.36)). Therefore, the Treg/Vγ9Vδ2 T cell ratio in peripheral blood can predict the capacity of γδ T cells to proliferate in response to phosphoantigens. In conclusion, these findings support a role for Treg in blunting the γδ T cell arm of the innate immune response in cancer patients and highlight the potential of Treg depletion (e.g. by anti-CD25 antibodies, cyclophosphamide or fludarabine) to promote γδ T cell mediated antitumor activity.