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γδ T cells play critical roles in host defense against infections and cancer. Although advances have been made in identifying γδ TCR ligands, it remains essential to understand molecular mechanisms responsible for in vivo expansion of γδ T cells in periphery. Recent findings identified the expression of the inducible NO synthase (NOS2) in lymphoid cells and highlighted novel immunoregulatory functions of NOS2 in αβ T cell differentiation and B cell survival. In this context, we wondered whether NOS2 exerts an impact on γδ T cell properties. Here, we show that γδ T cells express NOS2 not only in vitro after TCR triggering, but also directly ex vivo. Nos2 deficient mice have fewer γδ T cells in peripheral lymph nodes (pLNs) than their wild-type counterparts, and these cells exhibit a reduced ability to produce IL-2. Using chemical NOS inhibitors and Nos2 deficient γδ T cells, we further evidence that the inactivation of endogenous NOS2 significantly reduced γδ T cell proliferation and glycolysis metabolism that can be restored in presence of exogenous IL-2. Collectively, we demonstrate the crucial role of endogenous NOS2 in promoting optimal IL-2 production, proliferation and glycolysis of γδ T cells that may contribute to their regulation at steady state., Competing Interests: The authors have declared that no competing interests exist.

Tumors affect myelopoeisis and induce the expansion of myeloid cells with immunosuppressive activity. In the MT/ret model of spontaneous metastatic melanoma, myeloid cells are the most abundant tumor infiltrating hematopoietic population and their proportion is highest in the most aggressive cutaneous metastasis. Our data suggest that the tumor microenvironment favors polarization of myeloid cells into type 2 cells characterized by F4/80 expression, a weak capacity to secrete IL-12 and a high production of arginase. Myeloid cells from tumor and spleen of MT/ret mice inhibit T cell proliferation and IFNγ secretion. Interestingly, T cells play a role in type 2 polarization of myeloid cells. Indeed, intra-tumoral myeloid cells from MT/ret mice lacking T cells are not only less suppressive towards T cells than corresponding cells from wild-type MT/ret mice, but they also inhibit more efficiently melanoma cell proliferation. Thus, our data support the existence of a vicious circle, in which T cells may favor cancer development by establishing an environment that is likely to skew myeloid cell immunity toward a tumor promoting response that, in turn, suppresses immune effector cell functions.