Notably, the IFN-γ-inducing

effect of splenic MDSCs is also clearly visible upon polyclonal (anti-CD3 + anti-CD28) T-cell activation, again with a predominant role for PMN-MDSCs, illustrating that antigen-specific contacts between MDSCs and T cells are not required (Supporting Information Fig. 16). Interestingly, however, the IFN-γ induction by MDSCs might be more prominent in the spleen as compared with that at the tumor site. Indeed, employing the Lewis Lung Carcinoma (LLC) Small molecule library cell assay model, tumor-infiltrating MO-MDSCs were shown to be strongly antiproliferative (to a large extent in an NO-independent fashion, data not shown) and did not allow for IFN-γ production (Supporting Information Fig. 17). By contrast, their splenic counterparts stimulated IFN-γ

production on a per cell basis, even though being antiproliferative through NO, thus phenocopying EG7-OVA-induced splenic MO-MDSCs. Along the same line, splenic MDSCs click here (both MO- and PMN-MDSCs) induced by RMA-OVA tumor growth tended to induce IFN-γ production by OT-1 CD8+ T cells (Supporting Information Fig. 15). Finally, unseparated MDSCs from EG7-OVA tumor-bearers also enhanced IFN-γ production at an early time point (Supporting Information Fig. 14). The exact mechanism of splenic MDSC-mediated IFN-γ induction remains speculative at present, but seems not to be mediated by IL-12 or T-bet. Other IFN-γ-inducing cytokines include IL-18, IL-23, IL-15, and IL-21 and could be tested for their involvement in future experiments. Alternatively,

monocytes and neutrophils might provide costimulatory signals for CD8+ T cells [34], as such contributing to the induction of IFN-γ. Interestingly, IL-2 secretion is lowered by both MDSC types from the spleen. Since IL-2 is critical for primary T-cell expansion, this strategy also fits in the antiproliferative program of MDSCs. In addition, downstream events of IL-2, such as CD25 expression and STAT-5 phosphorylation, are significantly inhibited by MO-, but not PMN-MDSCs, in an NO-dependent fashion, possibly explaining MO-MDSC’s superior antiproliferative capacity. Previously, immortalized myeloid suppressor lines were reported to affect IL-2R next signaling [35], and our data extend these findings to primary MDSCs. Moreover, we report an influence of splenic MO-MDSCs on the expression of several functionally important CD8+ T-cell activation markers, with a varying implication of NO. Of note, some activation markers are not affected by the presence of MDSCs, indicating that these cells do not cause an overall shut-down of T-cell activation, but rather target certain aspects of the T cell. For example, upregulation of the early activation marker CD69 is not prevented, and in the case of MO-MDSCs even stimulated at later time points.