Checkpoint Regulator VISTA plays a role in Suppression of B-Cell Responsiveness by Monocytic Myeloid Derived Suppressor Cells from LP-BM5 retrovirus-infected Mice

MDSC inhibition of tumor directed T cell responses is well described. We have reported an increase of monocytic MDSC (M-MDSC) during infection of B6 mice by LP-BM5 immunodeficiency causing retrovirus. These M-MDSCs suppressed T, and B cell responsiveness ex vivo. M-MDSC inhibition of stimulated T-cell proliferation and IFN-gamma production was ~100% iNOS/NO dependent; whereas suppression of B-cells was only ~50% dependent on iNOS/NO. An additional mechanism(s) for M-MDSC inhibition of B cell responsiveness involved V-domain Ig S uppressor of T cell Activation (VISTA), negative checkpoint regulator. Using anti-VISTA blocking mAb, ~50% of total MDSC suppression of B-cell responses was dependent on MDSC-expressed VISTA. The combination of iNOS/NO inhibitors and VISTA lead to additive, if not synergistic, blockade of M-MDSC suppression of B cell responsiveness. Regarding the LP-BM5-infection dependency of M-MDSCs, spleens from uninfected mice yielded ~3-fold fewer enriched M-MDSCs; and on a per-cell basis, the M-MDSCs from uninfected mice were also substantially less suppressive (4.3 fold); than those from infected mice. Consistent with a direct role of VISTA, in the absence of M-MDSCs, a VISTA-Ig fusion protein also partially inhibited B-cell responsiveness. Initial experiments show differential VISTA expression for the LP-BM5 M-MDSC CD11b +hi Ly6C +hi subpopulations we have recently defined (O’Connor et al, 2015, Virology). Thus, LP-BM5 infected, when compared to analogous subsets from uninfected mice, contain substantially more VISTA+ cells. These results suggest an unique role for M-MDSCs in LP-BM5 induced immunodeficiency, and highlight multiple suppressive pathways in the area of MDSC suppression of B cell responses.