MDSC suppression of B cell responses in murine retrovirus-induced immunodeficiency: a role for VISTA (IRC4P.603)

Abstract

Abstract Inhibition by myeloid derived suppressor cells (MDSC) of T cell responses is well established in tumor microenvironments. We demonstrated (Green et al., 2013) induction of monocytic MDSCs during infection of B6 mice by LP-BM5 retrovirus, which causes profound immunodeficiency. These MDSCs suppressed not only T, but also B cell, responsiveness ex vivo. Whereas MDSC inhibition of stimulated T cell proliferation and IFN-gamma production was ~100%, iNOS/NO dependent, MDSC suppression of B-cell responses was only ~50% dependent on iNOS/NO - as shown by using iNOS inhibitors and iNOS k.o. mice as a source of MDSCs. We then discovered an additional mechanism(s) in MDSC inhibition of (only) B cell responsiveness that involved VISTA, a new negative checkpoint regulator. Using anti-VISTA blocking mAb, or LP-BM5 infected VISTA-/- MDSCs, MDSC inhibition of B cell responses was dependent on MDSC-expressed VISTA — again accounting for ~50% of total MDSC suppression. Combining the use of reagents to block both iNOS/NO and VISTA lead to an additive, if not synergistic, abrogation of MDSC suppression of B cell responsiveness. Consistent with a direct role of VISTA, in the absence of MDSCs, a VISTA-Ig fusion protein also partially inhibited B cell responsiveness. These results were compatible with a role for MDSC in LP-BM5-induced immunodeficiency and highlight involvement of multiple and unique suppressive pathways in the under-studied area of MDSC suppression of B cell responses.