Suppression of B cell responsiveness by LP-BM5 retrovirus-induced myeloid derived suppressor cells generated during a murine acquired immunodeficiency syndrome: a role for negative checkpoint regulator expression on the MDSCs (VIR7P.1059)

Abstract

Abstract The inhibitory activities of myeloid derived suppressor cells (MDSC) against T cell responses are well established in tumor microenvironments. We published (2013, J. Virol. 87:2058-2071) on the induction of monocytic MDSCs during infection of susceptible B6 mice by LP-BM5 retrovirus, which causes a profound immunodeficiency. These MDSCs inhibited not only T, but also B cell, responsiveness to polyclonal stimulation in ex vivo suppression assays. Whereas MDSC inhibition of stimulated T cell proliferation and IFN-gamma production was largely iNOS/NO dependent, MDSC suppression of B cell responses was only partially (~50%) due to iNOS/NO - as shown by using iNOS inhibitors and LP-BM5 infected iNOS k.o. mice as a source of MDSCs. Here, we further study additional suppressive mechanism(s) in the MDSC inhibition of B cell responsiveness. Using MDSCs from LP-BM5 infected mice of negative checkpoint regulator knock-out origin, and specific blocking antibody, MDSC suppression of B cell responses was partly dependent on MDSC expression of certain checkpoint regulators. Combining the use of reagents to interrupt both iNOS/NO and checkpoint regulation lead to a synergistic blocking of MDSC suppression of B cell responsiveness. These results were compatible with a role for MDSC in LP-BM5 induced immunodeficiency and highlight the involvement of multiple and unique suppressive pathways in the under-studied area of MDSC suppression of B cell responses.