Selective Involvement of the Checkpoint Regulator VISTA in Suppression of B-Cell, but Not T-Cell, Responsiveness by Monocytic Myeloid-Derived Suppressor Cells from Mice Infected with an Immunodeficiency-Causing Retrovirus

Abstract

Ly6Cϩ/hi and granulocytic/polymorphonuclear-leukocyte-like Ly6G ϩ/hi Ly6C ϩ/Ϫ/lo , use differential suppressive mechanisms to inhibit T cells (12,13). MDSC inhibition of B-cell responses is studied rarely, if at all.Retroviruses are adept at co-opting immunoregulatory mechanisms. Human immunodeficiency virus type 1/simian immunodeficiency virus induction of PD-1 downregulates T effector cells (14,15), and murine Friend retrovirus infection-induced PD-1 and Tim-3 affect pathogenesis and retroviral loads (16, 17), sometimes with "functionless" T cells occurring (14,15). Viral infections can also induce CD4 ϩ FoxP3 ϩ regulatory T (Treg) cells (18), including in LP-BM5 murine retroviral pathogenesis (19-21). By 5 weeks postinfection (wpi), LP-BM5 causes profound immunodeficiency, with increased susceptibility to "opportunistic" infections and B-cell lymphomas (22, 23). Immunodeficiency requires "pathogenic" CD4ϩ T-effector cell expression of CD154 and ligation of CD40 (22,24,25), and PD-1/PD-L1 and IL-10 downregulate effector T-cell activity (21,26).A CD11b ϩ FcR␥III/II ϩ myeloid cell subset expands during LP-BM5 pathogenesis (26, 27). We recently defined these monocytic MDSCs as Gr-1 ϩ Ly6C ϩ/hi Ly6G ϩ/Ϫ/low CD11b ϩ with strong ex vivo inhibition of T-and B-cell responses used to measure LP-BM5-induced immunodeficiency (10). This robust direct MDSCinduced inhibition of B-cell responsiveness is novel for murine retrovirus-induced immunosuppression, if not generally. Also, a new negative-checkpoint regulatory ligand, VISTA (V-domain Ig suppressor of T-cell activation) (28-30), also designated PD-1H (31), with homology to PD-L1 has been defined. VISTA can be highly upregulated on myeloid-derived cells and can inhibit T-cell responses in autoimmunity and antitumor immunity in a nonredundant manner with PD-L1 (28).At 5 wpi with LP-BM5, regarding cell surface VISTA expression, the percentage of VISTA ϩ spleen cells had not expanded but VISTA mean fluorescence intensity (MFI) increased and the shape of the positive peak changed, consistent with the dominance of CD4 T-cell-expressed VISTA in uninfected B6 mice (28) and with CD11b ϩ VISTA ϩ cell expansion. Comparison of cells from wild-type (WT), iNOS Ϫ/Ϫ , and VISTA Ϫ/Ϫ B6 mice (32) at 5 wpi confirmed VISTA and CD11b coexpression by the highly enriched monocytic Ly6C ϩ MDSC population we have previously described (10), as depicted in the representative experiment in Fig. 1 (consistent with the average MFI and percent positivity over three experiments [legend to Fig. 1]). Of note, there was minimal contamination with other cells, particularly CD4ϩ Treg cells (legend to Fig. 1). Interestingly, similar monocytic MDSCs could be isolated from the spleens of uninfected mice. These MDSCs expressed levels of VISTA approaching (and, over three repeat experiments, not significantly statistically significantly different from) that of their

Glutathione Depletion Is Linked with Th2 Polarization in Mice with a Retrovirus-Induced Immunodeficiency Syndrome, Murine AIDS: Role of Proglutathione Molecules as Immunotherapeutics

“…It would be interesting, given the results of the effect of I-152 treatment on peritoneal macrophages obtained here, to test the activity of the molecule against other cells of the myeloid lineage in the context of LP-BM5 retrovirus-induced disease. Specifically, recent reports have detailed augmented monocytic myeloid-derived suppressor cell (M-MDSC) production during LP-BM5-induced disease, including immunodeficiency (69)(70)(71)(72). Because the mechanism of inhibition of T-cell responses by these M-MDSCs for inhibition of T-cell responses was fully dependent on iNOS/NO, whereas iNOS/NO was responsible for about 50% of the inhibition of B-cell responses, it would be interesting to study I-152 treatment to determine whether M-MDSC numbers and suppressive activity can be increased or decreased in LP-BM5-infected mice.…”

The Role of Myeloid-Derived Suppressor Cells in Viral Infection

“…The major mechanisms of suppression by MDSCs, primarily discovered in tumor systems, include catabolism of L-arginine (L-Arg) by arginase 1 (Arg1) and/or inducible nitric oxide synthase (iNOS); production of reactive oxygen species (ROS); production of immunosuppressive cytokines; or negative checkpoint regulators (i.e., V-domain Ig suppressor of T cell activation [VISTA], also known as PD-1H) (41,54,158).…”

“…Suppression of B cell responses by these same M-MDSCs from LP-BM5-infected mice was mediated not only, in part, by iNOS/NO (53) but also substantially by the negative checkpoint regulator VISTA (41,54,158), and by ROS, RNS, and TGF-b (Fig. 1a) (124).…”

“…VISTA, a newly identified negative checkpoint regulator ligand, expressed on several hematopoietic cell types, including myeloid lineages cells (158), was expressed on MMDSCs from LP-BM5-infected mice (54). Suppression of B cell, but not T cell, responses by M-MDSCs from LP-BM5-infected mice was partially blocked by the addition of blocking anti-VISTA mAb (54).…”

“…Use of iNOS and VISTA blocking reagents in combination revealed that the VISTA and iNOS/NO pathways were distinguishable and were the two primary effector mechanisms required for suppression by M-MDSCs (54). In support of (in part) a direct effector suppressive mechanism, a VISTA-Ig chimeric fusion protein also inhibited B cell responses (54).…”