Granulocytic Myeloid-Derived Suppressor Cells in Murine Models of Immune-Mediated Bone Marrow Failure
Authors: Feng Xingmin , Kim Jisoo , Gonzalez Matias Gladys , Wu Zhijie , Solorzano Sabrina , Batchu Sai , Gao Shouguo , Chen Jichun , Young Neal S. 
Published: November 2021

Abstract
Myeloid-derived suppressor cells (MDSCs) are a heterogeneous group of immature myeloid cells with immunoregulatory function. Limited published studies have reported conflicting data concerning the effects of MDSCs on autoimmune diseases and graft-versus-host disease. MDSCs can be divided into two major subsets, more abundant granulocytic (G-MDSCs) and monocytic (M-MDSCs). We examined G-MDSCs in murine models of human bone marrow failure (BMF).
We first characterized bone marrow (BM) MDSCs from C.B10 mice. CD11b +Ly6G +Ly6C low G-MDSCs suppressed in vitro proliferation of both CD4 and CD8 T cells from C57BL/6 (B6) mice, while Ly6G +Ly6C - cells had no effect and Ly6G -Ly6C + cells increased T cell proliferation (Fig. 1A). We then tested G-MDSCs in vivo utilizing antibody-mediated cell depletion. Lymph node (LN) cells from B6 donor mice were injected into sub-lethally irradiated major histocompatibility-mismatched CByB6F1 mice to induce BMF. Anti-Ly6G antibody injection worsened cytopenias and BM hypoplasia, and they increased BM CD4 and CD8 T cell infiltration. In contrast, anti-Ly6G antibody injection in the minor histocompatibility-mismatched C.B10 BMF model improved platelet counts and reduced BM CD8 T cells. The pathogenic and protective effects in the two models correlated with differential anti-Ly6G antibody modulation of G-MDSCs: in the CByB6F1 model, anti-Ly6G antibody eradicated G-MDSCs in blood and BM while in the C.B10 model the same antibody generated a novel G-MDSC cell population, of identical Ly6C lowCD11b + phenotype but intermediate Ly6G expression, which was not present in the CByB6F1 animals after antibody injection. When we examined the efficacy of G-MDSCs in C.B10 BMF: Ly6G + cells were enriched from BM of normal C.B10 donors (94%-97% Ly6C lowLy6G +CD11b +), and injected at the time of marrow failure initiation. Mice infused with Ly6G + cells had significantly higher levels of WBC, RBC, platelets, and total BM cells, decreased BM CD4 and CD8 T cell infiltration, and improved BM cellularity. These results indicated a protective role of G-MDSCs. When G-MDSCs were injected at day 3 after LN cell infusion, treated mice again had higher levels of WBC, RBC, platelets, and total BM cells at day 14, alleviating BMF. As both prophylaxis and therapy, G-MDSCs decreased Fas expression and Annexin V binding of residual BM cells, suppressed intracellular levels of gamma interferon and tumor necrosis factor alpha, as well as cell proliferation protein Ki67 levels in BM CD4 and CD8 T cells, relative to BMF control mice. TotalSeq simultaneously detecting surface proteins and mRNA expression in whole BM mononuclear cells in the therapy model showed an increased proportion of myeloid cells and reduced proportion of T cells in marrow from G-MDSC-treated mice based on cell surface markers and marker gene expression (Fig. 1B). Gene pathway analysis revealed down-regulation of Fas expression and reduced program cell death in total BM cells and decreased expression of genes related to cell cycle in infiltrating T cells from Ly6G + cell-treated mice-both results consistent with suppression by G-MDSCs of T cell proliferation and protection of target BM cells from apoptosis. In vitro culture of T cells from B6 mice with G-MDSCs which had been isolated from C.B10 BM cells showed dose-dependent suppression of T cell proliferation.
In conclusion, our results demonstrate an active role of G-MDSCs in protecting BM from immune-mediated destruction, by suppression of T cell proliferation in the BM. G-MDSCs might have clinical application as treatment in human aplastic anemia and other immune-mediated and autoimmune diseases.
Figure 1 Figure 1.

Disclosures
Young: Novartis: Research Funding.