IFNalpha polarizes granulocytic MDSCs from neutrophils by inducing protein translation

The extensive heterogeneity in the tumor immune microenvironment is a problem plaguing immune-based treatment failures for solid tumors because it prevents defining the specific vulnerabilities of the various cells to develop treatments with greater precision. Myeloid-derived suppressor cells (MDSCs) are a heterogenous collection of immunosuppressive monocytes and granulocytes (neutrophils) whose link to poor cancer survival is not well understood. Most MDSCs exhibit features of immature granulocytes (Gr-MDSCs) and exert their T cell suppression by secreting reactive oxygen and nitrogen species (RONS). Gr-MDSCs can survive for days by recruiting mechanisms to protect protein translation despite generation of RONS. One MDSC subtype of interest is defined by their polarization in response to type 1 interferon (IFNalpha) that we previously identified in chronic Helicobacter -infected mice with gastric metaplasia, suggesting that a Helicobacter -infected stomach polarizes neutrophils to acquire an immunosuppressive phenotype. We hypothesize that IFNalpha initiates neutrophil reprogramming by increasing protein synthesis required to alter their metabolic activities as they acquire the Gr-MDSC phenotype. We performed scRNA-Seq on biopsies from 5 groups of patients referred for endoscopy and showed no gastric abnormalities, were H. pylori + (Hp), had intestinal metaplasia (IM) or gastric adenocarcinoma (GAC) and identified that the IFN-polarized MDSC population initially present in IM increased substantially in GACs but was not detected in normal and Hp -infected patients, confirming the polarization of IFN-polarized Gr-MDSCs once IM appears. We showed that the IFN-polarized Gr-MDSCs re-programmed from neutrophils in vitro and required induction of protein translation. To analyze translation, we treated primary cultures of granulocytes from mouse bone marrow or human PBMCs with IFNalpha and quantified protein translation by flow cytometry after incubating the cultures with 20µM O-propargyl-puromycin (OPP) and proliferation with EdU. In vivo analysis was performed by administering OPP to mice before euthanization. In the in vitro neutrophil cultures, about 28% of the polarized neutrophils were CD11b + Gr-1 + MDSCs, and 85% of these Gr-MDSCs induced their protein synthesis coincident with increased proliferation. scRNA-Seq analysis of both mouse and human gastric tissues demonstrated induction of mediators of the unfolded protein response (UPR), e.g., CHOP. In summary, polarization of Gr-MDSCs by type 1 IFNs from neutrophils requires induction of protein translation, revealing possible metabolic pathways that may be targeted to reprogram the immunosuppressive tumor microenvironment.