IntroductionMDSCs express SCHLAFEN 4 (SLFN4) inHelicobacter-infected stomachs coincident with spasmolytic polypeptide-expressing metaplasia (SPEM), a precursor of gastric cancer. We aimed to characterize SLFN4+cell identity and the role of Slfn4 in these cells.MethodsSingle-cell RNA sequencing was performed on immune cells sorted from PBMCs and stomachs prepared from uninfected and 6-monthH. felis-infected mice. Knockdown of Slfn4 by siRNA or PDE5/6 inhibition by sildenafil were performed in vitro. Intracellular ATP/GTP levels and GTPase activity of immunoprecipitatedSlfn4complexes were measured using the GTPase-Glo assay kit. The intracellular level of ROS was quantified by the DCF-DA fluorescent staining, and apoptosis was determined by cleaved Caspase-3 and Annexin V expression.Gli1CreERT2 x Slfn4fl/flmice were generated and infected withH. felis. Sildenafil was administered twice over 2 weeks by gavagingH. felisinfected mice ~4 months after inoculation once SPEM had developed.ResultsSlfn4was highly induced in both monocytic and granulocytic MDSCs from infected stomachs. BothSlfn4+-MDSC populations exhibited strong transcriptional signatures for type-I interferon responsive GTPases and exhibited T cell suppressor function. SLFN4-containing protein complexes immunoprecipitated from myeloid cell cultures treated with IFNa exhibited GTPase activity. Knocking down Slfn4 or PDE5/6 inhibition with sildenafil blocked IFNa induction of GTP, SLFN4 and NOS2. Moreover, IFNa induction ofSlfn+-MDSC function was inhibited by inducing their reactive oxygen species (ROS) production and apoptosis through protein kinase G activation. Accordingly, in vivo disruption of Slfn4 inGli1CreERT2 x Slfn4fl/flmice or pharmacologic inhibition by sildenafil after Helicobacter infection also suppressed SLFN4 and NOS2, reversed T cell suppression and mitigated SPEM development.ConclusionTaken together, SLFN4 regulates the activity of the GTPase pathway in MDSCs and precludes these cells from succumbing to the massive ROS generation when they acquire MDSC function.