Systemic lupus erythematosus (SLE) is an autoimmune disorder disproportionally affecting women. A similar sex difference exists in the murine New Zealand Black/White hybrid model (NZBWF1) of SLE with all females, but only 30-40% of males, developing disease within the first year of life. Myeloid-derived suppressor cells (MDSCs) are prominent in NZBWF1 males and while depletion of these cells in males, but not females, promotes disease development, the mechanism of suppression remains unknown. S100a9, expressed by neutrophils and MDSCs, has previously been shown to exert immunosuppressive functions in cancer and inflammation. Here we investigated if S100a9 exerts immunosuppressive functions in NZBWF1 male and female mice. S100a9+/+, S100a9+/- and S100a9-/- NZBWF1 mice were followed for disease development for up to 8 months of age. Serum autoantibody levels, splenomegaly, lymphocyte activation, glomerulonephritis and proteinuria were measured longitudinally or at the time of harvest. In accordance with an immunosuppressive function of MDSCs in male mice, S100a9-deficient male NZBWF1 mice developed accelerated autoimmunity as indicated by increased numbers of differentiated effector B and T cells, elevated serum autoantibody levels, increased immune-complex deposition and renal inflammation, and accelerated development of proteinuria. In contrast, female mice showed either no response to S100a9-deficiency or even a slight reduction in disease symptoms. Furthermore, male, but not female, S100a9-/- NZBWF1 mice displayed an elevated type I interferon-induced gene signature, suggesting that S100a9 may dampen a pathogenic type I interferon signal in male mice. Taken together, S100a9 exerts an immunosuppressive function in male NZBWF1 mice effectively moderating lupus-like disease development via inhibition of type I interferon production, lymphocyte activation, autoantibody production and the development of renal disease.

All (NZB x NZW)F1 (BWF1) female mice develop lupus-like disease, compared to ~30% of male BWF1 mice. Our previous research described an immunosuppressive effect of Gr1hiCD11bhi cells in male BWF1 mice. Specifically, as BWF1 mice age, female Gr1hi cells lost their ability to suppress altogether, while antibody-mediated depletion of Gr1hi cells accelerated disease development in male mice only. Reverse transcriptase-PCR studies revealed upregulated S100a9 and S100a8 mRNA levels in male flow-sorted Gr1hi cells as compared to cells from female BWF1 mice and non-autoimmune stains, leading us to hypothesize that S100a8/a9 produced by male Gr1hi cells is immunosuppressive. To study this, we created S100a9-deficient BWF1 mice. Our results showed spleen weight and splenocyte count, indicators of lupus advancement, were significantly higher in S100a9−/−male mice. Additionally, increased IgG-IC deposition in kidney glomeruli of S100a9−/−male mice was observed along with a significant increase in serum anti-chromatin IgG levels. Deficiency of S100a9 also resulted in increased frequencies of memory B cells and plasma cells in male mice, but not in female mice. Interestingly, S100a9-deficiency caused an accumulation of Gr1hiCD11b+ splenocytes in male mice. We speculated that this is due to a lack of trafficking of Gr1hi cells into the glomerulus of the kidney due to S100a9-deficiency. Subsequent IHC confirmed the lack of S100a9 expression in kidneys from S100a9-deficient mice. Finally, H/E and Trichrome stainings indicated accelerated renal failure in S100a9−/−mice. In conclusion, S100a9 may function as a chemoattractant of Gr1hi cells exerting immunosuppression and protection against lupus-like renal disease in male BWF1 mice.