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.

Testosterone protects from SLE pathogenesis, yet the mechanism of protection remains unknown. Using the (NZB x NZW)F1 (BWF1) mouse model of SLE, we have previously shown that immunosuppressive Gr1+CD11b+ cells are elevated in protected male BWF1 mice compared with females. Female and male Gr1+ cells use different mechanisms of suppression: reactive oxygen and nitrogen species (ROS/NOS) (female), or a secreted component independent of ROS/NOS production (male). Furthermore, female Gr1+ cells lose their capacity to suppress as the mice age. In support hereof, in vivo depletion of Gr1+ cells accelerated disease development in male mice only. Calcium-binding protein S100a9 is induced by inflammation and has been shown to exert either proinflammatory or immunosuppressive functions. Finding that male-derived Gr1+ cells express 3-4 fold higher levels of S100a9 mRNA than females, we hypothesized that S100a9 is the immunosuppressive mechanism used by male Gr1+ cells. Using S100a9-/- BWF1 mice we show that male-derived Gr1+ cells indeed use S100a9 to suppress plasma cell differentiation in vitro. Moreover, BWF1.S100a9-/- males developed increased serum ANA levels and IgG-IC deposition in the kidney glomeruli, and accelerated renal failure, while only the kidney phenotype was somewhat affected in BWF1.S100a9-/- female mice. In conclusion, differential regulation of S100a9 by male and female Gr1+ cells may contribute to the differential development of lupus in male and female BWF1 mice.