Background Pre-clinical mouse models are invaluable tools to investigate the mechanisms driving systemic lupus erythematosus (SLE) and identify therapeutic targets to treat human disease. The BXSB.Yaa (Yaa) mouse spontaneously develops SLE-like disease, that is accelerated by the loss of CD8 T cells in BXSB.Yaa.CD8a-/-IL15-/- (DKO) mice, suggesting that CD8 T cells have a protective role against autoimmunity in this model. The role of CD8 T cells in human SLE remains unclear as studies have found conflicting associations with increased disease severity or with a loss of regulatory activity. Therefore, we carried out transcriptome analysis of Yaa and DKO mice to clarify the role of CD8 T cells in SLE.
Methods Six week old ‘pre-disease’ mice were examined to determine the impact of CD8 T cells before the development of inflammatory disease. Gene expression profiles from spleens of 6 week old BXSB.B6 (CTL), Yaa, and DKO mice were obtained by RNA-seq. Log2 gene expression values were normalized between datasets using housekeeping genes. Differential enrichment of immune cell and pathway gene modules curated for analysis of lupus mouse datasets was determined by Gene Set Variation Analysis (GSVA). Immune cell population GSVA scores were then correlated with functional pathways by linear regression.
Results Spleens from 6 week BXSB.Yaa mice showed evidence of extensive immune activation. GSVA revealed shared enrichment of SLE-associated lymphocyte populations including germinal center (GC) B cells and T follicular helper (Tfh) cells in Yaa compared to healthy CTL mice that was not further accentuated in DKO mice (figure 1A). Notably, plasma cells (PCs) were only enriched in DKO mice. Myeloid cells and specifically macrophages (Mφs) and dendritic cells (DCs) were not enriched in Yaa, but significantly enriched in DKO over both Yaa and CTL mice. Furthermore, enrichment of Mφ signatures in DKO mice was restricted to M2 but not M1 polarization markers and specifically to M2a, M2c, and M2d subsets (figure 1B). Comparison of normalized log2 gene expression of pro- and anti-inflammatory cytokines revealed that Il1a, Il18, and Il21 were elevated, whereas Il1b, Tnf, and Tgfb1 were decreased in DKO as compared to CTL and Yaa mice (figure 1C). The enriched M2 population significantly correlated with a number of inflammatory pathway signatures related to Mφ function including IFNA1, IFNB2, IFNG, phagocytosis, and fibrosis (figure 1D).
Conclusions Transcriptomic analysis of Yaa mice before onset of SLE like clinical disease and DKO mice that develop accelerated disease uncovered differences in immune profiles, which point to a role for CD8 T cells in protection from autoimmunity in this model. In the absence of CD8 T cells, DKO mice exhibited an increase in gene signatures of M2-like Mφs representing a unique functional subset correlated with pathologic pathway signatures. This analysis provides evidence for a protective, regulatory function of CD8 T cells against autoimmune pathology with implications for understanding their role in human SLE.
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