ABSTRACT: T cells from many patients with systemic lupus erythematosus are hypoproliferative in response to TCR ligation. This defect is mediated, at least in part, through down regulation of the TCR zeta chain. Investigation of this phenomenon in lupus-prone mice revealed that the hypoproliferative T cell phenotype apparent in BXSB males is mediated in part by the Yaa locus. The BXSB model of lupus is a recombinant inbred strain that was originally derived from a cross between a male SB/LE mouse and a female C57BL/6 mouse. BXSB is unique among lupus-prone mouse strains, in that males carry an autoimmune accelerating factor on the Y chromosome, termed Yaa, which accelerates disease onset in male BXSB mice so that fatal glomerulonephritis occurs around 6 months of age. It was recently reported that the Yaa locus is a translocation of a short stretch of genes including that encoding Toll-like receptor 7 (Tlr-7) from the X to the Y chromosome, leading to a two-fold over expression of a subset of these genes, including Tlr-7, and hypersensitivity of Yaa+ cells to TLR-7 agonists. To more fully understand the functional significance of the Yaa gene and how it plays a part in the modulation of CD4+ T cell responses, we performed microarray analysis using Affymetrix 430 2.0 arrays on spleens from B6 male B6.Yaa male samples. The analysis revealed an enhanced signature of innate immunity, including increased expression not only of Tlr7 and Tlr8, in line with reports that the Yaa locus leads to a duplication of the murine Tlr7 and Tlr8 genes , but also of Tlr2, Tlr5, Cd14, Lbp, C2, C3 and genes for several heat-shock proteins. T cells from B6.Yaa mice also displayed decreased TCRζ expression. Interestingly, cell surface levels of the inhibitory molecule B7-H1 were increased on B6.Yaa B cells and monocytes/macrophages. In line with increased TLR-7 expression reported in B6.Yaa mice, the TLR-7 agonist imiquimod increased B7-H1 expression on normal B cells. Restoration of proliferation in co-cultures of B6 T cells and B6.Yaa APC with B7-H1 blocking Ab demonstrated a mechanistic contribution of B7-H1 up-regulation to Yaa-mediated APC-dependent T cell hypoproliferation. In addition, T cell hypoproliferation was induced in normal B6 T cells in vitro by the application of imiquimod. However, this defect was not mediated by B7-H1 or secreted factors but was independent of APC and mediated through direct imiquimod stimulation of T cells themselves. We postulate that excessive TLR-7 stimulation in Yaa+ mice mediates T cell hypoproliferation in part through up-regulation of B7-H1 and possibly also through direct T cell-mediated effects. Keywords: differential gene expression, RNA, spleen, C57Bl/6J, B6.SB-Yaa/J, male, wild type, Yaa
