Endogenous antigens shape the transcriptome and TCR repertoire in an autoimmune arthritis model.

The development of pathogenic autoreactive CD4+ T cells, particularly in the context of impaired signaling, remains poorly understood. Unraveling how defective signaling pathways contribute to their activation and persistence is crucial for identifying new therapeutic targets. We profiled a highly arthritogenic subset of naïve CD4+ T cells using bulk and single-cell RNA and TCR sequencing from SKG mice, which develop CD4+ T cell mediated autoimmune arthritis driven by a hypomorphic mutation in Zap70-a key TCR signaling kinase. Despite impaired signaling, these cells exhibit heightened expression of T cell activation and cytokine signaling genes, but diminished expression of a subset of tolerogenic markers (Izumo1r, Tnfrsf9, Cd5, S100a11) compared to wild-type cells. The arthritogenic cells show an enrichment for TCR variable beta (Vβ) chains targeting superantigens from the endogenous mouse mammary tumor virus (MMTV) but exhibit diminished induction of tolerogenic markers following peripheral antigen encounter, contrasting with the robust induction of negative regulators seen in wild-type cells. In arthritic joints, cells expressing superantigen-reactive Vβs expand alongside detectable MMTV proviruses. Antiretroviral treatment and superantigen-reactive T cell depletion curtail SKG arthritis, suggesting that endogenous retroviruses disrupt peripheral tolerance and promote the activation and differentiation of self-reactive CD4+ T cells into pathogenic effector cells.