Cross-presentation of citrullinated antigens drives cytotoxic CD8⁺ T cell responses in rheumatoid arthritis

Rheumatoid arthritis (RA) is an autoimmune synovitis marked by anti-citrullinated protein antibodies (ACPAs) and infiltration of the synovium by activated immune cells. In ACPA-positive RA, CD8⁺ T cells are elevated in both the blood and synovium, and can be activated by MHC class I-restricted citrullinated autoantigens to mediate cytotoxic effector function. However, the mechanisms underlying the activation of cytotoxic CD8⁺ T cells in RA remain poorly understood. Here, single-cell transcriptomic and T cell receptor repertoire analysis of RA blood and synovial T cells revealed shared clonally expanded cytotoxic CD8⁺ T cell programs, with synovial enrichment of activated effector and proliferating populations and increased frequencies of GZMB ⁺ IFNG ⁺ CD8⁺ T cells. We demonstrated that RA-associated oral bacteria stimulate neutrophil extracellular trap (NET) formation, leading to the peptidyl arginine deiminases (PAD)-dependent generation of extracellular citrullinated bacterial and host proteins. We further demonstrated that these antigens can be cross-presented to CD8⁺ T cells via HLA class I molecules expressed by monocyte-derived dendritic cells (MoDCs) and autoreactive ACPA-expressing B cells. Toll-like receptor (TLR) signaling, particularly TLR4 activation by citrullinated antigens, enhanced cross-presentation of citrullinated antigens and promoted CD8⁺ T cell activation and clonal expansion. In turn, citrullinated antigens stimulated autoreactive B cells to produce IL-8, which recruited CXCR1/2⁺ cytotoxic CD8⁺ T cells and amplified B cell-CD8⁺ T cell interactions. These findings reveal a mechanistic pathway linking microbial triggers, antigen presentation, and cytotoxic CD8⁺ T cell responses that may drive joint destruction in RA.