Jae-Seung Moon^1,2,11, Shady Younis^1,2,3,11, Nitya S. Ramadoss^1,2, Radhika Iyer^1,2, Khushboo Sheth^1,2, Orr Sharpe^1,2, Navin L. Rao⁴, Stephane Becart⁵, Julie A. Carman⁴, Eddie A. James⁶, Jane H. Buckner⁶, Kevin D. Deane⁷, V. Michael Holers⁷, Susan M. Goodman^8,9, Laura T. Donlin^8,9, Mark M. Davis^3,10 & William H. Robinson^1,2,3

¹Division of Immunology and Rheumatology, Department of Medicine, Stanford University School of Medicine, Stanford, CA 94305, USA. 
²VA Palo Alto Health Care System, Palo Alto, CA 94304, USA. 
³Institute for Immunity, Transplantation and Infection, Stanford University, Stanford, CA, USA. 
⁴Immunology Discovery, Janssen Research and Development LLC, Spring House, PA 19477, USA. 
⁵Immunology Discovery, Janssen Research and Development LLC, San Diego, CA 92121, USA. 
⁶Center for Translational Immunology, Benaroya Research Institute, Seattle, WA 98101, USA. 
⁷Division of Rheumatology, University of Colorado Anschutz Medical Campus, Aurora, CO 80045, USA. 
⁸Hospital for Special Surgery, New York, NY 10021, USA. 
⁹Weill Cornell Medicine, New York, NY 10021, USA. 
¹⁰Department of Microbiology and Immunology, Stanford University, Stanford, CA 94305, USA. 
¹¹These authors contributed equally: Jae-Seung Moon, Shady Younis.

Corresponding author : Correspondence to William H. Robinson.

The immune mechanisms that mediate synovitis and joint destruction in rheumatoid arthritis (RA) remain poorly defined. Although increased levels of CD8⁺ T cells have been described in RA, their function in pathogenesis remains unclear. Here we perform single cell transcriptome and T cell receptor (TCR) sequencing of CD8⁺ T cells derived from anti-citrullinated protein antibodies (ACPA)+ RA blood. We identify GZMB⁺CD8⁺ subpopulations containing large clonal lineage expansions that express cytotoxic and tissue homing transcriptional programs, while a GZMK⁺CD8⁺ memory subpopulation comprises smaller clonal expansions that express effector T cell transcriptional programs. We demonstrate RA citrullinated autoantigens presented by MHC class I activate RA blood-derived GZMB⁺CD8⁺ T cells to expand, express cytotoxic mediators, and mediate killing of target cells. We also demonstrate that these clonally expanded GZMB⁺CD8⁺ cells are present in RA synovium. These findings suggest that cytotoxic CD8⁺ T cells targeting citrullinated antigens contribute to synovitis and joint tissue destruction in ACPA+ RA.