Researchers may have uncovered the immune adverse events and the specific type of CD8 T cells that characterize inflammatory arthritis induced by immune checkpoint inhibitors, according to a recent study published by Wang et al in Science Immunology. Researchers studied T cells from a patient with severe immune checkpoint inhibitor–associated arthritis who required bilateral knee replacement surgery. In addition to these two joint tissue samples, the researchers studied the synovial fluid of an additional 23 patients with the same condition. They then compared these samples to those from patients with rheumatoid arthritis or psoriatic arthritis. Patients whose cancer was treated with immune checkpoint inhibitors had high levels of this type of CD8 T cell, and additional analyses revealed that the cells traveled between the joints and blood. Further, the CD8 T cells remained in the bloodstream, providing one explanation for why immune checkpoint inhibitor–associated arthritis may persist after immune checkpoint inhibitors are discontinued.