Immune checkpoint inhibitor-associated inflammatory arthritis.

Understanding the cellular and molecular mechanisms of T cell activation has enabled the identification of immune checkpoints, such as PD-1 and CTLA-4, and the development immune checkpoint inhibitors (ICI) which have revolutionized cancer therapy. However, ICI cancer treatment is commonly associated with autoimmune side effects, including inflammatory arthritis (ICI-IA). ICI-IA occurs in ~6% of ICI-treated patients and often resembles rheumatoid arthritis phenotypically, although it is generally seronegative. Imaging often demonstrates joint inflammation in patients with ICI-associated joint pain, even in the absence of joint swelling. The ICI-IA synovium is characterized by clonal expansion of actively proliferating CD38