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Abstract: Immune checkpoint inhibitors (ICPis) have revolutionized cancer immunotherapy but also can induce autoimmune hemolytic anemia (AIHA), a severe disease with high mortality. However, the cellular and molecular mechanism(s) of AIHA secondary to ICPi therapy (ICPi-AIHA) are unclear, other than being initiated through decreased checkpoint inhibition. Herein, we report ICPi-AIHA in a novel mouse model that shows similar characteristics of known human ICPi-AIHA (eg, autoantibodies, hemolysis, and increased mortality). During ICPi-AIHA, there is the simultaneous reduction of 2 regulatory T-cell populations (FoxP3+ and Tr1 [type 1 regulatory cells]) and an increase in inflammatory T helper cell 17 (TH17). Moreover, a novel CD39+CD73-FoxP3-CD25- CD4+ T-cell subset (ie, CD39 single positive [CD39SP]) emerges, and early increases in CD39SP predict AIHA development; CD39 is an ectonuclease that breaks down adenosine triphosphate (ATP). Additionally, we found that boosting ATPase activity by injecting recombinant apyrase mitigates AIHA development and significant CD39SP reductions, both suggesting a functional role for CD39 and demonstrating a novel therapeutic approach. Importantly, CD39SP are detectable in multiple mouse models developing AIHA and in patients with AIHA, demonstrating applicability to idiopathic and secondary AIHA. Highlighting broader autoimmunity relevance, ICPi-treated NZB mice experienced accelerated onset and severity of lupus, including AIHA. Moreover, ICPi treatment of healthy B6 animals led to detectable CD39SP and development of autoantibodies against multiple autoantigens including those on red blood cells and platelets. Together, our findings provide further insight into the cellular and molecular mechanisms of ICPi-AIHA, leading to novel diagnostic and therapeutic approaches with translational potential for use in humans being treated with ICPi., (© 2024 American Society of Hematology. Published by Elsevier Inc. All rights are reserved, including those for text and data mining, AI training, and similar technologies.)