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Background: Autoimmunity is a fundamental consequence of impaired tolerance checkpoints in patients with hypomorphic mutations in recombination activating genes (RAG1 and 2), the major player in V(D)J recombination of antigen receptors. We recently published a cohort of 63 patients with partial RAG deficiency with autoimmune and/or hyperinflammatory manifestations. Autoimmune hemolytic anemia (AIHA) was most common, occurring in (60.3%) of patients at a median age of 1.9 years. The episodes of AIHA were often severe and refractory to first or second-line therapy. Despite the severity of early-onset disease, a large fraction of patients ware diagnosed later for an underlying genetic cause. This may be due to deceased awareness and lack of routine clinical biomarkers for underlying RAG deficiency. The natural course of AIHA episodes, diversity of antibodies targeting red blood cells (RBC) and the underlying mechanisms that promote AIHA remain elusive. Our previous study on RAG-deficient patients has revealed that autoreactive B cells are present early in B cell development, and autoantibodies are produced to a broad selection of self-antigens, including those on RBCs. A prevailing theory for the development of anti-RBC antibodies involves dysregulation of somatic hypermutation (SHM). There are specific regions of the immunoglobulin (Ig) that are naturally reactive to self-antigen in the germline state, and mutation away from self-reactivity occurs through SHM. On example is the germline Ig VH4-34 AVY segment that has been known to bind to the i/I antigen on RBCs. Increased awareness for detection and mechanistic understanding of the generation of anti-RBC antibodies are needed to promote targeted therapy in this highly vulnerable young patient population with AIHA and immune deficiency. Methods: Through retrospective chart review, we collected detailed information on the episodes of AIHA. This included clinical evidence of hemolysis, recurrence and severity of episodes, specifics on red blood cell reactivity, treatment and outcome. In addition, We performed B-cell repertoire studies to assess rate of SHM and "mutation away" from RBC reactivity. Results In our 42 patients with RAG deficiency and AIHA, we accumulated data on 20 patients. Many of the patients experienced the onset of AIHA after viral infections and often recurred (up to 8 times in lifetime). Most of the patients had warm AIHA with broad reactivity (direct, indirect, IgG, IgM, C3). In parallel, patients developed specific antibodies to cytokines targeting IFNa, IFNw and IL12. Second or third-line therapy was required in 50% of cases. One patient underwent HSCT due to severe refractory AIHA. The B-cell repertoire in selected RAG patients with AIHA contained more unmutated Ig VH4-34 segments (including the AVY segment), and demonstrate less evidence of SHM when compared to healthy donors. Conclusions Patients with partial RAG deficiency may develop severe, recurrent and treatment-refractory AIHA at early ages. Diagnosis may be expedited with increased awareness and serological evidence of broadly reactive antibodies to RBC and cytokines. Mechanistically, decreased SHM may promote the survival of naturally occurring antibodies to RBC. Studies should continue to advance understanding for targeted approach to serve as bridge therapy before HSCT is considered or indicated. Disclosures No relevant conflicts of interest to declare.