Background: We studied 2 unrelated patients with immune thrombocytopenia and autoimmune hemolytic anemia in the setting of acute infections. One patient developed multisystem inflammatory syndrome in children in the setting of a severe acute respiratory syndrome coronavirus 2 infection. Objectives: We sought to identify the mechanisms underlying the development of infection-driven autoimmune cytopenias. Methods: Whole-exome sequencing was performed on both patients, and the impact of the identified variants was validated by functional assays using the patients' PBMCs. Results: Each patient was found to have a unique heterozygous truncation variant in suppressor of cytokine signaling 1 (SOCS1). SOCS1 is an essential negative regulator of type I and type II IFN signaling. The patients' PBMCs showed increased levels of signal transducer and activator of transcription 1 phosphorylation and a transcriptional signature characterized by increased expression of type I and type II IFN-stimulated genes and proapoptotic genes. The enhanced IFN signature exhibited by the patients' unstimulated PBMCs parallels the hyperinflammatory state associated with multisystem inflammatory syndrome in children, suggesting the contributions of SOCS1 in regulating the inflammatory response characteristic of multisystem inflammatory syndrome in children. Conclusions: Heterozygous loss-of-function SOCS1 mutations are associated with enhanced IFN signaling and increased immune cell activation, thereby predisposing to infectionassociated autoimmune cytopenias. (J Allergy Clin Immunol 2020;nnn:nnn-nnn.)

Genetic susceptibility to Multisystem Inflammatory Syndrome in Children (MIS-C) SARS-CoV-2 SOCS1 XIAP CYBB Asymptomatic MIS-C inflammation Monogenic variants in: No genetic risk factors XIAP, X-linked inhibitor of apoptosis CYBB, Cytochrome b(-245), beta subunit SOCS1, Suppressor of cytokine signaling 1

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