Poor outcomes in COVID‐19 correlate with clinical and laboratory features of cytokine storm syndrome. Broad screening for cytokine storm and early, targeted antiinflammatory therapy may prevent immunopathology and could help conserve limited health care resources. While studies are ongoing, extrapolating from clinical experience in cytokine storm syndromes may benefit the multidisciplinary teams caring for patients with severe COVID‐19.

BACKGROUND. Pediatric SARS-CoV-2 infection can be complicated by a dangerous hyperinflammatory condition termed multisystem inflammatory syndrome in children (MIS-C). The clinical and immunologic spectrum of MIS-C and its relationship to other inflammatory conditions of childhood have not been studied in detail. METHODS.We retrospectively studied confirmed cases of MIS-C at our institution from March to June 2020. The clinical characteristics, laboratory studies, and treatment response were collected. Data were compared with historic cohorts of Kawasaki disease (KD) and macrophage activation syndrome (MAS). RESULTS.Twenty-eight patients fulfilled the case definition of MIS-C. Median age at presentation was 9 years (range: 1 month to 17 years); 50% of patients had preexisting conditions. All patients had laboratory confirmation of SARS-CoV-2 infection. Seventeen patients (61%) required intensive care, including 7 patients (25%) who required inotrope support. Seven patients (25%) met criteria for complete or incomplete KD, and coronary abnormalities were found in 6 cases. Lymphopenia, thrombocytopenia, and elevation in inflammatory markers, D-dimer, B-type natriuretic peptide, IL-6, and IL-10 levels were common but not ubiquitous. Cytopenias distinguished MIS-C from KD and the degree of hyperferritinemia and pattern of cytokine production differed between MIS-C and MAS. Immunomodulatory therapy given to patients with MIS-C included intravenous immune globulin (IVIG) (71%), corticosteroids (61%), and anakinra (18%). Clinical and laboratory improvement were observed in all cases, including 6 cases that did not require immunomodulatory therapy. No mortality was recorded in this cohort. CONCLUSION. MIS-C encompasses a broad phenotypic spectrum with clinical and laboratory features distinct from KD and MAS.

Tetramethylpentadecane (TMPD, or commonly known as pristane)-induced lupus is a murine model of systemic lupus erythematosus (SLE). Renal disease and autoantibody production strictly depend on signaling through the interferon (IFN)-I receptor. The major source of IFN-I is immature monocytes bearing high levels of the surface marker Ly6C. Interferon production is mediated exclusively by signaling through TLR7 and the adapter protein MyD88. It is likely that endogenous TLR7 ligands such as components of small nuclear ribonucleoprotein complexes are involved in triggering disease. Lupus autoantibodies are produced in ectopic lymphoid tissue developing in response to TMPD. This model is well suited for examining links between dysregulated IFN-I production and the pathogenesis of human SLE, which like TMPD-lupus, is associated with high levels of IFN-I.

Increased type I interferon (IFN-I) production and IFN-stimulated gene (ISG) expression are linked to the pathogenesis of systemic lupus erythematosus (SLE). Although the mechanisms responsible for dysregulated IFN-I production in SLE remain unclear, autoantibodymediated uptake of endogenous nucleic acids is thought to play a role. 2,6,10,14-tetramethylpentadecane (TMPD; also known as pristane) induces a lupus-like disease in mice characterized by immune complex nephritis with autoantibodies to DNA and ribonucleoproteins. We recently reported that TMPD also causes increased ISG expression and that the development of the lupus is completely dependent on IFN-I signaling (Nacionales, D.C., K.M. Kelly-Scumpia, P.Y. Lee, J.S. Weinstein, R. Lyons, E. Sobel, M. Satoh, and W.H. Reeves. 2007. Arthritis Rheum. 56:3770 -3783). We show that TMPD elicits IFN-I production, monocyte recruitment, and autoantibody production exclusively through a Tolllike receptor (TLR) 7 -and myeloid differentiation factor 88 (MyD88) -dependent pathway. In vitro studies revealed that TMPD augments the effect of TLR7 ligands but does not directly activate TLR7 itself. The effects of TMPD were amplifi ed by the Y-linked autoimmune acceleration cluster, which carries a duplication of the TLR7 gene. In contrast, deficiency of Fc ␥ receptors (Fc ␥ Rs) did not affect the production of IFN-I. Collectively, the data demonstrate that TMPD-stimulated IFN-I production requires TLR7/MyD88 signaling and is independent of autoantibody-mediated uptake of ribonucleoproteins by Fc ␥ Rs.