Multisystem inflammatory syndrome in children (MIS-C) is a rare and severe condition that follows benign COVID-19. We report autosomal recessive deficiencies of OAS1 , OAS2 , or RNASEL in five unrelated children with MIS-C. The cytosolic double-stranded RNA (dsRNA)–sensing OAS1 and OAS2 generate 2′-5′-linked oligoadenylates (2-5A) that activate the single-stranded RNA–degrading ribonuclease L (RNase L). Monocytic cell lines and primary myeloid cells with OAS1, OAS2, or RNase L deficiencies produce excessive amounts of inflammatory cytokines upon dsRNA or severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) stimulation. Exogenous 2-5A suppresses cytokine production in OAS1-deficient but not RNase L–deficient cells. Cytokine production in RNase L–deficient cells is impaired by MDA5 or RIG-I deficiency and abolished by mitochondrial antiviral-signaling protein (MAVS) deficiency. Recessive OAS–RNase L deficiencies in these patients unleash the production of SARS-CoV-2–triggered, MAVS-mediated inflammatory cytokines by mononuclear phagocytes, thereby underlying MIS-C.

MIS-C is characterized by a polyclonal activation of Vβ21.3⁺CD4⁺and CD8⁺T cells that poorly respond to SARS-CoV-2 antigens.

Objectives: Impairment of type I interferon (IFN‐I) immunity has been reported in critically ill COVID‐19 patients. This defect can be explained in a subset of patients by the presence of circulating autoantibodies (auto‐Abs) against IFN‐I. We set out to improve the detection and the quantification of IFN‐I auto‐Abs in a cohort of critically ill COVID‐19 patients, in order to better evaluate the prevalence of these Abs as the pandemic progresses, and how they correlate with the clinical course of the disease.Methods: The concentration of anti‐IFN‐α₂ Abs was determined in the serum of 84 critically ill COVID‐19 patients who were admitted to ICU in Hospices Civils de Lyon, France, using a commercially available kit (Thermo Fisher, Catalog #BMS217).Results: A total of 21 of 84 (25%) critically ill COVID‐19 patients had circulating anti‐IFN‐α₂ Abs above cut‐off (> 34 ng mL⁻¹). Among them, 15 of 21 had Abs with neutralising activity against IFN‐α₂, that is 15 of 84 (18%) critically ill patients. In addition, we noticed an impairment of the IFN‐I response in the majority of patients with neutralising anti‐IFN‐α₂ Abs. There was no significant difference in the clinical characteristics or outcome of with or without neutralising anti‐IFN‐α₂ auto‐Abs. We detected anti‐IFN‐α₂ auto‐Abs in COVID‐19 patients' sera throughout their ICU stay. Finally, we also found auto‐Abs against multiple subtypes of IFN‐I including IFN‐ω.Conclusions: We reported that 18% of critically ill COVID‐19 patients were positive for IFN‐I auto‐Abs, whereas all mild COVID‐19 patients were negative, confirming that the presence of these antibodies is associated with a higher risk of developing a critical COVID‐19 form.

Myeloid-derived suppressor cells (MDSC) are a heterogeneous population of immature myeloid cells with immunosuppressive properties. In cancer patients, the expression of lectin-type oxidized LDL receptor 1 (LOX-1) on granulocytic MDSC identifies a subset of MDSC that retains the most potent immunosuppressive properties. The main objective of the present work was to explore the presence of LOX-1+ MDSC in bacterial and viral sepsis. To this end, whole blood LOX-1+ cells were phenotypically, morphologically, and functionally characterized. They were monitored in 39 coronavirus disease-19 (COVID-19, viral sepsis) and 48 septic shock (bacterial sepsis) patients longitudinally sampled five times over a 3 wk period in intensive care units (ICUs). The phenotype, morphology, and immunosuppressive functions of LOX-1+ cells demonstrated that they were polymorphonuclear MDSC. In patients, we observed the significant emergence of LOX-1+ MDSC in both groups. The peak of LOX-1+ MDSC was 1 wk delayed with respect to ICU admission. In COVID-19, their elevation was more pronounced in patients with acute respiratory distress syndrome. The persistence of these cells may contribute to long lasting immunosuppression leaving the patient unable to efficiently resolve infections.

Juvenile idiopathic arthritis is the most common chronic rheumatic disease in children, and its etiology remains poorly understood. Here, we explored four families with early-onset arthritis carrying homozygous loss-of-expression mutations in LACC1. To understand the link between LACC1 and inflammation, we performed a functional study of LACC1 in human immune cells. We showed that LACC1 was primarily expressed in macrophages upon mTOR signaling. We found that LACC1 deficiency had no obvious impact on inflammasome activation, type I interferon response, or NF-κB regulation. Using bimolecular fluorescence complementation and biochemical assays, we showed that autophagy-inducing proteins, RACK1 and AMPK, interacted with LACC1. Autophagy blockade in macrophages was associated with LACC1 cleavage and degradation. Moreover, LACC1 deficiency reduced autophagy flux in primary macrophages. This was associated with a defect in the accumulation of lipid droplets and mitochondrial respiration, suggesting that LACC1-dependent autophagy fuels macrophage bioenergetics metabolism. Altogether, LACC1 deficiency defines a novel form of genetically inherited juvenile arthritis associated with impaired autophagy in macrophages.

Introduction: Biological disease-modifying anti-rheumatic drugs (bDMARDs) targeting interleukin (IL)-6 and IL-1β represent a steroid-sparing first-line therapy used in systemic-onset juvenile idiopathic arthritis (sJIA). Recently, the occurrence of pulmonary alveolar proteinosis (PAP) in sJIA patients was reported with early-onset and exposure to bDMARDs as potential risk factors. We report on a new case with longitudinal immunomonitoring successfully treated by Janus Kinase inhibitors (JAKi) and review past clinical descriptions of this new entity. Methods: We report one case of pulmonary alveolar proteinosis and macrophage activation syndrome (PAP-MAS) with longitudinal immunomonitoring. We then conducted a review of the literature of seven publications reporting 107 cases of PAP-MAS sJIA, and included the main characteristics and evolution under treatment. Results: Of the seven articles analyzed, the incidence of PAP-MAS among sJIA patients varied from 1.28% to 12.9%. We report here a single case among a cohort of 537 sJIA patients followed in the pediatric department of the Hospices Civils de Lyon over the last 15 years. This child presented with all clinical and immunological characteristics of PAP-MAS. After several lines of treatment, he benefited from JAKi and improved with respect to both systemic symptoms and lung disease. In the literature, strategies with monoclonal antibodies targeting either INF-γ or IL-1β/IL-18 have been tested with variable results. Orally taken JAKi presents the advantage of targeting multiple cytokines and avoiding parenteral injections of monoclonal antibodies that may contribute to the pathogenesis. Conclusions: JAKi represent a promising option in the treatment of lung disease associated with sJIA.