Immune dysregulation and multisystem inflammatory syndrome in children (MIS-C) in individuals with haploinsufficiency of SOCS1

Abstract

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 infection-associated autoimmune cytopenias.

Keywords: COVID-19; Evans syndrome; MIS-C; SARS-CoV-2; SOCS1; autoimmune hemolytic anemia; immune thrombocytopenia.

Fig 1

Two unique heterozygous truncation SOCS1 variants in 2 unrelated children with ES. A, Indicated laboratory values for patient 1 since initial diagnosis. B, Pedigree of the family of patient 1, who presented at age 5 months with anemia and neutropenia. WES identified a de novo SOCS1 variant. C, Indicated laboratory values for patient 2. The arrow indicates the recent hospital admission for SARS-CoV-2 infection. D, Pedigree of the family of patient 2, who presented with thrombocytopenia at age 14 years and recent development of autoimmune hemolytic anemia. WES identified a paternally inherited SOCS1 variant. E, Schematic of domains for wild-type (WT) SOCS1 and the truncation variants found in the patients. ALC, Absolute lymphocyte count; ANC, absolute neutrophil count; WES, Whole-exome sequencing.

Fig 2

Enhanced IFN and proapoptotic signaling associated with loss-of-function SOCS1 variants. Patient 2 was on corticosteroids (CSs) for 2 months at the time of sampling for the assays shown. A, Schematic of the interaction between SOCS1 and the JAKs. B, Quantification of phospho-STAT1 expression in CD14⁺ monocytes from healthy controls and patients before and after treatment with IFN-β (25 ng/mL) or IFN-γ (25 ng/mL) for 20 minutes. Histograms are representative of 2 independent experiments; the isotype control was comparable between controls and patients. Plotted data are combined from 2 independent experiments, each with 4 controls and 2 patients. C, Expression of the IFN-stimulated gene CD64 compared with CD32, which is independent of IFN signaling, on CD14⁺ monocytes from healthy controls and patients. The flow cytometry histogram (left) is representative of 2 experiments, which were pooled for quantification (right). The MFI for each sample was normalized to the average expression of healthy controls. Gray shading indicates the 25% to 75% quartiles of control values. D. Quantitative PCR analysis of 2 additional IFN-stimulated genes, IFI44 and ISG15, compared with TNF, which is IFN-independent, in PBMCs from controls and patients. Fold expression for each sample was normalized to the average expression of healthy controls. Gray shading indicates the 25% to 75% quartiles of control values. Data are combined from 2 independent experiments. E and F, Heat-map of type I and type II ISGs (Fig 2, E) as well as proapoptotic vs antiapoptotic genes (Fig 2, F) in 4 controls and patients. MFI, Mean fluorescence intensity.