Deep Immune Profiling of Patients with STAT1 Gain-of-Function: Revealing New Mechanisms of Pathology

Background: Patients with heterozygous signal transducer and activator of transcription 1 (STAT1) gain-of-function (GOF) pathogenic variants exhibit an array of clinical phenotypes including susceptibility to multiple infections, autoimmunity, and cancer predisposition. Previous studies to characterize the pathways involved and explore therapeutic interventions have been constrained by the technology to perform in-depth immunophenotyping. Mass cytometry has allowed us to perform extensive immune profiling of patients with inborn errors of immunity (IEI) to help gain a better understanding of disease pathology. STAT1 gain-of-function (GOF) mutations have demonstrated higher levels of phosphorylated STAT1 in response to type I and II interferons, but the response to other cytokines is less understood. Using advanced cytometry, we demonstrate a unique pattern of STAT1 phosphorylation in response to IL-6 stimulation in T-cell subsets and this differential pattern may play a role in T-cell differentiation and memory in STAT1 GOF patients. Cases: We report two patients with heterozygous STAT1 GOF mutations in the coiled-coil domain. For both patients, the clinical phenotype was largely consistent with other STAT1 GOF patients, one (P1, c.800C>T; p.ala267Val) presented with secondary HLH due to histoplasmosis, and the second (P2, c.866A>G; p.Tyr289Cys) presented with presumed vaccine strain varicella zoster virus (VZV) meningitis and subsequent history of recurrent herpes simplex virus (HSV) skin lesions. Patient peripheral blood mononuclear cells (PBMCs) were evaluated by fluorescence flow cytometry and cytometry by time of flight (CyTOF) as previously described (Roussel et al. J Leukoc Biol. 2017) to evaluate the impact of STAT1 GOF mutations on T-cell immunophenotype and cytokine signaling. Results: Utilizing cytometric data, we were able to identify similar patterns of T cell distribution on t-distributed stochastic neighbor embedding (t-SNE) plots for both patients with STAT1 GOF that were distinct compared to healthy controls (Fig 1a). In the T-cell compartment, both patients had decreased Th17 and Treg populations and an increased Th1/Th2 ratio compared to healthy donor (Fig 1b). In response to stimulation with IFNg or IL-6, there were also clear patterns with the two patients compared to healthy controls. Levels of p-STAT1 and p-STAT3 were assessed in STAT1 GOF and health donor PBMCs at several times points between 15 and 120 minutes, after stimulation wi