Investigating type I interferon signalling regulation in the context of human Mendelian autoinflammatory disease

Abstract

Type I interferons (IFN-Is) are primary antiviral cytokines. Homeostasis of IFN-I signalling is under tight regulation, with too little or too much IFN-I activity resulting in significant pathology. While impaired IFN-I signalling is characterised by immunodeficiency, inappropriate upregulation of IFN-I signalling results in a set of autoinflammatory states termed the type I interferonopathies. In this work, through the study of real-world patients, distinct but related regulatory mechanisms important for homeostatic IFN-I signalling were investigated.

In the first part of the thesis, I studied a patient presenting with features consistent with a type I interferonopathy, who we identified to carry a rare homozygous missense variant p.(A219V) in STAT2. Through in vitro testing, I showed that while STAT2 p.(A219V) maintained the ability to transduce an IFN-I signal, its negative regulatory function was impaired due to defective binding of USP18.

In the second part of this study, I report a novel cohort of patients with a highly stereotyped clinical phenotype comprising normal early development followed by the onset of subacute neuro-regression, with increased IFN-stimulated gene (ISG) expression in whole-blood and raised neopterin levels in cerebrospinal fluid. We found these patients to harbour rare heterozygous variants in PTPN1 resulting in loss of mRNA and/or protein expression, suggesting that haploinsufficiency of PTPN1 leads to upregulated IFN-I signalling. Indeed, cells deficient in PTPN1 demonstrate an upregulation of ISG expression at baseline, and ‘hypersensitivity’ to stimulation with IFNα2b and the STING agonist diABZI. Underlying mechanisms could involve enhanced STING signalling, as evidenced by an overexpression of IFNB1 upon diABZI stimulation, and/or enhanced IFN-I signalling, where higher levels of STAT1 phosphorylation upon IFNα2b stimulation result in overproduction of ISGs.

Overall, this thesis describes the study of IFN-I mediated autoinflammation, with a particular focus on IFN signalling downstream of the IFN-I receptor. This work thus contributes new knowledge relevant to the physiological regulation of IFN-I signalling in humans, with potentially important implications for clinical testing and treatment.