Gain-of-function STAT1 mutations impair STAT3 function and underlie susceptibility to Chronic Mucocutaneous Candidiasis

Abstract

Background: Signal transducer and activator of transcription (STAT)3 activation triggers transcription of interleukin (IL)-17 which is crucial for mounting protective immune responses against fungi. Several mutations affecting the STAT3/IL-17 pathway have been reported, resulting in selective susceptibility to fungal (Candida) infection characteristic of Chronic Mucocutaneous Candidiasis (CMC). Patients with autosomal dominant (AD)-CMC have defective T helper (Th)-17 responses (Ng et al, JACI 2010) and harbour gain-of-function (GOF) STAT1 (rather than STAT3) mutations (van de Veerdonk et al NEJM 2011), leading to hyper-phosphorylation of STAT1 (Smeekens et al PLosOne 2011). How this affects STAT3 or leads to decreased IL-17 production is unknown.

Objective: To assess how GOF-STAT1 mutations affect STAT3 activation, DNA-binding, gene expression, cytokine production and whether this can be epigenetically modified.

Approaches: Peripheral blood mononuclear cells (PBMCs) or Epstein-Barr virus (EBV)-transformed cell lines were stimulated with various cytokines (IL-23, IL-6, IL-21, IL-27, IFN-α, IFN-γ) in the absence or presence of fludarabine (a STAT1 inhibitor) or trichostatin A (a histone deacetylase inhibitor, HDACi). Activation of STAT1 and STAT3 was measured by western blotting (WB). DNA-binding of STAT1 and/or STAT3 was evaluated using electrophoretic mobility shift assay (EMSA), TransAm STAT3 kit and chromatin immunoprecipitation (ChIP) assay. STAT3-inducible gene (RORc, IL-17, IL-22, IL-10, c-Fos, SOCS3, c-Myc) and STAT1-inducible gene (CXCL10, IRF1) expression was assessed using quantitative real time-polymerase chain reaction (qRT-PCR). Gene silencing was performed with small interfering RNA (siRNA) targeting HDAC1, 2 and 3. Candida albicans-induced cytokine (IL-17, IL-22, IL-10) production was measured by enzyme-linked immunosorbent assay (ELISA). Key Findings: GOF-STAT1 mutations lead to hyper-phosphorylation of STAT1 and delayed dephosphorylation. Mutations decrease STAT3-induced gene expression and Th-17-mediated cytokine production without affecting STAT3 phosphorylation, nuclear accumulation or DNA-binding to a STAT-consensus binding site - human sis-inducible element (hSIE). The disrupted STAT3 function can be modified either by enhancing acetylation or inhibiting STAT1 activation.

Conclusion: GOF-STAT1 mutations impair STAT3 function, which can be rescued by enhancing acetylation and/or inhibiting STAT1. These findings are the likely mechanisms underlying decreased Th-17 (IL-17A and IL-22) cytokine production and susceptibility to fungal infections in CMC