Heterozygous signal transducer and activator of transcription 1 (
STAT1) gain-of-function (GOF) mutations have been discovered in 2011 [
1,
2] as the cause of most inherited chronic mucocutaneous candidiasis (CMC), but follow-up studies have soon demonstrated that the STAT1-GOF disorder is extremely heterogeneous and determines an immune dysregulation syndrome characterized by a broad spectrum of bacterial, mycobacterial, viral, or other invasive fungal infections, autoimmunity or autoinflammatory manifestations, vascular aneurysms, and malignancies [
3‐
5]. Hypothyroidism is the most frequent endocrine dysfunction, as reported in the 44% of patients described in the largest cohort study [
3], albeit a wide variety of other multiorgan autoimmune diseases may complicate the phenotype, including autoimmune cytopenia, Systemic Lupus Erythematosus, inflammatory bowel diseases, or dermatological conditions such as alopecia, vitiligo, and psoriasis [
3,
5,
6].
STAT1 GOF mutations mostly affect the coiled-coin domain (CCD) or the DNA-binding domain (DBD) of the gene. The gain-of-function activity was initially ascribed to the hyperphosphorylation of STAT1 at the amino acid residue Y701 after stimulation by type I and type II interferons (IFNs) and IL-27 [
2], and furtherly demonstrated through impaired STAT1 dephosphorylation in response to IFNs and IL-27, or increased STAT1 protein levels with normal pattern of dephosphorylation [
7,
8]. The hyperactivity of STAT1 in response to IFNs and IL-27 signals associates with PD-L1 up-regulation, reduced expression of STAT3-dependent genes including SOCS3, and, consequently, inhibition of Th17 differentiation that leads to the susceptibility to fungal infections [
9,
10]. Immunological abnormalities may vary and include T and B lymphopenia, and low IgG levels often with impaired antibody production to protein antigens. Recently, a more profound combined immunodeficiency that associates with more severe infectious, autoimmune phenotype and increased mortality has been correlated to
STAT1 mutations in the DNA-binding domain (DBD) [
11]. Autoimmunity may depend upon the enhanced response to type I IFNs and a stronger IFNs-JAK-STAT1-dependent genes circuit in these patients. STAT1 signals throughout Janus kinases (JAKs) that are activated following the cytokine binding to the receptor; the JAK-mediated phosphorylation of the cytokine receptor allows the recruitment of STAT1 molecules that are phosphorylated, dimerize, and translocate to the nucleus to regulate gene expression. It follows that the inhibition of JAKs may regulate the augmented STAT1 signaling. Since 2015, the use of JAK inhibitors (JAKinib) has been reported in STAT1-GOF patients with favorable results, both with the JAK1/JAK2 inhibitors ruxolitinib (42 patients, 29 < 18 years and 4 adults) or baricitinib (6 patients, 4 < 18 years and 2 adults), or also with tofacitinib (2 children), a selective JAK3 inhibitor with limited selectivity against JAK1 and JAK2 [
12‐
25]. Particularly, resolution or amelioration of CMC and alopecia were reported, together with the improvement of autoimmune and inflammatory manifestations. A reduction of IL-6, an increase of IL-17A and IL-17F production, and a sustained improvement of lymphopenia was demonstrated with JAKinib [
13]. Conventional management includes the use of topic and systemic antifungal or antibiotic prophylaxis and therapies; variable resistance to azoles has been reported (10–39% of patients) [
13], but the risk of antimicrobial resistance remains to be evaluated in follow-up studies. There is a limited experience with hematopoietic stem cell transplantation (HSCT) that was reported in 17 STAT1-GOF patients who exhibited severe disease unresponsive to conventional treatment; HSCT was burdened by secondary graft failure and death. On this aspect, the use of ruxolitinib as immunosuppressive therapy in a patient before transplant was associated to full immune reconstitution with no secondary graft loss, thus supporting the role of JAK inhibitors in controlling IFNγ-induced inflammation [
16]. We have previously described nine Italian STAT-1-GOF patients who exhibited a heterogeneous phenotype whose prognosis in adolescence and adulthood depended upon the recurrence of infections, the occurrence of autoimmunity, and the development of lymphopenia. Particularly, the latter may affect long-term infectious morbidity and survival. For this reason, in the present study we aimed at investigating if T cell lymphopenia could be caused by increased apoptosis following cellular stimulation, and if the JAK inhibitor ruxolitinib could revert the pathogenesis of lymphopenia, in vitro and in vivo. Meanwhile, we chose to analyze the interferon gene signature as a biomarker of active autoimmune/autoinflammatory diseases and aimed at evaluating the effect of the JAK inhibitor ruxolitinib in modulating the IFN-driven hyper-inflammatory activation in STAT1-GOF patients.