Discussion
IL-17 immunity is critical for the induction of mucosal pro-inflammatory cytokines and chemokines involved in mucocutaneous protection againstC. albicans infections [6]. Patients with low proportions of Th17 cells (e.g AD STAT3, AR ZNF341 or ROR-γ/γT deficiency, or STAT1 GOF), or defects in IL-17 signaling (e.g. AR IL-17RA, IL-17RC, ACT1, AD IL-17F, JNK1 deficiencies) are particularly susceptible to CMC(Figure 1) [2-4, 7, 8]. Genetic analysis of our index case revealed variants in STAT1 and TRAF3IP2 (Figure 2) . Considering that STAT1 GOF mutations have been widely described to cause CMC, we evaluated the functional impact of this variant. High levels of STAT1 expression and/or STAT1 phosphorylation upon activation of the IFN pathway are typical immunological features of STAT1 GOF patients [4, 17, 18]. Based on the similar STAT1 and pSTAT1 levels found in patient’s cells and healthy controls, and considering that her father, harboring the same mutation, was asymptomatic, we classified this variant as likely not pathogenic (Figure 2-3) . We therefore studied the ACT1 variants and their possible disease-causing impact. The ACT1 SEFIR domain allows for the recruitment of ACT1 to the IL-17RA/IL-17RC upon IL-17A stimulation [8, 10-12]. By overexpression, the patient’s alleles were expressed at similar levels as compared to the WT protein, with p.K454Fsf11* leading to the expression of a truncated protein, impairing its interaction with IL-17RA, whereas p.D451G showed a normal interaction (Figure 4A/B). After ACT1-IL-17RA/IL-17RC interaction upon IL-17A/IL-17F stimulation, the kinase TAK1 and the E3 ubiquitin ligase TRAF6 are recruited to the receptor to facilitate the activation of the transcription factor NF-κB (Figure 1) [10, 11]. By luciferase, we showed that both patient’s ACT1 alleles strongly impaired constitutive and IL-17-dependent NF-κB activation(Figure 4C) . Furthermore, patient’s SV40-fibroblasts displayed abolished responses to IL-17A, comparable to that of patients with AR IL-17RA or IL-17RC deficiency (Figure 4D) [5, 7, 12]. In contrast, all of them retained the ability to respond to TNF-α and IL-1β, suggesting that D451G and K454fs11* ACT1 mutations specifically affect the IL-17A-induced ACT1-mediated GRO-α production, known to play a protective role in fungal infection through the recruitment of neutrophils [19]. Finally, as previously reported [12, 16], the patient displayed enhanced proportions of IL-17A-and IL-22-producing CD4+ cells upon stimulation whereas baseline Th17 proportions were normal. This observation highlights the importance of functional assays, as the pattern of T cell differentiation is variable. Two previous reports [12, 16], as well as our case, revealed normal or slightly reduced IFN-γ-producing memory CD4+ cells whereas others observed reduced Th1 proportion [14]. Furthermore, and in contrast to previous cases [14, 16], levels of IL-4-producing Th2 cells were higher in our patient than those found in relatives, and healthy controls (Figure 5) .
Clinical and laboratory findings, and management of all eleven previously reported patients with AR ACT1 deficiency, including our report, are summarized in Figure 6 and, in more detail, insupplementary Table 3 [12-16]. Of note, detailed information was not available for all patients. The overall phenotype was characterized by CMC in early childhood (9/11, before age 2 years) requiring medical attention, treatment, or antifungal prophylaxis for a prolonged time in most cases. Dermatologic manifestations were common, and Staphylococcus aureus was specifically reported in the context of skin lesions in 5/11 patients. Treatment responses of CMC were satisfactory when documented, and no fatal cases have been described (supplementary Table 3 and Figure 6) . The clinical course of our patient was similar, as she suffered from CMC, chronic cheilitis, recurrent wheezing, and skin lesions (supplementary Table 3) . Importantly, standard immune phenotyping and immunoglobulin levels were unremarkable in most patients when reported. Raised Th17 percentages appear to be the most common finding (6/7) when tested. In contrast hyper IgG (P1-P3), hyper IgE (P3, P4), or eosinophilia (P1, P4) was found in only few cases.
In conclusion, we describe a patient with AR ACT1 deficiency, caused by novel compound heterozygous (D451G and K454fs11*) variants ofTRAF3IP2 . Although only K454fs11* apparently affected ACT1 binding to IL-17RA, patient’s cells were unresponsive to IL-17A, explaining the clinical phenotype of CMC, further highlighting the importance of performing detailed functional studies to understand the clinical impact of genetic variants found in patients, particularly in the field of immune dysregulation syndromes.