The Th17/IL-17 Axis and Host Defense Against Fungal Infections

The pathobiology of human fungal infections

Human fungal infections are a historically neglected area of disease research, yet they cause more than 1.5 million deaths every year. Our understanding of the pathophysiology of these infections has increased considerably over the past decade, through major insights into both the host and pathogen factors that contribute to the phenotype and severity of these diseases. Recent studies are revealing multiple mechanisms by which fungi modify and manipulate the host, escape immune surveillance and generate complex comorbidities. Although the emergence of fungal strains that are less susceptible to antifungal drugs or that rapidly evolve drug resistance is posing new threats, greater understanding of immune mechanisms and host susceptibility factors is beginning to offer novel immunotherapeutic options for the future. In this Review, we provide a broad and comprehensive overview of the pathobiology of human fungal infections, focusing specifically on pathogens that can cause invasive life-threatening infections, highlighting recent discoveries from the pathogen, host and clinical perspectives. We conclude by discussing key future challenges including antifungal drug resistance, the emergence of new pathogens and new developments in modern medicine that are promoting susceptibility to infection.

Monozygotic triplets with juvenile-onset autoimmunity and 18p microdeletion involving PTPRM

Abnormal gene dosage from copy number variants has been associated with susceptibility to autoimmune disease. This includes 18p deletion syndrome, a chromosomal disorder with an estimated prevalence of 1 in 50,000 characterized by intellectual disability, facial dysmorphology, and brain abnormalities. The underlying causes for autoimmune manifestations associated with 18p deletions, however, remain unknown. Our objective was to investigate a distinctive case involving monozygotic triplets concordant for developmental delay, white matter abnormalities, and autoimmunity, specifically juvenile-onset Graves' thyroiditis. By chromosomal microarray analysis and whole genome sequencing, we found the triplets to carry a de novo interstitial 5.9 Mb deletion of chromosome 18p11.31p11.21 spanning 19 protein-coding genes. We conducted a literature review to pinpoint genes affected by the deletion that could be associated with immune dysregulation and identified PTPRM as a potential candidate. Through dephosphorylation, PTPRM serves as a negative regulator of STAT3, a key factor in the generation of Th17 cells and the onset of specific autoimmune manifestations. We hypothesized that PTPRM hemizygosity results in increased STAT3 activation. We therefore performed assays investigating PTPRM expression, STAT3 phosphorylation, Th1/Th2/Th17 cell fractions, Treg cells, and overall immunophenotype, and in support of the hypothesis, our investigations showed an increase in cells with phosphorylated STAT3 and higher levels of Th17 cells in the triplets. We propose that PTPRM hemizygosity can serve as a contributing factor to autoimmune susceptibility in 18p deletion syndrome. If confirmed in unrelated 18p/PTPRM deletion patients, this susceptibility could potentially be treated by targeted inhibition of IL-17.

Inflammatory cytokine signalling in vulvovaginal candidiasis: a hot mess driving immunopathology

Protective immunity to opportunistic fungal infections consists of tightly regulated innate and adaptive immune responses that clear the infection. Immune responses to infections of the vaginal mucosa by Candida species are, however, an exception. In the case of vulvovaginal candidiasis (VVC), the inflammatory response is associated with symptomatic disease, rather than that it results in pathogen clearance. As such VVC can be considered an inflammatory disease, which is a significant public health problem due to its predominance as a female-specific fungal infection. Particularly, women with recurrent VVC (RVVC) suffer from a significant negative impact on their quality of life and mental health. Knowledge of the inflammatory pathogenesis of (R)VVC may guide more effective diagnostic and therapeutic options to improve the quality of life of women with (R)VVC. Here, we review the immunopathogenesis of (R)VVC describing several elements that induce an inflammatory arson, starting with the activation threshold established by vaginal epithelial cells that prevent unnecessary ignition of inflammatory responses, epithelial and inflammasome-dependent immune responses. These inflammatory responses will drive neutrophil recruitment and dysfunctional neutrophil-mediated inflammation. We also review the, sometimes controversial, findings on the involvement of adaptive and systemic responses. Finally, we provide future perspectives on the potential of some unexplored cytokine axes and discuss whether VVC needs to be subdivided into subgroups to improve diagnosis and treatment.

Card9 and MyD88 differentially regulate Th17 immunity to the commensal yeast Malassezia in the murine skin

The fungal community of the skin microbiome is dominated by a single genus, Malassezia. Besides its symbiotic lifestyle at the host interface, this commensal yeast has also been associated with diverse inflammatory skin diseases in humans and pet animals. Stable colonization is maintained by antifungal type 17 immunity. The mechanisms driving Th17 responses to Malassezia remain, however, unclear. Here, we show that the C-type lectin receptors Mincle, Dectin-1, and Dectin-2 recognize conserved patterns in the cell wall of Malassezia and induce dendritic cell activation in vitro, while only Dectin-2 is required for Th17 activation during experimental skin colonization in vivo. In contrast, Toll-like receptor recognition was redundant in this context. Instead, inflammatory IL-1 family cytokines signaling via MyD88 were also implicated in Th17 activation in a T cell-intrinsic manner. Taken together, we characterized the pathways contributing to protective immunity against the most abundant member of the skin mycobiome. This knowledge contributes to the understanding of barrier immunity and its regulation by commensals and is relevant considering how aberrant immune responses are associated with severe skin pathologies.

Increased type 1 inflammation in gynecologic cervicovaginal samples in patients with APS-1

BACKGROUND

Inborn errors of immunity offer important insights into mucosal immunity. In autoimmune polyendocrine syndrome type-1 (APS-1), chronic mucocutaneous candidiasis has been ascribed to neutralizing IL-17 autoantibodies. Recent evidence implicates excessive T-cell IFN-γ secretion and ensuing epithelial barrier disruption in predisposition to candidiasis, but these results remain to be replicated. Whether IL-17 paucity, increased type I inflammation, or their combination underlies susceptibility to chronic mucocutaneus candidiasis in APS-1 is debated.

OBJECTIVE

Our aim was to characterize the immunologic features in the cervicovaginal mucosa of females with APS-1.

METHODS

Vaginal fluid was collected with a flocked swab from 17 females with APS-1 and 18 controls, and cytokine composition was analyzed using Luminex (Luminex Corporation, Austin, Tex). Cervical cell samples were obtained with a cervix brush from 6 patients and 6 healthy controls and subjected to transcriptome analysis.

RESULTS

The vaginal fluid samples from patients with APS-1 had IFN-γ concentrations comparable to those of the controls (2.6 vs 2.4 pg/mL) but high concentrations of the TH1 chemokines CXCL9 and CXCL10 (1094 vs 110 pg/mL [P < .001] and 4033 vs 273 pg/mL [P = .001], respectively), whereas the IL-17 levels in the samples from the 2 groups were comparable (28 vs 8.8 pg/mL). RNA sequencing of the cervical cells revealed upregulation of pathways related to mucosal inflammation and cell death in the patients with APS-1.

CONCLUSION

Excessive TH1 cell response appears to underlie disruption of the mucosal immune responses in the genital tract of patients with APS-1 and may contribute to susceptibility to candidiasis in the genital tract as well.

The ouroboros of autoimmunity

Human autoimmunity against elements conferring protective immunity can be symbolized by the 'ouroboros', a snake eating its own tail. Underlying infection is autoimmunity against three immunological targets: neutrophils, complement and cytokines. Autoantibodies against neutrophils can cause peripheral neutropenia underlying mild pyogenic bacterial infections. The pathogenic contribution of autoantibodies against molecules of the complement system is often unclear, but autoantibodies specific for C3 convertase can enhance its activity, lowering complement levels and underlying severe bacterial infections. Autoantibodies neutralizing granulocyte-macrophage colony-stimulating factor impair alveolar macrophages, thereby underlying pulmonary proteinosis and airborne infections, type I interferon viral diseases, type II interferon intra-macrophagic infections, interleukin-6 pyogenic bacterial diseases and interleukin-17A/F mucocutaneous candidiasis. Each of these five cytokine autoantibodies underlies a specific range of infectious diseases, phenocopying infections that occur in patients with the corresponding inborn errors. In this Review, we analyze this ouroboros of immunity against immunity and posit that it should be considered as a factor in patients with unexplained infection.

Inborn errors of immunity: A field without frontiers

The study of primary immunodeficiencies or inborn errors of immunity continues to drive our knowledge of the function of the human immune system. From the outset, the study of inborn errors has focused on unraveling genetic etiologies and molecular mechanisms. Aided by the continuous growth in genetic diagnostics, the field has moved from the study of an infection dominated phenotype to embrace and unravel diverse manifestations of autoinflammation, autoimmunity, malignancy, and severe allergy in all medical disciplines. It has now moved from the study of ultrarare presentations to producing meaningful impact in conditions as diverse as inflammatory bowel disease, neurological conditions, and hematology. Beyond offering immunogenetic diagnosis, the study of underlying inborn errors of immunity in these conditions points to targeted treatment which can be lifesaving.

Short-term risk and long-term incidence rate of infection and malignancy with IL-17 and IL-23 inhibitors in adult patients with psoriasis and psoriatic arthritis: a systematic review and meta-analysis

The risk of infection and malignancy may be a concern for patients with psoriasis receiving interleukin (IL)-17 and IL-23 inhibitors, particularly with long-term treatments. We aimed to estimate the short-term risks and long-term incidence rates of infection and malignancy with IL-17 or IL-23 antagonists in adult patients with psoriasis and psoriatic arthritis through this comprehensive meta-analysis (PROSPERO registration number: CRD42022363127). We searched PubMed, MEDLINE, Web of Science and ClinicalTrials.gov until May 17, 2023 for randomized placebo-controlled trials and long-term (≥ 52 weeks) open-label extension studies. The estimates of short-term risk ratios (RRs) and long-term exposure-adjusted incidence rates (EAIRs) were pooled using R software 4.1.1 and STATA 16.0. This review included 45 randomized placebo-controlled studies and 27 open-label extension studies. Short-term RRs of serious infection, overall infection and malignancy were 1.45 (95% confidence intervals, 95% CI: 0.81-2.59), 1.20 (95% CI: 1.06-1.35), 0.83 (95% CI: 0.41-1.71) with IL-17 inhibitors; and 0.68 (95% CI: 0.38-1.22), 1.13 (95% CI: 1.00-1.28), 0.87 (95% CI: 0.37-2.04) with IL-23 inhibitors. Increased short-term risks of nasopharyngitis and Candida infection with IL-17 inhibitors were found. Long-term EAIRs of serious infection, overall infection, nonmelanoma skin cancer (NMSC), malignancies excluding NMSC, nasopharyngitis and upper respiratory tract infection were 1.11/100 patient-years (PYs), 57.78/100PYs, 0.47/100PYs, 0.24/100PYs, 15.07/100PYs, 8.52/100PYs, 3.41/100PYs with IL-17 inhibitors; and 1.09/100PYs, 48.50/100PYs, 0.40/100PYs, 0.43/100PYs, 10.75/100PYs, 5.84/100PYs with IL-23 inhibitors. Long-term EAIR of Candida infection was 3.41/100PYs with IL-17 inhibitors. No active or reactivated tuberculosis was ever reported in all the trials, and only a few cases of latent tuberculosis, hepatitis, and herpes zoster were reported during the long-term extension periods. No evidence of increased EAIRs of infection and malignancy with longer durations was found. Our study suggested that short-term risk and long-term incidence of infections and malignancies in psoriasis patients receiving IL-17 inhibitors and IL-23 inhibitors are generally low. However, close monitoring is required for nasopharyngitis and Candida infection with IL-17 inhibitors.

Systematic Review Registration

https://www.crd.york.ac.uk/PROSPERO/, identifier CRD42022363127.

Targeted Treatment of Diseases of Immune Dysregulation

Increasing molecular knowledge of autoinflammatory and autoimmune disorders has enabled more targeted treatment of these conditions. Treatment of inflammasomopathies is often aimed at interleukin-1 (IL-1) blockade, with potential use of other inhibitors targeting cytokines such as IL-18 and IL-6. Interferonopathies and some disorders with overlap features of autoimmunity and autoinflammation may improve with Janus kinase inhibition. Autoimmune conditions may also respond to inhibition of different cytokines, as well as to inhibition of T and B lymphocytes. Effective treatment is increasingly possible through targeted/precision medicine approaches.

Risk of candidiasis associated with interleukin-17 inhibitors: Implications and management

The application of interleukin-17 (IL-17) inhibitors, including secukinumab, ixekizumab, brodalumab, and bimekizumab, are associated with elevated risk of candidiasis. These medications interfere with the IL-17 pathway, which is essential for maintaining mucosal barriers and coordinating the immune response against Candida species. The observational data and clinical trials demonstrate the increased incidence of candidiasis in individuals treated with IL-17 inhibitors. Brodalumab and bimekizumab pose a greater risk than secukinumab in eliciting candidiasis, whereas the data regarding ixekizumab are equivocal. Higher doses and prolonged treatment duration of IL-17 inhibitors increase the risk of candidiasis by compromising the immune response against Candida species. Prior to prescribing IL-17 inhibitors, healthcare professionals should comprehensively evaluate patients' medical histories and assess their risk factors. Patients should be educated on the signs and symptoms of candidiasis to facilitate early detection and intervention. Future research should focus on identifying the risk factors associated with candidiasis in patients receiving IL-17 inhibitors. Prospective studies and long-term surveillance are required to explore the impact of specific inhibitors on the incidence and severity of candidiasis and to evaluate the effectiveness of combination therapies, such as concurrent use of IL-17 inhibitors and prophylactic antifungal agents.

Targeting Th17 cells: a promising strategy to treat oral mucosal inflammatory diseases

With the improved quality of life, oral health is under increased pressure. Numerous common oral mucosal diseases, such as oral lichen planus(OLP) and gingivitis, are related to the destruction of the oral immune barrier. The cytokines secreted by T-helper 17 (Th17) cells are essential for maintaining oral immune homeostasis and play essential roles in immune surveillance. When antigens stimulate the epithelium, Th17 cells expand, differentiate, and generate inflammatory factors to recruit other lymphocytes, such as neutrophils, to clear the infection, which helps to maintain the integrity of the epithelial barrier. In contrast, excessive Th17/IL-17 axis reactions may cause autoimmune damage. Therefore, an in-depth understanding of the role of Th17 cells in oral mucosa may provide prospects for treating oral mucosal diseases. We reviewed the role of Th17 cells in various oral and skin mucosal systemic diseases with oral characteristics, and based on the findings of these reports, we emphasize that Th17 cellular response may be a critical factor in inflammatory diseases of the oral mucosa. In addition, we should pay attention to the role and relationship of "pathogenic Th17" and "non-pathogenic Th17" in oral mucosal diseases. We hope to provide a reference for Th17 cells as a potential therapeutic target for treating oral mucosal inflammatory disorders in the future.