Recognition of Candida albicans and Role of Innate Type 17 Immunity in Oral Candidiasis

Oral Microbiota Alterations in Subjects with SARS-CoV-2 Displaying Prevalence of the Opportunistic Fungal Pathogen Candida albicans

The oral cavity remains an underappreciated site for SARS-CoV-2 infection despite the myriad of oral conditions in COVID-19 patients. Recently, SARS-CoV-2 was shown to replicate in the salivary gland cells causing tissue inflammation. Given the established association between inflammation and microbiome disruption, we comparatively profiled oral microbial differences at a metagenomic level in a cohort of hospitalized COVID-19 patients and matched healthy controls. Specifically, we aimed to evaluate colonization by the opportunistic fungal pathogen Candida albicans, the etiologic agent of oral candidiasis. Comprehensive shotgun metagenomic analysis indicated that, overall, COVID-19 patients exhibited significantly reduced bacterial and viral diversity/richness; we identified 12 differentially abundant bacterial species to be negatively associated with COVID-19, and the functional pathways of certain bacteria to be highly associated with COVID-19 status. Strikingly, C. albicans was recovered from approximately half of the COVID-19 subjects but not from any of the healthy controls. The prevalence of Candida is likely linked to immune hypo-dysregulation caused by COVID-19 favoring Candida proliferation, warranting investigations into the interplay between Candida and SARS-CoV2 and potential therapeutic approaches directed toward oral candidiasis. Collectively, our findings prompt a reassessment of oral opportunistic infection risks during COVID-19 disease and their potential long-term impacts on oral health.

SARS-CoV-2 Infection of Salivary Glands Compromises Oral Antifungal Innate Immunity and Predisposes to Oral Candidiasis

Saliva contains antimicrobial peptides considered integral components of host innate immunity, and crucial for protection against colonizing microbial species. Most notable is histatin-5 which is exclusively produced in salivary glands with uniquely potent antifungal activity against the opportunistic pathogen Candida albicans. Recently, SARS-CoV-2 was shown to replicate in salivary gland acinar cells eliciting local immune cell activation. In this study, we performed mechanistic and clinical studies to investigate the implications of SARS-CoV-2 infection on salivary histatin-5 production and Candida colonization. Bulk RNA-sequencing of parotid salivary glands from COVID-19 autopsies demonstrated statistically significant decreased expression of histatin genes. In situ hybridization, coupled with immunofluorescence for co-localization of SARS-CoV-2 spike and histatin in salivary gland cells, showed that histatin was absent or minimally present in acinar cells with replicating viruses. To investigate the clinical implications of these findings, salivary histatin-5 levels and oral Candida burden in saliva samples from three independent cohorts of mild and severe COVID-19 patients and matched healthy controls were evaluated. Results revealed significantly reduced histatin-5 in SARS-CoV-2 infected subjects, concomitant with enhanced prevalence of C. albicans. Analysis of prospectively recovered samples indicated that the decrease in histatin-5 is likely reversible in mild-moderate disease as concentrations tended to increase during the post-acute phase. Importantly, salivary cytokine profiling demonstrated correlations between activation of the Th17 inflammatory pathway, changes in histatin-5 concentrations, and subsequent clearance of C. albicans in a heavily colonized subject. The importance of salivary histatin-5 in controlling the proliferation of C. albicans was demonstrated using an ex vivo assay where C. albicans was able to proliferate in COVID-19 saliva with low histatin-5, but not with high histatin-5. Taken together, the findings from this study provide direct evidence implicating SARS-CoV-2 infection of salivary glands with compromised oral innate immunity, and potential predisposition to oral candidiasis.

Implications of innate lymphoid cells in oral diseases

Innate lymphoid cells (ILCs), as newly discovered antigen-independent innate immune cells, respond promptly to stimuli by secreting effector cytokines to exert effector functions similar to those of T cells. ILCs predominantly reside at mucosal sites and play critical roles in defending against infections, maintaining mucosal homeostasis, regulating inflammatory and immune responses, and participating in tumorigenesis. Recently, there has been a growing interest in the role of ILCs in oral diseases. This review outlines the classifications and the major characteristics of ILCs, and then comprehensively expatiates the research on ILCs in oral cancer, primary Sjogren's syndrome, periodontal diseases, oral lichen planus, oral candidiasis, Behcet's disease, and pemphigus vulgaris, aiming at summarising the implications of ILCs in oral diseases and providing new ideas for further research.

γδ T cells in oral diseases

OBJECTIVE

γδ T cells are a distinct subset of unconventional T cells, which link innate and adaptive immunity by secreting cytokines and interacting with other immune cells, thereby modulating immune responses. As the first line of host defense, γδ T cells are essential for mucosal homeostasis and immune surveillance. When abnormally activated or impaired, γδ T cells can contribute to pathogenic processes. Accumulating evidence has revealed substantial impacts of γδ T cells on the pathogenesis of cancers, infections, and immune-inflammatory diseases. γδ T cells exhibit dual roles in cancers, promoting or inhibiting tumor growth, depending on their phenotypes and the clinical stage of cancers. During infections, γδ T cells exert high cytotoxic activity in infectious diseases, which is essential for combating bacterial and viral infections by recognizing foreign antigens and activating other immune cells. γδ T cells are also implicated in the onset and progression of immune-inflammatory diseases. However, the specific involvement and underlying mechanisms of γδ T cells in oral diseases have not been systematically discussed.

METHODS

We conducted a systematic literature review using the PubMed/MEDLINE databases to identify and analyze relevant literature on the roles of γδ T cells in oral diseases.

RESULTS

The literature review revealed that γδ T cells play a pivotal role in maintaining oral mucosal homeostasis and are involved in the pathogenesis of oral cancers, periodontal diseases, graft-versus-host disease (GVHD), oral lichen planus (OLP), and oral candidiasis. γδ T cells mainly influence various pathophysiological processes, such as anti-tumor activity, eradication of infection, and immune response regulation.

CONCLUSION

This review focuses on the involvement of γδ T cells in oral diseases, with a particular emphasis on the main functions and underlying mechanisms by which γδ T cells influence the pathogenesis and progression of these conditions. This review underscores the potential of γδ T cells as therapeutic targets in managing oral health issues.

Ethyl caffeate combined with fluconazole exhibits efficacy against azole-resistant oropharyngeal candidiasis via the EFGR/JNK/c-JUN signaling pathway

Ethyl caffeate (EC) is a phenylpropanoid compound derived from Elephantopus scaber. In our previous work, EC was investigated to have a strong synergistic antifungal effect against azole-resistant strains of Candida albicans when combined with fluconazole (FLU). However, the protective effect and mechanism of EC + FLU on oropharyngeal candidiasis (OPC) caused by drug-resistant strains of C. albicans have not been investigated. This study aimed to investigate the protective effect and mechanism of EC combined with FLU against C. albicans-resistant strains that lead to OPC. An OPC mouse model revealed that EC + FLU treatment reduced fungal load and massive hyphal invasion of tongue tissues, and ameliorated the integrity of the tongue mucosa. Periodic acid-Schiff staining results showed more structural integrity of the tongue tissues and reduced inflammatory cell infiltration after EC + FLU treatment. Phosphorylation of EGFR (epidermal growth factor receptor) and other proteins in the EFGR/JNK (c-Jun N-terminal kinase)/c-JUN (transcription factor Jun) signaling pathway was significantly downregulated by EC + FLU. EGFR and S100A9 mRNA expression were also reduced. The above results were verified in FaDu cells. ELISA results showed that the concentration of inflammatory factors in the cell supernatant was significantly reduced after EC combined with FLU treatment. Molecular docking revealed that EC exhibited high binding energy to EGFR. In conclusion, EC enhances the susceptibility of azole-resistant C. albicans to FLU, and the underlying mechanism is related to the inhibition of the EGFR/JNK/c-JUN signaling pathway. This result suggests that EC has potential to be developed as an antifungal sensitizer to treat OPC caused by azole-resistant C. albicans.

Astragalus regulates the intestinal immune response during sepsis by mediating ILC3 proliferation through RORγt

Background

Sepsis is a common complication of many diseases and is associated with high morbidity and mortality rates. Astragalus can improve humoral and innate immunity, inhibit inflammatory responses, and protect immune cells and organs from damage. However, to the best of our knowledge there are no reports on whether astragalus can regulate intestinal innate immune function during sepsis.

Methods

In this study, a rat cecal ligation and puncture model of sepsis was used to investigate the effects of astragalus treatment, following which the apoptosis rate of lymphocytes from Peyer's patches (PP) was determined. Type 3 innate lymphoid cells (ILC3) were cultured in vitro to further evaluate the effects and mechanisms of astragalus.

Results

The apoptosis level of lymphocytes from PP in rats with sepsis was significantly increased, and the number of ILC3 was significantly reduced, compared with the sham operation group, which aggravated intestinal injury and ultimately led to the death of rats. Astragalus treatment significantly inhibited the apoptosis of lymphocytes from PP, increased the number of ILC3, and improved the intestinal inflammatory environment compared to the sepsis group. RT-PCR revealed that astragalus and the retinoic acid-related orphan receptor γt (RORγt) agonist LYC-55716 both promote the expression of interleukin (IL)-17A, IL-17F, IL-22, interferon-γ, and granulocyte-macrophage colony-stimulating factor mRNA. Mechanistically, astragalus promotes the proliferation of ILC3 through RORγt, thereby reducing intestinal inflammatory damage.

Conclusion

Astragalus, via RORγt, promotes the generation of ILC3, improves the inflammatory environment in rats with sepsis.

The effect of herbal medicine in innate immunity to Candida albicans

Candida albicans (C. albicans) is an opportunistic pathogenic fungus that often causes mucosal and systemic infections. Several pattern recognition receptors (PRRs), such as Toll-like receptors (TLRs) and C-type lectin receptors (CLRs), have been implicated in the host recognition of C. albicans. These PRRs recognize the pathogen-associated molecular patterns (PAMPs) of C. albicans to activate innate immune cells, thereby rapidly inducing various inflammatory responses by activating intracellular signaling cascades. Herbal medicine and its active components deserve priority development due to their low toxicity and high antibacterial, antiviral and antifungal activities. This review discussed the activities of herbal compounds against C. albicans and their related mechanisms, especially their regulatory role on innate immune cells such as neutrophils, macrophages, and dendritic cells (DCs) implicated in C. albicans infections. Our work aims to find new therapeutic drugs and targets to prevent and treat diseases caused by C. albicans infection with the mechanisms by which this fungus interacts with the innate immune response.

The interaction between innate immunity and oral microbiota in oral diseases

INTRODUCTION

Innate immunity serves as the frontline to combat invading pathogens. Oral microbiota is the total collection of microorganisms colonized within the oral cavity. By recognizing the resident microorganisms through pattern recognition receptors, innate immunity is capable of interacting with oral microbiota and maintaining homeostasis. Dysregulation of interaction may lead to the pathogenesis of several oral diseases. Decoding the crosstalk between oral microbiota and innate immunity may be contributory to developing novel therapies for preventing and treating oral diseases.

AREAS COVERED

This article reviewed pattern recognition receptors in the recognition of oral microbiota, the reciprocal interaction between innate immunity and oral microbiota, and discussed how the dysregulation of this relationship leads to the pathogenesis and development of oral diseases.

EXPERT OPINION

Many studies have been conducted to illustrate the relationship between oral microbiota and innate immunity and its role in the occurrence of different oral diseases. The impact and mechanisms of innate immune cells on oral microbiota and the mechanisms of dysbiotic microbiota in altering innate immunity are still needed to be investigated. Altering the oral microbiota might be a possible solution for treating and preventing oral diseases.

The role of group 3 innate lymphoid cell in intestinal disease

Group 3 innate lymphoid cells (ILC3s), a novel subpopulation of lymphocytes enriched in the intestinal mucosa, are currently considered as key sentinels in maintaining intestinal immune homeostasis. ILC3s can secrete a series of cytokines such as IL-22 to eliminate intestinal luminal antigens, promote epithelial tissue repair and mucosal barrier integrity, and regulate intestinal immunity by integrating multiple signals from the environment and the host. However, ILC3 dysfunction may be associated with the development and progression of various diseases in the gut. Therefore, in this review, we will discuss the role of ILC3 in intestinal diseases such as enteric infectious diseases, intestinal inflammation, and tumors, with a focus on recent research advances and discoveries to explore potential therapeutic targets.

Immunological effects of anti-IL-17/12/23 therapy in patients with psoriasis complicated by Candida infections

Biologics that block the T-helper-17 pathway are very effective in the treatment of psoriasis and other inflammatory diseases. However, interleukin-17 is also crucial for antifungal host defense, and clinical trial data suggest an increase in the incidence of Candida infections during IL-17 inhibitor (IL-17i) therapy. We investigated the innate and adaptive immune responses of psoriasis patients with a history of skin and/or mucosal candidiasis during IL-17i or IL-12/23i therapy, comparing those responses to healthy controls. Psoriasis patients with IL-17i showed significantly lower CD4+Th1-like (CCR6^-CXCR3⁺CCR4^-) and Th1Th17-like (CD4⁺CCR6⁺CXCR3⁺CCR4^-) cell percentages. Patient cells stimulated with Candida albicans produced significantly lower IL-6 in the IL-12/23i group and IL-1β production in the IL-17i group, while the release of TNF-α and ROS was similar between patients and controls. IFN-γ and IL-10 production in response to several stimuli after 7 days was particularly decreased in patients receiving IL-17i therapy. Finally, after stimulation with the polarizing cytokines IL-1β and IL-23, the Th17 cytokine response was significantly lower in the IL-17i patient group. These innate and adaptive immune response defects can diminish antifungal host immune response and thereby increase susceptibility to candidiasis in patients treated with IL-17i or IL-12/23i.

Single-Cell RNA Sequencing Analysis for Oncogenic Mechanisms Underlying Oral Squamous Cell Carcinoma Carcinogenesis with Candida albicans Infection

Oral squamous cell carcinoma (OSCC) carcinogenesis involves heterogeneous tumor cells, and the tumor microenvironment (TME) is highly complex with many different cell types. Cancer cell-TME interactions are crucial in OSCC progression. Candida albicans (C. albicans)-frequently pre-sent in the oral potentially malignant disorder (OPMD) lesions and OSCC tissues-promotes malignant transformation. The aim of the study is to verify the mechanisms underlying OSCC car-cinogenesis with C. albicans infection and identify the biomarker for the early detection of OSCC and as the treatment target. The single-cell RNA sequencing analysis (scRNA-seq) was performed to explore the cell subtypes in normal oral mucosa, OPMD, and OSCC tissues. The cell composi-tion changes and oncogenic mechanisms underlying OSCC carcinogenesis with C. albicans infec-tion were investigated. Gene Set Variation Analysis (GSVA) was used to survey the mechanisms underlying OSCC carcinogenesis with and without C. albicans infection. The results revealed spe-cific cell clusters contributing to OSCC carcinogenesis with and without C. albicans infection. The major mechanisms involved in OSCC carcinogenesis without C. albicans infection are the IL2/STAT5, TNFα/NFκB, and TGFβ signaling pathways, whereas those involved in OSCC carcinogenesis with C. albicans infection are the KRAS signaling pathway and E2F target down-stream genes. Finally, stratifin (SFN) was validated to be a specific biomarker of OSCC with C. albicans infection. Thus, the detailed mechanism underlying OSCC carcinogenesis with C. albicans infection was determined and identified the treatment biomarker with potential precision medicine applications.

The Role of the Oral Immune System in Oropharyngeal Candidiasis-Facilitated Invasion and Dissemination of Staphylococcus aureus

Candida albicans and Staphylococcus aureus account for most invasive fungal and bacterial bloodstream infections (BSIs), respectively. However, the initial point of invasion responsible for S. aureus BSIs is often unclear. Recently, C. albicans has been proposed to mediate S. aureus invasion of immunocompromised hosts during co-colonization of oral mucosal surfaces. The status of the oral immune system crucially contributes to this process in two distinct ways: firstly, by allowing invasive C. albicans growth during dysfunction of extra-epithelial immunity, and secondly following invasion by some remaining function of intra-epithelial immunity. Immunocompromised individuals at risk of developing invasive oral C. albicans infections could, therefore, also be at risk of contracting concordant S. aureus BSIs. Considering the crucial contribution of both oral immune function and dysfunction, the aim of this review is to provide an overview of relevant aspects of intra and extra-epithelial oral immunity and discuss predominant immune deficiencies expected to facilitate C. albicans induced S. aureus BSIs.

Type 3 Innate Lymphoid Cells as Regulators of the Host-Pathogen Interaction

Type 3 Innate lymphoid cells (ILC3s) have been described as tissue-resident cells and characterized throughout the body, especially in mucosal sites and classical first barrier organs such as skin, gut and lungs, among others. A significant part of the research has focused on their role in combating pathogens, mainly extracellular pathogens, with the gut as the principal organ. However, some recent discoveries in the field have unveiled their activity in other organs, combating intracellular pathogens and as part of the response to viruses. In this review we have compiled the latest studies on the role of ILC3s and the molecular mechanisms involved in defending against different microbes at the mucosal surface, most of these studies have made use of conditional transgenic mice. The present review therefore attempts to provide an overview of the function of ILC3s in infections throughout the body, focusing on their specific activity in different organs.

Immune Sensing of Candida albicans

Candida albicans infections range from superficial to systemic and are one of the leading causes of fungus-associated nosocomial infections. The innate immune responses during these various infection types differ, suggesting that the host environment plays a key role in modulating the host–pathogen interaction. In addition, C. albicans is able to remodel its cell wall in response to environmental conditions to evade host clearance mechanisms and establish infection in niches, such as the oral and vaginal mucosa. Phagocytes play a key role in clearing C. albicans, which is primarily mediated by Pathogen Associated Molecular Pattern (PAMP)–Pattern Recognition Receptor (PRR) interactions. PRRs such as Dectin-1, DC-SIGN, and TLR2 and TLR4 interact with PAMPs such as β-glucans, N-mannan and O-mannan, respectively, to trigger the activation of innate immune cells. Innate immune cells exhibit distinct yet overlapping repertoires of PAMPs, resulting in the preferential recognition of particular Candida morphotypes by them. The role of phagocytes in the context of individual infection types also differs, with neutrophils playing a prominent role in kidney infections, and dendritic cells playing a prominent role in skin infections. In this review, we provide an overview of the key receptors involved in the detection of C. albicans and discuss the differential innate immune responses to C. albicans seen in different infection types such as vulvovaginal candidiasis (VVC) and oral candidiasis.

Microbial interactions and immunity response in oral Candida species

Oral candidiasis are among the most common noncommunicable diseases, related with serious local and systemic illnesses. Although these infections can occur in all kinds of patients, they are more recurrent in immunosuppressed ones such as patients with HIV, hepatitis, cancer or under long antimicrobial treatments. Candida albicans continues to be the most frequently identified Candida spp. in these disorders, but other non-C. albicans Candida are rising. Understanding the immune responses involved in oral Candida spp. infections is a key feature to a successful treatment and to the design of novel therapies. In this review, we performed a literature search in PubMed and WoS, in order to examine and analyze common oral Candida spp.-bacteria/Candida-Candida interactions and the host immunity response in oral candidiasis.