Secretory component mediates Candida albicans binding to epithelial cells

Extracellular vesicle production: A bidirectional effect in the interplay between host and Candida fungi

Candida fungi exploit various virulence strategies to invade the human host, while host cells employ diverse mechanisms to maintain homeostasis and respond to infection. Extracellular vesicles (EVs) are integral components of the multifaceted landscape of host-pathogen interactions, with their abundant production by all contributors involved in these complex and dynamic relations. Herein, we present the current state of knowledge regarding the host response by releasing EVs in reaction to Candida, as well as the influence of fungal EVs on the functionality of the confronted host cells. Fungal vesicles contribute to enhanced adhesion of pathogens to human cells as evidenced for C. auris, and may modulate the production of several cytokines, including IL-6, IL-8, IL-10, IL-12p40, TGF-β and TNF-α, thereby exerting pro-infective and pro-inflammatory effects, as described for C. albicans and other Candida species. Whereas the biosynthesis of EVs by host cells can dynamically modulate the proliferation and viability of fungal cells and affect the candidacidal functionality of other effector cells. The reciprocal influence of EVs from host cells and Candida pathogens is a key focus, explaining their significant role in cell signaling and interkingdom communication.

Surface-expressed phosphoglycerate mutase of Candida albicans binds to salivary DMBT1

Candida albicans colonizes oral tissues and causes infectious diseases. Colonization of C. albicans on the oral mucosa and tooth enamel surfaces is established via the interaction between C. albicans adhesins and salivary proteins, forming a film on the oral tissues. Deleted in malignant brain tumors 1 (DMBT1), also known as salivary agglutinin or gp-340, belongs to the scavenger receptor cysteine-rich (SRCR) superfamily. In the oral cavity, immobilized DMBT1 on oral tissues causes microbial adherence. Recently, we demonstrated that C. albicans binds to DMBT1 and isolated a 25-kDa C. albicans adhesin involved in the interaction with the binding domain of DMBT1, namely, SRCRP2. In the present study, we searched for additional DMBT1-binding adhesins in C. albicans. The component isolated here had a molecular mass of 29 kDa and was found to be phosphoglycerate mutase (Gpm1). Isolated Gpm1 inhibited C. albicans binding to SRCRP2 and directly bound to SRCRP2 in a dose-dependent manner. Gpm1 localization on the C. albicans cell wall surface was confirmed by immunostaining. These results suggest that surface-expressed Gpm1 functions as an adhesin for the establishment of C. albicans cells on the oral mucosa and tooth enamel by binding to DMBT1.

A novel mannose-containing sialoprotein adhesin involved in the binding of Candida albicans cells to DMBT1

Candida albicans colonizes the oral cavity and causes oral candidiasis and early childhood caries synergistically with cariogenic Streptococcus mutans. Colonization of oral tissues with C. albicans is an essential step in the initiation of these infectious diseases. Deleted in malignant brain tumors 1 (DMBT1), also known as salivary agglutinin or gp-340, belongs to the scavenger receptor cysteine-rich (SRCR) superfamily and has important functions in innate immunity. In the oral cavity, DMBT1 causes microbial adherence to tooth enamel and oral mucosa surfaces, but the adherence of C. albicans to DMBT1 has not been examined. In this study, we investigated the binding of C. albicans to DMBT1 and isolated the fungal components responsible for the binding. Candida albicans specifically bound to DMBT1 and strongly bound to the peptide domain SRCRP2. Binding to SRCRP2 was inhibited by N-acetylneuraminic acid and mannose and by lectins recognizing these sugars. The isolated component had a molecular mass of 25 kDa, contained sialic acid and mannose residues, and inhibited C. albicans binding to SRCRP2. The localization of the 25-kDa protein on the surface of C. albicans cell walls was confirmed by immunostaining and a cell ELISA using an antiserum to the protein, and Western blotting revealed the presence of the 25-kDa protein in the cell wall fraction of C. albicans. These results suggest that the isolated adhesin is localized on the surface of C. albicans cell walls and that sialic acid and mannose residues in the adhesin play a significant role in the binding reaction.

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.

Oral Fungal Infections: Past, Present, and Future

Oral fungal infections have afflicted humans for millennia. Hippocrates (ca. 460-370 BCE) described two cases of oral aphthae associated with severe underlying diseases that could well have been oral candidiasis. While oral infections caused by other fungi such as cryptococcosis, aspergillosis, mucormycosis, histoplasmosis, blastomycosis, and coccidioidomycosis occur infrequently, oral candidiasis came to the fore during the AIDS epidemic as a sentinel opportunistic infection signaling the transition from HIV infection to AIDS. The incidence of candidiasis in immunocompromised AIDS patients highlighted the importance of host defenses in preventing oral fungal infections. A greater understanding of the nuances of human immune systems has revealed that mucosal immunity in the mouth delivers a unique response to fungal pathogens. Oral fungal infection does not depend solely on the fungus and the host, however, and attention has now focussed on interactions with other members of the oral microbiome. It is evident that there is inter-kingdom signaling that affects microbial pathogenicity. The last decade has seen significant advances in the rapid qualitative and quantitative analysis of oral microbiomes and in the simultaneous quantification of immune cells and cytokines. The time is ripe for the application of machine learning and artificial intelligence to integrate more refined analyses of oral microbiome composition (including fungi, bacteria, archaea, protozoa and viruses—including SARS-CoV-2 that causes COVID-19). This analysis should incorporate the quantification of immune cells, cytokines, and microbial cell signaling molecules with signs of oral fungal infections in order to better diagnose and predict susceptibility to oral fungal disease.

Role of Polymeric Immunoglobulin Receptor in IgA and IgM Transcytosis

Transcytosis of polymeric IgA and IgM from the basolateral surface to the apical side of the epithelium and subsequent secretion into mucosal fluids are mediated by the polymeric immunoglobulin receptor (pIgR). Secreted IgA and IgM have vital roles in mucosal immunity in response to pathogenic infections. Binding and recognition of polymeric IgA and IgM by pIgR require the joining chain (J chain), a small protein essential in the formation and stabilization of polymeric Ig structures. Recent studies have identified marginal zone B and B1 cell-specific protein (MZB1) as a novel regulator of polymeric IgA and IgM formation. MZB1 might facilitate IgA and IgM transcytosis by promoting the binding of J chain to Ig. In this review, we discuss the roles of pIgR in transcytosis of IgA and IgM, the roles of J chain in the formation of polymeric IgA and IgM and recognition by pIgR, and focus particularly on recent progress in understanding the roles of MZB1, a molecular chaperone protein.

Mediation of Mucosal Immunoglobulins in Buccal Cavity of Teleost in Antibacterial Immunity

The buccal mucosa (BM) of vertebrates is a critical mucosal barrier constantly exposed to rich and diverse pathogens from air, water, and food. While mammals are known to contain a mucosal associated lymphoid tissue (MALT) in the buccal cavity which induces B-cells and immunoglobulins (Igs) responses against bacterial pathogens, however, very little is known about the evolutionary roles of buccal MALT in immune defense. Here we developed a bath infection model that rainbow trout experimentally exposed to Flavobacterium columnare (F. columnare), which is well known as a mucosal pathogen. Using this model, we provided the first evidence for the process of bacterial invasion in the fish BM. Moreover, strong pathogen-specific IgT responses and accumulation of IgT⁺ B-cells were induced in the buccal mucus and BM of infected trout with F. columnare. In contrast, specific IgM responses were for the most part detected in the fish serum. More specifically, we showed that the local proliferation of IgT⁺ B-cells and production of pathogen-specific IgT within the BM upon bacterial infection. Overall, our findings represent the first demonstration that IgT is the main Ig isotype specialized for buccal immune responses against bacterial infection in a non-tetrapod species.

Effects of Candida albicans infection on defense effector secretion by human oral mucosal epithelial cells

OBJECTIVE

The aim of this study was to investigate the effects of Candida albicans on the production of defense effector molecules by human oral mucosal epithelial cells in vitro.

DESIGN

Immortalized human oral mucosal epithelial (Leuk-1) cells and C. albicans strain 5314 were cocultured at different cell-to-C. albicans ratios. The viability of Leuk-1 cells was determined by MTT and RTCA measurements. The secretory levels of multiple defense effector molecules were determined by Enzyme-linked immunosorbent assay (ELISA).

RESULTS

Our results indicated that C. albicans significantly decreased the secretion of IgG, cystatin C, lactoferrin, and TGF-β1 in a dose-dependent manner and remarkably reduced the production of IgA independent of the cell-to-C. albicans ratio. However, C. albicans clearly enhanced the secretion of IgM, galectin-3, P-selectin, granzyme B and perforin.

CONCLUSION

These results suggest that C. albicans may exert a regulatory role in the defense response of oral mucosal epithelial cells by altering secretory levels of defense effector molecules.

The Role of the Polymeric Immunoglobulin Receptor and Secretory Immunoglobulins during Mucosal Infection and Immunity

The gastrointestinal tract houses millions of microbes, and thus has evolved several host defense mechanisms to keep them at bay, and prevent their entry into the host. One such mucosal surface defense is the secretion of secretory immunoglobulins (SIg). Secretion of SIg depends on the polymeric immunoglobulin receptor (pIgR), which transports polymeric Ig (IgA or IgM) from the basolateral surface of the epithelium to the apical side. Upon reaching the luminal side, a portion of pIgR, called secretory component (SC) is cleaved off to release Ig, forming SIg. Through antigen-specific and non-specific binding, SIg can modulate microbial communities and pathogenic microbes via several mechanisms: agglutination and exclusion from the epithelial surface, neutralization, or via host immunity and complement activation. Given the crucial role of SIg as a microbial scavenger, some pathogens also evolved ways to modulate and utilize pIgR and SIg to facilitate infection. This review will cover the regulation of the pIgR/SIg cycle, mechanisms of SIg-mediated mucosal protection as well as pathogen utilization of SIg.

Elucidating the role of hyposalivation and autoimmunity in oral candidiasis

INTRODUCTION

Oral candidiasis (OC) is a potential oral complication in Sjögren's syndrome (SS). Some studies indicate that the low stimulated salivary flow and not low unstimulated salivary flow is associated with OC in SS, while others report that the underlying autoimmune disorders contribute to OC, based solely on correlation coefficients. Given the conflicting and limited existing evidence, we purposed to ascertain the role of both salivary gland dysfunction (hyposalivation based on unstimulated and stimulated flow rates) and autoimmunity (SS, other autoimmune disorders) in OC among those with SS, other salivary gland dysfunction, and non-salivary gland dysfunction controls (NSGD).

METHODS

A nested case-control study was designed within a larger NIH/NIDCR cohort. Descriptive analyses, nonparametric tests, comparative analyses, and multivariate logistic regression analyses were undertaken.

RESULTS

Data on 1526 subjects (701 SS, 247 ISS, 355 Sicca, and 223 NSGD) were obtained from the source cohort of 2046 and analyzed for this study. The median whole unstimulated salivary flow rate (WUS, ml 15 min^-1 ) was lower in SS (0.8, interquartile range (IQR) 1.8) compared to ISS (5.5, IQR: 5.2, P < 0.001) and NSGD (3.8, IQR: 3.8, P < 0.001) but comparable with that of Sicca (1.0, IQR: 1.5, P = 0.777) participants. The median total stimulated salivary flow rate (TSS, ml 15 min^-1 ) was lowest in SS (7.0, IQR: 12.4, P < 0.001) compared to other groups. Of the 45 OC cases in this cohort, 71.1% (n = 32) were from the SS group. The prevalence of OC was highest in the SS group (4.6%, P = 0.008). SS group had twice the risk of OC than NSGD (OR = 2.2, 95%CI: 1.1-4.2, P = 0.02) and Sicca (OR = 2.2, 95% CI: 1.0-4.8, P = 0.03), adjusting for confounders; hyposalivation [WUS (OR = 5.1, 95%CI: 2.5-10.4, P < 0.001), TSS (OR = 1.9, 95%CI: 1.0-3.5, P = 0.04)], history of other autoimmune disorders (OR = 4.4, 95%CI: 1.7-11.3, P = 0.002), medications for extraglandular manifestations (OR = 2.3, 95%CI: 1.1-4.9, P = 0.03), and diabetes mellitus (4.2, 95%CI: 1.2-15.2, P = 0.02) were independent predictors of OC; females had a lower risk than males (OR = 0.29, 95%CI: 0.13-0.67, P = 0.004). Age, race, anti-SSA/SSB autoantibodies, focus score, other medications, anxiety, fatigue, cigarette smoking, alcohol, and caffeine use were not associated with oral candidiasis.

CONCLUSION

Salivary gland dysfunction (hyposalivation with WUS being a stronger predictor than TSS) and autoimmunity (SS, other autoimmune disorders, medications, i.e., DMARDS) are both independent predictors of OC. Diabetes mellitus is an independent predictor of OC among those with salivary gland dysfunction. Our findings suggest that these independent predictors should be considered in the prevention and management of OC in this population.