Trafficking of Candida albicans through oral epithelial endocytic compartments

Subversion strategies of lysosomal killing by intracellular pathogens

Many pathogenic organisms need to reach either an intracellular compartment or the cytoplasm of a target cell for their survival, replication or immune system evasion. Intracellular pathogens frequently penetrate into the cell through the endocytic and phagocytic pathways (clathrin-mediated endocytosis, phagocytosis and macropinocytosis) that culminates in fusion with lysosomes. However, several mechanisms are triggered by pathogenic microorganisms - protozoan, bacteria, virus and fungus - to avoid destruction by lysosome fusion, such as rupture of the phagosome and thereby release into the cytoplasm, avoidance of autophagy, delaying in both phagolysosome biogenesis and phagosomal maturation and survival/replication inside the phagolysosome. Here we reviewed the main data dealing with phagosome maturation and evasion from lysosomal killing by different bacteria, protozoa, fungi and virus.

Perspective on receptor-associated immune response to Candida albicans single and mixed infections: Implications for therapeutics in oropharyngeal candidiasis

Oropharyngeal candidiasis (OPC), commonly known as 'thrush', is an oral infection that usually dismantles oral mucosal integrity and malfunctions local innate and adaptive immunities in compromised individuals. The major pathogen responsible for the occurrence and progression of OPC is the dimorphic opportunistic commensal Candida albicans. However, the incidence induced by non-albicans Candida species including C. glabrata, C. tropicalis, C. dubliniensis, C. parapsilosis, and C. krusei are increasing in company with several oral bacteria, such as Streptococcus mutans, S. gordonii, S. epidermidis, and S. aureus. In this review, the microbiological and infection features of C. albicans and its co-contributors in the pathogenesis of OPC are outlined. Since the invasion and concomitant immune response lie firstly on the recognition of oral pathogens through diverse cellular surface receptors, we subsequently emphasize the roles of epidermal growth factor receptor, ephrin-type receptor 2, human epidermal growth factor receptor 2, and aryl hydrocarbon receptor located on oral epithelial cells to delineate the underlying mechanism by which host immune recognition to oral pathogens is mediated. Based on these observations, the therapeutic approaches to OPC comprising conventional and non-conventional antifungal agents, fungal vaccines, cytokine and antibody therapies, and antimicrobial peptide therapy are finally overviewed. In the face of newly emerging life-threatening microbes (C. auris and SARS-CoV-2), risks (biofilm formation and interconnected translocation among diverse organs), and complicated clinical settings (HIV and oropharyngeal cancer), the research on OPC is still a challenging task.

Fungal invasion of epithelial cells

Interaction between host cells and invasive Candida plays a large role in the pathogenicity of Candida species. Fungal-induced endocytosis and active penetration are the two distinct, yet complementary invasion mechanisms of invasive candidiasis. Induced endocytosis is a microorganism-triggered, epithelial-driven, clathrin-mediated and actin-dependent process. During the fundamental pathological process of induced endocytosis, invasins (Als3 and Ssa1), which mediate the binding of host epithelial surface proteins, are expressed by Candida species on the hyphal surface. Sequentially, the interaction between invasins and host epithelial surface proteins stimulates the recruitment of clathrin, dynamin and cortactin to the sites where Candida enters epithelial cells, which in turn induce the actin cytoskeleton reorganization. Actin cytoskeleton provides the force required for fungal internalization. Parallely, active penetration of Candida can directly pass through epithelial cells possibly due to progressive elongation of hyphae and physical forces. Several molecules, such as secreted hydrolases and Als3, can affect the protective barrier of the epithelium and make Candida actively penetrate into epithelial cells through intercellular gaps of epithelial layers.

Early Endosome Antigen 1 (EEA1) decreases in macrophages infected with Paracoccidioides brasiliensis

Paracoccidioidomycosis (PCM) is a chronic granulomatous disease caused by the dimorphic fungus Paracoccidioides brasiliensis, endemic in Latin America. P. brasiliensis has been observed in epithelial cells in vivo and in vitro, as well as within the macrophages. The identification of the mechanism by which it survives within the host cell is fertile ground for the discovery of its pathogenesis since this organism has the ability to induce its own endocytosis in epithelial cells and most likely in macrophages. The study of the expression of endocytic proteins pathway and co-localization of microorganisms enable detection of the mechanism by which microorganisms survive within the host cell. The aim of this study was to evaluate the expression of the endocytic protein EEA1 (early endosome antigen 1) in macrophages infected with P. brasiliensis. For detection of EEA1, three different techniques were employed: immunofluorescence, real-time polymerase chain reaction (PCR) and immunoblotting. In the present study, decreased expression of EEA1 as well as the rearrangement of the actin was observed when the fungus was internalized, confirming that the input mechanism of the fungus in macrophages occurs through phagocytosis.

Candida albicans interactions with epithelial cells and mucosal immunity

Candida albicans interactions with epithelial cells are critical for commensal growth, fungal pathogenicity and host defence. This review will outline our current understanding of C. albicans-epithelial interactions and will discuss how this may lead to the induction of a protective mucosal immune response.

The interaction between Candida krusei and murine macrophages results in multiple outcomes, including intracellular survival and escape from killing

Candida krusei is a fungal pathogen of interest for the scientific community for its intrinsic resistance to fluconazole. Little is known about the interaction of this yeast with host immune cells. In this work, we have characterized the outcome of the interaction between C. krusei and murine macrophages. Once C. krusei was internalized, we observed different phenomena. In a macrophage-like cell line, C. krusei survived in a significant number of macrophages and induced filamentation and macrophage explosion. Phagocytosis of C. krusei led to actin polymerization around the yeast cells at the site of entry. Fluorescent specific staining with anti-Lamp1 and LysoTracker indicated that after fungal internalization, there was a phagolysosome maturation defect, a phenomenon that was more efficient when the macrophages phagocytosed killed yeast cells. Using cell line macrophages, we also observed macrophage fusion after cell division. When we used primary resident peritoneal macrophages in addition to macrophage explosion, we also observed a strong chemotaxis of uninfected macrophages to regions where C. krusei-infected macrophages were present. We also noticed yeast transfer phenomena between infected macrophages. Primary macrophages inhibited pseudohypha elongation more efficiently than the macrophage-like cell line, suggesting that C. krusei infection was better controlled by the former macrophages. Primary macrophages induced more tumor necrosis factor alpha (TNF-α) and interleukin-6 (IL-6) than the macrophage-like cell line. Our results demonstrate that C. krusei can exploit the macrophages for replication, although other different outcomes are also possible, indicating that the interaction of this pathogen with phagocytic cells is very complex and regulated by multiple factors.