Molecular Insights Into Candida albicans Commensalism and Pathogenesis: Exploring the Role of Immune Regionalization in the Murine Stomach

Abstract

The polymorphic fungus Candida albicans is a ubiquitous member of the human microbiome with a remarkable ability to adapt to diverse host environments. C. albicans colonization has even been documented in the stomach, most microbial growth is restricted by an extremely low pH and proteolytic activity. Though integral to early digestion and pathogen defense, the role of the stomach and gastric microbiota in the development of gastrointestinal diseases and chronic infection has been largely overlooked. Our laboratory and others have demonstrated that C. albicans can robustly colonize and cause gastritis in the limiting ridge of the murine stomach – a mucosal fold separating the forestomach from the glandular stomach. However, the existing model to describe the interplay between C. albicans and the gastric mucosa is built off of the assumption that the murine stomach operates as a unified entity with homogeneity in its immune regulation and activity. We hypothesized that mucosal immunity in the murine stomach is regionalized, leading to distinct patterns of commensal or pathogenic colonization. Thus, the primary objective of this research was to evaluate the mucosal responses to C. albicans in different anatomical areas of the murine stomach. First, we investigated the impact of C. albicans colonization on mucosal responses in the glandular stomach, where we do not observe fungal invasion or inflammation, using a custom, high-throughput qPCR assay. As expected, C. albicans did not induce a strong inflammatory response, though there was upregulation of Il18, calprotectin (S100a8 and S100a9), and a number of antimicrobial peptides. We were additionally interested in determining whether the glandular stomach is a site for food allergy induction, and if allergic inflammation would alter the mucosal response to C. albicans. Our model of experimental oral antigen-induced food allergy resulted in a robust type 2 immune response and significant tissue remodeling in the glandular stomach. Interestingly, C. albicans did not exacerbate this response, though the underlying food allergy augmented the expression of Il1b, Il12a, Tnf, indicating an activation of innate and type 3 immunity against C. albicans. Next, we focused our analysis on the limiting ridge, a region prone to fungal infection and inflammation. C. albicans elicited a strong type 3 immune response, characterized by increased expression of Il17a, Il17f, Il1b, Tnf, and Il36g. This response was accompanied by widespread chemokine induction and a substantial influx of neutrophils, monocytes/macrophages, and eosinophils, leading to extensive tissue damage and remodeling. Notably, colonization and subsequent gastritis required transient amoxicillin-mediated dysbiosis, underscoring the role of the microbiota in inhibiting fungal pathogenesis. Complementing these studies, we examined the transcriptional adaptations of C. albicans to anaerobic environments – crucial for its survival in a host. RNA sequencing of anaerobically and aerobically cultivated C. albicans revealed a strong induction of the alternative oxidase Aox2 and genes encoding mitochondrial enzyme complexes, reflecting a shift to non-oxidative respiration. This coincided with the downregulation of genes associated with cell growth, metabolism, cell wall remodeling, and virulence. These findings were replicated in two different strains of C. albicans, highlighting a species-conserved adaptative response to anaerobic environments. In summary, we are the first to report that the mucosal response to C. albicans in the murine stomach is regionalized, influencing its varied lifestyles within a host, as well as potential mechanisms by which C. albicans is able to adapt to oxygen-deprived environments like the gastrointestinal tract.