# The combinatorial action of hyphal growth and candidalysin is critical for promoting Candida albicans oropharyngeal infection

## TL;DR

This study shows that hyphal growth and candidalysin work together to cause Candida albicans infections in the mouth, and targeting these could lead to new treatments.

## Contribution

The study reveals the cooperative role of hyphal growth and candidalysin in C. albicans oropharyngeal infection using a combinatorial gene deletion approach.

## Key findings

- Deleting HGC1 and ECE1 genes led to 100% survival in mice, compared to 15% with wild-type C. albicans.
- Hyphal growth and candidalysin are critical for fungal burden, immune response, and mortality in oropharyngeal infection.
- Combination mutants of HGC1 and ECE1 showed no disease progression in the mouse model.

## Abstract

Candida albicans is one of the most common fungal pathogens, yet much remains unknown about how its virulence factors cooperate to promote pathogenicity. To investigate this, CRISPR-Cas9 technology was used to create a panel of 19 single, double, triple, and quadruple deletion mutant strains targeting four established virulence factors: ALS3 (adhesin/invasin), ECE1 (candidalysin toxin), HGC1 (hypha formation regulator), and SAP2 (protease). In vitro, the deletion of each gene had differing impacts across multiple characterization assays. The hgc1∆/∆ mutant was unable to form hyphae under inducing conditions, leading to downstream impairment of epithelial invasion. The als3∆/∆ mutant exhibited significantly reduced adhesion and invasion into epithelial cells, resulting in attenuated cellular damage. The ece1∆/∆ mutant displayed significantly reduced epithelial damage, cell signaling, and immune activation. The phenotype of the sap2∆/∆ mutant resembled that of wild type but was unable to degrade protein. In an immunocompromised murine model of oropharyngeal infection, hyphal growth and candidalysin production were the dominant drivers of elevated fungal burden, innate immune responses, and mortality. Following a 5-day infection with hgc1∆/∆ and ece1∆/∆ single gene deletion strains, mice had survival rates of 100% and 80%, respectively, compared to 15% in wild-type infected mice. Notably, 100% survival was also observed following challenge with all hgc1∆/∆ and ece1∆/∆ combination mutants. This study demonstrates that specific C. albicans virulence attributes act in combination to promote mucosal infection, with hyphal growth and candidalysin production being a critical driver of oropharyngeal infection.

Candida albicans has been classified by the WHO as a “critical priority” pathogen, highlighting the urgent need for a greater understanding of the mechanisms that enable it to cause disease. C. albicans possesses numerous virulence attributes, but how they synergize during infection is not well understood. Here, using reverse genetics, we dissect the individual and combinatorial roles of four C. albicans virulence factors (Als3p, candidalysin, hyphal growth, and Sap2p) in vitro and in an in vivo murine model of oropharyngeal candidiasis. Increasing the number of C. albicans gene deletions correlated with reduced oral fungal burden, with hyphal growth and candidalysin together being critical for infection, inflammation, and mortality during oropharyngeal infection. These findings demonstrate that virulence attributes act cooperatively as a collective network to promote pathogenicity, a finding also observed in plant fungal pathogens. Our approach has identified specific fungal virulence factors that can be targeted for new treatment strategies against C. albicans infections.