Discovery of CO2 tolerance genes associated with virulence in the fungal pathogen Cryptococcus neoformans

Virulence factors released by extracellular vesicles from Cryptococcus neoformans

Cryptococcus neoformans, a prominent opportunistic pathogen, is equipped with unique mechanisms to evade host immune defenses, notably through its capsule and the secretion of extracellular vesicles (EVs). Despite significant understanding of its pathogenesis, the precise role of EVs in virulence and their molecular components remain underexplored. This review synthesizes current research on the virulence factors encapsulated within EVs of Cryptococcus, highlighting their contribution to fungal survival and pathogenicity. By analyzing the biochemical composition of these vesicles, we found the presence of enzymes (e.g., Urease, laccase), toxins (e.g. Melanin), and genes (e.g. Ssa1) associated with pathogenicity factors. Furthermore, we discuss the implications of these findings for developing therapeutic interventions. This work advances the field by providing a comprehensive overview of EV-mediated mechanisms in Cryptococcus, offering new insights into potential targets for antifungal strategies.

Microevolution of Cryptococcus neoformans in high CO2 converges on mutations isolated from patients with relapsed cryptococcosis

Cryptococcus neoformans is an environmental fungus that causes an estimated 180,000 deaths annually and transitions from the external environment to the host environment to cause disease. CO2 concentrations in the atmosphere (0.04%) are dramatically lower than in mammalian tissues (5%). Environmental C. neoformans strains that cannot tolerate 5% CO2 are less virulent than CO2-tolerant strains. Microevolution at elevated CO2 generates loss-of-function mutations in the nucleotide binding protein Avc1 that confer CO2 tolerance to CO2-intolerant strains. Mechanistically, Avc1 positively regulates the expression of plasma membrane transporters, including PDR9, a phospholipid floppase that negatively modulates CO2 fitness. Deletion of AVC1 in five CO2-intolerant environmental strains increases competitive fitness in host CO2 and in a mouse infection model. Importantly, strains with similar AVC1 mutations emerge in patients with relapsed cryptococcosis. Therefore, this microevolutionary convergence strongly suggests that adaptation to host CO2 is a significant driver of C. neoformans fitness during infection.

Insight into the Mechanisms and Clinical Relevance of Antifungal Heteroresistance

Antifungal resistance poses a critical global health threat, particularly in immuno-compromised patients. Beyond the traditional resistance mechanisms rooted in heritable and stable mutations, a distinct phenomenon known as heteroresistance has been identified, wherein a minority of resistant fungal cells coexist within a predominantly susceptible population. Heteroresistance may be induced by pharmacological factors or non-pharmacological agents. The reversible nature of it presents significant clinical challenges, as it can lead to undetected resistance during standard susceptibility testing. As heteroresistance allows fungal pathogens to survive antifungal treatment, this adaptive strategy often leads to treatment failure and recurring infection. Though extensively studied in bacteria, limited research has explored its occurrence in fungi. This review summarizes the current findings on antifungal heteroresistance mechanisms, highlighting the clinical implications of fungal heteroresistance and the pressing need for deeper mechanism insights. We aim to bring together the latest research advances in the field of antifungal heteroresistance, summarizing in detail its known characteristics, inducing factors, molecular mechanisms, and clinical significance, and describing the similarities and differences between heteroresistance, tolerance and persistence. Further research is needed to understand this phenomenon and develop more effective antifungal therapies to combat fungal infections.