Faster Cryptococcus melanization increases virulence in experimental and human cryptococcosis

Abstract

Cryptococcus spp. are important human pathogens responsible for about 180,000 deaths per year. Studying their virulence attributes can lead to better cryptococcosis prevention and treatment. In this work, we systematically investigated virulence attributes of Cryptococcus spp. clinical isolates and correlated them with patient data. We collected 66 C. neoformans and 19 C. gattii isolates from Brazilian patients and analyzed multiple phenotypes related with their capsule, production of laccase, melanin and extracellular vesicles. We also tested their virulence in Galleria mellonella and ability to evade macrophage LC3-associated phagocytosis (LAP). All phenotypes analyzed varied widely among the isolates, but C. neoformans isolates tended to melanize faster and more intensely and produce thinner capsules in comparison with C. gattii. We also observed correlations that match previous studies, such as that between secreted laccase – but not total melanin production – and disease outcome in patients. The most striking results, though, came from our measurements of Cryptococcus colony melanization kinetics, which followed a sigmoidal curve for most isolates. Faster melanization correlated positively with LAP evasion, virulence in G. mellonella and worse prognosis in humans. These results suggest that the speed of melanization, more than the total amount of melanin Cryptococcus spp. produces, is crucial for virulence.

Graphical abstract