Acetylation and accessibility of Cryptococcus neoformans cell wall chitosans influence the strength of host immune...

The fungal pathogen Cryptococcus neoformans causes fatal cryptococcal meningitis, a significant global health risk. Unlike most fungi, C. neoformans expresses chitin deacetylases that convert its cell wall chitin into chitosan, helping to evade chitin-triggered host immunity. However, it is unclear how the properties and accessibility of these chitosans vary between strains and growth conditions, and how they influence the host immune response. To show how chitin deacetylase activity and growth conditions shape the properties of cell wall chitosans, we present the first comprehensive analysis of the chitosan content, fraction of acetylation (FA), enzymatic degradation products by host chitinases, and accessibility in various Cryptococcus strains, including chitin deacetylase mutants, cultivated under different conditions. Certain chitin deacetylase knockouts, as well as challenging growth conditions, lead to higher FAs and enhanced exposure, associated with stronger host immune responses. The response to cryptococcal chitin/chitosan ranges from low (resulting in cryptococcosis) to adequate (providing effective host defense) and excessive (leading to hyper-inflammation). Correlation between the strength of the immune response and the abundance of chitin-binding proteins was used to rapidly screen a multitude of strains and growth conditions for their immunogenic potential. Our approach provides insight into Cryptococcus-related disease caused by insufficient and excessive immune responses, and may accelerate the development of effective cell-based vaccines.