The Cryptococcus neoformans titanide is a pathogenic morphotype that arises from typical yeast cells in response to host-relevant conditions

Abstract

Fungal morphogenesis is a major driver of disease outcome. For the opportunistic fungal pathogen Cryptococcus neoformans, extreme morphological heterogeneity within the lung environment drives dissemination, immune evasion, and drug resistance. One such morphotype is the small, oval, titanide (2-3 {micro}m). This poorly understood cell type is prevalent in lung histology and under in vitro conditions that mimic the host environment to generate cellular heterogeneity. Titanides appear after 24 hours post-induction and by 72 hours are the dominant morphotype. Despite their prevalence and the significance of cryptococcal morphogenesis for virulence, their origin remains unclear and their biology unstudied. Using TEM and fluorescence microscopy we demonstrate that titanides display distinct morphological features; they possess thin cell walls and capsules with reduced Pathogen Associated Molecular Pattern exposure and altered distribution. These features distinguish titanides from previously described C. neoformans small cells such as in vivo seed cells (4-6 {micro}m, high cell-wall mannan content) and micro cells (round, <1 m, with thick cell walls). Using microfluidics, we answer key questions pertaining to their origin and cell fate and further our understanding of C. neoformans typical cell and titanide morphological plasticity in host-relevant conditions. Interestingly, despite their thin cell walls, we show that titanides display an increased resistance to cell wall/membrane stressors and are an effective infectious propagule in a murine model. Together, these findings establish a definition and essential characterisation of the titanide morphotype and provide new insight into a key player in C. neoformans pathogenesis.