The Cryptococcus neoformans Titan Cell: From In Vivo Phenomenon to In Vitro Model

Abstract

Purpose of Review During infection, the human fungal pathogen Cryptococcus neoformans undergoes an unusual change in size, from small haploid yeast to large polyploid Titan cells. This transition is now well recognized as a virulence factor, but significant questions remain about how Titanisation is regulated and how it influences disease progression. Progress has been impeded by the lack of an in vitro model for the yeast-to-Titan transition, a challenge that was recently overcome by three independent groups. Recent Findings Here, we review Titanization in the context of patient samples and animal models and set the stage for three new reports describing in vitro Titan cell induction assays. We compare and contrast key findings, place them in the broader research context, and identify areas of further interest. Summary New in vitro models will allow pressing questions about molecular mechanisms driving the yeast-to-Titan transition and their influence on drug resistance and pathogenesis to be addressed.

mSphere of Influence: Positive Research Culture Enables Excellence and Innovation

“…The example first established by Nielsen and Casadevall benefited not only the PIs on the three papers, but also our teams, particularly graduate students Hommel and Trevijano-Contador, who led the work in the Alanio and Zaragoza labs. We all continue to work in this space, contributing to a broader understanding of the many interacting signals that drive titanization (13). It has also served as an example for others facing similar competitive choices in our field (14)(15)(16)(17).…”

Host environmental conditions induce small fungal cell size and alter population heterogeneity in Cryptococcus neoformans

“…Cryptococcosis occurs when desiccated yeast or spores are inhaled, proliferate in the host lung and further disseminate into the central nervous system (2, 3). In response to the earliest interactions with host lung stimuli , C. neoformans cells undergo an unusual morphogenic switch, changing from small haploid yeast to large polyploid titan cells (4–9). While titan cells are defined as those cells larger than 10 μm in diameters, in vivo they can reach up 50 to 100 μm; typical yeast cells are around 5 μm (4, 5, 10).…”

“…In the past, one of the biggest challenges hampering further understanding of the molecular mechanisms driving titanization and their contribution to disease outcome was the lack of an in vitro titan cell induction protocol (10, 15–17). The process of titanization is triggered by environmental stimuli from the host lung, and our group and others have demonstrated that host-relevant stimuli are able to induce titan cells in vitro (9, 15–17). These in vitro generated titan cells exhibited similar features to in vivo induced titan cells, including: enlarged cell sizes, tightly compacted capsule, thicker cell wall, high ploidy and the presence of a single, large vacuole (9, 10).…”

“…The process of titanization is triggered by environmental stimuli from the host lung, and our group and others have demonstrated that host-relevant stimuli are able to induce titan cells in vitro (9, 15–17). These in vitro generated titan cells exhibited similar features to in vivo induced titan cells, including: enlarged cell sizes, tightly compacted capsule, thicker cell wall, high ploidy and the presence of a single, large vacuole (9, 10). In our lab, the in vitro titan cell induction protocol is a two-step process: first Cryptococcus cells are incubated under nutrient starvation medium to mimic environmental conditions before entering into host, and then titan cells are induced with PBS+10%HI-FCS at low cell density (OD 600 =0.001) (9, 15).…”

Redistribution of a glucuronoxylomannan epitope towards the capsule surface coincides with Titanisation in the human fungal pathogenCryptococcus neoformans

“…The composition of this capsule is highly variable and differs between yeast cells and Titan cells, the latter representing a subset of giant cells (>10 μm in cell body diameter) formed by C. neoformans within the host lung (7–9). Titan cells contribute to pathogenesis by resisting phagocytosis, enhancing dissemination of yeast to the central nervous system and altering host immune status (7, 9–13).…”