Impact of Cryptococcus titan cells on pathogenesis

Project Summary Cryptococcus neoformans (Cn) infections cause almost half of deaths due to fungal infection and are responsible for 20% of HIV-related mortality. Even with access to the best available antifungals, cryptococcosis mortality rates of 30% remain unacceptably high. Although natural immunity to fungal infection is quite efficient in healthy individuals, disease in advanced HIV/AIDS patients is a complex interaction between 1) failure to control fungal replication and 2) a qualitative shift towards a deleterious immune response. The high mortality observed clinically suggests current antifungal drug treatments are inadequate in immunocompromised individuals. This paradox suggests we are missing critical components of the Cn-host interaction. Characterization of cryptococcal cells during infection has revealed that Cn produces a unique cell type - referred to as “titan cells” - during infection that alter the host-pathogen interaction. These titan cells are produced in response to the host pulmonary environment and are 5-10x larger than typical-sized Cn cells. We demonstrated previously that titan cell production is critical for virulence and impacts dissemination to the CNS. We further showed titan cell formation alters the host response by reducing phagocytosis and stimulating a detrimental Th2-mediated response. The interaction between pathogens and their human hosts can be very complex, and the outcome depends on both host and pathogen responses. The host must sense pathogen associated molecular patterns (PAMPs), and then produce an appropriate immune response to kill the pathogen. Conversely, the pathogen must sense and respond to the host environment to promote its own survival. This proposal aims to identify critical alterations involved in the genesis of titan cells and their progeny, as well as define how their unique cellular structure impacts the host immune response and thereby, pathogenesis. Titan cells have a number of unique characteristics, thus the focus of our proposed investigations is to determine how these traits influence: 1) pathogen adaptation/survival in the host, and 2) the host immune response. Our previous studies showed titan cells undergo ploidy changes associated with their formation and replication, and lead us to hypothesize these changes are critical for adaptation and survival in the host environment. Therefore, our first aim is to define the genetic changes in titan cell progeny cells that lead to enhanced adaptation and survival in the host. In our second aim, we will determine how titan cells undergo reductive division, testing the hypothesis that titan progeny contain only the newly synthesized copy of DNA. The third aim will determine how titan cell formation alters the immune response to promote latent infection establishment and reactivation upon immune dysfunction. These studies will ultimately coalesce into multi-faceted antimicrobial therapies that combine targeting patho- gen-specific processes to limit pathogen adaptation and modulation of the host immune response to maximize beneficial host responses to reduce disease.