Fetal bovine serum-triggered Titan cell formation and growth inhibition are unique to theCryptococcusspecies complex

Abstract

Dylag, Colon-Reyes, and Kozubowski 1 1 2 3 Fetal bovine serum-triggered Titan cell formation and growth inhibition are unique 4 to the Cryptococcus species complex 5 6 ABSTRACT 18In the genus Cryptococcus, C. neoformans and C. gattii stand out by the number of virulence 19 factors that allowed those fungi to achieve evolutionary success as pathogens. Among the factors 20 contributing to cryptococcosis is a peculiar morphological transition to form giant (Titan) cells. 21 Formation of Titans has been described in vitro. However, it remains unclear whether non-C. 22 neoformans/non-C. gattii species are capable of titanisation. Based on a survey of several 23 basidiomycetous yeasts, we propose that titanisation is unique to C. neoformans and C. gattii. In 24 addition, we find that under in vitro conditions that induce titanisation, fetal bovine serum (FBS) 25 possesses activity that inhibits growth of C. gattii and kills C. neoformans if incubation is 26 conducted in the absence of 5% CO 2 . Moreover, lowering of pH or addition of an antioxidant, 27 rescues growth in the presence of FBS and allows titanisation even in the absence of 5% CO 2 . 28Strikingly, acapsular mutants, cap10Δ and cap59Δ, show complete or partial reduction of 29 titanisation, respectively and a partial resistance to growth inhibition imposed by the FBS. Our 30 data implicate titanisation as a unique feature of C. neoformans and C. gattii and provide novel 31 insights about the nature of this unusual morphological transition. 32 33 34 35 36 37 42 Trichosporon 1 . In particular, two species belonging to the Cryptococcus genus (formerly 43 Filobasidiella), C. neoformans and C. gattii are the etiological agents of fatal systemic mycoses. 44 In recent years, nearly one million cases of cryptococcal meningitis occur annually resulting in 45 over 600,000 deaths globally 2 . Moreover, cryptococcosis is responsible for 15-17% AIDS-46 related deaths on a global scale 3,4 . While C. neoformans can cause mainly opportunistic 47 infections in immunocompromised patients, C. gattii is capable of infecting also 48 immunocompetent individuals 5-8 . Moreover, it is well established that among C. neoformans, 49 serotype A (varietas grubii) is responsible for the majority of cryptococcal infections, while 50 serotype D (varietas neoformans) is less common 7 . Among Cryptococcus species other than C. 51 neoformans and C. gattii, the following species have been described as causing occasional 52 infections in humans: C. laurentii 9-11 , C. albidus 9,12,13 , C. curvatus 14,15 , C. uniguttulatus 16 , and 53 C. adeliensis 17 . Such casuistic infections, reviewed in literature most recently by Smith et al. 18 54 can be also systemic in case of strains able to grow at 37°C. What makes those selected 55 Cryptococcus species capable of infecting humans is an important question, the answer to which 56 remains incomplete. Among the best-described cryptococcal characteristics necessary for 57 pathogenicity are the ability to proliferate at 37°C, melanisation, formati...