The Tools for Virulence of Cryptococcus neoformans

Carolina Coelho*,†; Anamelia Lorenzetti Bocca§; Arturo Casadevall*,1    * Department of Microbiology and Immunology, Albert Einstein College of Medicine of Yeshiva University, New York, USA
† Centre for Neuroscience and Cell Biology of Coimbra, Institute of Microbiology, Faculty of Medicine, University of Coimbra, Coimbra, Portugal
§ Department of Cellular Biology, Institute of Biological Sciences, University of Brasília, Brasília, Brazil
1 Corresponding author: email address: arturo.casadevall@einstein.yu.edu

1 Introduction to Cryptococcus neoformans and Cryptococcosis

Cryptococcus neoformans is the causative agent of cryptococcosis, an often fatal disease characterized by an initial pneumonia that can progress to fatal meningoenchepalitis. Although cryptococcosis was described over a century ago (Barnett, 2010) this disease rose to spotlight as an AIDS-associated opportunistic disease. It is now reported to cause at least half a million of AIDS-associated deaths each year (Park et al., 2009). While access to highly active antiretroviral therapy has dramatically reduced the prevalence of AIDS-related cryptococcosis, other risk factors have arisen in recent years. Any individual undergoing immunosupressive therapy, such as a transplant or a cancer patient, is at risk for cryptococcal disease, which can affect up to 20% of solid organ transplant recipients in the United States (Singh et al., 2008).

C. neoformans is a ubiquitous organism with a worldwide distribution that includes both environmental and urban settings, particularly trees, soil, and avian guano (Barnett, 2010; Lin & Heitman, 2006). The genus Cryptococcus is remarkable for having a polysaccharide capsule (Barnett, 2010), which is unique among the pathogenic fungi. Although other species in the genus Cryptococcus are occasionally associated with human disease, the only consistently pathogenic species in this genus are C. neoformans and C. gattii. Originally with a geographical distribution restricted to subtropical regions, C. gattii has recently expanded to North America (Barnett, 2010; Lin & Heitman, 2006). An unexpected outbreak in the American Pacific Northwest has caused deaths in human hosts with no apparent immune defect (Hoang, Maguire, Doyle, Fyfe, & Roscoe, 2004).

The events leading to cryptococcal disease can be roughly summarized as follows. An infectious particle becomes airborne from their environmental reservoirs, are inhaled by an unknowing host and deposited in the lungs (Giles, Dagenais, Botts, Keller, & Hull, 2009). Both spores and/or desiccated yeast cells possess the physical characteristics that allow inhalation and lung deposition. This was proved experimentally when spores were isolated and found to have the same virulence characteristics as yeast cells (Giles et al., 2009). Inhalation of fungi by humans is a common occurrence, since up to 80% of 5-year olds in an urban setting manifest serological reactivity to C. neoformans consistent with prior exposure (Davis et al., 2007; Goldman et al., 2001). While primary infection is believed to be asymptomatic or to pass unnoticed as one of the many illnesses of childhood (Goldman et al., 2001), the current prevailing thought is that in many individuals the infection is not completely cleared and instead it persists in a latent asymptomatic state (see Fig. 1.1). This is supported by both epidemiological data (Dromer, Ronin, & Dupont, 1992; Garcia-Hermoso, Janbon, &Dromer, 1999; Singh et al., 2008) and animal studies. In the rat model it is possible to establish latent infections that can be reactivated by subsequent immune suppression (Goldman, Lee, & Casadevall, 1994; Goldman, Lee, Mednick, Montella, & Casadevall, 2000). Latency occurs by persistence of intracellular fungal forms within the lung, site of primary infection (Lindell, Ballinger, McDonald, Toews, & Huffnagle, 2006), but the possibility of latency in other tissues, in particular the brain, has not been excluded. In the event of host immunosuppression, the latent form begins to proliferate in the lung and from here it can disseminate to extrapulmonary sites. Management of the resulting cryptococcosis is difficult and very aggressive treatment is required once the yeast crosses the blood–brain barrier (Longo et al., 2011). Despite adequate medical care, fatalities still amount to 10–25% of the cases (Hoang et al., 2004; Jarvis & Harrison, 2007; Longo et al., 2011; Park et al., 2009; Schwarz, Dromer, Lortholary, & Dannaoui, 2006; Singh et al., 2008). For individuals with reversible immune deficiencies, the best course of action is restoration of immune function (Longo et al., 2011; Schwarz et al., 2006). In individuals with severe immunodeficiency, therapy may not be curative, that is, eradication of fungus is not achieved, and the clinical goal is to reduce symptoms with prolonged, if not lifelong, antifungal therapy.

2 What Tools Allow C. neoformans to Become a Pathogen?

2.1 The host–pathogen duo and virulence as an emergent property

C. neoformans survival and proliferation within a mammalian host is made possible by the combination of microbial virulence factors and host susceptibility (Casadevall & Pirofski, 1999, 2003; Steenbergen & Casadevall, 2003). Virulence factors include all the mechanisms that allow the fungus to efficiently divide and survive inside the host, factors that allow the fungus to resist host immune attack, and lastly, factors that eventually damage the host. The classical example of a condition predisposing individuals to cryptococcosis is a CD4+ T-cell deficiency, such as AIDS patients (Jarvis et al., 2013), but many others exist. There is a large body of literature showing that X-linked immunodeficiency (Szymczak et al., 2013) predisposes to cryptococcal disease. Equally Fc receptor (Rohatgi et al., 2013) and IL-4 (Müller et al., 2012) polymorphisms have recently been associated with altered susceptibilities to human cryptococcosis. When considering the phenomenon of virulence, it is important to remember that disease is an outcome of an interaction between a host and a microbe (Casadevall & Pirofski, 1999), since the host aims for effective control of the pathogen with minimization of tissue damage. Consequently, the phenomenon of virulence should not be viewed in isolation from the host. In this chapter, we attempted to frame the discussion of each virulence attribute with its host counterpart (Alanio, Desnos-Ollivier, & Dromer, 2011). However, we will focus primarily on the fungal attributes, the virulence factors, of C. neoformans, their hypothesized evolutionary origins, and the complex interactions that resulted in such a remarkable pathogen.

2.2 Thermotolerance

Growth at host body temperature is an absolute requirement for virulence. C. neoformans isolates manifest considerable thermotolerance (Martinez, Garcia-Rivera, & Casadevall, 2001; Perfect, 2006; Robert & Casadevall, 2009), which might simultaneously explain its worldwide distribution and its capacity for growth at mammalian temperatures. Because thermotolerance is a...