1  Introduction
Cryptococcus neoformans and Cryptococcus gattii are environmental yeasts that cause opportunistic infection. The most common and serious type of cryptococcal disease is pulmonary cryptococcosis that is uncontrolled and leads to cryptococcal meningitis (CM) or meningoencephalitis. CM is almost exclusively observed in those with severely impaired immunity. Rarely other forms of cryptococcal infection are seen (e.g. cutaneous infection, peritonitis; (Christianson et al., 2003; El-Kersh et al., 2013) but this review will focus on infection and immunity to CM (Jarvis and Harrison, 2007).
C. neoformans infection is a leading cause of mortality within AIDS patients worldwide, particularly in sub-Saharan Africa (Park et al., 2009). In addition to HIV infected individuals, C. neoformans is also associated with other immunocompromised groups such as those with haematopoietic malignancy, solid transplant patients (e.g. glucosteroid therapy) and therapy for rheumatoid arthritis (e.g. anti-TNFα mAb). In contrast, C. gattii is capable of infecting immunocompetent individuals, most notably in the Vancouver Island outbreak (Kidd et al., 2004), and is generally absent in the immunocompromised (Chen et al., 2000; Mitchell and Perfect, 1995). There are a number of counter examples e.g. C. gattii infection of HIV infected individuals (Springer et al., 2014; Steele et al., 2010) and certain C. neoformans genotypes infecting the immunocompetent (Chau et al., 2010). C. neoformans is found globally and has been isolated from pigeon excrement, soil and rotting vegetables, whereas C. gattii is predominantly restricted to tropical and sub-tropical areas, and is associated with eucalyptus trees (Bovers et al., 2008; Voelz and May, 2010).