6  Intracellular cryptococci in macrophages
After phagocytosis, the newly formed phagosome undergoes a series of vesicle fusion and maturation steps to generate a highly antimicrobial environment. Many intracellular pathogens, for example Mycobacterium tuberculosis, interfere with the maturation of the phagosome. In contrast, cryptococci apparently do not evade the acidified phagosome, as they survive within Lamp1 positive vesicles at low pH (Johnston and May, 2010; Levitz et al., 1999; Vandal et al., 2009).
In addition to the classical phagosome maturation pathway, the cell content recycling pathway of autophagy has been implicated in the response to a number of pathogens particularly those that can escape the phagosome (Cemma and Brumell, 2012). Components of the autophagy pathway were first implicated in the handling of cryptococci in a Drosophila S2 cell RNAi screen (Qin et al., 2011) and the autophagy effector LC3 has been observed associated with cryptococcal phagosomes (Nicola et al., 2012). Conditional macrophage ATG5 deficient mice showed no difference in survival, although histology of lung lesions showed reduced inflammation (Nicola et al., 2012). There are multiple routes into activation of autophagy and the lack of strong phenotype may be explained by compensation of alternative autophagy activation pathways (Cemma and Brumell, 2012).
C. neoformans is able to proliferate within the macrophage phagosome (Tucker and Casadevall, 2002; Voelz et al., 2009). This suggests that C. neoformans is able to inhabit the macrophage as a protective niche within the host. C. neoformans is also able to travel from one macrophage to another, whereby the receiving macrophage accepts the cryptococcal cell in an actin dependant manner (Alvarez and Casadevall, 2007; Ma et al., 2007). This is a rare event in vitro that has yet to be observed in vivo. The cause of this phenomenon, and if it benefits the host or the pathogen is unknown. The receiving cell may be assisting a potentially moribund macrophage or lateral transfer between macrophages allows evasion of extracellular host immune responses by cryptococci.
Cryptococci can escape macrophages by lysis or through expulsion. The mechanism of lysis is unknown. In some cases this appears to result from intracellular replication leading to large numbers of intraphagosomal cryptococci causing rupture of the host cell membrane (Tucker and Casadevall, 2002). Several studies have identified that the cryptococcal phagosome is permeabilised, but by two different mechanisms: Pores to the cytoplasm (Tucker and Casadevall, 2002) and extracellular phagosome emptying (Carnell et al., 2011; Johnston and May, 2010). Expulsion of cryptococci is described as a non-lytic escape from macrophages via an actin independent exocytosis (Alvarez and Casadevall, 2006; Johnston and May, 2010; Ma et al., 2006). Cryptococci leave the macrophage without causing damage to the host cell. Again as with lateral transfer the ultimate beneficiary is unknown.
The direct role of all these macrophage parasitic phenomena in vitro has yet to be demonstrated in pathogenesis but they support the hypothesis that macrophages are a controlling measure that require wider host immunity to clear cryptococci. In addition, macrophages may act as Trojan horses for the dissemination of cryptococci, a model that has been elegantly supported by showing that mice infected intravenously with cryptococci within macrophages showed much faster dissemination than free yeast (Charlier et al., 2009).