The Cryptococcus gattii species complex:Unique pathogenic yeasts with understudied virulence mechanisms

Members of Cryptococcus gattii/neoformans species complex are the etiological agents of the potentially fatal human fungal infection cryptococcosis. C. gattii and its sister species cause disease in both immunocompetent and immunocompromised hosts, while the closely related species C. neoformans and C. deneoformans predominantly infect immunocompromised hosts. To date, most studies have focused on similarities in pathogenesis between these two groups, but over recent years, important differences have become apparent. In this review paper, we highlight some of the major phenotypic differences between the C. gattii and neoformans species complexes and justify the need to study the virulence and pathogenicity of the C. gattii species complex as a distinct cryptococcal group.</p

Loss of the scavenger receptor MARCO results in uncontrolled vomocytosis of fungi from macrophages

Vomocytosis, also known as nonlytic exocytosis, is a process whereby fully phagocytosed microbes are expelled from phagocytes without discernible damage to either the phagocyte or microbe. Although this phenomenon was first described in the opportunistic fungal pathogen Cryptococcus neoformans in 2006, to date, mechanistic studies have been hampered by an inability to reliably stimulate or inhibit vomocytosis. Here we present the fortuitous discovery that macrophages lacking the scavenger receptor MAcrophage Receptor with COllagenous domain (MARCO), exhibit near‐total vomocytosis of internalised cryptococci within a few hours of infection. Marco−/− macrophages also showed elevated vomocytosis of a yeast‐locked C. albicans strain, suggesting this to be a broadly relevant observation. We go on to show that MARCO's role in modulating vomocytosis is independent of its role as a phagocytic receptor, suggesting that this protein may play an important and hitherto unrecognised role in modulating macrophage behaviour

Amoeba predation of <i>Cryptococcus</i>:A quantitative and population genomic evaluation of the accidental pathogen hypothesis

The “Amoeboid Predator-Fungal Animal Virulence Hypothesis” posits that interactions with environmental phagocytes shape the evolution of virulence traits in fungal pathogens. In this hypothesis, selection to avoid predation by amoeba inadvertently selects for traits that contribute to fungal escape from phagocytic immune cells. Here, we investigate this hypothesis in the human fungal pathogens Cryptococcus neoformans and Cryptococcus deneoformans. Applying quantitative trait locus (QTL) mapping and comparative genomics, we discovered a cross-species QTL region that is responsible for variation in resistance to amoeba predation. In C. neoformans, this same QTL was found to have pleiotropic effects on melanization, an established virulence factor. Through fine mapping and population genomic comparisons, we identified the gene encoding the transcription factor Bzp4 that underlies this pleiotropic QTL and we show that decreased expression of this gene reduces melanization and increases susceptibility to amoeba predation. Despite the joint effects of BZP4 on amoeba resistance and melanin production, we find no relationship between BZP4 genotype and escape from macrophages or virulence in murine models of disease. Our findings provide new perspectives on how microbial ecology shapes the genetic architecture of fungal virulence, and suggests the need for more nuanced models for the evolution of pathogenesis that account for the complexities of both microbe-microbe and microbe-host interactions

Human immune polymorphisms associated with the risk of cryptococcal disease

Cryptococcus neoformans is an opportunistic fungal pathogen that can cause lethal cryptococcal meningitis in immunocompromised individuals such as those with HIV/AIDS. In addition, cryptococcal infections occasionally arise in immunocompetent individuals or those with previously undiagnosed immunodeficiencies. The course of cryptococcosis is highly variable in both patient groups, and there is rapidly growing evidence that genetic polymorphisms may have a significant impact on the trajectory of disease. Here, we review what is currently known about the nature of these polymorphisms and their impact on host response to C. neoformans infection. Thus far, polymorphisms in Fc gamma receptors, mannose‐binding lectin, Dectin‐2, Toll‐like receptors and macrophage colony‐stimulating factor have been associated with susceptibility to cryptococcal disease. Notably, however, in some cases the impact of these polymorphisms depends on the genetic background of the population; for example, the FCGR3A 158 F/V polymorphism was associated with an increased risk of cryptococcal disease in both HIV‐positive and HIV‐negative white populations, but not in Han Chinese patients. In most cases, the precise mechanism by which the identified polymorphisms influence disease progression remains unclear, although impaired fungal recognition and phagocytosis by innate immune cells appears to play a major role. Finally, we highlight outstanding questions in the field and emphasize the need for future research to include more diverse populations in their genetic association studies

Schematic diagram illustrating CGSC distinct phenotypic virulence traits as compared to <i>C</i>. <i>neoformans</i>.

Upon inhalation from the environment, CGSC yeast cells/spores responds to the lung extracellular niche by exhibiting phenotypic traits including larger capsule (with less immunogenic properties), larger cell body (with higher degree of homogeneity), and thinner but more compacted cell wall with higher chitosan content than C. neoformans. The manner in which CGSC strains exhibit these host-adaptive traits is perhaps responsible for its low affinity to dissemination from the lungs to the brain. Within the host macrophage, the intracellular phenotypes (mitochondrial fusion and “division of labour” proliferation mechanism mediated by extracellular vesicles) exhibited by C. deuterogattii (which drives the fatal Pacific Northwest outbreak) are absent in C. neoformans.</p

The role of Dectin-3 in host response to <i>C</i>. <i>deuterogattii</i> infection <i>in vitro</i> and <i>in vivo</i>.

(A) The C-type lectin receptor, Dectin-3, recognises C. deuterogattii (Cd) capsular glucuronoxylomannan (GXM) [64]. The recognition of GXM leads to the activation of NF-κB and ERK signalling pathways to drive proinflammatory cytokine production. (B) Dectin-3 deficient mice showed increased susceptibility to C. deuterogattii infection [64]. Figure created with BioRender.com.</p

List of virulence-related phenotypic traits whose underlying molecular, genetic, and metabolic mechanism has been studied in <i>C</i>. <i>neoformans</i> but not <i>C</i>. <i>gattii</i>.

List of virulence-related phenotypic traits whose underlying molecular, genetic, and metabolic mechanism has been studied in C. neoformans but not C. gattii.</p

Comparison of morphological attributes between 70 clinical isolates of cryptococci from HIV/AIDS patients in Botswana-Africa (adapted from [37] and summarized).

Comparison of morphological attributes between 70 clinical isolates of cryptococci from HIV/AIDS patients in Botswana-Africa (adapted from [37] and summarized).</p

Micrograph showing the budding nature of <i>C</i>. <i>deuterogattii</i> yeast (left panel) vs. titan (right panel) cells [44].

Scale bar = 5 μm.</p