Aspartyl proteases in Candida glabrata are required for suppression of the host innate immune response

Abstract

A family of 11 cell surface-associated aspartyl proteases (CgYps1–11), also referred as yapsins, is a key virulence factor in the pathogenic yeast Candida glabrata. However, the mechanism by which CgYapsins modulate immune response and facilitate survival in the mammalian host remains to be identified. Here, using RNA-Seq analysis, we report that genes involved in cell wall metabolism are differentially regulated in the Cgyps1–11Δ mutant. Consistently, the mutant contained lower β-glucan and mannan levels and exhibited increased chitin content in the cell wall. As cell wall components are known to regulate the innate immune response, we next determined the macrophage transcriptional response to C. glabrata infection and observed differential expression of genes implicated in inflammation, chemotaxis, ion transport, and the tumor necrosis factor signaling cascade. Importantly, the Cgyps1–11Δ mutant evoked a different immune response, resulting in an enhanced release of the pro-inflammatory cytokine IL-1β in THP-1 macrophages. Further, Cgyps1–11Δ–induced IL-1β production adversely affected intracellular proliferation of co-infected WT cells and depended on activation of spleen tyrosine kinase (Syk) signaling in the host cells. Accordingly, the Syk inhibitor R406 augmented intracellular survival of the Cgyps1–11Δ mutant. Finally, we demonstrate that C. glabrata infection triggers elevated IL-1β production in mouse organs and that the CgYPS genes are required for organ colonization and dissemination in the murine model of systemic infection. Altogether, our results uncover the basis for macrophage-mediated killing of Cgyps1–11Δ cells and provide the first evidence that aspartyl proteases in C. glabrata are required for suppression of IL-1β production in macrophages.

Candida glabrata: A Lot More Than Meets the Eye

“…The ability of C. glabrata to form biofilms has also been found to be enhanced in the high-iron environment [57]. Moreover, recent studies have shown a close association between the capacity to form biofilms and colonize murine organs in the systemic candidiasis model [57,58], however, the precise mechanisms underlying this relationship are unknown.…”

“…CgYPS genes have been shown to be regulated by various environmental conditions including low pH, thermal stress, macrophage and neutrophil internalization [60,61,62,63]. Consistently, CgYapsins are pivotal to the regulation of several cellular processes such as maintenance of the cell wall architecture, pH and vacuole homeostasis, biofilm formation and interaction with the host [58,60,61,64]. The latter includes suppression of the production of pro-inflammatory cytokine IL-1β in human THP-1 macrophages, facilitating survival in macrophages and virulence in the models of murine systemic candidiasis and Drosophila melanogaster [58,60,65,66].…”

“…Furthermore, infection with the Cgyps11 mutant, generated by the CRISPR-Cas system, resulted in slower death of fruit flies lacking adapter of the Toll signaling pathway MyD88 [67]. Lastly, the Cgyps1-11Δ mutant displayed sunken cell walls containing higher chitin and reduced β-glucan and mannan levels (Figure 1) [58,64]. CgYapsins have also been implicated in shedding Epa1 adhesin off the cell wall, as Epa1 release into the medium was drastically reduced in the Cgyps1-11Δ mutant compared to wild-type cells (Figure 1) [60].…”

“…In accordance, macrophages did not produce TNF-α, IL-6, IL-8, IL-12, and IFN-γ pro-inflammatory cytokines, however, GM-CSF production has been reported upon C. glabrata infection [65]. Additionally, the spleen tyrosine kinase, Syk, is known to be phosphorylated in response to C. glabrata infection which resulted in the NLRP3 inflammasome-dependent production of IL-1β in human THP-1 macrophages [58]. The family of eleven cell surface-associated proteases (CgYapsins) has been shown to be required to keep the Syk pathway activation in check, as lack of these proteases led to increased IL-1β release and killing of C. glabrata cells [58].…”

“…Furthermore, the continuous contact with macrophages during microevolution studies led to a change in the morphology of C. glabrata cells from yeast to pseudohyphae which was attributed to a mutation in the chitin synthase-encoding gene CgCHS2 [151]. Altogether, because of its ability to subvert the immune response and replicate in macrophages in vitro, macrophages are assumed to be the Trojan horses for C. glabrata [58,143,151]. Figure 4 summarizes key aspects of C. glabrata -macrophage interaction.…”