Candida albicans and related Candida spp. are responsible for ~400,000 invasive infections/year, which have an ~50% mortality rate. A crucial virulence trait of C. albicans, and other fungi, is the ability to diminish their detection by their hosts. The cell wall carbohydrate (1,3)-glucan is an important epitope that the immune systems of humans and other mammals use to recognize and respond to fungal infections through receptors like Dectin-1 and complement receptor 3 (CR3). Fungi like C. albicans diminish their detection from immune cells through masking (1,3)-glucan under an outer layer of mannosylated glycoproteins (mannan). The virulence of C. albicans is compromised in conditions where (1,3)-glucan is more exposed (unmasked). For example, echinocandin antifungal drugs, like caspofungin, inhibit (1,3)-glucan synthase and cause cell lysis in vitro, but also induce exposure of (1,3)-glucan, even at sublethal concentrations. In addition, a number of mutants that exhibit increased exposure of (1,3)-glucan have decreased virulence. However, a major research challenge is to understand the impact of (1,3)-glucan exposure on virulence during caspofungin treatment. It has been difficult to differentiate between cidal effects of the drug and the impact of (1,3)-glucan exposure. A challenge closely related to this is that the mechanism by which caspofungin causes (1,3)-glucan exposure is unknown. We have found that we can decouple caspofungin's cidal effects from unmasking, which allows us to address both of these challenges. This can be done by activating caspofungin-responsive signaling pathways using a genetic approach rather than the drug, and we have discovered that at least one of these pathways causes unmasking. The Cek1 MAP kinase (MAPK) pathway is activated by caspofungin treatment, and we have discovered that genetic activation of this cascade causes unmasking when hyperactivated, even in the absence of caspofungin. However, unlike the drug, activation of this pathway does not compromise viability. Thus, we can meet the second challenge by using this pathway to dissect the mechanism through which unmasking occurs. Moreover, we can meet the first challenge by using the Cek1 pathway as tool to probe how the immune system responds to unmasking during mouse systemic infections because, unlike caspofungin, it is not cidal. We will address these challenges in three specific aims.
In Aim 1 we will elucidate the mechanisms by which the Cek1 cascade regulates (1,3)-glucan exposure. There are two main transcription factors downstream of Cek1 and we will determine how the pathway chooses a particular one (Cph1) using a combination of genetic, epistasis, and cell biology techniques that will identify how Cek1- Cph1 is activated to cause unmasking.
In Aim 2 we will determine how transcriptional targets of Cek1-Cph1 alter the cell wall to cause unmasking.
In Aim 3, we will elucidate how exposure of (1,3)-glucan causes decreased virulence in mice. We will use transgenic mice to define how neutrophils, macrophages, Dectin-1 and/or CR3 participate to reduce the virulence of unmasked C. albicans.
Candida albicans is the most common cause of human fungal infections, including life-threatening invasive infections. Improvement in patient outcomes will be possible by enhancing immune responses to the fungus. This proposal will study a signaling pathway in C. albicans that can be activated to cause the fungus be more readily visible to the immune system.