Activation of Cph1 causes ss(1,3)-glucan unmasking in Candida albicans and attenuates virulence in mice in a neutrophil-dependent manner

Masking the immunogenic cell wall epitope ss(1,3)-glucan under an outer layer of mannosylated glycoproteins is an important virulence factor deployed by Candida albicans during infection. Consequently, increased ss(1,3)-glucan exposure (unmasking) reveals C. albicans to the host's immune system and attenuates its virulence. We have previously shown that activation of the Cek1 MAPK pathway via expression of a hyperactive allele of an upstream kinase (STE11(Delta N467)) induces unmasking. It also increased survival of mice in a murine disseminated candidiasis model and attenuated kidney fungal burden by >= 33 fold. In this communication, we utilized cyclophosphamide-induced immunosuppression to test if the clearance of the unmasked STE11(Delta N467) mutant was dependent on the host immune system. Suppression of the immune response by cyclophosphamide reduced the attenuation in fungal burden caused by the STE11(Delta N467) allele. Moreover, specific depletion of neutrophils via 1A8 antibody treatment also reduced STE11(Delta N467)-dependent fungal burden attenuation, but to a lesser extent than cyclophosphamide, demonstrating an important role for neutrophils in mediating fungal clearance of unmasked STE11(Delta N467) cells. In an effort to understand the mechanism by which Ste11(Delta N467) causes unmasking, transcriptomics were used to reveal that several components in the Cek1 MAPK pathway were upregulated, including the transcription factor CPH1 and the cell wall sensor DFI1. In this report we show that a cph1 Delta Delta mutation restored ss(1,3)-glucan exposure to wild-type levels in the STE11(Delta N467) strain, confirming that Cph1 is the transcription factor mediating Ste11(Delta N467)-induced unmasking. Furthermore, Cph1 is shown to induce a positive feedback loop that increases Cek1 activation. In addition, full unmasking by STE11(Delta N467) is dependent on the upstream cell wall sensor DFI1. However, while deletion of DFI1 significantly reduced Ste11(Delta N467)-induced unmasking, it did not impact activation of the downstream kinase Cek1. Thus, it appears that once stimulated by Ste11(Delta N467), Dfi1 activates a parallel signaling pathway that is involved in Ste11(Delta N467)-induced unmasking.

Automated summary

Activation of the Cek1 MAPK pathway and unmasking of immunogenic cell wall epitope ss(1,3)-glucan by specific genetic mutations weaken the virulence of Candida albicans. Neutrophils play a critical role in the clearance of unmasked fungal cells.