Candida albicans Infection of Caenorhabditis elegans Induces Antifungal Immune Defenses

Abstract

Candida albicans yeast cells are found in the intestine of most humans, yet this opportunist can invade host tissues and cause life-threatening infections in susceptible individuals. To better understand the host factors that underlie susceptibility to candidiasis, we developed a new model to study antifungal innate immunity. We demonstrate that the yeast form of C. albicans establishes an intestinal infection in Caenorhabditis elegans, whereas heat-killed yeast are avirulent. Genome-wide, transcription-profiling analysis of C. elegans infected with C. albicans yeast showed that exposure to C. albicans stimulated a rapid host response involving 313 genes (124 upregulated and 189 downregulated, ∼1.6% of the genome) many of which encode antimicrobial, secreted or detoxification proteins. Interestingly, the host genes affected by C. albicans exposure overlapped only to a small extent with the distinct transcriptional responses to the pathogenic bacteria Pseudomonas aeruginosa or Staphylococcus aureus, indicating that there is a high degree of immune specificity toward different bacterial species and C. albicans. Furthermore, genes induced by P. aeruginosa and S. aureus were strongly over-represented among the genes downregulated during C. albicans infection, suggesting that in response to fungal pathogens, nematodes selectively repress the transcription of antibacterial immune effectors. A similar phenomenon is well known in the plant immune response, but has not been described previously in metazoans. Finally, 56% of the genes induced by live C. albicans were also upregulated by heat-killed yeast. These data suggest that a large part of the transcriptional response to C. albicans is mediated through “pattern recognition,” an ancient immune surveillance mechanism able to detect conserved microbial molecules (so-called pathogen-associated molecular patterns or PAMPs). This study provides new information on the evolution and regulation of the innate immune response to divergent pathogens and demonstrates that nematodes selectively mount specific antifungal defenses at the expense of antibacterial responses.

The Contribution of Candida albicans Vacuolar ATPase Subunit V 1 B, Encoded by VMA2 , to Stress Response, Autophagy, and Virulence Is Independent of Environmental pH

“…However, the pH-dependent phenotype of vma mutants makes interpretation of virulence models difficult, as in most model systems the physiological pH is restrictive for the growth of vma mutants. Therefore, we tested tetR-VMA2 virulence in a C. elegans model of infection (34). C. elegans is an ideal organism for the study of virulence in C. albicans vma mutants, as the pH of the larynx and intestine of C. elegans nematodes, where C. albicans infection occurs (34,35), ranges from 3.5 to 5.5 (57).…”

“…elegans infection assays. C. elegans solid medium killing assays were performed as described previously (34), with some important modifications. First, all infection assays were completed at 30°C rather than 25°C due to the cold sensitivity of the tetR-VMA2 strain upon DOX repression (see Fig.…”

“…2A). Further, previous studies have utilized either C. elegans glp-4;sek-1 nematodes rather than wild-type nematodes (35) or treatment with 5-fluoro-2=-deoxyuridine (34) to avoid the complicating presence of eggs and larvae during the infection assay. However, because our experiments were conducted at 30°C, a temperature at which C. elegans reproduction, but not viability, is completely inhibited (36), we were able to use the C. elegans wild-type strain, N2.…”

“…However, because our experiments were conducted at 30°C, a temperature at which C. elegans reproduction, but not viability, is completely inhibited (36), we were able to use the C. elegans wild-type strain, N2. To ensure that exposure to chronic heat stress or doxycycline was not the primary cause of worm mortality, we utilized negative controls in which nematodes were exposed to heat-killed C. albicans cells, which have previously been shown to be avirulent (34). Negative controls were maintained at an elevated temperature (30°C) with or without DOX for the duration of the experiment.…”

“…Plates of YPD (pH 4) with or without DOX then were seeded with a lawn of 50 l THE1-CIp10 or tetR-VMA2 cells that had been pretreated with DOX or left untreated. For negative controls, lawns of heat-killed cells were prepared on agar plates of YPD, pH 4, with or without DOX as described previously (34). The plates were incubated at 30°C for approximately 16 h. Synchronized L4 worms were washed from plates with sterile M9 buffer and collected via centrifugation for 1 min at 6,000 rpm.…”