An unusual Toll/MyD88-mediated Drosophila host defence against Talaromyces marneffei

Talaromycosis, caused by Talaromyces marneffei (T. marneffei, formerly known as Penicillium marneffei), is an opportunistic invasive mycosis endemic in tropical and subtropical areas of Asia with high mortality rate. Despite various infection models established to study the immunological interaction between T. marneffei and the host, the pathogenicity of this fungus is not yet fully understood. So far, Drosophila melanogaster, a well-established genetic model organism to study innate immunity, has not been used in related research on T. marneffei. In this study, we provide the initial characterization of a systemic infection model of T. marneffei in the D. melanogaster host. Survival curves and fungal loads were tested as well as Toll pathway activation was quantified by RT-qPCR of several antimicrobial peptide (AMP) genes including Drosomycin, Metchnikowin, and Bomanin Short 1. We discovered that whereas most wild-type flies were able to overcome the infection, MyD88 or Toll mutant flies failed to prevent fungal dissemination and proliferation and ultimately succumbed to this challenge. Unexpectedly, the induction of classical Toll pathway activation readouts, Drosomycin and Bomanin Short 1, by live or killed T. marneffei was quite limited in wild-type flies, suggesting that the fungus largely escapes detection by the systemic immune system. This unusual situation of a poor systemic activation of the Toll pathway and a strong susceptibility phenotype of MyD88/Toll might be accounted for by a requirement for this host defence in only specific tissues, a hypothesis that remains to be rigorously tested.