Dual Disruption of the Immune Cytokine Spätzle Facilitates Fungal Infection of Diverse Insect Hosts

Insect innate immunity has been well studied in Drosophila melanogaster. However, the mechanisms of immune invasion and host adaptation mediated by entomopathogens remain understudied. Here, it is reported that the Drosophila immune cytokine Spätzle (Spz, a Toll receptor ligand) can be targeted by two divergent virulence effectors (ETSs) of Metarhizium robertsii, a fungus that infects a wide range of invertebrates. Mechanistically, the M28-family aminopeptidase ETS1 degrades Spz and its mature ligand form C106, while the hypothetical protein ETS6 only binds C106. Both effectors, particularly ETS6, attenuate or disable Spz interaction with its processing enzyme, the formation of the C106 dimer, and ligand-receptor interaction. Mutagenesis of ETS6 revealed its structural uniqueness in hijacking C106. While mutant Drosophila lacking functional Spz are similarly killed by wild-type and mutant strains of M. robertsii, transgenesis with either ETS1 or ETS6 reduced fly resistance to fungal colonization. Both effectors can target the sequence-divergent yet structurally similar orthologous ligands of other invertebrates, unveiling a fungal mechanism for infecting and killing diverse host species. These findings reveal a rare instance of multiple effectors targeting a single immune factor in fungus-animal interactions, and offer a mechanistic insight into the manipulation of parasite host range.