The peritrophic matrix (PM) forms a layer composed of chitin and
glycoproteins that lines the insect intestinal lumen. This physical
barrier plays a role analogous to that of mucous secretions of the
vertebrate digestive tract and is thought to protect the midgut
epithelium from abrasive food particles and microbes. Almost
nothing is known aboutPMfunctions in Drosophila, and its function
as an immune barrier has never been addressed by a genetic approach.
Here we show that the Drosocrystallin (Dcy) protein, a putative
component of the eye lens of Drosophila, contributes to adult
PM formation. A loss-of-function mutation in the dcy gene results
in a reduction of PM width and an increase of its permeability. Upon
bacterial ingestion a higher level of expression of antibacterial peptides
was observed in dcy mutants, pointing to an influence of this
matrix on bacteria sensing by the Imd immune pathway. Moreover,
dcy-deficient flies show an increased susceptibility to oral infections
with the entomopathogenic bacteria Pseudomonas entomophila
and Serratia marcescens. Dcy mutant flies also succumb faster than wild type upon ingestion of a P. entomophila toxic extract. We show that this lethality is due in part to an increased deleterious action of Monalysin, a pore-forming toxin produced by P. entomophila. Collectively, our analysis of the dcy immune phenotype indicates that the PM plays an important role in Drosophila host defense against enteric pathogens, preventing the damaging action of pore-forming toxins on intestinal cells.