The role of the peritrophic matrix (PM) in the development of
Leishmania major infections in a natural vector, Phlebotomus
papatasi, was investigated by addition of exogenous chitinase to
the bloodmeal, which completely blocked PM formation.
Surprisingly, the absence of the PM was associated with the loss
of midgut infections. The chitinase was not directly toxic
to the parasite, nor were midgut infections lost due to premature expulsion
of the bloodmeal. Most parasites were killed
in chitinase-treated flies within the first 4 h after feeding.
Substantial early killing was also observed in control flies,
suggesting that the lack of PM exacerbates lethal conditions which
normally exist in the blood-fed midgut. Early parasite
mortality was reversed by soybean trypsin inhibitor. Allosamadin, a specific
inhibitor of chitinase, led to a thickening of
the PM, and also prevented the early parasite mortality seen in infected
flies. Susceptibility to gut proteases was extremely
high in transitional-stage parasites, while amastigotes and fully
transformed promastigotes were relatively resistant. A
novel role for the PM in promoting parasite survival is suggested, in
which the PM creates a barrier to the rapid diffusion
of digestive enzymes, and limits the exposure of parasites to these
enzymes during the time when they are especially
vulnerable to proteolytic damage.