Mosquito-Plasmodium interactions in response to immune activation of the vector.

During the development of Plasmodium sp. within the mosquito midgut, the parasite undergoes a series of developmental changes. The elongated ookinete migrates through the layers of the midgut where it forms the oocyst under the basal lamina. We demonstrate here that if Aedes aegypti or Anopheles gambiae, normally susceptible to Plasmodium gallinaceum and P. berghei, respectively, are immune activated by the injection of bacteria into the hemocoel, and subsequently are fed on an infectious bloodmeal, there is a significant reduction in the prevalence and mean intensity of infection of oocysts on the midgut. Only those mosquitoes immune activated prior to, or immediately after, parasite ingestion exhibit this reduction in parasite development. Mosquitoes immune activated 2-5 days after bloodfeeding show no differences in parasite burdens compared with naive controls. Northern analyses reveal that transcriptional activity for mosquito defensins is not detected in the whole bodies of Ae. aegypti from 4 h to 10 days after ingesting P. gallinaceum, suggesting that parasite ingestion, passage from the food bolus through the midgut, oocyst formation, and subsequent release of sporozoites into the hemolymph do not induce the production of defensin. However, reverse transcriptase-PCR of RNA isolated solely from the midguts of Ae. aegypti indicates that transcription of mosquito defensins occurs in the midguts of naive mosquitoes and those ingesting an infectious or noninfectious bloodmeal. Bacteria-challenged Ae. aegypti showed high levels of mature defensin in the hemolymph that correlate with a lower prevalence and mean intensity of infection with oocysts. Because few oocysts were found on the midgut of immune-activated mosquitoes, the data suggest that some factor, induced by bacterial challenge, kills the parasite at a preoocyst stage.