Defense reactions of mosquitoes to filarial worms: potential mechanism for avoidance of the response by Brugia pahangi microfilariae

Mosquito infection responses to developing filarial worms

Human lymphatic filariasis is a mosquito-vectored disease caused by the nematode parasites Wuchereria bancrofti, Brugia malayi and Brugia timori. These are relatively large roundworms that can cause considerable damage in compatible mosquito vectors. In order to assess how mosquitoes respond to infection in compatible mosquito-filarial worm associations, microarray analysis was used to evaluate transcriptome changes in Aedes aegypti at various times during B. malayi development. Changes in transcript abundance in response to the different stages of B. malayi infection were diverse. At the early stages of midgut and thoracic muscle cell penetration, a greater number of genes were repressed compared to those that were induced (20 vs. 8). The non-feeding, intracellular first-stage larvae elicited few differences, with 4 transcripts showing an increased and 9 a decreased abundance relative to controls. Several cecropin transcripts increased in abundance after parasites molted to second-stage larvae. However, the greatest number of transcripts changed in abundance after larvae molted to third-stage larvae and migrated to the head and proboscis (120 induced, 38 repressed), including a large number of putative, immunity-related genes (∼13% of genes with predicted functions). To test whether the innate immune system of mosquitoes was capable of modulating permissiveness to the parasite, we activated the Toll and Imd pathway controlled rel family transcription factors Rel1 and Rel2 (by RNA interference knockdown of the pathway's negative regulators Cactus and Caspar) during the early stages of infection with B. malayi. The activation of either of these immune signaling pathways, or knockdown of the Toll pathway, did not affect B. malayi in Ae. aegypti. The possibility of LF parasites evading mosquito immune responses during successful development is discussed.

Filarial worms that cause human lymphatic filariasis (LF) are transmitted by many species of mosquitoes. Within susceptible mosquitoes, Brugia malayi develop from microfilariae (mf) to infective-stage larvae (L3s), in approximately eight days. These nematodes develop as intracellular parasites within mosquito flight muscle cells, in which they ingest cellular material and eventually cause cell death when L3s migrate to the mosquito's proboscis. We examined the effects of B. malayi parasitism on Aedes aegypti by analyzing changes in mosquito gene expression at different stages of parasite development. We found that a few genes were differentially expressed at the RNA level relative to non-infected controls. The majority of changes occurred at two time periods, when the filarial worms began feeding and when the L3s were in the head and proboscis. Many transcriptional changes in the later group concur with documented descriptions of tissue damage, clean-up and repair that occurs in mosquitoes infected with filarial worms. In addition, we activated two innate immunity signaling pathways and observed the effects on filarial worm development. B. malayi seems to be capable of evading these immune responses, because its development was not impeded by the activation of either the Toll or Imd signal pathways in Ae. aegypti.

Penetration of the mosquito midgut is not required for Brugia pahangi microfilariae to avoid the melanotic encapsulation response of Armigeres subalbatus

Insect vectors of disease have the capacity to respond to, and prevent further development of, parasites and pathogens using a response known as melanotic encapsulation. The naturally-occurring Armigeres subalbatus-Brugia spp. system provides an excellent way to investigate melanotic encapsulation and immune recognition in a mosquito host, because Brugia malayi microfilariae (mf) acquired via a blood meal are rapidly melanized in the body cavity of Ar. subalbatus, but Brugia pahangi mf evade or suppress the immune response and develop normally into infective stage larvae. Previous studies have suggested that B. pahangi mf are changed in some manner in the process of exiting the mosquito gut, thereby facilitating escape from, or suppression of, the melanotic encapsulation response. By inoculating mosquitoes with parasites, thus circumventing the midgut, we show that approximately 88% of B. pahangi mf escape the melanotic encapsulation response while approximately 90% of inoculated B. malayi mf are melanized. Methods to isolate parasites for this procedure are described. These results mimic those observed in Ar. subalbatus against Brugia spp. mf that are ingested following blood feeding, and demonstrate that midgut penetration is not required for B. pahangi mf to avoid the melanotic encapsulation response of Ar. subalbatus.

Pathology of malaria-infected mosquitoes

Concepts of the basic case reproduction rate of malaria, or the vectorial capacity of malaria vectors, tend to assume that the behaviour of infected and non-infected mosquitoes will be similar. However, recent years have seen a series of studies demonstrating that mosquitoes infected with malaria or other parasites show many pathological features with important effects on their behaviour and on the transmission dynamics of the parasite. Parasitology Today will be featuring a series of reports discussing these effects and attempting to unravel the expected effects on parasite transmission dynamics; this article sets the scene.