Mosquito Immune Response Research Articles

Dirofilaria immitis: Effect on hemolymph polypeptide synthesis in Aedes aegypti during melanotic encapsulation reactions against microfilariae

[ 35S]Methionine-labeled hemolymph polypeptides from adult, female Aedes aegypti Liverpool strain mosquitoes inoculated with the microfilariae of the filarial nematode Dirofilaria immitis were compared with those from saline-inoculated and uninoculated controls by sodium dodecyl sulfatepolyacrylamide gel electrophoresis (SDS-PAGE) followed by fluorography. SDS-PAGE analysis of cell-free hemolymph collected via perfusion at 6, 12, 24, 48, 72, and 96 hr postinoculation (PI) detected the enhanced expression of an 84-kDa polypeptide. This polypeptide, expressed constitutively in the hemolymph of all three groups of mosquitoes, increased considerably in inoculated mosquitoes as time progressed as compared with uninoculated controls. Moreover, the 84-kDa polypeptide was expressed at higher levels in D. immitis-inoculated mosquitoes than in saline-inoculated controls. This stimulation of de novo biosynthesis of the 84-kDa polypeptide in inoculated mosquitoes may play a role in the immune response of mosquitoes. Since it is likely that the wound healing response in insects involves many of the same chemical processes as occur in melanotic encapsulation reactions of mosquitoes against filarial worms, the preferential expression of the 84-kDa polypeptide in saline-inoculated mosquitoes seen in this study may reflect a wound healing response. The greater increase in synthesis of this protein in D. immitis-inoculated mosquitoes may reflect production of melanotic material required for parasite destruction as well as for wound healing.

The efficacy of melanotic encapsulation as a defense against parasite-induced mortality in snowpool mosquitoes (Diptera: Culicidae) infected by mermithid nematodes

The effects of host reactions to parasitism were studied in two species of mosquitoes (Diptera: Culicidae) infected with mermithid nematodes (Romanomermis sp.). No immune reactions were observed in Aedes impiger and virtually all (98.5%) infected larvae died before pupation. Mermithid infections of Aedes nevadensis resulted in high host mortalities (99.3% failed to pupate) when none of the parasites were encapsulated. Melanotic encapsulations of the nematodes were found in 67% of infected Aedes nevadensis, and larvae of this species that encapsulated all of their parasites had the same rate of survival as uninfected conspecifics. Survivorship of Aedes nevadensis larvae that encapsulated some but not all of their Romanomermis parasites was intermediate. Encapsulation also suppressed parasite-induced reductions in host size, and larvae that encapsulated all parasites were significantly larger than other Aedes nevadensis. This is the first quantitative demonstration of the efficacy of mosquito immune responses as defenses against mermithid parasites.

Hemocyte Monophenol Oxidase Activity in Mosquitoes Exposed to Microfilariae of Dirofilaria immitis

Monophenol oxidase (MPO) activity in hemocytes collected from Aedes aegypti Liverpool strain and Aedes trivittatus intrathoracically inoculated with saline alone, inoculated with Dirofilaria immitis microfilariae (mff), or from uninoculated mosquitoes was compared using a radiometric tyrosine hydroxylation assay. Hemocyte MPO activity in mff-inoculated (= immune-activated) mosquitoes was significantly increased at 24 hr postinoculation (PI) in A. aegypti and at 6, 12, and 24 hr PI in A. trivittatus as compared with saline-inoculated controls. Baseline and immune-activated levels of hemocyte MPO activity in A. trivittatus were significantly higher compared with those seen in A. aegypti. Baseline hemocyte population levels were similar in both species, but immune activation did not elicit increases in total hemocyte populations in A. trivittatus as has been demonstrated for A. aegypti. Likewise, immune activation by the inoculation of mff did not significantly alter plasma MPO activity in A. trivittatus as compared with uninoculated or saline-inoculated mosquitoes. Plasma MPO activity in A. aegypti, however, appears to constitute a major component of the immune response. The importance of phenol oxidase(s) in the immune response of mosquitoes against mff and the relationship of observed differences in MPO activity to differences in immunological capability between A. aegypti and A. trivittatus are assessed.