Humoral response to Anopheles gambiae salivary protein gSG6: a serological indicator of exposure to anopheline mosquitoes.

Human antibody response to Anopheles saliva has been suggested as a potential serological marker of exposure to vector bites and malaria risk. However, saliva is a complex mixture: thus cross-reactivity with other antigens may be misleading. Moreover, obtaining large amounts of mosquito saliva is neither easy or reproducible. We have shown that An. gambiae saliva contains several anopheline-specific proteins, i.e. not found in culicine mosquitoes or other blood feeders. These proteins, if immunogenic, may represent ideal serological indicators of exposure to Anopheles bites. To test this hypothesis we measured the humoral immune response to the An. gambiae s.s. salivary protein gSG6 in human sera collected during three consecutive years in rural malaria hyperendemic areas of Burkina Faso. We found that gSG6 is immunogenic and elicits in the exposed population an IgG response that varies according to the level of malaria transmission. Interestingly, a drop in the IgG levels was observed during the dry low transmission season, suggesting that this response is short-lived. Significantly different IgG levels were found in the two sympatric ethnic groups, Mossi and Fulani, previously shown to differentially respond to several P. falciparum antigens. The IgG response also varied according to age: surprisingly, it was higher in children up to ten years old and progressively decreased, a trend that is in conflict with the typical response to malaria antigens. Furthermore, the application of pyrethroid-impregnated bednets attenuated some of the above-described differential responses. In conclusion, our study shows that antibody response to anopheline-specific salivary antigens may be a reliable marker of exposure to malaria vectors. This indicator may represent a complementary tool for epidemiological studies and for monitoring anti-vector control campaigns, especially in settings where the assessment of classical entomological parameters would be difficult or impossible.