Anopheles gambiae salivary proteins as a tool to evaluate spatial and temporal variation of human exposure to malaria vectors.

Saliva of blood sucking arthropods, injected into host skin during feeding, evokes an antibody response that may be used as serological indicator of exposure to disease vectors. Studies on mosquito salivary repertoires highlighted that Anopheles saliva carries genus-specific proteins (i.e. not found in Aedes/Culex or in other blood feeders). These proteins, if immunogenic, may be ideal candidates as markers of exposure to anopheline vectors, a tool which would be very useful for malaria epidemiological studies. Human plasma samples collected in an hyperendemic area of Burkina Faso (Ouagadougou) and in an area of moderate malaria endemicity in Tanzania (Korogwe) were used to evaluate by ELISA the IgG, IgG1 and IgG4 antibody responses to the anopheline-specific salivary proteins gSG6 and cE5 from the malaria vector Anopheles gambiae. The anti-gSG6 IgG response varied according to transmission/rainy season and was positively associated with the level of Anopheles exposure. The cE5 protein was more immunogenic than gSG6, but no variation of the IgG response was found during the transmission season, most likely because of its longer lasting nature. Moreover, the anti-gSG6 IgG level decreased with age, suggesting the induction of tolerance mechanisms, which was not the case for the anti-cE5 IgG response. Finally, the analysis of IgG1 and IgG4 subclasses pointed out the differential responses evoked by these two salivary proteins in exposed individuals with (i) gSG6 triggering a short-lived response of the Th2-type, with high IgG4 levels and induction of tolerance and (ii) cE5 eliciting a longer-lived Th1-type response, with high IgG1 levels and no tolerance induction. The Entomological Inoculation Rate, which measures the number of infectious bites/person/unit of time, is presently the gold standard for the assessment of malaria transmission intensity. However, its determination may be problematic or not fully reliable (logistic constraints, low vector density, micro heterogeneity, low infection level, sampling technique, etc.) and additional/alternative tools would be very helpful. Our studies provide clear evidence that the An. gambiae gSG6 salivary protein can be used in malaria transmission areas as a reliable marker to evaluate spatial and temporal variation of human exposure to Afrotropical vectors. The cE5 protein showed higher sensitivity and may be of use in conditions of low or transient exposure to Anopheles vectors (i.e. low vector density, travellers). In conclusion, since parasite antigens (AMA-1, MSP-1, CSP) are already in use for the serological assessment of malaria transmission, the addition of Anopheles salivary antigens for measuring vector exposure may allow for comprehensive epidemiological profiling using just serological techniques.