Humoral response to mosquito salivary proteins as serological indicator of exposure to disease vectors: IgG response to the Anopheles gambiae gSG6 and malaria.

Saliva of hematophagous arthropods, injected into the vertebrate during blood feeding, triggers the production of anti-saliva IgG and IgE antibodies. This host immune response to vector saliva (from ticks, sand flies, triatomines, tsetse and mosquitoes) may be used as a marker of exposure to vector bites and indicator of disease risk. Indeed, the potential to estimate vector density through serological assays may represent a very valuable tool for epidemiological studies. Human humoral response to Anopheles saliva has been correlated with the exposure to anopheline vectors and with malaria transmission (Remoue F et al., 2006; Waitayakul A et al., 2006; Orlandi-Pradines E et al., 2007). However, saliva is a complex mixture and cross-reactivity with antigens from other mosquitoes and/or other blood-feeding arthropods may be misleading. Moreover, obtaining large amounts of mosquito saliva or salivary extracts is tedious and not easily reproducible. In this respect recent studies on the salivary transcriptomes of blood feeding vectors provided some interesting perspectives. In fact, comparative analysis clarified that the mosquito salivary repertoire includes both salivary proteins that are widely shared among Culicidae family members and proteins that are genus-specific, i.e. found only in Anopheles, but not in Aedes or Culex species, and viceversa. This finding implies that anopheline-specific proteins, if immunogenic, may be ideal serological indicators of exposure to Anopheles bites and, similarly, that Aedes-specific proteins could be used as markers of exposure to Aedes-mosquitoes. The An. gambiae salivary protein gSG6 is a small protein specifically expressed in adult female glands where it plays some important role in blood feeding (Lanfrancotti A et al., 2002; Lombardo F et al., 2009). So far gSG6 has been found only in species of the An. gambiae complex and in a few additional anophelines (An. stephensi, An. funestus and An. freeborni), whereas it is absent in culicine mosquitoes and in other blood feeding arthropods. Preliminary indications suggested that gSG6 is immunogenic (Poinsignon et al., 2008) encouraging a deeper evaluation of its possible use as epidemiological marker of exposure to Anopheles vectors. To test this hypothesis we measured the humoral immune response to the An. gambiae salivary protein gSG6 in human sera collected during three consecutive years in rural malaria hyperendemic areas of Burkina Faso. We confirmed that gSG6 is immunogenic and found that it elicits, in the exposed population, an IgG response that varies according to the level of malaria transmission. Interestingly, this anti-gSG6 IgG response: (a) is short-lived, since a drop in the IgG levels was observed during the dry low transmission season; (b) is significantly different in the two sympatric ethnic groups, Mossi and Fulani, previously shown to differentially respond to several P. falciparum antigens (Modiano D et al., 1996; Torcia MG et al., 2008); (c) starts very early, reaching a maximum in one-two years old children, and then decreases according to age. This is to our knowledge the first study employing a mosquito recombinant salivary protein for a large scale malaria epidemiological analysis. We believe that our study provides a solid indication that antibody response to anopheline-specific salivary antigens may be a reliable marker of exposure to malaria vectors suggesting that, similarly, culicine-specific salivary antigens may be exploited as markers of exposure to Aedes and Culex vectors. In conclusion salivary antigens from mosquito disease vectors may represent valuable complementary tools for epidemiological studies in settings where the assessment of classical entomological parameters would be difficult or impossible.