A novel biomarker of human exposure to Aedes albopictus based on the Ag5-3 salivary protein from the tiger mosquito

Background: Mosquito-borne arboviral diseases represent a growing threat and serious worldwide concern for public health authorities. Host immunoglobulin G (IgG) responses to mosquito salivary antigens emerged as a useful additional tool to evaluate human-vector contact, which is crucial for transmission risk assessment and planning vector control interventions. We previously reported that IgG responses to the Aedes albopictus 34k2 salivary protein (al34k2) are suitable, although with some limitations, to reveal variation of human exposure to the tiger mosquito. In this study we evaluated the Ae. albopictus Ag5-3 (alAg5-3), an Antigen 5 family member specifically and abundantly expressed in the saliva of adult females. Methods: IgG responses to recombinant alAg5-3, as well as to a combination of alAg5-3 and al34k2, were measured in a set of sera previously collected from healthy human blood donors before and after the summer season of exposure to mosquito bites. Surveys were conducted in two districts of Northeast Italy, Padua and Belluno, with different density and history of colonization by the tiger mosquito Ae. albopictus. Results: A preliminary pilot study, performed on a small subset of individuals from Padua, indicated that alAg5-3 was more immunogenic than al34k2 and may be suitable to detect variations of exposure to Ae. albopictus. Analysis of the whole set of 523 sera showed that anti-alAg5-3 IgG levels significantly increased, in both study areas, after the summer period of high mosquito density. However, differences between the two study sites were only found when a mixture of the two antigens, alAg5-3 and al34k2, was used. Conclusions: IgG responses to alAg5-3 represent a novel appropriate marker to evaluate seasonal variation of human exposure to Ae. albopictus and, because of its higher sensitivity, it appears preferable to al34k2, especially for longitudinal studies in conditions of low-to-moderate mosquito density. However, the combination of both antigens may be a better surrogate of Ae. albopictus saliva since it allows the detection of both temporal and spatial variations of exposure to Ae. albopictus bites. The high conservation of the Ag5-3 protein among Aedes species suggests it may be exploited to also reveal exposure to Aedes aegypti and perhaps to other Aedes species.