Human immune response against salivary antigens of Simulium damnosum s.l.: A new epidemiological marker for exposure to blackfly bites in onchocerciasis endemic areas
Abstract
Author summary Measuring biting rates of bloodsucking arthropods has been proved to be a crucial tool in understanding transmission dynamics and evaluating control measures against vector-borne diseases. Blackflies of the S. damnosum sensu lato (s.l.) complex are the prime vectors of Onchocerca volvulus, a filarial nematode and causative agent of human onchocerciasis (or river blindness) in Africa.
Blackfly biting rates are typically measured by human attractant vector collectors who capture host-seeking female flies as they alight on their skin but before they take a blood meal. Apart from the ethical concerns raised by this method, estimates thus obtained do not reflect true exposure to vector bites.
An understanding of whether there are people who receive more bites than others and who may be at higher risk of acquiring the infection is crucial to modelling control interventions. Therefore, in the present study we developed IgG- and IgM-based immunoassays to quantify antibody responses in human populations to salivary antigens of S. damnosum s.l. in onchocerciasis-endemic communities of Ghana.
The proteome of S. damnosum s.l. salivary glands was also investigated to potentially identify salivary antigens that could be employed as exposure markers against this important vector group. Background Simulium damnosum sensu lato (s.l.) blackflies transmit Onchocerca volvulus, a filarial nematode that causes human onchocerciasis.
Human landing catches (HLCs) is currently the sole method used to estimate blackfly biting rates but is labour-intensive and questionable on ethical grounds. A potential alternative is to measure host antibodies to vector saliva deposited during bloodfeeding.
In this study, immunoassays to quantify human antibody responses to S. damnosum s.l. saliva were developed, and the salivary proteome of S. damnosum s.l. was investigated. Methodology/Principal findings Blood samples from people living in onchocerciasis-endemic areas in Ghana were collected during the wet season; samples from people living in Accra, a blackfly-free area, were considered negative controls and compared to samples from blackfly-free locations in Sudan.
Blackflies were collected by HLCs and dissected to extract their salivary glands. An ELISA measuring anti-S. damnosum s.l. salivary IgG and IgM was optimized and used to quantify the humoral immune response of 958 individuals.
Both immunoassays differentiated negative controls from endemic participants. Salivary proteins were separated by gel-electrophoresis, and antigenic proteins visualized by immunoblot.
Liquid chromatography mass spectrometry (LC-MS/MS) was performed to characterize the proteome of S. damnosum s.l. salivary glands. Several antigenic proteins were recognized, with the major ones located around 15 and 40 kDa.
LC-MS/MS identified the presence of antigen 5-related protein, apyrase/nucleotidase, and hyaluronidase. Conclusions/Significance This study validated for the first time human immunoassays that quantify humoral immune responses as potential markers of exposure to blackfly bites.
These assays have the potential to facilitate understanding patterns of exposure as well as evaluating the impact of vector control on biting rates. Future studies need to investigate seasonal fluctuations of these antibody responses, potential cross-reactions with other bloodsucking arthropods, and thoroughly identify the most immunogenic proteins.