Abstract
Malaria affects millions of people worldwide and hundreds of thousands of people each year in Brazil. The mosquito Anopheles aquasalis is an important vector of Plasmodium vivax, the main human malaria parasite in the Americas. Reactive oxygen species (ROS) have been shown to have a role in insect innate immune responses as a potent pathogen-killing agent. We investigated the mechanisms of free radicals modulation after A. aquasalis infection with P. vivax. ROS metabolism was evaluated in the vector by studying expression and activity of three key detoxification enzymes, one catalase and two superoxide dismutases (SOD3A and SOD3B). Also, the involvement of free radicals in the mosquito immunity was measured by silencing the catalase gene followed by infection of A. aquasalis with P. vivax. Catalase, SOD3A and SOD3B expression in whole A. aquasalis were at the same levels of controls at 24 h and upregulated 36 h after ingestion of blood containing P. vivax. However, in the insect isolated midgut, the mRNA for these enzymes was not regulated by P. vivax infection, while catalase activity was reduced 24 h after the infectious meal. RNAi-mediated silencing of catalase reduced enzyme activity in the midgut, resulted in increased P. vivax infection and prevalence, and decreased bacterial load in the mosquito midgut. Our findings suggest that the interactions between A. aquasalis and P. vivax do not follow the model of ROS-induced parasite killing. It appears that P. vivax manipulates the mosquito detoxification system in order to allow its own development. This can be an indirect effect of fewer competitive bacteria present in the mosquito midgut caused by the increase of ROS after catalase silencing. These findings provide novel information on unique aspects of the main malaria parasite in the Americas interaction with one of its natural vectors.
Highlights
Malaria is an important health problem that affects millions of people and causes millions of deaths worldwide each year
Following evidence for a role of reactive oxygen species (ROS) in A. stephensi and A. gambiae immunity, we investigated the recruitment of ROS as an immune defense of the Brazilian natural malaria vector A. aquasalis against P. vivax, the main human malaria parasite in the Americas
A 1989 bp full-length A. aquasalis catalase cDNA (AqCAT) was obtained, including a 1515 bp coding region, which translates into a 505 amino acid protein, as well as a 161 bp 59 untranslated region (UTR) and 313 bp 39 UTR (Figure S1)
Summary
Malaria is an important health problem that affects millions of people and causes millions of deaths worldwide each year. For malaria transmission to occur the parasite needs to complete a complex life cycle inside the insect vector that includes: differentiation of gametes, fertilization, passage through the midgut epithelial cells, establishment in the midgut basal lamina as oocysts, multiple cell divisions with breakdown of the oocysts and the release of hundreds of sporozoites into the hemolymph, invasion of the salivary gland, differentiation, and inoculation into a new vertebrate host [3] During these steps the parasite interacts with diverse insect tissues causing activation of the mosquito powerful innate immune defenses, which are responsible for major parasite losses [4,5]. This mucous protection may prevent the deleterious effect of the immune response to the host itself and to commensal gut bacteria
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