Abstract
BackgroundMosquitoes respond to infection by mounting immune responses. The primary regulators of these immune responses are cells called hemocytes, which kill pathogens via phagocytosis and via the production of soluble antimicrobial factors. Mosquito hemocytes are circulated throughout the hemocoel (body cavity) by the swift flow of hemolymph (blood), and data show that some hemocytes also exist as sessile cells that are attached to tissues. The purpose of this study was to create a quantitative physical map of hemocyte distribution in the mosquito, Anopheles gambiae, and to describe the cellular immune response in an organismal context.ResultsUsing correlative imaging methods we found that the number of hemocytes in a mosquito decreases with age, but that regardless of age, approximately 75% of the hemocytes occur in circulation and 25% occur as sessile cells. Infection induces an increase in the number of hemocytes, and tubulin and nuclear staining showed that this increase is primarily due to mitosis and, more specifically, autonomous cell division, by circulating granulocytes. The majority of sessile hemocytes are present on the abdominal wall, although significant numbers of hemocytes are also present in the thorax, head, and several of the appendages. Within the abdominal wall, the areas of highest hemocyte density are the periostial regions (regions surrounding the valves of the heart, or ostia), which are ideal locations for pathogen capture as these are areas of high hemolymph flow.ConclusionsThese data describe the spatial and temporal distribution of mosquito hemocytes, and map the cellular response to infection throughout the hemocoel.
Highlights
Mosquitoes respond to infection by mounting immune responses
In both the culicine and anopheline mosquito lineages there are several morphologically distinct classes of hemocytes: granulocytes are involved in the phagocytosis response, oenocytoids are involved in the melanization response, and prohemocytes are small cells of unknown function that have been hypothesized to serve as hematopoietic progenitors [14-16]
Circulating and sessile hemocyte numbers: effect of age and infection Examination of fluorescently labeled hemocytes revealed that at two days post-eclosion, naïve mosquitoes contain an average of 3,811 circulating hemocytes and that this number decreases to 1,781 and 1,720 by six and sixteen days after emergence, respectively (Figure 1A)
Summary
Mosquitoes respond to infection by mounting immune responses. The primary regulators of these immune responses are cells called hemocytes, which kill pathogens via phagocytosis and via the production of soluble antimicrobial factors. Mosquito hemocytes are circulated throughout the hemocoel (body cavity) by the swift flow of hemolymph (blood), and data show that some hemocytes exist as sessile cells that are attached to tissues. The immune cells, called hemocytes, exist within an open circulatory cavity called the hemocoel [4,5], where they phagocytose foreign organisms and help coordinate the humoral response to infection [1,3]. Because of its effect on transmission dynamics, the mosquito immune response could be harnessed for the control of mosquito-borne diseases [11-13] In both the culicine and anopheline mosquito lineages there are several morphologically distinct classes of hemocytes: granulocytes are involved in the phagocytosis response, oenocytoids are involved in the melanization response, and prohemocytes are small cells of unknown function that have been hypothesized to serve as hematopoietic progenitors [14-16]. A strong consensus indicates that naïve adult mosquitoes contain between 1,000 and 3,000 circulating hemocytes [14,18,20-22], which is similar to the number of hemocytes present in the related dipteran, Drosophila melanogaster [23,24]
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