Physiological and Molecular Characterization of the Interactions between the Cellular Immune Response and the Circulatory System of the Mosquito Anopheles gambiae
Sigle, Leah Theresa
Mosquitoes transmit pathogens that cause deadly and debilitating diseases. When a pathogen invades the hemocoel (body cavity) of Anopheles gambiae, a primary vector of malaria, innate immune responses are activated. A subset of circulating hemocytes (immune cells) attach to the surface of the heart, and migrate to the regions surrounding the ostia (valves) and become sessile. Hemolymph (blood) enters the heart through the ostia, circulates through the aorta in the thorax and then empties into the head. At the ostia, aggregates of hemocytes phagocytose and kill pathogens and are called periostial hemocytes. This is the only location where hemocytes aggregate, suggesting that the association with these regions of high hemolymph flow is advantageous, but the mechanisms of this response are unknown. I infected mosquitoes with different pathogens and assessed for periostial hemocyte aggregation and characterized the spatial distribution of hemocytes and their activity on the heart. I found that all infections induce hemocyte aggregation, and the ostia experiencing the highest hemolymph flow contain the highest numbers of hemocytes and phagocytic activity. Furthermore, I characterized the structure of the aorta, and found that hemocytes do not aggregate here or at any other location on the heart, supporting that hemocytes specifically associate with the abdominal ostia. To elucidate mechanisms signaling periostial hemocyte aggregation, at four hours following an infection I compared gene expression in the periostial hemocytes and heart, circulating hemocytes in the hemolymph, and the abdomen. I found that infection induces differential gene expression in the periostial hemocytes and the heart, and fifteen genes are specifically upregulated only in these tissues. This includes three immune genes associated with the immune deficiency pathway, implicating this signaling cascade in the periostial hemocyte response. Lastly, I showed that two genes that were previously shown to be enriched in hemocytes positively affect periostial hemocyte aggregation and pathogen killing by phagocytosis. Together these studies reveal factors involved in the interactions of the immune and circulatory systems and may provide novel strategies to prevent mosquito-borne diseases.