Mosquito infection with various bacterial species induces periostial hemocyte aggregation and reduces heart contraction rates

Mosquito-borne pathogens must traverse the hemocoel prior to being transmitted. This process is affected by hemolymph circulation, as flow influences both pathogen movement and the movement of mosquito-produced cellular and humoral immune factors. A recently described immune tissue in Anopheles gambiae, called periostial hemocytes, associates closely with the circulatory system, as hemocytes in circulation attach to the heart near the ostia (valves), and form large cellular aggregates that phagocytose pathogens. The aggregation of hemocytes at the periostial regions has been observed during Escherichia coli and Plasmodium infection, however, little is known about the mechanisms of hemocyte aggregation or how heart physiology changes following infection. Here, we examined the heart for periostial hemocyte aggregation in mosquitoes infected with Staphylococcus aureus, Staphylococcus epidermidis and Micrococcus luteus. Periostial hemocyte aggregation occurred during all infections, confirming the fundamental role of this immune response. Periostial hemocyte aggregation was not uniform along the length of the heart: larger hemocyte aggregates consistently formed in abdominal segments 4, 5, and 6. Furthermore, melanization was evident on the surface of the heart, with the areas containing the largest melanin deposits corresponding to the segments where hemocyte accumulation was the highest. Assessment of heart physiology following infection then revealed that periostial hemocyte aggregation is accompanied by a decrease in the heart contraction rate. In summary, changes in hemocyte distribution and heart physiology following infection suggest that the interface between circulation and immunity is highly dynamic.