Periostial hemocyte aggregation patterns correlate with differential pathogen phagocytosis on the heart of Anopheles gambiae

Insects have an innate immune system comprised of cellular and humoral factors, with the cellular response consisting of sessile and circulating hemocytes. In Anopheles gambiae a novel immune tissue, called periostial hemocytes, associates with the circulatory system, as hemocytes attach to the heart near the ostia (valves) and form phagocytic cellular aggregates. Hemocytes aggregate at the six periostial regions of the heart during Escherichia coli and Plasmodium infection, suggesting this interaction is essential for cellular-mediated immunity; however, the mechanisms directing this active hemocyte aggregation are unknown. To assess the fundamental role of periostial hemocyte aggregation in pathogen killing, in this study mosquitoes were infected with different bacterial species and hemocyte and pathogen aggregation were both monitored. Periostial hemocyte aggregation was observed during all infections. Furthermore, the distribution across the heart was not uniform: the largest periostial hemocyte aggregates consistently formed in the periostial regions of abdominal segments 4, 5, and 6. These regions also accumulated the highest densities of E. coli and were the regions of highest phagocytosis of both E. coli and Staphylococcus aureus. In summary, periostial hemocytes aggregate on the heart in a distinct pattern. They form the largest aggregates in the mid-abdominal segments, and this distribution correlates with the intensity of local cellular immune responses on the surface of the heart. These results suggest that mosquito immune responses are influenced by hemolymph flow patterns thereby affecting pathogen sequestration and killing.