Effects of blood meal on mosquito cellular immunity

Malaria continues to be a global public health problem, especially in sub-Saharan Africa where the mosquito Anopheles gambiae s.s. serves as the major vector for the protozoan Plasmodium falciparum. A vital aspect of vector competence for malaria is the mosquito immune system. Hemocytes, a critical component of mosquito immunity, represent the cellular arm of immunity and produce soluble immune factors that either support or prevent malaria parasite development. However, despite their importance in vector competence, understanding of their basic biology is lacking. Due to the obligatory nature of the anautogenous life style of An. gambiae, we set out to assess effects of blood meal on mosquito hemocytes. We found the blood meal alone activates mosquito blood cells. Blood cell activation was determined by conserved activation markers found in mammals and Drosophila including an increase in: (i) hemocyte number, (ii) DNA replication, (iii) size, (iv) granularity, (v) surface moiety changes, and (vi) Ras-MAPK activation. Specific to mosquito immunity, blood meal caused an increase in expression of critical components of complement and melanization, namely TEP-1 and PPO6. Temporal analysis of hemocyte activation was transient, peaking at 1 day post blood meal. Additionally, reverse genetics analyses found Ras-signaling to play a pivotal role in hemocyte homeostasis. This study demonstrates that in An. gambiae, hemocyte numbers and activation are tightly linked with blood meal, indicating an overlooked critical mechanistic link between blood feeding and immunity. We posit that blood meal induced effects on mosquito hemocytes plays a major role in vector competency.