Malaria is a disease of poverty and continues to plague a great part of the world’s population. An increased understanding of the interactions between the vector mosquito, the malaria parasite, and also the mosquito gut microbiota are pivotal for the development of novel measures against the disease. The first aim of this thesis was to gain a deeper knowledge of the microbial compounds that elicit immune responses in the main malaria vector Anopheles gambiae and also using the model organism Drosophila melanogaster. The second aim was to analyse the genome characteristics in silico of a bacterial symbiont from the mosquito midgut. In Paper I, we investigated the immunogenic effects of (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate in Anopheles. This compound is the primary activator of human Vγ9Vδ2 T cells and is only produced by organisms that use the non-mevalonate pathway for isoprenoid synthesis, such as Plasmodium and most eubacteria but not animals. We show that the parasite releases compounds of this nature and that provision of HMBPP in the bloodmeal induces an immune response in the mosquito. In Paper II, we investigated whether bacteria inactivated by gamma-irradiation could still stimulate potent immune responses in Drosophila cells. We show that E. coli retains the capacity to synthesize and release peptidoglycan de novo for several days after the irradiation event. When cells were stimulated with supernatants from irradiated bacteria, however, a unique response was observed. In Paper III, we presented the draft genome sequence of Elizabethkingia anophelis, a predominant gut symbiont of An. gambiae recently described in our lab and subsequently found in another laboratory strain of the mosquito. The genome data were then annotated in Paper IV to gain insights into the symbiotic characteristics of the bacterium, as well as the genetic background for its strong antibiotic resistance. In conclusion, this thesis work has shed light on novel modes for stimulating immune responses in insects and also led to the characterization of a predominant bacteria in the mosquito gut that may be used in future malaria intervention strategies.
At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Manuscript. Paper 4: Manuscript.