The role of NF -kappaB transcriptional factor REL2 in the mosquito innate immune response

The insect immune system is an efficient, non-self recognition mechanism that resembles that of mammals. Pathogen recognition leads to activation of immune signaling pathways and results in synthesis of an array of effector molecules, including antimicrobial peptides (AMPs). Imd is one of the two major insect immune signaling pathways, in which NF-kappaB transcriptional factors are the key regulators. The Imd pathway controls the activity of the NF-kappaB factor Relish and is required for resistance against Gram-negative bacteria. Signaling through Imd results in proteolytic cleavage and nuclear translocation of Relish, which then binds to the target promoters.;Mosquito REL2, a homologue of Drosophila Relish and mammalian NF-kappaB factors p100 and p105, is a combined protein consisting of a functional RHD domain and an inhibitory ankyrin domain, which has to be cleaved for REL2 activation. The REL2 gene of the mosquito Aedes aegypti gives rise to three alternative isoforms: predominant full-length REL2-Long; REL2-Short containing a RHD domain, but lacking an ankyrin domain; and IkappaB-type lacking a RHD but containing an ankyrin domain. In the current study, I investigated the role of Ae. aegypti REL2 in AMP expression, and in anti-bacterial and anti-Plasmodium immunity.;I used mosquito transgenesis to overexpress the REL2-Short isoform under the control of the fat-body-specific, blood-meal-activated vitellogenin promoter. Blood-meal-activated overexpression of REL2-Short led to transcriptional activation of AMPs---Defensins A, C, and D, and Cecropins A and N---as well as to secretion of Defensin A protein into the hemolymph. Moreover, the induction of REL2 resulted in increased resistance of the mosquito to Gram-negative and Gram-positive bacteria and to Plasmodium gallinaceum infection.;Next, I studied the individual immune functions of Ae. aegypti REL2 isoforms. Using RNAi, I knocked down both REL2-Long and REL2-Short simultaneously or REL2-Long individually and monitored AMP expression and resistance to bacteria and P. gallinaceum. I found that REL2 isoforms may have complementary functions and that REL2 is essential for mosquito antibacterial resistance and is an important component of anti-Plasmodium immunity.;Melanization is another key component of the insect innate immune response and one of the mechanisms of anti-Plasmodium resistance in mosquitoes. I investigated the role of the Ae. aegypti serine protease inhibitor Serpin2 in the regulation of melanization and immunity against P. gallinaceum. I showed that Ae. aegypti Serpin 2 is a negative regulator of melanization, although it does not affect the development of Plasmodium oocysts. Together, my findings contribute to current knowledge of the roles that NF-kappaB factor REL2 and Serpin 2 (a modulator of melanization) play in mosquito innate immunity.