A PGRPLC1/Rel2-F axis controls Anopheles gambiae resistance to systemic infections with Gram-positive bacteria containing Lys-type peptidoglycan

In the Afrotropical malaria vector Anopheles gambiae s.l., the Imd pathway plays pleiotropic roles in immunity, including resistance to malaria parasites, that are mediated by its NF-κB transcription factor Rel2. Rel2 exists as a full-length form (Rel2-F) containing the Rel-homology domain (RHD) and the C-terminal inhibitory ankyrin (Ank) and death domains (DD), and a shorter alternatively spliced form (Rel2-S) proposed to encode a constitutively active protein containing only the RHD. Despite its important roles in immunity, there are still multiple uncertainties concerning the identity and function of key components of the pathway as well as its overall contribution to mosquito resistance to systemic bacterial infections. Here, we show that Rel2 is critical for limiting the burden of Gram-negative and Gram-positive bacterial proliferation in An. gambiae s.s. after systemic infections and this function is attributed to the endoproteolytic activation of Rel2-F in the fat body but not to Rel2-S. Interestingly, while Rel2-F activation in the fat body regulates Cecropin 1 and Defensin 1 expression, its activation in the midgut after oral infections is dispensable for their regulation. We provide direct evidence that PGRPLC1 is necessary and sufficient for Rel2-F activation in the fat body in response to infections with Gram-positive bacteria containing Lysine-type peptidoglycan, however sensing of Gram-negative bacteria and Gram-positive bacilli containing DAP-type peptidoglycan is more complex and may be mediated by various PGRPLC isoforms, indicating that the mosquito Imd pathway integrates distinct receptor modules to sense Gram-positive and Gram-negative bacterial infections.