Cellular and Molecular Mechanisms of Insect Immunity

Multicellular organisms constantly encounter potentially harmful microorganisms. Although insects lack an adaptive immune system, they do have powerful means of fighting infections. Cellular responses involve phagocytosis of bacteria and encapsula ‐ tion of parasites. In addition, insects can mount a humoral response against pathogens. This is characterized by the secretion of antimicrobial peptides into the hemolymph. Recognition of foreign pathogens involves specific receptors for sensing infection. These include peptidoglycan recognition proteins (PGRPs) and β‐glucan recognition proteins (βGRPs). Engagement of these receptors starts signaling pathways that activate the genes that encode antimicrobial peptides. These pathways include the Toll, the Imd, and the JAK‐STAT. This chapter describes the innate immunity of insects including both the cellu ‐ lar and humoral responses to bacteria, fungi, and parasites. In addition, recent advances in insect antivirus immune responses are discussed. for activating the IKK complex, which phosphorylates the NF‐kB‐like nuclear factor Relish. DREDD is also required for mediating the cleavage of the precursor Relish. Upon cleavage and phosphorylation, free Relish can translocate into the nucleus, where it activates transcription of specific antimicrobial peptides (AMP). Monomeric peptidoglycan can be recognized intracellularly by the receptor PGRP‐LE, and also activate the Imd pathway. tyrosine residues on the cytoplasmic part of the receptor. These phosphorylated tyrosines become docking sites for the STAT transcription factor, Stat92E. Hopscotch also phosphorylates Stat92E at tyrosine residues, allowing it to form dimers and then translocate into the nucleus, where it binds the promoters of their target genes. This pathway is also regulated by a negative feedback loop involving the suppressor of cytokine signaling (SOCS) protein Socs36E, which is upregulated by STAT‐JAK signaling. In addition, eye transformer (ET), a nonsignaling receptor for upd, is able to associate with the receptor complex, interacting with both Dome and hop. Thus, ET seems to inhibit intracellular signaling. peptidoglycan and inducing Imd signaling or autophagy independently of Imd. PGRP‐LF functions as an inhibitor of the Imd pathway. PGRP‐LB and ‐SC1a/1b/2 cleave DAP‐type peptidoglycan to inactive fragments, thus preventing activation of the Imd pathway. In addition, PGRP‐SC1a acts as an opsonin for phagocytosis of bacteria. PGRP‐SB1 is directly bactericidal due to its DAP‐type peptidoglycan‐specific amidase activity. of TOR. This kinase phosphorylates autophagy‐related (Atg) 13 protein at multiple sites. This prevents Atg13 binding to Atg1 and other Atg proteins like Atg17, for assembly of the preautophagosomal structure (PAS), which leads to autophagy. Different Atg proteins accumulate with Atg1 under normal growing conditions to generate cytoplasm to vacuole targeting (Cvt) vesicles. During an infection with vesicular stomatitis virus (VSV), the receptor Toll‐7 detects the virus, and the PI3K‐Akt‐TOR signaling pathway is inhibited. This activates autophagy and in turn decreases viral replication.