Antimicrobial peptides: a key component of honey bee innate immunity

Honey bee immune responses are composed of a complex suite of individual immune mechanisms and special types of behavioral adaptations. The main focus of this paper is innate immunity in the honey bee, and specifically, the role and function of antimicrobial peptides (AMPs). Insect innate immunity constitutes evolutionary conserved defense strategies that provide immediate responses against invading pathogens. It consists of the three levels of resistance: physical barriers as the first line of defense, cell-mediated immunity, and cell-free humoral immunity, a complex network of intracellular signaling pathways leading to activation of a variety of humoral factors. Among those, AMPs are recognized as key components of humoral immunity in many types of organisms. The two basic mechanisms of action are: (1) the generation of leaks into prokaryotic membranes; and (2) either inhibition of bacterial protein translation or folding. Recently, four families of AMPs (i.e., apidaecins, abaecin, hymenoptaecin and defensins) have been described in the honey bee. One of the defensins, defensin1, was originally isolated from royal jelly, and therefore named royalisin. In addition, several bioactive peptides (e.g., apamin and melittin) were identified in bee venom. The expression of bee AMPs is regulated mainly by two intracellular signaling pathways Toll and Imd/JNK. However, the extent of gene expression and peptides synthesis is affected by a number of different biotic and abiotic factors. In this review paper, we have attempted to discuss factors involved in activation of the honey bee AMPs and their role in bee resistance to microbial pathogens and environmental stress such as exposure to pesticides. We also discuss recent knowledge on the molecular regulation of bee AMPs. Although recent advances in genomics have produced a new understanding of bee immunity in general, the exact mechanisms of gene regulation within each of the immune signaling pathways and the complex network of these signaling pathways still await further investigations.