The innate immune and systemic response in honey bees to a bacterial pathogen, Paenibacillus larvae

Abstract

BackgroundThere is a major paradox in our understanding of honey bee immunity: the high population density in a bee colony implies a high rate of disease transmission among individuals, yet bees are predicted to express only two-thirds as many immunity genes as solitary insects, e.g., mosquito or fruit fly. This suggests that the immune response in bees is subdued in favor of social immunity, yet some specific immune factors are up-regulated in response to infection. To explore the response to infection more broadly, we employ mass spectrometry-based proteomics in a quantitative analysis of honey bee larvae infected with the bacterium Paenibacillus larvae. Newly-eclosed bee larvae, in the second stage of their life cycle, are susceptible to this infection, but become progressively more resistant with age. We used this host-pathogen system to probe not only the role of the immune system in responding to a highly evolved infection, but also what other mechanisms might be employed in response to infection.ResultsUsing quantitative proteomics, we compared the hemolymph (insect blood) of five-day old healthy and infected honey bee larvae and found a strong up-regulation of some metabolic enzymes and chaperones, while royal jelly (food) and energy storage proteins were down-regulated. We also observed increased levels of the immune factors prophenoloxidase (proPO), lysozyme and the antimicrobial peptide hymenoptaecin. Furthermore, mass spectrometry evidence suggests that healthy larvae have significant levels of catalytically inactive proPO in the hemolymph that is proteolytically activated upon infection. Phenoloxidase (PO) enzyme activity was undetectable in one or two-day-old larvae and increased dramatically thereafter, paralleling very closely the age-related ability of larvae to resist infection.ConclusionWe propose a model for the host response to infection where energy stores and metabolic enzymes are regulated in concert with direct defensive measures, such as the massive enhancement of PO activity.

Mechanisms of stable lipid loss in a social insect

“…The sample was clarified for 10min at 16,100 g at 4°C and the pelleted debris was discarded, while the supernatant proteins were precipitated by adding ethanol and sodium acetate as described elsewhere (Foster et al, 2003). Hemolymph was processed as described previously (Chan et al, 2006) and hemolymph and fat body proteins were prepared for mass spectrometry, isotopically labeled and analyzed on an LTQOrbitrapXL (ThermoFisher, Loughborough, Leics, UK) exactly as described before (Chan et al, 2009). The reported relative protein expression average was calculated by averaging across at least two of the three biological replicates, provided that the total number of measured peptides for that protein was not less than three.…”

Characterization of antimicrobial peptide genes from Japanese honeybee Apis cerana japonica (Hymenoptera: Apidae)

“…Furthermore, Chan et al (2009) detected increased levels of prophenoloxidase (proPO) and lysozyme in the larval hemolymph, suggesting that ProPO and lysozyme play important roles, involving killing pathogens directly. Neither abaecin nor defensin was detected in honeybee hemolymph in response to P. larvae infection (Chan et al, 2009), which is in agreement with the results of our gene expression analysis.…”

“…This weak antimicrobial activity of abaecin seems to function as a back-up for apidaecin-resistant bacteria (Casteels et al, 1993). Recently, it was reported that the response of honeybees to P. larvae is involved with another immune effector molecule, hymenoptaecin (Chan et al, 2009). The authors were only able to quantify hymenoptaecin in the hemolymph of P. larvae-infected European honeybees.…”

Interaction between pesticides and other factors in effects on bees

“…by the American foulbrood organism Paenibacillus larvae. (Chan et al, 2009;Evans, 2004). This has been identified in colonies dosed with fenoxycarb which was observed to predispose the treated hives to European foulbrood and sacbrood (a virus) (Marletto et al, 1992).…”

“…The capacity for immune function varies with age in honeybees but not in bumble bees (James and Xu, 2011). While there are no differences in encapsulation response between developmental stages (Wilson-Rich et al, 2008) honeybee larvae and pupae have the highest total haemocyte counts and lowest levels of phenoloxidase (PO) activity but high levels of catalytically inactive pro-PO (prophenoloxidase) (Laughton et al, 2011) that is proteolytically activated upon infection (Chan et al, 2009). Phenoloxidase activity increases post-emergence reaching a plateau within the first week and does not decline with age in workers despite the reported reduction in haemocyte numbers (Schmid et al, 2008).…”