Innate humoral immune defences in mammals and insects: The same, with differences ?

Abstract

The insect immune response demonstrates many similarities to the innate immune response of mammals and a wide range of insects is now employed to assess the virulence of pathogens and produce results comparable to those obtained using mammals. Many of the humoral responses in insects and mammals are similar (e.g. insect transglutaminases and human clotting factor XIIIa) however a number show distinct differences. For example in mammals, melanization plays a role in protection from solar radiation and in skin and hair pigmentation. In contrast, insect melanization acts as a defence mechanism in which the proPO system is activated upon pathogen invasion. Human and insect antimicrobial peptides share distinct structural and functional similarities, insects produce the majority of their AMPs from the fat body while mammals rely on production locally at the site of infection by epithelial/mucosal cells. Understanding the structure and function of the insect immune system and the similarities with the innate immune response of mammals will increase the attractiveness of using insects as in vivo models for studying host - pathogen interactions.

Keywords: Antimicrobial peptide; Toll; coagulation; humoral immunity; insect; mammal; melanization.

Figure 1.

Diagramatic representation of the similarities between invertebrate Toll signalling and vertebrate toll-like signalling. Upon activation of invertebrate toll receptor and the homologous toll-like receptor in vertebrates, a cascade is induced where the homologous transciption facotors Nf-κB and Dif are activated in vertebrates and invertebrates, respectively. Upon translocation of these transcription factors, AMPs are produced in invertebrates while co-stimulatory molecules and pro-inflammatory cytokines such as IL-1β, IL-6 and IL-8 are produced in vertebrates.

Figure 2.

Comparision of insect IMD pathway and mammalian TNF-α pathway. The IMD pathway is activated by binding of peptidoglycan (PGN) to peptidoglycan-recognition proteins (PGRPs) which results in recruitment and formation of a IMD, dFADD and DREDD complex and results in IMD cleavage and subseqeunt activation of TAB2/TAK1. This results in Relish phosphorylation and ultimately the production of AMPs (e.g cecropin). Alternatively in mammals, TNF-α is bound by the tumor necrosis factor receptor 1 (TNF-R1) which results in recuirment of RIPP, FADD and caspase 8. This complex activates TAK1 which activates the IKK complex resulting in phosphoylation and degradation of the inhibitor protein IκB. NF-κB is released for translocation to the nucleus resulting in pro-inflammatory cytokine production.

Figure 3.

Sequence comparison between human (h) von Willebrand factor and Drosophila (d) Hemolectin protein sequences using emboss needle pairwise sequence alignment. Highlighted in yellow are conserved cysteine residuals. hVWF and dHemolectin are up to 30.9% similar protein sequences. * (asterics); indicates a single, fully conserved residue,: (colon); indicates conservation between groups of strongly similar properties, . (full stop); denotes conservation between groups of weak similar properties).

Figure 4.

Schematic comparison of the hemolymph/blood clotting system in insects versus mammals. Hemolymph clotting in insects involves co-ordination between plasmatocytes, transgluatimase mediated activation of hemolectin, eig71Ee and fondue as well as phenoxidase activation. During the mammalian blood clotting a series of enzymatic reactions result in the formation of thrombin and subsequently the conversion of fibrinogen to fibrin. Activations of the complement cascade also feeds into mammalian coagulation in the same way as phenoloxidase activation in insects. Similarities can be seen in both systems in that TG is homologous to factor XIIIa. Both factors contribute to the formation of a hemolymph/fibrin network.

Figure 5.

Schematic diagram of the proPO-system for melanin production in insects. PAMPs such as β-1,3 glucan, LPS and peptidoglycan amongst others bind pathogen recognition receptors such as β-1,3 glucan-binding protein (βG-bp), lipopolysaccharide-binding protein (LPD-BP) and peptidoglycan-binding protein (PG-BP), respectively. This results in the activation of the serine protease cascade which initates the conversion of prophenoloxidase-activating enzyme from its pro-form (pro-ppA) to its active form (ppA). PpA then catalyzes the conversion of prophenoloxidase (proPO) to phenoloxidase (PO). PO in combination with phenols and O₂ results in the formation of quinones which polymerize to form melanin. Similarily, the alternative complement pathway generates C3b by C3 convertase which with other proteins froms the C5 convertage. This enzyme cleaves C5 to C5a and C5b, the latter of which recuits and assembles C6, C7, C8 and multiple C9 molecules to form a pore forming membrane attack complex which is deposited on the microbial cell surface ultimately resulting in cell lysis.

Figure 6.

Schematic diagram of the mechanism of action and function of antimicrobial peptides in insects and mammals. Both insect and mammalian AMPs display direct microbicidal activity by initiating cell lysis at the cell surface or interfering with intracellular targets. Some AMPs possess anti-biofilm activity (e.g. LL-37), inhibit protein synthesis (e.g. apidaecin) or inhibit microbial proteases (e.g. Histatin-5). Some AMPs also possess pleotropic cell-modulatory activities such as angiogenesis, re-epithelization, chemotaxis, anti-inflammatory and growth effects depending on cell type.