Humoral immune response

Insect humoral immune responses involve secretion of antimicrobial peptides by fat bodies that is functionally equivalent to the mammalian liver, into the hemolymph in response to challenges to the immune system. Most of our knowledge of the insect humoral immune response is derived from studies of Drosophila. To date, seven classes of antimicrobial peptides, including attacin, cecropin, defensin, diptericin, drosocin, dro-somycin, and metchnikowin, have been identified in Drosophila, and their expression has been found to be regulated by two NF-kB signaling pathways, Toll pathway and immune deficiency (Imd) pathway (reviewed by Bulet et al., 2004; Leclerc and Reichhart, 2004). The humoral signaling pathway is also triggered by the binding of PAMPs to PGRPs and GNBPs which is involved in the upstream infection recognition. The Toll pathway has long been recognized to be a critical signaling pathway during Grampositive bacterial and fungal infections. The Toll transduction cascade is activated when the ligand, Spatzle, binds to the transmembrane Toll receptor and induces the recruitment of a protein complex consisting of MyD88, Tube, and Pelle. The recruitment of the protein complex leads to the proteasome-dependent degradation of cactus. The degradation of cactus allows translocation of two NF-kB transcription factors, Dif and Dorsal, to the nucleus, causing rapid expression of gene-encoding antimicrobial peptides such as defensin, drosomycin, and metchnikowin. Imd signaling pathway, in contrast, is specific for Gram-negative bacteria although it is activated in a similar fashion to the Toll pathway. The Imd pathway activates a transcription factor, Relish, and the processed Relish then enters the nucleus where it regulates the expression of the gene-encoding antibacterial peptides such as attacin, cecropin, diptericin and drosocin. A study by Zambon et al. (2005) reported that both the Toll and Imd pathways were activated in Drosophila by an infection of Dro-sophila X virus (DXV), a dsRNA virus. Their studies showed that Toll pathway was required for the inhibition of DXV replication and that the inactivation of the Toll pathway could result in increases in virus titer and death in infected flies. This study clearly indicates that the Toll pathway was an essential part of the antiviral response in Drosophila.

Another study conducted by Dostert et al. (2005) showed that infection of Drosophila C virus (DCV), a member of the genus Cripavirus and the family Dicistroviridae, that several honey bee viruses also belong to, induced a set of genes distinct from those regulated by the Toll and Imd pathways and triggered a Janus Kinase-signal transducer and activator of transcription (Jak-STAT) DNA-binding activity. Therefore, they suggested that a Jak-STAT signaling pathway is required for an antiviral response in Drosophila (Dostert et al., 2005). The Jak-STAT pathway is ubiquitous amongst vertebrates. The signaling pathway takes part in the regulation of cellular responses to a variety of cytokines and growth factors to alter gene expression. The binding of a cytokine or growth factor to its receptor activates Jak, a cytoplasmic tyrosine kinase, and triggers it to phosphory-late and stimulate STAT, a gene regulatory protein, to detach from the receptor and translocate to the nucleus. Different STATs accumlated in the nucleus form hetero- and homodimers that induce expression of their target genes. Studies by Dostert et al. clearly indicated that in addition to Toll and Imd pathways for defense against bacterial and fungal infections, another evolutionarily conserved innate immune pathway, Jak-STAT pathway, exists in Drosophila and participates in the function of antiviral infections.

Several antimicrobial peptides including abaecin, apidaecin, hyme-noptaecin, and defensin have been identified in the hemolymph of honey bees on induction of bacterial infections (Casteels et al., 1989, 1990; Casteels-Josson et al., 1994). These peptides do appear to be involved in the bee immune response to pathogen infections. A recent genome-wide analysis of honey bee immunity indicates that honey bees possess ortho-logues for the core members involved in different recognition and signaling pathways including Toll, Imd, Jak-STAT, as well as JNK, which is also a pivotal actor in the Drosophila immune response and involves the activation of transcriptional factor, Basket, though the functions of most honey bee components in these pathways remain to be validated (Evans et al., 2006). The data generated from Drosophila studies indicate that insects have an effective innate immune system that is able to respond not only to bacterial and fungal infections but also to viral infections. Knowledge of the antiviral immunity demonstrated in Drosophila should provide us with important insight into the relationship between virus infections and host immune responses in honey bees.

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