In
D. melanogaster, NF-κB-dependent AMP induction through the Toll and Imd pathways is activated by detection of microbial components via different pattern recognition receptors (PRRs). PRRs are soluble or membrane-bound proteins that bind to specific microbe associated molecular patterns (MAMPs) such as lipopolysaccharide (LPS), lipoteichoic acid (LTA), peptidoglycan (PGN) or β-1,3-glucan that are released from or are found on the cell surfaces of bacteria or fungi [
55]. Upon interaction with MAMPs, PRRs can directly agglutinate pathogens or trigger proteolytic signaling cascades and cytokine release, which in turn lead to the activation of downstream cellular and humoral pathways, including pro-PO activation and AMP gene expression [
16,
65,
76].
PGN recognition proteins (PGRPs) and β-1,3-glucanase-related proteins (βGRPs) were discovered in the lepidopteran silkworm (
B. mori) by assaying for plasma components that activate the proPO cascade [
77]. PGRPs were subsequently shown to be conserved across mammals and insects [
78], and in
D. melanogaster their role in the induction of AMP gene expression through Toll and IMD pathways in response to PGN has been well documented [
79,
80,
81,
82,
83]. Similarly, βGRPs have been shown to induce AMP expression through Toll pathway in response to fungal infections [
79,
84]. In contrast, there is a dearth of literature linking specific PGRPs or βGRPs to AMP induction in Lepidoptera [
85]. Such a link is possible, since PGN and β-1,3-glucan can activate AMP gene expression in
M. sexta and
B. mori [
85,
86,
87,
88,
89,
90] and multiple infection-induced PGRP- and βGRP-encoding genes have been identified in Lepidoptera [
32,
38,
54,
55,
91,
92,
93,
94]. However, there are numerous hints that Lepidoptera and Diptera may have evolved divergent mechanisms of linking pathogen detection to conserved Toll and IMD signal transduction cascades. First, a genome comparison between
B. mori and
D. melanogaster failed to identify 1:1 PGRP orthologs [
54]. Similarly,
B. mori gram-negative binding protein (GNBP) and
M. sexta microbe binding protein (MBP), members of the β-1,3-glucanase-related protein superfamily [
76,
95], appear to be distantly related to
D. melanogaster GNBPs [
76], suggesting divergence of this group of proteins.
M. sexta MBP expression is strongly up-regulated in fat body after immune challenge and shows specific binding to LTA, LPS, DAP-PGN [
76]. Also, in contrast to the situation in
D. melanogaster, highly purified LPS and LTA are inducers of AMP gene expression in Lepidoptera, though not as potently as crude LPS (with contaminating PGN) or purified PGN [
85,
90,
96,
97]. This raises the possibility that different MAMPs or combinations of MAMPs are most efficacious in eliciting AMP gene expression in Lepidoptera relative to Diptera. Also, since purified LPS can elicit AMP expression in Lepidoptera but not
D. melanogaster, Lepidoptera have either a distinct repertoire of PRRs responsible for LPS-dependent triggering of Imd or Toll pathways, or an as-yet undiscovered pathway that links LPS to AMP induction. Testing these ideas awaits the identification of the suite of PRRs and signal transduction pathways responsible for transducing LPS, LTA, PGN, or combinatorial microbial signals to AMP gene expression.
One class of lepidopteran PRR that may mediate infection-dependent induction of AMPs is the C‑type lectins (CTLs), Ca
2+-dependent, secreted proteins that have carbohydrate-binding capabilities. Lepidopteran CTLs are involved in immunity. Similar to some CTLs of
D. melanogaster [
98], several CTLs of
M. sexta [
55] and
B. mori [
54,
99] are reported to mediate induction of cellular responses and the proPO cascade. Although the nomenclature quickly becomes confusing, CTLs include lipopolysaccharide-binding protein (LBP)
. B. mori LBP binds LPS and triggers cellular responses (nodulation) [
100]. Finally, immulectins (IML) are also CTLs.
M. sexta IML-1 binds to Gram-positive and Gram-negative bacteria as well as yeast [
101], IML-2 shows specific binding to LPS [
102], IML-3 and IML-4 show specific binding to LPS and LTA, and IML-3 can also bind laminarin, a ß-1,3-glucan [
103,
104]. Diversity in CTL carbohydrate-binding specificities may result in lineage-specific pathogen recognition-signal transduction connections.
Of particular relevance to the theme of this review are PRRs present in Lepidoptera but not Diptera (
Table 1). In general, both orders of insects encode βGRPs and PGRPs. However, specific representatives of each class are restricted to Lepidoptera (
Table 1). For example, the Lepidopteran βGRP-2, which binds fungal cell wall β-1,3 glucans [
55] and LTA [
105], is absent from Diptera [
54]. Such derived βGRP and PGRPs may contribute to lepidopteran-specific transduction of signals to downstream pathways. Other Lepidoptera-specific PRRs are hemolin and hemolymph proteinase-14 precursor (proHP14) (
Table 1). Like IML C-type lectins, hemolin is an LPS- and LTA-binding PRR [
58] with roles in mediating cellular responses and as an opsonin to enhance phagocytosis [
59]. HP14 has been shown to detect and bind a broad range of MAMPs, and may coordinate with βGRP1 or βGRP2 to activate proPO [
60,
106]. The potential role of the PRRs discussed above in mediating the expression of AMP genes remains to be determined, and further study of the Lepidoptera-specific immune surveillance proteins and divergent activities of conserved PRRs likely will yield novel avenues for pest-control.