Insect Lectin-mediated immune responses: Comparison
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Please note this is a comparison between Version 1 by Pengyu Chen and Version 2 by Rita Xu.

Lectins are carbohydrate-binding proteins that recognize and selectively bind to specific sugar structures. Interaction of lectin with sugars on cell surface can activate multiple cellular responses, including the immune response. Many insect lectins have been identified or predicted but without in-depth analysis.

  • insect lectin
  • innate immunity
  • cellular immunity
  • humoral immunity

1. Introduction

Lectins are unique proteins that are characterized by their ability to selectively bind to specific carbohydrate residues. These sugar structures can be monosaccharides, disaccharides, or polysaccharides, and can be present as free sugars or as glycoconjugates linked to proteins and lipids. In the past, lectins were found to agglutinate red blood cells; therefore, they were often referred to as “hemagglutinins” or “agglutinins” [1]. Subsequent research indicated that agglutination is not universal for all lectins. Only some plant lectins will agglutinate certain types of cells, and this aggregation of cells can be blocked by preincubation with specific sugars. Consequently the word “lectin”, meaning “to select”, was introduced to replace the term hemagglutinin [2].
Because of their selectivity in carbohydrate binding, lectins play crucial roles in a multitude of biological processes in plants, animals, and microorganisms. For example, many plant lectins serve as defense proteins and are harmful to insects or pathogens [3]. Similarly, some animals can secrete lectins that can kill bacteria by forming pore structures on their membranes [4]. Bacteria use their surface lectins to adhere to host cells for invasion [5]. Inside cells, lectins participate in protein quality control [6]. In the extracellular matrix, some lectins alter ion transport [7]. Secreted lectins have also been reported to be involved in host immunity due to their ability in pathogen recognition [8][9][8,9].

2. Insect Innate Immunity

Animals are frequently challenged by invading pathogens such as fungi, bacteria, viruses, parasites, etc. Furthermore, they also harbor a microbiome in tissues such as the intestine and the hemolymph [10]. To maintain homeostasis and system integrity, animal hosts must regulate their own microbiota and eliminate pathogen infection through an elaborate immune system [11]. While mammalians have both an adaptive (depending on memory immune cells) and an innate immunity system, insects mainly depend on innate immunity when threatened by pathogens. Nonetheless, insects have evolved to be very successful organisms, occupying almost every habitat and ecological niche. This is due to a strong innate immune system consisting of a cellular and a humoral component (reviewed by [12][13][14][12,13,14]). The cellular defense is initiated instantly when pathogens are detected and results in the phagocytosis of smaller pathogens or encapsulation of bigger invaders [13][14][13,14]. The humoral defense is a relatively slow response and involves the production of a series of antimicrobial peptides (AMPs), complement proteins, lysozymes, protease inhibitors, reactive oxygen species (ROS), and enzyme cascades leading to the formation of melanin and clotting [12][14][12,14].
The cellular or humoral immunity system depends on the presence of immune cells of different types. These immune cell types can differ between insect species. For example, the mosquito Aedes aegypti has more kinds of immune cells identified than Drosophila [15]. The immune cells, called hemocytes due to their presence in the hemolymph, have differentiated from prohemocytes and are mainly composed of three highly differentiated cell types: the plasmatocytes, crystal cells, and lamellocytes [14][16][14,16]. Plasmatocytes represent more than 90% of the hemocyte pool. These cells have been shown in vitro to possess strong adhesive features, enabling them to surround and engulf pathogens, and to produce antimicrobial peptides (AMPs) for the humoral defense [14][15][14,15]. Unlike plasmatocytes, crystal cells are not adhesive, but they can express phenoloxidase, the key enzyme in the formation of melanin involved in wound healing and melanization [17]. Lamellocytes are large adhesive cells that are only present in larva or in infected adults, and are involved in melanization and encapsulation [15].

3. Insect Lectins

Insects are the largest and most diverse group of animals, and more and more insect lectins are being discovered. Lectin classification is important to cope with the diversity of these proteins. Insect lectins can be grouped according to the animal classification system, which encompasses 16 families of lectins, each with a characteristic carbohydrate-recognition domain (CRD) [18].
In insects, most of the identified endogenous lectins belong to the C-type lectin (CTL) family. Canonical CTLs bind sugars through their CRD, and this interaction is dependent on Ca2+, hence the name “C-type lectins”. The CRD motif of CTLs is versatile, resulting in broad range of carbohydrate-binding interactions. For example, the Glu-Pro-Asn (EPN) motif in the CRD binds mannose, N-acetylglucosamine, L-fucose, and glucose, while galactose and N-acetylgalactosamine are recognized by the Gln-Pro-Asp (QPD) motif [19][20][19,20]. Many other motifs have been identified in insects, such as QPS, QPN, APD, and MPP, among others [21], but their carbohydrate-binding activities need to be confirmed. According to their complexity, CTLs can further be classified into subfamilies such as collectins (collagen-containing C-type lectins), endocytic receptors, selectins, etc. [22]. Based on sequence homology, proteins with a CTL domain have been identified in at least 12 insects belonging to different orders, including model organisms such as Drosophila melanogaster, Bombyx mori, Manduca sexta, Tribolium castaneum and Nilaparvata lugens [9][21][9,21]. Expression of some of these putative lectins was verified by quantitative real-time PCR [21]. In each of these insect genomes, about 7–40 putative CTLs have been identified and most contain a signal peptide, indicating these proteins are probably secreted extracellularly [9]. The majority of these CTLs have a single CRD, but M. sexta, Helicoverpa armigera and Spodoptera litura possess lectins with a dual-CRD structure (also named the immulectin family). The CTL domain can be linked to other functional domains (CTL-X) such as an epidermal-growth-factor-like domain (EGF) or a chitin-binding domain (CBM), which greatly increases the functional diversity among CTLs [9]. Being the largest lectin family in insects, CTLs are involved in a broad range of processes, especially the immune responses (Table 1).
Malectin and calnexin/calreticulin are protein chaperones located in the ER (endoplasmic reticulum). During translation, an N-glycan precursor (Glc3Man9GlcNAc2) is attached to the newly synthesized polypeptide. The processing of the precursor glycan by glucosidases yields bi-, mono-, and non-glucosylated N-glycans, which creates signals for glycoprotein folding and quality control mediated by the chaperone lectins. Malectin binds to Glc2-N-glycans, whereas calnexin/calreticulin binds to Glc1-N-glycans [6][23][24][6,23,24]. Malectins in the invertebrate scallop Chlamys farreri and big-belly seahorse Hippocampus abdominalis are regulated by pathogen infection [25][26][25,26], suggesting their participation in immunity. Orthologs of malectins have been identified in D. melanogaster and A. aegypti, but have not been studied yet [27][28][27,28]. Calnexin/calreticulin chaperones have been identified in B. mori [29][30][29,30] and D. melanogaster [31][32][31,32]. In Drosophila, calnexin was reported to be related to neuron functions and sodium channel regulation [31][32][31,32].
F-type lectins (FTL) preferentially bind to fucose through a carbohydrate-binding domain composed of the HX(26)RXDX(4)R/K sequence motif [33][34][33,34]. The first FTL identified in insects was the lectin encoded by the Drosophila furrowed gene, and the furrowed protein is associated with a CTL domain and Sushi repeats [33][35][36][33,35,36]. Drosophila furrowed participates in planar cell polarity signaling and is crucial for cell adhesion [37]. The F-type lectin domain is also predicted in Anopheles gambiae, but its function has not been verified yet [35][36][35,36].
Chitinase-like proteins (CLPs) gained their name due to their chitin-binding ability. In contrast to chitinases, these proteins lack the enzymatic activity to digest chitin due to the absence of essential catalytic residues in the consensus motif [38]. In Drosophila, the most notable CLPs are the imaginal disc growth factors (IDGFs), composed of six glycoproteins which participate in cellular functions like proliferation, mobility, and immune recognition [38][39][38,39]. Sequences encoding CLPs have been predicted in at least in 10 insects including model insects like the red flour beetle, T. castaneum, N. lugens, and mosquito, A. gambiae; sequences encoding CLPs were predicted, but since the homology search is based on a motif of catalytic residues, some of these CLPs identified are actually true chitinases [40][41][42][43][40,41,42,43], which are normally not considered to be lectins [44][45][44,45].
L-type lectins are soluble ER luminal compounds which contain a CRD similar to those of leguminous plant lectins such as concanavalin A (Con A), and some L-type lectins are responsible for glycoprotein sorting and trafficking [20][46][20,46]. Drosophila has a homolog of ER–Golgi intermediate compartment 53 (ERGIC-53), a human L-type lectin responsible for cargo transport of glycoproteins [47][48][47,48], which may be related to the adhesion protein talin [49]. B. mori also has an ERGIC-53 homolog which responds to insecticide treatment [50]. The L-type lectin LvLTLC1 was reported to be upregulated after pathogen stimuli in shrimp [46], but this was not reported in insects.
Galectins or S-type lectins contain a CRD that specifically binds to β-galactosides [51], although other carbohydrate ligands have also been reported. For example, the galectin Agalectin from A. gambiae caused agglutination that was inhibited by gangliosides, sulfated polysaccharides, and sialic acid-containing glycans [52][53][52,53]. Galectins in human can be further classified into three major groups: prototypical galectins, chimeric lectins, and tandem-repeat galectins, according to their CRD organization [54]. Many animal lectins are glycosylated, but the galectin family seems to be an exception [55][56][55,56]. Galectins have been reported in a few insects, including D. melanogaster, A. gambiae, A. aegypti, and the sand fly Phlebotomus papatasi [57][58][59][60][61][57,58,59,60,61]. Galectins expressed in the insect gut have been shown to participate in the neutralization of bacterial toxins [57][58][57,58].
I-type lectins belong to the immunoglobulin gene superfamily (IgSF). Hemolins, the well-studied I-type lectins of D. melanogaster, S. exigua, and M. sexta, recognize lipopolysaccharides, and their expression was shown to be induced after bacterial infection [62]. Further studies suggest that hemolin facilitates phagocytosis of bacteria and encapsulation of synthetic beads [62][63][64][62,63,64].
R-type lectins have a CRD similar to ricin, the toxic plant lectin from castor bean. Most R-type CRDs are ligated to other functional domains, including the CTL domain (mannose receptor family), pore-forming domain, and GalNAc-transferase domain. In the genome of D. melanogaster, 14 GalNAc-transferases have been identified containing R-type CRDs at their carboxy terminals. A QxW repeat in the CRD was supposed to be an important motif for carbohydrate binding [65][66][65,66].
Other lectin families common in animals, such as P-type and X-type lectins, are seldom identified in invertebrates [18] although previous searches in insect genome sequences predicted their existence [20].
Table 1.
Overview of insect lectins.
Lectin Families Insect Species Gene/Protein a Lectin Functions Experiment

Verification		 Predicted by GO/Homology References
RNA b Protein c
CTL Aedes aegypti AaeCTLs; CTL-20; mosGCTL-7 Pathogen recognition; interacts with phosphatase; reduces exogenous toxin toxicity + +   [9][67][68][69][9,67,68,69]
  Tribolium castaneum TcCTL6, TcCTL3 Responds to pathogen infection; regulates AMP expression   +   [70][71][70,71]
  Spodoptera litura SliCTLs Responds to pathogen infection +     [21]
  Mythimna separata EPL Promotes encapsulation +     [72]
  Ostrinia furnacalis OfCTLs, OfIMLs   +     [73]
  Spodoptera exigua Se-LLs, Se-BLLs Responds to virus infection +     [74]
  Thitarodes xiaojinensis CTL-S, CTL-X, IMLs Responds to pathogen infection +     [75]
  Helicoverpa armigera Ha-lectin, HaCTL Regulates ecdysone and juvenile hormone signaling; regulates AMP expression; promotes phagocytosis   +   [76]
  Drosophila melanogaster Slf, DL2-3 Organizes the cuticle layers; enhances encapsulation   +   [77][78][77,78]
  Antheraea pernyi Ap-CT Binds PAMPs; activates PO   +    
  Bombyx mori BmIML, BmMBP, CTL-S3, BmEL-1, 2, 3 Recognizes PAMPs; activates PO; promotes melanization;   +    
  Hyphantria cunea Hdd15   +      
  Periplaneta americana LPS-BP Responds to E. coli   +    
  Heliothis virescens MBL     +   Reviewed by [9]
  Manduca sexta MsIML-1, 2, 3, 4 Responds to pathogens; binds PAMPs; activates PO; enhances encapsulation   +    
  Anopheles gambiae AgamCTLs Responds to pathogens        
  Nilaparvata lugens   n.d.        
  Plutella xylostella   n.d.        
  Apis mellifera   n.d.        
  Acyrthosiphon pisum   n.d.        
Chitinase like Acyrthosiphon pisum AcypiCht1 (IDGF homologue) Expresses in bacteriocyte and midgut +     [41]
  Anopheles gambiae AgIDGF2, AgIDGF4 Expresses in different developmental stages and tissues +     [79]
  Bombyx mori BmIDGF Expresses in eggs, hemocytes, fat body, and silk gland   +   [80][81][80,81]
  Drosophila melanogaster IDGF1-6 Participates in would healing and wing development + +   [38][39][82][38,39,82]
  Nilaparvata lugens NlIDGF Expresses in female reproductive organs and fat body +     [42]
  Tribolium castaneum TcIDGF2, 4 Acts in adult eclosion +     [83]
  Plutella xylostella PxIDGF n.d.     + [84]
  Manduca sexta MsIDGF1 n.d.     + [85]
  Bemisia tabaci BtIDGF1-3 Highly abundant in adults +     [86]
Galectin Drosophila melanogaster Dmgal Expresses in hemocytes and in different developmental stages   +   [59][87][59,87]
  Phlebotomus papatasi PpGalec Strong expression in adult female; binds pathogen       [61]
  Anopheles gambiae Agalectin, GALE6-8 Expresses in salivary gland; Responds to viral infection + +   [52][88][52,88]
  Bombyx mori BmGalectin-4 Responds to bacteria in fertilized eggs; binds bacteria   +   [89]
  Aedes aegypti galectin-6, galectin-14 Reduces exogenous toxin toxicity   +   [57][58][57,58]
  Anopheles darlingi   n.d.        
  Anopheles stephensi   n.d.        
  Culex quinquefasciatus   n.d.        
  Drosophila ananassae   n.d.        
  Drosophila mojavensis   n.d.        
  Drosophila pseudoobscura   n.d.        
  Drosophila virilis   n.d.     + Predicted by [87]
  Drosophila willistoni   n.d.        
  Drosophila yakuba   n.d.        
  Glossina morsitans   n.d.        
  Malus domestica   n.d.        
malectin Aedes aegypti   n.d.     + [27][28][27,28]
  Drosophila melanogaster   n.d.     +
Calnexin/calreticulin Bombyx mori Calr/Canx; BmCNX Responds to ER stress + +   [30][90][30,90]
  Drosophila melanogaster Cnx Regulates the function of sodium channel paralytic   +   [32]
F-type lectin Drosophila melanogaster Furrowed Functions in planar cell polarity   +   [37]
  Anopheles gambiae   n.d.       Reviewed by [36]
I-type (immuno-globulin fold) Drosophila melanogaster hemolin n.d.   +   Reviewed by [91]
  Manduca sexta HEM Recognizes PAMPs; promotes nodulation, hemocyte aggregation, and phagocytosis       [63]
  Spodoptera exigua SeHem Acts as opsonin; regulates phagocytic activities and encapsulation +     [62]
  Plodia interpunctella PiHem Function related to gut bacteria +     [92]
  Bombyx mori Hemolin n.d.     + [93]
  Actias selene As-HEM Mediates immune response   +   [94]
  Antheraea pernyi Hemolin Regulates innate immunity   +   [95]
L-type Drosophila melanogaster ERGIC-53 homolog n.d.       [48], reviewed by [96]
  Bombyx mori ERGIC-53 Responds to ER stress +     [50]
R-type (ricin B type) Drosophila melanogaster lectin domain of GalNAc Transferase Binds glycopeptides   +   [97], reviewed by [65]

a some publications have predicted lectins but did not assign names for these lectins; therefore, there are some blanks in the table. b RNA verification studies included RT-qPCR, dsRNA silencing, and transcriptome analysis. c Protein verification included immunoblotting, recombinant protein production, etc.

4. Endogenous Insect Lectins as Immune Modulators

4.1. Pathogen Recognition

Before hemocytes can activate the immune response, the pathogen or immune target must be recognized. During pathogen invasion, pathogen-associated molecular patterns (PAMPs) such as bacterial peptidoglycan or fungal β-1,3-glucan are recognized by specialized proteins called pattern-recognition receptors (PRRs) [98]. The Gram-negative binding proteins (GNBPs), and peptidoglycan-recognition proteins (PGRPs) are the two major PRR families. GNBPs mainly recognize fungal and Gram-negative bacterial PAMPs, while PGRPs mainly respond to Gram-positive bacteria [98]. Since many PAMPs are carbohydrate structures, lectins constitute important parts of the membrane-bound or extracellular PRRs of hosts.
Lectins have been reported to bind and aggregate pathogens such as bacteria because of their recognition of carbohydrate structures. CTLs of H. armigera and M. sexta were shown to bind various PAMPs, such as lipopolysaccharide (LPS), fungal glucan, and peptidoglycan, to activate the humoral and cellular immune defenses [9][76][99][100]. In Drosophila, CTLs such as DL2 and DL3 can either be secreted or bound to the plasma membranes of hemocytes, and they were shown to bind some Gram-negative bacteria and agglutinate them [15]. While many insect PRRs belong to the C-type lectin family, lectins from other families can also function as PRRs. For example, galectins have been reported to recognize and bind pathogen surface glycans [53]. The silkworm B. mori possesses a dual-CRD galectin which can bind a series of PAMPs, such as LPS, LTA (lipoteichoic acid), peptidoglycan, and laminarin, and was shown to agglutinate E. coli, Staphylococcus aureus, and Bacillus subtilis [89][101].

4.2. Lectin-Induced Cellular Immunity

4.2.1. Phagocytosis

Many hemocytes can engulf invading pathogens as well as dead cells or other entities in a process called phagocytosis [13][102]. Upstream events of phagocytosis include the recognition of the targets by the PRRs, which activates downstream events including receptor cross-linking, membrane remodeling, phagosome formation, and maturation, and finally phagosome fusion with the endosomes and lysosomes to kill the pathogens via the acidic environment, AMPs, digestive enzymes, etc. [103]. To increase the efficiency of phagocytosis, hemocytes sometimes rely on opsonins, molecules that can coat and aggregate pathogens such as bacteria and viruses to limit their mobility and promote recognition [103]. Lectins have been proven to stimulate phagocytosis by acting as PRRs to detect pathogens or as opsonins to coat the invaders. For example, rHa, a CTL lectin obtained from H. armigera, has two CRDs which are both required for lectin agglutination of rabbit erythrocytes, but any single domain is sufficient for aggregation of Gram-negative bacteria, Gram-positive bacteria, and fungi. Injection of rHa lectin together with Bacillus thuringiensis bacteria in insects efficiently decreased the B. thuringiensis number in vivo, and hemocytes of H. armigera engulfed more B. thuringiensis in the presence of rHa lectin [104]. CTL-mediated phagocytosis has also been observed in mammalians and shrimps [105][106]. Besides the CTLs, the I-type lectin hemolin SeHem from S. exigua also helped the host cells to eliminate bacteria by enhancing phagocytosis [62]. Galectins from crustaceans have been reported to enhance host phagocytosis [107] but there are no such reports for insect galectins.

4.2.2. Encapsulation

When the invading targets are too large, such as parasitoids or nematodes, a group of hemocytes is recruited to surround the target, forming a capsule-like structure in a process termed encapsulation. In Drosophila, lamellocyte precursor cells are activated upon infection with parasitic wasp eggs and will differentiate into mature forms [16][108]. These cells are recruited to the site of infection, attach to the surface of the parasites, and undergo morphological changes to spread around the parasitoids [16]. The process in which the lamellocytes are flattened is called cell spreading and relies on phosphatase/kinase-mediated cytoskeleton rearrangement and activation of adhesion proteins [109][110][111]. Cell spreading is a very fast reaction; a mere 20 min after stimulation, most Drosophila S2 cells had already entered this spreading state [112]. The spread cells cover the parasite to form the capsule. Stabilization of the capsule depends first on intercellular septate junctions. These ladder-like structures are composed of multiple adhesion proteins such as contactin, neurexin, fibronectin, etc. [113]. Second, melanization follows to strengthen the capsule and to kill the parasites. Melanization is a process in which phenols are oxidized to quinones that can be polymerized to form melanin [114]. The deposition of melanin will darken the capsule [115]. Encapsulated targets are restricted in their movement and are finally killed directly by melanization-derived toxic components such as quinones, reactive oxygen intermediates, and AMPs [115], or indirectly by nutrient deprivation [116].
Insect lectins have been shown to be involved in both encapsulation and melanization. One common method used to study encapsulation in vitro is the use of synthetic beads incubated with isolated hemocytes. Synthetic beads such as agarose or Sephadex can attract hemocytes which form capsules around the beads that can be easily observed under a microscope [109]. Coating of these beads with stimulating proteins can accelerate and increase encapsulation [78]. For example, recombinantly produced Drosophila CTLs DL2 and DL3 were coated onto Ni-NTA agarose beads which can attract hemocyte attachment. These hemocytes aggregated to the bead surface to form capsules and became darker colored after longer incubations, and this process was blocked by antibodies targeted against the recombinant proteins [78]. Besides the in vitro test, injecting the coated beads into an insect hemocoel also validated the hypothesis. In H. armigera, a CTL, HaCTL3, was coated onto Sephadex A-25 beads and injected into the H. armigera larval hemocoel. After 12 h, the beads were found to be extensively encapsulated and melanized [99]. Besides CTLs, the I-type lectin SeHem was also reported to coat nonself targets for encapsulation [62]. While many lectins have been reported to participate in encapsulation and melanization [9], it is not very clear which receptors on the plasma membrane are responsible for the effect. One possible explanation is through interaction with integrins. Evidence suggests that silencing of β-integrin, a hemocyte membrane protein participating in cell–cell adhesion and signal transduction, can effectively decrease the encapsulation of beads [99]. In addition, CTL-mediated melanization is suggested to be specific. In one in vitro test, the immune lectin MsIML from M. sexta was shown to be able to activate a protease cascade required for phenoloxidase activation, which only happens when this lectin binds to LPS. Phenoloxidase has been proven to be a key enzyme for melanization [114][117].

4.3. Lectin-Induced AMP Expression

Besides the cellular response, the insect host can secrete a series of extracellular effector molecules that can kill foreign invaders. Among these effectors, AMPs are the major participants [118]. AMPs are positively charged small peptides consisting of 15–45 amino acids, which can bind to the normally negatively charged surface of microbes and lead to membrane rupture and cell lysis [119]. The healthy host cells are protected from AMP damage mainly by the cholesterol-rich plasma membrane which makes healthy cells positively charged to repulse cationic AMP attachment [119]. However, when host cells are not healthy, they can be attacked by the AMPs. In a study of Drosophila tumor genesis, tumor cells tended to have a negatively charged cell surface due to the phosphatidylserine turning inside out, allowing the AMP defensin to locate and attack these cells to limit the tumor growth [120].
Classification of AMPs can vary based on different criteria, such as the type of the target microbe (antifungal or anti-Gram-positive/negative-bacterial AMPs) or based on the pathway by which they are activated (such as Toll-regulated or Imd-regulated AMPs) [118]. However, neither classification system can perfectly group different AMPs. While AMPs like drosomycin (Drs) are highly specific against fungal infections, other AMPs have a broader pathogen specificity. For example, metchnikowin (Mtk) can target all three groups of pathogens mentioned above. In addition, AMP regulation can also be complex; for example, while Drs is regulated by the Toll pathway, many others, such as defensin (Def), are coregulated by both pathways [118][119].
The insect fat body around the body cavity is the major tissue secreting AMPs [121]. When stimulated by a pathogen, the AMP titers in the hemolymph can drastically increase within 30 min and the concentration can reach up to 300 µM (reviewed by [15]). In addition to the fat body cells, hemocytes can also produce AMPs. For example, isolated hemocytes from the blue blowfly, Calliphora vicina, showed the same ability to produce AMPs such as defensin, cecropin, diptericins, and proline-rich peptides [111]. Drosophila hemocyte-like S2 cells have also been shown to produce all kinds of AMPs upon stimulation by E. coli or other protein stimuli [112][122].
Insect lectin-mediated pathogen recognition can trigger the production of AMPs. Insect lectins are commonly coregulated with AMPs [123], but a recent study gave more direct evidence that the insect lectin can regulate AMPs. After silencing of HaCTL3, a CTL from H. armigera participating in larval development, the fat body expressed far less AMPs than in the control group. HaCTL3 was found to regulate different AMPs, including lebocin, attacin, cecropin 1, pre-gloverin, pre-lebocin, and cecropin, of which the antimicrobial activities were confirmed by in vitro assays. Even more interestingly, the upstream PRRs, including PGRPs, β-1,3-GRPs, and even a CTL4, were also downregulated, suggesting that lectin-regulated AMP production might be initiated by affecting upstream recognition events [76].
Within host immunity, insect lectins and AMPs can have complex interactions. Insect lectins can protect the beneficial host microbiome against the toxic effects of AMPs. For example, silencing of A. aegypti C-type lectins (mosGCTLs), which are coregulated with AMP through the Imd pathway, leads to failure of colonization and maintenance of the gut microbial flora [124]. In addition, AMP toxicity significantly decreased when bacteria were pre-coated by mosGCTLs, which blocked AMP deposition on the bacterial surface [124]. Viruses are also reported to use host lectins. The West Nile virus (WNV), a pathogen causing West Nile fever and transmitted by mosquitos, can stimulate expression of an A. aegypti C-type lectin, mosGCTL-1, which can strongly interact with a mosquito phosphatase, mosPTP-1. WNV uses mosGCTL-1 to coat its surface and enters cells through interaction with mosPTP-1 [125].

4. Endogenous insect lectins as immune modulators

4.1. Pathogen recognition

Before hemocytes can activate the immune response, the pathogen or immune target must be recognized. During pathogen invasion, pathogen associated molecular patterns (PAMPs), such as bacterial peptidoglycan or fungal β-1,3-glucan, are recognized by specialized proteins called pattern recognition receptors (PRRs) [98]. The Gram-negative binding proteins (GNBPs), and peptidoglycan recognition proteins (PGRPs), are the two major PRR families. GNBPs mainly recognize fungal and Gram negative bacterial PAMPs, while PGRPs mainly respond to Gram positive bacteria [98]. Since many PAMPs are carbohydrate structures, lectins constitute an important part of the membrane bound or extracellular PRRs of hosts.

Lectins have been reported to bind and aggregate pathogens such as bacteria because of their recognition of carbohydrate structures. CTLs of H. armigera and M. sexta, were shown to bind various PAMPs, such as lipopolysaccharide (LPS), fungal glucan and peptidoglycan, to activate the humoral and cellular immune defenses [9,76,99,100]. In Drosophila, CTLs such as DL2 and DL3 can either be secreted or bound to the plasma membrane of hemocytes and they were shown to bind some Gram-negative bacteria and agglutinate them [15]. While many insect PRRs belong to the C-type lectin family, lectins from other families can also function as PRRs. For example, galectins have been reported to recognize and bind pathogen surface glycans [53]. The silkworm, B. mori, possesses a dual-CRD galectin which can bind a series of PAMPs, such as LPS, LTA (lipoteichoic acid), peptidoglycan and laminarin, and was shown to agglutinate E. coli, Staphylococcus aureus and Bacillus subtilis [89,101].

4.2. Lectin-induced cellular immunity

4.2.1. Phagocytosis

Many hemocytes can engulf the invading pathogens as well as dead cells or other entities in a process called phagocytosis [13,102]. Upstream events of phagocytosis include the recognition of the targets by the PRRs, which activates downstream events including receptor cross-linking, membrane remodeling, phagosome formation and maturation, and finally phagosome fusion with the endosomes and lysosomes to kill the pathogens by the acidic environment, AMPs, digestive enzymes, etc. [103]. To increase the efficiency of phagocytosis, hemocytes sometimes rely on opsonins, molecules that can coat and aggregate the pathogens, such as bacteria and viruses, to limit their mobility and promote recognition [103]. Lectins have been proven to stimulate phagocytosis, by acting as PRRs to detect pathogens or as opsonins to coat the invaders. For example, rHa, a CTL-lectin obtained from H. armigera, has two CRDs which are both required for lectin agglutination of rabbit erythrocytes, but any single domain is sufficient for aggregation of Gram-negative bacteria, Gram-positive bacteria, and fungi. Injection of rHa-lectin together with Bacillus thuringiensis bacteria in the insects, efficiently decreases the B. thuringiensis number in vivo, and hemocytes of H. armigera engulfed more B. thuringiensis in the presence of rHa lectin [104]. CTL-mediated phagocytosis was also observed in mammalians and shrimps [105,106]. Besides the CTLs, the I-type lectin hemolin, SeHem, from S. exigua also helped the host cells to eliminate bacteria by enhancing phagocytosis [62]. Galectins from Crustaceans have been reported to enhance host phagocytosis [107] but there are no such reports for insect galectins.

4.2.2. Encapsulation

When the invading targets are too large, such as parasitoids or nematodes, a group of hemocytes are recruited to surround the targets, forming a capsule-like structure in a process termed encapsulation. In Drosophila, lamellocyte precursor cells are activated upon infection with parasitic wasp eggs and will differentiate into mature forms [16,108]. These cells are recruited to the site of infection, attach to the surface of the parasites, and undergo morphological changes to spread around the parasitoids [16]. This process in which the lamellocytes are flattened is called cell spreading and relies on phosphatase/kinase mediated cytoskeleton rearrangement and activation of adhesion proteins [109–111]. Cell spreading is a very fast reaction, 20 min after stimulation most of the Drosophila S2 cells already have entered this spreading state [112]. The spreaded cells cover the parasite to form the capsule. Stabilization of the capsule depends first on intercellular septate junctions, these ladder-like structures are composed of multiple adhesion proteins such as contactin, neurexin, fibronectin, etc. [113]. Second, melanization follows to strengthen the capsule and to kill the parasites. Melanization is a process in which phenols are oxidized to quinones that can be polymerized to form melanin [114]. The deposition of melanin will darken the capsule [115]. Encapsulated targets are restricted in their movement and are finally killed directly by melanization derived toxic components, such as quinones, reactive oxygen intermediates, and AMPs [115], or indirectly by nutrient deprivation [116].

Insect lectins have been shown to be involved in both the encapsulation and the melanization. One common method to study the encapsulation in vitro is the use of synthetic beads incubated with isolated hemocytes. Synthetic beads such as agarose or Sephadex can attract hemocytes which form capsules around the beads that can be easily observed under the microscope [109]. Coating of these beads with stimulating proteins can accelerate and increase the encapsulation [78]. For example, recombinantly produced Drosophila CTLs, DL2 and DL3 were coated on Ni-NTA agarose beads which can attract hemocyte attachment. These hemocytes will aggregate to the bead surface to form capsules and become darker colored after longer incubation, and this process can be blocked by antibodies targeted against the recombinant proteins [78]. Besides the in vitro test, injecting the coated beads into the insect hemocoel can also validate the hypothesis. In H. armigera, a CTL, HaCTL3, was coated on Sephadex A-25 beads and injected into the H. armigera larval hemocoel. After 12h the beads were found to be extensively encapsulated and melanized [99]. Besides CTLs, the I-type lectin SeHem was also reported to coat non-self-targets for encapsulation [62]. While many lectins have been reported to participate in encapsulation and melanization [9], it is not very clear which receptors on the plasma membrane are responsible for the effect. One possible explanation is through interaction with integrins. Evidence suggests that silencing of β-integrin, a hemocyte membrane protein participating in cell-cell adhesion and signal transduction, can effectively decrease the encapsulation of beads [99]. In addition, CTL mediated melanization is suggested to be specific. In an in vitro test, the immune lectin MsIML from M. sexta was shown to be able to activate a protease cascade required for phenoloxidase activation, which only happens when this lectin binds to the LPS. Phenoloxidase is proven to be a key enzyme for melanization [114,117].

4.3. Lectin-induced AMP expression

Besides the cellular response, the insect host can secrete a series of extracellular effector molecules that can kill foreign invaders. Among these effectors, AMPs are the major participants [118]. AMPs are positively charged small peptides, consisting of 15-45 amino acids, which can bind to the normally negatively charged surface of microbes leading to membrane rupture and cell lysis [119]. The healthy host cells are protected from AMP damage mainly due to the cholesterol rich plasma membrane which makes healthy cells positively charged to repulse cationic AMP attachment [119]. However, when host cells are not healthy, they can be attacked by the AMPs. In the study of Drosophila tumor genesis, tumor cells tend to have a negatively charged cell surface due to the phosphatidylserine turning inside-out, allowing the AMP Defensin to locate and attack these cells to limit the tumor growth [120].

Classification of AMPs can vary based on different criteria, such as the type of the target microbe (anti-fungal or anti-Gram-positive/negative-bacterial AMPs) or based on the pathway by which they are activated (such as Toll regulated or Imd regulated AMPs) [118]. But neither classification system can perfectly group different AMPs. While AMPs like Drosomycin (Drs) are highly specific against fungal infections, other AMPs have a broader pathogen specificity. For example, Metchnikowin (Mtk) can target all three groups of pathogens mentioned above. In addition, the AMP regulation can also be complex, for example, while Drs is regulated by the Toll pathway, many others, such as Defensin (Def), are coregulated by both pathways [118,119].

The insect fat body around the body cavity is the major tissue to secrete AMPs [121]. When stimulated by the pathogen, the AMP titers in the hemolymph can drastically increase within 30 min and the concentration can reach up to 300 µM (reviewed by [15]). In addition to the fat body cells, hemocytes can also produce AMPs. For example, isolated hemocytes from the blue blowfly, Calliphora vicina, show the same ability to produce AMPs such as defensin, cecropin, diptericins, and proline-rich peptides [111]. Drosophila hemocyte-like S2 cells can also produce all kinds of AMPs upon stimulation by E. coli or other protein stimuli [112,122].

Insect lectin mediated pathogen recognition can trigger production of AMPs. Insect lectins are commonly coregulated with AMPs [123], but a recent study gives more direct evidence that the insect lectin can regulate AMPs. After silencing of HaCTL3, a CTL from H. armigera participating in larval development, the fat body expressed far less AMPs than in the control group. HaCTL3 was found to regulate different AMPs, including lebocin, attacin, cecropin 1, pre-gloverin, pre-lebocin and cecropin, of which the antimicrobial activities were confirmed by in vitro assays. And more interestingly, the upstream PRRs, including PGRPs, β-1,3-GRPs and even a CTL4, were also downregulated, suggesting lectin-regulated AMP production might be initiated through affecting upstream recognition events [76].

Within the host immunity, insect lectins and AMPs can have complex interactions. Insect lectins can protect the beneficial host microbiome against the toxic effects of AMPs. For example, silencing of A. aegypti C-type lectins (mosGCTLs), which are coregulated with AMP through the Imd pathway, leads to failure of colonization and maintenance of the gut microbial flora [124]. In addition, the AMP toxicity significantly decreased when bacteria were pre-coated by mosGCTLs, which blocked AMP deposition on the bacterial surface [124]. Viruses are also reported to use host lectins. The West Nile virus (WNV), a pathogen causing West Nile fever and transmitted by mosquitos, can stimulate expression of an A. aegypti C-type lectin, mosGCTL-1, which can strongly interact with a mosquito phosphatase, mosPTP-1. WNV uses mosGCTL-1 to coat its surface and enters in the cell through interaction with mosPTP-1 [125].

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