Keywords

1 Introduction

The C-type lectin superfamily comprising of transmembrane and soluble proteins like selectins, collectins, and asialoglycoprotein receptor has garnered attention since eons [1]. The family of mannose receptors is an integral part of the C-type lectin family. Multiple lectin domains in a single polypeptide structure make this family an unconventional member of the lectin superfamily [2]. The mannose family receptors are involved in antigen capture, recognition of mannosylated structures of pathogenic cell walls and may be overexpressed in certain diseased states. Targeting the mannose receptor provides an attractive strategy to combat number of infections and certain cancers [3, 4]. A complete understanding of the receptors, ligands, and binding interactions is quintessential for successful targeting applications. This chapter focuses on the mannose receptor family and its physiology in normal state and in pathologies. Drug delivery approaches to harness targeting effectively in the therapy of infectious diseases and cancer are also discussed.

2 Mannose Receptor Family

The family of mannose receptors comprises of four endocytic glycoprotein receptors, namely mannose receptor, M-type receptor for phospholipases A2 (PLA2R), DEC-205/CD205/gp200-MR6, and Endo180/uPARAP [5,6,7,8]. Mannose receptor, the first member of the family, was identified in the late 1970s. Multiple C-type lectin domains (CTLDs) in a single polypeptide backbone constitute a distinct feature of this receptor family. The members of mannose receptor family share mutual structural features, namely cysteine-rich domain, fibronectin type II domain, and CTLDs which vary from eight to ten. However, C-type lectin activity is not exhibited by all members. The C-type lectin activity for interacting with mannosylated moieties is displayed only by mannose receptor and Endo180. CTLD5 of PLA2R is involved in protein–protein interactions, a nonlectin activity. The cysteine-rich domain is involved in the recognition of sulfated carbohydrates whereas the fibronectin type II domain internalizes collagen. A functional cysteine-rich domain for binding to sulfated carbohydrates like galactose is present only in the mannose receptor. The receptors of mannose family terminate into short cytoplasmic domains. The receptors are rapidly internalized inside the cell and deliver the extracellular content to the intracellular compartments. Delivery to intracellular locations occurs via interactions between the motifs of terminal cytoplasmic domains and the endocytic machinery. The members of mannose family and their recognition domains are depicted in Fig. 15.1.

Fig. 15.1
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The mannose receptor family

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2.1 Receptor Recognition Domains

2.1.1 Cysteine-Rich Domain

Although present in all family members, the cysteine-rich domain lacks homology among the mannose receptor family. A 25–30% sequence identity is observed among the family members. Among the four receptors, only the mannose receptor has a functional N-terminal cysteine-rich domain which can exhibit binding to sulfated molecules. The receptor can bind to glycoproteins containing sulfated N-acetylglucosamine and sulfated galactose residues in hormones like lutropin and thyrotropin via this domain [9]. Binding to chondroitin sulfate A, chondroitin sulfate B, sulfated Lewis antigens, CD45, and sulfated transmembrane protein sialoadhesin is also reported [10, 11]. The binding is Ca2+ independent and occurs through a neutral binding site. The exact mechanism of binding is beyond the purview of this chapter and is well explained in the literature [2, 12].

2.1.2 Fibronectin Type II Domain

Fibronectin type II domain is the most conserved extracellular domain of mannose receptor family. This domain occurs in different proteins like matrix metalloproteinases (MMP) 2 and 9 [13]. This domain mainly binds to denatured collagen. The collagen binding may be a result of interaction between aromatic structures of the hydrophobic pocket exposed by the solvent with the nonpolar collagen residues leading to disruption of triple helix. The conserved amino acid (Arg34 and Asp36) residues can play a role in stabilizing this interaction [2]. Another hypothesis suggests the fibronectin type II domain can bring about N-terminal extension to bring the N-terminal in the vicinity of the C-terminal leading to stabilized interaction with collagen [14]. The binding of fibronectin type II domain of mannose receptor to collagen has been studied. An extended conformation at physiological pH and a compact conformation at acidic pH was reported. At physiological pH, a calcium-dependent binding was observed whereas acidic pH calcium did not affect the collagen binding [15]. This behavior could play a critical role in the intracellular trafficking of cargo delivered through mannose receptor endocytosis.

Mannose receptor demonstrates the ability to bind to collagens I, II, III, and IV while exhibiting a weak binding to collagen V [16]. Fibronectin type II domain of M-type PLA2 expressing cells binds to collagens I and IV, while Endo180 fibronectin type II domain preferentially binds to collagen V over collagens I and IV. No information is available regarding the ability of DEC-205 to recognize collagen, although this is a likely possibility.

2.1.3 C-Type Lectin Domains

CTLDs contain 120 amino acids. Noncovalent and covalent interactions between two antiparallel β sheets and two α helices lead to the formation of a hydrophobic fold. The carbohydrate interactions in functional CTLDs occur in the hydrophobic fold that imparts stability by hydrophobic core formation. Two disulfide bonds are also formed between cysteine residues. This hydrophobic fold of functional CTLDs permits interactions with sugars by facilitating contact with residues integral for coordination with Ca2+ and sugar moieties [2].

In the case of mannose receptor, the binding of terminal carbohydrate residues like mannose, fucose, and N-acetylglucosamine occurs in the presence of Ca2+. A higher affinity is demonstrated by mannose receptor CTLDs toward mannose and fucose whereas the binding affinity to N-acetylglucosamine and glucose is lower. Only the mannose receptor CTLD4 is involved in sugar binding. Similar to mannose receptor, CTLD2 of Endo180 shows binding dependent on Ca2+ to glycoconjugates, while CTLD5 of PLA2R is involved in binding to the nonglycosylated PLA2 ligand via Ca2+-independent pathways. Further, instead of lectin interactions, CTLD5 mediates protein–protein interactions. DEC-205 is devoid of C-type lectin activity.

2.2 Ligand Binding

In most multidomain receptors, domains which mediate ligand interaction are often stationed at a distance from the membrane. Surprisingly, among the mannose family receptors, CTLD4 and CTLD5 which exhibit a crucial role in binding are found in the central region of the mannose receptor and the PLA2 receptor, respectively. An extensive study of mannose receptor revealed an extracellular domain with a rigid and extended conformation and close interactions between neighboring CTLDs (CTLDs 1 and 2, CTLDs 4 and 5, CTLDs 7 and 8) with exposed flexible linker regions on either side of CTLDs 3 and 6 [2, 17]. CTLD5 of mannose receptor demonstrates weak binding to sugars in addition to CTLD4, the principal sugar-binding domain. The association of these two CTLDs results in the formation of a protease-resistant core. Such domain disposition enables binding to multiple sugar moieties, enhanced binding of CTLDs, and/or modulates the rigidity of CTLDs.

Closeness of the N- and C-terminal of fibronectin type II domain brings it near to the other domains, as suggested by the sequence analysis. A close association of the cysteine-rich domain, fibronectin type II domain, and C-type lectin domains is seen by protease studies. Such an arrangement stabilizes the interaction with collagen and projects the cysteine-rich domain away from the membrane. This projection is desirable for interactions of cellular sulfated glycoproteins and the domain.

Further, the ligand–receptor binding in mannose receptor is highly pH dependent. Mannose receptor shows poor binding of ligands at pH 5 and optimal binding at pH 7. Such pH dependency is prominent in ligands dissociating in the acidic endosomal compartments. The pH-dependent binding enables separation of the ligand and receptor and recycling of the free receptors to the cell surface. Additionally, the Ca2+ dependency in binding may aid in the endosomal dissociation [18].

2.3 Intracellular Internalization

The rapid internalization of mannose receptor family members mainly occurs via clathrin-mediated endocytosis. Internalization by phagocytosis is another pathway mediated by mannose receptor expressed on the macrophages. Under steady state, the cell surface receptors constitute 10–30% whereas remaining 70–90% receptors are intracellular.

2.3.1 Clathrin-Mediated Endocytosis

During endocytic uptake, ligands packed in clathrin-coated vesicles are internalized from the plasma membrane and are delivered in the endosomal system. Smaller particles (<0.2 μm) are taken up by this pathway. The mannose family receptors recycle about 10 times an hour.

Two endocytic motifs, namely tyrosine residue-based motif and dihydrophobic motif, are present in the cytoplasmic domain. Although directed to the same intracellular compartment, the mannose receptor and Endo180 mediate the transport via different motifs. Mannose receptor and PLA2R recruit tyrosine-based motif whereas Endo180 utilizes the dihydrophobic motif [19]. Internalization occurs from the clathrin-coated pits into the early endosomes. This is followed by transportation to late endosomes and fusion with lysosomes followed by release of cargo into the cytoplasm. A different destination of DEC-205 within the cells is reported. Whereas mannose receptor is located in the early endosomes, localization of DEC-205 is seen in the late endosomes [20].

2.3.2 Phagocytosis

The uptake of particles of >0.2 μm occurs via phagocytosis. Fc receptors and complement receptors, the opsonic receptors, initiate phagocytosis signaling resulting in extension of membrane around the particle via regulation of actin cytoskeleton [21]. A phagosome is formed which then fuses with endosomes/lysosomes leading to exposure of the cargo to hydrolytic enzymes. The direct role of mannose receptor in phagocytosis is questionable. Phagocytic pathway may proceed upon binding to a mannosylated residue which may in turn activate a classical phagocytosis receptor [2]. As PLA2R and DEC-205 are mainly involved in uptake of macromolecules and are not expressed on phagocytic macrophages, their involvement in phagocytic machinery is unlikely. Although Endo180 is expressed on macrophages, an involvement in phagocytosis analogous to the mannose receptor is not observed in vitro [19].

3 Receptor Location and Expression

Mannose receptor, a 175-kDa type I membrane glycoprotein receptor, was originally isolated in liver and alveolar macrophages [22]. The receptor is predominantly found in most tissue macrophages and dendritic cells (DCs). It is also located in endothelial cells of liver and splenic sinusoids [23]. The receptor is also expressed on the microvascular endothelial cells of the dermis [24], cells of Kaposi’s sarcoma [25], human keratinocytes [26], and retinal pigment epithelium [27]. Although initially termed as the macrophage mannose receptor, it is now designated as the mannose receptor, as the occurrence is not exclusively limited to macrophages. The involvement of mannose receptor in phagocytosis of mannosylated structures and pinocytosis of soluble molecules is reported. It also acts as pattern recognition receptor by recognizing the mannosylated ligands of microbes [28,29,30]. Other functions of this receptor constitute improved presentation of antigens, modulation of cellular trafficking, and maintaining homeostasis by scavenging nonessential mannoglycoproteins and circulating pituitary hormones like lutropin and thyrotropin. PLA2R is expressed on muscle cell membranes and internalizes PLA2, the lipolytic enzymes required for digestion of phospholipids [5]. DEC-205 which is expressed by dendritic cells has shown involvement in uptake of antigens and delivery of cargo to T cells whereas Endo180 is an endocytic receptor involved in remodeling of cellular membranes.

A comprehensive overview of the four mannose receptor family members is provided in Table 15.1.

Table 15.1 Mannose receptor family
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4 Pathophysiological Features

The expression of mannose receptor is regulated by macrophage differentiation pattern. Consequently, differentiated macrophages reveal abundant receptor expression whereas circulating monocytes do not express mannose receptor [31]. The physiological status also affects the expression pattern. Anti-inflammatory molecules (corticosteroids, IL-10) [32, 33], Vitamin D3 [34], prostaglandin E [35], and Th2 cytokines (IL-4, IL-13) upregulate the mannose receptor expression by promoting synthesis whereas interferon ɣ (IFN ɣ) [32], lipopolysaccharide (LPS) [36], and immune complexes [37] downregulate the expression by restricting the synthesis.

Binding of pathogenic mannosylated ligands to mannose receptor may induce interleukin (IL)-10 and curb IL-12, thereby inhibiting pathways that could enable protective immune responses [30]. Mannose receptor recognizes the mannosylated cell walls of bacteria, fungi, viruses, or parasites, enabling their internalization in the cells (Fig. 15.2a). Pathogens entering the cellular environment using the mannose receptor portal do not evoke an immune reaction.

Fig. 15.2
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Recognition of (a) mannosylated pathogenic cell walls by the mannose receptor present on macrophages and dendritic cells in infections and (b) mannose ligands of tumoral mucins by the mannose receptor present on tumor-associated macrophages (TAMs) and dendritic cells in cancers

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Although mainly associated with infections, mannose receptor also shows a peculiar expression pattern in cancers. The tumor site shows the presence of mannose receptor expressing macrophages accompanied by ligands for mannose receptor like tumoral mucins [38, 39]. Tumoral mucin MUC1, a ligand of mannose receptor positive cancer cells, comprises of mannose and galactose residues. The tumor cells express abnormal quantities or irregular forms of mucins compared to the healthy cells [40]. The tumoral mucins can invade the immune responses occurring in the tumor microenvironment by binding to mannose receptor on the DCs and tumor-associated macrophages (TAMs) (Fig. 15.2b) and lead to upregulation of IL-10 and suppression of IL-12, thus suppressing Th1-polarized responses, similar to that in infections.

5 Ligands

Mannose receptor binds to various endogenous ligands and acts as a homeostasis regulator by clearing the unwanted molecules from circulation [30, 41]. As discussed earlier, the cysteine region of mannose receptor binds to sulfated moieties whereas the CTLDs bind to glycoproteins rich in mannose oligosaccharides. The fibronectin type II domain shows collagen-specific binding. Classification of ligands based on binding domains is presented in Table 15.2. Utilization of mannose, the most popular ligand, and other ligands like sulfated residues of N-acetyl-D-galactosamine and mannans, etc., for targeted intracellular delivery of therapeutics/antigens is discussed in Sect. 6.

Table 15.2 Ligands for mannose receptor based on domain structure
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The mannose receptor also binds to exogenous ligands from several microbes and enables their entry into the cell. Microbes may target the mannose receptor to provoke an anti-inflammatory/immune-suppressive response and cause a resistant infection. Mannose receptor lacks the ability to distinguish between pathogenic and nonpathogenic strains, thus internalizing both, unlike the Toll-like receptors [28]. Pathogens, such as Mycobacterium tuberculosis [44], Leishmania donovani [45], Trypanosoma cruzi [46], Trichinella spiralis [47], Streptococcus pneumoniae [48], HIV virus [49], and influenza virus [50], enter the intracellular environment aided by the carbohydrate ligands on their cell membranes. In the case of Mycobacterium tuberculosis, lipoarabinomannan (LAM) , a glycolipid present in the mycobacterial cell wall, contains terminal mannose residues that can interact with the mannose receptors. The internalization of LAM-anchored polystyrene beads by mannose receptor mediated phagocytosis is reported. However, mannose receptors bind to virulent H37Rv and Erdman strains but do not bind to the avirulent H37Ra strain of Mycobacterium tuberculosis [44]. The biological responses attributed to LAM may be a result of interaction with mannose receptors or other receptors that recognize LAM-like CD14 receptors. In addition to bacterial and viral sugar residues, mannose receptor also recognizes several fungal ligands including glycoprotein A of Pneumocystis carinii [51] and mannan from Candida albicans [52].

The ligands for other mannose receptor family members are relatively few. Pancreatic sPLA2IB is reported as the only ligand of PLA2R; however, an interspecies variation in binding affinity was observed [53, 54]. Other potential ligands include sPLA2-V, sPLA2-IID, and sPLA2-X [55, 56]. Specific ligands for DEC-205 are not reported. DEC-205 cysteine-rich domain does not interact with sulfated sugars and also lacks the C-type lectin activity. Like the mannose receptor, Endo180 is also multifunctional and exhibits binding to a distinct set of ligands. Ca2+ - dependent binding of Endo180 to mannose, fucose, and N-acetylglucosamine is evident. Endo180 does not bind to galactose and sulfated sugars [57]. It exhibits binding to components of the extracellular protease systems (MMP13 and uPAR). An interaction of Endo180 with collagen via the fibronectin type II domain is reported.

6 Receptor Targeting Strategies

Among the family of mannose receptors, the most extensively exploited and studied receptor for targeted drug delivery is the mannose receptor. Hence, this section focuses mainly on mannose receptor enabled intracellular delivery. The endocytosis and phagocytosis of microbes in the macrophages occur by interaction of glycoproteins in the cell walls with the mannose receptor. Mannose conjugates and mannosylated nanocarriers target these intracellular pathogens by promoting uptake of the drug-loaded mannosylated constructs in the infected cells via mannose receptor. Nanocarrier-based strategies to target mannose receptor overexpression in tumor microenvironment are reported. Moreover, interaction of mannose ligands with mannose receptor expressed on macrophages/dendritic cells can lead to induction of immune signaling pathways, an approach of great importance in vaccine delivery [58]. Targeting desired cells via ligand-mediated approach can minimize systemic distribution and off-site toxicity. Decoration of surface of nanocarriers with ligands with high affinity to the mannose receptor is the strategy employed for targeting.

6.1 Mannose Conjugates

Mannose conjugates can be prepared by reaction between mannose derivatives and proteins or therapeutic agents like antigens. The stability of the conjugate within the body and release of the therapeutic agent at the site of action depend on the bond between the mannose derivative and the system. Most of the strategies studied involve use of endogenous mannose receptor ligands. Recent studies report utilization of synthetic ligands specific to mannose receptor expressed on macrophages or DCs. Polysaccharide from Bletilla striata (a glucomannan) having high affinity to mannose receptor expressing cells was conjugated to alendronate, a bisphosphonate. The conjugate revealed inhibition of angiogenesis and elimination of TAMs leading to suppressed tumor progression [59]. In some instances, mannose ligand has been employed to act as antigen and potentiate the immunogenicity of the conjugated protein/peptide molecule. A mannosylated vaccine formed by conjugation of glucuronoxylomannan, a polysaccharide found in Cryptococcus neoformans capsule and tetanus toxoid, elicited high levels of capsular antibodies [60]. Another study reports coupling of heptasaccharide oligosaccharide, the immunodeterminant of glucuronoxylomannan with human serum albumin which resulted in induction of immunogenic responses [61].

6.2 Mannosylated Nanocarriers

Mannosylated nanocarriers can be prepared by coating/conjugation of mannose ligands to the surface of nanocarriers like liposomes or nanoparticles. Such mannosylated systems enable targeting to the mannose receptor and permit the delivery of cargo (antigen/drug) at the site of interest. Furthermore, particulate nature of the nanocarriers accompanied by mannose association significantly improves uptake by the endocytic and phagocytic pathways.

6.2.1 Mannosylated Liposomes

Liposomes have been extensively studied in the literature as carriers for drugs, proteins, and even fluorescent markers. Mannosylation of liposomes enables their application in treatment of intracellular infections like tuberculosis (TB) and leishmaniasis or as vaccine candidates in cancers or infections. Mannosylated liposomes can be prepared by using mannose lipid conjugates, covalently attaching mannose derivatives to liposomes, or by adsorbing the ligand on liposomal surface [3]. The click reaction was used for the preparation of cytotoxic mannose click conjugates by reaction with aminobenzoic acid derivatives [62]. In one study, mannose-cholesterol conjugates were synthesized by click reaction for liposomal drug delivery systems [63]. Wang et al. studied the effect of varying the chain length of the polyethylene glycol (PEG) linker and the optimal mannose-cholesterol conjugates were used for liposomal messenger RNA (mRNA) delivery [64].

Drug-related issues like toxicity and resistance in leishmaniasis have been tackled by treatment with mannosylated liposomes. Amphotericin B liposomes coated with palmitoyl mannose (Man-Lip) or 4-sulfated N-acetyl galactosamine (sulf-Lip) revealed rapid intracellular uptake of Sulf-Lip and higher liver and spleen Amphotericin B levels indicating specificity of 4-sulfated N-acetyl galactosamine to resident macrophages [65]. In a similar study, mannosylated Amphotericin B loaded liposomes demonstrated maximum reduction in parasite load (78.8 ± 3.9%) compared to Amphotericin B solution (42.5 ± 1.8%) and cationic Amphotericin B loaded liposomes (61.2 ± 3.2%) in Leishmania donovani-infected golden hamster model [66]. Among three sugar grafted liposomes (mannose, glucose, and galactose), mannose liposomes loaded with pentamidine isethionate revealed superior reduction in parasite loads [67]. Sinha et al. reported reduced spleen parasitic burden with mannosylated andrographolide loaded liposomes when tested in experimental hamster leishmaniasis model [68]. A succinct summary of other liposome-based mannose receptor targeting for intracellular infections and cancer is provided in Table 15.3.

Table 15.3 Mannosylated liposomes for targeted delivery
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Mannosylated liposomes have been reported for vaccination against infections and cancers. Garcon et al. covalently coupled mannosylated albumin to the surface of dehydration–rehydration vesicles (DRVs). These mannosylated DRVs containing tetanus toxoid revealed selective binding to mouse peritoneal macrophages compared to nonmannosylated DRVs with an augmented immunoadjuvant activity in Balb/c mice [77]. In another study, liposomes coated with neoglycolipids (mannopentose or mannotriose) revealed high serum levels of soluble leishmanial antigen (SLA)-specific IgG2a antibody titer and low level of IgG1 antibody titers in comparison to uncoated liposomes along with a delayed footpad swelling progression [78]. In contrast to uncoated liposomes, subcutaneous immunization with oligomannose residue coated liposomes encapsulating peptides representing epitopes of gp120 (a HIV1 envelope glycoprotein) induced MHC class I-restricted CD8+ cytotoxic T-lymphocyte response [79].

A protective immune response against cancer can be elicited by association of immunostimulants or immunomodulators with mannosylated antigen loaded liposomes. Mannosylated liposome–plasmid DNA complex (Man-lipoplex), prepared as a potential DNA vaccine for melanoma, revealed greater pUb-M gene transfection into antigen-presenting cells than uncoated liposomes and demonstrated prolonged survival coupled with melanoma inhibition in mice model [80]. A similar study performed by White et al. revealed mannosylated liposomes of lipid core peptide with Quil A adjuvant acted as prophylactic anticancer vaccines and protected mice against tumors [81].

6.2.2 Mannosylated Nanoparticles (NPs)

Mannosylated NPs are widely investigated in infections and cancers akin to the mannosylated liposomes. Mannosylation of polyanhydride NPs can be performed by techniques such as desolvation or direct coating. Iron oxide NPs may be coated by precipitation of iron salts by incubation with D-mannose solution or by oxidation of NPs followed by addition of D-mannose solution. Chemical modification of polymers with mannosylated ligands is also reported [3].

The mannose receptor is profusely overexpressed on the macrophages, DCs, and foamy cells which constitute the TB granuloma. This permits utilization of mannosylated NPs for targeted intracellular delivery in TB. A multilayer mannosylated drug delivery system for intracellular delivery of first-line antibiotics Rifampicin and Isoniazid has been developed [82]. Isoniazid loaded mannosylated gelatin NPs reduced drug hepatotoxicity and significantly decreased bacterial burden in lungs and spleen of infected Balb/c mice [83]. In an analogous study, licorice loaded mannosylated gelatin NPs revealed enhanced uptake in RAW 264.7 cells and reduced spleen and lung bacterial loads in Mycobacterium tuberculosis H37Rv-infected mice compared to untreated animals [84]. Other strategies employing mannosylated NPs for drug delivery in infections and cancer are enlisted in Table 15.4. A summary of mannosylated NPs employed as vaccine carriers is presented in Table 15.5.

Table 15.4 Mannosylated NPs in infections and cancer
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Table 15.5 Mannosylated NPs-based vaccines
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6.3 Miscellaneous Applications

6.3.1 Mannosylated SPIONs as MRI Contrast Agents

Superparamagnetic iron oxide nanoparticles (SPIONs) are reported as promising magnetic resonance imaging (MRI) contrast agents. Surface modification of SPIONs becomes essential owing to their drawbacks such as aggregation in water, chemical instability, and nonspecific targeting. To overcome these issues, SPIONs were coated with mannan to enable recognition by mannose receptor present on macrophages [106]. Mannan-coated SPIONs of 28.4 ± 7.2 nm size demonstrated low cytotoxicity in RAW 264.7 cells. Surface coating with mannan prevented aggregation of SPIONs enabling selective delivery into antigen-presenting cells, suggesting applicability as macrophage-targeted MRI contrasting agent.

6.3.2 Two-Photon Photodynamic Therapy

Photodynamic therapy combined with two-photon excitation offers a noninvasive alternative approach to chemo- and radiotherapy to reduce small solid tumors. The photosensitizer was covalently attached to mesoporous silica NPs followed by mannose coating. A single injection aided targeting to tumor site by mannose receptor and two-photon photodynamic therapy led to reduction in tumor size [107].

6.3.3 Biomarker for Pulmonary TB Patients

The serum and pleural concentrations of mannose receptor (CD206) were monitored in pulmonary TB subjects. An increased CD206 level was observed in sera but not in pleura with a sensitivity of 77.3% and specificity of 86.5%. This presents a new application of mannose receptor as a biomarker of pulmonary TB [108].

6.3.4 Lysosomal Targeting in Storage Diseases

Therapeutic enzymes were conjugated to yeast cell wall, a natural source of mannose-6-phosphate (M6P) glycan for utilization in glycogen storage diseases like Pompe disease. Recombinant acid α-glucosidase, a therapeutic in Pompe disease when conjugated to M6P glycans from cell wall of glyco-engineered yeast, revealed efficient intracellular localization and improved accumulated glycogen digestion [109].

7 Clinical Studies

Although targeting to the mannose receptor has been widely investigated, very few mannosylated candidates have entered the clinical trials. Herein, we discuss a promising mannose-based targeted strategy DermaVir, a topical preparation for the treatment of HIV/AIDS and FDA-approved radiopharmaceutical 99mTc-tilmanocept for sentinel lymph node mapping.

DermaVir (Genetic Immunity) is currently enrolled for Phase III clinical trials, set to begin in 2019. It represents topical immunotherapy for the treatment of HIV/AIDS comprising of plasmid DNA-based mannosylated particles [110, 111]. The mannosylated particles are formed by complexation of DNA with a cationic polymer (PEIm) while glucose present in the formulation acts as an aggregation inhibitor and stabilizer. The formulation when applied on the epidermal layer penetrates the skin surface and triggers immune responses.

Staging of cancer progression relies on the mapping of lymph node metastases. Mapping of the sentinel lymph node requires an agent that quickly clears the injection site, rapidly enters, and retains in the sentinel lymph node, without entering the distal lymph nodes. FDA-approved 99mTc-tilmanocept by Navidea Biopharmaceuticals is a mannose-targeted radiopharmaceutical for the detection of sentinel lymph node and lymphatic mapping in tumors [112, 113]. The radiopharmaceutical has crossed several clinical trials [114,115,116] and is now employed for stage determination of cancers under the trade name “Lymphoseek.”

Chemically, it is 99mTc-diethylenetriaminepentaacetic acid–mannosyl–dextran comprising of diethylenetriaminepentaacetic acid and mannose units covalently linked to a 10-kDa dextran backbone. The binding occurs via the mannose residues to the receptors expressed by the myeloid cells. Following injection, 99mTc-tilmanocept enters the lymphatic channels and localizes in the sentinel lymph node by binding to the mannose receptor, thus enabling the mapping of lymph.

8 Advantages and Limitations Related to Specific Targeting through Through Mannose Receptor

As mannose receptor is predominantly located on macrophages, the abode of intracellular infections, specific targeting via mannosylated conjugates and mannose decorated nanocarriers can improve the efficacy of therapeutics and vaccine candidates. Additionally, mannose receptor mediated targeting could provide a practical approach for development of intracellular vaccines. Nevertheless, mannose-targeted vaccines would need to be coupled with other agents to enhance immune response. Interestingly, vaccines based on mannose receptor endocytosis may not only enhance immune responses against cancer and infectious diseases but could also find application in autoimmune disease therapeutics [58]. Surface-modified mannosylated constructs could provide the additional advantage of both phagocytic and endocytic uptake to augment intracellular drug concentrations. However, the ubiquitous presence of macrophages all over the body could provide challenges in targeting specific macrophages through mannosylated carriers.

9 Conclusion

Targeting the mannose receptors represents an exciting therapeutic strategy for infections and cancers overexpressing the receptors. Extrapolating this strategy to vaccines provides exciting opportunities in the design of targeted therapeutics.