(TACAs)-based Antitumor Vaccines - ACS Publications - American

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Recent Advance in Tumor-associated Carbohydrate Antigens (TACAs)-based Antitumor Vaccines Danyang Feng,†,‡ Abdul Sami Shaikh,§ and Fengshan Wang*,†,‡ †

Key Laboratory of Chemical Biology of Natural Products (Ministry of education), Institute of Biochemical and Biotechnological Drug, School of Pharmaceutical Sciences, Shandong University, Jinan 250012, Shandong, Peoples’ Republic of China ‡ National Glycoengineering Research Center, Shandong University, Jinan 50012, Shandong, Peoples’ Republic of ChinaChina § Institute of Clinical Pharmacology, Qilu Hospital of Shandong University, Jinan 250012, Shandong, Peoples’ Republic of China ABSTRACT: Cancer cells can be distinguished from normal cells by displaying aberrant levels and types of carbohydrate structures on their surfaces. These carbohydrate structures are known as tumor-associated carbohydrate antigens (TACAs). TACAs were considered as promising targets for the design of anticancer vaccines. Unfortunately, carbohydrates alone can only evoke poor immunogenicity because they are unable to induce T-cell-dependent immune responses, which is critical for cancer therapy. Moreover, immunotolerance and immunosuppression are easily induced by using natural occurring TACAs as antigens due to their endogenous property. This review summarizes the recent strategies to overcome these obstacles: (1) covalently coupling TACAs to proper carriers to improve immunogenicity, including clustered or multivalent conjugate vaccines, (2) coupling TACAs to T-cell peptide epitopes or the built-in adjuvant to form multicomponent glycoconjugate vaccines, and (3) developing vaccines based on chemically modified TACAs, which is combined with metabolic engineering of cancer cells.

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glycan structures on the tumor cell surface result from the combined action of glycotransferases and glycosidases which are abnormally expressed in tumor cells.5 TACAs can be divided into two classes (Figure 1): (1) glycoprotein antigens including Tn, Thomsen−Friendreich (TF), and sailyl-Tn (sTn) linked to the hydroxyl group of serine or threonine residues of protein; and (2) glycolipid antigens, which are carbohydrates linked to ceramide lipids and anchored to the lipid bilayer on the cell surface through hydrophobic interactions. The glycolipids can be further classified into several families: the gangliosides such as GD2, GD3, GM2, GM3, and fucosyl-GM1; globo class such as Globo-H, Gb3, Gb4, and Gb5; blood group determinants such 68 as Lewisx, Lewisy, and their sialylated derivates. Very recently, a TACA derivate of Globo H, stagespecific embryonic antigen-4 (SSEA-4), has become a potential therapeutic target in astrocytoma.6 TACAs play important roles in tumor cell metastasis and signal transduction.7,8 TACAs are barely expressed on normal tissues due to their tumorspecificity. Take for instance sLex (short for sialyl Lewisx, a tetrasccharide antigen) antigen, which only presents in cancer patients’ sera9−12 but not in the normal sera.13,14 Furthermore, TACAs are shared by many cancer cell types,15 and it is even more important that the overexpression of TACAs on the cancer cell surface is a sign of tumor progression and

ancer is one of the most serious public health problems all around the world. According to a report from American Cancer Society,1 cancer constitutes an enormous burden for developed countries as well as developing countries. For example, about 14.1 million new cancer cases and 8.2 million deaths occurred in 2012 worldwide. Significant progress has been made in traditional methods of cancer therapy, including chemotherapy, radiation, and surgery during past decades. However, cancer still remains extremely difficult to cure or prevent. Therefore, it is imperative to develop novel methods to combat cancer. Built on the great success in vaccines against infectious diseases, cancer vaccines and immunotherapy have become attractive alternative approaches for cancer prevention and treatment. Early in 1893, William Coley injected live or inactivated Streptococcus pyogenes and Serratia marcescens into tumor tissues and observed that an immune response could eliminate established sarcomas.2 This is the first report on cancer immunotherapy. In recent decades, passive immune treatment with antitumor monoclonal antibodies (mAbs) has been used successfully in the clinic.3,4 These advances shed light on the development of durable and long-lasting immune response cancer vaccines.

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TUMOR-ASSOCIATED CARBOHYDRATE ANTIGENS Tumor-associated carbohydrate antigens (TACAs) are molecular markers overexpressing on human tumor cells, which can be used to distinguish tumor cells from normal cells. These © 2016 American Chemical Society

Received: January 27, 2016 Accepted: February 19, 2016 Published: February 19, 2016 850

DOI: 10.1021/acschembio.6b00084 ACS Chem. Biol. 2016, 11, 850−863

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ACS Chemical Biology

Figure 1. Representative structures of TACAs.

Figure 2. Immunological response to carbohydrate antigens. (a) Carbohydrates bind B-cell receptors, initiating a weak T-cell independent response. (b) Induction of T-cell-dependent response by conjugated vaccines.

still facing serious problems: first, the availability of a sufficient amount of TACAs. The carbohydrate antigens applied to cancer vaccines were isolated from tumor cells initially,18 and the heterogeneity of the sugar chains on the cell surface makes the separating operation strenuous and time-consuming. However, this problem has been solved basically with the booming development of glycobiology and oligosaccharide synthesis in recent years. Solid-phase synthesis of oligosaccharides,19 one-pot synthesis depending on the activity of glycosyl

metastasis,16 and that is frequently correlated with poor prognosis.17 Therefore, TACA-based vaccines become promising candidates for cancer immunotherapy.

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PROBLEMS ASSOCIATED WITH TACA-BASED VACCINES

Despite the great potential of TACA-based antitumor vaccines as a highly potent and specific cancer immunotherapy, they are 851



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donors,20 activation strategy independent of activity of glycosyl donors,21 chemo-enzymatic synthesis of oligosaccharides,22 and a series of oligosaccharide synthesis innovation provide new paths for the synthesis of abundant, high purity and structurally uniform TACAs. Second, most TACAs are T-cell-independent antigens. They do not bind with the major histocompatibility complexes (MHCs) directly and cannot activate T-cells by themselves.23 In contrast to peptides and proteins, carbohydrate antigens fail to induce T-cell-mediated immunity, which is critical for cancer immunotherapy. TACAs alone can only weakly activate the Bcells. Without additional help from helper T-cells (Th-cells), low titers of the low-affinity IgM antibodies are generated (Figure 2a). In order to generate IgG by B-cells, two signals are required: an antigen-specific signal delivered via cross-linking of B-cell receptors (BCRs) on the cell surface and a costimulatory cytokine signal delivered by Th-cells. To convert the immunity type to IgG-type, cells need to interact with Th-cells. It is known that peptides are the principle epitopes to evoke the Tcells response through binding with MHCs.24 Therefore, a classical design of carbohydrate-based vaccines is to conjugate TACAs with a protein or peptide carrier containing established T-cell epitopes.25−29 Antigen-presenting cells (APCs) will capture and internalize the conjugate vaccines when they enter the human body and cleave them into peptides. Then, the peptide epitopes are presented onto the cell surface as complexes with MHC class II (MHC II) molecules. These complexes present antigens to T-cells, and they are essential to T lymphocyte activation. The activated T-cells will release cytokines, and Th-cells will promote the B-cell proliferation and producing IgG antibodies,30 and differentiating to memory B lymphocytes which can survive longer periods. Memory B lymphocytes can response rapidly and produce high affinity IgG antibodies when the same antigens present31 (Figure 2b). In addition to the T-cell and B-cell activation, the immune system also requires some “danger signals” to provoke adaptive immune response, which is usually achieved by using adjuvants in the vaccine formulation. Adjuvants and ligands of toll-like receptors (TLRs) can stimulate the production of these “danger signals” to promote APCs maturation and cytokines expression. They stimulate APCs to release cytokines through activating the TLRs on macrophage surfaces to induce inherent immunity.32 Another immunological challenge is the low immunogenicity of TACAs. Different from exogenous causative agents like bacteria or viruses, tumor cells are endogenous host cells. Although TACAs are overexpressed on cancer cell surfaces, they are also present in small amounts on normal cells, thus perceived as self-antigens by the immune system. Immunotolerance and immunosuppression are more easily induced due to their endogenous properties. One of the key points of the investigation of a carbohydrate-based antitumor vaccine is how to break the immunotolerance. Recently, tremendous efforts have been devoted to overcome these obstacles. As previously mentioned, carbohydrate-based antitumor vaccines based on coupling TACAs to carriers or immunostimulant components, and using chemically modified TACAs to overcome the imunotolerance, have been well summarized.33−44 This review will refer to the methods to improve the immunological character of the TACA constructs and the latest advances.

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CONJUGATED VACCINES BASED ON NATURAL TACAS TACAs Conjugated to Immunogenic Protein Carriers. Covalently conjugating TACAs with protein or peptide carriers containing T-cell epitopes to convert them to T-cell-dependent antigens can remarkably improve their immunogenicity and stimulate the immune system to produce high-affinity IgG antibodies. Therefore, the method of conjugating synthetic TACAs to protein carriers is widely used in carbohydrate-based antitumor vaccines. Over the years, many kinds of proteins such as bovine serum albumin (BSA), human serum albumin (HSA), ovalbumin (OVA), and tetanus toxoid (TTox) have been used as carrier proteins in synthetic tumor vaccines. Kagan and coworkers45 have shown that keyhole limpet hemocyanin (KLH) is one of the most classic TACA carriers. Recently, Shinefield46 summarized that the CRM197 protein has become another promising conjugated vaccine carrier protein. It is demonstrated that different linkers used to conjugate carbohydrates and proteins have different effects on the immunological properties of resultant conjugates.47 Protein conjugated antitumor vaccines based on TACAs can be classified into three types according to the number and type of TACAs attached to a carrier protein. The monomeric vaccines only contain one type of TACAs, while the monovalent clustered vaccines contain clusters of a single type of TACAs, and multivalent vaccines contain several different types of TACAs. So far, most studies were based on monomeric vaccines. Globo H antigen conjugated to KLH showed beneficial results in phase I clinical trials for breast cancer patients.48 Lewisy-KLH (Ley-KLH) conjugate was well tolerated with no obvious side effects in clinical trials. However, the immune responses against Ley were low.49 To overcome the weak immunogenicity of Ley, the Ley−Lex heterodimer (KH-1 antigen) was explored as the carbohydrate antigen. Mice immunized with the KH-1−KLH generated high levels of IgM and IgG antibodies against KH-1 and significant titers of IgG against Ley, while there was little recognition for Lex.50 Recently, a phase III clinical study of GM2 conjugated to KLH with QS-21 as the adjuvant for patients with stage II melanoma showed that the vaccine was ineffective and could even be detrimental to stage II melanoma patients.38,51 STn−KLH plus QS-21, named Theratope, was tested in more than 1000 women with breast cancer. The resulta confirmed its safety, but no survival benefit was presented.52 However, Kunz and co-workers synthesized a series of TACAs conjugates consisting of TTox and a tandemrepeat glycopeptide containing tumor associated mucin MUC1, which induced a strong and highly selective immune response in the mice model53 as well as in human breast tumor tissues.54 In order to identify the optimal carrier and adjuvant combination, Huang and co-workers chemically synthesized and linked Globo H to a carrier protein, including KLH, CRM197, TTox, and BSA, and combined it with the αgalactosylceramide C34 as an adjuvant, and the mice immunization study indicated that, compared with the Globo H-KLH/QS21, the Globo H-CRM197/C34 vaccine elicited more IgG antibodies, which are more selective for Globo H and the Globo H-related epitopes, stage-specific embryonic antigen 3 (SSEA3), and SSEA4, all of which were specifically overexpressed on breast cancer cells and breast cancer stem cells. They further developed SSEA4-CRM197/C34 as a vaccine 852


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Figure 3. Structure of Gb3−MUC5AC(c)−KLH vaccine construct.

Figure 4. Unimolecular hexavalent vaccine containing Globo-H, Lewisy, sTn, Tn, GM2, and TF antigens.

cancer. The MUC5AC was incorporated as both a linker and a marker, which was designed not only as a B-cell epitope for the production of antibodies against mucins, but also as a Th-cell epitope to activate T-cells.59 A new direction has been explored to prepare a multivalent vaccine that contains a broad range of different tumor antigens. Multivalent vaccines may increase the likelihood to cure cancer or improve their chances of being clinically effective.60 Initially, researchers physically mixed several monovalent carbohydrate conjugated vaccines, and high titer IgM and IgG antibodies were induced against each of the antigens administered in mice.61,62 But physically mixed polyvalent vaccines take in large amounts of protein carriers, and the content of carbohydrate antigens are indeterminate. To tackle this problem, researchers synthesized polyvalent-monomolecular vaccines. Danishefsky’s group has synthesized a series of multivalent vaccines successively. Polyvalent glycopeptides containing Tn antigen, Globo-H antigen, and Ley antigen were synthesized by his group originally.63 Then, they synthesized a new pentameric vaccine targeting a specific cancer, containing prostate tumorassociated antigens, Tn, TF, sTn, Ley, and Globo-H.64

candidate, and after immunization, it was found that the elicited antibodies are also IgG-dominant and very specific for SSEA4.55 It has been indicated that clustering of TACAs on the vaccine construct to increase local concentrations of TACAs is helpful to induce antibodies with a higher affinity to the natural antigen.56 Tn clustered-KLH (Tn(c)−KLH) was synthesized by Livingston’s group. Mice immunized with the Tn(c)−KLH generated high IgG titers toward the Tn antigen, while Tn− KLH only raised a few antibodies.45 In phase I clinical trials, Tn(c)−KLH elicited significant titers of IgM and IgG with prostate cancer patients. The vaccination induced declines of the slopes of prostate-specific antigen in several patients.57 Clustered TF-antigen conjugated with KLH plus QS21 as an adjuvant had entered phase II clinical trial. All doses induced high-titer IgM and IgG antibodies against TF antigen, and an antitumor effect in the form of a change in post-treatment versus pretreatment logPSA slopes was also observed.58 In addition, Zhu and co-workers had designed and synthesized a vaccine that incorporated alternating repeats of the Gb3 antigen and the MUC5AC-based peptide marker (Figure 3). MUC5AC is a mucin-derived peptide associated with breast and gastric 853


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Figure 5. Functionalization of the Tn-CMPV conjugate.

workers72 that demonstrated that a variety of carbohydrate antigens including Globo-H, sLex, and blood group A tetrasaccharides were arrayed on the exterior surface of CPMV. Inoculation of chickens with these virus conjugates produced a large amount of polyclonal antiglycan IgY antibodies. The polyclonal antiglycan IgY antibodies are the functional equivalent of mammalian IgG’s that displayed excellent avidity and specificity in binding the carbohydrate antigens from which they were generated. Therefore, VLPTACA conjugates are promising candidates for diagnostic and immunotherapeutic application. The attention for gold nanoparticles (AuNPs) has recently grown, especially regarding its applications in cancer treatment and immunization by nanovaccines. The interest for this type of nanoparticles is given by their ability to penetrate blood vessels and tissue barriers and to be targeted to a specific cell by means of specifically functionalized molecules. Parry and co-workers describe the design and construction of peptide-free multivalent glycosylated nanoscale constructs as potential synthetic cancer vaccines that generate significant titers of antibodies selective for aberrant mucin glycans. A polymerizable version of the Tn antigen glycan was prepared and converted into well-defined glycopolymers by Reversible Addition−Fragmentation chain Transfer (RAFT) polymerization. The polymers were then conjugated to AuNPs, yielding “multicopy-multivalent” nanoscale glycoconjugates. Immunological studies indicated that these nanomaterials generated strong and long-lasting production of antibodies that are selective to the Tn antigen glycan and cross-reactive toward mucin proteins displaying Tn.76 Very recently, Biswas and co-workers synthesized TF antigen-glycoamino acid conjugates attached to AuNPs through a combined alkane/PEG linker, where the TF antigen was attached to either a serine or threonine amino acid. They showed that the particles bearing the saccharides selectively inhibited tumor cell growth of the Gal-3 positive cells significantly more than the Gal-3 negative cells. In addition, the threonine-attached TF particles were more potent than the serine-attached constructs. These results support the use of AuNPs as antitumor therapeutic platforms, targeted against cell lines that express specific lectins that interact with TF antigen.77 Zwitterionic Polysaccharide-based TACA Delivery. A majority of carbohydrate antigens are known as T-cellindependent antigens which elicit immune responses independently of T-cells. A recent study disclosed that the zwitterionic capsular polysaccharide (ZPS) from capsules of several bacteria can utilize MHC II presentation to activate Th cells in vitro and in vivo.73 Several ZPSs were isolated from different bacterial species. These ZPSs share an important structure that is crucial for binding to MHC II and presentation to Th cells. Wang et al. demonstrated that PS A1, a ZPS isolated from Bacteroides f ragilis strain 9343, could be identified as a TLR-2 agonist, which played a fundamental role in pathogen

Remarkably, another anticancer vaccine has been prepared via the conjugation of unimolecular pentavalent vaccine, containing five prostate and breast cancer associated carbohydrate antigens, Globo-H, GM2, sTn, TF, and Tn, to maleimidemodified carrier protein KLH. The immunization in mice with this vaccine in the presence of QS-21 as an adjuvant showed high promise in inducing IgG and IgM antibodies against each of the five individual carbohydrate antigens. In addition, fluorescence activated cell sorter (FACS) analysis indicated that these antibodies were able to react with MCF-7 breast cancer cell lines which expressed all five of these carbohydrate antigens.65 Ragupathi and co-workers66 synthesized unimolecular hexavalent vaccines targeting prostate and breast cancer, which is the most complex unimolecular polyvalent vaccine up to now (Figure 4). In recent years, scientists have developed new approaches for the synthesis of multivalent antitumor vaccines. Cai and co-workers have synthesized a vaccine composed of a MUC1 glycopeptide with both Tn and sTn antigens, which was further conjugated to BSA carriers. Immunization of Balb/c mice with this vaccine candidate could promote a remarkable immune response with predominant IgG1 isotype antibodies and the induced antibodies showed a strong binding to breast MCF-7 tumor cell lines.67 Nanoparticle-based TACA Delivery. Research has shown that antigen bound to inert polystyrene microspheres (0.5−2.0 μm) is efficiently presented by MHC class I on APC, leading to subsequent CD8+T-cell activation, inducing high antibody titers in mice.68 Conjugation of antigens to these nanobeads induced responses that were significantly higher than those elicited by other bead sizes, and higher than a range of currently used adjuvants.69 Virus-like particles (VLPs) are multiprotein structures that fall within a similar particle size range (0.02−0.06 μm) that mimic the envelopes or capsids of authentic native viruses but lack the viral genome. VLPs are composed of structural proteins self-assembled in a highly ordered manner, which potentially yield safer and cheaper vaccine samples.70 Huang’s group described the application of the cowpea mosaic virus (CPMV) capsid as the immunogenic TACA carrier.71 They synthesized Tn antigen derivatives with either a maleimide or a bromoacetamide linker that was conjugated selectively to cysteine residues of CPMV (Figure 5). CMPV does not contain endogenous carbohydrates, and each Tn-CPMV contained an average of 60 copies of Tn. The Tn-CPMV was then injected into mice and generated high total antibody titers and high IgG titers specific for Tn antigen. The antibodies generated were able to recognize naturally occurring Tn antigens presented on the surfaces of both MCF-7 breast cancer cells and the multidrug resistant breast cancer cell line NCI-ADR RES, suggesting that TACA-CPMV based vaccines can be a potentially innovative therapeutic agent toward drug-resistant cancer. Another study was reported by Kaltgrad and co854


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Figure 6. Synthesis of the Tn-PS A1 vaccine.

Figure 7. Structures of synthetic GD3 derivatives.

recognition and activation of innate immunity.74 The ability of binding to MCH II and activating Th cells make ZPSs a novel carrier for the development of TACA-based cancer vaccines. A vaccine candidate containing a PS A1 as a TLR-2 agonist and a Tn antigen as a B-cell epitope has been synthesized and evaluated by De Silva and co-workers (Figure 6).75 The vicinal hydroxyl groups in D-galactofuranose of PS A1 were selectively oxidized. The resulting aldehyde was conjugated to a Tn antigen through an oxime bond. The mice immunization results indicated that chemical modification of PS A1 does not alter the recognition sequence responsible for MHC II-mediated Tcell-dependent immune response. The Tn-PS A1 conjugate construct could generate high titer IgG3 against Tn antigen. The addition of adjuvant for vaccination formulation could further boost the antibody titer.75

Gangliosides are prone to form lactones at low pH since they are hydroxyl acids. Various ganglioside lactones can be formed in vivo, especially in tumor tissues. The lower pH values in a tumor microenvironment may favor the lactone formation while the expression of ganglioside lactones is rare in normal tissues.78 In addition, the lactones have rigid and distorted conformations and may be better recognized by BCR.79,80 The first report on ganglioside lactone synthesis and evaluation was in 1987, by Nores et al.79 The mice immunized with a GM3 lactone formed a high number of hybridomas secreting antibodies which recognized both GM3 and GM3 lactone. However, mice immunized with GM3 under the same conditions did not result in hybridoma formation. In view of this finding, Livingston and co-workers prepared a series of GD3 derivatives (Figure 7), but the antibodies to the GD3 derivatives did not cross-react with GD3 in clinic trials.81,82 In their continuing research, the melanoma patients were immunized with GD3 lactone-KLH conjugates plus QS-21 as an adjuvant. After immunization, high titers of IgM and IgG antibodies were detected against both GD3 and GD3 lactone. These antibodies also strongly cross-reacted with GD3.83 Recently, Arcangeli and co-workers designed and synthesized a hydrolytically stable GM3 lactone mimetic, which was remarkably simpler than the native GM3 lactone. After conjugation of the GM3 lactone mimetic to KLH, the resulting

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THE MODIFICATION OF TACAS IN ANTITUMOR VACCINES Vaccines based on Rare or Unnatural TACAs. As natural TACAs are inherent elements in the human body, immunotolerance and immunosuppression are more easily induced. In contrast to naturally occurring TACA structures, the rare TACA derivatives or unnatural TACA mimics may be more immunogenic and can generate a robust antibody response cross-reactive to the natural TACA antigens. 855


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Figure 8. Structures of mannosamine derivatives used as precursors for cancer glycoengineering and the unnatural TACA analogs expressed on cancer cells.

enzymatic degradation in vivo. Therefore, to improve the stability toward glycosidases and enhance the bioavailability, structural modifications of native antigens have been proposed.93 Hoffmann-Röder and co-workers synthesized new vaccines consisting of TTox and MUC1 glycopeptide with a Tantigen or a more stable T-antigen mimetic. The T-antigen mimetic was characterized by two fluorine substituents in 6and 6′-positions of the pyranose rings. Of note, strong and selective immune reactions were elicited by both vaccines, showing that the primary hydroxy groups of the T-antigen can be replaced by fluorine without reducing the immunogenicity and selectivity of the construct.94 Yang and co-workers synthesized numerous structurally modified sTn antigens. The vaccination results on mice showed that some fluorinecontaining modifications on the sTn antigen can significantly increase the anti-sTn IgG titers and improve the ratios of antisTn IgG/IgM. The antisera could recognize the tumor cells expressing the native sTn antigen.95 Cancer Vaccines based on Modified TACAs Combining with Metabolic Glycoengineering. It is reported that mammalian cells could uptake various N-acylmannosamine analogs from the cultural medium as biosynthesis precursors and convert them to unnatural N-acyl neuraminic acids, which were further incorporated into glycosphingolipids on the cell surface.96 Zuo et al. synthesized vaccines containing either GD3 or N-butyryl GD3 (GD3Bu) conjugated with KLH as the protein carrier. The GD3Bu−KLH conjugate raised the highest IgG titers without any cross-reactivity to unmodified GD3 in mice immunization with MPLA as the adjuvant. The SK-MEL28 cells expressing GD3Bu could be lysed by mAb and GD3Bu antiserum in the presence of a complement.97 Guo’s group synthesized a series of GM3 derivatives and found that KLH− GM3 elicited low levels of immune response in mice immunization, while the KLH conjugates of N-propionyl, Nbutanoyl, N-iso-butanoyl, and N-phenylacetyl (NPhAc) GM3s induced robust immune reactions. The modified GM3−KLH conjugates generated multiple isotypes of antibodies, suggesting that significant T-cell-dependent immune responses were elicited.98 Soon afterward, they modified forms of GM3 expressed on tumor cell surfaces by incubating tumor cells with different N-acyl-D-mannosamines. N-phenylacetyl-D-mannosamine (ManPhAc) was efficiently incorporated in several tumor cell lines, which expressed bioengineered GM3 on their

conjugate successfully elicited specific antimelanoma antibodies in mice.84 Besides the GD3 lactone, O-acetylated GD3 including 9-Oacetyl GD3 and 7-O-acetyl GD3 has been observed on the surface of several cancer cells.85 Livingston et al. investigated the antibody response of melanoma patients to several O-acetyl GD3 derivatives from different source in clinical trials and found only immunization with the buttermilk-derived O-acetyl GD3 (mixture of 7-O-acetyl GD3, 9-O-acetyl GD3 and 7,9-diO-acetyl GD3) preparation resulted in consistent production of IgM antibodies. However, the induced antibodies reacted with the immunogen and with 7-O-acetyl GD3 derived from hamster melanoma but not with 9-O-acetyl GD3 or human melanoma cells expressing 9-O-acetyl GD3 on their cell surface.86 Therefore, use of the O-acetylated GD3 as an effective antitumor vaccine still has a long way to go. N-glycolylneuraminic acid (Neu5Gc) is a derivate of Nacetylneuraminic acid (Neu5Ac). Neu5Gc is biosynthesized from Neu5Ac in the presence of cytidine monophospho-Nacetylneuraminic acid hydroxylase (CMAH) in most mammals.87 However, human tissues normally lack Neu5Gc due to deletion of the CMAH gene.88 However, Neu5GcGM3 was found highly expressed on several types of human tumor cell surfaces.89 It is reported that Neu5GcGM3 down-regulated CD4 expression in murine and human T lymphocytes especially in nonactivated T-cells.90 These results indicated that Neu5GcGM3 could be a possible target for immunotherapy of melanoma. The subsequent studies demonstrated this hypothesis as two constructs targeting Neu5GcGM3 are undergoing clinical trials. One is anti-idiotype murine mAb racotumomab (1E10) against Neu5Gc-containing ganglisosides for nonsmall or small cell lung cancer,91 and another one is Neu5GcGM3-VSSP (very small size proteasome) for melanoma and breast cancer. 92 The phase Ib/IIa study of Neu5GcGM3-VSSP for advanced melanoma patients demonstrated that the vaccine construct was tolerated. Vaccination induced specific anti-NeuGcGM3 IgM, IgG, and IgA antibodies responses and the IgM antibodies showed significant crossreactivity with Neu5AcGM3. It is noteworthy that 7 out of 22 patients were alive for more than 2 years. This is incredible, for they were advanced melanoma patients. Synthetic Modifications of TACAs. The glycosyl linkages in TACAs are susceptible to acid-catalyzed hydrolysis and 856


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Figure 9. Structure of the clustered Tn-antigen mimetics multiantigenic vaccine candidates.

provoked stronger T-cell-dependent IgG1 immune responses.105 All these investigations have proved that the combination of modification of TACAs and metabolic glycolengineering is an efficacy strategy to cure cancer.

cell surface, including K562, SKMEL-28, and B16−F0. These tumor cell lines exhibited ManPhAc-dependent sensitivity to cytotoxicity mediated by anti-PAcGM3 immune serum and complement.99 Nevertheless, to verify the principal of the new immunotherapeutic strategy for cancer, a monoclonal antibody 2H3 against GM3NPhAc (N-phenylacetyl GM3, a modified form of the GM3 antigen) was prepared and employed to demonstrate that murine melanoma cell B16F0 could be effectively glycoengineered by ManPhAc to express GM3NPhAc. It turned out that 2H3 was highly cytotoxic to the glycoengineered B16F0 cell in the presence of complements. These results indicated that the B16F0 cell line can be glycoengineered efficiently by incubating with ManPhAc100 (Figure 8). The same group also synthesized a series of GM3NPhAc derivatives and coupled them to KLH. The resulting glycoconjugates were evaluated as vaccines in mice and compared to the KLH conjugate of GM3NPhAc. All the new GM3 derivatives were proved to be more immunogenic than GM3NPhAc conjugates and elicited stronger T-celldependent immune responses.101 Furthermore, vaccination with a conjugate vaccine made of GM3NPhAc followed by ManNPhAc treatment could significantly suppress tumor growth and prolong the survival of tumor-bearing mice.102 A series of unnatural N-acyl sTn derivatives were also designed and synthesized, and the immunological properties of of N-acyl sTn-based vaccines were evaluated by Guo’s group. The KLH conjugate of sTn elicited low levels of IgM antibodies; however, the KLH conjugates of N-iso-butanoyl sTn and N-phenylacetyl sTn (especially the latter) induced high titers of antigen-specific IgG antibodies. The N-iso-butanoyl sTn and N-phenylacetyl sTn based vaccines generated a T-cell-dependent response that is critical for the antitumor activity.103 5′-N-Phenylacetyl sTn (sTnNPhAc) and its α-linked protein conjugates were prepared. The antiserum derived from sTnNPhAc α- or sTnNPhAc β-KLH inoculated mice was similarly reactive to the corresponding antigens, but the antisera showed very little reactivity to sTn or other partial structures. It was concluded that the sTnNPhAc-elicited immunity was specific for the whole antigen rather than the partial structures of sTnNPhAc.104 Furthermore, a series of modified sTnNPhAc’s and their protein conjugates were synthesized. The immunological properties of these sTn derivatives in the form of KLH conjugate were evaluated in mice and compared to that of sTnNPhAc. It was shown that all the modified structures are much more immunogenic than sTnNPhAc and that they

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FULLY SYNTHETIC CARBOHYDRATE CANCER VACCINE Although the protein conjugate vaccines showed promising potential in cancer immunotherapy, the inherent immunogenicity of the carrier protein is known to be responsible for the suppression of the immune response toward the TACAs or glycoconjugate epitopes.40,45,106 To avoid anticarrier immune responses, fully synthetic vaccines, which contain the exact Th epitope and TACA as a B-cell epitope, have been developed in recent years Multiple Antigen Glycopeptide. Lo-man’s group prepared a multiple antigen glycopeptide (MAG) composed of four copies of the Tn antigen and a mouse Th-cell peptide epitope derived from poliovirus (PV) known as MAG:Tn-PV. Mice immunization with MAG:Tn-PV generated high titers of IgG antibodies in the presence of adjuvant. T-cell stimulation was enhanced, suggesting that the incorporation of Tn in the construct significantly improved the binding of PV peptide to MHC II.107 To further increase the immune response, a MAG based on three consecutive Tn moieties, MAG:tri-Tn-PV, was prepared. The Abs induced by this tri-Tn-PV recognized murine and human tumor cell lines expressing the Tn antigen. Prophylactic vaccination using tri-Tn-PV protected the mice against tumor challenge. When used in active specific immunotherapy, the tri-Tn-PV was much more efficient than the Tn-PV in promoting the survival of tumor-bearing mice.108 The superior efficacy of the glycocluster of three Tn’s on the STT tripeptide backbone over the traditional KLH glycoconjugate to elicit an IgG response against Tn antigen was demonstrated. The MAG:Tn vaccines induced strong tumorspecific anti-Tn antibodies not only in mice but also in two nonhuman primate species. The antisera could mediate antibody-dependent cell cytotoxicity against human tumor cells. In another study, Freire et al. demonstrated the capacity of Tn-glycosylated antigens and the MAG:tri-Tn candidate vaccine to target mouse and human MGL+DCs, especially dermal DCs. In mice studies, MGL+ CD103− dermal DCs efficiently captured and processed glycosylated Tn antigen in vivo and induced a potent MHC II-restricted T-cell response. 857


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Figure 10. Construct of lipid A (1) and the structure of N. meningitis lipid A (2) and the conjugates of MPLA derivatives with sTnNPhAc (3−6).

QS21 as the adjuvant can elicit exceptionally high titers of IgG antibodies in mice.113 In a subsequent study, the construct of the three-component vaccine was improved by substitution of components. Two variations of the Pam3Cys (Pam2CysSK4 or Pam3CysSK4) was used as a TLR2 ligand. Instead of the Tn antigen, Tn-containing glycopeptide MUC1 was incorporated. The YAF peptide was substituted by Th epitope PV.114 Mice immunization showed that Pam3CysSK4 glycopeptide elicited higher titers of anti-MUC1 IgG antibodies, which displayed strong recognition of MCF-7 cells. The immune response was further boosted with the coadministration of QS-21 as the adjuvant. In another study, the authors synthesized the same glycopeptide linked with an immunosilent lipid anchor to further examine the effect of formulation on immune responses.115 They found that immunizations with glycolipopeptide, which contains lipidated amino acids instead of a TLR2 ligand, gave significantly lower titers of IgG antibodies. This demonstrates that TLR engagement is critical for optimum antigenic responses. Although when coadministrated with Pam3CysSK4 or monophosphoryl lipid A (MPLA) similar titers of IgG antibodies were elicited, the resulting antisera had impaired ability to recognize cancer cells. Sucheck et al. synthesized a novel three-component vaccine containing the Tn antigens, a YAF peptide, and an l-rhamnose (Rha) moiety (Rha-YAF-Tn).116 Human serum has been reported to contain large amounts of naturally occurring antiRha antibodies. To mimic this condition, mice were immunized with Rha-ovalbumin (Rha-OVA) to induce high titers of antiRha antibodies in mice, which generated anti-Rha antibody titers 100 fold higher than the control groups. Mice producing anti-Rha were challenged with Rha-YAF-Tn or YAF-Tn. Sera collected from the groups initially immunized with Rha-OVA and later challenged with Rha-YAF-Tn showed a 2-fold increase in anti-Tn titer at 1/100 serum dilution relative to mice not immunized with Rha-OVA. An in vitro T-cell proliferation study using cells primed with either Rha-YAF-Tn or YAF-Tn was done to examine possible differences in antigen uptake and presentation due to anti-Rha antibody and chemical modification. Proliferation of T-cells was stimulated by a 10-fold lower antigen concentration in the presence of Rha antibodies. The results strongly suggested that T-cells present in the spleen were presented with higher concentrations of Rha-YAF-Tn as a result of the presence of the anti-Rha antibodies.

Intradermal immunization with MAG:tri-Tn induced high levels of Th2 cytokines and was associated with increased expansion of the germinal center B-cell population. Therefore, MGL acts as an efficient endocytic antigen receptor on dermal DCs in vivo, able to prime Tn-specific T- and B-cell responses. Moreover, even in the absence of adjuvant, immunization with MAG:tri-Tn induced high levels of anti-Tn antibody responses, which could recognize human tumor cells.109 Very recently, Richichi et al. synthesized a new vaccine composed of four clustered Tn-antigen mimetics (MIM_Tn) and a Th-cell peptide epitope (OvaPADRE) that are conjugated to a cyclopeptide carrier (regioselectively addressable functionalized template, RAFT; Figure 9). The immunization of mice with this vaccine induced a strong and long-lasting Tn-specific response with IgM/IgG antibodies and produced a significant antibody-dependent regression of tumors and conferred protection.110 Self-adjuvanting, multicomponent vaccines. Despite great progress made in the development of various carbohydrate-based vaccines, adjuvants are still needed to boost immune responses in many cases. Currently, only a few adjuvants are approved for human vaccine formulation. Therefore, self-adjuvanting vaccines have been pursued. Recently, Guo’s group coupled the GM3 antigen and the modified GM3 derivate GM3NPhAc with the adjuvant MPLA. Although this construct lacks a peptide Th epitope, the MPLATACA conjugates elicited strong IgG3 antibody responses without the use of an external adjuvant. The induced antibodies reacted strongly with glycoengineered SKMEL-28 cells. The addition of an external adjuvant greatly reduced the immunological activity of the conjugates.111 In another study, the conjugates of MPLA derivatives with a modified form of the sTn antigen (sTnNPhAc) were synthesized simultaneously. These derivatives elicited high titers of antigen-specific IgG antibodies in mice immunization, indicating a T-cell-dependent immune response, in the absence of an external adjuvant (Figure 10).112 It was concluded that MPLAs could be utilized as potent vaccine carriers and built-in adjuvants to create fully synthetic self-adjuvanting carbohydrate-based cancer vaccines. Boon’s group designed a three-component vaccine composed of the lipopeptide adjuvant Pam3Cys (a TLR2 agonist), the YAF peptide (derived from an outer-membrane protein of Neisseria meningitides) as a Th epitope, and a Tn antigen as a Bcell epitope. Immunization with the glycolipopeptide with 858


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ACS Chemical Biology Wilkinson et al.117,118 synthesized a series of multicomponent constructs containing MUC1 glycopeptide bearing multiple copies of Tn and TF antigens tethered to the Pam3CysSer as the built-in adjuvant, with or without the TTox derived peptide as the Th-cell epitope. The corresponding dicomponent lipopeptides without glyosylation in the MUC1 were prepared for comparison. The tricomponent construct (with TTox derivatives) gave much higher antibody titers than the corresponding dicomponent one (without TTox derivatives). Glycosylation of the lipopeptides has great effects on specificity of the humoral response. Mice immunized with the TFsubstituted lipoglycopeptide elicited significant IgG antibodies, while the one substituted with Tn generated a lower IgG response. The antisera from all three constructs exhibited significant binding to MCF7 (22−48% of cells bound). In contrast, antisera from the tricomponent vaccines did not bind to native MUC1 isolated from human breast milk, suggesting that antibodies raised from these vaccines are tumor-selective and do not recognize the more heavily glycosylated MUC1 epitopes displayed on normal cells. Recently, Cai et al. synthesized another glycopeptide-lipopeptide antitumor vaccine containing the TLR2 lipopeptide Pam3CSK4 combined with up to four copies of MUC1 tandem repeat peptides bearing Tn or sialyl-Tn antigens (Figure 11). The immuniza-

structural analogues of HER-2 glyco-lipopeptide (HER−GLP) were designed with palmitic acid attached either at the Nterminal (linear HER−GLP) or between the Th and Tc cell epitopes (branch HER−GLP). Immunological studies on mice have revealed that linear HER−GLP elicited a more potent HER-specific IFN-χ-producing Tc-cell response, while the branched HER-GLP generated a stronger tumor-specific IgG response. Further investigations on cellular and molecular mechanisms have indicated that the position of the lipid moiety in the peptide sequence significantly affects the magnitude of Bcell and cytotoxic T lymphocytes (CTLs) responses. Both of the two HER−GLPs exhibited tumoral regression and a significant increase of survival without administration of external adjuvant. Interestingly, the combination of branched and linear HER−GLP led to higher tumor therapeutic efficiencies than individual ones.122 Abdel-Aal et al. developed lipopeptides incorporating two 16carbon-containing (C16) lipoamino acid (LAA) residues to activate TLR2. The use of LAAs as immunostimulants is advantageous because of their compatibility with standard solidphase peptide synthesis (SPPS) protocols. They designed and synthesized a number of fully synthetic new chimeric constructs incorporating (1) a TLR2 targeting immunostimulant having two C16 LAAs and two serine residues at the C-terminus of the constructs, (2) a cluster of three Tn antigens as B-cell epitopes, (3) a universal CD4+ Th epitope (PV, KLFAVWKITYKDT) derived from polio virus, and (4) OVA257−264 (SIINFEKL) peptide as a CD8+ T-cell epitope. Vaccine candidates were able to induce significantly strong antibody responses in mice without the need for any additional adjuvant or carrier protein.123

Figure 11. Structure of a vaccine containing the TLR2 ligand lipopeptide conjugated to four copies of TACAs bearing MUC1 glycopeptides.

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CONCLUSION AND PROSPECT

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AUTHOR INFORMATION

Taking a panoramic view of tumor immunotherapy, carbohydrate-based synthetic vaccines represent milestones in the field of nontraditional cancer therapies. In spite of many efforts, there are still no carbohydrate-based cancer vaccines approved for clinical use, and many cancer vaccine candidates failed in clinical trials. This may be associated with the ability of tumor cells to escape from endogenous immune response, and this type of tumor cell is more easily responsive to the reaction that causes targeting immunoregulation paths than antigen stimulation. Therefore, mechanism research on tumor immunogenicity should be processed as well as the clinical research. To sum up, the use of multiple TACAs within the same vaccine and the employment of chemically modified mimetic antigen structures combined with metabolic engineering, should be emphatically accounted for developing safe and efficient therapeutic TACA based vaccines against cancer. Additionally, optimization of vaccine formulation in terms of built-in adjuvants or innovative delivery systems may lead to significant improvements in utilization of currently available carbohydrate antigens and open new perspectives for cancer treatment, as well as other diseases which are lacking efficacious either preventative or therapeutic vaccines.

tion in mice with these candidates showed a significant cluster effect. It induced prevailing IgG2a antibodies without the use of any external adjuvant in mice, which bind to MCF-7 breast tumor cells and initiate the killing of these tumor cells by activation of the complement-dependent cytotoxicity complex119 In view of the success of the three-component vaccines, a self-adjuvanting four-component vaccine, incorporating an additional peptide epitope for cytotoxic T-cell (Tc-cells), has been described by Renaudet et al.120,121 This new vaccine prototype displays within a cyclopeptide scaffold, and conjugates with a cluster of Tn antigen as B-cell epitope, a Th peptide epitope (PADRE), a Tc epitope OVA257−264, and a palmitic acid moiety as a built-in adjuvant. The final prototype, delivered in adjuvant-free PBS in mice, induced robust RAFTspecific IgG/IgM that recognized breast cancer cells MCF-7. The vaccine also generated local and systemic OVA257−264specific Tc cells and strong PADRE-specific Th-cell responses. Importantly, the vaccination not only elicited a reduction of tumor size in mice inoculated with syngeneic murine MO5 carcinoma cells and a protection from lethal carcinoma cell challenge but also significantly inhibited the growth of preestablished MO5 tumors. In another study, the authors used a human tumor specific Tc epitope HER420−429 substituted the OVA antigen and two

Corresponding Author

*Tel.: +86-0531-88382589. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 859


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(8) Hakomori, S.-I., and Zhang, Y. (1997) Glycosphingolipid antigens and cancer therapy. Chem. Biol. 4, 97−104. (9) Okuno, K., Kawai, I., Hirai, N., Narimatsu, H., and Yasutomi, M. (2003) Role of sialyl Lewis X in liver metastasis in view of liverassociated immunity. Hepatogastroenterology 50, 756−760. (10) Mizuguchi, S., Nishiyama, N., Iwata, T., Nishida, T., Izumi, N., Tsukioka, T., Inoue, K., Kameyama, M., and Suehiro, S. (2007) Clinical value of serum cytokeratin 19 fragment and sialyl-Lewis x in non-small cell lung cancer. Ann. Thorac. Surg. 83, 216−221. (11) Sozzani, P., Arisio, R., Porpiglia, M., and Benedetto, C. (2008) Is Sialyl Lewis x antigen expression a prognostic factor in patients with breast cancer? Int. J. Surg. Pathol. 16, 365−374. (12) Balmaña, M., Sarrats, A., Llop, E., Barrabés, S., Saldova, R., Ferri, M. J., Figueras, J., Fort, E., de Llorens, R., Rudd, P. M., and Peracaula, R. (2015) Identification of potential pancreatic cancer serum markers: Increased sialyl-Lewis X on ceruloplasmin. Clin. Chim. Acta 442, 56− 62. (13) Fukushima, K., Hirota, M., Terasaki, P. I., Wakisaka, A., Togashi, H., Chia, D., Suyama, N., Fukushi, Y., Nudelman, E., and Hakomori, S. (1984) Characterization of sialosylated Lewisx as a new tumorassociated antigen. Cancer Res. 44, 5279−5285. (14) Malagolini, N., Santini, D., Chiricolo, M., and Dall’Olio, F. (2007) Biosynthesis and expression of the Sda and sialyl Lewis x antigens in normal and cancer colon. Glycobiology 17, 688−697. (15) Hamilton, W. B., Helling, F., Lloyd, K. O., and Livingston, P. O. (1993) Ganglioside expression on human malignant melanoma assessed by quantitative immune thin-layer chromatography. Int. J. Cancer 53, 566−573. (16) Bresalier, R. S., Niv, Y., Byrd, J. C., Duh, Q. Y., Toribara, N. W., Rockwell, R. W., Dahiya, R., and Kim, Y. S. (1991) Mucin production by human colonic carcinoma cells correlates with their metastatic potential in animal models of colon cancer metastasis. J. Clin. Invest. 87, 1037−1045. (17) Werther, J. L., Tatematsu, M., Klein, R., Kurihara, M., Kumagai, K., Llorens, P., Guidugli Neto, J., Bodian, C., Pertsemlidis, D., Yamachika, T., Kitou, T., and Itzkowitz, S. (1996) Sialosyl-Tn antigen as a marker of gastric cancer progression: an international study. Int. J. Cancer 69, 193−199. (18) Helling, F., Shang, A., Calves, M., Zhang, S., Ren, S., Yu, R. K., Oettgen, H. F., and Livingston, P. O. (1994) GD3 vaccines for melanoma: superior immunogenicity of keyhole limpet hemocyanin conjugate vaccines. Cancer Res. 54, 197−203. (19) Plante, O. J., Palmacci, E. R., and Seeberger, P. H. (2001) Automated Solid-Phase Synthesis of Oligosaccharides. Science 291, 1523−1527. (20) Sears, P., and Wong, C. H. (2001) Toward automated synthesis of oligosaccharides and glycoproteins. Science 291, 2344−2350. (21) Huang, X., Huang, L., Wang, H., and Ye, X.-S. (2004) Iterative One-Pot Synthesis of Oligosaccharides. Angew. Chem., Int. Ed. 43, 5221−5224. (22) Monsan, P., Remaud-Simeon, M., and Andre, I. (2010) Transglucosidases as efficient tools for oligosaccharide and glucoconjugate synthesis. Curr. Opin. Microbiol. 13, 293−300. (23) Danishefsky, S. J., and Allen, J. R. (2000) From the Laboratory to the Clinic: A Retrospective on Fully Synthetic Carbohydrate-Based Anticancer Vaccines. Angew. Chem., Int. Ed. 39, 836−863. (24) Germain, R. N., and Margulies, D. H. (1993) The biochemistry and cell biology of antigen processing and presentation. Annu. Rev. Immunol. 11, 403−450. (25) Haurum, J. S., Arsequell, G., Lellouch, A. C., Wong, S. Y., Dwek, R. A., McMichael, A. J., and Elliott, T. (1994) Recognition of carbohydrate by major histocompatibility complex class I-restricted, glycopeptide-specific cytotoxic T lymphocytes. J. Exp. Med. 180, 739− 744. (26) Abdel-Motal, U. M., Berg, L., Rosen, A., Bengtsson, M., Thorpe, C. J., Kihlberg, J., Dahmen, J., Magnusson, G., Karlsson, K. A., and Jondal, M. (1996) Immunization with glycosylated Kb-binding peptides generates carbohydrate-specific, unrestricted cytotoxic T cells. Eur. J. Immunol. 26, 544−551.

ACKNOWLEDGMENTS This work was financially supported by the Major State Basic Research Development Program of China (973 Program) (No. 2012CB822102) and the Major Projects of Science and Technology of Shandong province (No. 2015ZDJS04001).

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KEYWORDS Tumor-associated carbohydrate antigen: Oligosaccharides overexpressing on human tumor cells, which can be used to distinguish tumor cells from normal cells Cancer immunotherapy: Cancer immunotherapy is the use of the immune system to treat cancer. Cancer immunotherapeutic approaches exploit the fact that cancer cells often have subtly different molecules on their surface (e.g., carbohydrates) that can be detected by the immune system known as cancer antigens. Glycoconjugate: Biomolecule obtained by covalent linkage of a glycan to a protein, which provides T cell epitopes for eliciting a memory response of the immune system against the saccharide MHCI and MHCII: Major histocompatibility complex class I and II; protein complexes whose function is to display fragments of intracellular proteins or glycoproteins to T cells Zwitterionic polysaccharide: Polysaccharide presenting sugars with alternated positive and negative charges Glycosylation: The reaction that allows binding of a carbohydrate (glycosyl donor) to the hydroxyl or other functional group of another molecule (glycosyl acceptor). In biology, the acceptor is usually a protein, a lipid, or other organic molecules. Metabolic glycoengineering: A synthetic monosaccharide similar in structure to a natural precursor in a biosynthesis pathway for a cell-surface glycan, but bearing an unnatural chemical group, is incubated with cells or introduced into live animals. The modified monosaccharide enters a cell and is processed by the biosynthetic enzymes in a manner similar to the natural precursor, and the resulting cell-surface glycan bears the unnatural functional group Self-adjuvanting vaccine: While traditional vaccines are formulated into mixtures of an antigen plus an adjuvant, vaccines in which the two moieties are contained within a single molecule are dubbed self-adjuvanting vaccines

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