Relative to conventional cancer vaccines, some nanomaterials can protect antigens from enzyme-mediated degradation and are able to delay the release of antigens, which is conducive to the continued role of the antigen (8).

Song et al (13) combined a PEG-b-PLACL aqueous solution with Span^®85 and squalene to form a stable isotropic PELC emulsion. Subsequently, the pre-emulsified material was dispersed in RAH polypeptide antigen solution to form uniform nanoparticles. Experiments revealed that the PELC formulation could delay the release of antigens in vitro and in vivo. PELC formulation protected the antigen from rapid degradation and enhanced the effectiveness of the vaccine. Furthermore, the vaccine effectively increased the number of CD4⁺ T cells and RAH-specific CD8⁺ T cells, promoted the secretion of IFN-γ, enhanced the cytotoxic T-cell response and antitumor effect and inhibited tumor growth in mice.

In cancer vaccine immunotherapy, simultaneous delivery of the antigen and the adjuvant can improve the efficiency of cancer vaccines (14). Nanomaterials deliver antigens into the body, activate the immune system and induce cellular and humoral immune responses. The main functions of the adjuvant are to accelerate a lasting immune response, reduce the dosage of the antigen and activate a specific cellular immune response.

Standley et al (15) synthesized nanoparticles with acrylamide as a monomer, that can degrade in the acidic lysosomal environment and covalently bind CpG and encapsulate ovalbumin (OVA) to co-deliver antigen and CpG. Compared with free OVA, OVA-NPs or OVA + CpG, DCs can significantly increase the expression of IL-12 and costimulatory molecules, resulting in stronger antigen-specific T-cell immune responses. Fox et al (16) used liposomes to carry the TLR4-agonist glucopyranoside A and the TLR7-agonist imiquimod. The Th1 phenotype of CD4⁺ T cells increased significantly compared with the use of a single agonist. This finding indicates that nanoparticles that co-deliver multiple immunostimulants possess a synergistic effect.

Lymph nodes located in lymphatic drainage pathways play an important role in the humoral immune response and immune cell activity (17). Nanocarriers can effectively drain the vaccine to the lymphoid tissue, increase the concentration of the vaccine in the lymph nodes, prolong the residence time of the vaccine in lymphoid tissue and control the release of the vaccine and the adjuvant. This function facilitates the antigen presentation and induces T-cell activation to enhance the effects of cancer vaccines.

Liu et al (9) used albumin as a carrier to target a cancer vaccine to the lymph nodes. In vivo, serum albumin is abundant and can transport fatty acids from the blood into lymphatic vessels and lymph nodes. Using this function of albumin, it was conjugated with a lipid, a peptide antigen and a CpG to form an amphiphilic molecular cancer vaccine (AMPH vaccine) that was inoculated into tumor-bearing mice. After inoculation of the structurally optimized CpG-DNA/AMPH vaccine in mice, it significantly accumulated in the lymph nodes, thereby enhancing the antitumor effect of T cells, while significantly reducing systemic toxicity.

APCs are of critical importance in transporting antigens from peripheral circulation to local organized lymphoid tissue. DCs are the most powerful APCs and are at the heart of initiation, regulation and maintenance of immune responses. They can present antigens through MHC class I and MHC class II molecular pathways, provide sufficient co-stimulatory signals and play a key role in initiation and regulation of the T-cell antitumor immune response. DCs express many surface receptors, such as mannose, DEC-205, CD40, CD11c and DC-SIGN receptors (18). The surface of nanoparticles encapsulating an antigen can bind to targeting ligands (mannose, anti-CD11c and anti-DEC205) to specifically deliver the antigen to DCs, thereby promoting binding and endocytosis of the targeting ligands to enhance the effect of the vaccine (19,20).

Guo et al (20) developed erythrocyte membrane-enveloped PLGA nanoparticles containing the antigenic peptide (hgp10025-33) and the TLR4 agonist monophosphoryl lipid A (MPLA). The nanoparticle surface was mannose-loaded, allowing the nanoparticles to be actively targeted by lymphoid organ DCs and promote accumulation in the lymph nodes. The vaccine was inoculated into melanoma-bearing mice and prolonged the antitumor time effect, inhibited tumor growth and metastasis and effectively increased IFN-γ secretion and CD8⁺ T cell response.

Antigens in cancer vaccines are exogenous antigens. In theory, the antigens activate CD4⁺ T cells through an MHC II presentation pathway in vivo. However, the antitumor effect is mainly produced by activation of the CD8⁺ T cell-mediated immunity and CD4⁺ T cells play a supporting role (21). Some nanovaccines can release antigens into the cytoplasm after being taken up by DCs. Subsequently the antigen escapes degradation by lysosomes and thus, can be presented by MHC class I molecules through the endogenous pathway and the CD8⁺ T cells can be activated. Exogenous antigens are presented via MHC I molecules instead of MHC II molecules, which is called cross-presentation (22) (Fig. 2). After entering the cell, the antigen is not damaged by the acidic lysosomal environment and the large number of enzymes and may produce biological effects to activate CD8⁺ cytotoxic T cells through cross-presentation to kill cancer cells. Therefore, it is important to assist the antigen escape from lysosomes in order to improve the vaccine effect. Nanocarriers are often designed based on this premise.

Shen et al (23) demonstrated that PLGA nanoparticles can enhance exogenous antigen escape from the endosome to the cytoplasm, leading to presentation of the antigen in conjunction with MHC I molecules through cross-presentation. Saluja et al (24) encapsulated the MART-127-35 peptide (melanoma-associated antigen) with PLGA nanoparticles and modified the nanoparticle surface with DEC-205 ligands to obtain a nanovaccine capable of targeting DCs. The results indicated that PLGA nanoparticles promoted the antigen uptake by DCs and facilitated the lysosomal escape, thus promoting cross-presentation of the exogenous antigens as well as activation and proliferation of CD8⁺ T cells.

Yuba et al (25) synthesized highly pH-sensitive liposomes that were stable at neutral pH and sensitive to slight changes in pH in a weakly acidic environment, which rendered them unstable. The nanomaterials were combined with OVA to treat tumor-bearing mice. The nanovaccine could effectively be taken up by DCs, where it then fused with the endosomal membrane in the acidic environment and was released into the cytoplasm, leading to cross-presentation, which effectively induced an antigen-specific cellular immune response.

TME refers to the internal environment of tumors, consisting of tumor, stromal and immune cells, microvessels and interstitium and is infiltrated by biological molecules (26). Interactions between the TME and tumor cells have an important effect on tumor growth and metastasis (27). Due to the special physical and chemical properties of nanoparticles, which can be enriched in the TME, tumor blood vessels have enhanced permeability and retention (EPR) (28). Nanoparticles in the TME can regulate the cellular and non-cellular components and then produce antitumor effects.

TAMs are macrophages that have infiltrated tumor tissue and are the most abundant immune cells in the TME. Via cytokines in the TME, macrophages differentiate into different types of TAMs, mainly divided into the M1 and M2 type (29). M1 TAMs can release a variety of pro-inflammatory cytokines, immune activators and chemokines and play an antitumor role through induction of acute inflammation, immune activation and phagocytosis via the following mechanisms: i) release of NO and ROS and direct elimination of tumor cells; ii) antibody-dependent cell-mediated cytotoxicity (ADCC); and iii) induction of a specific immune response (29). M2 type TAMs exert immunosuppressive effects through the following mechanisms: i) secretion of cytokines, such as CXCL-8 and IL-10, that promote tumor cell growth (30); ii) production of matrix metalloproteinases (MMPs) that promote tumor invasion and metastasis; iii) secretion of cytokines, such as IL-10 and TGF-β, that inhibit T-cell activation and promote the differentiation of regulatory T cells (Tregs) (31); and iv) secretion of VEGF, PDGF, and MMPs that promote neovascularization of tumor cells (32) (Fig. 3).

Zanganeh et al (33) found ‘hidden functions’ of ferumoxytol, which has been approved by the FDA for the treatment of anemia, such as the production of antitumor effects by modulating TAMs in the TME, while being non-toxic to normal cells. Experiments revealed that ferumoxytol can recruit more macrophages and induce M1 polarization. Notably, apoptotic cancer cells continue to induce M1 polarization to form an autocrine feedback loop which maintains the production of tumor cytotoxic substances, thereby killing tumor cells. In vivo, local and systemic inoculation of the nanoparticles significantly inhibited tumor cell growth as well as treated and prevented distant metastasis of the tumor. Notably, the experiments clarified that the anticancer mechanism of ferumoxytol includes the promotion of macrophage recruitment for M1 macrophage polarization, inhibiting M2 macrophages.

The rapid growth of tumors requires neovascularization to provide adequate nutrient supply. Tumor neovascularization is dependent on angiogenic factors, mainly vascular endothelial growth factor (VEGF) and platelet-derived growth factor (PDGF), which are widely overexpressed in tumors (34). Therefore, regulation of angiogenic factors can inhibit tumor growth. Some nanomaterials can modulate some of the components of the TME without the need for binding biomolecules. Numerous studies have revealed that gadolinium-containing fullerene nanoparticles are not toxic to normal cells, however inhibit tumor growth and metastasis (35,36). Metal fullerene nanoparticles can act on multiple angiogenic factors concurrently, effectively inhibiting angiogenesis and decreasing the microvessel density in tumors (35). In addition, they significantly decreased the activity of MMPs and promoted the formation of a fiber cage, which can block signal transduction between TAMs and tumor cells (36).

Some nanomaterials, due to their particular physical and chemical properties, can kill cancer cells under certain conditions (37,38). The photothermal effect of certain nanomaterials, for instance, has become a topic of great interest in cancer treatment studies (37,39). Photothermal therapy refers to the use of a material with a high photothermal conversion efficiency that is injected into the body and under irradiation of an external light source (usually near infrared light), the light energy is converted into heat energy that kills cancer cells.

Guo et al (37) designed HCuSNPs for the combination of photothermal therapy and immunotherapy. After the recombination, the compound is more stable, easily remains at the tumor site and is easily taken up by DCs. Furthermore, photothermal ablation induced apoptosis of local cancer cells and some tumor antigens were released into the surrounding environment and synergized with chitosan-CPG nanocomplexes to activate antigen-specific antitumor immune responses. Similarly, Duan et al (38) revealed that Zn-pyrophosphate (ZnP) nanoparticles loaded with the photosensitizer pyrolipid (ZnP@pyro) can induce apoptosis via light irradiation, destroy the tumor vasculature and increase tumor immunogenicity.

Tumor cell vaccines include autologous tumor whole cell vaccines and allogeneic tumor cell vaccines. Autologous tumor cell vaccines are prepared by isolating tumor cells from patients, processing them into vaccines in vitro, and then administering the vaccine to the same patient (40). Allogeneic tumor cell vaccines, which often contain two or three established human tumor cell lines, can be used to overcome many of the limitations of autologous tumor cell vaccines (41). Tumor cell vaccines can present all the tumor antigens, which induce an immune response to the tumor antigens. There are many tumor antigens on the cancer cell membrane surface and it is difficult to achieve these functions through traditional synthesis technology, although this would be an ideal vaccine (42). However, due to the low immunogenicity of tumor vaccines alone, they can only stimulate a small immune response in the body and therefore, it is essential to find a suitable adjuvant to enhance the immunogenicity of a tumor cell vaccine (43).

Fang et al (43) combined purified tumor cell membrane with PLGA nanoparticles to form cancer cell membrane-coated nanoparticles (CCNPs). Combination of monophosphoryl lipid A (MPLA) with CCNPs in a vaccine promoted the maturation of DCs, the uptake of antigen and induced immune activation. Similarly, Liu et al (44) designed a multi-adjuvant whole cell tumor vaccine (WCTV) based on PLGA nanoparticles modified with a cell penetrating peptide, which could promote transportation of GM-CSF and IL-2 into tumor cells. The experimental data revealed that the compound adjuvants promoted DC recruitment, antigen presentation and T-cell activation.

The use of DCs as the carrier in a DC cancer vaccine is recognized as one approach for tumor immunotherapy. A tumor antigen-loaded DC vaccine is considered to have enhanced potential for tumor immunotherapy (45). However, these vaccines mainly use virus or virus-like particles (VLPs) as carriers to transfer antigens into DCs. Thus, they often cause an associated immune response, producing specific neutralizing antibodies to prevent viral infection and therefore, their use is risk-bearing (46). Because of their unique physicochemical properties, nanomaterials have exhibited great potential as vaccine carriers/adjuvants to enhance the immunogenicity of antigens by promoting antigen uptake by DCs, protecting antigens from enzymatic degradation and regulating immune cells (47).

Zeng et al (48) transfected DCs with AFP1 cDNA (with a signal peptide) and AFP2 cDNA (without a signal peptide) using calcium phosphate nanoparticles and induced maturation with recombinant mouse granulocyte macrophage colony-stimulating factor (rmGM-CSF), creating a DC vaccine. The in vivo and in vitro results revealed that the DC vaccine induced activation and proliferation of T lymphocytes to promote secretion of the cytokine IFN-γ, which induced a highly effective and specific immune response to liver cancer. In addition, Matsuo et al (49) encapsulated OVA in biodegradable nanoparticles (γ-PGA NPs) and delivered it to DCs in vitro. The in vivo experimental results revealed that the use of OVA/γ-PGA NP-pulsed DCs induced TAA-specific CTLs, resulting in a strong antitumor effect.

Cancer peptide vaccines have the advantage of direct stimulation, high specificity of the immune response, no autoimmune response or immunosuppression, as well as lack of carcinogenic risk and have wide prospective applications (50). However, cancer peptide vaccines also have shortcomings, such as weak immunogenicity, a short half-life and being prone to immune tolerance. Nanomaterials can be used as carriers in cancer peptide vaccines and as vehicles for co-delivery of antigens and immune adjuvants to target the immune system, which increases peptide vaccine immune function (51).

Zhuang et al (52) used lipid-coated zinc phosphate hybrid nanoparticles (LZnP NPs) to deliver polypeptides (TRP2180-188 and HGP10025-33) and Toll-like receptor 4 agonists. The combination of H-2K (b) and H-2D (b)-restricted peptides provided multiple epitopes as targets of specific MHC alleles and then, the immune system was better able to monitor the tumor. The data revealed that the LZnP nanovaccine increased the secretion of cytokines and the population of CD8⁺ T cells with IFN-γ secretion. The antitumor effect of the nanovaccine was significant in the treatment and prevention of a melanoma-mouse model compared with free antigens and a single peptide-loaded nanovaccine (52).

Genetic vaccines, also known as nucleic acid vaccines, contain recombinant genes encoding a tumor antigen that are inserted into vectors and then introduced into host cells, which then express the exogenous antigenic proteins that can induce an immune response against the antigen, thereby preventing and treating the disease. Genetic vaccines are divided into DNA vaccines and RNA vaccines, however DNA vaccines are the most researched nucleic acid vaccines. DNA vaccines are promising therapeutic vaccines for tumors, but conventional vaccination with plasmids is less than ideal and has potential safety concerns (53). To this end, the use of synthetic novel non-viral vector materials for the delivery of DNA vaccines has become an intensely studied area of research (54,55).

Liu et al (54) designed alginic acid-coated chitosan nanoparticles (ACNPs) as an oral delivery carrier for the legumain-DNA vaccine. The experimental data revealed that ACNPs were better than chitosan nanoparticles (CNPs) in protecting DNA from degradation in an acidic solution (pH 1.5). Furthermore, the results indicated that the vaccine could avoid degradation of DNA by gastric acid and was effectively taken up and expressed by APCs and the tumor volume was significantly reduced.

Li et al (55) used four DNA strands that self-assembled into DNA nanostructures with well-defined structures and uniform sizes. Unmethylated CpG bound to the end of DNA strands by base pairing to form three-dimensional DNA tetrahedra carrying an adjuvant. Experiments demonstrated that the DNA nanostructures effectively entered macrophage-like RAW264.7 cells without transfection reagents. DNA nanostructures have high plasticity, a precise structure, stable properties, low toxicity, and resistance to nuclease degradation. Therefore, DNA nanostructures provide an unprecedented opportunity for the design of safe and effective nanovaccine carriers.