1 Introduction
Cancer is still one of the most difficult diseases to cure in the world. The traditional methods of cancer treatment are mainly divided into three categories: surgical therapy, chemotherapy and radiotherapy. Surgical treatment, that is, surgical resection of tumors in vivo, has a good effect on early tumors that have not spread, but with the development and metastasis of malignant tumors, the effect and prognosis of advanced tumors are poor. Although chemotherapy and radiotherapy can effectively treat some tumors to a certain extent, they are not specific, have great damage to normal cells and immune system of the body, and have the shortcomings of low drug utilization, poor stability and toxic side effects, so they greatly limit the application of cancer treatment. In this context, the new idea of tumor immunotherapy has greatly changed the treatment of malignant tumors, so it was listed as the top ten scientific and technological breakthroughs in Science in 2013, and has become a promising new strategy for clinical treatment of malignant tumors in recent years
[1].
Immunotherapy refers to the treatment of diseases by artificially enhancing or inhibiting the immune function of the body in response to the low or hyperactive immune state of the body. Immunotherapy, based on the function of the immune system to protect the host and its specificity and memory, has been considered as a potential method to treat diseases. Inspired by the above characteristics of immunotherapy, researchers are using this characteristic to carry out basic and clinical research on various infectious diseases, autoimmune diseases, transplant rejection and cancer diseases. In these fields, tumor immunotherapy has achieved remarkable results in clinical practice.
2 Nanocarrier and tumor immunotherapy
Under normal circumstances, the immune system can recognize and eliminate tumor cells in the tumor microenvironment. However, in order to survive and reproduce, tumor cells can adopt different strategies to suppress the human immune system, so that it can not normally recognize and kill tumor cells, so as to survive in all stages of anti-tumor immune response, which is called immune escape. In order to better understand the relationship between the multiple links and steps of tumor immunity, researchers have proposed the concept of tumor-immunity cycle, which mainly includes the following seven links: (1) tumor antigen release
[2]; (2) tumor antigen presentation; (3) initiating and activating effector T cells; (4) migration of T cells to tumor tissues; (5) T cell infiltration in tumor tissue; (6) T cells recognize tumor cells; (7) eliminating tumor cells. The abnormality of any link may lead to the failure of anti-tumor immune cycle and the immune escape of tumors. Different tumors can induce abnormalities in different links to inhibit the effective recognition and killing of tumor cells by the immune system, resulting in immune tolerance, and even promote the growth, diffusion and metastasis of tumors
[3,4].
Tumor immunotherapy is a therapeutic method to control and eliminate tumor cells by restarting and maintaining the tumor-immune cycle and restoring the body's normal anti-tumor immune response
[5]. Its therapeutic principle is to expose the deception of tumor cells, activate the immune system, target and attack tumor cells, and remind the immune system to remove the invasive tumor cells in the blood circulation and prevent their micrometastasis to distant organs
[6]. The key difference between tumor immunotherapy and traditional treatment methods is that its target is the body's own immune system, not the tumor itself.
In tumor immunotherapy, it is necessary for the carrier system to carry immunomodulatory drugs into the body, deliver them to the target area and regulate the immune function, and nano-carriers can meet this requirement. Nanocarrier refers to the carrier with nanometer scale (1 ~ 1000 nm), which is a kind of material carrier delivery system belonging to the nanoscale category, and can realize the carrying and targeted delivery of a variety of drug molecules, immune adjuvants or related antigens and antibodies and other immunomodulatory substances. Nanocarriers have special scale characteristics and physical and chemical properties, so that they can help drugs pass through various barriers in vivo through high permeability and retention effect (EPR effect) to achieve the effect of enrichment in tumor sites. Through the antibody or drug modification on the surface of the nano-carrier, the drug targeting can be improved, and the drug distribution can be effectively improved; Through more efficient delivery of immunomodulatory substances, the intensity of immune response is regulated to achieve effective inhibition of tumor growth. Therefore, in order to further optimize the effect of tumor immunotherapy, the application of nano-carriers in the field of tumor immunotherapy has become a new research hotspot.
3 Cell Membrane Modified Nanocarrier and Its Mechanism of Action
Although nanocarriers have become a key part of cancer immunotherapy, their application in vivo still faces risks such as immune clearance. The immune system is the most powerful defense weapon in the organism, which can identify and eliminate foreign invaders such as bacteria, viruses and microorganisms to ensure the smooth operation of the body. However, while the immune system successfully removes the threat of foreign invasion, the nano-carriers involved in drug transport in vivo will also be interfered by the immune system, resulting in drug transport obstacles. The adsorption of plasma proteins on the surface of nanoparticles is the first defense barrier of the immune system, which can lead to various changes in the surface properties of nanoparticles, such as aggregation and charge neutralization, biochemical activation of defense cascades, and triggering phagocytes and other immune cells to clear them
[7]. Therefore, how to help nano-carriers escape from the body's immune clearance and prolong their circulation time in organisms has become a major research focus for researchers
[8].
In the past, the strategy used to improve the internal circulation time of nanocarriers was "evasion", such as modifying the surface of nanoparticles with PEGs, forming a flexible polymer coating on the surface of nanoparticles through PEGylation,The spatial form of the PEG-molecular chain is utilized to generate a brush-like boundary around the PEG-molecular chain to form a hydrophilic barrier and provide steric hindrance to block receptor interaction, thereby reducing protein adsorption and antibody binding and avoiding the clearance of immune cells
[7,9]. However, in clinical studies, it has been found that repeated application of PEG-acylated nanoparticles can produce antibodies in vivo, trigger anti-PEG immune response, and induce faster blood clearance. With the increase of the number of times of use, the efficacy of the nano-carrier will become worse and the cycle time will become shorter
[10⇓~12]. In order to solve this problem, a new strategy, "camouflage", has gradually come into people's vision. Zhang Liangfang and others at the University of California, San Diego, took the lead in proposing the "camouflage" strategy, which is to transfer the recognition and dialogue mechanism between red blood cells and immune cells to nanomaterials through cell membrane modification.Thereby obtaining camouflage to escape the clearance of immune cells to the nano carrier, and deriving the biomimetic delivery platform of the nano carrier modified by the cell membrane
[13].
In the current research, the use of cell membrane to modify the nanocarrier is mainly divided into three steps, including the extraction of cell membrane, the preparation of nanoparticle "core", and the fusion of the two into a core-shell structure nanocarrier. The extraction of cell membrane mainly includes the following steps: firstly, destroying cells by hypotonic dissolution, centrifuging and separating the substances inside the cells to obtain cell membrane fragments, then forming the cell membrane into vesicles through a porous membrane in a form of combining ultrasonic and mechanical extrusion, and finally co-extruding the vesicles and the nanoparticles to form the cell membrane-coated nanoparticles
[14].
Due to the inherent charge asymmetry of biological cell membrane, it has been proved that the negatively charged nanoparticle surface is most conducive to the coating of the membrane, and has less damage to the membrane structure and fluidity. The positively charged nanoparticles will interact strongly with the negatively charged cell membrane to form polydisperse aggregates with large size. At the same time, the researchers found that the electrostatic repulsion between the negatively charged nanoparticle surface and the asymmetrically charged cell membrane also provided a driving force, which was conducive to the outward orientation of the cell membrane surface, making most of the membrane proteins, surface glycans and so on in an outward state. A large number of hydrophilic surface glycans on the cell membrane also provide a good stabilizing effect for the cell membrane coated nanoparticles. The unstable polymer surface with high surface energy easily interacts with the glycan-stabilized membrane, thus minimizing the overall energy, while the glycan-stabilized membrane surface prevents further membrane interactions
[15,16].
For the approach of cell membrane-coated nanoparticles, membrane extrusion and sonication are the two most commonly used methods to efficiently fuse the cell membrane obtained by centrifugation and hypotonic treatment with the nanoparticle core and to avoid drug leakage and protein denaturation. The membrane extrusion method generally mixes the cell membrane with the nanoparticles and coextrudes the mixture through various nano-sized polycarbonate porous membranes for a plurality of times to obtain the cell membrane-coated nano-carrier. Although this method is very effective, it is still a challenge to apply it to large-scale preparation, so people continue to explore the use of ultrasonic treatment to induce the fusion of cell membrane and nanoparticles, which is more convenient and has great application value. In addition, microfluidic electroporation methods have been reported in recent years to achieve complete membrane coverage of nanoparticles, which together with the above methods construct a biomimetic platform for cell membrane modification of nanocarriers
[17].
The platform not only contains the physical and chemical properties of the nano-carrier itself, but also has the biological properties similar to natural cells. The application of the platform in tumor immunotherapy mainly has the following effects:
[18⇓⇓⇓⇓~23](1) The nano-carrier modified by cell membrane can be disguised as endogenous substances, retain the corresponding functions and surface physical and chemical characteristics of the original cell membrane structure, avoid immunological recognition, such as reducing the uptake of reticuloendothelial system, improve the anti-immune clearance ability, and have better long-term blood circulation ability.
And (2) by design and regulating that cell membrane modify nano carrier, the medicine can be protected from being degrade, the stability of the medicine is increased, the in-vivo activity and the administration efficiency of the medicine are improved, the controllable continuous slow release of the medicine is realized, and the dosage of the treatment medicine is reduced.
And (3) the nano carrier can maintain good biocompatibility of cell membranes, has relatively low cytotoxicity and immunogenicity, does not cause significant side effects and rejection reaction, and reduces the accumulation and toxic and side effects of drugs in internal organs.
(4) With the help of specific recognition units (proteins, polysaccharides, lipids, etc.) On the cell membrane, the nano-carrier obtains specific targeting and homologous targeting capabilities, effectively improves the enrichment in specific areas of the body, enhances the efficiency of drug targeted transport, and improves the biodistribution of drugs.
(5) The nano-carrier system modified by partial cell membrane can also be used for imaging and prevention of specific tumors, so as to realize the integration of diagnosis, prevention and treatment of tumors.
And (6) the nano carrier and the cell membrane have good modifiability, so that the biomimetic delivery system has the potential of combined drug loading, co-targeted delivery and the like, and realizes tumor treatment by combining immunotherapy, chemotherapy, photothermal therapy and other therapies.
Different cell membranes have different biological functions in the body, and corresponding cell membrane modified nano carriers can be selected according to specific requirements,The original function of the nano carrier is kept unchanged, and the biological property of the cell membrane is transferred, so that the nano carrier achieves better tumor immunotherapy effect on the basis of the original function
[24].
Based on the research of nano-carriers modified by erythrocyte membrane, researchers have developed a series of nano-carriers modified by platelet membrane, macrophage membrane, stem cell membrane, tumor cell membrane and bacterial membrane. These new bionic nanocarriers have more diversified functions, such as long circulation in blood and drug loading, and integrate the characteristics of active targeting, immune antigen and controllable sustained release, which enhance the application potential of nanocarriers in tumor diagnosis and treatment
[8].
4 Application of cell membrane-modified nanocarrier in tumor immunotherapy
As the most basic unit of life activity, cells have various important functions needed by the human body, and each cell has its own unique features. Cell membrane is an important medium for information exchange and signal transmission in organisms, and its abundant recognition units (such as proteins, sugars, etc.) On its surface endow it with high biological specificity. In the field of tumor immunotherapy, some achievements have been made in the research of using various cell membrane modified nanocarriers to carry immunomodulatory drugs. The following is an introduction to various cell membrane modified nanocarriers, including erythrocyte membrane, platelet membrane, immune cell membrane and tumor cell membrane, and their corresponding roles in tumor immunotherapy.
4.1 Red blood cell membrane modified nano carrier
Red blood cells (RBC) are the most common cells in the blood, which are mainly responsible for transporting oxygen in the body and can flow with the blood throughout the body, and their greatest feature is the ability of long circulation.Because of its good biocompatibility, elastic plasticity, non-immunogenicity and abundant resources, it is the earliest cell used for surface modification of drug carriers. CD58 and CD59 proteins retained on the surface of erythrocyte membrane are the corresponding ligands of CD2 protein. CD2 is widely present on the surface of T lymphocytes and is a member of the immunoglobulin superfamily, which plays a key role in cell-cell adhesion. CD58 and CD59 are also believed to play an important role in the interaction between T cells and target cells
[25]. In addition, the surface of red blood cells contains a large number of self-recognition markers such as proteins, carbohydrates and sialic acid groups, which can effectively inhibit immune attack
[26]. Erythrocyte membranes isolated from red blood cells can better protect the activity of the transported substances, giving them a longer and more controllable life cycle and avoiding the elimination of the immune system. By inheriting the advantages of erythrocytes, such as long circulation ability, erythrocyte membrane-modified nanocarriers have become a valuable transport carrier
[27].
In order to improve the targeting and antigen presentation efficiency of dendritic cells (DCs), Guo et al. developed erythrocyte membrane-encapsulated poly (D, L-lactide-co-glycolide) (PLGA) nanoparticles for carrying hgp10025-33 antigen peptide and Toll-like receptor 4 agonist monophosphoryl lipid A (MPLA)
[28]. By constructing mannose-embedded membrane structures to actively target antigen-presenting cells (APCs) in lymphoid organs, redox-sensitive peptide-bound PLGA nanoparticles were finally prepared that are easily cleaved in the intracellular environment (fig. 1). The nano vaccine has specific proteins such as CD47 retained by the erythrocyte membrane, which can be combined with a receptor SIRPα mainly expressed on the surface of immune cells such as macrophages to interact with each other and induce the generation of an inhibition signal of "don't eat me" to inhibit the phagocytic activity of the macrophages, thereby avoiding being phagocytized and eliminated and prolonging the circulation time in vivo
[29]; The mannose structure inserted into the cell membrane has the ability to target the mannose receptor on the surface of lymphocytes. In vitro experiments show that it can enhance the uptake of lymphocytes, achieve the effect of antigen aggregation at the site of administration, and promote its retention in draining lymph nodes. The vaccine can also effectively enhance the secretion of interferon gamma and the response of CD8
+T cells, and the content of cytotoxic T lymphocytes (CTL) induced by the vaccine can reach 36.6%, which is obviously higher than 27.6% of free peptide + MPLA (hgp + M) and 28.4% of PLGA-NP
hgp+M. The results of this experiment show the superiority of using polymer nanocarriers coated with erythrocyte membranes for antigen delivery in tumor immunotherapy.
Basal-like breast cancer has a triple-negative phenotype, and the prognosis is poor even with conventional chemotherapy and anti-human epidermal growth factor receptor (HER) therapy. However, immunotherapy is very suitable for this kind of refractory tumor with high mutation rate. Liang et al. Developed a biomimetic black phosphorus quantum dots (BPQDs) nanoformulation to induce apoptosis of breast cancer cells in situ by near-infrared (NIR) laser irradiation to mobilize the immune system to eliminate residual and metastatic tumor cells
[30]. BPQD was coated with red blood cell membrane (RM) to form BPQD-RM nanovesicle biomimetic formulation (BPQD-RMNV), which showed longer circulation time and better tumor enrichment in vivo. Basal-like 4T1 breast tumors undergo apoptosis and necrosis under NIR laser irradiation and recruit dendritic cells (DCs) to capture tumor antigens in vivo. In addition, the use of programmed cell death protein 1 (PD-1) antibody (.alpha.PD-1) in the nanovesicles prevented CD8
+T cell failure. In conclusion, BPQD-RMNV-mediated photothermal therapy (PTT) combined with immune checkpoint blocking antibodies can enhance the infiltration and activity of CD8
+T cells in tumors and directly inhibit the growth of basal-like breast tumors.
Biomimetic nanogels with tumor microenvironment response characteristics can also be used for anti-tumor synergistic therapy combined with chemotherapy and immunotherapy. Song et al. Used hydroxypropyl β-cyclodextrin acrylate and two kinds of reverse charge chitosan derivatives to prepare nanogels, which were used to entrap paclitaxel, an anti-cancer drug, and precisely control the pH response of nanogels
[31]. The nanogels are modified by red blood cell membranes, so that the delivery of the immunotherapeutic interleukin-2 can be realized under the condition of not reducing the biological activity. By responsively releasing the drug in the tumor microenvironment, the nanogel significantly improves the drug permeability and anti-tumor activity, induces the exposure of calreticulin, and enhances anti-tumor immunity. Low-dose combination therapy can improve the tumor microenvironment, enhance the infiltration of immune effector cells, and reduce immunosuppressive factors.
Multifunctional therapeutic nanocarrier systems have been designed to address the shortcomings of cancer monotherapy, including the use of immunotherapy to stimulate the host immune system to eradicate cancer,And provides additional phototherapeutic effects in combination with the photoactive components in the nanosystem, thereby enhancing the anti-tumor effect by minimizing off-target effects and tumor-induced immunosuppression. However, the practical application of combination therapy has been hampered by problems such as systemic toxicity and thermotoxicity, as well as the difficulty of metabolizing the nanocarrier components. Therefore, there is still a need to design more powerful multifunctional nanocarriers to enhance targeting, provide durable and mild combination therapy without significant side effects, and thus achieve effective treatment of tumors.
Ou et al. developed a nanocarrier consisting of a substrate core (poly-L-histidine [H] -grafted black phosphorus [BP]) and a surface shell (erythrocyte membrane [EM]) to provide a mildly photoresponsive (near infrared) activity with erythrocyte membrane camouflage
[32]. Ephrin-A2 receptor-specific peptide (YSA, targeting cancer cells), interleukin-1α silencing small interfering RNA (ILsi, limiting the transport of regulatory T cells) and paclitaxel (X, inducing persistent chemotherapy) were integrated into the basal core-shell structure to form a BP-H-ILs structure, which provided a more intelligent nano-carrier for cancer combination therapy. The biomimetic functionalization of the erythrocyte membrane endows the nanocarrier with the ability to evade macrophage uptake and prolong blood circulation, thereby ensuring its effective accumulation at the tumor site and further enhancing the effect of tumor combination therapy
[33].
4.2 Platelet membrane modified nano carrier
Platelets, like red blood cells, are an important component of mammalian blood, responsible for hemostasis, healing and immune response, and can adhere to many disease-related substrates
[34]. Platelets play an important role in anti-tumor metastasis due to the widespread presence of related antigens and functional proteins, which are associated with immune defense and targeting of damaged vasculature, as well as the ability to respond to invasive microorganisms
[35]. Platelet surface modification and erythrocyte membrane surface modification have the same ability to prolong the blood circulation time of nanocarriers and escape immune clearance
[36]. Moreover, due to the presence of specific targeting recognition proteins such as p-selectin on the membrane, the platelet-membrane-modified nanocarrier can be significantly taken up by tumor cells (rather than normal fibroblasts) and has active targeting capability, thereby increasing the enrichment efficiency of drugs in tumors
[37]. Based on these excellent properties, platelet-membrane-modified nanocarriers have been designed and manufactured for a variety of biomedical applications, including tumor immunotherapy.
In order to protect nanoparticles from phagocytosis by immune cells such as macrophages and deliver drugs to specific tumor sites, Zhang Liangfang et al developed a biomimetic nanoparticle delivery system based on platelet membrane (PNP-Dtxl), which wrapped PLGA nanoparticles with 100 nm size with platelet membrane and carried anticancer drug docetaxel
[18]. The biomimetic nanoparticle completely retains the biological function of the platelet membrane, and the integrin related protein CD47 protein existing on the membrane can send camouflage signals to immune cells to escape immune attack; It can gather in the tumor area through the targeting effect of platelets, kill tumor cells, regulate tumor-related microenvironment, and immunosuppress tumor growth. In order to prove that PNP can use the inherent targeted adhesion mechanism of platelet membrane to deliver drugs, they used Staphylococcus aureus (MRSA252), which can express platelet adhesin, as a research model. After experimental comparison, they found that the adhesion retention rate of PNP increased by 12 times compared with pure NP. Moreover, the retention rate of RBCNP was more than 5 times lower than that of PNP, indicating that the adhesion mechanism was membrane-specific. In addition, in terms of vascular effects, free docetaxel produced an intima-to-media ratio (I/M) of 0.76 ± 0.18 and a lumen occlusion rate of 33.6%, while PNP-Dtxl produced lower values of 0.18 ± 0.06 and 8.0%, respectively, demonstrating the benefit of targeted PNP administration in improving drug localization in diseased vessels.
Circulating tumor cells (CTCs) are the primary cause of hematogenous metastasis to distant organs, which accounts for more than 90% of cancer-related mortality
[38]. Previous basic studies have shown that neutralizing CTCs in the circulation may be an effective strategy to prevent tumor metastasis. Blood circulation by CTCs lacks the same tumor microenvironment as that of solid tumors, but rather a unique "microenvironment" created by the introduction of different types of tumor cells into the blood circulation, in which activated platelets and fibrin are physiologically related to tumor cells in the blood
[39]. Influenced by this, Li et al. Functionally modified the activated platelet membrane-derived vesicles with biocompatible silica nanoparticles, which retained the complete composition of membrane proteins and glycans related to CTC targeting on the platelet membrane.As well as CD47 protein and other substances involved in host cell self-recognition, in order to achieve the purpose of targeting CTC and escaping phagocytosis mediated by phagocytes
[40]. In addition, the binding of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), a major tumor killer cytokine, to platelet-coated nanoparticles induced TRAIL to be highly expressed on the surface of tumor-killing natural killer cells, activated neutrophils, and cytotoxic T cells
[41]. The biomimetic nano-carrier utilizes the adhesion of CTC and activated platelets and the tumor killing activity of immune cells to generate a targeted therapeutic effect; Animal experiments have shown that the implantation of the biomimetic nanoparticles into CTC-related pulmonary vascular microthrombus in a mouse breast cancer metastasis model can significantly reduce lung metastasis.
4.3 Immune cell membrane modified nano carrier
Although erythrocyte membrane and platelet membrane modified nanocarriers have shown excellent performance in evading reticuloendothelial system (RES) -mediated uptake and immune recognition monitoring, surface modification of nanocarriers with other types of cell membranes still has specific biological functionality and applications. Based on this, researchers have noticed that white blood cells with immune functions, such as macrophages, neutrophils, dendritic cells and T cells, are important components of the innate immune system to resist the invasion of pathogens.
The membrane structure of immune cells is more complex, containing a variety of specific proteins and carbohydrates that other cell membranes do not have, which can initiate active immune response to eliminate inflammation and inhibit the occurrence of tumors. Tumor tissues are essentially inflammatory sites, so immune cells such as macrophages and neutrophils can target and enrich in tumor sites. For example, circulating tumor cells (CTCs) can induce the expression of intercellular adhesion molecule-1 (ICAM-1), thereby promoting the adhesion and recruitment of CTCs to host organs. Mac-1 (CD11b,α
Mβ
2), which is highly expressed on the membrane surface of neutrophils, can respond to such inflammatory signals. Therefore, the interaction between neutrophil Mac-1 and CTC-ICAM-1 is an attractive target
[42,43]. Therefore, the strategy of using immune cell membranes containing specific proteins to construct biomimetic nanocarriers has unique value in tumor immunotherapy
[44⇓~46].
Macrophages, as the first line of defense in organisms, are activated to recruit, engulf and digest harmful invaders as soon as they detect the signal of infection or tissue damage. Based on these unique characteristics, macrophage membranes are well suited for coating nanoparticles to construct biomimetic carrier systems for biomedical applications. With the help of specific recognition proteins on macrophage membrane, nanocarriers can effectively improve their aggregation in specific parts of the body and enhance the efficiency of drug targeted transport
[47⇓~49].
At present, nano-drugs based on high permeability and retention effect (EPR effect) are often not enough to meet the goal of targeting in cancer therapy.Liu et al. Reported a laser-responsive shape-variable nanoparticle drug I-P @ NPs @ M coated with macrophage membrane, which promotes the long-term circulation and tumor targeting of the nanoparticle drug
[50]. Chlorin E6 (photosensitizer Ce6) in I-P @ NPs @ M can convert 650 nm laser into reactive oxygen species (ROS), which can convert spherical nano-micelles into linear nano-fibers, and has strong retention in the tumor area, which can localize and release drugs continuously for a long time. ROS can kill tumor cells and induce immunogenic cell death by stimulating dimeric paclitaxel (PTX) to produce monomer PTX through photodynamic therapy, and inhibit indoleamine 2,3-dioxygenase (IDO) production pathway synergistically with indolimod (IND) to activate immune response. Through the combination of chemotherapy, photodynamic therapy and immunotherapy, I-P @ NPs @ M showed good anti-tumor effect and significant inhibition of lung metastasis.
Neutrophils are the most abundant type of white blood cells, play a central role in acute inflammation caused by pathogens, and can naturally target circulating tumor cells (CTCs) and tumor metastasis sites through changes in the expression of cell surface adhesion molecules
[51]. Activated neutrophils can use their cell membrane function to track cytokines and chemokines produced by wounds or inflammation, and accumulate to exert anti-inflammatory effects
[52⇓~54]. At the same time, experimental results have shown that neutrophil membrane binding protein mixtures can be used to capture circulating tumor cells in vivo. PLGA nanoparticles coated with neutrophil membranes effectively improve the capture efficiency of CTCs, promote the return of tumors to the pre-metastatic state, and inhibit the formation of metastatic lesions
[42]. Based on this chemotactic capture behavior, biomimetic nanosystems can be constructed by coating activated neutrophil membranes on nanoparticles, which shows great potential in drug delivery systems.
In the tumor microenvironment, neutrophils can interact with the tumor metastasis environment and CTCs through intercellular adhesion molecules
[55]. Based on this finding, the researchers prepared neutrophil membrane-derived nanovesicles and loaded the nanovesicles with polymer nanoparticles (NM-NP-CFZ) containing carfilzomib, a second-generation proteasome inhibitor, to enhance the targeting and elimination of CTCs and tumor vasculature (Figure 2). This study shows that NM-NP can bind to CTC and localize to pre-metastatic endothelial cells in vitro, and has good targeting and aggregation in the pre-metastatic environment in vivo. Considering the static state, fluid state, temperature and other factors, the experiment proved that the targeting adhesion efficiency of NM-NP on mouse breast cancer cells (4T1 cells) was 2 to 4 times higher than that of pure NP and PLGA-PEG-NP. The number of 4T1 cells treated with NM-NP-CFZ (23.0% ± 3.2%) was also significantly higher than those treated with NP-CFZ and free CFZ (1.88% ± 0.30% and 1.58% ± 0.27%, respectively). After NM-NP-CFZ treatment, the survival rate of leukocytes was 94.6%, indicating that the toxicity of NM-NP-CFZ to leukocytes was greatly reduced. In addition, current cancer therapies, such as radiotherapy and photodynamic therapy (PDT), can spontaneously elicit an immune response that activates neutrophils to infiltrate tumors. Thus, neutrophil membrane-encapsulated polymer nanoparticles are a powerful tumor immunotherapy approach that can prevent tumor metastasis through a variety of different mechanisms
[42].
Lymphocytes can also carry drugs, and immune T lymphocytes have specific tumor immune recognition ability compared with other cells. Immune recognition proteins on T cell membrane, such as T cell antigen receptor (TCR), can selectively recognize tumor surface antigens and activate T cells, thus showing high affinity and killing ability to tumors. Considering the tumor-specific recognition and immunological function of T cells, T cell membrane-modified nanocarriers will be an excellent carrier system for tumor-targeted drug delivery
[56].
In order to combine the advantages of nanoscale drug delivery system with the targeting ability of CAR-T cells, Ma et al., based on cell membrane-coated nanoparticles and cell membrane targeting modification technology, coated CAR-T cell membranes capable of specifically recognizing GPC3 protein on the surface of liver cancer cells on mesoporous silica nanoparticles loaded with IR780 to prepare a new type of nanomaterial
[57]. Through in vitro and in vivo experiments, it was verified that CAR-T cell membrane-coated nanoparticles had superior targeting and less systemic toxicity compared with mesoporous silica nanoparticles loaded with IR780, and could play a greater role in photothermal immunotherapy of tumors.
Influenced by the high mutagenicity of tumor cells and the changes of immune microenvironment, the phenomenon of tumor immune escape is difficult to eliminate, and the dual targeting strategy can provide a more reliable and effective method to improve the tumor targeting therapeutic effect of nanocarriers. Researchers used biological orthogonal technology to construct an artificial target, BCN group (bicyclo [6.1.0] nonyne), at the tumor site, and used glycometabolism technology to embed azide group (N
3) into T cell membrane, and then wrapped the extracted N
3 functionalized T cell membrane on the surface of polymer nanoparticles loaded with indocyanine green (ICG) to construct biomimetic nanoparticles (N
3-TINPs). The special protein (T cell antigen receptor, TCR) on the T cell membrane endows N
3-TINPs with the ability of immune recognition of tumor cells, and the functional N
3 group can react with the artificial target BCN group marked by glucose metabolism on tumor cells efficiently and specifically. The N
3-TINPs utilizes the artificial targeting and natural target dual-guidance treatment technology to greatly enhance the enrichment of ICG at the tumor site and effectively overcome the "off-target effect" of the tumor, thereby improving the anti-tumor efficacy of the nano-carrier tumor targeting and immunomodulatory drugs
[56].
The main challenge of traditional cancer therapy, including surgery, radiotherapy and chemotherapy, is how to induce tumor cell death and make full use of the host immune system to achieve anti-tumor effect. In order to overcome the problem of tumor treatment, Yu et al. Developed a kind of iron oxide magnetic nanoparticles (MNP @ MDSC) wrapped by bone marrow-derived suppressor cell (MDSC) membrane, which can effectively improve the immune escape, magnetic resonance imaging and photothermal therapy (PTT) -induced tumor killing effect of nanoparticles, especially showing superior performance in the active targeting of tumors by MDSC membrane
[58]. After targeting the tumor microenvironment, MNPs @ MDSC can be combined with photothermal therapy and immunotherapy to reprogram tumor-infiltrating macrophages, reduce tumor metabolic activity and enhance anti-tumor immune response by inducing immunogenic cell death.
4.4 Tumor cell membrane modified nano carrier
The use of tumor cell membrane modified nanocarriers for cancer therapy has also attracted the attention of many researchers. Tumor cells have a variety of unique functions, including strong replication potential, resistance to cell death, immune escape ability, long circulation time and homologous binding ability, which make them novel modified materials for the design of anti-tumor therapeutic systems
[59]. Therefore, the diversity of antigens and specific functional proteins associated with tumor cell membrane surface are also very important, and these factors must be taken into account in the construction of anti-tumor vaccines
[60]. There are specific homology-binding adhesion molecules on tumor cell membranes, such as N-cadherin, galectin-3, epithelial cell adhesion molecule (EpCAM) and so on, which make them have the ability of specific homology-binding with tumor cells of the same membrane protein, and can be used for surface functionalization of nanoparticles to provide homologous targeting
[61]. Nanocarriers modified by tumor cell membranes not only represent an effective way to introduce membrane-bound antigens, but also promote tumor-specific immune responses and targeted aggregation processes. It also represents a new drug delivery method and shows the prospect of cancer treatment from simple drug delivery to carrier immunotherapy. In addition, tumor cell membrane modification may also be a new personalized strategy for the development of specific nano-vaccines for different types of tumors
[62].
Biomimetic nanoparticles (BNPs) combine the high flexibility and functionality of synthetic and biological materials for drug delivery and immunomodulation. The researchers developed and characterized poly (lactic-co-glycolic acid) (PLGA) nanoparticles bound to tumor cell membrane components CCMFs (consisting of U87 glioma cells and human breast cancer cells) into CCMF-modified PLGA nanoparticles (CCMF-PLGA NPs) (fig. 3). After immunotherapy of mice, near-infrared fluorescence imaging confirmed the migration of CCMF-PLGA NPs to the proximal draining lymph nodes (LNs), and a higher proportion of CD8
+ and CD4
+ cytotoxic T lymphocyte populations were observed in the spleen and lymph nodes of mice. The results of experimental characterization showed that CCMF-PLGA NPs contained intact membrane-associated proteins, including CXCR4 and CD44, and had the ability to disrupt the interaction between tumor cells and stromal cells and initiate the immune response induced by the immune system, inhibit the migration of tumor cells to human breast fibroblasts, and reduce the migration rate of U87 cells by about 30%
[63,64].
Acute myeloid leukemia (AML) is the most common acute leukemia in adults, also known as blood cancer. One obstacle in current AML therapies is how to target minimal residual cancer cells during the "remission-relapse" period. Recent studies on antigen presentation and immunosuppression of cancer cells have shown that tumor immunotherapy is expected to activate the immune response against residual target cancer cells. In view of this, Johnson et al. Developed and characterized an AML cell membrane-coated nanoparticle (AMCNP) that carries the same surface antigens as the source leukemia cells used as an anti-cancer vaccine
[65]. C1498-mOVA was used as a model antigen by modifying the cell membrane of C1498 mouse AML, and C1498-mOVA was used to coat the nanoparticles, and CpG oligonucleotides were used as immunostimulatory adjuvants to form C1398-mOVA-AMCNPs with core-shell structure. The nanoparticle retain AML specific antigen, effectively stimulates DC-OVA-MHC clas I cross expression after in vivo inoculation, induces enhanced antigen-specific immune response, and improve that immune response to AML.
Liposomes are clinically used as drug carriers for cancer therapy, but they are prone to unnecessary leakage and poor tumor targeting, which may cause toxic side effects on healthy cells and lead to failure of cancer therapy. To overcome these limitations, Kim et al. functionalized liposomes with a photosensitizer (Killer Red, KR) -intercalated cancer cell membrane (CCM) and embedded a lipid adjuvant to promote an anti-tumor immune response
[66]. The results showed that KR protein could be expressed on CCM without leakage from the lipid complex, and the tumor targeting efficiency of liposomes in vivo was greatly improved due to the isotype affinity of CCM to the source cancer cells. The bionic liposome can produce cytotoxic active oxygen in photodynamic therapy, effectively induce tumor immune response, and inhibit primary tumor growth and lung metastasis of tumor-bearing mice.
In recent years, tumor vaccine has been one of the hot spots in the field of tumor immunotherapy. Yang et al. Proposed a strategy to construct a tumor vaccine by encapsulating immunoadjuvant nanoparticles with mannose-modified tumor cell membranes
[67]. Toll-like receptor 7 agonist imiquimod (R837) was first loaded onto PLGA nanoparticles. The adjuvant nanoparticle (NP-R) is then coated on a tumor cell membrane (NP-R @ M), the surface protein of which can be used as a tumor-specific antigen; Finally, by further modifying the mannose moiety, the obtained nanovaccine (NP-R @ M-M) can be taken up in large quantities by antigen presenting cells (such as dendritic cells) and can be stimulated to a mature state to trigger an anti-tumor immune response. The results showed that NP-R @ M-M nanovaccine combined with immune checkpoint blockade therapy had good immunotherapy efficacy and could greatly delay the progress of tumors. Fontana et al. prepared adjuvant-based porous silicon (PSi) nanovaccines by nanoprecipitation in a glass capillary microfluidic device, and used tumor cell membranes as vesicles to encapsulate PSi polymer nanosystems bound to model antigens
[68]. The multilevel nanovaccine has biocompatibility in a wide concentration range, shows immunostimulatory properties in human cells, and is capable of promoting the expression of stimulatory signals and the secretion of anti-inflammatory cytokines.
In the field of tumor vaccine-mediated immunotherapy, dendritic cells (DC) are expected to become the target of tumor vaccine development because of their effective antigen processing and presentation characteristics. Ni et al. developed a multifunctional calcium carbonate (CC) nanoparticle (MC) encapsulated in a tumor cell membrane (CCM) capable of generating in situ tumor-associated antigens (TAAs) for DC vaccination
[69]. Low doses of doxorubicin hydrochloride (DOX) can be encapsulated in the CC core of the nanoparticles to trigger immunogenic cell death (ICD); The commonly used photosensitizer Ce6 was loaded into the nanoparticle CCM, and finally the nano-vaccine (MC/DOX/Ce6) was constructed, which could effectively carry out photodynamic therapy (PDT) through the generation of reactive oxygen species (ROS). In-depth studies have shown that MC/DOX/Ce6 treatment can induce TAAs population and DC recruitment, triggering a subsequent immune cascade, and that recruited DCs can be stimulated in situ for effective vaccination.
Effective immune stimulation usually requires the delivery of both adjuvants and tumor antigens into the body.In order to construct an effective vaccine formulation that delivers multiple tumor antigens and immune adjuvants, and deliver these antigens to antigen-presenting cells to stimulate endogenous immune responses, researchers report the preparation of a biomimetic nanoparticle anti-tumor vaccine. The nano-vaccine uses B16-F10 mouse melanoma cell membrane as the source of a full range of tumor antigens, covers them on PLGA nanoparticles, and delivers them together with MPLA, a highly immunostimulatory adjuvant, which can maximize the ability of tumor antigens and immune adjuvants to promote antigen presentation and activate downstream immune processes
[62,70]. Experiments prove that the nano preparation can induce effective anti-tumor immune response in vivo, and can be added with other immunomodulatory drugs such as immune checkpoint inhibitors (such as programmed cell death-1 antibody) for synergistic effect, so as to better inhibit the growth of tumors. The composite nano-system combined with multiple treatment methods can overcome the tumor microenvironment of immunosuppression better than presenting tumor antigens alone, so as to achieve better effect of tumor immunotherapy
[71,72].
Compared with the traditional tumor cell lysis immunization strategy, the individualized tumor cell nano-vaccine has good prospects and advantages, and can induce more effective and lasting anti-tumor immune response. By using surgically resected tumors, an individualized photothermal vaccine combined with PD-1 checkpoint blocking antibodies can be constructed to prevent tumor recurrence and metastasis. The researchers prepared black phosphorus quantum dot nanovesicles (BPQD-CCNV) coated with patient tumor cell membranes and loaded them into a heat-sensitive hydrogel containing GM-CSF and lipopolysaccharide (LPS). After subcutaneous injection of BPQD-CCNVs, the sustained release of GM-CSF can effectively induce dendritic cells to capture tumor antigens, while near-infrared light and lipopolysaccharide can stimulate the expansion and activation of DCs, which then reach lymph nodes and present antigens to CD8
+T cells. In addition, the combination of PD-1 antibody can significantly enhance tumor-specific CD8
+T cells and eliminate residual and metastatic tumors after operation
[73,74]. Ochyl et al. Also attempted to develop a simple and effective method for therapeutic vaccination using autologous whole tumor cell lysates, which were modified with endogenous cell membranes extracted from tumor cells to form PEG-NPs
[75]. PEG-NPs showed good serum stability in vitro, good drainage effect on regional lymph nodes when administered subcutaneously in vivo, and could induce more powerful antigen-specific cytotoxic CD8
+T lymphocyte response. More importantly, in combination with anti-programmed death-1 (αPD-1) IgG immunotherapy, the PEG-NP vaccine induced a higher frequency of antigen-specific T cell responses, which protected the surviving cells from reattack by tumor cells.
In order to realize the complementary advantages and functional amplification of various treatment methods, there are still many studies on the combination of immunotherapy with chemotherapy, photothermal therapy and other treatment methods. Wang et al combined immunogenic nanoparticle-mediated photodynamic therapy (PDT) with magnetic hyperthermia to synergistically enhance the anti-metastatic efficacy of immunotherapy
[76]. The Janus nanobullet is designed to integrate a photosensitizer (Ce6) -loaded disulfide-bridged magnetic head mesoporous organic silica body (M-MONs @ Ce6), which can be used for redox/pH-induced photosensitizer release and substrate degradation. The coating of tumor cell membrane makes M-MONs @ Ce6 have good tumor targeting and longer blood circulation time. The combination of PDT and magnetic hyperthermia has a strong synergistic anti-tumor effect, and at the same time causes a series of immunogenic cell death, resulting in a synergistic tumor-specific immune response. When combined with anti-CTLA-4 antibody, the biomimetic nanosystem can significantly eradicate primary and deep metastatic tumors with low systemic toxicity, which may facilitate the combined treatment of magnetic thermotherapy, photodynamic therapy and immune checkpoint blocking therapy to combat the development of tumor metastasis.
Although the effects of immunogenic cell death (ICD) and antigen cross-presentation (AC) have been documented in photodynamic therapy (PDT), the overall immune efficacy is rather limited. In order to realize photodynamic immunotherapy, Wang et al. realized the nano-assembly of ovalbumin antigen (OVA) by driving the disulfide bond network between ovalbumin molecules, and then the OVA nanoparticles (ON) loaded with photosensitizer Ce6 were encapsulated by B16-OVA tumor cell membrane to form membrane-encapsulated ON (MON)
[77]. ROS produced by laser irradiation significantly enhanced the efficiency of cross-presentation of OVA antigen. At the same time, due to the coverage of tumor cell membrane, MON has the homologous targeting to tumor. In the treatment of B16-OVA tumor-bearing mice, MON can effectively trigger the immune cascade reaction, completely eliminate the tumor under laser irradiation, and stimulate long-term anti-tumor immune memory effect. The design of the antigen nano-carrier is expected to effectively improve the effect of traditional photodynamic therapy by enhancing the immune cascade.
In order to improve the efficacy of anti-PD-1 immunotherapy in the treatment of melanoma, Xie et al. Proposed a combination of immunotherapy and "starvation therapy" to achieve better anti-tumor effect
[78]. A biomimetic nanosystem CMSN-GOx (fig. 4) was constructed by loading glucose oxidase ( [GOx]) on mesoporous silica nanoparticles (MSN), which can catalyze the conversion of intracellular glucose into gluconic acid and H
2O
2, and inhibit the growth of tumors by blocking the supply of nutrients, and modifying the surface of MSN with melanin tumor cell membranes. Through the bionic surface of functionalized MSN, nanoparticles that can escape the monitoring of host immune system are synthesized, and their tumor targeting and enrichment in tumor tissues are improved, which can eliminate tumors and induce the maturation of dendritic cells, thus stimulating anti-tumor immune response. The percentage of CD80 and CD86 co-stimulatory molecules, which are widely used as biomarkers of DC maturation, was observed to increase from 21.48% to 54.70% after CMSN-GOx in combination with PD-1.The results of DCs maturation show that this treatment regimen can produce strong immune response, showing its potential to improve the effect of immunotherapy. In general, this combination therapy has better anti-tumor therapeutic effect than "starvation therapy" or immunotherapy alone.
In addition to a single drug, the tumor cell membrane modified nanocarrier can also encapsulate multiple drugs with synergistic effects to ensure that multiple drugs are delivered to the same cells in the tumor site to improve the synergistic anti-tumor effect. Chen et al. developed tumor cell membrane-coated nanoparticles (CCNPs) to co-load DOX and small interfering RNA (siRNA) against PD-L1
[79]. DOX can directly target tumor cells by CCNP, while PD-L1 is overexpressed in tumor sites, and its inhibition can reduce the viability of tumor cells and lead to enhanced anti-tumor immune response. In the experiment, the two drugs can be loaded into the PLGA nanoparticle core at the same time, and the PLGA-NP is wrapped by the HeLa cervical cancer cell membrane to achieve efficient homotypic targeting, so as to realize the combined treatment of chemotherapy and immunotherapy.
4.5 Bacterial membrane modified nano carrier
Similar to biomimetic nanosystems based on cell membranes, bacterial membranes containing specific proteins can also be combined with functional nanocarriers for drug delivery, immune stimulation, prolonged circulation, and tumor imaging and therapy
[80⇓~82]. After years of research, a large number of bacteria have been found to be able to form high affinity with specific mammalian cells and tissues through their ligands, and some bacteria naturally have tumor targeting characteristics mediated by adhesion proteins, antigens or specific molecules on the surface of protein coat. Moreover, based on the ability of bacteria to survive in anaerobic environments, bacterial-mediated vector systems can follow oxygen concentration migration to anoxic tumors in vivo
[83]. In addition, there are a large number of pathogen-associated molecular patterns (PAMP) on the surface of bacterial membranes, such as lipopolysaccharide, teichoic acid, peptidoglycan, mannose, spirochete lipoprotein, porin, pilus and lectin.It can induce the recognition and binding of highly expressed pattern recognition receptors (PRR) on the surface of innate immune cells, especially a series of Toll-like receptors (TLR), and then activate the immune system (such as DC, APC and other immune cells) to target and eliminate tumors
[84]. For example, host cell lectins can bind to beta-glucan and LPS on bacterial membranes; Complement receptors CR3 and p150, 95 on the surface of APC and related molecules such as leukocyte function antigen 1 (LFA-1) have multiple specific binding sites for different carbohydrate moieties on the bacterial membrane, of which C1q can bind to mannose-binding lectins on the bacterial membrane. Scavenger receptor a (SR-a) on the surface of macrophages and DCs can recognize various bacterial membrane components such as lipoproteins, LPS, galactose/N-acetylglucosamine, and then produce various targeting effects
[85⇓⇓~88]. In view of the excellent performance of these bacteria, some researchers use the "special cell membrane" obtained from bacteria, bacterial membrane, to achieve the strategy of cell membrane modified nanocarrier, which not only simplifies the current bacteria-based drug delivery system, but also reduces the biological hybrid carrier to nanoscale. Compared with the cell membrane modified nanosystems, the bacterial membrane modification strategy provides a new idea for the design and construction of biomimetic platforms for biomedical applications
[8].
Among the many tumor antigens recognized by T cells, the new antigens induced by random mutations are particularly important. In "cold tumors" that are not infiltrated by tumor-infiltrating lymphocytes (TILs), the limited recognition of tumor neoantigens by T cells leads to a lack of corresponding tumor immune response. Radiotherapy (RT) can enhance the immune recognition of "cold tumor" and produce more diverse anti-tumor T cell responses, but it is difficult to produce systemic anti-tumor immune responses by radiotherapy alone. Patel et al., therefore, developed a multifunctional bacterial membrane-coated nanoparticle (BNP) consisting of an immune-activated PC7a/CpG multimeric core coated with a bacterial membrane and imide groups that enhances antigen presentation (fig. 5)
[89]. In mice with homologous melanoma or neuroblastoma, the use of BNP in combination with radiotherapy and immunotherapy can capture tumor neoantigens after radiotherapy, enhance their uptake in dendritic cells (DCs), and promote their cross-presentation to stimulate anti-tumor T cell responses. Experimental tests showed that 87.5% and 100% of mice treated with BNP alone and BNP + RT, respectively, showed complete regression of primary tumors, with significantly lower metastasis rates and higher survival rates compared with mice treated with RT alone or no drug. In the anti-tumor memory study, mice treated with BNP + RT showed 75% tumor rejection after rechallenge, compared to 50% in mice treated with BNP alone and 15% in the no-drug control group. These results suggest that BNP + RT combination therapy produces significant tumor regression and specific anti-tumor immune memory through the activation of DCs and effector T cells. This bacterial membrane-coated nanocarrier facilitates in situ immune recognition of irradiated tumors by T cells, allowing novel personalized tumor immunotherapy using existing strategies.
At present, the research of bacterial membrane modified nano-carriers is still in the optimization stage, and has not yet reached the versatility of the conventional cell membrane modification process. Therefore, the research work in this field still needs to be further carried out and deepened.
4.6 Other cell membrane modified nano carrier
In addition to the above common cell membrane modified nanocarriers, there are also some special cell membrane or composite membrane modified nanocarrier systems, which are being widely explored and studied by researchers.
For example, stem cells, an important class of cells in organisms, can differentiate into various types of cells to produce desired biological functions. Many types of stem cells, such as mesenchymal stem cells (MSCs) and neural stem cells (NSCs), have been shown to have the ability to specifically target tumors
[90]. Stem cells can be induced by chemokines secreted by tumor cells to accumulate in tumor areas, and the molecular recognition mechanism is complex, including stromal cell-derived factor 1 (SDF1).Also known as CXCL12) and its receptor CXC chemokine receptor 4 (CXCR4), PI3K signaling pathway, urokinase-type plasminogen activator (uPA) -uPA receptor (uPAR), vascular endothelial growth factor receptor 2 (VEGFR2) and matrix metalloproteinase 1 (MMP1) -protease-activated receptor 1 (PAR1)
[91⇓~93]. The modified stem cells can secrete factors such as tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), interferon-beta (IFN-beta), IL-12/18 and the like in tumor tissue regions to inhibit the proliferation of tumor cells
[94⇓~96]. Mesenchymal stem cells target tumor cells mainly through cell-cell surface contact and chemotaxis, and their targeting mechanism is tumor-specific targeting rather than species-specific targeting. Different types of mesenchymal stem cells can be isolated to target different types of tumor cells. In addition, stem cells have low immunogenicity, so different mesenchymal stem cell membranes can be used to modify the surface of nano-carriers to prepare stem cell membrane-modified nano-gel particles, which can improve the anti-immune clearance ability, long-term blood circulation ability and tumor targeting recognition ability of nano-carriers in vivo
[97][98,99].
In recent years, researchers have tried to extend the life of nanocarriers in blood circulation by chemically modifying nanocarriers. However, chemically-modified nanocarriers still can not completely avoid the occurrence of immune clearance reaction, and still have some toxic and side effects. In order to solve this problem, Gao et al. Developed a drug nano-delivery system (SCMGs) with high tumor targeting and biocompatibility by modifying mesenchymal stem cell membranes on gelatin nanogels by physical extrusion using top-down preparation method, which has the biological functions of natural stem cell membranes and significant stability in vitro and in vivo
[100]. The experimental results show that the system can efficiently transport the anti-tumor drugs to the tumor site, realize pH-sensitive release, greatly increase the accumulation of drugs in tumor cells, and significantly improve the killing effect on tumor cells. The nano-delivery system is loaded with immunomodulatory drugs and precisely targets the microenvironment of tumor sites, which will play a greater role in tumor immunotherapy.
In order to further enhance the functionality of cell membrane modified nano-carriers, Zhang Liangfang and others made a breakthrough in fusing two different cell membranes (erythrocyte membrane and platelet membrane) and wrapping them on the surface of nanoparticles.The protein markers of the two cell membranes (such as the characteristic protein CD235a on the erythrocyte membrane, CD41 and CD61 on the platelet membrane, which play an important role in its adhesion properties, and CD44, which is expressed on both cells and can resist phagocytosis by macrophages of the immune system) can be retained on them
[36]. For the first time, they constructed a new fusion cell membrane wrapped nano-carrier with two kinds of cell membrane biological functions, and characterized its structure and function in detail. Studies have shown that that nanoparticle encapsulated by the fusion cell membrane retain the structural similarity with the nanoparticle encapsulated by a single cell membrane, and show perfect combination in respective specific functions, and have good biocompatibility, anti-immune clearance capability, tumor targeting and long circulation time in vivo.
This research has achieved a breakthrough on the basis of the existing biomimetic nanosystem, overcome the functional limitations of a single cell membrane modified nanocarrier, and verified the feasibility of different cell membranes coating nanoparticles to construct nanocarriers. Its potential integration ability and the possibility of further enhancing the functional modification of various cell membranes provide an effective technical platform for the construction of biomimetic nanosystems based on disease personalization in the future, which can be widely used in drug delivery, immune regulation, poison adsorption and many other fields.
Subsequently, Liu et al. Also proposed the concept of using dendritic cells (DC) and tumor cell-derived fusion cells (FCs) biological recombinant membranes (FM) to modify tumor nanovaccines (fig. 6)
[101]. The researchers injected FM modified MOF nanoparticles (NP @ FM), simple nanoparticles NP, dendritic cell membrane-coated nanoparticles (NP @ DM) and tumor cell membrane-coated nanoparticles (NP@ CM) into mice respectively, and then inoculated 4 T1 tumor cells. The experimental results showed that the proportion of tumor-free mice in NP @ FM group was as high as 60%, while the control group pretreated with NP @ CM was zero, and the control group pretreated with NP or NP @ DM was 20%. Moreover, in order to observe the secretion of immunostimulatory cytokines that can promote CTL production, the experiment found that the corresponding proportion of CTLs in NP @ FM group was 14.45%, which was higher than that in NP group (9.53%), NP @ CM group (9.69%) and NP @ DM group (11.02%), indicating the functional superiority of the fusion recombinant membrane. The fusion of these two immune-related cells can highly express tumor antigen complexes and immune costimulatory molecules on the fusion membrane, which enables NP @ FM to function as antigen presenting cells (APCs) for T cell immune activation. In addition, DCs can biologically recognize NP @ FM with tumor antigens, thereby inducing DC-mediated T cell immune activation. The combination of these two immune activation pathways provides a powerful anti-tumor immune response, which provides more options for the development of nano-vaccines against multiple tumor types.
Bone marrow-derived suppressor cells (MDSCs) are often produced after tumor immunotherapy, which can mediate immunosuppression.Recent studies have shown that MDSCs are sensitive to docetaxel, 5-fluorouracil, doxorubicin and other chemotherapeutic drugs.Chemotherapy can enhance the host immune response to tumor by inducing tumor cell death and antigen shedding and presentation
[102][103]. Therefore, the combination of chemotherapy and immunotherapy is a promising synergistic anti-tumor strategy. Wu et al. reported a novel S-CM-HPAD nanoparticle with supramolecular polymer DOX/polyethyleneimine-modified (2-hydroxypropyl) -gamma-cyclodextrin (HPAD) as the core, which was cooperatively modified by tumor cell membrane-Lactobacillus membrane S-layer protein for effective tumor clearance and metastasis inhibition in combination with chemoimmunotherapy
[104]. In the S-CM-HPAD NPs nanocarrier system, the encapsulated mouse melanoma B16F10 tumor cell membrane endows the S-CM-HPAD NPs with the ability of homologous targeting and tumor-associated antigen characteristics; By loading DOX, not only tumor cells can be killed, but also tumor-induced MDSCs can be eliminated, and the anti-tumor immunotherapy effect can be enhanced; As a natural adjuvant, the S-layer protein of Lactobacillus was modified on the surface of S-CM-HPAD NPs by self-assembly, which changed the tumor microenvironment by promoting T cell proliferation and cytokine secretion, protected the antigen and activated the anti-tumor immune response. The results suggest that S-layer protein-enhanced chemoimmunotherapy can provide a new strategy for cell membrane modified nanocarriers in cancer therapy.
Tumor-associated macrophages (TAMs) are a promising target for tumor immunotherapy.However, the use of nano-drug delivery systems (NDDS) to target TAMs is very challenging. In order to overcome this technical barrier, Wang et al. prepared a novel pH-responsive copolymer nanomicelle (dextran-graft-polyhistidine copolymer) modified by erythrocyte-cancer cell composite membrane, and loaded selective colony stimulating factor 1 receptor (CSF-1R) inhibitor BLZ-945 to form a nanocarrier system DH @ ECm
[105]. DH @ ECm has a small enough particle size (about 190 nm) and good immunomodulation and tumor targeting.In the acidic tumor microenvironment, DH @ ECm also has pH-responsive characteristics and unique membrane escape effect, which can promote the recognition and internalization of TAMs through the specific interaction between dextran and CD206 receptor, and then release CSF-1R inhibitor to clear TAMs. In vivo studies, DH @ ECm can improve the tumor immune microenvironment, enhance the expression level of CD8
+T cells, and has a good effect on tumor immunotherapy.
5 Conclusion and prospect
With the further study of the mechanism of tumorigenesis and the development of biotechnology, the treatment of cancer has been constantly improved. More and more experimental and clinical studies have shown that tumor immunotherapy is a very effective anti-tumor treatment. For a series of challenges in tumor immunotherapy, nanocarrier delivery system has shown important application prospects in improving anti-tumor immune effect and overcoming immunosuppression, and the application of new biomimetic nanocarrier in tumor immunotherapy has become the focus of current research.
In this paper, the research progress of biomimetic nanocarrier system modified by cell membrane in tumor immunotherapy is systematically introduced. Based on biomimetic strategies, specific functions and biological properties of cell membranes (such as long circulation time, immune escape, anti-bioadhesion and tumor-specific targeting) are transferred to nanocarriers by using cell membrane fusion and self-assembly technologies. At the same time, the diversity of parental cells (red blood cells, platelets, immune cells, tumor cells and bacteria) provides a rich choice of coating modification for various tumor immunotherapy applications. In addition, most synthetic nanocarriers can be surface modified with cell membranes to form biomimetic nanodelivery systems that are biocompatible and non-immunogenic.
In addition, when designing and using cell membrane modified nanocarriers, many factors must be considered, including the size, shape, charge, surface coating of nanocarriers and the binding of antigens and adjuvants, as well as the specific functions of various cell membranes and the problems of antigen or drug modified nanocarrier.O that the bionic nano system can precisely target a target tissue, enhance the uptake capacity of the target tissue, realize the controllable and continuous release of drugs, activate immune cells such as dendritic cells and the like, improve the inhibitory tumor microenvironment and exert the maximum anti-tumor immune effect.
On the basis of current research, in order to improve the application of cell membrane modified nano-carriers in tumor immunotherapy and enhance the effect of tumor immunotherapy, the following aspects can be carried out or deepened:
(1) Continuously explore new immune drugs, nano-carriers and new cell or bacterial membranes, study and design the best load matching and multi-drug co-transport system according to their respective characteristics, regulate the mixing ratio of cell membranes and nanoparticles to optimize the fusion coating process, and achieve higher encapsulation efficiency and good controlled release ability.
(2) Strengthen the theoretical research on the mechanism of immunology and biology, determine the unknown role of some specific functional proteins and structural units in the cell membrane, and try to identify, remove or add specific antigen proteins on the surface of the cell membrane, so as to modify the nano-carrier more accurately.
(3) Strengthen the research on the mechanism of tumor immunotherapy and nano-delivery, and understand the human immune system and tumor microenvironment in detail, which is conducive to the design of more accurate tumor immunotherapy.
(4) To explore the use of more mixed types of composite membrane components to modify nano-carriers, or to incorporate soluble ligands composed of antibodies, DNA/RNA, proteins or enzymes into cell membrane modified coatings for additional surface functionalization, to introduce multiple biological functions beyond the pure cell membrane, and to enhance the synergistic performance of cancer immunotherapy applications.
(5) To explore the use of cell membrane modified nano-carriers prepared by patients'own cells for individualized drug administration and treatment, so as to achieve the effect of precise treatment of tumors.
(6) Strengthen the extensive combination of immunotherapy with other treatment methods such as chemotherapy, radiotherapy and photothermal therapy, and organically combine the advantages of other therapies in directly killing tumors and immunotherapy in stimulating the immune system to resist tumors, so as to realize the integration of various tumor treatment methods.
In conclusion, the research of cell membrane-based nanocarrier platform in tumor diagnosis and treatment is still in its infancy, and the transition from experimental research to clinical application must overcome many challenges. With the continuous optimization of nanomaterials and the vigorous development of nanotechnology, more innovative and efficient nanocarrier systems based on cell membrane modification will be developed, which will provide a platform and opportunity for the breakthrough of cancer immunotherapy.
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