Cancer immunotherapy (immuno-oncology) is a kind of treatment that aims to restore the capacity of the immune system to identify and reject cancer. Immunotherapy is considered to be a promising new generation of therapy, since immunotherapy can eliminate cancer cells by activating adaptive immunity and innate immunity of patients, with higher specificity and less toxicity, Compared with traditional therapies, such as chemotherapy and radiotherapy [1
]. Presently, cancer immunotherapy mainly includes cellular immunotherapy (Provenge, CAR-T), antibody therapy (Alemtuzumab, Durvalumab), cytokine therapy (interferon, interleukin) and oncolytic viruses. However, despite the encouraging results in tumor treatment, cancer immunotherapy still faces lots of challenges, which may be mainly attributed to tumor heterogeneity, immune cell dysfunction, tumor microenvironment, acquired resistance to immunotherapy, and immunotoxicity [3
]. Therefore, it is necessary to improve the effectiveness and safety of tumor immunotherapy. Recent trends in cancer immunotherapy have focused on developing immunocyte membrane-based nanomaterials.
Cell membranes are composed primarily of lipids, proteins and carbohydrates, and they give cells their structure, protect intracellular components from the extracellular environment, and regulate the materials that enter and leave the cell [4
]. The cell membrane also plays an important role in cell-cell contact, surface recognition, and cell signaling and communication [8
]. The protein content of cell membrane is very high, usually about 50% of membrane volume [10
]. These membrane proteins are crucial to the cellular survival and function because they are responsible for many vital biological events, such as energy storage, cytoskeleton contact, signaling, enzymatic activity, substance transport, and information transduction [11
]. Moreover, the membrane proteins can differ substantially across different cell types, and even the same type of cells from different individuals can have completely different glycosylation modifications [10
]. Hence, as membrane markers, membrane proteins and their glycosylation, which allow cells to recognize each another, are of great importance for cell-to-cell communication. On the one hand, cell-cell recognition is critical for cellular signaling processes that can affect formation and development of tissues and organs in early stage of ontogeny. On the other hand, cell-to-cell communication based on membrane proteins plays a very key role in the distinction between “self” and “non-self” in subsequent immune responses [14
As a lipid bilayer mixed with proteins, the cell membrane is actually a perfect two-dimensional nanomaterial with various functions, since the thickness of cell membrane is only about 10 nm [15
]. Moreover, owing to the lipid bilayer’s spontaneously “self-sealing” behavior, broken cell membranes can also naturally form nearly spherical nanovesicles with an internal, aqueous lumen [10
]. It is feasible and would be significant to use cell membranes to coat nanomaterials for more effective drug delivery. Numerous nanomaterials coated with cell membrane have been fabricated from many different types of cells, such as red blood cells [16
], cancer cells, immunocytes, stem cells, platelets [21
], and bacteria. These cell membrane-based biomimetic nanomaterials not only retain the complex biological functions of natural cell membranes, but they also maintain the highly adjustable physicochemical properties of the synthesized nanomaterials [22
Natural cell membranes that camouflage the nanoparticles’ antigenic diversity from the source cells can have a variety of source cell-relevant functions, such as “self” markers, biological targeting, communication and negotiation with the immune system, and homing to specific regions [25
]. Cell membrane-coated nanoparticles’ unique abilities to biomimic and biointerface with cell membranes not only give them certain physiochemical properties, such as high cargo loading and great stability under high shear-stress conditions [24
], but also make them tunable to have certain biological functions, such as long circulation, targeted recognition, enhanced accumulation in disease sites, and deep tumor penetration [30
In addition, bare nanoparticles usually adsorb biomolecules in plasma and/or intracellular fluid in vivo and form a biological coating on their surface, namely protein corona owing to high surface free energy of nanomaterials [33
]. The composition of protein corona varies depending on the composition, size, and surface modification of the nanomaterial, as well as the environment surrounding the nanomaterial. This protein corona may shield the specific surface structure and cover the targeting ligand, thus hindering the specific reaction between the nanoparticle and its target, which greatly affects the nanoparticles’ fate and may results in removal of nanoparticles from the bloodstream [34
]. Therefore, it is crucial to avoid the formation of protein corona in the development of nanomaterials for biological or biomedical application. However, having a cell membrane coating around the nanoparticles can successfully prevent nanomaterials from forming a protein corona. As the interface between the cell and the outside world, the cell membrane is perfectly compatible with biofluids, effectively blocking the interactions between the nanomaterials and biological system. This strategy allows nanoparticles to navigate more effectively within the body, thereby limiting off-target side effects, significantly regulating immune responses and, ultimately, enhancing treatment efficacy and expanding the application range of nanomaterials [37
Herein, we summarize the recent progress in research on biomimetic immunocyte membrane-coated nanoparticles for cancer immunotherapy. This article will introduce macrophage membrane, T-cell membrane, Natural killer membrane and dendritic cell membrane-based nanoparticles (neutrophil membrane and platelet membrane-based nanomaterials have been introduced in a recent review, so this article will not discuss these in detail). We will highlight their novelty, analyze their potential prospects in the biomedical field, and finally discuss the challenges in their large-scale preparation and application.
2. Immunocyte Membrane Molecules Contributing to Nanomaterials’ Anti-Tumor Immune Effects
Immune response depends on the communication and mutual recognition between immunocytes, and between immunocytes and other cells. Immunocyte recognition is based on immunocyte membrane molecules, usually known as cell surface markers. There is a wide variety of immunocyte surface markers related to tumor immunity, including receptors, antigens, adhesion molecules, and other molecules on the cell surface. Table 1
summarizes some of the major cell membrane surface markers that may contribute to nanomaterials’ anti-tumor immune effects. Because of these specific surface markers, immunocyte membranes have unique functions and can play a special role in assisting drug delivery, especially in tumor cell recognition and anti-tumor immunity.
Because nanoparticles are foreign substances, one of the fundamental problems and technical barriers to using them is uptake by the reticuloendothelial system (RES) or the mononuclear phagocytic system (MPS), which is part of the immune system and consists of phagocytic cells such as monocytes, macrophages in lymph nodes and spleen, and Kupffer cells in liver [39
]. When nanoparticles enter the body, they are first “opsonized” and coated by non-specific proteins that make them more recognizable to phagocytic cells such as macrophages, monocytes, and dendritic cells. Once opsonization, phagocytosis will occur, by which the nanoparticles are engulfed and eventually destroyed or removed from the bloodstream [42
]. The most common strategy to reduce RES uptake is to shield nanoparticles with polyethylene glycols (PEGs) or other polymers. This is effective, but it cannot avoid uptake completely [43
]. However, when nanoparticles are covered with immunocyte membranes, especially macrophage membranes, the RES system can be completely avoided, because the immune system recognizes nanoparticles camouflaged with macrophage membranes as “self” rather than “foreign.”
In addition, some specific receptors and adhesion molecules on macrophage membranes, such as C-C chemokine receptor 2 (CCR2), vascular cell adhesion molecule-1 (VCAM-1, CD106), and intercellular adhesion molecule-1 (ICAM-1; CD54), can guide nanoparticles coated with membranes to inflammatory and tumor sites. Notably, the binding of ICAM-1with macrophage adhesion ligand-1 (Mac-1; ITGAM), leukocyte function-associated antigen-1 (LFA-1), and fibrinogen can facilitate the transmigration of leukocytes across vascular endothelia [44
]. ICAM-1 and soluble ICAM-1 also have antagonistic effects on the tight junctions and thus promote nanoparticles coated with membranes to cross the blood–brain barrier (BBB) [46
The second valuable cell membrane is natural killer (NK) cell membranes. NK cells are unique lymphocytes that can recognize and kill aberrant cells without antibodies or major histocompatibility complex (MHC) [48
]. Therefore, NK cells are key for tumor cell surveillance, because tumor cells that are missing MHC I markers cannot be detected and destroyed by other immune cells, such as T-cells [49
]. Although NK cells lack antigen-specific cell surface receptors, they have many alternative receptors that can recognize tumor cells, including NKG2D, NKp44, NKp46, NKp30, and DNAM [50
]. NKG2D, for example, is a disulfide-linked homodimer that recognizes several ligands, including UL16-binding protein (ULBP) and MHC class I chain-related gene A (MICA), which are typically expressed on tumor cells [52
T-cell membranes are helpful for recognizing and targeting tumor cells because of the T-cell receptor (TCR) on the cell surface. As a protein complex, TCR recognizes fragments of antigens as peptides bound to MHC molecules, thus allowing T-cells to target both surface and intracellular tumor neoantigens [53
]. Neoantigens are mutated antigens specifically expressed by tumor tissue, but not expressed on the surface of normal cells, so they are highly specific for individuals [55
]. Since TCRs can recognize neoantigens and then target tumor cells or tumor tissue, nanoparticles coated with T-cell membranes could target tumor cells or tumor tissue with highly specificity for individuals.
Furthermore, PD-1, CTLA4, and other specific checkpoint inhibitory receptors on T-cells are harmful to tumor immunotherapy, but these checkpoint inhibitory receptors can also identify the corresponding ligands, such as PDL-1, on tumor cells [56
]. Hence, TCRs are beneficial to the delivery of nanodrugs, because they can target tumor cells.
The greatest advantage of mature dendritic cell (DC) membranes is that they possess the antigen presentation function of whole DCs and can specifically activate T-cells, because they have a broad spectrum of peptide/MHC complexes on their surface [58
]. DCs can also express co-stimulatory molecules and adhesion molecules, such as ICAM-3/CD50, CD40, CD44, and integrin family members [60
]. These markers can reduce the negative charge on the cell surface and mediate cell adhesion, thus promoting the interaction between DCs and T-cells [62
5. NK Cell Membranes
NK cells can lead to immune surveillance against cancer and eliminate small tumors early. Because they can engage tumor targets without needing specific antigens, NK cells’ therapeutic potential has been broadly explored to control the metastatic dissemination of malignancies [96
]. NK cells lack genetically rearranged antigen receptors [97
], but they can still recognize and directly lyse abnormal cells without prior sensitization [98
]. This occurs through a well-established and unique set of receptors expressed on NK cells’ surface, which trigger cell lysis activity by interacting with ligands on infected and transformed cells [100
]. The receptors on NK membranes, especially activated receptors such as NKp30, NKp44 NKp46, DNAM-1 (CD226), and NKG2D, are closely associated with tumor recognition and tumor killing [101
]. NKp30 recognizes B7-H6 tumor antigens, while NKp44 binds proliferating cell nuclear antigen (PCNA) and other tumor-associated ligands [100
]. NKp46 inhibits metastatic growth in mice [103
]. DNAM-1 is an adhesion and co-stimulatory molecule that promotes NK cell cytotoxic activity upon binding to its ligands, CD112 and CD155 [104
]. The NKG2D form transduces activating signals to initiate cytotoxic activity upon binding to specific stress-induced ligands, MICA and MHC class I polypeptide-related sequence B (MICB), which are selectively expressed on tumor cells [105
5.1. Targeting Tumors
Aryal Murali et al. developed an NK cell membrane-cloaked fusogenic liposomal delivery system (NKsome) for targeted cancer therapy [108
]. The engineered NKsomes have a variety of receptor proteins on their surface, thus they exhibited not only high binding ability to tumor cells in vitro, but also powerful tumor-homing efficiency in vivo. They also have outstanding biocompatibility and long blood circulation time (18 h). Further in vivo experiments showed that NKsome had promising therapeutic effects on MCF-7 tumor xenograft models. This study demonstrated that biomimetic nanocarriers based on NK cell membrane can partly communicate like immune cells and thus have therapeutic advantages through enhancing tumor drug delivery.
5.2. M1 Polarization and Induction of Immunogenic Cell Death (ICD)
Our group reported on NK cell membrane-cloaked photosensitizer TCPP-loaded nanoparticles (NK-NPs) that could eliminate primary tumors and inhibit distant tumors [109
]. NK cell membranes on the surface of NPs enabled the NK-NPs to target 4T1 tumors via NK cell membrane receptors, such as NKG2D and DNAM-1 (Figure 2
). Furthermore, NK cell membrane proteins such as RAB-10, IRGM1, RANKL Galectin-12, and CB1, can interact with macrophage surface receptors such as tumor necrosis factor or receptor toll-like receptor 4, to prompt or increase pro-inflammatory M1 macrophage polarization, which would kill tumor cells directly by secreting reactive oxygen species (ROS) and nitrogen radicals [110
At the same time, the photosensitizer TCPP inside the NPs, after being irradiated with light, can destroy the cancer cells and induces immunogenic cell death (ICD). These dying cancer cells secrete damage-associated molecular patterns (DAMPs), promote activation of DCs and initiate an adaptive immune response. Results from animal experiments confirmed that NK-NPs selectively accumulate in 4T1 tumors and could eliminate primary tumor growth and produce an abscopal effect (a distant anti-tumor activity induced by local treatments), and then inhibit distant tumors. Thus, this NK cell membrane-based method offers a promising strategy for tumor immunotherapy.
5.3. Penetrating the BBB
Treating brain tumors with drugs is extremely difficult because the BBB hinders the delivery of systemic therapies into the tumor. Many strategies have been proposed for improving drug delivery across the BBB [113
]. Since immunocytes, such as macrophages, neutrophils, T-cells, and NK cells, use specialized mechanisms to penetrate the BBB without compromising their structural integrity [115
], using these immune cells as vehicles to deliver drugs through the BBB has recently become a research hotspot in this field.
Our group developed aggregation-induced emission (AIE) characteristic nanodots decorated with NK membranes ([email protected]
) for near-infrared-II fluorescence-guided glioma theranostics [117
]. The binding of [email protected]
with cell adhesion molecules (CAMs) on brain microvascular endothelial cells by LFA-1 (lymphocyte function-associated antigen 1) and VLA-4 (very late antigen-4) on NK cell membranes could trigger an intracellular signaling cascade, which would disrupt tight junctions and reorganize actin cytoskeletons to form intercellular gaps at the BBB. Then, [email protected]
could cross the BBB through the paracellular pathway. Furthermore, [email protected]
could target malignant glioma cells (U-87 MG) through the receptors (DNAM-1 and NKG2D) on the NK membrane. The sufficient accumulation and high quantum yield of [email protected]
, on the one hand, allow them to perform high contrast and through-skull tumor NIR-II fluorescent imaging, and on the other hand, create [email protected]
localized hyperthermia effects with laser irradiation, which could effectively inhibit glioma growth.
We have summarized the current research on immunocyte membrane-coated nanoparticles (Table 2
). As a new biomimetic drug delivery platform, the immunocyte membrane covering approach is a feasible way to overcome the limitations of introducing foreign materials into the immune system by providing a unique biological interface, and would thus have a wide range of advantages for activating the innate and adaptive immune responses. The cell membranes discussed herein include those originating from macrophages, NK cells, T-cells, and dendritic cells. Macrophages are the main component cell of the RES, so macrophage membranes can help nanoparticles escape the phagocytosis of RES perfectly. In addition, some specific receptors and adhesion molecules can guide nanoparticles camouflaged with macrophage membranes to inflammatory and tumor sites. NK membranes can guide particles to target tumor sites and cross the BBB by a well-established and unique set of receptors expressed on the surface of NK cells. T-cell membranes display some distinctive properties, including binding to neoantigens and displaying specific receptors, which endow biomimetic nanoparticles with immune escape and good targeting abilities. Modified DC membranes enable the membrane-based nanocarriers to present antigens and then activate T-cells. Moreover, by combining with other therapeutic agents such as chemotherapeutic drugs or photosensitizers to treat tumors, cell membrane-coating approaches could enhance cancer therapy.
However, there are some critical issues that must be addressed for the further development and translation of immunocyte membrane-based nanocarriers. First, there is an urgent need for a standard protocol to guide the preparation of immunocyte membrane-based carriers, owing to the difficulty of controlling the parameters of natural materials. Since the functional proteins on cell membranes are susceptible to inactivation, the extraction of cell membranes needs to be done carefully. The whole membrane extraction mainly includes cell lysis, removal of intracellular contents, purification of membranes, and coating cell membrane onto nanoparticle core. The detailed preparation parameters vary according to the cell type. To generate cell membranes-derived nanoparticles with reproducibility and scalability, a more standard protocol needs to be proposed to guide the synthesis of biomimetic nanomaterials.
Second, more researchers should study the integrity of membranes on artificial particles in vivo. The incomplete membrane may lead to the exposure of naked nanomaterials, which in turn may affect the biological effectiveness of cell membranes-derived nanoparticles. There are no relevant research data concerning the extent to which the non-integrity of cell membrane actually affects the function of cell membranes-derived nanomaterials. Therefore, more research should be done in the integrity of membranes on nanoparticles.
Third, measures should be taken to further transform and functionalize immunocyte membranes to broaden the application of membrane-derived nanoparticles. These methods, such as membrane hybridization, lipid insertion, metabolic engineering, and genetic modification, would contribute diverse functions in a nondisruptive fashion while preserving the natural function of the cell membranes. In addition, Immunocyte-derived exosomes also are considered to be very promising system for drug delivery and are receiving a great deal of attention due to their naturally biocompatible characteristics [126
]. The exosomes based on monocytes and macrophages could prolong blood circulation time and avoid immune phagocytosis [127
]. DC-derived exosomes show attractive application prospects in vaccine delivery, and they have been proven safe in multiple phase I trials in different types of cancers [128
]. Nevertheless, the difficulty in purifying exosomes maybe restrict their large-scale application.
Finally, the biological safety of immunocyte membranes in nanoparticles in vivo should be investigated carefully. First, the accumulation of nanomaterials with a large proportion in normal tissues can be harmful. Second, mismatch of allogeneic MHC between donor and recipient of immunocyte membranes may lead to serious safety problems. Thirdly, modified membranes may also raise health risks, such as TNF α-binding macrophage membranes, which maybe induce hyperinflammation.