Tailoring biomaterials and applications targeting tumor-associated macrophages

IntroductionIt has been illustrated that tumor initiation and progression are not only related to the genomic changes of cancerous cells, but are also affected by the tumor microenvironment (TME) (1, 2). Tumor associated macrophages (TAMs), as a key regulator in TMEs, are made up of a mix of tissue resident and exudative macrophages in varying proportions based on the type, location, and stage of the tumor (3). Typically, TAMs can be designated as M1- and M2-polarized macrophages. M1 macrophages produce pro-inflammatory cytokines, including interleukin‐1β (IL‐1β), IL‐6, IL‐12, tumor necrosis factor‐α (TNF‐α), and interferon‐γ (IFN‐γ), etc., which activate host immune responses against microbes and viruses, subsequently leading to the suppression of tumor progression (4). While M2 macrophages secrete anti‐inflammatory cytokines, including IL‐10, IL‐13, and transforming growth factor‐β (TGF‐β), exerting the promotion of cancer occurrence and development (5). However, there are still some situations where M2 macrophages might be an inhibitor in tumor progression. For instance, Rakaee et al. provided evidence showing that high level of CD204+/CD68+ M2 macrophages would be an independent positive prognostic marker of prolonged survival in lung cancer (6), indicating the exact function of the TAMs highly depends on cellular phenotype. To support this notion, in our preliminary study, we found that oral cancer metastasis in clodronate-treated mice was not significantly reduced by M1/2 macrophage reduction (7), suggesting that TAMs with specific surface markers, instead of the board M1/2 macrophages, exert various cellular functions in cancers. Although the controversy of TAMs subtypes in tumor biological behaviors, TAMs are still extensively studied and regarded as a great potential target. Importantly, various TAMs-related materials have been created for anti-tumor therapy not only in the basic research but also in the pre-clinical settings.Biological materials based on TAMs have recently been divided into three main aspects as follows: materials targeting TAMs directly, and those targeting TAMs indirectly through cancer cells and through immune cells (8, 9). An increasing number of studies of biological materials targeting TAMs have focused on experimental and pre-clinical anti-tumor approaches with encouraging signs; however, there are still a number of challenges to overcome before they can be employed in clinical practice. For instance, materials aiming at TAMs polarization might be oversimplified and problematic, as TAMs cannot be readily split into M1/M2 macrophages due to the existence of more nuanced phenotypes (10, 11). The cellular marker cluster of differentiation (CD68) has been widely used as a pan-macrophage marker in most studies; however, it has been reported that CD68 occasionally expressed in dendric cells, stromal cells, even cancer cells (12), indicating that any biological materials targeting CD68+ TAMs might be off-target. Furthermore, there are still some obstacles to the optimization of biocompatibility and efficacy in the application of nanomaterials targeting TAMs. Thus, to comprehensively understand the current progress of TAMs based on materials, in this review, we systematically summarized the design and application of materials based on TAMs from various routes and targets, providing new insights for anti-cancer therapeutic avenues.Materials targeting tumor-associated macrophages directlyTAM phenotypic heterogeneity and its relationship with materials applicationTAMs are highly heterogeneous stromal cells, and these distinct phenotypic characteristics are well established by techniques including immunohistochemistry, flow cytometry, single-cell sequencing, etc. Recently, it has been proposed that tissue macrophages arise from not only blood monocytes, but also the embryonic precursors deriving from the yolk sac and/or fetal liver (13, 14). The differentiated cell type’s chromatin landscape, among other epigenomic traits, represents the macrophage developmental origins (15), indicating that epigenetic modification and ontogeny can influence its identity development and thus dictate phenotypic heterogeneity. Furthermore, the TME in different cancers could also significantly alter macrophages phenotypes in distinct anatomical regions. For instance, TAMs are formed of a heterogeneous population of macrophages in hepatocellular carcinoma and breast cancer (16, 17). Interestingly, even in the same TME, the majority of the TAMs population differs in phenotype, which is related to the distances between cells, and systemic toxicity might result if all types of TAMs are targeted. To support this, using mass cytometry with extensive antibody panels, Chevrier et al. found that there were 17 unique macrophage phenotypes in the TME of human renal cell carcinoma, and even that the same type of macrophage not only expressed the CD169 (as an anti-tumorigenic marker), but also co-expressed with pro-tumorigenic markers including CD163, CD206, etc. (10), suggesting that the application of targeting materials should focus on, or be aware of, the most important subsets and paradoxical behaviors to optimize the therapeutic efficacy. Another potential hypothesis for the different phenotypic heterogeneity in the same type of tumor is that the environment caused by distinct sites might modulate macrophage phenotypes. For instance, in gastric cancer, Huang et al. provided data showing that CD68+IRF8+ macrophages dominated in the closest sites to the cancer cells, while the CD68+CD163+CD206+ macrophages dominated in the furthest sites (18), demonstrating the macrophages marker expression differences in tumor areas. Due to heterogeneity, TAMs release different mediators: either anti-tumor, including IL-6, IL-1β, chemokine (C-C motif) ligand 2 (CCL2), and TNF, or pro-tumor, including IL-10, TGF-β, vascular endothelial growth factor (VEGF), platelet-derived growth factor (PDGF), CCL17, CCL22, and CCL24 (19, 20). By using the monoclonal and specific antibodies for cellular markers, various cytokines and transcriptional profiles have been utilized for targeting TAMs (21). In summary, the phenotypic heterogeneity of TAMs depends on cellular origins, cancer type, and cellular distribution in the TME, etc., and with the discovery of more specific markers, we will be able to more accurately identify the subpopulations and develop the targeting materials for anti-cancer therapy.TAM co-culture systems and their relationship with materials applicationTo monitor the biological behavior of TAMs, co-culture systems, which are necessary in recreating TMEs, could provide a promising human in vivo-like tissue model, including two-dimensional (2D) and three‐dimensional (3D) cell cultures (Figure 1A). In vitro 2D cell cultures, such as Transwell inserts with Matrigel, have been widely used to explore the polarization and pertinent signaling pathways of TAMs in vitro (22). However, limited by physical structure and components, existing 2D models might remodel cells and their internal cytoskeleton, and affect cell arrangements on a flat substrate (23), making the exploration of cell performance and the simulation of natural environments in vivo difficult. Thus, 3D co-culture systems have arisen to mimic the situation in vivo, in which the spatial organization of cells is more reliable for the physiological relevance of experiments. At present, the types of 3D co-culture models related to TAMs can mainly be classified into spheroids, scaffold-based models, and microfluidic-based 3D models (Figure 1A). The 3D multi-cellular spheroid model, using the ‘hanging drop’ approach, or aggregate cultures with a matrix construction including heterogeneous populations of cells could be devised with hypoxia and necrotic patches to imitate tumor features in vivo (24, 25), leading to better comprehension of TAM performance as influenced by the reactive oxygen species (ROS) and hypoxic region in the TME. Regarding scaffold-based models, a large number of either organic or inorganic matrices and scaffolds have been employed to mimic the extracellular matrix due to good biocompatibility. For instance, Matrigel is composed of a reconstituted basement membrane extract (BME) secreted by a mouse sarcoma, and the natural scaffolds consists of purified proteins such as type I collagen, while artificial scaffolds are made up of polyethylene glycol (PEG)-based hydrogels and synthetic alternatives to
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