Introduction

Cancer immunotherapy has become a potent disease treatment option that helps advanced cancer patients survive longer while removing any chance of returning tumors [1, 2]. In cancer patients, immune cells are ineffective against cancer cells and promote tumor growth, decreasing treatment effectiveness [3, 4]. Among innate system cells, macrophages play a crucial role in normal homeostasis, inflammation, and phagocytosis [5, 6]. However, macrophages have been shown to play a negative role in the progression of oncogenesis and neoplastic disease by promoting genetic instability and angiogenesis. [7]. Macrophages are divided into the M1 and M2 subgroups based on morphological, phenotypic, and functional variability. The M2 macrophages have been shown to support tumor growth and metastasis, whereas the M1 macrophages play a crucial role in antitumor immunity and largely orchestrate pro-inflammatory activities in the tumor microenvironment (TME) [8, 9]. Tumor-associated macrophages (TAMs), the most diversified immune cells in the TME that are essential for tumor formation, include the M2 macrophages and a small population of M1 macrophages [10]. In this line, tumor cells secrete chemokines and growth factors to draw in macrophages and change them into the M2 type. Therefore, it was also discovered that significant dynamic changes in macrophage subpopulations were related to the efficacy of immunotherapy [30]. Moreover, the secretion of STAT3 by TAMs into the TME, with their increasing numbers in the stroma, can lead to CD8+ T cell exhaustion [31]. TAMs and myeloid-derived suppressor cells can also suppress immune function through cell-to-cell contact, stimulating myeloid-derived suppressor cells (MDSCs) to secrete IL-10 and inhibit IL-12 production via dendritic cells [32]. The TAMs also play a role in inhibiting T cell recruitment, so targeting certain pathways, such as colony-stimulating factor-1 (CSF1/CSF1R) [3], can obstruct macrophage recruitment and promote T cell infiltration [33].

TAMs targeting breast cancer therapy

Currently, CSF-1R is inhibited by PLX3397 to diminish M2 macrophage recruitment, which is utilized to treat malignancies such as glioblastoma, breast cancer, and other tumors. There was high tolerability in a phase 1 study of the CSF-1R inhibitor LY3022855 in metastatic breast cancer [34]. Twenty-two medicines that target CXCR4 are now in the active development phase; most of these are small molecule antagonists; however, there are also antibody-based medications, gene therapies, and CAR-T cell treatments. Eighteen of these medications are being developed to treat solid tumors and hematological malignancies. Mozobil (Plerixafo), a small molecule antagonist that targets CXCR4, was introduced in 2018. It is first utilized with granulocyte colony-stimulating factor (G-CSF) to provoke hematopoietic stem cells for therapy of multiple myeloma and non-lymphoma Hodgkin’s.

To evaluate IMM2902’s safety and effectiveness in HER2+ advanced solid tumors, clinical trials have approved the drug's primary indication of the lung, gastric HER2-positive breast, and other solid tumors (NCT05076591). SIGLEC10 interacts with CD24 in renal clear cell carcinoma, triple-negative breast, and ovarian cancers to prevent tumor cell phagocytosis and immune cell activation. Blocking SIGLEC10hi TAMs in HCC decreased the expression of immunosuppressive molecules and increased the cytotoxic effects of CD8+ T cells. It also supported Pembrolizumab as an anti-tumor drug that targets PD-1 molecules [74]. Wei, C. et al. showed that IL-6 secretion by TAMs stimulates EMT, thereby improving CRC invasion and migration ability through the modulation of the JAK2/STAT3/miR-506-3p/FoxQ1 axis [80, 81]. For instance, in a 30-patient Japanese CRC cohort, a lower density of CD68+ in the tumor stroma and invasive front were linked to more progressive cancer, whereas high levels of TAMs were linked to a favorable prognosis [82]. Similar relationships got observed in European cohorts. For instance, in a tissue microarray of 100 colon cancer patients in Germany demonstrated decreased CD68+ macrophages in higher-stage tumors [83]. In a Bulgarian cohort conducted on 210 patients with primary CRC, a lower density of CD68+ TAMs in the invasive tumor front which is considerably associated with the advanced tumor stage (III and IV stages), distant metastases, and local lymph nodes specific metastases was observed [84]. A lesser number of CD68+ TAMs were also reported in cancer patients where the tumor cells migrated and invaded the blood circulation, lymph vessels, and perineural tissues.

Additionally, a high CD206/CD68 ratio has been linked to improved recurrence-free survival rates in patients with stage II of CRC after receiving adjuvant chemotherapy [85]. On the other hand, both VEGF-expressing and CD68-TAMs have been found to predict improved survival rates in individuals with stages II and III of CRC. So, only TAM infiltration cannot fully explain the degree of disease recurrence. A recent meta-analysis of 6115 CRC cases from 27 separate studies indicated a high density of TAMs in CRC as an independent favorable predictor for 5-year OS but not for DFS. The TAM density and additional prognostic markers may be a more accurate predictor of CRC relapse. In this line, traditional methods of analyzing TAMs have relied on the expression of CD68, a pan-macrophage marker, but recent studies have used double immunofluorescence staining to identify different subsets of TAMs using other markers such as CD86, CD163, and CD206 [86, 87]. Literature has shown that the presence of M2 macrophages (CD163+) is correlated with poorer overall survival and disease-free survival/recurrence-free survival in CRC. In addition, high levels of CD163+ TAMs and a high CD163/CD68 ratio have been linked to an aggressive phenotype and poor prognosis in CRC [80]. Additionally, the expression of CD86 TAMs and TNM stage were found to be independent prognostic factors for recurrence-free survival and overall survival in CRC [88].

Potential applications of TAMs in CRC therapy

Blocking monocyte infiltration in CRC

Blocking the infiltration of mononuclear cells, such as TAMs, in the inflammatory tissues associated with tumors has been identified as a potential therapeutic method for primary cancers. Chanmee et al. demonstrated that TAMs, specifically those associated with colon cancer, induce CXCR4, CXCL-12, and HIF-1 in the hypoxic TME. Moreover, the accumulation of TAMs is blocked by targeting the HIF-1/CXCR4 axis effectively. Mantovani et al. revealed that TAMs derived from colon cancer monocytes could differentiate, highlighting the need for combination therapies that block differentiation to target these cells effectively. In this line, the TNF-α has been found to induce the recruitment of monocytes and simultaneously inhibit the differentiation of monocytes or macrophages into TAMs in the TME of colon cancer in vivo [89]. Another strategy for targeting TAMs is the inhibition of their recruitment or infiltration. SIX1, a protein that is overexpressed in various types of cancer and promotes the recruitment of pro-tumor TAMs to the region of colorectal cancer (CRC) [79], can be silenced through the use of its inhibitor, Nitazoxanide, which suppresses the WNT/CTNNB1 pathway [90, 91]. Trifluridine/Tipiracil, an anti-metabolism drug, has been observed to effectively exhaust M2 macrophages when combined with oxaliplatin, leading to the infiltration of cytotoxic CD8+ T cells and the lysis of tumor cells [92].

Repolarizing TAMs

TAMs predominantly exhibit an M2 phenotype, simultaneously promoting immunosuppression and angiogenesis. They can be re-educated via M2 to M1 polarization. For instance, TAMs mediated inhibition of macrophage receptor expression with collagenous structure (MARCO) repolarized TAMs to the M1 phenotype and caused antitumor activity in the MC38 colon cancer mice model [93]. By altering the number and frequency of myeloid cells infiltrating the tumor, tasquinimod-based immunotherapy can reduce the immunosuppressive potential of TME [94]. It has been demonstrated by Olsson et al. that tasquinimod targets early-stage myeloid cells that tend to penetrate tumors, causing M2 myeloid cells to adopt an M1 macrophage phenotype, altering the tumor microenvironment, preventing angiogenesis, and inhibiting metastatic spread [95].CRC can be diagnosed and treated using long non-coding RNAs (lncRNA) as noninvasive biomarkers and targeted molecules. For instance, cells secreting lncRNAs, such as RPPH1, promote M2 polarization and tumor metastasis but can't be directly targeted [90, 91]. Cathepsin K (CTSK), which binds to TLR4 and activates mTOR, is synthesized by intestinal microflora and modulates the expression of long noncoding RNAs in various tissues [92]. The CTSK-specific inhibitor Odanacatib has been reported to curb the pro-tumor effects and improve the prognosis of CRC patients [96]. Moreover, researchers found that Ru@ICG-BLZ nanoparticles effectively repolarize TAMs to M1 macrophages due to their CRC specificity and low toxic properties as a new approach [97].

Targeting TAMs in immunotherapy

Immune checkpoint inhibitors, T cells-based treatment, and autologous tumor vaccines are the key components of immunotherapy in CRC treatment [98,99,100,101]. These strategies target immune checkpoint inhibitors with matching targets, including CTLA-4, PD-1, and PD-L1 [3, 10, 102]. The co-inhibitory molecule CTLA-4, produced by T cells, binds to the ligand CD80/86 on adenomatous polyposis coli (APC) to produce an inhibitory signal [103]. When PD-1, an immunosuppressive receptor on T cells, binds to PD-L1, it significantly reduces the activity of antigen-specific T cells [104]. PD-L1 is expressed mainly by aggressive primary tumor cells and by CD68/CD163-positive M2 macrophages in patients with colorectal cancer with high microsatellite instability [105]. Gordon and his team discovered that when the illness worsens, TAMs express more PD-1. Additional research revealed the ability of TAMs to phagocytose is inversely linked with PD-1 expression, and in vivo, inhibition of PD-1-PD-L1 improved macrophage phagocytosis, slowed tumor growth, and lengthened mouse survival [16]. On the other hand, evidence suggests that the PD-1/PD-L1 axis plays a role in skewing TAMs from the M1 to M2 phenotype, and M2 TAMs have been found to contribute to resistance to PD-1/PD-L1 blockade [133, 134]. Repolarizing TAMs by immunomodulators such as imiquimod and IFNs may inhibit melanoma tumor growth as TAMs generated by CCL17 and CCL22 attract Tregs to melanoma tumor locations [135, 136]. Certain chemokines, including IL-8, CCL4, CCL17, and CXCL10, in the cerebrospinal fluid may predict brain metastases in melanoma patients. The use of certain chemotherapy agents, such as nimustine hydrochloride, dacarbazine, and vincristine, has been shown to decrease CCL22 production in B16F10 melanoma mice [134]. These findings suggest that TAM-derived chemokines produced in the tumor stroma under the influence of POSTN (a protein involved in extracellular matrix organization) may contribute to melanoma-specific TILs in melanoma patients [137].

Targeting TAMs-derived angiogenic factors

TAMs have been shown to produce various angiogenic factors, including platelet-derived growth factor (PDGF), VEGF, TGFβ, and matrix metalloproteinases (MMPs) [138]. These factors can promote neovascularization by recruiting TAMs to the location of skin tumors in mouse models [139]. The VEGF and MMPs have been identified as crucial indicators of skin cancer progression [140], with high concentrations of POSTN and CD163+ TAMs in the tumor stroma of skin malignancies leading to increased production of MMP1 and MMP12 in skin lesions [141]. TAMs stimulated by tumor stromal factors may therefore serve as potential targets for molecular targeted therapy in the treatment of cancers [133].

Effects of anti-cancer agents on TAMs

Recent studies have also concentrated on the immunomodulatory impacts of chemotherapeutic drugs on TAMs. For instance, in mouse melanoma models, non-cytotoxic dosages of paclitaxel could reduce MDSCs and even prevent their ability to suppress the immune response [142]. It has been discovered that chemotherapeutic agents and drugs with low molecular weight co-localize along with TAMs at tumor locations. Hu-Lieskovan et al. showed combination therapy with dabrafenib and trametinib with synergistic impacts of immune checkpoint inhibitors. In contrast, dabrafenib and trametinib monotherapy led to elevated Tregs and decreased TAMs in melanoma, respectively [143].

In a different study, the collagen-structured anti-macrophage receptor was discovered to induce TAMs polarization into pro-inflammatory phenotypes, leading to anti-tumor immunological responses in B16 melanomas [36]. Furthermore, Gordon et al. showed that suppression of PD-1/PD-L1 in vivo promoted macrophage phagocytosis, decreased tumor progression, and improved macrophage survival [16, 144]. Lymphocyte-Activation Gene 3 (LAG-3, CD223) is another important immune checkpoint molecule that participates in T-cell exhaustion similar to Cytotoxic T-Lymphocyte Antigen 4 (CTLA-4) and Programmed cell death protein 1 (PD-1) [145]. It is expressed on the surface of activated CD4+ and CD8+ T cells and other immune cells, such as natural killer cells, regulatory T cells, and macrophages [146, 147]. TAMs release chemokines that lead to the recruitment of immune-suppressive cells towards the tumor microenvironment, which influence other stromal cells, like fibroblasts, to synthesize chemokines. Young et al. demonstrated that granulocytic MDSCs are recruited to tumor sites by the CXCR2 ligand, produced by fibroblasts, after being stimulated by IL-1β from TAMs [148]. Moreover, combination therapy with anti-CD115 Abs and CXCR2 agonists might inhibit B16F10 melanoma in vivo by preventing the enrollment of granulocytic MDSCs and removing immature TAMs [149]. Notably, emactuzumab, an anti-human CD115 Ab, reduced CD163+ CD206+ M2 macrophages in melanoma cases by eliminating immature TAMs before being stimulated by IL-4 [150]. These findings imply that anti-CXCR2 agonists and emactuzumab may trigger the anti-melanoma immune response by lowering M2 polarized TAMs. These results highlight the necessity of understanding how chemotherapeutic agents affect TAMs. The ICIs, in combination with TAMs targeting agents, provide promising outcomes for melanoma treatment. Data from preclinical research provided good explanations for clinical trials in which elimination or repolarization of immunosuppressive TAMs are being investigated to overcome ICI resistance and improve their anti-tumor functions [151]. Studies using ICIs and immunomodulatory factors that block M2-TAMs activities have been performed or are still being conducted in melanoma patients. Decreases in M-CSF (CSF-1) and increases in GM-CSF levels are two strategies being investigated in conjunction with ICIs to re-polarize M2-TAMs into M1-TAMs. For example, the phase 2 studies of recombinant human analog (sargramostim) as a GM-CSF agonist in combination with ipilimumab for the treatment of unresectable stage III or IV metastatic melanoma has been completed and revealed increased survival [152,153,154]. Talimogene laherparepvec (T-VEC), a modified oncolytic herpes virus, is another GM-CSF agonist that increases the anti-tumor responses and has been approved for local treatment of advanced melanoma. The T-VEC exclusively infects and replicates in tumor cells and results in immune-mediated lysis of tumor cells via encoding human GM-CSF, as well as the susceptibility of melanoma to ICIs. Combination therapy of melanoma with T-VEC plus nivolumab and pembrolizumab has reached phase 2 clinical trial [155,156,157]. OPTiM is also a phase III trial of talimogene laherparepvec, in which T-VEC had long-term efficacy in contrast to GM-CSF in advanced melanoma [158, 159]. Moreover, ONCOS-102 is an engineered oncolytic adenovirus encoding GM-CSF that has shown synergistic effects for metastatic or unrespectable melanoma treatment in combination with pembrolizumab (anti-PD-1 Ab) [160, 161]. Designing antagonists against M-CSF cytokine is another strategy that might lead to the depletion of M2-TAMs and an improvement in ICIs functions. In addition, M-CSF contributes to metastatic melanoma resistance to BRAF-targeted therapies. Therefore, M-CSF acts as a therapeutic target in BRAFV600E melanoma. Monoclonal antibody lacnotuzumab, an anti-M-CSF, has been studied alone and in combination with ICI spartalizumab (an anti-PD-1 mAb) [162,163,164,165]. The M-CSF receptor (CSF1R) provides another therapeutic target to deplete the immunosuppressive functions of TAMs. Some examples include BLZ945 (CSF1R inhibitor) combined with PDR001 (anti-PD-1 mAb), LY3022855 (CSF1R inhibitor) combined with tremelimumab or durvalumab ICIs, emactuzumab (CSF1R inhibitor), and cabiralizumab (a humanized mAb against CSF1R) [166,167,168,169,170]. APX005M is a humanized CD40 agonist mAb that activates immune responses by stimulating IFN-γ secretion [170]. INCB001158 is an arginase inhibitor used as monotherapy or combined with pembrolizumab in solid metastatic tumors such as melanoma. It has been suggested that inhibition of metabolic enzymes, such as ARG-1, could restore T-cell activities by filling arginine storage [171]. Moreover, it has been reported that PI3K-γ inhibition can re-polarize M2-TAMs into pro-inflammatory M1-TAMs. Moreover, IPI-549 is a PI3K inhibitor used alone or in combination with nivolumab (Fig. 5) [172, 173].

Fig. 5
figure 5

Role of TAMs in melanoma occurrence and therapy. Melanoma cells can elicit an immune response through the release of various cytokines, including CXCL-2, CCL-2, CSF-1, GM-CSF, A9, S100A8, and VEGFA. Some of these cytokines, such as GM-CSF and VEGFA, can stimulate the activation of macrophages, transforming these cells into TAMs. The activation of macrophages also results in the release of a series of factors, including TGF-β, CCL-22, and IL-10, which can influence dendritic cells and T-lymphocytes. In addition, TAMs can release TNF-α and interferon-γ to target cancer cells. It is worth noting that matrix metalloproteinases 9 and 2 (MMP9 and MMP2) can break down collagen in the tissue surrounding the melanoma mass, contributing to its decomposition

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Macrophages in glioma cancer (GBM)

Glioma is a type of primary brain tumor, including glioblastoma, astrocytoma, and oligodendroglioma [174]. The microenvironment of glioma is characterized by the presence of macrophages and microglia, known as tumor-associated macrophages [175, 176]. Microglia, which are phagocytes of the central nervous system, exist in three forms: amoeboid, ramified, and reactive [177, 178]. Amoeboid microglia are involved in embryonic central nervous system development [179], while ramified microglia are found in large quantities in the brain parenchyma with the ability to transform into neurons, astrocytes, or oligodendrocytes [180, 181]. Reactive microglia, which are rod-like with non-branching processes and numerous lysosomes and phagosomes, are associated with brain injury and neuroinflammation [182, 183]. They also secrete MHC class II antigens and produce inflammatory mediators [184, 185]. Macrophages in the central nervous system can be classified according to their location as perivascular macrophages, meningeal macrophages, macrophages of the circumventricular organs, or macrophages of the choroid plexus. Among the brain cancers mentioned above, glioma is a particularly aggressive and untreatable type of brain tumor with a poor prognosis, and current treatments have not been successful in improving outcomes [186, 187]. Therefore, there is a need for further research into the mechanisms behind the invasiveness and recurrence of glioma and the development of new therapeutic approaches, including immunologic treatment [188, 189].

Glioma-associated macrophages (GAMs) are a key component of the tumor microenvironment in gliomas [190] that can be derived from microglia as well as bone marrow-derived macrophages [191,192,193,194]. The number and characteristics of GAMs can vary significantly, with evidence from single-cell sequencing showing that GAMs are made up of 59.05% and 27.87% of immunocytes in primary and recurrent glioblastomas, respectively [195]. Various signaling molecules, growth factors, transcription factors, and epigenetic and post-transcriptional modifications influence the phenotype and activation state of GAMs. Depending on their origin, these cells can exhibit different characteristics, with some derived from brain-resident microglia [196] and others from bone marrow-derived monocytes [197]. The GAMs play a role in various aspects of glioma progression, such as cell motility, proliferation, survival [188], and immune suppression [198, 199]. They can also produce a range of growth factors and pro-inflammatory cytokines that contribute to the supportive matrix for tumor cell metastasis and the development of an immunosuppressive microenvironment [200]. Understanding the role of GAMs in the tumor microenvironment may provide insights into potential therapeutic approaches for gliomas. In this context, Woolf et al. demonstrated using single-cell imaging that P2RY12 and TMEM119 label microglia in GBM, and they further demonstrated that these markers could be used to distinguish microglia from BMDM. P2RY12 protein expression is associated with longer survival rates in patients. Activation of P2Y12 receptors has been linked to the extension of microglial cell processes [201, 202]. Moreover, another study that analyzed marker genes in GAMs found that only a small number of genes were consistently present, indicating the diverse responses observed in different settings. In this regard, Tgm2 and Gpnmb genes were the only ones that were common across the analyzed data sets, highlighting the need for further research to understand the functional state of GAMs.

GAMs regulating GBM malignancy

In the presence of glioblastoma (GBM) cells, the functions of microglia may be impaired, leading to the initiation or growth of tumors. This has been demonstrated through comparative transcriptome analysis. It was found that GBM-bearing mice's microglia are less sensitive and impaired at monitoring immunity due to a reduction in a group of genes that encode receptors for various antigens, chemokines, and cytokines [203]. Additionally, microglia engage in reciprocal molecular crosstalk with glioblastoma stem cells, exhibiting a more direct pro-tumorigenic function through the secretion of TGF-β [204]. Microglia activated by GM-CSF can release CCL5, a chemokine that upregulates the secretion of MMP2 in GBM cells, thereby promoting tumor migration and invasion [205]. This effect may be mediated by the secretion of interferon-gamma (IFNγ) by infiltrating microglia, which leads to the stable expression of a specific transcriptional program in GBM cells that is associated with myeloid cells [206]. This epigenetic immunoediting may also be present in human mesenchymal subtype glioblastoma stem cells (GSCs) [207]. The TAMs also play a role in GBM invasion through the expression of CCL8 and the activation of signaling pathways in GBM cells through the binding of CCL8 to CCR1 and CCR5 receptors [208], the secretion of CSF-1 [209] and epidermal growth factor (EGF) by GBM and microglia, respectively, have also been shown to stimulate GBM invasion through the recruitment of TAMs and activation of signaling pathways in GBM cells through the binding of EGF to epidermal growth factor receptors (EGFR) [210].

GAMs in angiogenesis of GBM

The resistance of GBM to anti-VEGF therapy, which targets a protein involved in angiogenesis, has been linked to the macrophages infiltration into the tumor (Fig. 6) [211]. It depends on the activation state of the immune cells and whether they promote or suppress angiogenesis. Immunosuppressive macrophages like M2 promote angiogenesis, while pro-inflammatory macrophages like M1 suppress it [212]. Depletion of TAMs in animal models has been shown to reduce the blood vessel density in GBM, suggesting a role of these cells in GBM angiogenesis [213]. Resident microglia may be particularly important in this process, as their selective depletion has been shown to reduce blood vessels in GBM to a greater extent than the depletion of all TAMs [213]. The TAMs isolated from a specific type of glioma have been found to overexpress proangiogenic factors such as VEGF and CXCL2, both of which have been linked to angiogenesis. The interaction of the receptor for advanced glycation end products (RAGE) with its ligands has also been shown to promote angiogenesis in GBM through the activation of TAMs-specific signaling pathways [214].

Fig. 6
figure 6

Anti-tumor/pro-tumor activity of macrophages in GBM

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GAMs in drug resistance of GBM

Resistance to temozolomide (TMZ) has been reported as a common obstacle to GBM patients’ treatment, where the resistance rate is approximately 60% [215]. Literature evidence suggests that genetic factors and GAMs may contribute to this resistance [216]. The interleukin-11 (IL-11) produced by microglia and macrophages activates STAT3-MYC signaling in GBM cells, leading to TMZ resistance [216]. By inhibiting GAM recruitment and IL-11 secretion through ABP1 ablation or genetic inactivation, TMZ resistance has been reversed in a murine model of GBM [217]. Additionally, different subpopulations of GAMs may have distinct effects on treatment responses [218]. For instance, M2-like GAMs contribute to resistance through secretion of exosomal miR-21-5p, while M1-like polarization of GAMs induced by GBM-derived extracellular HMGB1 has been shown to restore sensitivity to TMZ. In addition to chemoresistance, GAMs have also been implicated in resistance to radiotherapy and antiangiogenic therapy [219]. The impact of GAMs on treatment responses may be mediated by the expression of PD-L1, which interacts with CD80 on T-cells and leads to CD4+ T-cell suppression, Treg expansion, and immune checkpoint blockade resistance [220]. The role of CD73-expressing macrophages in ICB resistance has also been demonstrated in a murine model of GBM [221].

GAM-targeted therapy in GBM

Several approaches have been identified and tested in experimental and clinical settings for targeting TAMs in glioblastoma (GBM). These approaches can be divided into three categories: TAM re-education, TAM education, and TAM depletion. TAM education involves activating pro-inflammatory pathways, which can also be delivered through gene therapy or direct administration, while TAM depletion involves targeting key molecules to achieve the unbiased depletion of TAMs or to inhibit macrophage infiltration. These TAM-targeting strategies can potentially counter immunotherapies and influence glioma progression [174].

Anti-angiogenic treatment

Tumor-infiltrating myeloid cells may play a role in the limited effectiveness of anti-angiogenic therapies by expressing alternative proangiogenic factors that bypass VEGF-mediated pathways [222]. The MerTK inhibitor MRX-2843 has been shown to have therapeutic benefits by promoting the polarization of macrophages away from immunosuppressive conditions, inhibiting neo-angiogenesis in the glioblastoma microenvironment, and inducing tumor cell death [223]. The metalloprotease-disintegrin ADAM8, which is highly expressed in tumor cells and associated immune cells in glioblastomas, is related to angiogenesis and is associated with a poor clinical prognosis [224]. The regulation of osteopontin mediates the angiogenic potential of ADAM8 in glioblastoma cells/primary macrophages, so targeting ADAM8 may be a viable approach for modulating angiogenesis in glioblastoma [225].

PD-L1 signaling pathway

Pembrolizumab monotherapy, which targets the PD-1 protein, cannot elicit an effective immune response in most GBM patients, likely due to the low number of T cells within the tumor microenvironment and the abundance of CD68 + macrophages [226]. Besides that, in a recent study, it was reported that GBM cells secrete interleukin-11 (IL11) in response to glial-derived neurotrophic GAMs, activating signal transducer and activator of transcription 3-MYC signaling. This signaling pathway leads to the induction of stem cell states, which increase tumorigenicity and resistance to temozolomide (TMZ) in GBM cells. In mouse GBM models, PI3K inactivation or inhibition reduces microglia recruitment and IL11 secretion, resulting in improved TMZ response [227]. Anyway, in the late stage of temozolomide (TMZ) treatment or relapse, treatment with an anti-PD-L1 antibody significantly reduced the infiltration of CD163-positive macrophages into tumors. In contrast, a combination of a PD-L1 antibody and IPI-549 (a selective PI3Kγ inhibitor) therapy effectively inhibited tumor growth [228]. Treatment with rapamycin and hydroxychloroquine (RQ) decreased the polarization of M2 macrophages, increased phagocytic ability, and increased the accumulation of lipid droplets. This treatment enhanced the ratio of anti-tumoral to pro-tumoral immune cells within the tumor and the ratio of CD8 to CD4 T cells. The combination of RQ and anti-PD1 treatment was found to be synergistic in action [229]. Saha et al. tested a triple combination of anti-CTLA-4, anti-PD-1, and G47Δ-mIL12 (oncolytic herpes simplex viruses armed with angiostatin and IL-12) in mouse GBM models. This treatment was associated with an influx of macrophages, an anti-tumoral, macrophage-like polarization of these cells, and an increase in the ratio of T-effector to T regulatory cells. This treatment was able to cure most mice with gliomas. Immune cell depletion studies showed that CD4+ and CD8+ T cells and macrophages are all required for the synergistic curative activity of this treatment [230].

Combination therapy

Several studies have reported the potential of targeting pro-tumoral macrophages in the treatment of GBM. Almahariq et al. found that the BLZ-945, a CSF-1R inhibitor, reduced pro-tumoral macrophage polarization and improved the response to radiotherapy in respected tumors with a high baseline population of pro-tumoral macrophages [231]. The results of Zhu et al. showed that when debulking plus anti-CD47 tumors were compared with non-debulking plus IgG tumors, macrophages with CD68-positive labels were recruited more, pro-inflammatory cytokines like CXCL10 were increased, and angiogenic proteins were decreased, indicating that surgical resections coupled with anti-CD47 blocking immunotherapy promote inflammation and prolong survival [232]. As a result of lipopolysaccharide and interferon-gamma stimulation of bone marrow macrophages and brain-resident macrophages, Herting et al. have found that dexamethasone prevented the production of IL-1. These findings suggest that IL-1 signaling may be a useful therapeutic target in the management of GBM-associated cerebral edema [233] (Table 1).

Table 1 Macrophages-based therapeutic strategies in four different cancers: breast, glioma, colorectal, and melanoma
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Conclusion

The use of cancer immunotherapy for removing residual tumors has emerged as an effective way to improve the survival of patients with advanced-stage cancers, as it enhances the immune system's ability to eliminate minimal residual tumors. As a result of ineffective immune cells against cancer cells, patients with cancer are more likely to develop tumors, which reduces the effectiveness of therapeutic measures. The macrophage is one of the most important innate system cells contributing to normal homeostasis, inflammation, and phagocytosis. Several studies have shown, however, that macrophages promote genetic instability and angiogenesis in the development of oncogenesis and neoplasms. The M2 macrophages promote tumor growth and metastasis. Among the most diverse immune cells in the TME are the M2 macrophages, which along with the M1 macrophages, are called TAMs. The pro-tumorigenic M2 macrophages are attracted to tumor cells by chemokines and growth factors. Therefore, immunotherapy efficacy is also strongly influenced by changes in macrophage subpopulations. The TAMs have been implicated as a therapeutic target in numerous biological studies due to their ability to deplete, inhibit recruitment, and influence polarization status. In addition, TAMs limit the efficacy of immunotherapy approaches, such as anti-PD1 treatment, because they are linked to resistance to well-known antitumor therapies, such as chemotherapy and radiotherapy. Anyway, many preclinical studies using small molecules or antibodies to block each of mentioned factors/pathways individually have demonstrated significant improvement in response to a wide variety of tumors to therapy, indicating that their blockage is generally well tolerated. However, more research is needed to overcome macrophage-based cancer therapy, particularly in nanoparticles and drug delivery. In this line, the use of small molecules or antibodies to block specific factors or pathways associated with TAMs has shown promising results in preclinical studies, leading to improved responses to a wide variety of tumors. These approaches have generally been well tolerated. However, more research is needed, especially in the field of nanoparticles and drug delivery, to advance macrophage-based cancer therapy further. As the role of TAMs in cancer therapy is increasingly recognized, several crucial gaps in the field necessitate further investigation. TAM heterogeneity, plasticity, and their interactions with other immune cells remain areas of exploration. Understanding the underlying mechanisms of TAM-mediated immunosuppression and identifying reliable biomarkers for patient stratification and treatment response assessment is paramount. Additionally, optimizing TAM-targeted therapies and validating their clinical effectiveness are essential for translating preclinical findings into meaningful treatments.