Chronic inflammation, cancer development and immunotherapy

1A functional linking between cancer and chronic inflammation has long been noted. In 1863, Virchow first described leukocyte infiltration within tumors and hypothesized that cancer originated from the sites of chronic inflammation. Tissue damages and the consequent chronic inflammation caused by certain types of irritants enhance cell proliferation, which might potentiate neoplastic risk when orchestrated with other risk factors of cancer. Currently, tumor-associated inflammation is considered as the seventh biological feature of cancer. Emerging evidences demonstrate that tumor-associated inflammation could affect almost every stage of cancer: cancer development, metastasis, drug resistance and cancer recurrence by inducing genomic instability, driving self-renewal of cancer stem-like cells (CSCs) and angiogenesis. In other words, chronic inflammation is showed to interfere with tissue homeostasis through the accumulation of various immune cells, thereby evolving a more lethal clonal form of tumor. In addition, various pro-inflammatory mediators from immune cells or cancer cells can promote cancer development and drug resistance. Therefore, it is crucial to investigate how chronic inflammation is associated with cancer development and develop new-targeted therapies.

2In the inflammatory tumor microenvironment (TME), innate immune cells such as monocytes and adaptive immune cells (e.g., T lymphocytes) directly contact or produce a variety of inflammatory mediators [e.g., tumor necrosis factor (TNF), IL-6] in response to aberrant signals from the tumor. These inflammatory mediators further activate multiple pivotal signaling pathways including Janus kinase/signal transducers and activators of transcription (JAK/STAT) and nuclear factor kappa B (NF-κB), which restricts the efficacy of chemotherapy by promoting cancer development and suppressing anti-tumor immunity. Thus, many studies have focused on improving therapeutic efficacy by targeting and remodeling TME. This review discusses how chronic inflammation affects the following aspects of malignancy: cancer development, immune escape, and drug resistance. We also review research advances in TME characterization and immune checkpoint. Finally, we summarize the application of targeting TME, immune checkpoint therapy (ICT) and combination therapy in the clinical management of cancers.

3Cancer development requires intrinsic and extrinsic factors such as genomic instability, abnormalities in proliferation and senescence, reprogramming energy metabolism, evasion of immune destruction, epithelial mesenchymal transition (EMT). Only 5%–10% of cancer cases driven by germline mutations, while the rest of cancers are caused by acquired factors such as chronic infections, dietary factors, obesity, inhalation of pollutants, smoking and autoimmune related factors (Table 1). These carcinogenic factors have common features of disrupting tissue homeostasis and producing a continuous protective response---chronic inflammation. The environment rich in inflammatory cells, cytokines and DNA damage leads to the various epigenetic changes, and accumulation of mutations in adjacent epithelial cells produces procarcinogenic effects such as immunosuppression and angiogenesis, thereby promoting sustained cell proliferation and tumor initiation (Figure 1).

4Persistent chronic inflammation activates pro-oncogenes and/or inactivate suppressor genes, cause genomic instability and induce tumors by promoting tumor cell proliferation and survival. Chronic inflammation is a step-by-step process that involves damage, repair and regression. Inflammatory cells such as lymphocytes, tumor-associated macrophages (TAMs), myeloid-derived suppressor cells (MDSCs) and fibroblasts, are recruited to inflammatory sites where they target pathogens and release cytokines to further elicit inflammation, immune responses and cell proliferation. Excess cytokines, chemokines, growth factors and proteases can stimulate tissue regeneration and reconstruct the extracellular matrix (ECM). These factors and mediators can also cause cellular DNA damage and impair DNA damage repair response, thus causing genomic instability and contributing to transformation and tumorigenesis. For instance, reactive oxygen species (ROS) produced in monocyte or cytokines IL-1β, transforming growth factor beta (TGF-β) and TNF were demonstrated to cause abnormal expression of activation-induced cytidine deaminase (AID), a key B cell enzyme mediating somatic hypermutation, class switching and antibody diversification. AID can induces TP53 and MYC mutations in normal cells, thereby promoting cancer development. In addition, chronic inflammation may directly disrupt the cell cycle checkpoints and inhibit DNA damage repair responses, thereby leading to the accumulation of random genetic alterations and genomic instability. Chronic inflammation is also reported to induce P53-mediated inactivation of genomic surveillance and acceleration of mutations in cancer cells. In addition, chronic inflammation is showed to inhibit mismatch repair (MMR) pathway through various mechanisms and increase the DNA replication error rates of the genome. Inflammatory mediators [prostaglandin E2 (PGE2), ROS, TNF, and IL-1β] induce hypoxia-inducible factor 1α (HIF1α) to downgrade mutS homolog 2 (MSH2) and MSH6 expression by displacing c-MYC from the MMR gene. Altogether, it is shown that inflammation affects genomic instability and promotes the cancer development.

5Conversely, cancer-associated genes are related with the recruitment and activation of pro-inflammatory cells such as bone marrow monocytes to coordinately alter the cancer transcriptional program. For example, the oncogenic gene RAS drives multiple inflammatory cytokines (IL-1, IL-6, IL-8, IL-23, etc.)/chemokines [chemokine (C-X-C motif) ligand 1 (CXCL1), CXCL2, CXCL5, CXCL8, and C-X-C motif chemokine receptor 2 (CXCR2), etc.] and signaling pathways [STAT3, NF-κB, mitogen-activated protein kinase (MAPK)] that promote tumor progression. Oncogenic RAS-induced secretion of IL-6 can link chronic inflammation and cancer. IL-6-mediated activation of the JAK/STAT3 signaling pathway by autocrine or paracrine means is required for malignant tumor proliferation. The activated cancer associated fibroblasts (CAFs) promote the immune checkpoint molecule expression and ECM remodeling, thereby suppressing the tumor immune response and promoting tumor development. Another important cancer-related gene is MYC. MYC can directly regulate tissue remodeling, angiogenesis and chronic inflammation to exert oncogenic activity. In a reversibly switchable mouse transgenic model of MYC-dependent β-cell carcinogenesis, the dramatic activation of MYC in β-cells triggers the release of IL-1β, induces the proliferation of adjacent endothelial cells, thereby leading to the generation of new complex vessels. Activation of MYC also promotes the recruitment of mast cells to tumor sites, which is required for tumor angiogenesis and macroscopic expansion.

6Tumor suppressor proteins can also suppress cancer development by inhibiting the production of inflammatory mediators. Von Hippel-Lindau (VHL) largely exerts tumor suppression by targeting HIF1α and HIF2α to oxygen-dependent ubiquitination and proteasome-dependent degradation. The inactivation of VHL by germline or somatic mutations causes the constitutive accumulation and activation of HIF-1α and HIF-2α, which induce gene expression programs, alter cellular metabolism, induce angiogenesis, promote metastasis and consequently contribute to the development of clear cell renal cell carcinoma (CCRCC). HIF1α also functionally interacts with the NF-κB, producing cytokine TNF-α and the chemokine receptor CXCR4, which are closely associated with the development of human renal-cell carcinoma and other malignant cancers.

7Genomic instability and aberrant proliferative responses caused by chronic inflammation can lead to cancer development, but not the main cause of death in cancer patients. Amount of studies have demonstrated that metastasis of tumors is the main cause of death in cancer patients, thus it is vital to investigate the inflammatory mechanisms involved in the metastatic process. Of note, cancer metastasis requires interactions between tumor cells and immune cells, inflammatory cells or interstitial components.

8In response to inflammatory factors (TGF-β, IL-1, and IL-1β) secreted by immune cells, tumor cells undergo EMT. Subsequently, TAMs typically accumulates around blood vessels and tumor margins and produces various proteases such as urokinase-type fibrinogen activator (uPA) and metalloproteinase 9 (MMP9) that regulate adaptive immunity to promote ECM remodeling, degradation of basement membrane and tumor cell invasion. In addition, CAFs and MDSCs are also recruited to the tumor margin where they exert immunosuppressive effects by interfering with the differentiation of dendritic cells (DCs).

9Through EMT, the direct reciprocal stimulation between cancer cells and TAMs promotes the infiltration of cancer cells into blood and lymphatic vessels. Inflammatory cells produce some mediators such as CXCR4, C-C chemokine receptor 7 (CCR7), and CCR9 which upregulates fibronectin expression and increase the permeability of tumor vessels. In addition, inflammatory mediators may increase the survival of circulating metastatic “seeds”. Circulating platelets establish protective aggregates with tumor cells that interfere with natural killer cells (NK cells)-mediated cytotoxicity through increasing fibrin deposition and hindering immune cell identification, which allow tumor cells to reach secondary organs. Inflammatory cells such as monocytes and neutrophils subsequently upregulate adhesion molecules to promote the extravasation. Tumor-derived C–C motif chemokine ligand 2 (CCL2) binds to CCR2+/Ly6Chi endothelial cells to increase vascular permeability and recruit inflammatory monocytes in a p38/MAPK-dependent manner, thereby promoting the extravasation and seeding of metastatic cancer cells. Finally, cancer cells recruit large numbers of vascular endothelial growth factor (VEGF)-positive hematopoietic progenitors (CD133+/CD34+/VEGFR3+) and colonize distant lateral metastatic sites (Figure 2).

10Surgery, chemotherapy, radiotherapy and immunotherapy are currently the main options for cancer treatment. However, pro-inflammatory mediators produced by these treatments in turn recruit immunosuppressive cells to reconstitute TME, which further enhances chronic inflammation, subsequently enriches CSCs and induces tumor resistance. CAFs are highly correlated with drug resistance in cancer. Tumor-derived IL-1β and constitutive IL-1 receptor-associated kinase 4 (IRAK4) activate the NF-κB signaling pathway in CAFs and pancreatic cancer cells, thereby attenuating the sensitivity to gemcitabine. CAFs-derived IL-6 could upregulate CXCR7 in esophageal squamous cell carcinoma and promote resistance to cisplatin via the JAK/STAT3 pathway. CAFs promotes EMT and resistance to cisplatin in non-small cell lung cancer (NSCLC) via the TGF-β/IL-6 axis; cisplatin treatment also increases the secretion of TGF-β in cancer cells, leading to CAFs activation and increased IL-6 secretion. TAMs are also a major cause of drug resistance. found that CCL2 secreted by TAMs increased TAMs recruitment and formed a positive feedback loop with TNF-α secreted from breast cancer cells via activating PI3K/Akt/mTOR signaling pathway, promoting tamoxifen resistance in breast cancer cells. IL-6, secreted by TAMs and induces signal transduction and JAK/STAT3, MAPK/ERK or PI3K/AKT activation in tumor, leading to drug resistance. TAMs are carriers of immune checkpoint ligands, which can interfere with programmed cell death ligand 1(PD-1)/programmed death-ligand 1(PD-L1) and cytotoxic T-lymphocyte antigen 4 (CTLA-4)/CTLA-4 ligand binding by overexpressing PD-L1 or CTLA-4 ligand, and inhibit the immune response of CD8+ T lymphocytes and the effect of immunotherapy. MDSCs impede the anticancer activity of immunotherapies including ICT by negatively regulating the expression of immune checkpoint molecules. Higher levels of MDSC numbers are associated with melanoma and other tumors responses to CTLA-4 inhibitors (ipilimumab). In addition, the recruitment of chemokines and cytokines to MDSCs has made MDSCs also resistant to targeted therapies, chemotherapy, and oncolytic viruses-based therapies. Chemokines such as CCL2 cause the resistance of melanoma to BRAF inhibitors by recruiting MDSCs as well. TGF-β1 secreted by MDSCs increases the sensitivity of NSCLC cells to cisplatin by reversing EMT (Table 2).

11Mutations, genomic instability and epigenetic modifications can affect the expression of tumor-specific antigen in tumor progenitor cells and activate anti-tumor immunity called “immune surveillance”. During the process, CD8+ CTL and CD4+ helper T (Th)1 cells prevent cancer development through some mechanisms in which the secretion of interferon (IFN)-γ and cytotoxins are usually involved. However, chronic inflammation can also accumulate pro-inflammatory mediators to produce the tumor immunosuppressive microenvironment (TIME). TIME can promote cancer development by promoting cancer cell proliferation, decreasing immunogenicity and evading immune elimination. For instance, IFN-γ activates STAT3 signaling, protects epithelial cells from CD8+ T cytotoxic cytokinesis, upregulates the expression of T cell depletion and induces PD-L1 on the transformed epithelial cells recognized by T cells. Similarly, inflammatory signals may increase the adaptation and decrease the expression of “stress ligands” on cancer cells, which are required for proper identification. In addition, chronic inflammation and injury trigger the remodeling of tissues, thereby creating space for malignant tumor growth. After circumventing innate surveillance and establishing a tolerant environment, tumors evade adaptive immune responses in a variety of ways by turning the genetic instability to their advantage. The mechanisms mainly involve the downregulation of MHC molecules, tumor antigens or the secretion of cytokines such as TGF-β and IL-10. Cancer cells also kill aggressive lymphocytes by expressing cell surface molecules that induce apoptosis, such as Fas ligand (FasL).

12It is widely believed that chronic inflammation promotes cancer development, distant metastasis, drug resistance and immunosuppression by providing a tumor-supportive microenvironment termed as the TME. The TME includes the cancer cells, intrinsic immune cells (TAMs, MDSCs, neutrophils, mast cells, DCs, and NK cells) and adaptive immune cells, as well as surrounding stromal cells (CAFs, endothelial cells, pericytes, mesenchymal cells). Crosstalk between cancer cells and immune cells ultimately form an environment that prompts cancer development. Recognizing the nature of this dialog may improve therapies and patient outcomes via simultaneously targeting multiple components in the TME.

13Monocytes are the main immune cells in TME and are found in almost all tumors with varying degrees of infiltration. Inflammatory infiltrating monocytes are divided into macrophages and MDSCs. The macrophages infiltrating in the TME are called “tumor-associated macrophages (TAMs)”. TAM has two polarization states, namely M1-macrophages (classically activated) and M2-macrophages (alternatively activated). M1 macrophages have the positive anti-tumor activity by upregulating the production of cytokines such as IL-6 and IL-1, which activate the T helper type 1 (Th1) reaction and further produce TNF-α, ROS, RNS, and other toxic substances to kill tumors and pathogenic microorganisms. Macrophages are induced polarization into the M2 type by Th2 cytokines such as IL-4, IL-10, and IL-13. M2 type macrophages are highly endocytic and phagocytic and involved in tissue repair, humoral immunity and metabolism. However, due to TAM is highly heterogeneous in TME, this simplified M1/M2 polarization distinction cannot strictly delineate the phenotypic and functional boundaries of TAMs. TAMs appear to share M1 and M2 polarization signatures. TAMs affects tumor progression from the following aspects: 1) TAMs produces various factors such as pro-angiogenic factors [e.g., vascular endothelial growth factor A (VEGFA)], TNF, TGF-β, chemokines (CXCL8 and CXCL12) and thymidine phosphorylase, which promote the generation of vascular networks in TME through recruitment and activation of endothelial cells or other cells (e.g., fibroblasts or pericytes). 2) TAMs directly enhances the migration and the aggressiveness of tumor cells through the paracrine loop between macrophages and tumor cells, in which the secretion of colony stimulating factor 1 (CSF1) by tumor cells and epidermal growth factor (EGF) family ligands by macrophages are critically involved. In addition, TAMs produce histone proteases and metalloproteinase such as MMP-12, MMP-9, MMP-7, and MMP-2 to remodel ECM and increase immunosuppressive cells infiltration. Then tumor cells could be attracted to the vasculature and come into contact with perivascular tunica interna endothelial cell kinase 2 (TIE2) macrophages that act as conduits for tumor cell escape by expressing VEGFA to increase vascular permeability. 3) TAMs mediate immunosuppression and promote immune escape. TAMs suppress the immune microenvironment by secreting chemokines and cytokines such as IL-10, TGF-β, and recruit Tregs to tumor sites to promote cancer development. TAM agonist homologous ligand protein S (PROS1) secreted by TAMs can bind to TAM receptors MER proto-oncogene, tyrosine kinase (MERTK), AXL and TYRO3, which promote malignant characteristics of tumor cells and promote immune escape by upregulating PD-L1 expression.

14MDSCs consist of a heterogeneous population of immature myeloid cells (IMC) with potent immunosuppressive activity. Among the major immunosuppressive cells in the TIME, MDSCs are able to activate the NF-κB, STAT3, and promote cancer development. Cancer cells, activated T cells and tumor-associated stromal cells express cytokines such as VEGF, TGF-β, and PGE2, granulocyte-macrophage colony-stimulating factor (GM-CSF) and TNFα, which might cooperatively disrupt myeloid cell maturation, promote the differentiation toward MDSCs, and enhancing MDSCs activation in the TME. Subsequently, MDSCs are recruited to tumor sites peripheral and lymphoid organs, then promote cancer development by different mechanisms. MDSCs are demonstrated to remodel TME, regulate EMT process, promote angiogenesis and establish a metastatic ecological niche for cancer dissemination, thereby promoting cancer development and metastasis. For instance, MDSCs allow immune escape by inducing incompetence in CD4+, CD8+ T cells and NK cells. Thus, MDSCs promote immune tolerance to TME and impede the efficacy of cancer immunotherapy.

15Tregs are immunosuppressive subset of T cells identified as CD4+ T cells with high CD25/IL-2 receptor α expression.. Tregs promote cancer development by exerting suppressive activities on T effector cells. In TME, Tregs is thought to be an important mechanism for successful evasion of the immune system in melanoma and head and neck squamous cell carcinoma. Tregs are highly activated in tumors and characterized by upregulation of the expression of forkhead box protein P3 (FoxP3), immune checkpoints (e.g., CTLA-4, PD-1) and cytokines (e.g., TGF-β). Tregs interact with monocytes, B regulatory cells and CAFs in the TME that drives a TIME. For example, Tregs-derived IL-10 alters the macrophages polarization state, causing macrophages to M2-macrophage and exert immunosuppressive effects. Tregs can inhibit the lytic activity of CD8+ through TGF-β, thus effectively suppressing the early tumor-specific immune response of CD8+ cells. Tregs can act on NK cells impairs their effector function by secreting TGF-β. Tregs were reported to release granzyme B and perforin and reduce the number of T effector cells in tumors by inducing cytolysis. In summary, Tregs play an immunosuppressive role in TME, which enhances the infiltration and differentiation of immunosuppressive cells, and limits the activity of cytotoxic NK cells and T cells. Immunosuppressive cells secrete cytokines in response to Treg stimulation, which in turn promote Treg expansion and promote tumor immune escape.

16In addition to cellular components, TME includes interstitial cells, microvasculature and cytokines that infiltrate into nearby areas, causing the environment for tumor survival under hypoxia, low pH, high interstitial pressure, and fibrosis. In this state, immune/inflammatory cells communicate with cancer cells through secreting large amounts of interleukins, immune checkpoint factors, growth factors, chemokines, colony-stimulating factors, TNF superfamily, and other cytokines to form a local environment conducive to cancer development, angiogenesis and therapeutic resistance.

17The inflammatory mediators involve in a complex signaling network coordinated by multiple signaling molecules and transcription factors. Among them, NF-κB and STAT3 signaling pathways are present throughout the TME. In the canonical NF-κB signaling pathway, excitatory signals activate IL-1 receptors, Toll-like receptors and TNF receptors via lipopolysaccharide, IL-1 and TNF-α, respectively. Various bridging proteins and signaling kinases can activate IκB kinase β (IKKβ) in the IKK complex, which in turn degrade IκBα in a phosphorylation-dependent manner. Subsequent transfer of homo- or heterodimers of NF-κB to the nucleus and activation of target gene transcription. The target genes include the auto-inhibitory proteins IκBα of negative feedback mechanism, anti-apoptotic genes, adhesion factors, MMP, cyclooxygenase-2 (COX-2), various cytokines and chemokines, thus affecting immunity, chronic inflammation and cell proliferation, apoptosis and metastasis. STAT3 is plays a crucial role in the induction and maintenance of the procarcinogenic inflammatory microenvironment. In the STAT3-related pathway, when cells are stimulated by inflammatory cytokines (IL-6, IL-22, etc.), growth factors [VEGF, EGF, hepatocyte growth factor (HGF), etc.] and other factors (UV, stress and infection), STAT3 is phosphorylated at Tyr 705 residue by JAK, intrinsic receptor tyrosine kinase and several non-receptor tyrosine kinases. The phosphorylation of STAT3, in turn, promotes homodimerization through the interaction between SH2 domain and phosphoryl-Y 705. Then the homodimer transfers to the nucleus and binds to the promoter of target genes, regulating the transcription of target genes. In tumor cells, over-activated STAT3 decreases the expression of immunostimulatory factors, including pro-inflammatory cytokines (IL-12, TNF-α), IFN and chemokines (CCL5, CXCL10), while increasing the expression of certain cytokines (IL-6, TGF-β, and VEGF), thus playing an immunosuppressive effect.

18IL-6, a multifunctional and pleiotropic cytokine, is secreted by various cells including tumor cells, monocytes, CAFs, adaptive immune cells and vascular endothelial cells. IL-6 is involved in the regulation of the inflammation, hematopoiesis, immune response, bone metabolism, oncogenesis and even in the “cytokine storm” caused by the novel coronavirus infections. High IL-6 levels in the TME reveals the close relationship between chronic inflammation and malignancy. IL-6 signaling (mainly via the JAK/STAT3 signaling pathway in epithelial and immune cells) can promote chronic inflammation and cancer development. IL-6 forms a complex with IL-6 receptor (IL-6R) and gp130, which mediates cancer cell development through the SHP2-ERK-MAPK and STAT3-BCL-2 pathways. Besides, IL-6 can enhance transcriptional induction of molecular targets that affect cell cycle and survival, such as MYC, the apoptosis inhibitor Survivin. In advanced stages of cancer, IL-6 induces transcriptional activators of EMT (zinc finger proteins Snail1 and Twist), accelerating the metastatic spread of invasive cancer cells. Importantly, the growth-enhancing effects of IL-6 on cancer cells also extend to cancer stem cells, capable of self-renewal and expansion, which require STAT3 to act synergistically with stem cell transcription factors such as NANOG. These interactions ultimately lead to multidrug resistance and progression of multiple tumors, including lung, bladder, breast, ovarian, colorectal and hematological malignancies.

19MIFs are secreted by activated T lymphocytes that inhibits monocyte/macrophage migration and maintains macrophage viability. Under physiological conditions, MIF functions as a regulator of inflammatory and immune responses, participating in embryonic development, repairing after tissue trauma and glucocorticoid resistance. Studies have shown that MIF, known as a proto-oncogene, is highly expressed in several tumors such as hepatocellular carcinoma, breast cancer, lung cancer and ovarian cancer, and it is also highly associated with the malignant degree of tumors. MIF binds to and activates the receptors CD74/CD44, CXCR4, and CXCR7 by autocrine and paracrine manners. Upon binding to the receptors, MIF inhibits p53 expression and activates various signaling pathways (JNK-cJun, PI3K-AKT pathways, etc.) to promote cell proliferation, metastasis, angiogenesis, anti-apoptosis and immune suppression, leading to accelerate cancer development.

20Tumor immune escape directly regulates cancer development. Immune checkpoints, including PD-L1/PD-1, CTLA4, lymphocyte activation gene 3 (LAG-3), IDO1 and galectin-9/TIM-3, which is the main mechanism of tumor immune escape via inhibiting the activation of effector T lymphocytes. Among these immune checkpoints, blocking PD-1/PD-L1-mediated tumor immune escape has become a new clinical strategy for tumor therapy.

21PD-1 (CD279), a 55 kDa transmembrane protein composed of 288 amino acid residues, is expressed in activated T cells, NK cells and B lymphocytes, monocytes, DCs and macrophages. PD-L1, the first ligand of PD-1, is also widely expressed in activated T cells, B cells, DCs and macrophages. Besides, PD-L1 is found to be highly expressed in cancer cells and therefore considered to be the main factor contributing to tumor immune escape. Many cytokines (e.g., INF-γ, TNF-α, IL-17, and IL-12) and tumor-derived exosomes in the TME can induce PD-L1 expression and cause tumor immune escape. PD-1 inhibits the activation of T lymphocytes, and in turn, T cell receptor-mediated activation rapidly induces PD-1 expression. PD-L1/PD-1 can be regulated by the following signaling pathways in the TME: PI3K/AKT pathway, NF-κB pathway, WNT pathway, MAPK pathway, JAK/STAT pathway and Hedgehog (Hh) pathway. These signaling pathways enhance tumor immune escape by promoting the expression of PD-1/PD-L1 axis.

22The standard treatments for advanced malignancies, such as chemotherapy and radiotherapy, have made some improvements over the past few decades, but clinical results remain unsatisfactory due to resistance and side effects. The use of immune checkpoint inhibitors for immunotherapy is a promising approach to cancer therapy, although response rates are limited due to the immunosuppressive TME. Emerging evidence supports that optimal therapies should include elimination of tumor cells, disruption of tumor-induced immunosuppression through targeting immunosuppressive cells as well as reactivation of tumor-suppressed effector T cells by checkpoint inhibitors (Table 3).

23TAMs, known as the major infiltrating inflammatory cells in the TME of tumors, contribute to the TIME that promotes cancer development and drug resistance. Therefore, TAMs are attractive targets for tumor therapy aimed at reducing chronic inflammation and TAM-coordinated immunosuppression. Several major strategies targeting TAMs have been applied for cancer treatment so far: inhibition of TAMs recruitment (CCL2/CCR2 axis, CCL5/CCR5 axis, etc.), depletion of TAMs [Colony-stimulating factor 1 (CSF1)/CSF1R axis, etc.], induction of M1 macrophage polarization [CD47/Signal regulatory protein alpha (SIRPα) axis, CD40/CD40L, etc.] and enhancement of macrophage-mediated phagocytosis [PD-1/PD-L1axis, MHC-1/leukocyte immunoglobulin like receptor B1 (LILRB1), etc.]. A number of preclinical and clinical studies with small molecule inhibitors and antibodies (such as PF-04136309, Maraviroc, BLZ945, Hu5F9 -G4, Duvelisib, FPA008, etc.) have shown that the TAM-targeted therapy is an effective anti-tumor strategy. For example, PF-04136309, a small molecule CCR2 inhibitor, inhibits tumor-associated macrophage infiltration and enhances endogenous anti-tumor immunity, and has performed well in clinical studies for advanced pancreatic cancer. Additionally, some drugs have been reported to induce differentiation of macrophages to a pro-inflammatory phenotype and inhibit tumor progression, such as Imiquimod, the only clinically approved toll-like receptor (TLR) ligand, exerts anti-tumor activity in basal cell carcinoma, melanoma and breast cancer.

24MDSCs, main components of the immunosuppressive TME, which induce unresponsiveness of NK cell, CD4+ and CD8+ T cell, thereby promoting immune tolerance to tumors. The expand of MDSCs has been reported to be associated with cancer development, therapeutic resistance, reduced efficacy of immunotherapy and poor prognosis of patients. Thus, MDSCs are considered as promising targets for cancer therapy. The main strategies to target MDSCs include: 1) depletion of circulating and tumor-infiltrating MDSCs [blocking VEGF and receptor tyrosine kinase (c-Kit) signaling; anti-CD33]; 2) prevention of MDSC recruitment and trafficking (CCL3, CCL4, CCL5-CCR5; CCL2, CCL5-CXCR2; CSF-1R/CSF-1); 3) Inhibition of MDSC immunosuppressive function [disruption of COX-2/PGE2 signaling; activation of nuclear factor erythroid 2-related factor 2 (Nrf2)]; 4) Differentiation of MDSCs into a non-suppressive immune state (promotion of IMC differentiation; epigenetic reprogramming). Currently, small molecule inhibitors and antibodies targeting the above-mentioned strategies (cabazitaxel, bevacizumab, rapalisin, paclitaxel, leronlimab + carboplatin, tadalafil, omadacyclovir, GTB-3550 TriKE™, etc.) have achieved good results in phase II clinical trials. For instance, MDSCs express S100A8/A9 and the receptor for advanced glycation endproducts (RAGE), which in turn recruit MDSCs and enhance their immunosuppressive function constituting positive feedback loop. Tasquinimod, an oral S100A9 inhibitor, that causes depletion of blood mononuclear cells, suppression of MDSCs recruitment and inhibition of metastasis, has been demonstrated to improve progression-free survival in patients with metastatic debulking-resistant prostate cancer (Li et al., 2021c).

25T cell-specific cancer immunotherapies are currently available to release the antitumor efficacy of T cells by suppressing immune checkpoints or to enhance adaptive immunity by using genetically engineered T cells equipped with chimeric antigen receptors (CARs) or T cell receptors. In recent years, immune checkpoint inhibitors with CTLA-4, PD-1 and PD-L1 monoclonal antibodies has become a successful treatment for several advanced cancers. To date, there are seven immune checkpoint inhibitors have been approved by U.S. food and drug administration (FDA) for certain cancer treatment (melanoma, NSCLC, lymphoma, urothelial carcinoma, etc.), including CTLA-4 inhibitors (Ipilimumab), PD-1 inhibitors (Pembrolizumab, Nivolumab, Cymplimab), PD-L1 inhibitors (Atezolizumab, Avelumab and Durvalumab). In addition, the combination of immune checkpoint inhibitors, such as Ipilimumab and Nivolumab, has recently been approved for unresectable malignant pleural mesothelioma, hepatocellular carcinoma, metastatic NSCLC, advanced renal cell carcinoma, esophageal squamous cell carcinoma, metastatic colorectal cancer and melanoma. Even so, the approved indications for immune checkpoint inhibitors are expected to increase, and several new agents are under investigation in clinical trials.

26Cytokines such as MIF, IL-6, and TGF-β used as adjuvants in cancer treatment has been extensively investigated in many clinical studies. The current therapeutic strategy targeting MIF is anti-MIF monoclonal antibody. Imalumab, an anti-MIF monoclonal antibody, is the only candidate that has progressed and has been well tolerated in clinical trials for the treatment of solid tumors. IL-6 inhibitors include Tocilizumab, an anti-IL-6R monoclonal antibody, and Siltuximab, an anti-IL-6 monoclonal antibody. Tocilizumab overcomes chemoresistance in mesenchymal stem cell-like breast cancer. Currently, drugs targeting TGF-β in clinical trials contains neutralizing antibodies, small molecule inhibitors and so on. For example, the TGF-β chelator GC1008 (also known as Fresolimumab) is one of the most characterized monoclonal anti-TGF-β1-3 antibodies, showing an acceptable safety and antitumor activity for patients with malignant melanoma or renal carcinoma in a phase one clinical trial. Vactosertib has a good suppressive effect on sclerotic fibroma as a potential small-molecule TGF-β type I receptor kinase inhibitor.

27Studies have showed that chemotherapy or immunotherapy alone promotes immunosuppression of tumors through TME, resulting in drug resistance and treatment failure. Thus, a growing number of preclinical and clinical trials have focused on the combination of TME-targeted agents with chemotherapy or immunotherapy. Recently, TME-targeted agents combined with immunotherapy in clinical studies has demonstrated new possibilities for long-term tumor control in patients. It is a promising therapeutic strategy by the combined inhibition of immune checkpoint and TGF-β signaling pathways because these critical pathways have independent and complementary immunosuppressive functions. Vactosertib in combination with Durvalumab demonstrates safety and efficacy in patients with metastatic NSCLC and urothelial carcinoma. Liver X receptors β (lxRβ) agonist RGX-104 induces depletion of immunosuppressive MDSCs (granulocytes and monocytes). Phase I clinical trials are ongoing to evaluate the effectiveness of the combination of RGX-104 and immune checkpoint inhibitors such as Nivolumab or Ipilimumab in patients with lymphoma. Galunisertib, a heterotetrameric complex of paired type I (TβRI) kinase inhibitor, has been used in combination with multiple chemotherapies (such as paclitaxel, gemcitabine, sorafenib, etc.), which can improve anti-tumor efficacy of patients with triple-negative breast cancer, glioblastoma and pancreatic ductal adenocarcinoma.

28Increasing evidences show that chronic inflammation can influence all aspects of cancer development along with the response to therapy. TME consisting of innate and adaptive immune cells, pro-inflammatory cytokines and chemokines accompanies the entire phase of cancer development and drug resistance. Under normal conditions, the immune system identifies and eliminates both pathogens and tumor cells to inhibit tumor growth. However, inflammatory cells and cytokines may contribute to cell survival, proliferation, invasion and angiogenesis as tumor promoters during chronic inflammation. Chronic inflammation also induces cancer development by altering the TME and important signaling pathways such as NF-κB and STAT. Oncogenic alterations can enable tumors to promote an inflammatory environment, which benefits tumors. Therefore, therapies targeting chronic inflammation will add new weapons to the arsenal of cancer therapy. In terms of therapeutic interventions, TME an impact on cancer development, immunosuppression and drug resistance that make it an attractive target for sensitizing tumors to conventional therapies as well as an alternative option for drug-resistant tumors. Targeting inflammatory infiltrating cells and cytokines in the TME is a potentially useful adjunct to immunotherapy and conventional treatment of tumors. However, TME and key signaling pathways vary widely between tumor types and tissues, how to manage the diversity and response to cancer treatment infecting by different TME will be an important area for future research.

29While immune checkpoint therapy has a positive therapeutic effect and potential cure for a small number of patients with advanced high-grade tumors, TME evolved dynamically through a compensatory feedback mechanism blocks the effect of immunotherapy, generates drug resistance and cancer development. To address this limitation of immunotherapy, we can try to design multi-targeted immunotherapy strategies and develop new integrated immune activation strategies. It is also possible to modulate TME to kill cancer with autoimmune potential and to reduce the possible adverse effects of specific targeted therapies.