The relationship between the immune system and tumors is one of the most studied topics in the field of cancer. Previously, it was thought that cancer only acted on the tumor itself; However, we now know that it requires the support of different cells and cytokines, the tumor microenvironment (TME). Among the components that make up the TME, immune cells that are responsible for supporting tumors play a key role in this tumor environment. While under normal circumstances, immune cells can recognize and destroy nascent tumor cells during cancer immune monitoring, they may be affected by different factors present in the tumor during cancer immune editing. The fact that immune cells can act as guardians (anti-tumor immunity) or bystanders or supporters of tumors (pro-tumor immunity) makes them a "double-edged sword" in the TME.
Tumor microenvironment
For a long time, almost all research focused on a single tumor cell, and it was thought that a tumor was a mass of tissue made up of proliferating cancer cells; However, in the 19th century, Stephen Paget's description of TME through seed and soil theory confirmed that tumors contained a variety of non-cancerous cells. Over the past 25 years, tumors have been studied as complex organs, showing that the entire tumor component (tumor cells) is not malignant in itself, but they require the action of the tumor stroma (non-tumor cells) to promote tumorigenesis, including ongoing inflammation. Indeed, in its initial stages, tumor cells are able to direct TME to form blood vessels to recruit trophic factors, cell-derived vesicles, and immune cells, thereby contributing to the development of many cancer features.
The tumor stroma that make up TME and tumor cells includes basement membrane, extracellular matrix (ECM), fibroblasts, various cancer-associated fibroblasts (CAFs), angiogenetic vascular cells (AVCs), endothelial cells, Sertoli mucinus cells, glial cells, smooth muscle, epithelial cells, adipocytes, and infiltrating immune cells (IICs). In addition, TME contains cytokines, chemokines, growth factors, and antibodies.
There is a growing body of research on how TME affects tumorigenesis, cancer progression, and metastasis. At present, existing studies have shown that tumors can change their microenvironment, and the microenvironment can affect the growth and spread of tumors. TME plays a key role in regulating the immune response in cancer patients. Tumor cells and their microenvironment often produce large numbers of immunomodulatory molecules that have a negative (inhibitor) or positive (activator) effect on the function of immune cells. Thus, TME is able to shift the immune response from a tumor-destructive mode to a tumor-promoting mode based on the composition of the TME. Among them, immune cells, soluble mediators (cytokines, chemokines, angiogenesis, lymphangipoietic factors, and growth factors) and cell receptors in TME play a key role in the immune response.
The dual role of immune cells in TME
Immune cells include different cell populations: white blood cells are divided into granulocytes (neutrophils, eosinophils, and basophils), monocytes, and lymphocytes (T lymphocytes and B lymphocytes), as well as red blood cells, platelets, mast cells, dendritic cells (DCs), and innate lymphocytes (ILCs). All of these cells are derived from hematopoietic stem cells (HSCs) in the bone marrow.
Among the factors involved, tumor-derived cytokines, chemokines, and even metabolic conditions (pH, oxygen levels, and nutrients) can affect immune cell function in the TME. Depending on the TME, the immune cells will secrete different factors accordingly, thus determining their anti-tumor or pro-tumor effects.
Granulocytes
Several studies have shown that tumor-associated centriocytes (TANs) in TME promote tumor cell proliferation, extravasation, and migration. TAN releases particulate components, such as elastase, to promote the proliferation and invasion of cancer cells. TAN also promotes the extravasation of tumor cells into the metastasis niche by secreting IL-1β and MPPs, thereby promoting the spread of cancer cells. In addition to pro-tumor effects, TAN mediates cytotoxicity in tumor cells by producing ROS and TRAIL.
Neutrophils with antitumor effects are called "N1" TAN, while neutrophils with pro-tumor effects are called "N2" TAN. Studies have shown that TME affects the balance of N1 and N2 subsets by secreting various cytokines, for example, TGF-β, IL-6, G-CSF and IL-35 can induce pro-tumor polarization of TAN, and IFN-β and IL-12 can induce anti-tumor polarization of TAN. Studies have also shown that TAN interacts with lymphocytes in TME and regulates their function. In 4T1 tumor-bearing mice, N2 TAN inhibits NK cell-mediated tumor cell clearance, thereby promoting tumor metastasis. N2 TAN also recruits Treg into TME by secreting CCL17, thereby promoting tumor growth. Conversely, N1 TAN produces chemokines, such as CCL3, CXCL9, CXCL10, to recruit CD8+ T cells into the TME and secretes cytokines (such as IL-12, TNF-α, and GM-CSF) to activate the cytotoxicity of CD8+ T cells, thereby providing anti-tumor effects.
Mast cells
The prognostic value of tumor-associated mast cells (TAMCs) in human solid tumors is unclear and controversial. However, MCs are known to promote inflammation, inhibit tumor cell growth, and induce tumor cell apoptosis by releasing IL-1, IL-4, IL-6, IL-8, TNF-α, IFN-γ, TGF-β, MCP-3, MCP-4, leukotriene B4 (LTB4), and chymotrypsin. In addition, prostaglandin D2 (PGD2) secretion was associated with inhibition of angiogenesis and vascular permeability in mouse models of lung cancer. Currently, they are known to be associated with a good prognosis for some human cancers, such as non-small cell lung cancer, breast cancer, and prostate cancer.
When activated as a pro-tumor cell, TAMC is able to promote: (1) tumor cell proliferation and survival; (2) Angiogenesis and lymphangiogenesis are achieved by secreting angiogenesis, including IL-8, TNF-α, TGF-β, nerve growth factor (NGF), VEGF, fibroblast growth factor (FGF-2), and urokinase-type plasminogen activator (uPA); (3) promote invasion and metastasis by releasing MMP, tryptase, and chymotrypsin; (4) TME immunosuppression. Interestingly, intratumoral MCs are associated with poor prognosis in this setting. They have pro-tumor activity due to their ability to disrupt anti-tumor immunity in human gastric cancer by expressing significantly higher levels of the immunosuppressive molecule PD-L1, thereby inhibiting T cell immunity. Thus, MCs are associated with a worse prognosis in patients with CRC, gastric cancer, PDAC, breast, lung, and prostate cancer
Macrophages
Tumor-associated macrophages (TAMs) are functionally heterogeneous, divided into two major subsets, M1 and M2 macrophages. In response to lipopolysaccharide (LPS), IFN-γ, and GM-CSF, M1 macrophages undergo classical activation and preferentially secrete antimicrobial molecules and pro-inflammatory cytokines, including reactive oxygen species (ROS), nitric oxide (NO), and IL-6. M1 macrophages are the first line of defense against microbial infection. M1 macrophages also maintain a robust antigen presentation capacity, inducing a strong Th1 response.
In contrast, M2 macrophages undergo selective activation by the action of IL-4, IL-13, IL-10, and CSF-1 and preferentially secrete anti-inflammatory cytokines, including transforming growth factor β (TGF-β), IL-10, and proteases such as arginase-1 and MPPs. M2 macrophages play a key role in limiting immune responses, inducing angiogenesis, and tissue repair. Thus, the presence of M2-like TAMs correlates with pro-tumor activity, while the presence of M1-like TAMs correlates with antitumor activity.
TAM creates a mutagenic microenvironment conducive to tumor initiation by secreting pro-inflammatory mediators, such as TNF-α and ROS.
In patients with non-small cell lung cancer, the expression of the immune checkpoint ligand PD-L1 is upregulated in tumor-infiltrating immune cells, which are rich in M2-like TAM. Mechanistic studies have shown that the expression of PD-L1 on TAMs is upregulated after exposure to lactate. The interaction of PD-L1 with PD-1 on T cells inhibits T cell proliferation and induces T cell apoptosis, leading to immune tolerance. At the same time, TAM inhibits the expansion of CD4+ helper T cells by expressing PD-L1 and secreting the anti-inflammatory cytokine IL-10.
In addition, TAM enhances the function of Treg by secreting the chemokine CCL22 to recruit Treg and secreting TGF-β. Human TAMs also secrete epidermal growth factor (EGF) to enhance the aggressiveness of tumor cells. TAM up-regulates MMP, which degrades mesenchymal collagen and upregulates collagen synthesis and assembly to remodel TME, which is conducive to the invasion of tumor cells.
Dendritic cells
Dendritic cells are the most potent antigen-presenting cells (APCs), bridging innate and adaptive immunity. DCs are phenotypic and functionally heterogeneous under physiological conditions. In response to microbial infection, extracellular microbial proteins are typically engulfed or endocytosed by mature DCs and presented to CD4+ T cells via class II major histocompatibility complex (MHC) molecules. In contrast, cytosolic microbial proteins are typically presented to CD8+ T cells via class I MHC molecules. DCs that infiltrate the TME include distinct subpopulations at different developmental stages. These tumor-associated dendritic cells exert immunostimulatory or immunosuppressive effects depending on the dendritic cell subset and tumor stage.
Conventional DCs (cDCs): cDCs are composed of two phenotypically and functionally distinct subpopulations. Human cDC1 expresses CD11c, MHC-II, BDCA3, CD141, XCR1, CLEC9A, and DNGR1. Human cDC1 expresses a toll-like receptor (TLR) and secretes pro-inflammatory cytokines, including IL-12p70 and IFN-α, in response to infection-induced Th1 responses. cDC1 stimulates anti-tumor immune responses, cDC1-specific gene markers have been used as positive prognostic markers in cancer patients, and the presence of cDC1 in tumors is associated with good clinical outcomes.
cDC2 was more abundant, expressing CD11c, MHC-II, BDCA1, CD172a (SIRPα), CD115 (CSF-1R) and CD11b. Human cDC2 produces various cytokines, such as IL-10 and IL-23, and presents antigens to CD4+ helper T cells, thereby activating effector T cells, including Th2 cells and Th17 cells. cDC1 initiates CD8+ T cells in different regions of dLN, while cDC2 initiates CD4+ T cells in different regions of dLN. Although cDC2-activated effector T cells are predominantly immune-tolerant Tregs, expression of cDC2 gene markers is associated with a positive prognosis in both human and mouse models. Therefore, both cDC1 and cDC2 are beneficial for anti-tumor immunity.
Plasma cytoid DCs (pDCs): DCs that have the same morphology as the plasma cells that secrete antibodies are called pDCs. Human pDCs are CD123+CD303+CD304+CD11c, and in response to viral infection, pDCs recognize viral nucleic acids through TLRs, secrete type I interferon IFN-α, and play a key role in antiviral defense. However, tumor-invasive pDCs lead to the reduction of IFN-α and maintain the expansion of FoxP3+ Tregs in vivo, promoting immune tolerance and tumor progression. In addition, pDCs in melanoma patients express indoleamine 2,3-dioxygenase, which can consume tryptophan, resulting in T cell incompetence and immune tolerance. These results suggest that pDCs can induce immunosuppressive immune responses.
In addition to immunosuppressive effects, some studies have shown that pDCs can induce an immunostimulatory response. CD123+ pDCs are located in the peritumoral region of primary melanoma and are in close contact with CD8+ T cells. Functional analysis showed that both human and mouse pDCs could excite CD8+ T cells, leading to their activation and differentiation into cytolytic and IFN-γ-producing effector T cells, and tumor regression in vivo.
Human CD2highpDCs in TME express high levels of granzyme B, TRAIL, and lysozyme, which limit tumor cell proliferation and mediate contact-dependent killing of tumor cells. In addition, CD2highPDC also effectively secretes IL-12p40, which stimulates naïve T cells, leading to T cell expansion and immune response. Therefore, pDCs play both pro-tumor and anti-tumor roles in the process of tumor development.
Monocyte-derived DCs (MoDCs): In response to infection, circulating monocytes enter tissues and differentiate into DCs, These DCs are called MoDCs or inflammatory DCs (inf DCs) and express CD1a, BDCA1, CD11c, MHC II, and CD64 in humans. MoDCs have been observed in TME in some cancers, and MODC is able to efficiently express TNF-α and inducible nitric oxide synthase (iNOS), and iNOS-mediated NO inhibits T cell proliferation, suggesting that MoDCs induce immunosuppressive responses. In addition, in mouse models of lymphoma, adoptively metastatic CD8+ T cell-mediated anti-tumor effects rely on nitric oxide synthase 2 (NOS2) expressed by DCs, suggesting that MODC has immunostimulatory effects. Therefore, the function of MoDCs in tumors needs to be further studied.
NK cells
NK cells can function spontaneously without prior sensitization. In addition, their ability to rapidly produce IFN-γ and TNF-α when triggered by cells leads to an early response to oncogenic transformation. Therefore, its immediate intervention on tumor cells, including its role in promoting inflammation, is related to their active participation in the immune monitoring of tumors.
NK cells are involved in oncolysis through their cytotoxic responses, and the effector immune response of NK cells involves the secretion of multiple cytokines (IFN-γ, IL-10, IL-5, IL-13, TNF-α, and GM-CSF) and chemokines (MIP-1α, MIP-1β, IL-8, and CCL5). It is worth noting that molecules such as CD96, CD161 and CD244 expressed by NK cells can act as co-receptors to regulate cytolytic activity and cytokine production, and IFN-γ is one of the most common cytokines in NK, which is secreted by NK by activating its surface receptor NKG2D, which plays a key role in antitumor activity.
Although NK cells' contribution to antitumor activity is well known, they can be modulated and manifest as a pro-tumor phenotype due to TME's attempt to evade the effects of NK cell surveillance. Its mechanism of action as a pro-tumor cell is related to immune editing and immunosuppression.
On the one hand, TME has an effect on NK receptors, and a study showed that neutralizing TME restored the cytotoxic activity of NK cells and enhanced the expression of NKG2D. On the other hand, TME induces an immunosuppressive environment through tolerant immune cells (e.g., Treg, MDSCs, TAMs) and soluble factors (e.g., PGE2, TGF-β, IL-6, etc.), which can lead to NK cell dysfunction. In this case, the immunosuppressive effects of TME cause NK cells to function in a pro-tumor manner, which has been reported in some patients with CRC.
B cells
Since the 70s of the 20th century, it has been widely believed that B cells only have pro-tumor functions, because in some mouse models, the behavior of B cell antibodies may favor the development and spread of cancer. However, in recent years, further studies have shown its role in anti-tumor immunity. In fact, in an analysis of 54 cancer studies, 50% reported a positive prognostic effect on tumor-infiltrating B cells, with the remaining 9% being poor and 41% neutral. In metastatic melanoma, TIL B cells are considered to be the second best disease prognostic predictor after CD8+TIL. In addition, TILs B cells are also capable of exhibiting antigen-driven clonal expansion, class switching, and affinity maturation in the cancer environment.
TIL B cells and antibodies produced within tumors promote antitumor activity through the following mechanisms: (1) presentation of tumor-derived antigens to CD4+ and CD8+ T cells; (2) alter the function of its antigenic target on cancer cells; (3) promote the opsonization of tumor cells, leading to the uptake of antigens through the Fcγ receptor on DCs and enhancing the presentation of DCs; (4) activation of the complement cascade; (5) through ADCC, resulting in macrophage and NK cell activation; (6) play a role through ADCP; (7) Produce IFN-γ and IL-12 with cytotoxic immune responses; (8) Tumor cells are killed by granzyme B and TRAIL.
In terms of protumor, B cells can induce pro-tumor activity by activating complement through the production of IL-10. One of the mechanisms that promotes cancer growth and progression is the production of immune complexes in cancer, which can be produced by anti-tumor antibodies. In addition, macrophages can also be activated and bind to immune complexes previously generated by B cells through pro-inflammatory mediators, promoting pro-tumor activity.
Regarding cytokine production, B cell-derived immunosuppressive cytokines can inhibit CD8+ CTL and NK cell function. These cytokines include: IL-10, TGF-β, IL-35, IL-21, and TNF-β. In addition, Breg cells are also able to produce adenosine, which is involved in inhibiting T cell activation. In recent studies, higher levels of Breg cells have shown a poor prognosis in patients with urothelial bladder and gastric cancers.
CD4+ T cells
Th cells, as the central coordinator of immune responses, play a crucial role in the amplification and regulation of cellular immune responses. With regard to tumors, Th cells have a multifaceted role: (1) by secreting cytokines, such as IFN-γ and TNF-α, helping B and NK cells and CD8+ CTL; (2) support humoral and cytotoxic responses; and (3) direct antitumor activity. Because of these characteristics, their fundamental role in developing and maintaining effective anti-tumor immunity, modulating adaptive immune responses against cancer, and serving as key regulators in the tumor immune microenvironment is unquestionable. In addition, they are also targets for immunotherapy, with a focus on their ability to activate CD8+ CTL responses.
In terms of pro-tumorial, CD4+ T cells may be affected by TME to produce cytokines that support tumor survival. They may act in combination with other types of cells, such as MDSCs, TAMs. In addition, Th1-Treg transformation is characteristic of mouse and human lung cancers due to its plasticity that can be converted to Treg.
Th2 cells are primarily associated with pro-tumor activity. They work by enhancing angiogenesis, inhibiting cell-mediated immunity and killing tumor cells. There are some key factors in Th2-mediated immunity that can exert pro-tumor effects, such as eosinophils (IL-10 inhibits CTL lysis of tumor cells), IL-13 (NKT cell-derived IL-13 inhibits CTL activation); Type 2 CD8+ T cells (IL-10-secreting Tc2 cells), B cells (inhibition of CTL-mediated tumor clearance through IL-10 secretion and formation of immune complexes).
Regulatory T cells (Tregs) are another Th subset characterized by immunosuppressive activity and tolerance in both the allostate and inflammation. Because they are considered immunosuppressive in nature, they do not have the same anti-tumor activity as traditional T cells on their own. They are considered to support TME because they can inhibit the anti-tumor immune effector response in TME through the production of IL-10 and TGF-β.
CD8+ T cells
CD8+ T cells are the primary effector cells of the anti-tumor immune response, and the original CD8+ T cells are activated upon specific recognition of APC-presented antigenic peptides by MHC-I and then differentiated into cytotoxic effector T cells. They perform effector functions through cytotoxic effects, leading to target cell death. Similar to NK cells, CD8+ T cells can sequentially bind to and attack multiple target cells. Thus, their ability to kill tumor cells makes them play a key role in anti-tumor immunity.
In terms of protumor, there are multiple subsets of CD8+ T cells (Tc1, Tc2, Tc9, Tc17, Tc22), but not all subsets have cytotoxic functions. Tc17 cells (characterized by IL-17A, IL-17F, IL-21, IL-22 production, and low levels of granzyme B) exhibit dual roles in different cancers, with anti-tumor activity in patients with esophageal squamous cell carcinoma and pro-tumor activity in patients with HNSCC. In addition, a low Tc1/Tc2 ratio is associated with a poor prognosis in patients with salivary gland tumors. Tc22 increases IL-22 and is positively correlated with pro-tumor activity because of its ability to induce tumor growth in transplant-related SSC patients.
Another mechanism by which TME influences and regulates is "CD8+ T cell exhaustion". This process results in a gradual loss of effector function, resulting in defectively activated tumor-specific T cells. This loss of effector function involves deletion of TNF-α, IFN-γ [519], and high-level expression of immune checkpoint molecules. Therefore, due to the presence of immune tolerance and immunosuppressive mechanisms, tumor-specific depleted T cells are unable to control tumor progression at an advanced stage.
brief summary
Currently, it is known that the immune system is capable of killing tumors and transforming cells through its anti-tumor immunity. However, this ability can be compromised through a process called "cancer immune editing," resulting in tumor cells that can evade immune surveillance, triggering tumor growth. In this process, in order to survive, the tumor creates an immunosuppressive microenvironment that allows immune cells that originally had anti-tumor effects to be transformed into dysfunctional, bystander, and even pro-tumor cells
In this case, TME and immune cells influence each other's fate in a bidirectional manner. As a result, TME has become one of the most important areas of research in cancer biology. A better understanding of the immune evasion process and the relationship between immune cells and TME may help us develop drugs to control immune cells and end immune evasion in tumors.
Bibliography:
1.Heterogeneous Myeloid Cells in Tumors. Cancers(Basel). 2021 Aug; 13(15): 3772.
2. Dual Effect of Immune Cells within TumourMicroenvironment: Pro- and Anti-Tumour Effects and Their Triggers. Cancers(Basel). 2022 Apr; 14(7): 1681.