Tumor Microenvironment: Immunosuppressive Cells

Many cell types are thought to contribute to the generation of an immunosuppressive tumor microenvironment (TME) including tumor-associated M2 macrophages (TAMs), regulatory T cells (Tregs), myeloid-derived suppressor cells (MDSCs), tolerogenic dendritic cells (tolDCs), and cancer-associated fibroblasts (CAFs). Read below for a brief description of each cell type and the mechanisms by which they contribute to immune suppression. Click the links to browse our offering of relevant proteins, antibodies, ELISAs, multiplex kits, and small molecules to support tumor microenvironment research.

Tumor-associated Macrophages

Tumor-associated M2 macrophages (TAMs) are a heterogeneous population of cells that display a range of phenotypes depending on the type of tumor and their locations in the tumor microenvironment (TME). TAMs are commonly the most abundant infiltrating leukocyte in most tumors and are predominantly thought to have pro-tumor effects. These include both immunosuppressive effects in addition to pro-angiogenic and metastatic effects. The mechanisms by which TAMs mediate immunosuppression include: 

Amino acid depletion and the production of immunosuppressive metabolites: TAMs express indoleamine 2,3-dioxygenase (IDO), which depletes L-tryptophan through kynurenine production. This metabolic pathway inhibits the activity of CD4⁺ and CD8⁺ T cells and natural killer (NK) cells, while promoting the differentiation of regulatory T cells (Tregs), thereby supporting tumor immune evasion.

Expression of non-classical HLA class I molecules: TAMs express HLA-E and HLA-G, which engage inhibitory receptors on T cells and NK cells, suppressing their cytotoxicity, proliferation, and cytokine production. HLA-G additionally induces tolerogenic dendritic cells, which promote Treg development, collectively fostering an immunosuppressive microenvironment.

Engagement of T cell inhibitory and apoptotic receptors: TAMs express B7-1/CD80, B7-2/CD86, PD-L1, PD-L2, and B7-H4 that bind to co-inhibitory receptors and suppress T cell activation, proliferation, and cytokine production. TAMs also induce T cell apoptosis via Fas L and TRAIL interactions with their T cell-expressed receptors.

Production of inhibitory cytokines and PGE₂: TAMs contribute to immunosuppression by secreting IL-10 and TGF-β1, which inhibit effector T cell functions, promote regulatory T cell expansion, impair NK cell activity, and suppress dendritic cell maturation. They also produce prostaglandin E2 (PGE₂) that further inhibits NK and T cell cytotoxicity, promotes MDSC expansion, and skews immune responses toward immunosuppressive phenotypes. Factors secreted by TAMs not only inhibit the anti-tumor immune response, but also drive tumor cell proliferation, angiogenesis, invasion, and metastasis.

Inhibition of tumor cell phagocytosis: TAMs promote immune evasion by expressing SIRPα, which interacts with overexpressed tumor cell CD47 to deliver a “don’t eat me” signal that inhibits phagocytosis.

Chemokine-mediated recruitment of Tregs: TAMs indirectly suppress anti-tumor immunity by secreting chemokines that recruit regulatory T cells, which then inhibit effector T cells, NK cells, and antigen-presenting cells (APCs) through diverse immunosuppressive mechanisms.

Regulatory T Cells

Regulatory T cells (Tregs) are a heterogeneous subset of CD4⁺ T cells with suppressive properties that play a central role in maintaining immune homeostasis and self-tolerance, dampening inflammation, and preventing autoimmunity. They also represent a significant suppressive cell population in the tumor microenvironment. They function by inhibiting the activities of CD4⁺ and CD8⁺ effector T cells, natural killer cells, NKT cells, and antigen-presenting cells (APCs) through multiple mechanisms including:

Secretion of immunosuppressive cytokines: Tregs suppress T cell responses by secreting immunosuppressive cytokines such as TGF-β1, IL-10, and IL-35, which inhibit Teff cell differentiation, proliferation, activation, and cytokine production, while promoting conversion to immunosuppressive phenotypes. Additionally, membrane-bound LAP-TGF-β1 and Galectin-1 induce Teff cell growth arrest and apoptosis through both soluble and contact-dependent mechanisms.

Induction of infectious tolerance: Cytokines secreted by Tregs including TGF-β1, IL-10, and IL-35 induce infectious tolerance by converting activated Tconv cells into regulatory cell phenotypes. TGF-β promotes FoxP3⁺ Tregs, IL-10 drives Tr1 cells, and IL-35 induces iTregs, thereby amplifying immunosuppression within the tumor microenvironment and facilitating tumor immune evasion.

IL-2 deprivation-mediated T effector cell apoptosis: Tregs suppress effector T cells by depleting local IL-2 via high CD25/IL-2 Rα expression, causing IL-2 deprivation-induced apoptosis.

Direct inhibition of T cell proliferation and IL-2 synthesis: Tregs also inhibit Teff cell proliferation and IL-2 synthesis by directly transferring inhibitory cAMP through the gap junctions of Teff cells. 

Production of adenosine: Tregs express CD39 and CD73 to degrade ATP/ADP into adenosine, which inhibits dendritic cell maturation, suppresses Teff and NK cell functions via A2a receptor signaling, and promotes both a tolerogenic dendritic cell phenotype and enhanced Treg numbers and suppressive activity.

Production of granzymes: Tregs contribute to immunosuppression by secreting granzymes, particularly granzyme B, which mediates Teff cell apoptosis primarily through perforin-dependent pathways, with some evidence for perforin-independent mechanisms. Tregs also induce cytolysis of B cells, NK cells, and CD8⁺ T cells through granzyme B and perforin-dependent pathways.

Blocking the maturation or antigen-presenting capabilities of dendritic cells: Tregs indirectly suppress Teff cells by using CTLA-4 to downregulate dendritic cell expression of the co-stimulatory molecules, B7-1/CD80 and B7-2/CD86. Binding of CTLA-4 and CD80 or CD86 also triggers the production of indoleamine 2,3-dioxygenase (IDO) by dendritic cells, generating pro-apoptotic metabolites that suppress Teff activity. LAG-3 expressed by Tregs binds to MHC class II on immature dendritic cells, thereby blocking their maturation and limiting T cell activation.

Myeloid-derived Suppressor Cells

Myeloid-derived suppressor cells (MDSCs) are a heterogenous population of immature myeloid progenitor cells that fail to differentiate into granulocytes, macrophages, and dendritic cells, leading to an accumulation of immature cells. In tumor biology, immature myeloid cell accumulation is of significant interest as these cells display immunosuppressive properties that are thought to contribute to the inhibition of anti-tumor immune responses. MDSC-mediated mechanisms of immunosuppression include: 

Amino acid depletion and the production of immunosuppressive metabolites: MDSCs suppress anti-tumor immunity by depleting key amino acids necessary for T cell proliferation and functions. Increased uptake and metabolism of L-arginine via high levels of ARG1 activity in MDSCs leads to low levels present in the microenvironment, resulting in reduced expression of TCR-CD3 zeta and T cell proliferative arrest. Cystine is also sequestered by MDSCs, limiting its ability to be taken up by APCs and subsequently reduced and exported as cysteine to be provided to T cells during antigen presentation. This compromises T cell activation, proliferation, and differentiation. In the presence of MDSCs, L-tryptophan levels are also reduced through indoleamine 2,3-dioxygenase (IDO)-mediated degradation by the kynurenine pathway, which inhibits the proliferation and activity of CD4⁺ and CD8⁺ T cells and natural killer (NK) cells and promotes the differentiation of regulatory T cells (Tregs).

Generation of reactive oxygen and reactive nitrogen species: MDSCs inhibit anti-tumor immunity by producing reactive oxygen species (ROS) and reactive nitrogen species (RNS). Hydrogen peroxide, peroxynitrite, and nitric oxide are produced by MDSCs via NADPH oxidase, ARG1, and iNOS. Collectively, these molecules impair T cell activation, proliferation, and migration, and induce apoptosis, and nitration or nitrosylation of multiple target molecules including TCR, CD3, CD8, and CCL2, which inhibits T cell activation and intra-tumoral migration.

Direct engagement of T cell inhibitory and apoptotic receptors: MDSCs suppress T cell functions by expressing PD-L1, Fas Ligand, and Galectin-9, which engage T cell receptors PD-1, Fas/CD95, and Tim-3, respectively, leading to T cell exhaustion or apoptosis depending on the ligand-receptor interaction.

Production of inhibitory cytokines: MDSCs produce IL-10 and TGF-β1 to suppress effector T cell activation, differentiation, proliferation, and cytokine production while promoting suppressive T cell expansion. TGF-β1 induces FoxP3⁺ regulatory T cells, while IL-10 promotes the conversion of activated conventional T cells to IL-10-, TGF-β1-secreting Tr1 cells. Additionally, TGF-β1 down-regulates the expression of NKG2D and NKp30 on NK cells and CD8⁺ T cells, reduces NK cell proliferation and cytotoxicity, skews macrophages toward an M2 phenotype, and inhibits dendritic cell maturation, migration, co-stimulation, and IL-12 production.

Inhibition of T cell homing to the lymph nodes: MDSCs inhibit T cell activation by expressing ADAM17, a protease that cleaves L-Selectin/CD62L on naïve T cells, preventing their homing from the blood and lymphatics to the lymph nodes where activation occurs.

Production of extracellular adenosine and PGE₂: MDSCs express CD39 and CD73 to generate immunosuppressive extracellular adenosine, which inhibits effector T, NK, and NKT cell functions, promotes Treg expansion and suppressive activity, and prevents dendritic cell maturation and pro-inflammatory cytokine production via A2a/A2b receptor signaling. MDSCs also produce PGE₂ through the up-regulated expression of COX-2 and PGES1. PGE₂ signals through receptors expressed on the surface of MDSCs, enhancing their own suppressive activity and expansion. PGE₂ also inhibits NK cell cytotoxicity, dendritic cell differentiation, IL-2 signaling, and Th1/CD8⁺ T cell responses.

S100A8/A9-mediated accumulation: MDSCs enhance their own accumulation and migration via an autocrine loop where secreted S100A8/S100A9 proteins bind to RAGE and other glycoprotein receptors on MDSCs, driving their accumulation and recruitment to the tumor microenvironment.

Tolerogenic Dendritic Cells

Tolerogenic dendritic cells (tolDCs) are a specialized subset of dendritic cells, characterized by a stable, immature or semi-mature phenotype, that promote immune tolerance rather than activation. Unlike conventional dendritic cells that initiate immune responses against pathogens or abnormal cells, tolDCs actively suppress immune activation and contribute to immune homeostasis. In the tumor microenvironment, factors such as chronic inflammation, hypoxia, and tumor-derived signals induce the differentiation of dendritic cells into this tolerogenic state. These tolDCs then play a pivotal role in dampening anti-tumor immunity by modulating T cell responses and fostering an immunosuppressive environment that allows tumor cells to evade immune surveillance. Key mechanisms by which tolerogenic DCs mediate immunosuppression in the tumor microenvironment (TME) include:

Induction of regulatory T cells (Tregs) that suppress immune responses: Tolerogenic DCs promote the differentiation and expansion of Tregs, which in turn inhibit the activation and function of effector immune cells, creating an environment that favors tumor immune evasion. This process is often supported by high expression of immunosuppressive enzymes like indoleamine 2,3-dioxygenase (IDO), which contributes to Treg induction and function.

Secretion of anti-inflammatory cytokines: By releasing cytokines such as IL-10 and TGF-β1, tolerogenic DCs actively suppress inflammation and limit the activation of effector T cells and other immune cells, thereby dampening protective anti-tumor immunity.

Inhibition of effector T cell activation and proliferation through immune checkpoint molecules: Tolerogenic DCs express checkpoint ligands like PD-L1 that engage inhibitory receptors on T cells, leading to reduced T cell activation, proliferation, and cytokine production within the TME.

Impairment of antigen presentation capacity to limit T cell priming: By presenting antigens in a tolerogenic context characterized by low expression of co-stimulatory molecules such as B7-1/CD80 and B7-2/CD86, tolerogenic DCs fail to adequately activate naive T cells, resulting in poor priming and suboptimal anti-tumor immunity.

Promotion of T cell anergy or functional unresponsiveness: Through multiple signals, including the presentation of antigen with low co-stimulatory molecule expression, tolerogenic DCs induce a state in T cells where they become unresponsive to tumor antigens, preventing effective immune targeting of cancer cells.

Reduction of pro-inflammatory cytokine production in the tumor microenvironment: Tolerogenic DCs contribute to lowering levels of pro-inflammatory signals, which otherwise would stimulate anti-tumor responses, further skewing the TME towards immunosuppression.

Recruitment and support of myeloid-derived suppressor cells (MDSCs): Tolerogenic DCs facilitate the accumulation and function of MDSCs, which synergize to suppress T cell activity and sustain an immunosuppressive niche within tumors.

Cancer-associated Fibroblasts

Cancer-associated fibroblasts (CAFs) are activated fibroblasts abundant in the tumor stroma. CAFs remodel the extracellular matrix (ECM) and secrete signaling molecules such as growth factors, cytokines, and chemokines that influence tumor progression and the tumor microenvironment. They can be identified by expression of the membrane protein Fibroblast Activation Protein alpha/FAP. CAFs suppress anti-tumor immune responses by restricting T lymphocytes to the stroma and preventing them from accumulating in the vicinity of cancer cells by at least three mechanisms:

Physical barrier: CAFs produce dense ECM proteins like collagen and fibronectin, which increase stromal stiffness and create a mechanical barrier preventing T cell migration into the tumor core.

Secretion of immunosuppressive factors: CAFs release cytokines (e.g., TGF-β, IL-6) and chemokines that suppress T cell activation or attract immunosuppressive cells such as regulatory T cells (Tregs) and myeloid-derived suppressor cells (MDSCs).

Modulation of immune cell recruitment: CAF-derived factors can skew infiltrating immune cells towards immunosuppressive phenotypes, indirectly limiting effector T cell presence.

Related Research Areas

• Tumor Microenvironment
• Tumor Microenvironment: T Cell Exclusion
• Cancer Research
• Immune Checkpoint Targets
• Immunotherapy 
• Immuno-Oncology Research
• Hypoxia Research
• Angiogenesis Research