Abnormal cells such as tumor cells are typically recognized and eliminated by the immune system. Immune cells such as natural killer (NK) cells, natural killer T (NKT) cells, and gamma delta T cells provide the first line of defense against tumor cells by exerting direct cytotoxic effects and secreting high levels of IFN-gamma to promote tumor cell destruction. Under these conditions, macrophage polarization is skewed toward a tumoricidal M1 phenotype, which is associated with a high level of phagocytosis and secretion of pro-inflammatory cytokines that contribute to tumor cell elimination. Control of tumor cell growth is further achieved by activation of the adaptive immune response. Dendritic cells take up antigens from tumor cells and present these antigens in the presence of co-stimulatory signaling molecules and secreted cytokines to naïve CD8+ and CD4+ T cells to prime T cell activation. Like NK cells, activated CD8+ T cells have direct cytotoxic activity against tumor cells and both these cells and Th1 cells secrete high levels of IFN-gamma to drive tumor rejection. Despite the activities of these cell types, some tumor cells can escape this process of elimination over time, leading to tumor growth. The tumor microenvironment (TME) plays a central role in this process. The TME consists of multiple different cell types including fibroblasts, endothelial cells, and infiltrating leukocytes, whose functions can be exploited or altered to create conditions that are favorable for tumor progression. Within this complex environment, tumor growth can be driven by many different factors including direct tumor cell-mediated mechanisms of immune cell evasion or immunosuppression, the development of CD8+ T cell or NK cell exhaustion, the recruitment and expansion of immunosuppressive immune cell types, the presence of high levels of immunosuppressive cytokines and other immunosuppressive factors that impair immune cell functions, and/or a shift in polarization toward a type II immune response. Type II polarization is associated with Th2-like cytokine secretion, M2 macrophage polarization, and the presence of type II NKT cells and N2 type neutrophils in the TME, which blocks the CD8+ T cell/NKT type I/Th1/M1 anti-tumor immune response. The different sections of this poster show the characteristics of exhausted CD8+ T cells and NK cells, and the key mechanisms by which tumor cells, tumor-derived exosomes (TEXs), regulatory T cells (Tregs), myeloid-derived suppressor cells (MDSCs), and tumor-associated M2 macrophages (TAMs) mediate immunosuppression in the TME. Many of the molecules implicated in the immunosuppressive mechanisms detailed here are currently being investigated as targets for cancer immunotherapy.
CD8+ T Cell Exhaustion in the Tumor Microenvironment
CD8+ T cells become exhausted or dysfunctional in tumors due to persistent antigen exposure and chronic T cell receptor (TCR) signaling. Characteristics of exhausted T cells include up-regulated and sustained expression of multiple inhibitory immune checkpoint receptors, reduced proliferation, decreased production of effector cytokines, and reduced cytotoxicity, which limits the anti-tumor immune response. Inhibitory receptors that are up-regulated on exhausted CD8+ T cells include PD-1, CTLA-4, TIM-3, LAG-3, 2B4, BTLA, CD160, and TIGIT, which researchers are trying to target, either alone or in combination, to reverse the exhausted phenotype and restore T cell functions. While monoclonal antibodies against PD-1 or CTLA-4, have been shown to have powerful anti-tumor effects that can prolong the survival of some cancer patients, this strategy has only been effective in a minority of patients and many patients that initially respond to treatment later become resistant or relapse presumably due to an up-regulation of other inhibitory receptors. As a result, researchers are now trying to understand what factors are involved in driving the development of the exhausted T cell phenotype with the goal of identifying other potential targets for cancer immunotherapy.
Natural Killer (NK) Cell Exhaustion in the Tumor Microenvironment
Natural killer (NK) cells are part of the first line of defense against tumor cells and their activation depends on the integration of activating and inhibitory signals received from cell surface receptors. Under normal physiological conditions, NK cell activation is inhibited by ligands expressed on healthy cells that bind to inhibitory receptors on NK cells. A reduction in the expression of these ligands or an up-regulation of stress-induced ligands that can occur in tumor cells leads to NK cell activation, and results in the production of IFN-gamma and direct cytotoxic activity against these cells. Similar to CD8+ T cells, however, natural killer (NK) cells in tumors display an exhausted phenotype that is associated with reduced expression of specific activating receptors such as NKG2D, CD16/Fc gamma RIII, DNAM-1/CD226, CD94-NKG2C, NKp30, NKp44, NKp46, and NKp80, increased expression of inhibitory receptors such as PD-1, CD94-NKG2A, TIM-3, and TIGIT, decreased production of effector cytokines such as IFN-gamma, and reduced cytolytic activity associated with the reduced expression of perforin and granzymes. Although the specific mechanisms that drive this phenotype in NK cells are currently not well-understood, it has been suggested to be caused by sustained proliferation, weakened signals from activating receptors, or immunosuppressive effects in tumor microenvironment.
Learn more about NK cell activating and inhibitory receptors | Natural Killer Cell Receptors: Human Target Cell-NK Cell Ligand-Receptor Interactions
Regulatory T Cell (Treg)-Mediated Mechanisms of Immunosuppression
Regulatory T cells (Tregs) are a unique subset of CD4+ T cells with immunosuppressive properties that are important for maintaining immune homeostasis and self-tolerance, limiting excessive inflammation, and preventing autoimmunity. Tregs function by inhibiting the activities of multiple immune cell types including CD8+ and CD4+ effector T cells, natural killer (NK) cells, and dendritic cells to restore homeostasis following inflammation. In cancer, Tregs are recruited to the tumor microenvironment (TME) by chemokines expressed by tumor cells or tumor-associated macrophages (TAMs) and these and other immunosuppressive cells in the TME promote their expansion. Tregs inhibit the anti-tumor immune response through multiple mechanisms and are recognized as a major obstacle for cancer immunotherapy.
Myeloid-derived Suppressor Cell (MDSC)-Mediated Mechanisms of Immunosuppression
Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immature myeloid cells with immunosuppressive properties that accumulate in different disease states including cancer. MDSCs produce high levels of IL-10, TGF-beta, IDO, PGE2, arginase 1, adenosine, and both reactive oxygen and reactive nitrogen species (ROS and RNS), which together inhibit the activities of natural killer cells, and CD8+ and CD4+ T cells, and promote the expansion of regulatory T cells. As a result, MDSCs are thought to play a critical role in the suppression of the anti-tumor immune response in cancer patients. Further supporting their involvement in tumor progression is the observation that MDSCs are abundant in the bone marrow, blood, and secondary lymphoid organs of cancer patients and their accumulation correlates with clinical stage, metastatic burden, and chemoresistance. Consequently, these cells are now recognized to be a major barrier in the treatment of cancer.
Learn more in our recent blog | Targeting Myeloid-Derived Suppressor Cells (MDSCs) for Cancer Immunotherapy
Tumor-derived Exosome (TEX)-Mediated Mechanisms of Immunosuppression
Tumor-derived exosomes are small, membrane-bound vesicles released by tumor cells that contain tumor cell-associated lipids, proteins, and nucleic acids including double-stranded DNA, mRNAs, and microRNAs (miRNAs). Additionally, they express immunosuppressive molecules such as PD-L1, Fas L, TRAIL, TNF-alpha, IL-10, TGF-beta, and PGE2, as well as decoy ligands for receptors involved in natural killer cell and CD8+ T cell activation, which are thought to reduce tumor cell recognition by these cells. Tumor-derived exosomes are capable of delivering signals to target cells and influencing their functions through either receptor-ligand interactions, membrane fusion followed by the release of the exosomal cargo into the cytoplasm of the recipient cell, or endocytosis-mediated exosome uptake by the recipient cell. They have been shown to inhibit immune cell functions either directly by delivering suppressive or apoptosis-inducing signals or indirectly by promoting the development and accumulation of regulatory T cells and myeloid-derived suppressor cells.
Tumor-Associated Macrophage (TAM)-Mediated Mechanisms of Immunosuppression
Tumor-associated macrophages (TAMs) are M2-like macrophages with immunosuppressive properties that are abundant in the tumor microenvironment (TME). TAMs inhibit anti-tumor immunity through multiple mechanisms including the expression of non-classical MHC class I molecules (HLA-E and HLA-G), the expression of ligands for T cell inhibitory and apoptotic receptors such as PD-L1, TRAIL, Fas L, and B7-H4, and the expression of SIRP-alpha, which binds to CD47 on tumor cells and inhibits macrophage-mediated tumor cell detection and phagocytosis. Additionally, TAMs produce immunosuppressive factors including IL-10, TGF-beta, IDO, and PGE2, and secrete chemokines that promote the recruitment of regulatory T cells (Tregs) to the TME. Besides their abilities to suppress anti-tumor immune responses, TAMs further drive tumor progression by promoting angiogenesis and tumor cell migration and invasion.
Tumor Cell-Mediated Mechanisms of Immunosuppression
Tumor cells utilize multiple mechanisms to either evade immune cell detection or inhibit the anti-tumor immune response. One of the most common tumor escape mechanisms is the down-regulation or complete loss of MHC class I molecule expression on tumor cells, which prevents their recognition by activated T cells. Tumor cells are also thought to evade immune detection by releasing soluble ligands for NKG2D and NKp30, two receptors that are involved in mediating natural killer (NK) cell and CD8+ T cell activation. Additionally, tumors may escape immune destruction by preventing the recruitment of dendritic cells or effector T cells into the tumor or by altering the tumor microenvironment to suppress the activities of different immune cell types and selectively promote tumor progression. Like other cells in the tumor microenvironment (TME), tumor cells can directly inhibit the functions of T cells and NK cells through the up-regulated expression of ligands for T cell inhibitory and apoptotic receptors such as PD-L1, TRAIL, Fas L, Galectin-9, CD112, CD155, B7-H4, and RCAS1. Tumor cells also inhibit immune cell functions through metabolic reprogramming that takes place in the tumor cells themselves. Tumor cells derive energy from glucose by aerobic glycolysis and increased glutaminolysis, which depletes glucose and L-glutamine from the tumor microenvironment and leads to the production of lactic acid. This results in a low pH in the tumor microenvironment that can suppress the anti-tumor immune response by inhibiting the activities of T cells and NK cells, and favoring regulatory T cell differentiation, myeloid-derived suppressor cell (MDSC) expansion, and M2 macrophage polarization. Tumor cells can also produce high levels of Arginase 1, iNOS, and IDO, resulting in low levels of L-arginine and L-tryptophan in the TME, which are necessary for T cell proliferation and activity. The presence of the tryptophan catabolite, kynurenine and its derivatives in the TME also inhibits NK cell proliferation and activity. Like tumor-associated macrophages (TAMs), some tumor cells also suppress immune cell functions through the expression of non-classical MHC class I molecules (HLA-E and HLA-G), or CD47, which binds to SIRP-alpha on macrophages and inhibits their abilities to recognize and phagocytose tumor cells. Additionally, tumor cells secrete chemokines that directly recruit regulatory T cells to the TME, and produce multiple other immunosuppressive factors including reactive oxygen and nitrogen species (ROS and RNS), PGE2, adenosine, and IL-10 and TGF-beta that inhibit immune cell functions through many different mechanisms detailed in the poster.