Tumors are not simply aggregates of proliferating malignant cells, they behave more like aberrant, evolving organs. Over time, they undergo extensive remodeling, influenced by selective pressures such as nutrient limitation, immune surveillance, and therapeutic interventions.
At the heart of this ecosystem lies the tumor microenvironment (TME). Much like any tissue and organ, tumors exist in an ecosystem that fosters its survival and growth.
Role of tumor microenvironment in tumor progression
The tumor microenvironment (TME) is a complex system consisting of non-cancerous cells such as fibroblasts, immune cells, blood vessels, extracellular matrix (ECM) that support the tumor by supplying nutrients, metabolites, cytokines and growth factors necessary for it to thrive.
Studies using single cell and spatial technologies have demonstrated changes in the immune milieu already at the premalignant stage.
Early lesions have been shown to attract naive T cells, demonstrating that the immune system was sensing the transformation early in the process. But as the lesions progress, an increase of activated T cells and myeloid cells, together with upregulation of immunosuppressive genes dramatically shift the immune landscape. The result is a microenvironment enriched with myeloid cells, regulatory T cells (Tregs), and exhausted CD8⁺ T cells, as well as elevated expression of immune checkpoint molecules such as PD-L1 and CTLA4.
Molecular drivers of immune suppression or activation
During the early stages of tumorigenesis, immune surveillance is effective in identifying and eliminating cancer cells. The adaptive immune system recognizes neoantigen peptides binding to the HLA epitopes and elicits lymphocytes’ immune response.
Eventually, cancer adapts to the immune surveillance, becoming resistant to the physiological immune mechanisms.
But how does the tumor microenvironment not only evade, but eventually hijack the immune system?
Tregs are a suppressive CD4 T cells subset. They normally regulate lymphocyte expansion and innate cytotoxic cells by inhibiting the immune response in autoimmune or chronic inflammatory diseases. In the TME, Tregs suppress anti-tumor immunity via multiple mechanisms, including cytokine secretion, expression of immunomodulatory molecules, and metabolic control.
Moreover, in response to tumor antigens, Tregs can undergo an antigen-driven clonal expansion, which can lead to a more robust suppression of anti-tumor response, further facilitated by their metabolic advantage over effector T cells.
This transition also happens in other cells.
Tumor-associated macrophages (TAMs) are infiltrating immune cells in the TME with a critical role in cancer progression. Macrophages differentiate from monocytes. Macrophages are a highly plastic cell population, influenced by their environment, like hypoxia, or cytokines milieu: they engulf and digest foreign and harmful substances like cellular debris or tumor cells, or display an anti-inflammatory behavior involved in wound healing and angiogenesis. When monocytes are recruited to the tumor site they differentiate into macrophages responding to the environmental cues driving them toward a pro-tumor phenotype.
TAMs pro-tumor phenotype leads to the infiltration of cytotoxic T cells into the TME by downregulating the chemokines that recruit them, and by expressing immune checkpoint ligands that inhibit their activation and proliferation. They also promote infiltration and activation of Tregs which in turn reinforce the M2-like polarization of TAMs. Moreover, they contribute to angiogenesis through secretion of angiogenic factors.
TAMs secrete chemokines that recruit and promote the differentiation and survival of myeloid-derived suppressor cells (MDSCs)
MDSCs normally dampen inflammation and prevent excessive immune activation. But, in the TME, subsets of these cells undergo metabolic reprogramming, enhancing glycolysis and lipid uptake. They accumulate phosphoenolpyruvate to neutralize ROS and avoid apoptosis, while tumor-derived cytokines (G-CSF, GM-CSF) further support their survival and suppressive function.
MDSCs also skew T cell populations toward immunosuppressive Tregs via TGF-β and retinoic acid. Their plasticity offers potential for therapeutic targeting, but their suppressive mechanisms remain incompletely understood.
How Cancer Cells Modify the Tumor Microenvironment for Survival and Growth
The TME facilitates tumor cells survival and expansion by constantly creating a hospitable niche for tumor progression through several mechanisms.
One of these key mechanisms is inflammation, which significantly contributes to the establishment of a supportive microenvironment.
Persistent inflammation creates an immunosuppressive environment where immune cells like MDSCs and Th2-type cells secrete ROS, pro-inflammatory cytokines, growth and pro-angiogenic factors, leading to the disruption of tissue homeostasis. This toxic combination results in a pro-tumorigenic microenvironment that contributes to DNA damage, epithelial transformation, immune evasion, and blood vessel formation.
Ultimately, the TME remodels the ECM to thrive and eventually invade surrounding tissues and spread to distant organs.
While the molecular mechanisms of the cells that constitute the TME ecosystem are still being explored, no two tumors share the exact same cellular composition or organization. The high resolution insights that only technologies like scRNA-seq provide will be essential to unravel their behaviour and decipher their communication to drive the next generation of cancer treatments.
Cell-to-cell communication within the Tumor Microenvironment
Cancer cells communicate and influence the immune and stromal cells to create a polarized environment where the tumor thrives.
In a healthy tissue, cancer-associated fibroblasts (CAFs) are the primary source of the connective tissues components (e.g. collagen) and when they differentiate following injury or inflammation, they secrete cytokines and growth factors that support the healing process. But within the TME, CAFs exist in a state of constant activation and never undergo apoptosis. They modify the extracellular matrix (ECM) by secreting collagen and laminin to reshape the tissue architecture, enabling invasion, metastasis, chemotherapy resistance, and forming barriers to immune cell infiltration. Using scRNA-seq data, researchers discovered three functionally distinct subpopulations in triple-negative breast cancer (TNBC). These subtypes exhibit different functional profiles that contribute to the complexity of the tumor microenvironment and to TNBC’s inter- and intra-tumoral heterogeneity. The study also identified five CAF-related hub genes—CD74, SASH3, CD2, TAGAP, and CCR7—with prognostic and potential therapeutic relevance.
Cell type plasticity influences tumor heterogeneity and development of aggressive cancer subtypes
Cellular plasticity is the ability of tumor cells to adopt new phenotypes in response to environmental cues. It plays a crucial role in cancer progression and heterogeneity.
One striking example is vasculogenic mimicry, an angiogenic process that differed from the already known angiogenesis involving the vascular endothelium. The vasculogenic mimicry (VM) is a process where tumors form their own microvascular channels made of tumor cells, and used to bring blood and oxygen to the cancer. Hypoxia triggers the transition of cancer stem cells (CSCs) to acquire endothelial-like characteristics. Vascular mimicry is a characteristic of aggressive, advanced, metastatic cancers, as the cells undergoing the VM process are exposed directly to the bloodstream, facilitating their detachment into the bloodstream to colonize other organs.Vascular mimicry is not the only example. Small cell lung cancer (SCLC) has a high degree of inter- and intratumoral heterogeneity and plasticity. A recent Nature paper used snRNA-seq with combinatorial barcoding to reveal how neuronal-like gene programs in SCLC arise and contribute to tumor behavior. The authors discovered through single nuclei sequencing that SCLC cells retain and activate neuronal transcriptional networks, forming functional synapses that promote tumor growth
Decoding the Tumor Microenvironment: How Single Cell RNA Sequencing Unravels Cellular Complexity and Immune Evasion
An important aspect of cancer cells is their ability to over time manipulate their microenvironment. This demonstrates that tumor evolution is not a static snapshot, but a dynamic process. ScRNA-seq analysis has enabled researchers to observe individual cells’ own path and uncover asynchronous progression patterns, where some cells rapidly evolve and proliferate, while others remain quiescent.
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Decoding the Tumor Microenvironment with Single Cell Sequencing
Profiling tumor heterogeneity and the tumor microenvironment (TME) has revealed cancer to be far more dynamic than once thought. Malignant, immune, and stromal cells constantly interact, shaping tumor growth, metastasis, and therapy response. Yet, much of this cellular cross-talk remains poorly understood. Single-cell RNA sequencing provides a powerful way to uncover rare cell populations, resolve immune evasion strategies, and map communication networks within the TME. It highlights how much remains to be discovered and how single cell approaches can help researchers understand cancer thoroughly and uncover new therapeutic opportunities.
Profiling tumor heterogeneity and its microenvironment
- What rare tumor cell populations exist within a single tumor? How do they evolve over time?
- How do proliferative, quiescent, and stem-like cancer cells differ in their transcriptional states?
Role of the TME in tumor progression
- Which immune and stromal cell subsets are enriched at different tumor stages?
- How does the TME composition shift from premalignant to invasive states?
Molecular drivers of immune suppression or activation
- Which T cell subsets exhibit exhausted or suppressive transcriptional states in the TME?
- How do immune cells adapt their gene programs across tumor niches?
Tumor cell modification of the TME
- How do cancer cells reprogram immune or stromal cells?
- Which ligand–receptor interactions mediate immune evasion and angiogenesis?
Cell communication between cancer, immune, and stromal cells
- How do cytokine and chemokine networks coordinate cross-talk in the TME?
- What pathways enable cancer-associated stromal cells to create physical and immune barriers?
Cell type plasticity and aggressive phenotypes
- How do cancer stem cells acquire invasive or metastatic traits?
- How do gene expression programs enable cancer cell plasticity processes like epithelial-to-mesenchymal transition (EMT) and vasculogenic mimicry that fuel tumor progression?
TLDR: Tumors are composed of diverse malignant, immune, and stromal cell populations whose interactions determine how cancers grow, evade immunity, and respond to therapy. This diversity is not incidental. Differences in cell states, spatial organization, and metabolic programs create microenvironments that can either restrain or accelerate disease progression and shape treatment outcomes. Single cell RNA sequencing reveals this hidden complexity at cellular resolution, identifying rare populations, functional states, and communication networks that explain why tumors behave differently across patients and over time. This chapter explores how profiling tumor heterogeneity and the tumor microenvironment provides critical insight into cancer progression and therapeutic response.
