They are entities that have an adverse effect on the environment around them. They corrupt and attract non-malignant cells to the region, which leads to the creation of a tumor microenvironment (TME).
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The term “TME” refers to the region that surrounds the tumor and includes the tumor’s lymphatic and vasculature, pericytes, immune cells, fibroblasts, extracellular matrix, resident and infiltrating host cells, secreted factors, proteins, and occasionally adipocytes.
The surrounding milieu, which the tumor intimately interacts with, is known to have a significant influence on the course of the tumor by affecting mechanisms that result in the elimination of the tumor, enhanced metastasis, or the creation of latent micrometastases.
Since the TME influences the tumor’s resistance to treatment and reaction to it, novel treatments that target particular environmental elements are being explored. In order to create novel, successful therapies, a great deal of research and development is now being done to better understand the connection between various tumor types and the TME.
The main elements of the TME of the majority of human malignancies are covered here along with its significance for the emergence, metastasis, and reaction to therapy of the disease.
Adipocytes
Adipocytes in the TME have been reported to aid in the recruitment of malignant cells by the release of adipokines and to encourage the development of these cells by supplying them with fatty acids in intra-abdominal cancers that metastasis to the omentum as well as in some other types of tumors.
B lymphocytes
Research has indicated that B cells can occasionally be discovered near the invasive edge of tumors, however they are typically connected to the lymph node draining system and the lymphoid structures adjacent to the tumor microenvironment. A favorable prognosis for ovarian and breast cancer appears to be associated with B cell infiltration in the TME, according to research.
T lymphocytes
Since T cells may destroy tumor cells and are often “experienced” with antigens, their presence is highly correlated with a favorable prognosis. A number of malignant cells, including cancer stem cells, have been shown to be effectively killed by γδ T lymphocytes present in the TME.
Nevertheless, there is little experimental data to establish a clear correlation between the presence of γδ T cells in the TME with a favorable or unfavourable prognosis.
Cancer-associated fibroblasts
Fibroblasts normally induce organ fibrosis as a reaction to tissue damage. On the other hand, it has been observed that the TME’s fibroblast content increases the likelihood of cancer formation.
DCs, or dendritic cells
Although DCs are normally able to digest antigens, it is believed that those present in the TME are defective and unable to properly trigger the immune response to antigens linked with tumors. Additionally, it has been discovered that certain DCs inhibit T cells, which further worsens prognosis.
Lymphatic endothelial cells
Lymangiogenesis is stimulated by VEGFC or VEGFD that tumors produce. As a result, the TME experiences a proliferation of lymphatic channels and a rise in lymphatic endothelial cells, which aid in the spread of cancerous cells.
A increasing amount of data also points to the possibility that lymphatic endothelial cells modify the host immunological response to the tumor in order to mechanically regulate the TME.
MDSCs, or myeloid-derived suppressor cells
Immune suppressor cells known as MDSCs have elevated expression in a variety of human malignancies. It is believed that they suppress T cell activation, which subsequently deteriorates prognosis.
NKT and NK cells
The presence of natural killer (NK) and natural killer T (NKT) cells in the tumor microenvironment (TME) of colorectal, renal, gastric, liver, and lung malignancies is thought to indicate a favorable prognosis. However, other research indicates that NK cells may not be able to carry out their tumor-killing role in the TME because transforming growth factor-beta (TGF-β) from malignant cells activates their anergic phenotype.
Pericytes
Pericytes, or perivascular stromal cells, are recognized as being essential to the tumor vasculature, which supports the anatomical integrity of the blood vessels. Low pericyte coverage of the vasculature in malignancies including bladder and colorectal is linked to an increased risk of metastases and tumor progression over time, according to research.
TAMs, or tumor-associated macrophages,
TAMs often have a pro-tumorigenic effect on the majority of human malignancies, causing invasion, malignant cell migration, and metastasis. The hypothesis that a high concentration of TAMs in the TME is associated with a bad prognosis is supported by preclinical and clinical research.
Neutrophils linked to tumors (TANs)
Some researches have found that TANs have a role in the development and spread of primary tumors, whereas others have doubts. TANS have been linked in some studies to both decreased immune response and enhanced angiogenesis, while other studies have shown that cytokine stimulation results in TANs having an anticancer effect.
Vascular endothelial cells
The TME’s VEGFs, FGFs, chemokines, and platelet-derived growth factors (PDGFs) can all stimulate endothelial cells and the pericytes that cling to them. This mechanism promotes the growth of cancer and may result in the angiogenic factors being produced, which further facilitates the tumor’s development.
