Review Article

Immune Microenvironment in Tumor Progression: Characteristics and Challenges for Therapy

Figure 1

Multiple roles of the immune microenvironment during tumor development. The immune system initially eliminates tumor cells via cytotoxic T cell and NK cell killing mechanisms (immune surveillance). This is achieved with the help of antigen-presenting cells (APC) such as the dendritic cells, antibodies expressed by B cells, and inflammatory cytokines including IFN- 𝛾 and IL-2 which activate the local immune response. However, with the progressive accumulation of tumor cell mutations and modifications to the microenvironment, the tumor cells can eventually “escape” from immune surveillance. Multiple lineages of immune cells including myeloid-derived suppressor cells (MDSC), tumor-associated macrophages (TAM), and regulatory T cells (Treg), as well as various immune mediators such as TNF- 𝛼 , IL-6, CXCL-1, CXCL-5, VEGF, and MMP, are responsible for shaping a favorable microenvironment for tumor growth. Recent findings also show that the immune response continues to play an important role in established tumors via mechanisms that involve cytotoxic T cells and NK cells, as well as IFN- 𝛾 , CCL5, CXCL10, and toll-like receptors.
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