Altered macrophage differentiation and immune dysfunction in tumor development Tumors require a constant influx of myelomonocytic cells to support angiogenesis and stroma remodeling. This is mediated by tumor-derived factors that cause sustained myelopoiesis and accumulation of myelomonocytic cells, mostly macrophages, at the tumor site. These cells can induce lymphocyte dysfunction. Understanding the complex interplay between neoplastic and myelomonocytic cells may offer novel therapeutic targets to deprive tumor cells of growth support and enhance antitumor immune responses. Clinical trials of cancer vaccines show they can induce robust immune responses against tumor antigens, but clinical benefits are limited. This may be due to tumor immunosuppressive effects. Immune dysregulation in cancer patients involves tumor-derived factors conditioning peripheral immune niches and hematopoietic organs, leading to abnormal myelopoiesis and accumulation of immunosuppressive myelomonocytic cells at the tumor site. Tumor-specific T cell dysfunction occurs at two sites: locally at the tumor-host interface and systemically with expanded immature myeloid cells. Myeloid-derived suppressor cells (MDSCs) are a population of myelomonocytic cells that can suppress immune responses. They are characterized by the expression of CD11b and Gr-1 and have the potential to suppress immune responses in vitro and in vivo. MDSCs can be divided into monocytic and granulocytic fractions, with the monocytic fraction being more responsible for immune dysfunction. MDSCs can suppress T cell function by interfering with IFN-γ secretion and by producing reactive oxygen species. They can also induce the development of FOXP3+CD4+ Tregs. MDSCs can be activated by antigen-experienced T cells and can suppress both CD4+ and CD8+ T cells in an MHC-independent manner. They can also induce Tregs through a pathway requiring IFN-γ and IL-10. The mechanisms of MDSC suppression are complex and may involve direct cell-cell contact or indirect modification of the microenvironment. MDSCs and tumor-associated macrophages (TAMs) have a phenotype similar to alternatively activated macrophages (M2 macrophages). TAMs are derived from circulating monocytes recruited to tumors by chemotactic factors. TAMs express M2 macrophage markers and have protumoral functions, including inducing angiogenesis, producing growth factors, and suppressing immune responses. TAMs and MDSCs can be derived from the same myeloid progenitor cells and share some characteristics. They can be polarized by different cytokines, leading to either M1 or M2 macrophage phenotypes. The balance between these polarizing signals determines the function of TAMs and MDSCs. STAT1, STAT3, and STAT6 are key transcription factors involved in macrophage polarization. STAT1 isAltered macrophage differentiation and immune dysfunction in tumor development Tumors require a constant influx of myelomonocytic cells to support angiogenesis and stroma remodeling. This is mediated by tumor-derived factors that cause sustained myelopoiesis and accumulation of myelomonocytic cells, mostly macrophages, at the tumor site. These cells can induce lymphocyte dysfunction. Understanding the complex interplay between neoplastic and myelomonocytic cells may offer novel therapeutic targets to deprive tumor cells of growth support and enhance antitumor immune responses. Clinical trials of cancer vaccines show they can induce robust immune responses against tumor antigens, but clinical benefits are limited. This may be due to tumor immunosuppressive effects. Immune dysregulation in cancer patients involves tumor-derived factors conditioning peripheral immune niches and hematopoietic organs, leading to abnormal myelopoiesis and accumulation of immunosuppressive myelomonocytic cells at the tumor site. Tumor-specific T cell dysfunction occurs at two sites: locally at the tumor-host interface and systemically with expanded immature myeloid cells. Myeloid-derived suppressor cells (MDSCs) are a population of myelomonocytic cells that can suppress immune responses. They are characterized by the expression of CD11b and Gr-1 and have the potential to suppress immune responses in vitro and in vivo. MDSCs can be divided into monocytic and granulocytic fractions, with the monocytic fraction being more responsible for immune dysfunction. MDSCs can suppress T cell function by interfering with IFN-γ secretion and by producing reactive oxygen species. They can also induce the development of FOXP3+CD4+ Tregs. MDSCs can be activated by antigen-experienced T cells and can suppress both CD4+ and CD8+ T cells in an MHC-independent manner. They can also induce Tregs through a pathway requiring IFN-γ and IL-10. The mechanisms of MDSC suppression are complex and may involve direct cell-cell contact or indirect modification of the microenvironment. MDSCs and tumor-associated macrophages (TAMs) have a phenotype similar to alternatively activated macrophages (M2 macrophages). TAMs are derived from circulating monocytes recruited to tumors by chemotactic factors. TAMs express M2 macrophage markers and have protumoral functions, including inducing angiogenesis, producing growth factors, and suppressing immune responses. TAMs and MDSCs can be derived from the same myeloid progenitor cells and share some characteristics. They can be polarized by different cytokines, leading to either M1 or M2 macrophage phenotypes. The balance between these polarizing signals determines the function of TAMs and MDSCs. STAT1, STAT3, and STAT6 are key transcription factors involved in macrophage polarization. STAT1 is