Improving radioresponse through modification of the tumor immunological microenvironment

Radioresponse is influenced by factors apart from the targeted cancer cells; in fact, endothelial cells and infiltrating immune cells within the tumor microenvironment (TME) are the two main components affecting the outcome of radiotherapy. The benefits of fractionated radiotherapy are attenuated through the upregulation of hypoxia inducible factor-1 α and vascular endothelial growth factor. The therapeutic effect of antiangiogenic agents is counteracted by the mobilization of endogenous proangiogenic cells to the TME. This study highlights the importance of radiation timing within a vascular normalization window and discusses the importance of immune cells that comprise the microenvironment. A balance between favorable tumor-infiltrating immune cells, including cytotoxic T cells, natural killer cells, and dendritic cells, and the unfavorable cells, such as tumor-associated macrophages and regulatory T cells, determines the final tumor-control probability. The reciprocal complementation between combinations of radiotherapy and immunotherapy strategies through modulation of the tumor immunological microenvironment may yield promising results in the future.