Tumor immunotherapy: Mechanisms and clinical applications

Immunotherapy, which aims to enhance the functions of the host's immune system to eliminate invading pathogens and mutant cells, has been widely used for cancer treatment. Despite the enormous progress in immunotherapy, the efficiency of immunotherapy is urgent to be improved. The tumor microenvironment is composed of multiple types of infiltrating immune cells and stromal cells (such as endothelial cells, fibroblasts, and tumor cells), extracellular matrix, various cytokines, chemokines, growth factors, and metabolites, all of which play crucial roles in tumor progress, metastasis, relapse and the outcome of immunotherapy. Emerging evidence indicates a better understanding of the characteristics of the tumor microenvironment and its associated factors, which could lead to uncovering novel promising immunotherapy approaches. This review will summarize the current knowledge of heterogeneity and function of the immune cells in the tumor microenvironment, the current progress of immunotherapy focusing on immune checkpoint blockade and chimeric antigen T cell therapy, as well as the challenge and limitations of the current immunotherapy strategy. Overall, we aim to provide the conceptions of tumor immune microenvironment and application of immunotherapy, highlighting the potential strategies to improve tumor immunotherapy.