Tumor microenvironment: obstacles and opportunities for T-cell based tumor immunotherapies

Abstract The complex composition and dynamic change of the tumor microenvironment (TME), mainly consisting of tumor cells, immune cells, stromal cells and extracellular components, significantly impedes the effector function of cytotoxic T cells (CTLs) and thus represents a major obstacle for tumor immunotherapies. In this review, we summarize and discuss the impacts and underlying mechanisms of major elements in the TME (different cell types, extracellular matrix, nutrients and metabolites, etc.) on the infiltration, survival and effector functions of T cells, mainly CD8+ CTLs. Moreover, we also highlight recent advances that may potentiate endogenous anti-tumor immunity and improve the efficacy of T-cell based immunotherapies in cancer patients by manipulating components inside/outside of the TME. A deeper understanding of the effects and action mechanisms of TME components on the tumor-eradicating ability of CTLs may pave the way for discovering new targets to augment endogenous anti-tumor immunity and for designing combinational therapeutic regimens to enhance the efficacy of tumor immunotherapies in clinic.