Suppressed Factors to CD8+ Cytotoxic T Lymphocytes in Cancer Immunotherapy

Abstract

Cancer is a disease with a very high fatality rate. immunotherapy is a promising way to treating cancers, of which CTLs are important for the host immune system against tumor, directly killing tumor cells. However, the relative immunotherapy is limited by many factors that suppress the function and activity of CD8+ CTLs. This review aims to study the mechanisms by which CD8+ CTLs are suppressed in the tumor microenvironment. By collecting preclinical and clinical studies, this paper summarized the main factors that suppress CD8+ CTLs including the following factors: immune cells like regulatory T cell (Tregs), tumor-associated macrophage (TAM), myeloid-derived suppressor cells (MDSCs); metabolic factors like hypoxia, lactic acid accumulation, glucose competition, and immune checkpoints molecules (ICM) PD-1 and CTLA-4. The complex mechanisms of suppressing CD8+ CTLs in the tumor microenvironment are discussed in these findings, which may give clues to the relative treatment and help overcome these suppression effects on CD8+ CTLs in tumorigenesis.