Microenvironmental regulation of solid tumour resistance to CAR T cell therapy

Chimeric antigen receptor (CAR) T cell therapy holds significant promise for the treatment of cancer; however, its efficacy in solid tumours is substantially hindered by the immunosuppressive tumour microenvironment (TME). Solid tumours can resist immunotherapy by impairing T cell trafficking, function and persistence. One of the initial obstacles that CAR T cells encounter is the abnormal tumour vasculature, which restricts efficient T cell infiltration, further compounded by a dense extracellular matrix. CAR T cells that do infiltrate the tumours are outnumbered by immunosuppressive cells such as regulatory T cells, myeloid-derived suppressor cells and tumour-associated macrophages. Additionally, tumour cells can contribute to CAR T cell resistance by upregulating immune checkpoint molecules, such as PDL1 and CTLA4, and engage in metabolic competition. In this Review, we discuss how cellular and non-cellular components of the TME impair CAR T cell therapy and consider potential strategies to improve CAR T cell therapies for solid tumours, either by reprogramming the TME or by engineering CAR T cells to resist the immunosuppressive effects of the TME.