Prospects to improve chimeric antigen receptor T-cell therapy for solid tumors

Adoptive transfer of patient-derived T-cells engineered with a chimeric antigen receptor (CAR) targeting the pan-B-cell marker CD19 has led to complete remission in patients with B-cell leukemias while response rates are more modest for B-cell lymphomas. This can be attributed to the fact that the semi-solid structure of lymphomas impedes T-cell infiltration and that the immune suppressive microenvironment within these tumors dampens the effect of CAR T-cells. These obstacles are even more pronounced for solid tumors where dense and often highly immunosuppressive structures are found. This article focuses on different aspects of how to improve CAR T-cells for solid tumors, primarily by decreasing their sensitivity to the harsh tumor microenvironment, by altering the immunosuppressive microenvironment inside tumors and by inducing bystander immunity.