Development of novel avenues to overcome challenges facing CAR T cells.

There has been dramatic success in treating patients with adoptive transfer of autologous T cells genetically modified to express a chimeric antigen receptor redirecting them to the antigen CD19. Despite this success, the application of chimeric antigen receptor T-cell therapy in solid malignancies has encountered many challenges that need to be overcome if similar success across other cancers is to become a reality. These challenges can be classified into 6 categories: the heterogeneity of tumor cell clones and tumor-associated antigen expression; poor T-cell trafficking into the tumor site; poor T-cell survival and persistence; the presence of suppressive immune cells; the secretion of suppressive soluble factors in the tumor microenvironment; and the upregulation of T-cell intrinsic inhibitory pathways. We outline specific representative hurdles in each of these categories and summarize the progress made in understanding them and developing strategies to overcome them.