Fig. 4 |. Addition of armour or subtraction of suppressive genes or their transcripts enables TME resistance.

a | Many potential design solutions are available, including gene deletion (for example, using CRISPR–Cas9 genome editing to knock out PDCD1), RNA interference (for example, knockdown of the adenosine A_2A receptor gene ADORA2A to confer adenosine resistance) and addition of genes to enable metabolic self-sufficiency (for example, addition of ASS and OTC, which encode the enzymes argininosuccinate synthetase and ornithine transcarbamylase, respectively) or that counter specific checkpoints and suppressive cytokines. b | Receptor-based approaches to resist tumour microenvironment (TME) suppression include expression of a dominant negative form of an inhibitory receptor to act as a sink. Alternatively, the receptor’s inhibitory domain can be switched to a co-stimulatory domain to change the inhibitory signal into an activating one. Chimeric antigen receptor (CAR) design considerations include the choice of co-stimulatory domains; both natural and mutant co-stimulatory domains may enhance the ability of the engineered T cell to survive in the suppressive TME. Multiple strategies may be used in parallel and should be customized to the specific TME roadblocks encountered. LTR, long terminal repeat; shRNA, short hairpin RNA; TM, transmembrane domain; T_reg cell, regulatory T cell.