Perspective and future direction
Here, we reviewed studies highlighting the promising impact of new technologies in cancer immunotherapy (Figure 3 ). Editing CAR-T cells with CRISPR/Cas9 can overcome many challenges such as allogeneic reaction, tonic signaling, exhaustion, low performance in TME and toxicity. In addition, the large-scale genetic screens using the CRISPR/Cas9 system provides scalable methods to interrogate thousands of genes in T cells with high efficiency and specificity.
TCR and HLA knocked out T cells present great potential for developing allogeneic third-party T cell products. However, there is controversy over benefits of TCR deletion. In a recent study, TCRβ knocked-out CD19 CAR-T cells were compared to TCR intact CD19 CAR-T cells (Stenger, Stief, Kaeuferle, et al., 2020). Although knocking out the endogenous TCR in CAR-T cells strongly eliminated alloreactivity compared to TCR-expressing CAR-T cells; co-expression of endogenous TCR plus CAR led to superior persistence of T cells and significantly extended the control of leukemia in vivo. This data highlights that despite the benefits of TCR knocked out T cells in developing off-the-shelf cell therapies, presence of endogenous TCR might be better for long-term survival of T cells. Hence, a deeper understanding of T cell biology and TCR signaling provides useful insights for designing and engineering more effective CAR-T cells.
CRISPR/Cas9 technology can also indirectly assist in developing more effective CAR-T cell therapies. Using large scale CRISPR screening libraries, we can discover novel antigens to be targeted by CAR-T cells. Moreover, we can identify factors in T cells, tumor cells or TME which induce resistance to CAR-T cells (Dufva et al., 2020). By genome-wide genetic perturbation/CRISPR screen, Dufva et al. investigated genes which their loss in cancer cells impaired the effector function of CAR-T cells and revealed the essentiality of death receptor signaling for CAR-T cell cytotoxicity. Similar studies provide a better understanding of mechanisms influencing CAR-T cell function and the potential for modulation using combination therapy or genetic engineering strategies.
Although recently emerged, CRISPR/Cas9 nowadays is a “go to” method for gene editing in many research labs in academic and pharmaceutical industry. With this fast advancement in the CRISPR/Cas9 application and emergence of new gene-editing technologies, we hope to soon witness the success of CAR-T cells in treating many refractory cancers including solid tumors.