A new insight in chimeric antigen receptor-engineered T cells for cancer immunotherapy

Abstract

Adoptive cell therapy using chimeric antigen receptor (CAR)-engineered T cells has emerged as a very promising approach to combating cancer. Despite its ability to eliminate tumors shown in some clinical trials, CAR-T cell therapy involves some significant safety challenges, such as cytokine release syndrome (CRS) and "on-target, off-tumor" toxicity, which is related to poor control of the dose, location, and timing of T cell activity. In the past few years, some strategies to avoid the side effects of CAR-T cell therapy have been reported, including suicide gene, inhibitory CAR, dual-antigen receptor, and the use of exogenous molecules as switches to control the CAR-T cell functions. Because of the advances of the CAR paradigm and other forms of cancer immunotherapy, the most effective means of defeating the cancer has become the integration therapy with the combinatorial control system of switchable dual-receptor CAR-T cell and immune checkpoint blockade.

Keywords: Adoptive cell therapy; Adverse effects; Bifunctional molecule; Immune checkpoint blockade; Switchable dual-receptor T cell.

Fig. 1

Schematic diagram of TCR- and CAR-modified T cells in adoptive T cells therapy. a Activation, proliferation, and cytotoxicity of the T cell are dependent upon the dual signal pathway that includes the T cell receptors (TCRs) that recognize peptide antigens which were processed by the antigen-presenting cells and presented upon the major histocompatibility complex (MHC) of a target cells, and the costimulatory receptor of T cell simultaneously engages a ligand, such as CD28 and B7 molecules. b The first-generation CAR contains only the antigen recognition signal, CD3ζ domain, resulting in the transient activation and proliferation of the CAR-T cell based on scFv specificity. c–d The second- and third-generation CARs contain one and two additional costimulatory signaling domains, respectively, such as CD28, CD137 (4-1BB), and CD134 (OX40). The costimulatory signaling domains can facilitate greater proliferation of modified-T cell and greater cytotoxicity than first-generation CAR. e To significantly enhance the overall cytotoxicity of the modified-T cell, the fourth-generation CAR-T cell is generally modified to express CARs with an inducible cytokine genes, such as IL-12 or heparinase, which can stimulate T cell to reach the surface of tumor cells in degrading the extracellular matrix (ECM) within the tumor microenvironment and blocking the inhibitory signaling pathway. f The next-generation structure of the CARs with effective specificity for target cells lacking several side effects to the body will be generated in the near future, including reconstruction of endogenous structure and introduction of exogenous regulatory

Fig. 2

Building strategies to improve the safety of CAR-modified T cells therapy. For upper left, when the release of cytokines by CAR-T cells after killing tumor cells becomes more pronounced than at basic levels, such as interleukin-2 (IL-2) and interferon-γ (IFN-γ), the inducible caspase 9 (iCasp9) can be dimerized, which usually leads to the rapid apoptosis of T cells expressing the iCasp9 suicide gene by addition of a synthetic dimerizing drug AP1903. For upper middle, to achieve the precise regulation of the CAR-T cells, the inhibitory strategy usually harnesses an inhibitory receptor structure comprising an antigen recognition domain with specificity to antigens only presented on normal tissue, and an inhibitory signaling domain to abort T cell behavior despite concurrent engagement of an activating receptor. In this way, the iCAR-T cell can distinguish the tumor and off-tumor cells, and reversibly restrict CAR-T cell functions in an antigen-selective manner, such as the PD-1- and CTLA-4-based iCARs. For upper right, the modified-T cell cotransduced with a CAR, which stimulates an activation signaling pathway upon binding to the first antigen and a chimeric costimulatory receptor (CCR) that recognizes a second antigen to provide the costimulatory signal, can eliminate the cancer cells expressing both antigens rather than either antigen alone. This dual receptor pattern provides a safe path to restricting the activity of engineered T cell in vitro and in vivo. For lower left and lower middle, the split CAR-T cell with two physically separate ports exert the therapeutic functions in the presence of tumor antigens and a heterodimerizing small molecule, AP21967. For lower right, bispecific antibodies are used as a switch to control the interaction between the cancer cell and CAR-T cell. The lower three strategies are similar, in which the cytotoxicity of CAR-T cell are dependent upon presence of exogenous molecule and tumor antigens

Fig. 3

Combinatorial strategy for therapeutic T cell that integrates activation signaling pathway and inhibitory signaling pathway. In the promotion of the activation signaling pathway, this design conformation of the modified-T cell can yield safer therapeutic function upon the safe platform, including the dual receptor engineering T cell and bifunctional molecules. In this system, the T cell that carry two receptors can engage the molecules and tumor surface antigens, and the bifunctional molecule can target T cells and cancer cells. On the other hand, blocking the inhibitory signaling pathway by adding some monoclonal antibodies, such as tremelimumab and nivolumab with specificity for CTLA-4 and PD-1, respectively, was also found to improve the efficacy and persistence of the infused CAR-T cell in vitro and in vivo