Novel approaches to enhance the specificity and safety of engineered T cells

Abstract

T-cell therapies using engineered T cells show great promise for cancer immunotherapy, as illustrated by the CD19 paradigm. Much of the excitement about this approach, and second-generation CARs in particular, is due to the dramatic clinical results recently reported by a few centers, especially in acute lymphoblastic leukemia, and the applicability of this approach, in principle, to a wide range of cancers. Extending the use of CAR therapies to cancers other than B-cell malignancies will require selective tumor targeting with minimal or acceptable "on-target, off-tumor" effects. The identification of new CAR target antigens is thus one of the next big challenges to address. Recognizing the paucity of currently available tumor-specific targets, we have developed broadly applicable approaches to enhance the tumor selectivity and safety of engineered T cells. Here, we review 2 promising concepts. One is to improve tumor targeting based on combinatorial antigen recognition. The other uses receptors that provide antigen-specific inhibition, which we named iCARs, to divert T cells from the normal tissues one wants to protect.

Figure 1. Combinatorial Antigen Recognition Allows For Selective Tumor Eradication

By using a CAR to supply a CD3ζ signal (purple) upon binding respective antigen (“A”) and a CCR to supply CD28 and CD137 signals (green and red) upon binding respective antigen (“B”), selective tumor eradication can be accomplished. Engineered T cells will remain unresponsive to cells not expressing either antigen specific to the CAR or CCR (A). Upon binding of the CAR alone (B), T cells can receive only T cell activation that can result in short-term cell lysis depending on the affinity or efficacy of the CAR binding alone. By reducing the affinity or efficacy of the CAR, activity to single positive cells can be avoided. Having only the CCR bind cells single positive cells (C) will engage T cell costimulation, but without activation no response of CCR binding alone can be measured. Only when T cells encounter tumor cells identified to be double positive for CAR and CCR respective antigens (D) can both T cell activation and costimulation occur, resulting in complete eradication of double positive tumor cells.

Figure 2. iCARs Allow Dynamic T Cell Behavior At Tumor and Normal Tissue Sites

Each round of antigen exposure is schematically represented as blue for activation (driven by the activating receptor) and red for inhibition (delivered through the iCAR). The rounds of exposure are tracked in each individual cell by color changes (red->blue->red: inhibition->activation->inhibition). Initially, all the iCAR T cells seed both the tumor and off-target normal tissue equally. During the first exposure in the tumor tissue, the T cells are activated, while in the normal tissue they are repressed. The activation causes the T cells to expand, but there is also recirculation between the two sites, so that upon the second exposure redistribution has occurred. As the T cells continue to receive activating signals at the tumor site, they continue to proliferate and destroy the tumor aided by T cells that initially encountered the normal tissue. At the normal tissue, the iCAR continue to temporarily inhibit most T cells with some potentially permanently anergized.