Understanding T cell phenotype for the design of effective chimeric antigen receptor T cell therapies

Abstract

Rapid advances in immunotherapy have identified adoptive cell transfer as one of the most promising approaches for the treatment of cancers. Large numbers of cancer reactive T lymphocytes can be generated ex vivo from patient blood by genetic modification to express chimeric antigen receptors (CAR) specific for tumor-associated antigens. CAR T cells can respond strongly against cancer cells, and adoptive transferred CAR T cells can induce dramatic responses against certain types of cancers. The ability of T cells to respond against disease depends on their ability to localize to sites, persist and exert functions, often in an immunosuppressive microenvironment, and these abilities are reflected in their phenotypes. There is currently intense interest in generating CAR T cells possessing the ideal phenotypes to confer optimal antitumor activity. In this article, we review T cell phenotypes for trafficking, persistence and function, and discuss how culture conditions and genetic makeups can be manipulated to achieve the ideal phenotypes for antitumor activities.

Keywords: chimeric antigen receptor; exhaustion; memory; persistence; self-renewal; trafficking.

Figure 1

Phenotype definition and characteristics of T cell differentiation states. A widely accepted model of T cell differentiation in adoptive T cell transfer field dictates that T cells progress along a linear trajectory of development, with each differentiation phase displaying a unique phenotype with altered functionality and properties. This model proposes while some primed naïve T cell (T_N) cells will differentiate into effector T cells (T_EFF), some primed T_N cells will differentiate into memory T cells, including stem memory T cells (T_SCM), a subtype categorized by their ability to self-renew, while also possessing multipotent differentiation capacity, central memory T cells (T_CM, long-lived), effector memory T cells (T_EM), and tissue residential memory T cells (T_RM). These memory T cells can both self-renew, and give rise to a pool of T_EFF. Bcl 2, B-cell lymphoma 2; IL7Rα, interleukin receptors.

Figure 2

T cell homing events into tumor tissue. T cell infiltration into the tumor from the blood is a multistep process involving a variety of molecules such as (1) selectins and their ligands, (2) chemokines and their receptors, and (3) integrins and their ligands. Strategies manipulating these three family of molecules essential for T cell trafficking for enhanced CAR T cell infiltration into tumors are discussed in the main text. The first step in T cell infiltration requires the initiation of T cell rolling, or slowing of T cell movement, in the blood stream. This is facilitated by the interaction of selectin molecules on the blood vessel with their target ligands on the T cell. Next, chemokine receptors on the T cell bind to their cognate chemokines, produced by tissues or endothelium, thereby initiating the activation of integrins on the T cell. Integrins, when bound to their cognate ligands on the vessel surface, mediate firm arrest of T cell movement. Finally, the T cell is able to migrate through the cell junctions of the blood vessels into tissues, along a gradient of chemokines. CAR, chimeric antigen receptors; LFA-1, lymphocyte function-associated antigen-1; VLA-4, very late antigen-4.

Figure 3

Markers of T cell exhaustion. T cell dysfunction occurs when there is chronic exposure to antigen, such as in the context of cancer. This loss of function is referred to as T cell exhaustion. T cell exhaustion includes the loss of IL-2 production, decreased proliferative capacity, and reduced cytotoxic abilities, including impairment in granzyme B, IFN-γ and TNF-α production. Along with the loss of functionality, T cells increase the expression of inhibitory receptors such as PD1, LAG3, Tim3, CTLA4, and TIGIT. Some of the key regulators of T cell exhaustion include the transcription factors TOX and NR4A (NR4A1, NR4A2, and NR4A3). IFN-γ, interferon-γ; IL-2, interleukin 6; TNF-α, tumor necrosis factor-α.