CAR T Cells in Solid Tumors: Blueprints for Building Effective Therapies

Abstract

Genetic redirection of T lymphocytes with chimeric antigen receptors (CARs) has soared from treating cancers preclinically to FDA approval for hematologic malignancies and commercial-grade production scale in under 30 years. To date, solid tumors are less susceptible to CAR therapies and instead have been treated more successfully with immune checkpoint blockade or tumor-infiltrating lymphocyte therapy. Here, we discuss the current challenges in treating solid tumors with CAR T cells, and the obstacles within the host and tumor microenvironment hindering their efficacy. We present a novel three-pronged approach for enhancing the efficacy of CAR T cells whereby a single infusion product can synergize the power of an optimal CAR construct, a highly potent T cell subset, and rejuvenate the endogenous immune response to conquer therapeutically-resistant solid tumors.

Keywords: T cell; adoptive cell transfer; checkpoint; chimeric antigen receptor; solid tumor.

Figure 1

Three-pronged approach to improve chimeric antigen receptor (CAR) T cell therapies in solid tumors. A multi-faceted attack on solid tumors resistant to standard CAR T cell therapies may best augment their efficacy in clinical trials. The ultimate CAR T cell therapy should encompass three axes: (1) a CAR with high fidelity targeting of more than one tumor antigen and trafficking capacity, (2) selection of a T cell subset with potent self-renewal and migratory capacity for long-term persistence and immunity, and (3) ability to harness and rejuvenate the host response to tumor neoantigens. A single arm (CAR, subset, or host response) has not been sufficient for long-term responses against aggressive solid tumors to date.

Figure 2

“Fourth-generation” chimeric antigen receptor (CAR) constructs incorporate novel mechanisms to enhance targeted antitumor efficacy. (A) The third-generation CAR incorporates the extracellular scFv with intracellular CD3ζ signaling and two tandem costimulatory domains. (B) CAR T cells with additional chemokine receptors have improved trafficking to tumors. (C,G) T cells secreting additional cytokines or engineered with cytokine signaling domains have enhanced activation and can modulate surrounding microenvironment. (D) Armored CARs redirect suppressive signals from the tumor to activating signals to resist exhaustion. (E) Suicide genes and (F) bispecificity mitigate off-tumor toxicity through the ability to deplete transferred cells or enhance specific targeting to tumors, respectively. (H) Switchable CAR targeting via adaptor molecules provides versatile opportunity to control CAR activation, specificity, and longevity after transfer of cells. Abbreviations: Ag, antigen; PNE, peptide neo-epitopes.

Figure 3

Antitumor efficacy of memory CD8⁺ T cell subsets diminishes with differentiation. (A) Once activated with cognate antigen, CD8⁺ T cells progressively differentiate from stem-cell memory (SCM), with the highest capacity of self-renewing properties, to central memory (CM), effector memory (EM), and finally to terminal effector (EFF) phenotypes. (B) Antitumor immunity of T_SCM cells is enhanced due to establishing long-term memory responses to tumor antigens and heightened ability to persist. As cells become more differentiated through the T_CM, T_EM, and T_EFF stages, they lose capacity for self-renewal and become exhausted, resulting in poor antitumor immunity.

Figure 4

Two approaches for generating less-differentiated T cells after ex vivo expansion for adoptive cell transfer. (A) Naïve T cells sorted from peripheral blood can be activated and transduced with chimeric antigen receptors (CARs) for antigen specificity. Adding pharmacologic inhibitors of AKT, GSK-3β, PI3K, or mTOR to the T cell culture helps retain cells in a less-differentiated state as they expand. This approach can enrich T_SCM and T_CM phenotypes in CAR T cells from naïve populations before adoptive transfer to enhance long-term immunity. (B) Differentiated T cells can be reprogrammed with stem-like qualities using iPSC technology. In brief, bulk T cells are isolated from the blood, programmed into iPSCs, and transduced with a CAR before lymphoid differentiation into naïve T cells. The most efficient approaches for lymphoid differentiation into naïve phenotypes are still under development.

Figure 5

Antitumor immunity of CD4⁺ T cells is dependent upon the subset to which they are polarized. Regulatory T cells (Tregs) (top left) and Th2 cells (top right) are classically tumor promoting. Tregs downregulate effector T cell responses via secretion of suppressive cytokines or engagement of inhibitory checkpoint molecules like CTLA-4 or TIGIT. Th2 cells secrete suppressive cytokines that hinder a Th1-mediated antitumor response. Conversely, transfer of Th1 cells (bottom left) and Th17 cells (bottom right) enhance antitumor responses. Th1 cells produce IFN-γ and enhance CD8⁺ cell-mediated immunity. Th17 cells produce proinflammatory cytokines that have controversially been implicated in carcinogenesis; however, adoptive transfer of Th17 cells has shown robust immunity in several solid tumors. Transferred Th17 cells have stem-like self-renewal capabilities and enhanced persistence long term over Th1 cells.

Figure 6

CAR-mediated tumor destruction can synergize with host immunity through epitope spreading. (A,B) Chimeric antigen receptor (CAR)-mediated tumor cell lysis induces inflammation, and release of tumor antigens. (C) DAMPs from dying cells recruit APCs to tumor site, which take up and process the released antigens for presentation. (D) APCs present newly processed tumor antigens to naïve T cells in lymph nodes. Activated T cells migrate to the tumor site. (E) Tumor-specific lymphocytes synergize with CAR T cells to eradicate difficult to treat solid tumors.

Figure 7

The trifecta of successful chimeric antigen receptor (CAR) T cell therapies in solid tumors. The ultimate CAR T cell therapy has tumor specificity, potent migratory capacity and persistence, and improves the host immune response. (A) Bispecificity through syn-Notch technology augments targeting to tumor/tumor-specific tissue. (B) Engineering a T cell with enhanced persistence and migratory capacity—such as a Th17 or CD4⁺CD26^high cell—or with self-renewing properties—such as a CD8⁺ T_SCM cell—will enhance long-term memory responses to prevent tumor recurrence. (C) Secretion of PD-1 blockade and cytokines such as IL-12, IL-15, IL-18, or IL-21 locally could overcome the suppressive tumor microenvironment, reinvigorate the exhausted host immune response to other tumor antigens, and synergize with CAR-specific T cells to destroy large heterogenous solid tumors.