Mechanisms of resistance to chimeric antigen receptor-T cells in haematological malignancies

Abstract

Chimeric antigen receptor (CAR)-T cells have recently emerged as a powerful therapeutic approach for the treatment of patients with chemotherapy-refractory or relapsed blood cancers, including acute lymphoblastic leukaemia, diffuse large B cell lymphoma, follicular lymphoma, mantle cell lymphoma and multiple myeloma. Nevertheless, resistance to CAR-T cell therapies occurs in most patients. In this Review, we summarize the resistance mechanisms to CAR-T cell immunotherapy by analysing CAR-T cell dysfunction, intrinsic tumour resistance and the immunosuppressive tumour microenvironment. We discuss current research strategies to overcome multiple resistance mechanisms, including optimization of the CAR design, improvement of in vivo T cell function and persistence, modulation of the immunosuppressive tumour microenvironment and synergistic combination strategies.

Fig. 1 ∣. Overview of CAR-T cell resistance mechanisms.

Several mechanisms of resistance to chimeric antigen receptor (CAR)-T cell immunotherapy have been identified and can be broadly related to CAR-T cell dysfunction (1), tumour-intrinsic resistance (2) or the surrounding immunosuppressive tumour microenvironment (3). CAR-T cells from responders are characterized by a more naive and central memory phenotype, as opposed to exhausted or dysfunctional CAR-T cells from non-responders. In addition, many tumour-intrinsic resistance mechanisms, beyond antigen-negative relapse, have been characterized. A ‘hot’ tumour microenvironment with high CAR-T cell infiltration, polarization and trafficking is usually predictive of a better response.

Fig. 2 ∣. Mechanisms of CAR-T cell dysfunction.

Chimeric antigen receptor (CAR)-T cell exhaustion is characterized by the sequential reduction of T cell effector functions, such as their ability to induce tumour lysis, produce cytokines and proliferate. (1) CAR-T cell activation requires binding to antigen-positive tumour cells. Persistent antigen stimulation and/or tonic CAR signalling result in CAR-T cell exhaustion. (2) Other factors contributing to exhaustion are the presence of immunosuppressive cell types in the tumour microenvironment (myeloid-derived suppressor cells (MDSCs), regulatory T (T_reg) cells and tumour-associated macrophages (TAMs)) and immunosuppressive ligands (IL-10 and transforming growth factor-β (TGFβ)). (3) Gene expression and epigenetic changes consistent with CAR-T cell exhaustion include activation of nuclear factor of activated T cell (NFAT) and the thymus high-mobility group box protein (TOX)–NR4A axis that drive the exhaustion programme of T cells and promote the expression of multiple inhibitory receptors (IRs) such as programmed cell death 1 (PD1), lymphocyte activation gene 3 (LAG3), T cell immunoglobulin domain and mucin domain 3 (TIM3) and cytotoxic T lymphocyte antigen 4 (CTLA4). (4) External factors and signals coming from IRs (such as PD1) compromise CAR-T cell metabolism. Exhausted CAR-T cells have an impaired ability to use aerobic glycolysis and oxidative phosphorylation and upregulate the glucose transporter (GLUT1) to compensate for the lack of glucose. ROS, reactive oxygen species.

Fig. 3 ∣. Tumour-intrinsic mechanisms of resistance to CAR-T cells.

Several tumour-intrinsic mechanisms have been described as factors associated with chimeric antigen receptor (CAR)-T cell failure. Owing to tumour heterogeneity, pre-existing antigen-negative tumour cells can be responsible for resistance to CAR-T cell therapy. (1) For CD19, antigen point mutations or alternative splicing result in a different form of the antigen (such as truncated) that can no longer be recognized by CAR-T cells. Antigen loss might also be due to defects in target antigen maturation and trafficking due to a lack of appropriate chaperons. (2) Tumour expression of inhibitory ligands, such as programmed cell death 1 ligand 1 (PDL1) or CD80, inhibits CAR-T activation and induces exhaustion, independently of antigen-target recognition. (3) Impaired apoptotic machinery in the tumour cells or downregulation of cell death receptors at the tumour level can confer tumour resistance to apoptosis induced by CAR-T cells. (4) Lineage switch may be responsible for complete phenotypic changes, including the loss of the target antigen. (5) CAR sensitivity depends on CAR binding to the antigen and antigen level of expression, but it is also increased when the tumour cells express the correct ligands for CAR co-accessory molecules. Defects in the engagement of co-accessory molecules (such as the loss of CD58 expression on tumour cells) can result from inefficient accessory receptors and CAR-T cell activation. CTLA4, cytotoxic T lymphocyte antigen 4; IFNγR, IFNγ receptor; LAG3, lymphocyte activation gene 3; PD1, programmed cell death 1; TIM3, T cell immunoglobulin domain and mucin domain 3; TNF, tumour necrosis factor.

Fig. 4 ∣. Tumour microenvironment barriers to CAR-T cell therapy.

Chimeric antigen receptor (CAR)-T cells might encounter environmental barriers that prevent their ability to infiltrate the tumour and activate and kill the target cells. (1) Barriers to CAR-T cell infiltration include tumour vasculature with downregulated expression of endothelial adhesion molecules by endothelial cells, which is necessary for T cell migration from the endothelium into tumours, as well defects in vascular permeability and the deposition of a fibrotic extracellular matrix (ECM) that physically blocks T cell migration and lowers T cell motility. (2) Tumour-associated macrophages (TAMs) and regulatory T (T_reg) cells contribute to CAR-T cell exhaustion. T_reg cells present in the tumour microenvironment (TME), for example, often express the inhibitory ligand programmed cell death 1 ligand 1 (PDL1), which is able to suppress CAR-T cell functionality. In addition, immunosuppressive cells and tumour cells secrete inhibitory cytokines (IL-10, IL-4 and transforming growth factor-β (TGFβ)) that also diminish the effector function of CAR-T cells. (3) Finally, tumour cells compete with CAR-T cells for nutrients (such as glucose and tryptophan) and oxygen. MDSC, myeloid-derived suppressor cell; VEGF, vascular endothelial growth factor.