Chimeric antigen receptor T cell therapies for thoracic cancers—challenges and opportunities
Editorial on Immunotherapy and Tumor Microenvironment

Chimeric antigen receptor T cell therapies for thoracic cancers—challenges and opportunities

Jack D. Chan1,2, Aaron J. Harrison1, Phillip K. Darcy1,2, Michael H. Kershaw1,2, Clare Y. Slaney1,2

1Cancer Immunology Program, Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia; 2Sir Peter MacCallum Department of Oncology, University of Melbourne, Parkville, Victoria, Australia

Correspondence to: Clare Y. Slaney. Cancer Immunology Program, Peter MacCallum Cancer Centre, Melbourne, Victoria, 3000, Australia. Email: clare.slaney@petermac.org.

Submitted Dec 11, 2019. Accepted for publication Feb 04, 2020.

doi: 10.21037/jtd.2020.03.34


It is widely accepted that the immune system plays a critical role in controlling cancer. Immunotherapies exploit this complex interplay by activating the immune response to target and clear cancer cells. Chimeric antigen receptor (CAR) T cells are genetically engineered T cells that target and specifically recognise tumour antigens and have demonstrated curative responses in certain blood cancers (1,2). Currently CAR T cells have been approved by the Food and Drug for the treatment of certain B cell malignancies with rapidly increasing interest in solid tumours (3). This editorial will highlight CAR T cell design, therapeutic strategies and potential roadblocks to the application of CAR T cells for the treatment of thoracic cancers.


Adoptive cell transfer (ACT) based cancer therapy

Meta-analysis studies have demonstrated that elevated levels of tumour infiltrating lymphocytes (TILs) are often correlated with prolonged patient survival (4). Adoptive T cell transfer therapy (ACT) is a cell-based therapy that aims to increase the number of tumour specific immune T cells in cancer patients. Here, TILs are isolated, activated ex vivo, and adoptively transferred back into patients in order to facilitate improved patient outcomes (5). However, responses to TIL-based ACT have only demonstrated efficacy in certain cancers, such as melanoma, and can vary greatly between patients (6). Furthermore, it is not always possible to isolate and expand TILs from every patient (5). To circumvent these issues, methods for transducing the first CAR expressing T cells were developed in the early 1990s (7,8).


CAR T cell therapy

CAR T cells are generated from peripheral blood lymphocytes. Patient T cells are transduced ex vivo to express CARs cognate for tumour antigens, thereby directing T cells to specifically kill tumour cells (5). A CAR is composed of an antigen-specific derived, single-chain variable fragment (scFv) linked to intracellular signalling domains (8) (Figure 1). Direct recognition of cancer antigens through the scFv facilitates T cell activation and tumour cell killing without the requirement for tumour antigen presentation through the major histocompatibility complex. The first developed CAR T cells were engineered to include a single intracellular signalling domain such as CD3-ζ (8) (Figure 1). Second- and third-generation CAR T cells introduced additional intracellular co-stimulation signalling domains to achieve more efficacious CAR T cell activation and greater in vivo persistence (9,10) (Figure 1).

Figure 1 Schematic of CAR T cell design. (A) First generation CAR T cells express a scFv conjugated to a single intracellular signalling domain (typically CD3-ζ). (B) Second- and (C) third-generation CAR T cells contain additional one or two co-stimulatory domains respectively such as CD28 or 4-1BB (CD137). CAR T cells may be modified to express (D) dominant negative receptors (DNRs) or (E) switch receptors that abrogate immune checkpoint receptor signalling or can activate CAR T cells respectively.

The success of CAR T cell treatment has resulted in FDA approval for the use of two CD19-targeted CAR T cell therapies in 2017. These included tisagenlecleucel (KYMRIAH) for the treatment of children and adolescent’s acute lymphoblastic leukemia (ALL) and axicabtagene ciloleucel (YESCARTA) for adult relapsed-refractory large B-cell lymphoma (3). More recently, the FDA has provided its regenerative medicine advanced therapy designation to CAR T cell treatment of relapsed or refractory multiple myeloma also known as CT053. Favourable outcomes in these malignancies have led to the investigation of CAR T cell treatment in the context of a range of thoracic cancers.


The challenges of CAR T cell therapy

Despite favourable outcomes in the treatment of haematological malignancies, CAR T-cells targeting solid tumours have demonstrated inadequate efficacy in thoracic cancers (10,11). This is believed largely attributed to poor trafficking of CAR T cells to the tumour site, the immunosuppressive tumour microenvironment (TME), poor activation and persistence of CAR T cells in vivo (12).

Inadequate lymphocyte recruitment to the tumour may be due to various factors including aberrant tumour vasculature, endothelial anergy, and mismatch of the TME chemokine profile and CAR T cell chemokine receptors (13). CAR T cell exclusion can be overcome through transduction of T cells to overexpress the relevant chemokine receptors in addition to a CAR. This has been demonstrated in the treatment of malignant pleural mesothelioma (MPM), which has a high level of CCL2 chemokine secretion (14). The overexpression of chemokine receptor 2 (CCR2) by mesothelin (MSLN) specific CAR T cells improved pleural accumulation and anti-tumour activity of CAR T cells against MPM (15). Furthermore, chemokine exclusion of CAR T cells may be overcome through regional delivery of CAR T cells to appropriate sites, as opposed to intravenous infusion. Improved CAR T cell persistence and anti-tumour activity has been demonstrated in regional delivery to orthotopic MPM mouse tumours and is now being evaluated in phase I clinical trials (16).

In addition to inhibition of trafficking, CAR T cells must overcome a wide variety of secreted and cellular factors in the TME which further act to suppress T cell killing activity. These TME factors include immunosuppressive cells, inhibitory ligands and receptors, and immunosuppressive factors. These challenges constitute some of the biggest barriers to successful CAR T cell treatment of thoracic cancers. The generation and recruitment of immunosuppressive cells to the TME is a key characteristic of a developing tumour (17). Major immunosuppressive subsets associated with lung cancer and mesothelioma include myeloid derived suppressor cells (MDSCs), mesenchymal stromal cells (MSCs), tumour-associated macrophages (TAMs) and regulatory T cells (Tregs) (18). Whilst CAR T cells targeting immunosuppressive subsets have been generated and utilised in pre-clinical studies, such cells are yet to be tested in the clinical setting.

Another strategy to target the TME is to block immune checkpoint receptor-ligand interactions. Immune checkpoint receptors are naturally expressed by activated T cells and function to minimise collateral host tissue damage during the immune response. Immunosuppressive myeloid populations and tumour cells may both aberrantly express ligands for immune checkpoint receptors expressed by endogenous and CAR T cells, particularly, ligands for programmed death-1 (PD-1). Ligation of immune checkpoint receptors suppresses CAR T cell anti-tumour function and polarises CAR T cells to a state of functional exhaustion (19). Disruption of immune checkpoint receptor-ligand interactions has been a successful treatment strategy for solid tumours. Antagonistic, monoclonal antibodies against CTLA-4, PD-1, and programmed death ligand 1 (PD-L1) have been FDA approved for the treatment of range of cancers, including non-small cell lung cancer (NSCLC) (20). Therefore, the combination of CAR T cells with checkpoint inhibitors has great potential in treating patients with thoracic cancers. In fact, in pre-clinical studies, the combination of CAR T cell and immune checkpoint blockade therapies has demonstrated significant efficacy in syngeneic mouse models, driving significant tumour regression, improved survival and prevented CAR T cell exhaustion (21).

Another approach for immune checkpoint receptor inhibition is the modification of CAR T cells to express dominant negative receptors (DNRs) or switch receptors. DNRs are mutated receptors for immunosuppressive signals in the TME that abrogate signalling and negative regulation. DNRs generated for PD-1 have demonstrated increased CAR T cell resistance to immunosuppression and restored effector function (22). Moreover, CAR T cell DNRs can also can also compete with endogenous T cells expressing immune checkpoint receptors of endogenous T cells, reducing the inhibition of endogenous anti-tumour T cell responses. Conversely, switch receptors are composed of an extracellular antigen binding portion of an antibody specific for immunosuppressive molecules such as PD-1 or CTLA-4 conjugated to intracellular activation signalling domains (12). Expression of switch receptors against CTLA-4 has shown to increase T cell efficacy in mouse models (23). In summary, combination therapy of CAR T cells with different approaches to immune checkpoint receptor inhibition may prove to be a promising avenue for the treatment of a range of thoracic cancers.

As CAR T cells are antigen specific, tumour heterogeneity has proven to be one the of the greatest hurdles for effective CAR T cell therapy in the solid tumour settings. It is considered that recruitment of the endogenous anti-tumour response following CAR T cell activity is required for broad protection against solid tumours (24). In addition to immune checkpoint blockade that may rescue previously inhibited endogenous immune cell types, modified ‘armoured’ CAR T cells can also recruit endogenous immune components. Armoured CAR T cells are engineered to express molecular factors that may facilitate immune cell activation and recruitment. For example, armoured CAR T cells modified to express single chain IL-12 have demonstrated elevated efficacy compared to conventional CAR T cell therapy in xenograft models (25). The constitutive IL-12 signalling provided through armoured CAR T cells enhanced T cell cytotoxicity, cytokine secretion and resistance against Treg immunosuppression (25). However, armoured-CAR activity can cause severe cytokine release syndrome (CRS) and has yet to demonstrate safety and efficacy in clinical trials.


Clinical investigation of CAR T cell treatment for thoracic cancers

Whilst effective CAR T cell therapy requires an antigen target that has high tumour expression, solid tumour antigens targeted by CARs are often also expressed on normal, healthy tissue (26). This raises significant risks for off-target effects, where toxicity has been demonstrated in a number of CAR T cell clinical trials (26). A selection of clinical trials studying CAR T cells in thoracic cancer are listed in Table 1.

Table 1
Table 1 Selection of CAR T cell clinical trials for thoracic cancers
Full table

The focus of research for the field remains in the discovery of tumour associated or tumour specific target antigens. Human epidermal growth factor receptor 2 (Her2) and epidermal growth factor receptor (EGFR) have presented as popular solid tumour targets in both pre-clinical and clinical CAR T cell investigations in thoracic cancers. Her2 and EGFR belong to the ErbB family of receptor tyrosine kinases and are often overexpressed in a range of thoracic tumour contexts (27). Phase I clinical trials of CAR T cells targeting EGFR in relapsed/refractory NSCLC demonstrated patient tolerance to therapy, CAR T cell tumour infiltration and depletion of EGFR expressing tumour cells in tumour biopsy samples (28).

MSLN is normally expressed by mesothelial cells of the pleura, peritoneum and pericardium, but can have increased expression in MPM and lung adenocarcinoma cases (29). Advanced MPM patients in a phase I clinical trial that received MSLN targeted CAR T cells demonstrated no obvious off-tumour on-target toxicity, and trafficking to primary and metastatic tumour sites. Moreover, tumour cell lysis and the decrease in tumour cell numbers in ascites was suggested to be caused by CAR T cell killing (11).

Pre-clinical studies in mouse NSCLC xenograft models targeting prostate stem cell antigen (PSCA) and Mucin 1 (MUC1) have shown that dual therapy of CAR T cells targeting PSCA and MUC1 can synergistically eliminate tumours co-expressing both target antigens (30). Of note, MUC1 is a glycoprotein expressed by epithelial cells on mucosal surfaces and is aberrantly expressed in NSCLC and lung adenocarcinomas. Currently there are a number of active clinical trials involving MUC1 directed CAR T cells for the treatment of thoracic cancers (NCT03706326 and NCT03525782).

CAR T cells targeting PD-L1 expressed by immunosuppressive cells and tumour cells present as an interesting opportunity to eliminate both tumour cells and mechanisms of immunosuppression. Although clinical and pre-clinical data supports the use of checkpoint blockade antibodies or the use of CAR T cells with intrinsic resistance to PD-1 checkpoint blockade, the benefit of CAR T cells specifically targeting PD-L1 has yet to be elucidated. There are currently a number of clinical trials studying the efficacy of PD-L1 targeting CAR T cells in thoracic cancers (NCT03198052, NCT03330834 and NCT03060343).


Summary and future perspectives

With the clinical landscape for CAR T cell treatment of haematological malignancies becoming increasingly well defined, further understanding of the mechanisms by which the TME suppresses CAR T cells will be critical in shaping the success of CAR T cell treatment in thoracic malignancies. Moreover, immunosuppressive mechanisms, tumour antigen expression, metastasis and tumour cell metabolism can differ greatly between tumour types. As such, a modular system in which CAR T cells can be paired with the correct CAR, chemokine sensitivity and resistance to immunosuppression will be key in providing effective, patient specific care in thoracic cancers.


Acknowledgments

Funding: This work was supported by grants from the Peter MacCallum Cancer Centre Foundation, the National Health and Medical Research Council (NHMRC) of Australia (1176935, 1103352 and 1132373), the National Breast Cancer Foundation (NBCF) of Australia (IIRS-18-064 and IIRS-20-073) and Susan G. Komen Breast Cancer Foundation (16376637). JDC was supported by a Research Training Program (RTP) and Rosie Lew scholarships. PKD and MHK were supported by NHMRC Senior Research Fellowships. CYS was supported by a Postdoctoral Fellowship from the NBCF.


Footnote

Provenance and Peer Review: This article was commissioned by the Guest Editors (Peng Luo, Clare Y. Slaney and Jian Zhang) for the series “Immunotherapy and Tumor Microenvironment” published in Journal of Thoracic Disease. The article did not undergo external peer review.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at http://dx.doi.org/10.21037/jtd.2020.03.34). The series “Immunotherapy and Tumor Microenvironment” was commissioned by the editorial office without any funding or sponsorship. CYS served as the unpaid Guest Editor of the series. The authors have no other conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.


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Cite this article as: Chan JD, Harrison AJ, Darcy PK, Kershaw MH, Slaney CY. Chimeric antigen receptor T cell therapies for thoracic cancers—challenges and opportunities. J Thorac Dis 2020;12(8):4510-4515. doi: 10.21037/jtd.2020.03.34