Cell Therapy With TILs: Training and Taming T Cells to Fight Cancer

Abstract

The rationale behind cancer immunotherapy is based on the unequivocal demonstration that the immune system plays an important role in limiting cancer initiation and progression. Adoptive cell therapy (ACT) is a form of cancer immunotherapy that utilizes a patient's own immune cells to find and eliminate tumor cells, however, donor immune cells can also be employed in some cases. Here, we focus on T lymphocyte (T cell)-based cancer immunotherapies that have gained significant attention after initial discoveries that graft-versus-tumor responses were mediated by T cells. Accumulating knowledge of T cell development and function coupled with advancements in genetics and data science has enabled the use of a patient's own (autologous) T cells for ACT (TIL ACTs). In TIL ACT, tumor-infiltrating lymphocytes (TILs) are collected from resected tumor material, enhanced and expanded ex-vivo, and delivered back to the patient as therapeutic agents. ACT with TILs has been shown to cause objective tumor regression in several types of cancers including melanoma, cervical squamous cell carcinoma, and cholangiocarcinoma. In this review, we provide a brief history of TIL ACT and discuss the current state of TIL ACT clinical development in solid tumors. We also discuss the niche of TIL ACT in the current cancer therapy landscape and potential strategies for patient selection.

Keywords: T-lymphocytes; adoptive cell therapy (ACT); cell therapy; immunotherapy; response biomarkers; toxicity; tumor infiltrating lymphocyte (TIL).

Figure 1

Landmark discoveries that aided the development of T cell based ACTs.

Figure 2

General scheme for the expansion of naturally occurring TILs for use in ACTs: Protocol for the expansion of TILs for clinical use has been described in detail (15, 47). Under anesthesia, tumors are excised from patients and cut into small pieces or digested enzymatically to obtain single cell suspensions (15, 47). Tumor fragments are grown individually in high dose IL-2 (6000 IU/mL). Under the influence of IL-2, cytotoxic lymphocytes overgrow and kill tumors within 2-3 weeks (15). Cytotoxicity of pure lymphocyte cultures are tested by co-culturing IL-2 primed lymphocytes and tumor cells. Individual cultures with high toxicity against target tumors can be rapidly expanded in the presence of irradiated feeder lymphocytes, an antibody targeting the epsilon subunit within the human CD3, and IL-2. Using this approach, Rosenberg and colleagues harvested approximately 10¹¹ lymphocytes in approximately 5-6 weeks for infusion into patients. In later studies, a lymphodepletion preparative regimen consisting of 60mg/kg cyclophosphamide for 2 days and 25 mg/m² fludarabine administered for 5 days demonstrated remarkable outcome in effectiveness of ACTs. Patients were infused with cells and IL-2 at 720,000 IU/kg to tolerance after lymphodepletion (15).

Figure 3

A schematic of neoantigen discovery pipeline: Using next generation exome sequencing, exome sequences of healthy cells and tumor cell are obtained and comparative analysis results in identification of tumor associated mutations. RNA sequencing of the tumor ascertains expressed tumor variants. Appropriate in silico methods such as prediction of peptide binding to the patient’s MHC haplotypes, peptide cleavage products generated by proteosome etc. are applied to predict putative neoantigens. Use of mass spectrometry analysis of MHC-associated peptides in tandem with an in silico approach could greatly aid neoantigen prediction/discovery. Putative neoantigens are synthesized and screened for eliciting neoantigen specific T cell responses through multimer based screen or cytokine induction by peptide stimulation.

Figure 4

Potential strategy for selecting patients for TIL ACT administration. Among suggested candidates are patients with virally-infected tumors and those who acquired resistance on ICB therapy. Tumors with a high mutational burden may respond to ACT with unselected TIL, whereas tumors with poorly immunogenic tumor may benefit from identification of specific tumor neoantigens and generation of a specialized TIL-ACT products targeting those neoantigens.