Mesothelin-Targeted CARs: Driving T Cells to Solid Tumors

Abstract

Chimeric antigen receptors (CAR) are synthetic receptors that target T cells to cell-surface antigens and augment T-cell function and persistence. Mesothelin is a cell-surface antigen implicated in tumor invasion, which is highly expressed in mesothelioma and lung, pancreas, breast, ovarian, and other cancers. Its low-level expression in mesothelia, however, commands thoughtful therapeutic interventions. Encouragingly, recent clinical trials evaluating active immunization or immunoconjugates in patients with pancreatic adenocarcinoma or mesothelioma have shown responses without toxicity. Altogether, these findings and preclinical CAR therapy models using either systemic or regional T-cell delivery argue favorably for mesothelin CAR therapy in multiple solid tumors.

Significance: Recent success obtained with adoptive transfer of CAR T cells targeting CD19 in patients with refractory hematologic malignancies has generated much enthusiasm for T-cell engineering and raises the prospect of implementing similar strategies for solid tumors. Mesothelin is expressed in a wide range and a high percentage of solid tumors, which we review here in detail. Mesothelin CAR therapy has the potential to treat multiple solid malignancies.

Figure 1. Antigens targeted in solid tumor CAR T-cell therapy clinical trials

Antigens listed in the figure were compiled by search of the active clinical trials in the clincialtrials.gov. CEA: Carcinoembryonic Antigen; EGFR: Epidermal Growth Factor Receptor; FAP: Fibroblast Associated Protein; FR: Folate Receptor; GD2: Diasialoganglioside; GPC3: Glypican 3; HER2: Human Epidermal Growth Factor Receptor; L1-CAM: L1 Cell Adhesion Molecule; MSLN: Mesothelin; MUC-16: Mucin 16; VEGFR: Vascular Endothelial Growth Factor Receptor.

Figure 2. Frequency and distribution pattern of the mesothelin protein in solid malignancies

Mesothelin is expressed in a wide range of solid tumors. For most cancers, MSLN expression is homogeneously distributed on the cell-surface, with low cytoplasmic expression. For stomach and lung cancers, MSLN is frequently expressed in the cytoplasm, as well as on the cell-surface. The distribution of MSLN in cytoplasm and/or cell-surface was represented in the figure, where data was available. The frequency and distribution of MSLN were calculated from the published literature (Supplementary Table 2).

Figure 3. Mesothelin-targeted immunotherapy strategies

Several therapeutic strategies have been designed for targeting mesothelin on tumor cells (1) tumor vaccine strategy (2) antibody-based therapies (3) adoptive CAR T-cell therapy. These therapies are being evaluated in phase I and/or phase II clinical trials.

Figure 4. CAR T-cell design. (A) Structure of the CAR

The CAR typically contains a single chain fragment variable (scFv) binding domain specific for mesothelin fused to a transmembrane domain and intracellular signaling domain (CD3ζ and CD28 or 4-1BB). The CAR expressed into patient’s own T cells after transduction provides both specificity and effector function activation (B) Different generations of the MSLN CAR. Three generations of CAR T cells differing by their signaling domains have been designed to increase the activation strength of T cells.