Anti-Mesothelin CAR T cell therapy for malignant mesothelioma

Abstract

Malignant mesothelioma (MM) is a treatment-resistant tumor originating in the mesothelial lining of the pleura or the abdominal cavity with very limited treatment options. More effective therapeutic approaches are urgently needed to improve the poor prognosis of MM patients. Chimeric Antigen Receptor (CAR) T cell therapy has emerged as a novel potential treatment for this incurable solid tumor. The tumor-associated antigen mesothelin (MSLN) is an attractive target for cell therapy in MM, as this antigen is expressed at high levels in the diseased pleura or peritoneum in the majority of MM patients and not (or very modestly) present in healthy tissues. Clinical trials using anti-MSLN CAR T cells in MM have shown that this potential therapeutic is relatively safe. However, efficacy remains modest, likely due to the MM tumor microenvironment (TME), which creates strong immunosuppressive conditions and thus reduces anti-MSLN CAR T cell tumor infiltration, efficacy and persistence. Various approaches to overcome these challenges are reviewed here. They include local (intratumoral) delivery of anti-MSLN CAR T cells, improved CAR design and co-stimulation, and measures to avoid T cell exhaustion. Combination therapies with checkpoint inhibitors as well as oncolytic viruses are also discussed. Preclinical studies have confirmed that increased efficacy of anti-MSLN CAR T cells is within reach and offer hope that this form of cellular immunotherapy may soon improve the prognosis of MM patients.

Keywords: CAR T cells; Cancer; Immunotherapy; Malignant mesothelioma; Malignant pleural mesothelioma; Mesothelin; Tumor microenvironment.

Fig. 1

a Maturation of Mesothelin (MSLN). MSLN is initially expressed on the membrane of cancer cells as a 71 kDa precursor protein which is cleaved by the endoprotease Furin to release the 31 kDa Megakaryocyte Potentiating Factor (MPF). The mature 40 kDa-MSLN remains bound to the cell membrane via a glycophosphatidylinositol (GPI) anchor. Surface MSLN can also be released from the GPI anchor by proteases, resulting in a soluble form of MSLN (soluble MSLN-related peptide - SMRP). b Anti-MSLN CAR Generations. The scFv of MSLN CAR T cells binds membrane-bound MSLN and the signaling depends on the CAR generation used. The endodomain of first generation CARs contains CD3ζ only; in second generation CARs, there is the addition of a single costimulatory domain (often CD28 or 4–1-BB); and in third generation CARs, two costimulatory domains are incorporated

Fig. 2

Barriers to MSLN CAR T cell activity in MM (in red) and strategies to overcome them (in green). a Low tumor infiltration caused by the physical barrier of the stroma in MM could be overcome by intrapleural or intraperitoneal delivery of anti-MSLN CAR T cells or by expressing the CCR2b chemokine receptor on anti-MSLN CAR T cells, which attracts them to the tumor. Combination with a vaccinia virus expressing CXCL11 may also increase anti-MSLN CAR T cell trafficking to the tumor. b Soluble immunosuppression mediators prostanglandin E (PGE2), adenosine and TGF-β contribute to the reduction of anti-tumor activity of anti-MSLN CAR T cells. Strategies that successfully rescued anti-MSLN CAR T cell anti-tumor activity include the knock out (KO) of receptors for TGFβ (TGFBR2) and adenosine (A_2AR), the deletion of diacylglycerol kinase (dgk), and the insertion of a ‘regulatory subunit I anchoring disruptor’ (RIAD) in anti-MSLN CAR T cells. The combination of anti-MSLN CAR T cells with TGF-β-targeting oncolytic viruses and TNFα-IL2-producing oncolytic viruses has also helped enhance their efficacy. c Exhaustion due to PD-1/PD-L1 signaling can be counteracted via knock out of PD-1 via CRISPR/Cas9 or by inserting a PD-1 DNR or a PD-1/CD28 switch receptor. Engineering of CAR T cells to secrete anti-PD-1 antibodies has also been studied. d Anti-MSLN CAR T cells with improved CAR design have been shown to have increased persistence and efficacy in mouse models. e CAR T safety can be improved by limiting ‘on target/off tumor’ toxicities through the introduction of a suicide switch in anti-MSLN CAR T cells or the use of mRNA anti-MSLN CAR with transient expression