The Biology of Malignant Mesothelioma and the Relevance of Preclinical Models

Abstract

Malignant mesothelioma (MM), especially its more frequent form, malignant pleural mesothelioma (MPM), is a devastating thoracic cancer with limited therapeutic options. Recently, clinical trials that used immunotherapy strategies have yielded promising results, but the benefits are restricted to a limited number of patients. To develop new therapeutic strategies and define predictors of treatment response to existing therapy, better knowledge of the cellular and molecular mechanisms of MM tumors and sound preclinical models are needed. This review aims to provide an overview of our present knowledge and issues on both subjects. MM shows a complex pattern of molecular changes, including genetic, chromosomic, and epigenetic alterations. MM is also a heterogeneous cancer. The recently described molecular classifications for MPM could better consider inter-tumor heterogeneity, while histo-molecular gradients are an interesting way to consider both intra- and inter-tumor heterogeneities. Classical preclinical models are based on use of MM cell lines in culture or implanted in rodents, i.e., xenografts in immunosuppressed mice or isografts in syngeneic rodents to assess the anti-tumor immune response. Recent developments are tumoroids, patient-derived xenografts (PDX), xenografts in humanized mice, and genetically modified mice (GEM) that carry mutations identified in human MM tumor cells. Multicellular tumor spheroids are an interesting in vitro model to reduce animal experimentation; they are more accessible than tumoroids. They could be relevant, especially if they are co-cultured with stromal and immune cells to partially reproduce the human microenvironment. Even if preclinical models have allowed for major advances, they show several limitations: (i) the anatomical and biological tumor microenvironments are incompletely reproduced; (ii) the intra-tumor heterogeneity and immunological contexts are not fully reconstructed; and (iii) the inter-tumor heterogeneity is insufficiently considered. Given that these limitations vary according to the models, preclinical models must be carefully selected depending on the objectives of the experiments. New approaches, such as organ-on-a-chip technologies or in silico biological systems, should be explored in MM research. More pertinent cell models, based on our knowledge on mesothelial carcinogenesis and considering MM heterogeneity, need to be developed. These endeavors are mandatory to implement efficient precision medicine for MM.

Keywords: animal models; cell models; mesothelioma; molecular characteristics; preclinical models; thoracic cancer; tumor heterogeneity.

Figure 1

Available malignant mesothelioma (MM) preclinical models that use tumor samples or cell lines. Malignant samples are tumor fragments or more often MM cells obtained after tissue dissociation; these samples are used in vitro or transplanted/inoculated in immunosuppressed or immuno-compatible rodents, almost exclusively mice. In vitro, two-dimensional (2D) cultures or three dimensional (3D) spheroids can be grown from tumor tissue samples. In vivo, MM tumor cells in culture are inoculated either subcutaneously or orthotopically (intracavitary, in the pleura or peritoneum) to generate cell-derived xenograft (CDX) or isograft. Tumor fragments can be also engrafted into immunosuppressed mice (patient-derived xenograft [PDX]). Immunocompetent models mainly comprise syngeneic rodent models. Human immunocompetent models can be obtained using NOD-scid IL2Rγ^null (NSG) mice.

Figure 2

Evaluation of treatments using preclinical models of malignant mesothelioma (MM). Anti-cancer agents are delivered to the MM models, either in a native, or vectorized form (in nanoparticles, liposomes, or viruses). MM cells may be labeled or engineered to allow one to determine tumor growth by imaging methods, mostly bioluminescence. To determine the host response, the analytical method depends on the MM model. In vivo endpoints comprise measuring the tumor volume, quantifying and identifying tumor cells in the pleural effusion or ascites (if produced), or measuring others parameters (angiogenesis, immune infiltration, etc.). In vitro, the endpoints are cell viability, type of death, proliferation, motility, invasion, morphology, and volume (spheroids). Both cell and mouse models have been applied to test the efficiency of drugs in mono- or multimodality chemotherapies, immunotherapies and target therapies, oncovirotherapies, or cell and gene therapies. SPECT, single photon emission computed tomography; IPe, intra-pleural; Ipl: intra-peritoneal; PDX, patient-derived xenograft.

Figure 3

In vivo models of malignant mesothelioma (MM) in asbestos-treated rodents and genetically modified mice (GEM). MM can be obtained from wild type (WT) rats or mice, exposed to asbestos, or GEM. Asbestos-induced MM models are generated by the injection of asbestos fibers intracavitary, in the pleura or peritoneum, mostly peritoneal in mice. Conditional mutant mice are obtained by engineering the major genes altered in MM. Asbestos-recipient animals and GEM may be investigated for tumor incidence, survival, quantitative and qualitative analyses of cells in ascites or pleural fluid, and molecular alterations in tumors.