Characterisation of the Tumour Immune Microenvironment and Mechanisms of Checkpoint Blockade in Mesothelioma

Malignant pleural mesothelioma (MPM) is a lethal cancer causally associated with asbestos exposure. MPM carries a dismal prognosis and few effective treatment options currently exist. Immune checkpoint blockade (ICB) immunotherapies have repeatedly demonstrated efficacy in clinical trials for MPM since 2017 and have received regulatory approval in many industrialised countries since 2020. Despite its efficacy, the mechanisms behind these ICB responses remain unknown, partially because many canonical pan-cancer biomarkers are less reliable in MPM, but also due to a scarcity of translational data. This project therefore

aimed to characterise MPM’s complex tumour immune microenvironment and elucidate mechanisms of response and resistance to ICB, focussing on tumour-associated macrophages (TAMs) and cytotoxic T lymphocytes (CTLs). Bioinformatic deconvolution algorithms, a syngeneic mouse model and multiplex fluorescent immunohistochemistry were used to quantify these key leukocytes and evaluate their clinical impact. Deconvolution-defined TAMs were associated with poor clinical outcomes and lower CTL infiltration, though inter-algorithm discrepancies emerged. Murine mesotheliomas were dominated by myeloid cells, particularly macrophages, while anti-tumour lymphocytes were scarce, indicating failure to recapitulate the complex microenvironment of human MPM, which discouraged ICB experiments. However, multiplex staining of biopsies obtained before nivolumab administration revealed several differentiators of clinical response, including high CTL density within programmed death ligand 1 (PD-L1)+ tumours and tertiary lymphoid structures (TLS), especially mature TLS with distinctively zoned secondary follicles. TLSs also epitomised a ‘hot’ microenvironment, existing primarily in tumours with substantial anti-tumour infiltrates of B lymphocytes and naïve T lymphocytes. These latter findings shed light on mechanisms of ICB response in MPM in agreement with evidence from lung cancer and melanoma, further suggesting actionable candidate biomarkers for future patient stratification. Being the first histopathological data from ICB-treated MPM, and the first data implicating TLS in MPM drug response, these results should be validated in future patient cohorts and could be refined further when supplemented with connected ‘omics data.