Tumor microenvironment heterogeneity and novel therapeutic approaches in pleural mesothelioma

Pleural mesothelioma (PM) is a rare and aggressive cancer with a poor prognosis, primarily caused by asbestos exposure and originating from the mesothelial cells of the pleura. For almost two decades, the standard first-line treatment for unresectable PM has involved a combination of cisplatin or carboplatin with pemetrexed. More recently, immunotherapy with immune checkpoint inhibitors (ICIs) has emerged as a frontline option, especially for the sarcomatoid subtype. However, despite these advancements, the benefits remain limited, with only a subset of patients responding to ICIs and a modest median overall survival improvement. Consequently, a new treatment paradigm combining chemotherapy with ICIs is being explored in various clinical trials, although results have been disappointing so far. The underlying biological mechanisms driving this general lack of response remain poorly understood, with emerging evidence nevertheless pointing to the crucial influence of the tumor microenvironment.
In this context, this thesis project aimed to deepen our understanding of the immune landscape in PM, with a particular focus on tumor-associated macrophages (TAMs), using single-nucleus RNA sequencing of PM biopsies. Simultaneously, the project sought to develop innovative therapeutic strategies by exploring two approaches: re-programming monocytes with epigenetic modulators and evaluating the synergistic potential of metronomic chemotherapy (mCT) combined with doublet immunotherapy in preclinical models.
Through the snRNA-seq analysis of PM patient biopsies, the main immune cell types previously described in PM were identified, and the characterization of PM TAMs was significantly expanded. Notably, an association between hypoxic TAMs and poor survival in PM patients was identified. TGF-β3 emerged as a potential driver of the hypoxic TAM phenotype, suggesting novel therapeutic approaches. Moreover, comparative analysis with non-tumoral macrophages revealed a global angiogenic signature among TAMs, with CD93 and SPP1 identified as potential therapeutic targets.
In a complementary study, it was demonstrated that blood-derived monocytes exhibited inherent cytotoxic activity against PM cells, at a low level. Treatment with valproic acid (VPA), a histone deacetylase inhibitor, significantly enhanced this cytotoxicity as well as monocyte migration and their aggregation with tumor cells. Additionally, VPA down-regulated key receptors associated with an M2-like phenotype, such as CD163, CD206, and CD209, suggesting VPA as an interesting compound to improve PM treatment outcomes, through modulation of monocyte cytotoxicity.
Lastly, a preclinical study demonstrated that various chemotherapeutic agents administered on a metronomic schedule effectively reduced tumor growth in syngeneic preclinical PM models. Surprisingly, mice treated with a combination of ICIs and metronomic cisplatin and pemetrexed demonstrated heterogeneous responses, with stabilization of tumor growth observed in some cases. Notably, this variability in response did not correlate with differences in immune cell infiltration at the end of treatment, underscoring the necessity for further investigation into the underlying mechanisms driving this response variability.