Abstract 3715A: Effusions of mesothelioma patients: What's in it for immunotherapy
Authors: Marcq Elly , Waele Jorrit De , Audenaerde Jonas van , Lion Eva , Santermans Eva , Hens Niel , Pauwels Patrick , Meerbeeck Jan P. van , Smits Evelien L. 
Published: July 2017

Abstract
Malignant pleural mesothelioma (MPM) is an aggressive cancer with a poor prognosis for which new therapeutic strategies are available. Data derived from a small number of mesothelioma patients suggest that blocking immune checkpoints could offer new treatment opportunities. Gaining more insight in the immunological aspect of the tumor microenvironment (TME) in MPM is essential to develop an effective immunotherapy. In this context, we investigated expression of the immune checkpoint molecules TIM-3, LAG-3, PD-1 and its ligand PD-L1 on cells in human pleural (collected via thoracocentesis, n=6) and ascites (collected via paracentesis, n=5) MPM fluids and identified different subsets of immune cells in the TME using multicolor flow cytometry.
Different types of immune cells in the fluids could be detected, with predominant occurrence of CD3+CD4+ T cells, CD64+ macrophages and CD11c+CD303+ dendritic cells (DCs). CD3+CD8+ T cells, CD3-CD56+ natural killer (NK) cells and CD19+ B cells were also present to a lesser extent. While CD4+ and CD8+ T cells and CD64+ macrophages could be detected in all samples, this was not the case for the other cell types. Ascites fluids contained more podoplanin+ (PDPN) tumor cells compared to pleural fluids (average 3.0% vs 0.3%). PD-1+ T cells and NK cells could be detected in all pleural and 80% of the ascites samples. 64% of all samples contained LAG-3-expressing cells, most frequently in pleural fluids. In all samples CD4+ T-cells and NK cells showed TIM-3 expression and in 82% of the samples TIM-3 was also expressed on CD8+ T-cells. In all ascites samples PD-L1 was expressed on DCs, B cells, macrophages and PDPN+ MPM tumor cells. PD-L1 was also expressed on DCs in all pleural samples while in 67%, 83% and 83% of those samples CD19+, CD64+ and PDPN+ cells were PD-L1+, respectively. Based on the mean fluorescence intensity PDPN+ tumor cells showed the highest PD-L1 expression in both fluids.
In conclusion, we provide a detailed analysis of a diversity of immune cells present in MPM fluid samples. We are the first to demonstrate TIM-3 and LAG-3 expression in mesothelioma effusions. Statistical analysis is ongoing to investigate whether there are differences between both fluid types and whether there are associations between the cellular composition of the fluids and survival of the patients. The finding of expression of PD-1, PD-L1, TIM-3 and LAG-3 on immune cells in MPM fluids warrants further investigation of the effect of immune checkpoint blockade in MPM, with TIM-3 and LAG-3 as interesting new targets.
Citation Format: Elly Marcq, Jorrit De Waele, Jonas van Audenaerde, Eva Lion, Eva Santermans, Niel Hens, Patrick Pauwels, Jan P. van Meerbeeck, Evelien L. Smits. Effusions of mesothelioma patients: What's in it for immunotherapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3715A. doi:10.1158/1538-7445.AM2017-3715A