Malignant pleural effusion: tumor-host interactions unleashed

Abstract

Malignant pleural effusion (MPE) poses a significant clinical problem. Current nonetiologic management is suboptimal in terms of efficacy and safety. In light of recent research progress, we propose herein a new view of MPE development, which may rapidly translate into meaningful changes in therapeutics. In addition to tumor-induced impairment of pleural fluid drainage, pertinent findings point toward another pathway to MPE formation: a vicious loop of interactions between pleural-based tumor cells and the host vasculature and immune system that results in increased net fluid production via enhanced plasma extravasation into the pleural space. The ability of tumor cells to trigger this cascade likely rests on a specific and distinct transcriptional repertoire, which results in important vasoactive events in the pleural space. Although the characterization of tumor-derived factors responsible for MPE development is in the making, an additional, indirect path to MPE was recently demonstrated: tumor cells recruit and co-opt host cells and mediators, which, in turn, amplify tumor cell-primed fluid leakage and impact tumor cell functions. Importantly, recent evidence suggests that the biologic events that culminate in clinical MPE are likely amenable to therapeutic inhibition and even prevention. In this perspective, the scientific basis for an update of current concepts of MPE formation is highlighted. Key questions for future research are posed. Finally, a vision for novel, effective, safe, and convenient treatment modalities that can be offered to outpatients with MPE is set forth.

Figure 1. A revised concept of malignant pleural effusion (MPE) pathogenesis.

Primary or metastatic pleural tumor cells coexist with mesothelial, endothelial, myeloid lymphoid and other cells. Oncogene signals and/or transcription factor activation in tumor cells determine paracrine gene expression. The balance between vasoactive mediators (e.g., VEGF, TNF, CCL2, OPN, etc) and possible protective molecules (e.g., endostatin) in the pleural space dictates the occurrence of vasoactive signaling with subsequent MPE development. Moreover, this signal cocktail determines further host cell activation and recruitment. In turn, resident and incoming host cells exert a multitude of functions, including direct effects on tumor cells (transcription factor stimulation; rejection, tumor promotion, immunoediting and/or tumor escape) and indirect effects on the pleural vasculature, immune cell populations, and mesothelium to further impact inflammation, angiogenesis, vascular leakage, and/or intrapleural metastasis with establishment of additional pleural-based tumor foci.