Background/Aims: Tuberculous pleural effusion (TPE) is the second most common form of extrapulmonary tuberculosis (TB) and a common cause of pleural effusion in endemic tuberculous areas. However, it is still a difficult problem to make a precise and quick diagnosis of TPE from a parapneumonic pleural effusion (PPE). In this study, we evaluated myeloid-derived suppressor cells (MDSC) in the pleural fluid to distinguish TPE from PPE

Method: Patients diagnosed with pleural effusion were enrolled from January 2016 to February 2018. We analyzed various immune cells including T, B, NK, and MDSC in pleural fluid and peripheral blood.

Results: The frequency of lymphocytes (T, B, NK cells) in the pleural fluid was significantly higher in TPE than in PPE. In contrast, the frequency of polymorphonuclear (PMN)-MDSC in the pleural fluid was significantly lower in TPE compared to PPE. There was no significant difference in immune cell frequency in blood between TPE and PPE. An area under the ROC curve of 0.92 was obtained using PMN-MDSC frequency as the indicator for TPE identification. PMN-MDSC in PPE produced a high level of reactive oxygen species and suppressed the production of interferon-gamma from T cells after non-specific stimulation.

Conclusions: The levels of MDSCs in TPE were significantly lower than that in PPE. Therefore, the frequency of MDSCs in pleural fluid can be utilized to distinguish TPE from PPE.