Discussion
Pleural effusion is a common manifestation of SLE.2 3 Given multiple causes of pleural effusion, it is useful to identify biomarkers for discriminating lupus pleuritis from pleural effusion of other aetiologies. We found in the present study that pleural fluid levels of ANA, C3 and C4 are potentially useful in discriminating lupus pleuritis from pleural effusion of other aetiologies.
Circulating levels of HMGB1 were elevated in RA and SLE.9 11 23 Upregulated HMGB1 is also present in patients with infectious diseases. Several studies have reported markedly elevated serum levels of HMGB1 in patients with sepsis24 or severe sepsis,25 and a positive correlation between plasma levels of HMGB1 and organ dysfunction in septic shock.26 A prior study also reported elevated levels of HMGB1 in malignant and inflammatory pleural effusion compared with transudative pleural effusion.27 Although the median level of HMGB1 appeared higher in infection-related pleural effusion than lupus pleuritis in the present study, the elevated level was not statistically significant. sRAGE acts as a suppressor of the inflammatory response in the RAGE axis. Circulating levels of sRAGE were decreased in patients with RA and SLE.10 12 13 28 A previous study reported lower levels of pleural fluid sRAGE in patients with bacterial pneumonia compared with those with tuberculosis or lung cancer.29 We only observed a trend of higher levels of pleural fluid sRAGE with lupus pleuritis when compared with malignant pleural effusion.
ADA catalyses the deamination of adenosine, which is a crucial suppressor of the inflammation.30 Moreover, ADA is involved in the differentiation and maturation of the immune cells such as lymphocytes.31 Elevated serum ADA activity was found in patients with SLE.31 32 A previous study reported elevated ADA activities in tuberculous pleurisy compared with lupus pleuritis. Similarly, we observed a trend towards higher ADA activity in infection-related pleural effusion compared with lupus pleuritis. Taken together, ADA activity in the pleural fluid was upregulated in infection-related pleural effusion. Regarding pleural fluid cytokines, we observed a trend of a higher level of IL-17A in infection-related pleural effusion compared with lupus pleuritis. This finding is in line with its known role in bacterial infection.33
The significant biomarkers for lupus pleuritis included the higher proportion of ANA positivity and lower levels of C4. Notably, ANA positivity achieved a high sensitivity of 91%, a specificity of 83%, a positive predictive value (PPV) of 59% and a high negative predictive value (NPV) of 97% in discriminating lupus pleuritis from pleural effusion of all other aetiologies combined (data not shown). Our results are consistent with previous reports on high titre ANA (≧1:160) in pleural fluid being a sensitive but less specific indicator of lupus pleuritis,21 22 34 35 although we found the positivity for a low titre ANA (≧1: 80) having a better diagnostic performance over the other titre thresholds. Resonated with our findings, the newly updated diagnostic criteria for SLE have taken a low titre ANA (≧1: 80) as the entry criterion to improve its sensitivity.36 37 To be noted, 89% of patients with lupus pleuritis had pleural fluid/serum ANA ratio ≦1 (data not shown), which implied the origin of pleural fluid ANA being the circulating blood.
In agreement with previous studies,38–41 we revealed lower pleural fluid levels of C3 and C4 in patients with lupus pleuritis. Our results showed that either C3 or C4 level had good diagnostic performance in discriminating lupus pleuritis from infection-related or malignant pleural effusion. Furthermore, a parallel combination of C3 (<24 mg/dL) and C4 (<3 mg/dL) showed better diagnostic performance than either biomarker alone. This combination achieved a sensitivity of 82%, a specificity of 89%, a PPV of 75% and a high NPV of 93% in differentiating between lupus pleuritis and exudative pleural effusion (infection-related and malignant pleural effusion combined; data not shown). To be noted, the lupus pleuritis group had lower pleural fluid levels of C4 when compared with infection-related and malignant pleural effusion. This may be partly explained by the concomitant genetic deficiency of C4 in these patients with SLE.42 On the contrary, we demonstrated higher pleural fluid levels of C3 in lupus pleuritis when compared with fluid overload-related pleural effusion in patients both without and with SLE. The finding differed from earlier studies which had reported lower C3 levels in lupus pleuritis.38–41 However, most of these studies recruited few patients with SLE (<10), and fluid overload-related pleural effusion was under-represented in the control group. Besides, another study reported lower C4 levels in pleural effusion due to heart failure when compared with parapneumonic and malignant pleural effusion.41 Notably, we found lower serum levels of C3 in patients with SLE with fluid overload than the lupus pleuritis group (44.25 (IQR 33.05–61.25) mg/dL vs 89.8 (IQR 62.2–119) mg/dL; data not shown), which might be the result of a higher proportion of nephrotic range proteinuria in the fluid overload group. This partly explains the lower pleural fluid C3 levels in our patients with SLE with fluid overload. It was also likely that the complement components in the blood had entered the affected tissue (eg, the pleural space) only under inflammation like in exudative pleural effusion.
In our 11 patients with lupus pleuritis, only 1 (10%) had pleural fluid/serum C3 and C4 ratios >1. In addition, as demonstrated in table 2, pleural fluid levels of C3 and C4 adjusted by protein levels appeared lower in the lupus pleuritis group. Furthermore, most (78%) of them had lower protein-adjusted pleural fluid levels of C3 and C4 when compared with protein-adjusted serum levels of C3 and C4 (data not shown). These observations are in line with the results of previous studies which suggested activation of the complement cascade locally in lupus pleuritis.38 40 41 Interestingly, we found a significantly lower pleural fluid levels of C3 (14.2 (IQR 9.9–25.1) mg/dL vs 40.7 (IQR 24.1–53.0) mg/dL) and C4 (2.85 (IQR 0.95–4.80) mg/dL vs 8.0 (IQR 5.4–10.9) mg/dL) between exudative and transudative pleural effusion in our 59 patients (both p<0.001; data not shown). Their diagnostic performance in comparison with traditional Light’s criteria should be explored in the following studies.
There are some limitations to our study. First, our study is limited by the small number of patients with pleural effusion. Patients with lupus pleuritis are difficult to recruit owing to the few number of these cases in clinical practice. A larger multicentre study is required to validate our findings on biomarkers for lupus pleuritis. Nevertheless, we have recruited patients with pleural effusion due to different common aetiologies. We have also comprehensively analysed an array of potential biomarkers in these pleural fluid samples. Second, we did not recruit enough patients with SLE presenting with pleural effusion of other aetiologies. Therefore, our findings cannot be extrapolated to differentiation between autoimmune pleuritis and pleural effusion of other aetiologies in patients with SLE.