Association of immunoglobulin G4 and free light chain with idiopathic pleural effusion

Abstract

The cause of pleural effusion remains uncertain in approximately 15% of patients despite exhaustive evaluation. As recently described immunoglobulin (Ig)G4-related disease is a fibroinflammatory disorder that can affect various organs, including the lungs, we investigate whether idiopathic pleural effusion includes IgG4-associated etiology. Between 2000 and 2012, we collected 830 pleural fluid samples and reviewed 35 patients with pleural effusions undiagnosed after pleural biopsy at Yamaguchi-Ube Medical Center. Importantly, IgG4 immunostaining revealed infiltration of IgG4-positive plasma cells in the pleura of 12 patients (34%, IgG4⁺ group). The median effusion IgG4 level was 41 mg/dl in the IgG4⁺ group and 27 mg/dl in the IgG4^- group (P < 0·01). The light and heavy chains of effusion IgG4 antibodies of patients in the IgG4⁺ group were heterogeneous by two-dimensional electrophoresis, indicating the absence of clonality of the IgG4 antibodies. Interestingly, the κ light chains were more heterogeneous than the λ light chains. The measurement of the κ and λ free light chain (FLC) levels in the pleural fluids showed significantly different κ FLC levels (median: 28·0 versus 9·1 mg/dl, P < 0·01) and κ/λ ratios (median: 2·0 versus 1·2, P < 0·001) between the IgG4⁺ and IgG4^- groups. Furthermore, the κ/λ ratios were correlated with the IgG4⁺ /IgG⁺ plasma cell ratios in the pleura of the IgG4⁺ group. Taken together, these results demonstrate the involvement of IgG4 in certain idiopathic pleural effusions and provide insights into the diagnosis, pathogenesis and therapeutic opportunities of IgG4-associated pleural effusion.

Keywords: IgG4-related disease; fibrinous pleuritis; free light chain; pleural effusion.

Figure 1

Histopathological features of the parietal pleura of patients with idiopathic pleural effusion. (a,b) Fibrous thickening of the pleura and prominent lymphoplasmacytic infiltrate in the subpleural fibrous and adipose tissue (haematoxylin and eosin staining, magnification ×100). (c–h) Immunostaining for immunoglobulin (Ig)G or IgG4, magnification ×200. (a,e,f) Case 1, effusion IgG4: 1133·1 mg/dl. (b,g,h). Case 2, effusion IgG4: 20·3 mg/dl (Table 2).

Figure 2

Comparison of pleural fluid levels of immunoglobulins between the immunoglobulin (Ig)G4⁻ and IgG4⁺ groups. (−), IgG4⁻ group; (+), IgG4⁺ group. Median and interquartile ranges are shown.

Figure 3

Analysis of the clonality of the effusion immunoglobulin (Ig)G4 antibodies of patients in the IgG4⁺ group by two‐dimensional electrophoresis (2‐DE). Control IgG4 κ myeloma protein from Sigma (cat no. I4639) (a –c). Effusion IgG4 antibodies of representative cases with abnormal IgG4 levels (d–i). The H and L chains were probed with anti‐IgG4‐Fc (left), anti‐κ chain (middle) and anti‐λ chain (right) antibodies.

Figure 4

Comparison of pleural fluid levels of the κ and λ free L chains (FLC) (a,b) and κ/λ ratio (c). Median and interquartile ranges are shown. (−), Immunoglobulin (Ig)G4⁻ group; (+), IgG4⁺ group.

Figure 5

Receiver operating characteristic (ROC) analysis on diagnostic utility of immunoglobulin (Ig)G4 and κ/λ ratio for distinguishing patients between the IgG4⁻ and IgG⁺ groups. Cut‐off value for κ/λ ratio, 1·42; sensitivity, 0·87; specificity, 0·83. Area under the curve (AUC), 0·88; 95% confidence interval (CI) for the AUC, 0·74 – 1·00. Cut‐off value for IgG4/IgG ratio, 2·75%; sensitivity, 0·75; specificity, 0·74; AUC, 0·80; 95% CI for the AUC, 0·66–0·94.

Figure 6

Correlation of the effusion κ/λ free L chains (FLC) ratio with immunoglobulin (Ig)G4⁺ plasma cell counts (a) and IgG4⁺/IgG⁺ plasma cell ratio (b) in the pleura of patients in the IgG4⁺ group. *One‐tailed P‐value.