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

Summary 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.