Background Because of the extremely variable incidence and outcome of interstitial lung disease (ILD) in polymyositis/dermatomyositis (PM/DM), exploration and validation of biomarkers for diagnosis, prognosis, and response to treatment is mandatory. Lung epithelium-derived proteins such as Krebs von den Lungen-6 (KL-6) and surfactant protein-D (SP-D) are used as serum biomarkers for ILD. However, their usefulness in patients with PM/DM is not fully established.

Objectives We aimed to evaluate the usefulness of KL-6 as a biomarker of ILD with PM/DM, including that in the short time course after treatment.

Methods A total of 42 patients with active PM/DM who were admitted to our hospital from April 2010 through March 2015 were included: 22 with PM, 14 with classical DM, and 6 with clinically amyopathic dermatomyositis (CADM). PM/DM and CADM were diagnosed according to Bohan and Peter's criteria and Sontheimer's criteria, respectively. Overlap patients with other collagen tissue diseases were excluded to avoid potential confounding. ILD was defined as presence of a diffuse parenchymal lung disease on high-resolution CT such as reticular opacity, ground-glass opacity, and honeycombing. Clinical, radiological, and laboratory data were retrospectively collected from the medical records and statistically analyzed.

Results Of the 42 study subjects, 30 patients had ILD on high-resolution CT: 24 with PM/classical DM, and 6 with CADM. There was no significant difference in age, sex, or disease duration between patients with and without ILD. On admission (pre treatment), % vital capacity (%VC) and % diffusing capacity of the lungs for carbon monoxide (%DLCO) were not significantly different between patients with and without ILD (p =0.11 and 0.06, respectively). However, serum KL-6 and SP-D levels were already significantly higher in patients with ILD than in those without (p <0.01 in both comparisons). In addition, levels of serum KL-6 and SP-D showed moderate inverse correlations with %VC and %DLCO (r = -0.45 to -0.51). All the patients with ILD were treated with glucocorticoid. In addition, calcineurin inhibitors, namely cyclosporine and tacrolimus, and intravenous cyclophosphamide were administered simultaneously in 22 and 8 patients, respectively. Although 12 patients showed some clinical and/or radiological improvement in 4 weeks, levels of serum KL-6 were not significantly different between those at pre treatment and at 2 or 4 weeks post treatment (p =0.78 and 0.81, respectively). Concordance rate between clinical or radiological amelioration and decrease in serum KL-6 levels after treatment was low (Cohen's κ = -0.39).

Conclusions The present study validated that serum KL-6 is a useful biomarker for the diagnosis of ILD associated with PM/DM. However, the levels of serum KL-6 did not respond immediately by treatment even in the cases with clinical improvement. Serum KL-6 should be used as a biomarker for ILD with PM/DM while understanding these strengths and limitations.

Disclosure of Interest None declared