Poor prognostic factors for relapse of interstitial lung disease in microscopic polyangiitis: the Japanese multicentre REVEAL cohort study

This study investigated poor prognostic factors for the relapse of interstitial lung disease (ILD) in patients with microscopic polyangiitis (MPA) after remission induction therapy. We enrolled patients diagnosed with MPA complicated by ILD according to the Chapel Hill Consensus definition from 2001 to 2023 in multiple institutions in the REVEAL cohort. All patients who were treated with immunosuppressive therapy were followed up, and those who relapsed with ILD were extracted in this study. We explored the risk factors for predicting ILD relapse in patients with MPA-ILD by comparing the demographic, clinical, laboratory, and radiological findings and treatments between the relapsed and non-relapsed groups on admission. Of 243 patients with MPA, 134 (55.1%) with MPA-ILD were enrolled. Among them, 28 (20.9%) relapsed during a mean follow-up of 4.2 years. The initial serum Krebs von den Lungen-6 (KL-6) and surfactant protein-D (SP-D) levels and the prevalence of usual interstitial pneumonia (UIP) pattern were significantly higher in the relapsed group. The biomarkers were also risk factors for relapse in multivariate Cox regression analysis. The best cut-off values of KL-6, SP-D for predicting ILD relapse were 430 U/mL and 89.5 ng/mL, respectively. We created prediction models based on the best cut-off values for KL-6, SP-D, and the presence of the UIP pattern (KSU model). The 10-year relapse rate was significantly different among patients with MPA-ILD stratified by the number of risk factors based on the KSU model. A higher relapse rate was associated with higher all-cause mortality. The initial serum high KL-6 and SP-D levels and the prevalence of the UIP pattern were associated with ILD relapse in patients with MPA-ILD. Our multicentre cohort study indicated that the KSU model, which consists of KL-6 ≥ 430 U/mL, SP-D ≥ 89.5 ng/mL, and the presence of the UIP pattern, is a useful predictor of ILD relapse in patients with MPA after immunosuppressive therapy.