Local and systemic SP-D in IPF and SARD-ILD

Surfactant protein D (SP-D) is recognized as a potential biomarker for non-invasive evaluation of interstitial lung disease (ILD). Alterations in local and systemic cytokines associated with alveolar injury may mirror the underlying pathological processes in ILD. The study aimed to examine the correlation between plasma and sputum SP-D levels, lung function, and disease severity in patients suffering from ILD. SP-D concentrations were evaluated in plasma and induced sputum samples from 34 healthy controls and 84 ILD patients, including 54 with systemic autoimmune rheumatic disease-ILD (SARD-ILD) and 30 individuals with idiopathic pulmonary fibrosis (IPF). Associations with pulmonary function were examined. ILD patients showed significantly higher SP-D than controls. Both plasma and sputum levels were elevated in IPF and SARD-ILD, with plasma SP-D higher in IPF. Plasma and sputum SP-D were moderately correlated with ILD patients ( r = 0.2497, P < 0.05). Plasma SP-D was inversely associated with lung function, including reduced forced vital capacity ( r =- 0.3778, P = 0.0002) and diffusing capacity for carbon monoxide ( r =- 0.4248, P < 0.0001). These associations were significant in SARD-ILD and showed a non-significant trend in IPF. No clear link was found between sputum SP-D and pulmonary function. Both plasma and sputum SP-D levels were elevated in ILD patients; however, only plasma SP-D demonstrated a strong negative correlation with pulmonary function. This suggests that plasma SP-D, likely reflecting leakage from the injured lung interstitium into circulation, may serve as a practical tool for non-invasive disease severity assessment in routine clinical practice, potentially aiding in risk stratification and monitoring of disease progression.