Longitudinal assessment of circulating fibroblast activation protein in systemic sclerosis-associated interstitial...

Systemic sclerosis-associated interstitial lung disease (SSc-ILD) is difficult to manage due to the heterogeneous disease course. There is a high need for new biomarkers to identify patients at high risk for ILD progression. Fibroblast activation protein (FAP) has gained interest as a biomarker to reflect fibrotic activity. As circulating FAP (cFAP) can be measured in blood, we aimed to investigate the value of cFAP in SSc-ILD.

cFAP concentrations were determined in plasma samples of 210 patients with systemic sclerosis (SSc) using an enzyme-linked immunosorbent assay. We compared cFAP in baseline and repeated longitudinal samples between SSc patients with (n = 63) and without ILD (n = 147). Furthermore, we investigated the correlation between cFAP and ILD progression at follow-up. In an exploratory analysis, we also investigated if cFAP was associated with other disease-related clinical features and all-cause mortality.

cFAP levels were not different between SSc patients with- and without ILD, both at baseline (median 91.5 and 97.7 ng/mL respectively; p = 0.80) or during follow-up. Furthermore, we found no association between cFAP at baseline and ILD progression at 1, 2 and 5 years of follow-up. Notably, cFAP levels were elevated at baseline in patients with higher skin scores (mRSS ≥ 10 compared to mRSS < 10; p = 0.01). Finally, we did not find an association between cFAP and all-cause mortality at 5 and 10 years of follow-up.

In conclusion, cFAP does not seem useful as a biomarker in SSc-ILD. However, the association between cFAP and skin score deserves further investigation.