Reply: Circulating Fibrocytes as a Biomarker of Prognosis in Hermansky-Pudlak Syndrome

From the Author:

We thank Dr. Song for his interest in our work (1). We agree that a number of factors, among them serum amyloid P and low-molecular-weight heparins, can influence fibrocyte differentiation from circulating cells under cell culture conditions. Meaningful extrapolation from such in vitro experiments to in vivo fibrocyte biology, however, is problematic: first because the in vitro conditions are very different from the conditions fibrocytes encounter in vivo, and second because serum amyloid P and heparins have complex and manifold in vivo functions that are not captured in culture. It is therefore far from clear that these factors are relevant to the biology of human fibrocytes during disease states in general, and whether they have any influence on the concentration of circulating fibrocytes in interstitial lung diseases in particular. We thus contend that although these considerations may be interesting avenues for future investigation, they do not materially affect the conclusions of our manuscript; namely, that circulating fibrocyte concentrations predict survival in patients with Hermansky-Pudlak syndrome and may be useful as a biomarker for outcome of interstitial lung disease.