Objective: To study the pathological process and characteristics of bleomycin (BLM)-induced interstitial lung disease (ILD) model and collagen-induced arthritis (CIA) complicated by BLM-induced ILD (CIA-ILD) model in mice and explore the correlations with circulating fibrocytes.
Methods: Ninety mice were randomly divided into normal saline group (S group), BLM group (B group), CIA-BLM group (CB group), with 30 mice in each group. On the 2, 7, 14, 21 and 28 days after BLM challenge, mice of each group were sacrificed. HE staining was used to detect the degrees of acute inflammation and sirius red staining to detect lung fibrosis. Immunohistochemicstry was adopted to detect alpha-smooth muscle actin expression. The peripheral blood was obtained to count the proportion of circulating fibrocytes (CD45⁺COL1⁺) by flow cytometry. The correlations between circulating fibrocytes and pathological changes of ILD were analyzed statistically.
Results: Both of the ILD model groups showed the dynamic process from pulmonary alveolitis to fibrosis gradually. Inflammatory lesion in the lungs of B group was the most obvious on the 7-14th day, and then there was a slow rehabilitation process on the 21th and 28th day. CB group began to present alveoli destruction and collagen deposition on the 14th day, and the inflammation was the most severe on the 28th day. Pulmonary hydroxyproline content was obviously ascended in CB group as compared with B group (P<0.05) after 14 days. An increase in the total number of circulating fibrocytes was observed in both B and CB groups. It tended to increase at first, and subsequently declined. The level of circulating fibrocytes was higher in CB group than in B group on day 14, 21 and 28 (P<0.05). The immunohistochemical results showed that there were many myofibroblasts infiltrating in CB group from the 14th day to 28th day. The number of circulating fibrocytes in peripheral blood was positively correlated with pulmonary inflammation, fibrosis degree and hydroxyproline content (r=0.847, 0.826, 0.735, P<0.01).
Conclusion: The pathological process of CIA-ILD animal model is more close to rheumatoid arthritis-induced ILD development. Circulating fibrocytes may be involved in the inflammation and fibrosis progression of lung.