Background Circulating fibrocytes express both hematopoietic (CD45+ CD34+), and mesenchymal markers (Collagen+). The circulating “immature” fibrocytes can differentiate to “mature” fibrocytes in the target organ and there produce fibrosis components. Chronic inflammatory stimuli mediate differentiation of fibrocytes in diseased organs as observed in idiopathic pulmonary fibrosis (IPF) and asthma, where the levels of circulating fibrocytes correlated to disease severity. In Rheumatoid Arthritis (RA), interstitial lung disease (ILD) is an fibrotic disease manifestation with limited treatment options.
Objectives 1) To demonstrate that circulating CD45+ CD34+ CD11b+ cells can express Procollagen and thereby show that these markers can be used to identify circulating fibrocytes. 2) To measure the levels of CD45+, CD34+, CD11b+ cells in patients with RA, Fibrotic ILDs, severe asthma and in healthy controls (HC). 3) To compare the levels with number of peripheral blood mononuclear cells (PBMC) that differentiate to mature fibrocytes in vitro.
Methods CD45+, CD34+, CD11b+ cells from 4 patients were isolated by cell sorting, and stained for Procollagen. 30 patients (10 of each RA, fibrotic ILDs: IPF or non-specific interstitial pneumonitis, severe asthma) and 10 HC were included. Current medication, disease activity, lung function and radiographic data were collected. In 100 μL of lysed blood cells were enumerated by flowcytometry. Further, PBMC were isolated and cultured for 5 days. Coverslips from two wells were stained and mature fibrocytes counted and reported as fibrocytes per 106 PBMC originally added to the well.
Results A higher fraction of sorted CD45+ CD34+ CD11b+ cells stained positive for Procollagen, compared to CD45+ CD34+ CD11b- cells. All groups had higher level of CD45+ CD34+ CD11b+ cells compared to HC (all p<0.05). HC had a median of 1.02 cells/μL (IQR 0.71–1.32), RA 2.38 (IQR 1.93–4.39), asthma 3.45 (IQR 2.25–8.08) and fibrotic ILDs 5.89 (IQR 2.86–10.2). The number of CD45+ CD34+ CD11b+ cells correlated to number of PBMC that in vitro differentiated to mature fibrocytes, overall (r=0.62, p=0.0006), notably in the RA group (r=0.92, p=0.0005), the level of CD45+ CD34+ CD11b- cells did not.
Conclusions This study supports that the markers CD45+ CD34+ CD11b+ can be used to identify immature fibrocytes, as they can express Procollagen and are more abundant in fibrotic diseases compared to HC. Further, the level correlates to the number of PBMC that differentiate to mature fibrocytes in vitro. The results indicate that circulating CD45+, CD34+, CD11b+ cells could be involved in the development of organ fibrosis, and thereby be a biomarker of early fibrosis development in RA ILD. Associations to inflammatory markers, pulmonary function and disease activity will be presented at the conference.
Disclosure of Interest None declared
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