Higher Levels of Circulating Fibrocytes Are Associated with Lower Oxygen Saturation in Adults with Sickle Cell Disease

Background: In patients with sickle cell disease (SCD), tissue injury due to vaso-occlusion can result in fibrosis and organ dysfunction. Fibrocytes are circulating bone marrow-derived cells that can home to damaged organs, differentiate into fibroblasts and myofibroblasts and contribute to scarring. Fibrocytes have been implicated in the pathogenesis of pulmonary fibrosis in several animal models, including a mouse model of SCD. A preliminary study of patients with SCD suggests that the concentration of circulating fibrocytes is increased compared to controls. To build upon these findings, we tested the hypothesis that in adults with SCD higher levels of fibrocytes are associated with measures of lung disease.

Methods: In a prospective cohort study of steady state adults with SCD, peripheral blood fibrocytes were measured and subjects underwent a pulmonary assessment that included pulmonary function tests, resting pulse oximetry, a 6-minute walk test, 2-D echocardiography, and high-resolution chest CT. A control group of healthy, African Americans also provided fibrocyte measurements and had pulmonary function testing. Fibrocytes were identified as collagen-1+ CD45+ cells using quantitative flow cytometry. For statistical analyses, fibrocyte values were log-transformed. Continuous and dichotomous variables were tested with Pearson’s correlation and Mann-Whitney U, respectively.

Results: The cohort was comprised of 47 adults with SCD and 19 healthy African American controls. Sixty percent of the SCD subjects were HbSS or HbS-β-thalassemia⁰. Mean age (SD) of SCD cases was 35 years (±11) and 70% were female. Twenty-seven percent of SCD cases had asthma and 41% had a significant smoking history. Sixty-four percent of SCD cases had ground glass opacities on CT of the lung, 34% had bronchiectasis and 20% had evidence of pulmonary fibrosis. Compared to controls, patients with SCD had lower forced expiratory volume in 1 second (p<0.001), forced vital capacity (p<0.001), and pre- and post-bronchodilator 25-75% forced expiratory flow (FEF25-75%) (p=0.006). Fibrocyte levels also trended higher in patients with SCD compared to controls (median 1.2 x 10⁶ vs. 8.5 x 10⁵, p=0.07). Within patients with SCD, most fibrocytes expressed the chemokine receptor CXCR4 and smaller subsets expressed CCR2 or CCR7. Increased levels of fibrocytes in patients with SCD were associated with lower oxygen saturation (p=0.01) and higher reticulocyte counts (p=0.01). Lower oxygen saturation and higher reticulocyte counts were also associated with each other, independent of fibrocytes (p=0.007). Fibrocytes expressing CXCR4, a chemokine that mediates homing to the lungs, were associated with lower post-bronchodilator FEF25-75% (p=0.04). There was no association between fibrocyte levels and other measures of pulmonary function testing, degree of fibrosis on high-resolution CT, or abnormalities on 2-D echocardiogram or 6-minute walk.

Conclusion: In adults with SCD, higher fibrocyte levels were associated with lower resting oxygen saturation and FEF25-75%, a measure of lung obstruction. Significant lung pathology was observed in a high percentage of SCD patients. The associations between fibrocyte levels, hypoxia and a measure of pulmonary dysfunction suggest that fibrocytes contribute to this lung disease. Higher reticulocyte counts in patients with increased fibrocytes may be due to the association of both reticulocyte count and fibrocyte level with hypoxia. Ongoing longitudinal studies may better define the relationship between fibrocytes and evolving lung disease. If additional evidence is found in these longitudinal studies, fibrocytes could be a therapeutic target to prevent chronic pulmonary disease in adults with SCD.