Galectin-3 Is a Mediator of Pulmonary Fibrosis in Sickle Cell Disease: Novel Roles for Hemolysis and Acute Chest Syndrome

Introduction: Patients with sickle cell disease (SCD) are suspected to have more pulmonary fibrosis. High resolution computerized tomography can detect interstitial fibrosis although the etiology of this pathology is understudied. A restrictive pattern of pulmonary function is present in 74 percent of patients. We previously demonstrated profound pulmonary artery remodeling in SCD - which could undoubtedly contribute to the restrictive pulmonary findings. Interstitial fibrosis likely plays a role in impaired pulmonary function as well. However, pleural fibrosis is major cause of restrictive pulmonary function but is, as yet, poorly described in patients with SCD.

Objective: To interrogate the prevalence of pleural pulmonary fibrosis in SCD and investigate a novel potential mechanism of all forms of pulmonary fibrosis in these patients.

Methods: A cohort of 20 autopsies from patients with SCD was formed (10 subjects had acute chest syndrome (ACS) listed as cause of death) and clinical data abstracted from medical records. Pulmonary fibrosis and histology were scored using standard methodology. Standard techniques were used to detect pulmonary proteins and stain connective tissue. Plasma was collected from SCD patients at steady state and during ACS (n=24). Soluble analytes were quantified via ELISA.

Results: The pleiotropic and pro-fibrotic cytokine, TGF-β1, clearly plays a major role in pathology of SCD. We, and others, have found that total TGF-β1 is elevated in the plasma of adult patients with SCD. Although clearly also associated with ischemic injury, we now report that elevated TGF-β1 is activated and associates with levels of plasma heme in patients. Levels of TGF-β1 dramatically increase during ACS as do levels of plasma heme. Importantly, plasma galectin-3 - a known inducer of fibrosis, is (1) increased in patients with SCD, (2) associated with TGF-β1 and levels of heme, and (3) increased during ACS. These data suggest that ACS is promoting a pro-fibrotic state.

At autopsy, the most striking feature of the lung was pleural fibrosis. All adult patients exhibited significant thickening of the pleura with collagen extending into the interstitia up to 10 fold further than in control (AA) lungs. Pleural fibrosis was age-independent. Interstitial fibrosis was closely associated with advancing age. Further interrogation of lung tissue at autopsy revealed the presence of heme/hemosiderin-laden macrophages in the tissues. The number of such macrophages associated well with the degree of interstitial fibrosis. In patients who died of ACS, both the number of these macrophages and the intensity of the hemosiderin content was increased. We also observed collagen "trapped" macrophages throughout both pleural and interstitial fibrotic regions. Such macrophages co-localized with tissue galectin-3 staining. Upon scoring, the entire interstitium of the patients who died with ACS had higher galectin-3 levels than those who died without ACS. There was only modest staining in AA control lungs. In addition to heme/hemosiderin, galectin-3 was also noted in the macrophage and in numerous smooth muscle cells of pulmonary arteries with intimal hyperplasia. Monocytes embedded in the intimal layer expressed high levels of galectin-3 as did monocytes captured in the blood stream at autopsy.

Higher soluble and pulmonary galectin-3 was noted in the lungs of the Townes mouse model of SCD at steady state. Similar to patients with ACS, murine pulmonary galectin-3 levels increased upon treatment of the mice with lipopolysaccharide - an inducer of inflammation and lung injury.

Furthermore, both monocytes and differentiated macrophages treated in vitro with heme released galectin-3. Similar in vitro results were obtained with heme-induced TGF-β1 release.

Conclusions: Our results suggest that hemolysis may contribute to the restrictive pulmonary function in SCD via heme-induced release of the pro-fibrotic protein galectin-3. Thus, the data provide evidence that a pro-fibrotic "tone" is present in the illness at "steady state". Events such as ACS invoke intermittent, but significant fibrotic injury via galectin-3. Thus, targeting galectin-3 may prove to be effective in treating fibrotic exacerbations in patients. Importantly though, pleural fibrosis is likely a significant contributor to the overall pulmonary function deficit in patients with SCD.