Characterization of localized macrophages in bronchiolitis obliterans after allogeneic hematopoietic cell transplantation

Background Bronchiolitis obliterans syndrome (BOS) remains one of the most devastating manifestations of chronic graft-versus-host disease in hematopoietic cell transplantation (HCT). Recent findings of BOS after lung transplantation indicate that donor (lung)-derived lung-resident macrophages contribute to BOS, suggesting that differences in the origin of immune cells and localized antigen-presenting cells cause the onset of BOS. Methods We identified the phenotype and origin of infiltrating macrophages using immunohistochemistry and fluorescence in situ hybridization in eight sex-mismatched HCT recipients who underwent lung transplantation for BOS after HCT. Results Most of the infiltrating macrophages appeared to be derived from donor (hematopoietic) cells in patients who developed BOS following HCT. Macrophages observed in the early-stage region of BOS were positive for cluster of differentiation (CD)68 and inducible nitric oxide synthase (iNOS) and negative for CD163 and CD206, suggesting an M1 phenotype. In the late-stage region, macrophages were negative for CD68 and iNOS in all patients, but also positive for CD163 and CD206 in some patients. Conclusions Donor-derived M1-macrophages may be involved in the pathogenesis of the early-stage region of BOS. In addition, some macrophages in the late-stage region showed M2 polarization that might be involved in fibrosis.