Spatial Transcriptomics Analysis of Macrophage-to-Myofibroblast Transition in Bronchiolitis Obliterans Following Hematopoietic Stem Cell Transplantation.

Bronchiolitis obliterans (BO) is a severe, fibrotic manifestation of chronic graft-versus-host disease (GVHD) after hematopoietic stem cell transplantation (HSCT). Macrophage-derived TGF-β is linked to GVHD-related fibrosis; however, its role in BO remains unclear. Given emerging evidence of macrophage-to-myofibroblast transition (MMT) in fibrosis, this study aimed to investigate whether MMT contributes to BO development. We analyzed lung samples from 3 HSCT recipients with BO who underwent lung transplantation, with tissues from 2 patients with lung cancer as controls. Immunofluorescent staining for CD68, CD206, and α-SMA was used to identify cells undergoing MMT. Spatial transcriptomics was performed to profile gene expression in foamy macrophages across anatomical regions and histological stages, compared to control peribronchiolar regions. Immunostaining showed strong co-expression of CD68, CD206, and α-SMA in foamy macrophages within and surrounding bronchioles in BO samples, with minimal expression in controls, suggesting MMT involvement. In BO, spatial transcriptomics revealed upregulation of macrophage- and TGF-β signaling genes, with distinct stage- and region-specific gene expression patterns. Unsupervised clustering revealed a shift from inflammation to fibrosis. These findings indicate that MMT contributes to fibrosis in BO. Gene expression shifts from inflammation to fibrosis as the disease advances, underscoring the importance of early intervention.