BET inhibition targets pathogenic interstitial macrophages to limit antibody mediated fibrosis in bronchiolitis obliterans chronic GVHD
Chronic graft-versus-host disease (cGVHD) is a major cause of late mortality in allogeneic-hematopoietic stem cell transplantation with bronchiolitis obliterans syndrome (BOS) among the most lethal treatment-resistant complications. Despite approval of 4 second-line therapies, clinical response in moderate and severe BOS patients remains poor. Bromodomain and Extra terminal (BET) proteins are epigenetic readers driving oncogenic and inflammatory transcriptional programs in multiple cell types. We hypothesized that BET inhibition would be a feasible therapeutic option by restraining the direct inflammatory T cells as well as macrophage polarization to profibrotic phenotype known. We used an established BOS cGVHD model wherein we treated cGVHD mice with the translatable BET inhibitor: OPN-51107 (BETi). We found that under BETi, T and B cells had reduced germinal center (GC) trafficking ability, and GC activation resulting in reduced IgG1+ class switched B cells. Mice with cGVHD had elevated pathogenic IgM and IgG1 both in circulation and deposited onto lung tissue, which was attenuated under BET inhibition. Furthermore, antibodies produced in cGVHD were both allo- and auto-reactive. Single-cell RNA sequencing analysis revealed 22 distinct cell types in the BOS lung. In cGVHD mice, GSEA analysis revealed upregulation of anti-inflammatory Arginase1 and profibrotic Tgfb1 expression in alveolar macrophages (AM) and interstitial macrophages (IM), which was significantly reduced in BETi treated mice. Spectral flow cytometry confirmed that BETi targeted lung-infiltrating M2 macs, through diminishment of CD206+FcgR+ M2 IMs and AMs. Ultimately, this concluded in reduced collagen deposition and improved lung function in mice treated with BETi. We identify BET proteins as a key regulator of inflammation in BOS. BETi suppresses Tfh and GC B cells, thereby reducing Ab production and deposition. Within the lung, BETi selectively targets M2 IM macs to reduce FcgR and TGF-b expression, thereby regulating collagen deposition and fibrosis. These findings reveal a previously unrecognized mechanistic axis driving cGVHD and highlight BET inhibition as a promising therapeutic strategy. Overall design: Single cell RNA-sequencing of isolated lung cells via the 10X Genomics NextGem Fixed RNA Profiling kit.