Retinoic acid-driven expansion of CD16hiCD177+ neutrophils mediates steroid-resistant GI-GVHD

Steroid-resistant gastrointestinal graft-versus-host disease (SR-GI-GVHD) is a lethal complication of allogeneic hematopoietic stem cell transplantation with no clear therapeutic target. The role of neutrophil heterogeneity in its pathogenesis remains poorly defined. Through single-cell RNA sequencing of patient blood, we identified a specific neutrophil subset (CD16hiCD177+) that is markedly expanded in SR-GI-GVHD. This subset uses CD177 to bind endothelial CD31, facilitating transmigration into the intestine. Upon arrival, MyD88-mediated sensing of translocated gut bacteria triggers the release of neutrophil extracellular traps (NETs), directly causing epithelial damage. Mechanistically, we found that elevated levels of retinoic acid (RA) in patients drive the expansion and pathogenic programming of these neutrophils via the retinoic acid receptor α-Spi-1 proto-oncogene (RARA-SPI1) transcriptional axis. In a murine model of GVHD, genetic deletion of Cd177 or Myd88 in donor cells attenuated disease. Crucially, pharmacological inhibition of the RA receptor with AGN193109 not only ameliorated GI-GVHD and improved survival but also restored sensitivity to corticosteroids. Together, these findings delineate a complete pathogenic circuit, from the metabolic driver RA and the RARA-SPI1 transcriptional axis to the CD177+ neutrophil cellular effector and its NET formation-mediated tissue damage. This establishes the RA-driven CD177+ neutrophil subset as a key mediator of SR-GI-GVHD and identifies RARA inhibition as a promising therapeutic strategy.