Recipient Endothelial IRF1 mediates IFNγ-driven tissue tolerance in mouse models of acute Graft-versus-Host Disease.

Recipient endothelial cells (ECs) actively respond to inflammation during allogeneic hematopoietic cell transplant (allo-HCT), yet mechanisms by which ECs influence acute graft-versus-host-disease (GVHD) pathology remain incompletely defined. Single cell RNA-sequencing of ECs isolated from a GVHD target organ, the liver, showed rapid, subset-specific transcriptional reprogramming after allo-HCT, with induction of canonical interferon-γ (IFNγ)-inducible genes including interferon regulatory factor 1 (IRF1), MHC II, and PD-L1 in lymphatic ECs (LECs). In allo-HCT recipients, circulating IFNγ peaked on day 4 (early), decreased but remained elevated at day 14 (late), and declined by day 21, with a concordant induction of IRF1+ LECs in the liver and GI tract. IFNγ neutralization with anti-IFNγ monoclonal antibody at either early or late time points attenuated IRF1⁺MHCII⁺PD-L1⁺ LEC activation. Notably, early donor T cell expansion was IFNγ-independent, whereas late IFNγ blockade selectively impaired donor Treg, but not Th1, expansion, leading to accelerated GVHD. Using in vitro LEC-CD4 T cell co-cultures and allo-HCT in Irf1-/- bone marrow chimera recipients, we show that recipient ECs that cannot mediate IFNγ-IRF1 signaling exhibit reduced activation and apoptosis, but also have impaired Treg expansion, increased donor Th1/Treg ratios, and worsened GVHD severity. Finally, pharmacological JAK inhibition in allo-HCT recipient mice spares IRF1+ LECs and Tregs while reducing pathogenic Th1 cells, correlating with reduced GVHD severity and improved survival. Our findings identify a role for the lymphatic endothelial IFNγ-IRF1 axis in regulating early vascular remodeling, donor T cell mediated tolerance, underscoring a potential "goldilocks" level of pathway activation required to improve post-transplant outcomes.