Abstract
Antithymocyte globulin (ATG) is a widely used immunosuppressive agent, yet its off-target vascular effects remain a clinical challenge in part due to a lack of relevant human models. Here, we uncover a biphasic mechanism of ATG-induced microvasculopathy using a humanized liver organoid platform derived from induced pluripotent stem cells (iPSCs). We show that ATG triggers a rapid, complement-dependent phase of injury restricted to human iPSC-derived liver sinusoidal endothelial cells (iLSECs), leading to C3 deposition and acute thrombosis specifically in human but not in mouse vessels. Transcriptomics reveals a delayed transforming growth factor β (TGF-β) pathway-driven proinflammatory program that coincides with neutrophil recruitment and degranulation, while pharmacological TGF-β blockade attenuates thrombosis and flow disturbance. Our findings reveal a pathogenic sequence of complement and TGF-β pathway activation and establish a translational platform for dissecting human liver-specific microvasculopathy in vivo.