In vitro models of the gut-liver axis: what we've learned and what remains to be built.

The gut-liver axis maintains metabolic homeostasis and immune regulation through continuous bidirectional communication, and its dysregulation contributes to a range of metabolic, inflammatory, and immune-mediated diseases. Integrated gut-liver axis model systems offer unique tools for dissecting these complex interactions by isolating individual variables that are difficult to disentangle in vivo, while allowing flexible experimental controls over them. Here, we review advances in gut-liver axis models from static co-cultures to microfluidic systems and their applications in pharmacokinetic and mechanistic studies. We identify underexplored areas, including metabolite-mediated gut-liver crosstalk, immune-mediated interorgan communication, and disease-specific modeling, and outline technical challenges to achieving physiologically faithful and reliable integrated platforms. By addressing these challenges, gut-liver axis models will contribute to a mechanistic understanding of gut-liver pathobiology that is difficult to achieve through clinical studies, animal models, or individual organ systems alone.