ObjectiveLiver metastases are often resistant to immune checkpoint inhibitor therapy (ICI) and portend a worse prognosis compared with metastases to other locations. Regulatory T cells (Tregs) are one of several immunosuppressive cells implicated in ICI resistance of liver tumours, but the role played by Tregs residing within the liver surrounding a tumour is unknown.DesignFlow cytometry and single-cell RNA sequencing were used to characterise hepatic Tregs before and after ICI therapy.ResultsWe found that the murine liver houses a Treg population that, unlike those found in other organs, is both highly proliferative and apoptotic at baseline. On administration of αPD-1, αPD-L1 or αCTLA4, the liver Treg population doubled regardless of the presence of an intrahepatic tumour. Remarkably, this change was not due to the preferential expansion of the subpopulation of Tregs that express PD-1. Instead, a subpopulation of CD29⁺(Itgb1, integrin β1) Tregs, that were highly proliferative at baseline, doubled its size in response to αPD-1. Partial and full depletion of Tregs identified CD29⁺Tregs as the prominent niche-filling subpopulation in the liver, and CD29⁺Tregs demonstrated enhanced suppressionin vitrowhen derived from the liver but not the spleen. We identified IL2 as a critical modulator of both CD29⁺and CD29⁻hepatic Tregs, but expansion of the liver Treg population with αPD-1 driven by CD29⁺Tregs was in part IL2-independent.ConclusionWe propose that CD29⁺Tregs constitute a unique subpopulation of hepatic Tregs that are primed to respond to ICI agents and mediate resistance.