Mouse liver allografts mismatched at major histocompatibility complex (MHC) loci are frequently accepted without immunosuppression, and a 2007 study using Jalpha18-knockout donors showed that graft-resident invariant natural killer T (iNKT) cells are required for this spontaneous tolerance. What these donor-residual iNKT cells actually do — which antigen-presentation route and cellular interactions (e.g., with Kupffer cells or liver dendritic cells) they use, whether they act by inducing apoptosis of infiltrating recipient T cells or by supporting regulatory T cells, which soluble mediators (IL-4, IL-10, TGF-beta, IFN-gamma, or others) execute the effect, why iNKT cells are regulatory in liver grafts but accelerate rejection of other graft types such as intrahepatic islets, and whether iNKT monitoring or modulation can be translated to human liver transplantation — remains unresolved. Dedicated reviews from 2023, 2025, and 2026 consistently state that the precise mechanism remains elusive, that mediator assignment varies across models, and that NKT-cell-based transplant immunotherapy is still in its very early stages.

Background

The liver is the most tolerogenic of transplanted organs: in mice, orthotopic liver transplantation (surgical replacement of the liver, OLT) across MHC barriers often leads to long-term graft acceptance without any immunosuppressive drugs, a phenomenon called spontaneous tolerance. The liver is also uniquely enriched in invariant natural killer T (iNKT) cells — innate-like T lymphocytes that carry an invariant T-cell receptor alpha chain (Valpha14-Jalpha18 in mice) and recognize lipid antigens presented by CD1d, a non-polymorphic MHC class I-like molecule expressed on hepatocytes, liver sinusoidal endothelial cells, Kupffer cells (the resident macrophages of the liver), and dendritic cells (DCs). Upon activation, e.g. by the prototypic glycolipid agonist alpha-galactosylceramide (alpha-GalCer), iNKT cells rapidly secrete both T-helper-1-type cytokines (IFN-gamma, TNF) and T-helper-2/regulatory cytokines (IL-4, IL-10, TGF-beta), and can be further subdivided into functionally distinct populations (NKT1, NKT2, NKT17, and IL-10-producing NKT10). Before 2007, NKT cells had been shown to be required for tolerance induction in several transplant models (heart allograft tolerance induced by donor-specific transfusion, rat islet xenografts accepted into the mouse liver, corneal allografts), graft-derived NKT cells had been implicated in spontaneous acceptance of rat liver allografts, yet iNKT cells had also been shown to participate in rejection of islet allografts placed in the liver — an unexplained divergence. Morita and colleagues (Transplantation Immunology, 2007) addressed the liver-graft case directly in mice: liver allografts from Jalpha18-knockout donors (which genetically lack iNKT cells) across an MHC barrier developed portal infiltration, hemorrhage, and necrosis with significantly reduced survival and impaired tolerance induction compared with wild-type grafts, and donor iNKT deficiency dominated over recipient iNKT deficiency. This established that donor-residual iNKT cells act as immune regulatory cells needed for spontaneous liver allograft tolerance, but the experiments were limited to survival, histology, and tolerance phenotypes; no pathway, interaction partner, or effector molecule was identified. Subsequent work clarified iNKT biology in neighboring contexts — ischemia-reperfusion injury, hepatitis, and tumor immunity — and a rat study showed that pharmacologic activation of iNKT cells with alpha-GalCer prolongs liver allograft survival with a correlative IFN-gamma-down/IL-10-up shift, but that agonist-driven, correlative result in a different species did not identify the mechanism operating in the mouse spontaneous-tolerance model. Reviews of liver transplant tolerance through 2025 still cite the 2007 knockout phenotype itself, rather than any mechanistic successor, and a 2025 review of tissue-resident lymphocytes in liver transplantation describes the roles of donor iNKT and other tissue-resident cells in transplantation as substantively unknown.

Problem Statement

Identify the mechanism by which donor-resident invariant NKT cells induce and/or maintain spontaneous tolerance of MHC-mismatched liver allografts in mice, and determine whether it generalizes to human liver transplantation. Concretely: (1) which cellular and molecular pathway executes the effect — the antigen-presentation route engaging iNKT cells (CD1d-expressing Kupffer cells, liver dendritic cells, sinusoidal endothelial cells, or hepatocytes), and whether the regulatory outcome depends on induction of apoptosis in infiltrating recipient T cells, on support of regulatory T-cell function, or on another identifiable interaction; (2) which soluble mediator(s) or cytokine program (among candidates including IL-4, IL-10, TGF-beta, and IFN-gamma, or others) is necessary and/or sufficient for iNKT-dependent liver-graft tolerance in this model; (3) why iNKT cells are regulatory in liver allografts but act as effectors that accelerate rejection in other transplant contexts (e.g., intrahepatic islet allografts) — that is, what feature or combination of features (antigenic strength, subset composition, local microenvironment) governs the direction of the iNKT response; and (4) whether these findings have clinical relevance — whether modulation or monitoring of graft-resident iNKT cells can be translated into tolerance-promoting or tolerance-monitoring interventions in human liver transplant recipients. These four deliverables form one scientific objective: a causal, mechanism-to-translation account of the donor-resident-iNKT tolerance effect established by the 2007 knockout phenotype. Because the mechanism has not yet been demonstrated experimentally in this model, any account of it must be inferred from existing evidence.

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