Role of Invariant Natural Killer T Cells in Liver Sinusoidal Endothelial Cell-Induced Immunosuppression Among T Cells with Indirect Allospecificity

Strategies for Deliberate Induction of Immune Tolerance in Liver Transplantation: From Preclinical Models to Clinical Application

The liver exhibits intrinsic immune regulatory properties that maintain tolerance to endogenous and exogenous antigens, and provide protection against pathogens. Such an immune privilege contributes to susceptibility to spontaneous acceptance despite major histocompatibility complex mismatch when transplanted in animal models. Furthermore, the presence of a liver allograft can suppress the rejection of other solid tissue/organ grafts from the same donor. Despite this immune privilege of the livers, to control the undesired alloimmune responses in humans, most liver transplant recipients require long-term treatment with immune-suppressive drugs that predispose to cardiometabolic side effects and renal insufficiency. Understanding the mechanism of liver transplant tolerance and crosstalk between a variety of hepatic immune cells, such as dendritic cells, Kupffer cells, liver sinusoidas endothelial cells, hepatic stellate cells and so on, and alloreactive T cells would lead to the development of strategies for deliberate induction of more specific immune tolerance in a clinical setting. In this review article, we focus on results derived from basic studies that have attempted to elucidate the immune modulatory mechanisms of liver constituent cells and clinical trials that induced immune tolerance after liver transplantation by utilizing the immune-privilege potential of the liver.

Postoperative Portal Hypertension Enhances Alloimmune Responses after Living-Donor Liver Transplantation in Patients and in a Mouse Model

Controlling portal vein pressure in living-donor liver transplantation has received increased attention owing to its potential importance for graft survival. Portal hypertension may lead to the activation of liver-resident APCs, including liver sinusoidal endothelial cells (LSECs), which have immunological tolerogenic capacity. We investigated the effects of portal hypertension on graft survival and the antidonor immune response using clinical data and a mouse model. We categorized patients (n = 136) according to their portal vein pressure values at the end of surgery. Using propensity score-matching analyses, we found that portal hypertension was significantly associated with a higher antidonor immune response and incidence of acute rejection. To investigate the mechanism, we performed an allogeneic coculture assay using a 70% hepatectomized (HTx) mouse model with or without a portosystemic shunt. Liver cells from HTx mice without a shunt exhibited a significantly greater anti-BALB/c B6 T cell response than those from sham-operated mice or HTx mice with a shunt. LSECs from sham-operated mice, but not from HTx mice, suppressed the B6 T cell alloresponse in a dose-dependent manner. Furthermore, LSECs from HTx mice without a shunt showed significantly downregulated MHC class I/II and programmed death-ligand 1 expression, and those from mice with a shunt showed recovered expression of these molecules. Postoperative portal hypertension enhances alloimmune responses in recipients after living-donor liver transplantation, likely due, in part, to the impaired immune-suppression capacity of LSECs.

Adoptive transfer of allogeneic liver sinusoidal endothelial cells specifically inhibits T-cell responses to cognate stimuli

Although it is well known that liver allografts are often accepted by recipients, leading to donor-specific tolerance of further organ transplants, the underlying mechanisms remain unclear. We had previously used an in vitro model and showed that mouse liver sinusoidal endothelial cells (LSECs) selectively suppress allospecific T-cells across major histocompatibility complex (MHC) barriers. In the present study, we established an in vivo model for evaluating the immunomodulatory effects of allogeneic LSECs on corresponding T-cells. Allogeneic BALB/cA LSECs were injected intraportally into recombination activating gene 2 γ-chain double-knockout (RAG2/gc-KO, H-2(b)) mice lacking T, B, and natural killer (NK) cells. In order to facilitate LSEC engraftment, the RAG2/gc-KO mice were injected intraperitoneally with monocrotaline 2 days before the adoptive transfer of LSECs; this impaired the host LSECs, conferring a proliferative advantage to the transplanted LSECs. After orthotopic allogeneic LSEC engraftment, the RAG2/gc-KO mice were immune reconstituted intravenously with C57BL/6 splenocytes. After immune reconstitution, mixed lymphocyte reaction (MLR) assay using splenocytes from the recipients revealed that specific inhibition of host CD4(+) and CD8(+) T-cell proliferation was greater in response to allostimulation with irradiated BALB/cA splenocytes rather than to stimulation with irradiated third party SJL/jorllco splenocytes. This inhibitory effect was attenuated by administering anti-programmed death ligand 1 (PD-L1) monoclonal antibody during immune reconstitution in the above-mentioned mice, but not in RAG2/gc-KO mice engrafted with Fas ligand (FasL)-deficient BALB/cA LSECs. Furthermore, engraftment of allogeneic BALB/cA LSECs significantly prolonged the survival of subsequently grafted cognate allogeneic BALB/cA hearts in RAG2/gc-KO mice immune reconstituted with bone marrow transplantation from C57BL/6 mice. In conclusion, murine LSECs have been proven capable of suppressing T-cells with cognate specificity for LSECs in an in vivo model. The programmed death 1/PD-L1 pathway is likely involved in these suppressive effects.