Peacekeepers are cross‐dressed in the liver land
Authors: Yuan Zhai, Jerzy W. Kupiec‐Weglinski
Journal: Hepatology (2017)

Potential conflict of interest: Nothing to report. See Article On Page 1499 Liver is often considered an immune privileged organ. Indeed, major histocompatibility complex (MHC)–incompatible liver grafts can survive long term in mice and pigs without exogenous immunosuppression.1 Although humans do not accept liver grafts spontaneously, approximately 20% of transplant recipients may be weaned safely off immunosuppressive drugs without triggering rejection or impairing the hepatocellular function. Liver is also the site of persistent infections, with malarial parasite and hepatitis viruses exploiting the liver immune privilege site and eluding host immune surveillance and defense responses. Which mechanisms may be responsible for the unique liver tolerogenic properties? Two main hypotheses have been proposed, each with its own supportive experimental evidence. The deletion or inactivation mechanism suggests that T cells, particularly the cluster of differentiation 8 (CD8) subset, may become anergic or apoptotic when primed or activated by liver‐specific antigens (Ags) due to specific feathers of hepatic Ag‐presenting cells (APCs). An alternative immune regulation mechanism, based on data from murine liver transplant models, implies that regulatory T cells (Treg) of the CD4+CD25+FoxP3+ phenotype are critical in curtailing anti‐donor T‐cell responses. In the clinical setting, however, studies on putative mechanisms of liver tolerance are convoluted by concomitant administration of relatively unspecific immunosuppressive drugs, which by suppressing the rejection response may also impair or deplete the Treg pool. The frequency of peripheral Tregs decreases after liver transplantation, and stable allograft recipients are often characterized by lower numbers of circulating Tregs with the CD4+CD25+CD127low phenotype. Thus, although not excluding the significance of Treg function, a T‐cell deletion or inactivation mechanism seems to be more relevant in liver transplant patients. It is important to recognize that the antidonor immune response, elicited early in liver allografts, can persist systemically throughout in spontaneously tolerant murine liver transplant recipients. Multiple molecular mechanisms have been implicated in the induction of tolerance in experimental liver transplantation. The crosstalk between recipient Fas and donor Fas ligand has been shown to be critically important by down‐regulating antidonor cytotoxic T‐lymphocyte activity.2 It was surprising to find at the time that liver tolerance may also require adjunctive signaling of the key proinflammatory cytokine interferon‐γ as its deficiency in recipients and its receptor deficiency in the donor organ all triggered graft rejection.3 Later on, however, donor programmed death‐1 signaling was shown to be indispensable in the induction of apoptosis of liver‐infiltrating cells and tolerance acquisition,4 whereas interferon‐γ produced by alloreactive infiltrating T cells was in fact responsible for triggering donor programmed death ligand 1 (PD‐L1) expression.5 As to the cellular basis of tolerance, the liver is a quite unique organ. The sinusoids, thin‐walled blood vessels in the liver tissue, receive blood not only from the arterial system but also from the portal circulation with bacteria‐derived molecules. Anatomically, liver sinusoidal endothelial cells form a fenestrated endothelium, which allows direct contact of blood contents with the subendothelial space of Disse and the underlying hepatocytes. With liver‐resident macrophages, Kupffer cells, and dendritic cells (DCs) all residing within the vasculature, this distinctive architecture allows circulating T cells to directly interact with almost the entire liver cell repertoire in their primary activation by local Ags. Based on these circulatory and structural features, it was conceivable to propose that Ag presentation by nonprofessional APCs might result in liver tolerance. However, it turned out that it was liver professional APCs, the DCs, that play the key role in tolerance induction.6 Nonprofessional or semiprofessional APCs, such as hepatocytes and liver sinusoidal endothelial cells, are in fact highly immunogenic. Although Kupffer cells represent the largest local APC population, they do not significantly affect liver transplantation outcomes.1 Dr. Angus Thomson's group has pioneered research on the role of hepatic DCs in the acquisition of tolerance in liver transplantation.7 They have previously shown that functional modulation of donor myeloid DCs either by Ftk3 ligand–mediated activation or using DAP12‐deficient cells, resulting in a relative increase of interleukin‐12 versus interleukin‐10 production by activated DCs, altered the nature of recipients' immune response to liver allografts and triggered their rejection. More recently, they have used CD11c‐diphtheria toxin receptor donors to document that myeloid DCs are essential for liver tolerance induction. The current work in this issue of Hepatology8 not only extends in‐depth exploration of the tolerogenic mechanisms of hepatic DCs but also illuminates a new aspect in transplant immunology, i.e., cross‐dressing in recipients' APCs. Cross‐dressing is distinctive from cross‐presentation in that a preformed MHC–Ag complex rather than donor cell Ag is acquired and presented as a whole to the host immune system.9 This is particularly interesting in transplant immunology as intact donor MHC can now stimulate recipient T cells by both donor (classical direct pathway) and cross‐dressed recipient APCs. Indeed, the presence of cross‐dressed recipient APCs dominated quantitatively over donor‐derived APCs in local lymphoid tissues in most types of transplant recipients and played key roles in initiating alloreactive T‐cell response and rejection response.10 In the study by Ono et al.,8 mouse recipient DCs (C3H) were found to infiltrate orthotopic liver grafts (B6) and to completely replace donor DCs by day 7 posttransplant. Importantly, without use of any immunosuppressive agents, these DCs became cross‐dressed with donor MHC class I and expressed high PD‐L1 levels. Functionally, cross‐dressed DCs (CD‐DCs) were able to suppress donor‐reactive T‐cell proliferation and promote their death in vitro. In parallel, a large proportion of liver graft–infiltrating T cells exhibited a classic exhausted phenotype (PD‐1+TIM‐3+). The functional relevance of CD‐DCs with transplant tolerance was further illustrated when the numbers of CD‐DCs and the degree of cross‐dressing with donor MHC class I significantly decreased in the rejecting liver graft setting (DAP12‐deficient donor). Most importantly, the relative expression of coinhibitory PD‐L1 versus costimulatory CD86 molecules in the latter transplant model has reversed as well. The elegant study by the Pittsburgh group extends our appreciation of the novel role of CD‐DCs in the acquisition of immune tolerance in mouse liver transplant recipients. What determines the fate of the interacting T cells, i.e., alloimmune activation versus suppression, is the costimulation context of Ag presentation and relative expression levels of negative (PD‐L1) versus positive (CD86) molecules (Fig. 1). In the liver, recipient CD‐DCs have inherited immunoregulatory features from their local predecessors (donor liver DCs with impaired ability to activate T cells) with enhanced suppressive functions in situ (inhibit donor‐reactive T‐cell response by liver graft–derived, but not spleen‐derived, CD‐DCs). It remains to be determined how CD‐DCs gain the suppressive function when sequestered in the liver grafts but lose such potential when they emigrate from the hepatic tissue and enter the cell recirculation pool. It is conceivable that the liver graft microenvironment itself, the acquisition of intact donor MHC, as well as the proinflammatory conditioning (e.g., interferon‐γ to up‐regulate PD‐L1) may all contribute to the maturation of infiltrating DCs into a potent immunoregulatory type in the absence of immunosuppressive therapy.Figure 1: The costimulation context of APCs determines the fate of interacting T cells.