Immunological Determinants of Liver Transplant Outcomes Uncovered by the Rat Model

Abstract

For many individuals with end-stage liver disease, the only treatment option is liver transplantation. However, liver transplant rejection is observed in 24%-80% of transplant patients and lifelong drug regimens that follow the transplant procedure lead to serious side effects. Furthermore, the pool of donor livers available for transplantation is far less than the demand. Well-characterized and physiologically relevant models of liver transplantation are crucial to a deeper understanding of the cellular processes governing the outcomes of liver transplantation and serve as a platform for testing new therapeutic strategies to enhance graft acceptance. Such a model has been found in the rat transplant model, which has an advantageous size for surgical procedures, similar postoperative immunological progression, and high genome match to the human liver. From rat liver transplant studies published in the last 5 years, it is clear that the rat model serves as a strong platform to elucidate transplant immunological mechanisms. Using the model, we have begun to uncover potential players and possible therapeutic targets to restore liver tolerance and preserve host immunocompetence. Here, we present an overview of recent literature for rat liver transplant models, with an aim to highlight the value of the models and to provide future perspectives on how these models could be further characterized to enhance the overall value of rat models to the field of liver transplantation.

Graphical abstract

FIGURE 1.

Development of ACR symptoms common to LEW→BN and DA→LEW rat OLT models. OLT rat recipients in alloimmune strain combinations that present ACR typically show lethargy and loss of appetite by POD3, and symptoms progressively worsen to development of severe jaundice and ascites from POD5. At the cellular level, assessment of RAI shows mononuclear cell infiltration as early as POD1, which continues to expand to liver parenchyma, with indication of severe ACR at POD7. LEW→BN rat OLT recipients typically survive for an average MST of 13.66 d, while DA→LEW models survive on average 11.22 d. ACR, acute cellular rejection; BN, Brown Norway; DA, Dark Agouti; IRI, ischemia-reperfusion injury; LEW, Lewis; MST, median survival time; OLT, orthotopic liver transplantation; POD, postoperative day; RAI, Rejection Activity Index.

FIGURE 2.

The days that follow an LT are marked by a dialogue between the liver graft and the host, through direct, indirect, and semidirect allorecognition. In this process of getting acquainted, the communication between different cells and the signals in the microenvironment of the liver will determine whether tolerance or rejection will follow. An initial inflammatory response, marked by IL-2, IFN-γ, TNF, and VCAM-1, leads to intragraft leukocyte infiltration independent of the final outcome and, in the presence of hepatic signals, is key to prime the response seen afterward. For instance, IFN-γ secretion is key to upregulate PD-L1 expression by LSECs, hepatocytes, and KCs, which subsequently contributes to the establishment of a tolerogenic environment. If, however, the alloantigen load or inflammatory signals, such as IL-2, are overexpressed, then the response is skewed toward acute cellular rejection. KCs are found in a spectrum from anti-inflammatory to proinflammatory phenotypes and are important contributors to the final outcome following LT. Activation of transcription factors, such as NF-κB, NFAT, RORγt, T-bet, leads to an environment enriched in proinflammatory cytokines and guide T-cell differentiation toward Th17 and Th1 responses. On the other hand, the engagement of PIK3/AKT and the upregulation of coinhibitory molecules, such as PD-L1, lead to a tolerogenic environment, with predominance of T_reg and Th2 responses. APC, antigen-presenting cell; BD, bile duct; CTL, cytotoxic T lymphocyte; CTLA-4, cytotoxic T-lymphocyte–associated protein 4; DA, Dark Agouti; DC, dendritic cell; FasL, Fas ligand; FGL2, higher fibrinogen protein 2; FOXP3, forkhead box P3, HA, hepatic artery; HGF, hepatic growth factor; HSC, hepatic stellate cell; IFN-γ, interferon γ; KC, Kupffer cell; LEW, Lewis; LSEC, liver sinusoidal endothelial cell; LT, liver transplant; MHC, major histocompatibility complex; NF-κB, kappa-light-chain-enhancer of activated B cells; NFAT, nuclear factor of activated T cells; NK, natural killer; PD-1, programmed cell death protein 1; PD-L1, PD-1 ligand; PV, portal vein; RORγt, retinoic acid receptor (RAR)–related orphan receptor (ROR)γt transcription factor; T-bet, T-box transcription factor; TGF-β, tumor growth factor β; Th, T helper cell; TNF, tumor necrosis factor; T_reg, regulatory T cells; VCAM-1, vascular cell adhesion molecule-1.