Emily Ahmed

TLR Signals Promote IL-6/IL-17-Dependent Transplant Rejection

Acute allograft rejection has often been correlated with Th1 differentiation, whereas transplantation tolerance is frequently associated with induction of regulation. The discovery of the Th17 phenotype has prompted its scrutiny in transplant rejection. Although IL-17 has recently been observed in settings of acute allograft rejection and drives rejection in T-bet-deficient mice that have impaired type 1 T cell responses, there is little evidence of its requirement during acute rejection in wild-type animals. We and others have previously shown that TLR9 signaling by exogenous CpG at the time of transplantation is sufficient to abrogate anti-CD154-mediated acceptance of fully mismatched cardiac allografts. In this study, we investigated the mechanism by which acute rejection occurs in this inflammatory context. Our results indicate that CpG targets recipient hemopoietic cells and that its pro-rejection effects correlate both with prevention of anti-CD154-mediated conversion of conventional CD4+ T cells into induced regulatory T cells and with the expression of IFN-γ and IL-17 by intragraft CD4+ T cells. Moreover, the combined elimination of IL-6 and IL-17 signaling abrogated the ability of CpG to promote acute cardiac allograft rejection. Thus, proinflammatory signals at the time of transplantation can change the quality of the effector immune response and reveal a pathogenic function for IL-6 and IL-17 in wild-type recipients.

Prevention of Allograft Tolerance by Bacterial Infection with Listeria monocytogenes

Exposure to certain viruses and parasites has been shown to prevent the induction of transplantation tolerance in mice via the generation of cross-reactive memory T cell responses or the induction of bystander activation. Bacterial infections are common in the perioperative period of solid organ allograft recipients in the clinic, and correlations between bacterial infections and acute allograft rejection have been reported. However, whether bacterial infections at the time of transplantation have any effect on the generation of transplantation tolerance remains to be established. We used the Gram-positive intracellular bacterium Listeria monocytogenes (LM) as a model pathogen because its effects on immune responses are well described. Perioperative LM infection prevented cardiac and skin allograft acceptance induced by anti-CD154 and donor-specific transfusion in mice. LM-mediated rejection was not due to the generation of cross-reactive T cells and was largely independent of signaling via MyD88, an adaptor for most TLRs, IL-1, and IL-18. Instead, transplant rejection following LM infection was dependent on the expression of the phagosome-lysing pore former listeriolysin O and on type I IFN receptor signaling. Our results indicate that bacterial exposure at the time of transplantation can antagonize tolerogenic regimens by enhancing alloantigen-specific immune responses independently of the generation of cross-reactive memory T cells.

Infection with the Intracellular Bacterium, Listeria monocytogenes, Overrides Established Tolerance in a Mouse Cardiac Allograft Model

Infections and TLR signals at the time of transplantation have been shown to prevent the induction of tolerance, but their effect on allografts after tolerance has been established is unclear. We here report that infection with Listeria monocytogenes precipitated the loss of tolerance and the MyD88‐ and T cell‐dependent rejection of accepted cardiac allografts in mice. This loss of tolerance was associated with increases in the numbers of graft‐infiltrating macrophages and dendritic cells, as well as CD4+FoxP3− and CD8+ T cells. Rejection was also associated with increased numbers of graft‐infiltrating alloreactive as well as Listeria‐reactive IFNγ‐producing T cells. Rejection of the established grafts required both IL‐6 and IFNß, cytokines produced during acute Listeria infection. However, IL‐6 and IFNß alone, even when present at higher concentrations than during Listeria infection, were insufficient to break tolerance, while the combination of IL‐6 and IFNß was sufficient to break tolerance. These and in vitro observations that IL‐6 but not IFNß enhanced T cell proliferation while IFNß but not IL‐6 enhanced IFNγ production support a hypothesis that these cytokines play nonredundant roles. In conclusion, these studies demonstrate that the proinflammatory effects of infections can induce the loss of tolerance and acute rejection of accepted allografts.

IL-6 Induced by Staphylococcus aureus Infection Prevents the Induction of Skin Allograft Acceptance in Mice

Clinical correlations between bacterial infections and rejection suggest a hypothesis that innate immune stimulation by bacterial infections results in the production of inflammatory cytokine that facilitate bystander T‐cell activation, increased alloreactivity and inhibition of tolerance induction. Previous studies demonstrated that IFNβ produced during an infection with a model bacterium, Listeria monocytogenes, prevented the induction of transplantation tolerance in mice with anti‐CD154 and donor‐specific transfusion (DST) ( 1). We investigated the impact of two clinically relevant bacterial infections at the time of transplantation on the ability of anti‐CD154 and DST to induce skin allograft acceptance in mice. Staphylococcus aureus (SA) infection prevented skin allograft acceptance whereas maximally tolerated doses of Pseudomonas aeruginosa infection had no effect. SA induced an acute production of IL‐6, which was necessary and sufficient for the prevention of skin allograft acceptance. Furthermore, a single pulse of methylprednisolone modulated IL‐6 production during SA infection and facilitated skin allograft acceptance in SA‐infected recipients. Taken together, our results suggest that bacterial infections elicit specific proinflammatory cytokines signatures that can serve as barriers to tolerance induction, and that inhibiting the production of or neutralizing these inflammatory cytokines can synergize with costimulatory blockade‐based therapies to facilitate the development of transplantation tolerance.

Bacterial infections, alloimmunity, and transplantation tolerance.

Transplantation of solid organs across histocompatibility barriers in the absence of immunosuppression is invariably followed by acute allograft rejection. Although several immunosuppressive regimens have been developed to prevent allograft rejection, these global immunosuppressive agents effectively inhibit all T cells, leaving the host vulnerable to infections. Thus, a major goal in transplantation immunology is to induce donor-specific tolerance that results in the extended suppression of allograft-specific immune responses, while leaving the remainder of the immune system competent to fight infections and malignancies. Initial successes in identifying approaches that successfully induce transplantation tolerance in experimental models have led to a newer research focus of identifying potential barriers to the induction of such tolerance as well as events that may reverse established allograft tolerance. Both clinical and experimental studies have identified bacterial infections as a possible trigger of allograft rejection. Recently, experimental models of transplantation tolerance have identified that bacterial signals can promote acute allograft rejection either by preventing the induction of transplantation tolerance or by reversing tolerance after it has been stably established. This review summarizes experimental and clinical literature supporting the hypothesis that bacterial infections and innate immunity can qualitatively and quantitatively alter adaptive alloreactivity through effects on innate immune responses.

Antagonistic effect of toll-like receptor signaling and bacterial infections on transplantation tolerance *

The induction of donor-specific tolerance remains a major goal in the field of transplantation immunology. Therapies that target costimulatory molecules can induce tolerance to heart and pancreatic islet allografts in mouse models, but fail to do so after transplantation of skin or intestinal allografts. We have proposed that organs colonized by commensal bacteria such as skin, lung, and intestine may be resistant to such therapies as a result of bacterial translocation at the time of transplantation, which may promote antigen-presenting cell maturation and the production of proinflammatory cytokines, consequently enhancing responses of alloreactive T cells. Our results indicate that the inability to sense signaling by most toll-like receptors (TLRs), as well as by interleukin-1R and -18R, as a result of genetic ablation of myeloid differentiation factor 88 promotes the acceptance of skin allografts. Conversely, TLR signals and infections by a model bacterium, Listeria monocytogenes (LM), at the time of transplantation can prevent the induction of transplantation tolerance. The effects of the TLR9 agonist CpG are myeloid differentiation factor 88-dependent, whereas the prorejection capacity of LM depends on the intracellular sensing of LM and the production of type I interferon. Therefore, transiently targeting these innate, proinflammatory pathways may have therapeutic value to promote transplantation tolerance.

Weighing the risk: Obesity and outcomes following liver transplantation.

Obesity is on the rise worldwide. As a result, unprecedented rates of patients are presenting with end stage liver disease in the setting of non-alcoholic fatty liver disease (NAFLD) and are requiring liver transplantation. There are significant concerns that the risk factors associated with obesity and the metabolic syndrome might have a detrimental effect on the long term outcomes following liver transplantation. In general, short term patient and graft outcomes for both obese and morbidly obese patients are comparable with that of non-obese patients, however, several studies report an increase in peri-operative morbidity and increased length of stay. Continued studies documenting the long-term outcomes from liver transplantation are needed to further examine the risk of recurrent disease (NAFLD) and also further define the role risk factors such cardiovascular disease might play long term. Effective weight reduction in the post liver transplant setting may mitigate the risks associated with the metabolic syndrome long-term.

Role of bacterial infections in allograft rejection.

Advances in immunosuppressive strategies have been essential for organ transplantation becoming an established therapy for the treatment of end-stage organ failure. A global state of immunosuppression imparted by current regimens leaves the transplant recipient at a significantly increased risk for a multitude of infections. As such, recognition, prevention and treatment of infection have become primary concerns. Furthermore, as our understanding of the interaction between the innate and adaptive immune system expands, the potential for enhanced alloreactivity in the face of infection has become a topic of increasing interest. Recent studies have shown a correlation between viral infections and graft rejection. However, other categories of infection, including bacterial infections, have the potential to elicit similar immune responses. This review seeks to explore the clinical and scientific literature examining the role of bacterial infections in transplant rejection.

Minocycline toxicity requiring liver transplant.

Fulminant hepatic failure (FHF) is a life-threatening condition that manifests as rapid development of profound acute liver injury with impaired synthetic function and encephalopathy. A recent prospective cohort study found that FHF predominantly affected female patients (73%) and was caused by acetaminophen overdose (39%) followed by idiosyncratic drug reactions (13%), viral hepatitis (12%), and other indeterminate causes (17%); overall patient survival at 3 weeks was 67% [1].

Thromboprophylaxis With Heparin During Orthotopic Liver Transplantation: Comparison of Hepcon HMS Plus and Anti-Xa Assays for Low-Range Heparin

OBJECTIVES The purpose of this study was to compare the agreement between two heparin assays, Hepcon HMS plus/Kaolin-ACT and Anti-Xa, and their predictive power in detecting circulating heparin levels post-reperfusion of the liver graft when compared with thromboelastogram (TEG) r time ratio in patients undergoing orthotopic liver transplantation (OLT). DESIGN Prospective, observational cohort study design. SETTING Single center, university hospital. PARTICIPANTS Thirty-eight consecutive adults who had undergone liver transplant. INTERVENTIONS None. MEASUREMENTS AND MAIN RESULTS Paired arterial blood samples were collected before surgical incision, 5 minutes after administration of an average dose of 2,054±771 units of intravenous unfractionated heparin before caval cross-clamping, 5 minutes after portal reperfusion, 5 minutes after hepatic artery reperfusion, and 1 hour after hepatic artery reperfusion. The observations that heparin assay measurements were within the predetermined limits of agreement, strongly suggested the two heparin assays (Hepcon HMS plus and Anti-Xa assay) are interchangeable during prophylactic heparin dose therapy during OLT. Post-reperfusion, receiver operating characteristic curve analysis revealed high accuracy in measuring circulating heparin levels with both Anti-Xa and Hepcon HMS assays when compared with the TEG r time ratio assay. CONCLUSIONS The point-of-care Hepcon HMS plus/Kaolin-ACT (activated clotting time) assay appeared to be a reliable alternative to the more expensive and laboratory-required Anti-Xa assay in monitoring the response to intravenous heparin in patients undergoing OLT.