TCR Signaling and CD28/CTLA-4 Signaling Cooperatively Modulate T Regulatory Cell Homeostasis.

Foxp3+ T regulatory cells (Tregs), conventional CD4+Foxp3- T cells, and CD8+ T cells represent heterogeneous populations composed of naive phenotype (NP, CD44low) and memory phenotype (MP, CD44high) subpopulations. NP and MP subsets differ in their activation state, contribution to immune function, and capacity to proliferate in vivo. To further understand the factors that contribute to the differential homeostasis of NP/MP subsets, we examined the differential effects of CD28 and CTLA-4 interaction with CD80/CD86, as well as MHC class II-TCR interaction within mouse Treg pools and CD4+ and CD8+ T cell pools. Blockade of CD80/CD86 with CTLA-4-Ig markedly reduced the cycling and absolute numbers of MP Tregs and MP CD4+ T cells, with minimal effect on the NP T cell subpopulations. Blockade of MHC class II-TCR interaction led to selective expansion of MP Tregs and MP CD4+ and CD8+ T cells that was reversed upon cotreatment with CTLA-4-Ig. Treatment with anti-CTLA-4 mAb altered MP Treg and MP CD4+ and CD8+ T cell homeostasis in a manner similar to that observed with anti-MHC class II. We postulate a complex pathway in which CD28 is the primary driver of Treg proliferation and CTLA-4 functions as the main brake but is likely dependent on TCR signals and CD80/CD86. These findings have important implications for the use of biologic agents targeting such pathways to modulate autoimmune and neoplastic disease.

Endogenous mouse mammary tumor viruses (mtv): new roles for an old virus in cancer, infection, and immunity.

Mouse Mammary Tumor Viruses are beta-retroviruses that exist in both exogenous (MMTV) and endogenous (Mtv) forms. Exogenous MMTV is transmitted via the milk of lactating animals and is capable of inducing mammary gland tumors later in life. MMTV has provided a number of critical models for studying both viral infection as well as human breast cancer. In addition to the horizontally transmitted MMTV, most inbred mouse strains contain permanently integrated Mtv proviruses within their genome that are remnants of MMTV infection and vertically transmitted. Historically, Mtv have been appreciated for their role in shaping the T cell repertoire during thymic development via negative selection. In addition, more recent work has demonstrated a larger role for Mtv in modulating host immune responses due to its peripheral expression. The influence of Mtv on host response has been observed during experimental murine models of Polyomavirus- and ESb-induced lymphoma as well as Leishmania major and Plasmodium berghei ANKA infection. Decreased susceptibility to bacterial pathogens and virus-induced tumors has been observed among mice lacking all Mtv. We have also demonstrated a role for Mtv Sag in the expansion of regulatory T cells following chronic viral infection. The aim of this review is to summarize the latest research in the field regarding peripheral expression of Mtv with a particular focus on their role and influence on the immune system, infectious disease outcome, and potential involvement in tumor formation.

Involvement of natural killer T cells in halothane-induced liver injury in mice.

Drug-induced liver injury (DILI) causes significant patient morbidity and mortality, and is the most common reason for drug withdrawals. It is imperative to gain a thorough understanding of the underlying mechanisms of DILI to effectively predict and prevent these reactions. We have recently developed a murine model of halothane-induced liver injury (HILI). The aim of the present study was to investigate the role of hepatic natural killer T (NKT) cells in the pathogenesis of HILI. The degrees of HILI were compared between WT and CD1d(-/-) mice, which are deficient in NKT cells. The data revealed that CD1d(-/-) mice were resistant in developing HILI. This resistance appeared to be a direct result of NKT cell depletion rather than an indirect one due to the absence of cross-talk between NKT cells and other hepatic innate immune cells. Compared with WT mice, CD1d(-/-) mice exhibited a significantly lower number of hepatic infiltrating neutrophils upon halothane challenge (470,000+/-100,000/liver in WT vs. 120,000+/-31,500/liver in CD1d(-/-) mice). This result in conjunction with our previous finding of an indispensable role of neutrophils in HILI strongly suggests that NKT cells play a critical role in regulating neutrophil recruitment, thereby contributing to the development of HILI. Collectively, the current study and published reports indicate that this murine model of HILI provides an experimental system for the investigation of the underlying mechanisms of DILI. In addition, this model may yield the discovery of susceptibility factors that may control the development of liver injury in patients treated with halothane and potentially other drugs.

Exacerbation of acetaminophen-induced disturbances of liver sinusoidal endothelial cells in the absence of Kupffer cells in mice.

Although there has been considerable research in terms of liver sinusoidal endothelial cells (LSECs) and hepatic resident macrophages (Kupffer cells, KCs) during the overall pathogenesis of acetaminophen (APAP)-induced liver injury, little is known about their potential interaction and relationship. In the present study, employing the use of liposome/clodronate to deplete KCs, we were able to confirm the previously demonstrated hepato-protective role for KCs. Such a protective role may be mediated, in part, via regulation of LSEC homeostasis and integrity. The further aggravation of APAP-induced LSEC disturbance upon depletion of KCs correlated with increased hepatic vascular permeability and red blood cell accumulation. The depletion of KCs prior to APAP challenge also resulted in the increased expression of cellular adhesion molecules on LSECs. Such increased disturbance in the hepatic endothelial may represent a contributing factor in the exacerbation of APAP-induced liver injury in the absence of KCs. However, these disturbances may also be a result of the increased hepatic damage, and as such, does not rule out the potential for additional mechanisms of KC-mediated hepato-protection during the pathogenesis of APAP-induced hepatotoxicity.

The role of damage associated molecular pattern molecules in acetaminophen-induced liver injury in mice.

The idiosyncratic nature, severity and poor diagnosis of drug-induced liver injury (DILI) make these reactions a major safety issue during drug development, as well as the most common cause for the withdrawal of drugs from the pharmaceutical market. Elucidation of the underlying mechanism(s) is necessary for identifying predisposing factors and developing strategies in the treatment and prevention of DILI. Acetaminophen (APAP) is a widely used over the counter therapeutic that is known to be effective and safe at therapeutic doses. However, in overdose situations fatal and non-fatal hepatic necrosis can result. Evidence suggests that the chemically reactive metabolite of the drug initiates hepatocyte damage and that inflammatory innate immune responses also occur within the liver, leading to the exacerbation and progression of tissue injury. Here we investigate whether following APAP-induced liver injury (AILI) damaged hepatocytes release "danger" signals or damage associated molecular pattern (DAMP) molecules, which induce pro-inflammatory activation of hepatic macrophages, further contributing to the progression of liver injury. Our study demonstrated a clear activation of Kupffer cells following early exposure to APAP (1h). Activation of a murine macrophage cell line, RAW cells, was also observed following treatment with liver perfusate from APAP-treated mice, or with culture supernatant of APAP-challenged hepatocytes. Moreover, in these media, the DAMP molecules, heat-shock protein-70 (HSP-70) and high mobility group box-1 (HMGB1) were detected. Overall, these findings reveal that DAMP molecules released from damaged and necrotic hepatocytes may serve as a crucial link between the initial hepatocyte damage and the activation of innate immune cells following APAP-exposure, and that DAMPs may represent a potential therapeutic target for AILI.

Identification and characterization of infiltrating macrophages in acetaminophen-induced liver injury.

The role of macrophages in the pathogenesis of acetaminophen (APAP)-induced liver injury remains controversial, as it has been demonstrated that these cells display pro-toxicant and hepato-protective functions. This controversy may stem from the heterogeneity and/or plasticity of macrophages and the difficulty in distinguishing and differentially studying subpopulations of macrophages in the liver. In the present study, using flow cytometric analysis and fluorescence-labeled antibodies against specific cell surface macrophage markers, we were able to, for the first time, identify an APAP-induced macrophage (IM) population distinct from resident Kupffer cells. The data demonstrated that the IMs were derived from circulating monocytes that infiltrated the liver following APAP-induced liver injury. The IMs exhibited a phenotype consistent with that of alternatively activated macrophages and demonstrated the ability to phagocytize apoptotic cells and induce apoptosis of neutrophils. Furthermore, in the absence of the IMs, the resolution of hepatic damage following APAP-induced hepatotoxicity was delayed in CCR2(-/-) mice compared with wild-type mice. These findings likely contribute to the role of the IMs in the processes of tissue repair, including counteracting inflammation and promoting angiogenesis. The present study also demonstrated the ability of separating populations of macrophages and delineating distinct functions of each group in future studies of inflammatory disease in the liver and other tissues.

Role of IL-6 in an IL-10 and IL-4 double knockout mouse model uniquely susceptible to acetaminophen-induced liver injury.

Drug-induced hepatitis remains a challenging problem for drug development and safety because of the lack of animal models. In the current work, we discovered a unique interaction that makes mice deficient in both IL-10 and IL-4 (IL-10/4-/-) highly sensitive to the hepatotoxic effects of acetaminophen (APAP). Male C57Bl/6 wild type (WT) and mice deficient in one or more cytokines were treated with 120 mg/kg APAP. Within 24 h after WT, IL-10-/-, IL-4-/-, or IL-10/4-/- mice were administered APAP, 75% of the IL-10/4-/- mice died of massive hepatic injury while all other genotypes were resistant to liver toxicity at this dose of APAP. The unique susceptibility of IL-10/4-/- mice was associated with reduced levels of liver glutathione and remarkably high serum levels of IL-6 and several proinflammatory factors including TNF-alpha, IFN-gamma, macrophage inflammatory protein-1alpha (MIP-1alpha), monocyte chemoattractant protein-1 (MCP-1), macrophage inflammatory protein-2 (MIP-2), and osteopontin (OPN) as well as nitric oxide (NO). IL-6 appeared to have a causal role in controlling the unique susceptibility of IL-10/4-/- mice to APAP-induced liver disease (AILD) because IL-6 neutralizing antibody reversed the high sensitivity of these mice to AILD. Moreover, IL-10/4/6-/- mice were also resistant to the enhanced susceptibility to AILD and expressed relatively low levels of most proinflammatory factor genes that were elevated in the IL-10/4-/- mice. In conclusion, liver homeostasis following AILD appears to be highly dependent on the activities of both IL-10 and IL-4, which together help prevent overexpression of IL-6 and other potential hepatotoxic factors.

Mechanisms of drug-induced liver injury.

The idiosyncratic nature and poor prognosis of drug-induced liver injury (DILI) make this type of reaction a major safety issue during drug development, as well as the most common cause for the withdrawal of drugs from the pharmaceutical market. The key to predicting and preventing DILI is understanding the underlying mechanisms. DILI is initiated by direct hepatotoxic effects of a drug, or a reactive metabolite of a drug. Parenchymal cell injury induces activation of innate and/or adaptive immune cells, which, in turn, produce proinflammatory and tissue hepatotoxic mediators, and/or mount immune reactions against drug-associated antigens. Understanding the molecular and cellular elements associated with these pathways can help identify risk factors and may ultimately facilitate the development of strategies to predict and prevent DILI.