The physiological function of the liver--such as removal of pathogens and antigens from the blood, protein synthesis and metabolism--requires an immune response that is adapted to these tasks and is locally regulated. Pathogenic microorganisms must be efficiently eliminated while the large number of antigens derived from the gastrointestinal tract must be tolerized. From experimental observations it is evident that the liver favours the induction of tolerance rather than the induction of immunity. The liver probably not only is involved in transplantation tolerance but contributes as well to tolerance to orally ingested antigens (entering the liver with portal-venous blood) and to containment of systemic immune responses (antigen from the systemic circulation entering the liver with arterial blood). This review summarizes the experimental data that shed light on the molecular mechanisms and the cell populations of the liver involved in local immune regulation in the liver. Although hepatocytes constitute the major cell population of the liver, direct interaction of hepatocytes with leukocytes in the blood is unlikely. Sinusoidal endothelial cells, which line the hepatic sinusoids and separate hepatocytes from leukocytes in the sinusoidal lumen, and Kupffer cells, the resident macrophage population of the liver, can directly interact with passenger leukocytes. In the liver, clearance of antigen from the blood occurs mainly by sinusoidal endothelial cells through very efficient receptor-mediated endocytosis. Liver sinusoidal endothelial cells constitutively express all molecules necessary for antigen presentation (CD54, CD80, CD86, MHC class I and class II and CD40) and can function as antigen-presenting cells for CD4+ and CD8+ T cells. Thus, these cells probably contribute to hepatic immune surveillance by activation of effector T cells. Antigen-specific T-cell activation is influenced by the local microenvironment. This microenvironment is characterized by the physiological presence of bacterial constituents such as endotoxin and by the local release of immunosuppressive mediators such as interleukin-10, prostaglandin E2 and transforming growth factor-beta. Different hepatic cell populations may contribute in different ways to tolerance induction in the liver. In vitro experiments revealed that naive T cells are activated by resident sinusoidal endothelial cells but do not differentiate into effector T cells. These T cells show a cytokine profile and a functional phenotype that is compatible with the induction of tolerance. Besides sinusoidal endothelial cells, other cell populations of the liver, such as dendritic cells, Kupffer cells and perhaps also hepatocytes, may contribute to tolerance induction by deletion of T cells through induction of apoptosis.

Local control of the immune response in the liver.

The liver has vital metabolic and clearance functions that involve the uptake of nutrients, waste products and pathogens from the blood. In addition, its unique immunoregulatory functions mediated by local expression of co-inhibitory receptors and immunosuppressive mediators help to prevent inadvertent organ damage. However, these tolerogenic properties render the liver an attractive target site for pathogens. Although most pathogens that reach the liver via the blood are eliminated or controlled by local innate and adaptive immune responses, some pathogens (such as hepatitis viruses) can escape immune control and persist in hepatocytes, causing substantial morbidity and mortality worldwide. Here, we review our current knowledge of the mechanisms of liver targeting by pathogens and describe the interplay between pathogens and host factors that promote pathogen elimination and maintain organ integrity or that allow pathogen persistence.

Living in the liver: hepatic infections

DC-SIGN and DC-SIGNR are two closely related membrane-associated C-type lectins that bind human immunodeficiency virus (HIV) envelope glycoprotein with high affinity. Binding of HIV to cells expressing DC-SIGN or DC-SIGNR can enhance the efficiency of infection of cells coexpressing the specific HIV receptors. DC-SIGN is expressed on some dendritic cells, while DC-SIGNR is localized to certain endothelial cell populations, including hepatic sinusoidal endothelial cells. We found that soluble versions of the hepatitis C virus (HCV) E2 glycoprotein and retrovirus pseudotypes expressing chimeric forms of both HCV E1 and E2 glycoproteins bound efficiently to DC-SIGN and DC-SIGNR expressed on cell lines and primary human endothelial cells but not to other C-type lectins tested. Soluble E2 bound to immature and mature human monocyte-derived dendritic cells (MDDCs). Binding of E2 to immature MDDCs was dependent on DC-SIGN interactions, while binding to mature MDDCs was partly independent of DC-SIGN, suggesting that other cell surface molecules may mediate HCV glycoprotein interactions. HCV interactions with DC-SIGN and DC-SIGNR may contribute to the establishment or persistence of infection both by the capture and delivery of virus to the liver and by modulating dendritic cell function.

Hepatitis C Virus Glycoproteins Interact with DC-SIGN and DC-SIGNR

PURPOSE: Preclinical studies have shown good anticancer activity following targeting of a polymer bearing doxorubicin with galactosamine (PK2) to the liver. The present phase I study was devised to determine the toxicity, pharmacokinetic profile, and targeting capability of PK2. PATIENTS AND METHODS: Doxorubicin was linked via a lysosomally degradable tetrapeptide sequence to N-(2-hydroxypropyl)methacrylamide copolymers bearing galactosamine. Targeting, toxicity, and efficacy were evaluated in 31 patients with primary (n = 25) or metastatic (n = 6) liver cancer. Body distribution of the radiolabelled polymer conjugate was assessed using gamma-camera imaging and single-photon emission computed tomography. RESULTS: The polymer was administered by intravenous (IV) infusion over 1 hour, repeated every 3 weeks. Dose escalation proceeded from 20 to 160 mg/m2 (doxorubicin equivalents), the maximum-tolerated dose, which was associated with severe fatigue, grade 4 neutropenia, and grade 3 mucositis. Twenty-four ho...

Hepatic Drug Targeting: Phase I Evaluation of Polymer-Bound Doxorubicin

Novel stable, concentrated, biologically active and ready-to-use lipid-comprising drug delivery complexes and methods for their production are described. The biological activity of the complexes produced are comparable to the formulations prepared according to the prior art admixture method and upon purification, the complexes produced by the method of this invention are 50 to 500 fold more concentrated than the complexes formed by admixture. The method described herein provides for the large scale production of lipid-comprising drug delivery systems useful for gene therapy and other applications.

Assadollah Djovkar

Papers

Endocytosis via galactose receptors in vivo. Ligand size directs uptake by hepatocytes and/or liver macrophages.