Endothelial cell-mediated uptake of a hepatitis B virus: A new concept of liver targeting of hepatotropic microorganisms

Abstract

The liver is a target for many infectious agents, most notably hepatitis viruses. However, several receptor molecules identified so far for hepatitis viruses were found to be ubiquitously expressed and can thus not account for efficient liver targeting. Using a model hepatitis B virus, the duck hepatitis B virus (DHBV), we have obtained data indicating that scavenging liver sinusoidal endothelial cells (LSEC), rather than hepatocytes themselves, play the key role in the initial uptake of viral pathogens into the liver. Many microorganisms, most importantly hepatitis viruses, infect the liver with high efficiency, a process assumed to rely on expression of hepatocyte-specific receptor molecules mediating this tissue tropism. However, recently identified attachment receptors for hepatitis A virus (HAV): HAVcr-1, 1,2 for hepatitis C virus (HCV): CD81, 3 for duck hepatitis B virus (DHBV): carboxypeptidase D, 4-6 and putative receptors for hepatitis B virus 7-9 all lack a recognizable liver specificity. On the other hand, despite using a hepatocyte-specific receptor, asialoglycoprotein-receptor, the Marburg virus has a quite different tropism, infecting a variety of cells and tissues. 10 Thus, additional mechanisms must be considered to explain the efficiency by which some of these viruses are targeted to the liver.We have addressed this issue using DHBV as a model hepatitis virus. In this very efficient infection system, the virus attachment and uptake receptor, carboxypeptidase D, is a Golgi-resident protein that functions as a virus receptor by a recycling transport via the plasma membrane. 5 However, carboxypeptidase D is ubiquitously expressed, 4 which raises the question of how these viruses uniquely target hepatocytes, the only cell capable of replicating the virus. Finally, hepatocytes are not directly exposed to agents that pass the liver in the blood. Sinusoidal endothelial cells of the liver (LSEC) that line the hepatic sinusoids, although fenestrated, physically separate the sinusoidal blood from the hepatocytes 11 ( Fig. 1A and 1B). To reach the hepatocytes, blood-borne viruses have thus either to pass LSEC through the fenestrae or be actively transported across the endothelium.Using the DHBV model, we have obtained data that support the concept of an active transport of viruses across the liver endothelium, thereby suggesting a key role for LSEC in the uptake of viral pathogens into the liver. MATERIALS AND METHODS

Identification of a structural motif crucial for infectivity of hepatitis B viruses

“…Recent work suggests that DHBV is internalized by receptormediated endocytosis (10)(11)(12)18). Based on these data we hypothesized that TLM deficiency might result in retention of the virus within the endosomal compartment.…”

“…Published work suggested that hepadnaviral infection relies on an endocytic process (10)(11)(12)18), and the presence of enveloped viral particles in purified endosomal fractions at an early stage of infection was very recently visualized by electron microscopy (unpublished data). However, the mechanisms allowing escape of the virus and the viral nucleocapsid from the endocytotic 3, 5, and 7).…”

L-SIGN (CD 209L) is a liver-specific capture receptor for hepatitis C virus

“…Receptor-mediated transcytosis of HBV results in efficient delivery of virus from the circulation to the target cell population (23,25) but the identity of the transcytosing receptor has not been determined. If operative in the setting of natural infection, this process could facilitate the establishment of initial as well as chronic infection.…”

Inhibition of duck hepatitis B virus infection of liver cells by combined treatment with viral e antigen and carbohydrates

“…In search for a possible entry pathway of DHBV into hepatocytes, it was postulated that DHBV needs to be transported across LSECs to reach the hepatocyte compartment in the liver and that this pathway is conserved in vitro (Breiner et al, 2001). By co-staining with fluorescently labelled acLDL, we found DHBe but not DHBs from SVPs associated with LSECs.…”

“…Thus, another receptor or additional factors are required for DHBV infection. Also, the participation of another cell type, LSECs, which are in close contact with hepatocytes in vivo, has been discussed previously (Breiner et al, 2001). The observation that DHBe inhibited DHBV infection prompted us to find out which cells were able to bind or take up DHBe.…”