New insights into hepatitis B and hepatitis delta virus entry

Interferon impedes an early step of hepatitis delta virus infection

Hepatitis delta virus (HDV) infects hepatocytes, the major cell type of the liver. Infection of the liver may be either transient or chronic. The prognosis for patients with chronic HDV infection is poor, with a high risk of cirrhosis and hepatocellular carcinoma. The best antiviral therapy is weekly administration for at least one year of high doses of interferon alpha. This efficacy of interferon therapy has been puzzling in that HDV replication in transfected cell lines is reported as insensitive to administration of interferon alpha or gamma. Similarly, this study shows that even when an interferon response was induced by transfection of poly(IC) into a cell line, HDV RNA accumulation was only modestly inhibited. However, when the HDV replication was initiated by infection of primary human hepatocytes, simultaneous addition of interferons alpha or gamma at 600 units/ml, a concentration comparable to that achieved in treated patients, the subsequent HDV RNA accumulation was inhibited by at least 80%. These interferon treatments were shown to produce significant time-dependent increases of host response proteins such as for Stat-1, phosphoStat-1, Mx1/2/3 and PKR, and yet interferon pretreatment of hepatocytes did not confer an increased inhibition of HDV replication over interferon treatment at the time of (or after) infection. These and other data support the interpretation that interferon action against HDV replication can occur and is largely mediated at the level of entry into primary human hepatocytes. Thus in vivo, the success of long-term interferon therapy for chronic HDV, may likewise involve blocking HDV spread by interfering with the initiation of productive infection of naïve hepatocytes.

Purinergic receptor functionality is necessary for infection of human hepatocytes by hepatitis delta virus and hepatitis B virus

Hepatitis B virus (HBV) and hepatitis delta virus (HDV) are major sources of acute and chronic hepatitis. HDV requires the envelope proteins of HBV for the processes of assembly and infection of new cells. Both viruses are able to infect hepatocytes though previous studies have failed to determine the mechanism of entry into such cells. This study began with evidence that suramin, a symmetrical hexasulfated napthylurea, could block HDV entry into primary human hepatocytes (PHH) and was then extrapolated to incorporate findings of others that suramin is one of many compounds that can block activation of purinergic receptors. Thus other inhibitors, pyridoxal-phosphate-6-azophenyl-2′,4′-disulfonate (PPADS) and brilliant blue G (BBG), both structurally unrelated to suramin, were tested and found to inhibit HDV and HBV infections of PHH. BBG, unlike suramin and PPADS, is known to be more specific for just one purinergic receptor, P2X7. These studies provide the first evidence that purinergic receptor functionality is necessary for virus entry. Furthermore, since P2X7 activation is known to be a major component of inflammatory responses, it is proposed that HDV and HBV attachment to susceptible cells, might also contribute to inflammation in the liver, that is, hepatitis.

Diffusion-dependent mechanisms of receptor engagement and viral entry

Enveloped viruses attach to host cells by binding to receptors on the cell surface. For many viruses, entry occurs via membrane fusion after a sufficient number of receptors have engaged ligand proteins on the virion. Under conditions where the cell surface receptor densities are low, recruitment of receptors may be limited by diffusion rather than by receptor-ligand interactions. We present a receptor-binding model that includes the effects of receptor availability at the viral binding site. The receptor binding and unbinding kinetics are coupled to receptor diffusion across the cell membrane. We find numerical solutions to our model and analyze the viral entry probabilities and the mean times to entry as functions of receptor concentration, receptor diffusivity, receptor binding stoichiometry, receptor detachment rates, and virus degradation/detachment rates. We also show how entry probabilities and times differ when receptors bind randomly or sequentially to the binding sites on the viral glycoprotein spikes. Our results provide general insight into the biophysical transport mechanisms that may arise in viral attachment and entry.

Chapter 3. Replication of the hepatitis delta virus RNA genome

Hepatitis delta virus (HDV) is a subviral agent dependent upon hepatitis B virus (HBV). HDV uses the envelope proteins of HBV to achieve assembly and infection of target cells. Otherwise, the replication of the RNA genome of HDV is totally different from that of its helper virus, and involves redirection of host polymerase activity. This chapter is concerned with recent developments in our understanding of the genome replication process.