Torque teno virus in liver diseases: On the way towards unity of view

Abstract

The review presents the data accumulated for more than 20 years of research of torque teno virus (TTV). Its molecular genetic structure, immunobiology, epidemiology, diagnostic methods, possible replication sites, and pathogenicity factors are described. TTV is a virus that is frequently detectable in patients with different viral hepatitides, in cases of hepatitis without an obvious viral agent, as well as in a healthy population. There is evidence suggesting that biochemical and histological changes occur in liver tissue and bile duct epithelium in TTV monoinfection. There are sufficient histological signs of liver damage, which confirm that the virus can undergo a replicative cycle in hepatocytes. Along with this, cytological hybridization in TTV-infected cells has shown no substantial cytopathic (cell-damaging) effects that are characteristic of pathogenic hepatotropic viruses. Studying TTV has led to the evolution of views on its role in the development of human pathology. The first ideas about the hepatotropism of the virus were gradually reformed as new data became available on the prevalence of the virus and its co-infection with other viruses, including the viruses of the known types of hepatitides. The high prevalence of TTV in the human population indicates its persistence in the body as a virome and a non-pathogenic virus. It has recently been proposed that the level of TTV DNA in the blood of patients undergoing organ transplantation should be used as an endogenous marker of the body’s immune status. The available data show the polytropism of the virus and deny the fact that TTV can be assigned exclusively to hepatitis viruses. Fortunately, the rare detection of the damaging effect of TTV on hepatic and bile duct epithelial cells may be indirect evidence of its conditionally pathogenic properties. The ubiquity of the virus and the variability of its existence in humans cannot put an end to its study.

Keywords: Torque teno virus; Prevalence of torque teno virus; Torque teno virus in liver disease; Virome and torque teno virus; Torque teno virus and human immune status

Core tip: The review is dedicated to torque teno virus (TTV) discovered in the late 20th century. It presents data on the morphofunctional properties of the virus, its immunobiology, prevalence, transmission routes, possible replication sites, methods for diagnosis and co-infection in liver diseases. The authors tried to analyze the information accumulated in the literature on these issues and to show the evolution of views on the importance of the virus to humans: From its hepatotropism to virome and a marker for the human immune status. Along with this, the review gives data suggesting that there are biochemical and histological changes in the liver and bile duct epithelium during TTV monoinfection. On the one hand, this information points to the possible replication of the virus in the cells of the liver and the epithelium of the bile ducts. On the other hand, the high prevalence of TTV in the population indicates its persistence in the body as a virome and a non-pathogenic virus. Moreover, the fortunately rare identification of the damaging effect of the virus on the cells of the liver and the epithelium of the bile ducts may be indirect evidence of its conditionally pathogenic properties.