Torque Teno Virus: Lights and Shades

Abstract

Torque Teno Virus (TTV) is a highly prevalent non-pathogenic DNA virus whose plasma levels may be related to the host's immune status. TTV gained attention about 25 years ago, but its replication is not fully understood, nor is its relationship with the host's immune system. Despite this lack of knowledge, TTV is currently being investigated as a functional biomarker of the immune system in patients with immunological damage and inflammatory diseases. Monitoring TTV viral load over time may help clinicians in making therapeutic decisions regarding immunosuppression as well as the likelihood of infectious complications. This review summarizes what we do and do not know about this enigmatic virus.

Keywords: anelloviruses; immune marker; immunocompromised; solid organ transplant; torque teno virus (TTV); transplantation; viral load.

Figure 1

TTV genome organization. The conserved non-coding region (UTR) and the coding region sequence are illustrated. ORF1 is responsible for the transcription of the capsid protein and contains hypervariable regions (HVRs). ORF2 exerts regulatory effects on both innate and adaptive immunity, as well as the inflammatory response. ORF3 is responsible for the transcription of the TTV-derived apoptosis-inducing protein (TAIP), while ORF4 is a protein whose function remains to be elucidated.

Figure 2

Anelloviridae genera and their hosts. The 34 genera of the family Anelloviridae are shown. The nine genera identified as capable of infecting humans are shown in grey: Alphatorquevirus, Betatorquevirus, Gammatorquevirus, Hetorquevirus, Gyrovirus, Lamedtorquevirus, Memtorquevirus, Samektorquevirus, and Yodtorquevirus. For the other genera, mammalian and invertebrate hosts are indicated. As shown, some genera have a preferred host, while others have an extended host tropism.