TT virus infection: a novel virus-host relationship

This review examines TT virus (TTV), a recently discovered non-enveloped DNA virus that establishes a persistent, ubiquitous infection in humans and other primates. TTV is a small 30-32 nm virus with a circular single-stranded DNA genome of 3750-3900 bases encoding 4-5 genes. The virus exhibits remarkable genetic diversity, with at least 28 distinct genotypes classified into four main groups showing 50% nucleotide divergence, and closely related TTV-like minivirus (TLMV) variants. TTV infections are characterized by persistent viraemia with extremely high replication rates exceeding 10 billion virions daily. The virus replicates in multiple tissues, particularly bone marrow, lymphoid tissue, lung, and liver, suggesting dependence on dividing cells. While TTV is ubiquitous in humans and other primates with evidence of co-evolution, disease associations remain unclear. Some genotypes may associate with post-transfusion hepatitis, though this remains controversial. Higher viral loads occur in immunocompromised individuals, suggesting immune system control of replication. The virus's evolutionary relationship with chicken anaemia virus suggests that TTV-like viruses may infect all land vertebrates. The persistent, non-pathogenic nature of TTV in the human population suggests possible commensal or even symbiotic relationships, raising fundamental questions about viral-host adaptation.

Key findings

• TTV is a ubiquitous, persistent infection in all humans and primates, with extremely high daily replication rates exceeding 10 billion virions and 28+ distinct genotypes showing remarkable genetic diversity
• The virus replicates primarily in bone marrow, lymphoid tissue, lung and liver tissues, with viral loads controlled by immune system activity, as evidenced by higher loads in immunocompromised patients
• TTV exhibits co-evolutionary relationships with primate hosts, with viral genetic diversity patterns mirroring host evolutionary relationships, suggesting ancient establishment across mammalian species
• Disease associations with TTV remain unclear despite ubiquitous infection, questioning whether certain genotypes cause post-transfusion hepatitis or bone marrow complications
• TTV may represent a successful symbiotic or commensal virus-host relationship rather than a parasitic one, raising questions about potential immune and physiological consequences of persistent infection