Human T-cell leukemia virus 2 (HTLV‑2) is a human retrovirus belonging to the genus Deltaretrovirus, family Retroviridae. It is closely related to HTLV‑1 but differs in its tissue tropism and disease associations. The enveloped virion contains two identical copies of a positive‑sense single‑stranded RNA genome approximately 9 kb in length. Following entry, the RNA genome is reverse transcribed into DNA and integrated into the host genome as a provirus. HTLV‑2 primarily infects CD8+ T lymphocytes and encodes structural proteins (Gag, Pol, Env) and regulatory proteins including Tax‑2 and antisense transcripts that modulate viral replication and host cell function.
Molecular Features and Transmission
HTLV‑2 is classified with HTLV‑1 in the genus Deltaretrovirus but has distinct molecular features. Four major subtypes (HTLV‑2a to HTLV‑2d) have been described based on differences in the envelope and regulatory regions. Its genome is flanked by long terminal repeats that drive transcription of viral genes. The Tax‑2 protein activates viral gene expression and cell‑cycle regulatory pathways, although its transforming capacity is weaker than Tax‑1. Entry into cells uses the same receptor complex as HTLV‑1 (glucose transporter 1, neuropilin‑1 and heparan sulphate proteoglycans). HTLV‑2 is transmitted through parenteral exposure to infected lymphocytes, sexual contact and breastfeeding. Infections are concentrated among indigenous peoples in the Americas and among injection drug users in North America and Europe. Unlike HTLV‑1, HTLV‑2 infection is usually clinically silent and persists via clonal expansion of infected T cells.
Clinical Associations and Epidemiology
Although HTLV‑2 was first isolated from a patient with a variant form of hairy cell leukemia, subsequent studies have not shown a strong link between HTLV‑2 and malignant disease. A minority of infected individuals develop neurological disorders resembling HTLV‑1‑associated myelopathy, peripheral neuropathies or chronic inflammatory conditions such as arthritis and dermatitis. HTLV‑2 has also been associated with thrombocytopenia and mild immune dysregulation, but causal relationships remain uncertain. Global estimates suggest around 1–2 million people are infected, with prevalence ranging from 5–40 % in some Native American and Brazilian indigenous communities and around 1–5 % among people who inject drugs in Europe and North America. There is no specific antiviral therapy; management is supportive, and prevention strategies mirror those for HTLV‑1, including screening blood donors, discouraging breast‑feeding by seropositive mothers and harm‑reduction programs for injection drug users. Because most infections remain asymptomatic, awareness and surveillance are necessary to understand the long‑term outcomes of this virus. HTLV‑2 provides insight into the diversity of human retroviruses and illustrates how closely related viruses can have markedly different clinical impacts. Continued research is needed to clarify its pathogenic potential and to inform public health strategies for populations at risk. Related Terms: Human T‑Cell Leukemia Virus 1, Human T‑Cell Leukemia Virus 3, Human T‑Cell Leukemia Virus 4, Human Immunodeficiency Virus 1, Human Immunodeficiency Virus 2