Human T-cell leukemia virus 1 (HTLV‑1) is a human retrovirus belonging to the genus Deltaretrovirus, family Retroviridae. It has an enveloped particle containing two copies of a positive‑sense single‑stranded RNA genome about 9 kb long. After entry into host cells, HTLV‑1 reverse transcribes its genome into DNA and integrates into the host genome as a provirus, establishing a lifelong infection. It primarily targets CD4+ T lymphocytes and uses cellular glucose transporter 1 and neuropilin‑1 as receptors. The virus encodes structural genes (gag, pol, env) and regulatory proteins including Tax and HBZ that modulate viral replication and host cell proliferation.
Classification, Pathogenesis and Transmission
HTLV‑1 is classified within the genus Deltaretrovirus along with bovine leukemia virus and simian T‑cell leukemia virus. Its genome contains long terminal repeats that flank the coding region and serve as promoters for viral gene expression. The Tax protein acts as a potent transcriptional activator of viral and cellular genes, driving clonal proliferation of infected T cells, while HBZ is expressed from the antisense strand and contributes to persistence. Transmission occurs through prolonged breastfeeding, sexual contact, sharing of contaminated needles and transfusion of unscreened blood. Efficient spread requires cell‑to‑cell contact rather than cell‑free virions. Once integrated, the provirus replicates mainly by mitotic expansion of the infected cell. Epidemiological studies estimate 5–10 million people are infected worldwide, with endemic foci in southwestern Japan, the Caribbean, South America, West and Central Africa and central Australia. Most carriers remain asymptomatic, but about 2–7 % will develop serious disease.
Diseases and Public Health Significance
The main malignant outcome of HTLV‑1 infection is adult T‑cell leukemia/lymphoma, an aggressive neoplasm of CD4+ T cells that usually arises decades after infection. Patients present with lymphadenopathy, hypercalcemia and skin lesions. Neurological disease can manifest as HTLV‑1‑associated myelopathy/tropical spastic paraparesis, a progressive spinal cord inflammation leading to weakness and gait disturbance. The virus is also linked to uveitis, dermatitis, alveolitis and opportunistic infections due to immune dysregulation. No curative therapy exists; treatment of adult T‑cell leukemia often involves combination chemotherapy or antiviral therapy combined with interferon‑α and zidovudine, while management of myelopathy is mainly supportive with corticosteroids. Prevention relies on screening of blood donors, advising against prolonged breastfeeding by infected mothers and promoting safer sex practices. Because the infection is lifelong and asymptomatic carriers serve as reservoirs, public health measures focus on detection and counseling in endemic regions.
The discovery of HTLV‑1 in 1980 provided the first evidence that a human retrovirus could cause cancer. Understanding its transmission routes and disease spectrum has guided screening policies and highlighted the need for continued surveillance and support for affected communities. Better therapeutic strategies are being explored, but prevention remains the most effective control method.
Related Terms: Human T‑Cell Leukemia Virus 2, Human T‑Cell Leukemia Virus 3, Human T‑Cell Leukemia Virus 4, Human Immunodeficiency Virus 1, Human Immunodeficiency Virus 2