| Human T-lymphotropic virus | |
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| Virus classification | |
| Group: | Group VI (ssRNA-RT) |
| Family: | Retroviridae' |
| Subfamily: | Orthoretrovirinae |
| Genus: | Deltaretrovirus |
| Species: | Simian T-lymphotropic virus |
| Serotypes | |
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Human T-lymphotropic virus |
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Human T-cell Lymphotropic Virus or Human T-lymphotropic virus Type 1 (HTLV-I), also called the Adult T-cell lymphoma virus type 1, a virus that has been seriously implicated in several kinds of diseases including HTLV-I-associated myelopathy, Strongyloides stercoralis hyper-infection, and a virus cancer link for leukemia (see adult T-cell leukemia/lymphoma). Between one in twenty and one in twenty-five infected persons are thought to develop cancer as a result of the virus.
HTLV was discovered in 1977 in Japan. The virus was first isolated by Drs. Bernard Poiesz and Francis Ruscetti and their co-workers in the laboratory of Robert C. Gallo at the NCI.[1] It was the first identified human retrovirus.
Infection with HTLV-I, like infection with other retroviruses, probably occurs for life and can be inferred when antibody against HTLV-1 is detected in the serum.
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Signs and symptoms of HTLV myelopathy include:
Other neurologic findings that may be found in HTLV include:
HTLV-I infection in the United States appears to be about half as prevalent as HIV infection among IV drug users and about one-tenth as prevalent in the population at large. Although little serologic data exist, prevalence of infection is thought to be highest among blacks living in the Southeast. A prevalence rate of 30% has been found among black intravenous drug users in New Jersey, and a rate of 49% has been found in a similar group in New Orleans.[3] It is possible that prevalence of infection is increasing in this risk group.
HTLV-I infection in Australia is very high among the Indigenous peoples of central and northern Australia, with a prevalence rate of 10-30%. It is also high among the Inuit of Northern Canada, in Japan, northeastern Iran,[4] Peru, the Pacific coast of Colombia and Ecuador, the Caribbean, and in Africa.
Transmission of HTLV-I is believed to occur from mother to child via breastfeeding; by sexual contact; and through exposure to contaminated blood, either through blood transfusion or sharing of contaminated needles. The importance of the various routes of transmission is believed to vary geographically.
The term viral tropism refers to which cell types HTLV-I infects. Although HTLV-1 is primarily found in CD4+ T cells, other cell types in the peripheral blood of infected individuals have been found to contain HTLV-1, including CD8+ T cells, dendritic cells, and B cells. HTLV-I entry is mediated through interaction of the surface unit of the virion envelope glycoprotein (SU) with its cellular receptor GLUT1, a glucose transporter, on target cells.[7]
Individuals infected with HTLV-1 are at risk for opportunistic infections, diseases not caused by the virus itself, but by alterations in the host's immune functions.
HTLV-1, unlike the distantly related retrovirus HIV, has an immunostimulating effect which actually becomes immunosuppressive. The virus activates a subset of T-helper cells called Th1 cells. The result is a proliferation of Th1 cells and overproduction of Th1 related cytokines (mainly IFN-γ and TNF-α). Feedback mechanisms of these cytokines cause a suppression of the Th2 lymphocytes and a reduction of Th2 cytokine production (mainly IL-4, IL-5, IL-10 and IL-13). The end result is a reduction in the ability of the infected host to mount an adequate immune response to invading organisms that require a predominantly Th2 dependent response (these include parasitic infections and production of mucosal and humoral antibodies).
In the central Australian Aboriginal population, HTLV-1 is thought to be related to their extremely high rate of death from sepsis.
It is particularly associated with bronchiectasis, a chronic lung condition predisposing to recurrent pneumonia.
It is also associated with chronic infected dermatitis, often superinfected with Staphylococcus aureus and a severe form of Strongyloides stercoralis infection called hyper-infestation which may lead to death from polymicrobial sepsis.
HTLV-1 is also associated with adult T cell leukemia/lymphoma, and has been quite well studied in Japan. The time between infection and onset of cancer also varies geographically. It is believed to be about sixty years in Japan, and less than forty years in the Caribbean. The cancer is thought to be due to the pro-oncogenic effect of viral DNA incorporated into host lymphocyte DNA, and chronic stimulation of the lymphocytes at the cytokine level may play a role in development of malignancy. The malignancy ranges from a very indolent and slowly progressive lymphoma to a very aggressive and nearly uniformly lethal proliferative lymphoma.
HTLV-1 is also associated with a progressive demyelinating upper motor neurone disease known as HAM/TSP for HTLV-1 associated myelopathy/Tropical Spastic Paraparesis characterized by sensory and motor deficits, particularly of the lower extremities, incontinence and impotence.[8] Less than 2% of infected individuals develop HAM/TSP, but this will vary dramatically from one geographic location to another.
Treatment for opportunistic infections varies depending on the type of disease and varies from careful observation to aggressive chemotherapy and antiretroviral agents. Adult T cell lymphoma is a common complication of HLTV infection, and requires aggressive chemotherapy, typically R-CHOP. Other treatments for ATL in HLTV infected patients include interferon alpha, zidovudine with interferon alpha, and CHOP with arsenic trioxide. Treatments for HLTV myelopathy are even more limited, and focus mainly on symptomatic therapy. Therapies studied include corticosteroids, plasmapherisis, cyclophosphamide, and interferon, which may produce a temporary symptomatic improvement in myelopathy symptoms.Valproic acid has been studied to determine if it might slow the progression of HLTV disease by reducing viral load. Although in one human study it was effective in reducing viral load, there did not appear to be a clinical benefit. Recently, however, a study of valproic acid combined with zidovudine showed a major decrease in the viral load of baboons infected with HLTV-1. It is important to monitor for opportunistic infections such as cytomegalovirus, histoplasmosis, scabies, pneumocystis pneumonia, and staphylococcal infections in HLTV patients. HIV testing should also be performed, as some patients may be co-infected with both viruses.[9] Allogenic bone marrow transplantation has been investigated in the treatment of HLTV-1 disease, with varied results. One case report describes a HLTV-1 infected woman who developed chronic refractory eczema, corneal injury, and adult T cell leukemia. She was subsequently treated with allogenic stem cell transplantion, and had complete resolution of symptoms. One year posttransplant, she has had no recurrence of any symptoms, and furthermore has had a decrease in her proviral load.[10]
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