For many years, retroviruses were known to be the cause of many kinds of animal leukemias and hematopoietic tumors. In spite of the high expectation that this would also be true for humans, very little evidence for retroviral involvement in any human diseases was forthcoming.

In the late 1970s, however, due to the development of sensitive and specific molecular methods to identify retroviruses and to produce large scale cultures of T lymphocytes, HTLV-I was discovered and implicated as the cause of adult T cell leukemia, a particular and relatively infrequent leukemia prevalent in southern Japan and parts of the Caribbean, and tropical spastic paraparesis, a demyelinating neuropathy similar to multiple sclerosis. The discovery that HTLV-I can be transmitted by breast milk has led to a significant decline in HTLV-I infections in Japan.

Although no retroviruses have been identified to date in other human leukemias or related diseases, the efforts that resulted in the discovery of HTLV-I were critical in isolating HIV-1 and identifying it as the cause of AIDS. The ability to grow the HIV-1 in quantity allowed the development of a blood test that has saved countless lives. The development of effective anti-retroviral drugs has made HIV-1 infection a somewhat manageable chronic condition rather than a certain death sentence. Although vaccine trials thus far have been rather disappointing, an effective vaccine is one of our most important needs.

Further reading: Retroviruses: Molecular Biology, Genomics and Pathogenesis

Labels: aids, Anti-retroviral drugs, HIV-1, HIV-2, HTLV-I, Paraparesis, Vaccine trials

Extensive analysis of naturally occurring simian immunodeficiency viruses (SIVs) and comparative phylogenetic studies with human immunodeficiency viruses (HIVs) suggests that the latter are close relatives of the SIVcpz viruses of chimpanzees (HIV-1) or the SIVsmm viruses of sooty mangabys (HIV-2).

Crossing of species barriers resulted in adaptation to the human host and subsequent acquisition of a pathogenic phenotype. Naturally occurring T lymphocyte-tropic lentiviral infections are highly prevalent and productive but are not usually pathogenic for native hosts. Crossing species barriers may produce an abortive infection or, as in the case of the HIVs, may enhance virulence after several cycles of transmission.

The large number of species carrying these viruses may suggest that infection confers an evolutionary advantage to the host. The virulent T-lymphocyte-tropic lentiviruses have a similar genomic structure and exhibit comparable replication strategies. Their major targets are lymphocytes populating lymphoid organs and tissues, and antigen-presenting cells (dendritic cells, mononuclear phagocytes). Within these targets the virus can replicate to very high titres and thereby exhaust CD4+ T cells, producing profound immunodeficiency. Although the infection of lymphoid organs and tissue is the pathologic hallmark of HIV infection, this virus also infects cells of the central nervous system

Further reading: Retroviruses: Molecular Biology, Genomics and Pathogenesis

Labels: hiv, HIV-1, HIV-2, Immunodeficiency Virus, Simian immunodeficiency virus, SIV, SIVsmm