Human retroviruses: a common virology

There are five known human retroviruses: human T-lymphotropic virus-I (HTLV-I), HTLV-II, HTLV-V, human immunodeficiency virus-1 (HIV-1), and HIV-2. These are related to animal lentiviruses. The simian retroviruses, simian T-lymphotropic virus-I (STLV-I) and STLV-III are related closely to HTLV-I and HIV-2 respectively. HTLV-I and HTLV-II and, possibly, HTLV-V are transforming agents that immortalize the CD4 cell. In contrast, HIV-1 and HIV-2 cause this cell to lyse, resulting in immunodeficiency (ID). HIV-1 and HIV-2 cause severe ID resulting in acquired immunodeficiency syndrome (AIDS). In HTLV-I and HTLV-II, ID is less severe and rarely progressive. Both of these retroviruses induce proliferation of CD4 cells. In HTLV-I, this results in acute T cell leukemia and mycosis fungoides (MF) with hypercalcemia. HTLV-V produces a less severe form of MF without hypercalcemia. Associated lymphomas (AL) occur with HTLV-I. HIV-1 and HIV-2 produce AL as well as Kaposi's sarcoma. Both also cause subcortical dementia because they are neurotropic. All human retroviruses appear to be transmitted sexually and by blood. Transfusional AIDS may be almost entirely eliminated by serologic testing of the blood supply, and transfusional lymphoma can be almost entirely eliminated by universal testing for HTLV-I.