Human retroviruses in health and disease

Before 1980, infectiously transmitted human retroviruses were unknown. Since then three types of human T-lymphotropic viruses (HTLVs) have been discovered and are under intensive study. Further types may well come to light. HTLV-I is etiologically associated with adult T-cell leukemia-lymphoma (ATLL), HTLV-II has been isolated from a patient with hairy T-cell leukemia, and HTLV-III is the cause of acquired immune deficiency syndrome (AIDS). Viruses related to HTLVs occur in several species of macaque monkeys. It appears that only a small minority of infected subjects develop the associated malignancy or immunodeficiency. Little is known of HTLV transmission, although it is clear that the viruses can be transmitted sexually and also iatrogenically through blood. HTLV-I and HTLV-II appear to be highly conserved across different human host populations, whereas HTLV-III shows greater polymorphism and elicits different immune responses. HTLVs have a tropism for T-helper/inducer cells. While HTLV-I and HTLV-II can penetrate many cell types and the T-cell tropism may reside in the activity of the X gene product following infection, initial infection of HTLV-III is restricted to cells bearing the T4 cell surface antigen. The T4 antigen is an essential component of the receptor for the virus, which is consistent with its tropism and the nature of the immunodeficiency it causes.