HTLV: the family of human T-lymphotropic retroviruses and their role in leukemia and AIDS.

Since 1980 four members of a new retrovirus family called the human T-lymphotropic viruses (HTLV-I to -IV) have been discovered and to a large extent characterised. Important common features of these viruses are: transmission by sexual contact or blood, tropism for the T4-lymphocyte and the effects of a viral transactivating gene (tat). They differ in their pathogenic effects, HTLV-I and -II being associated with lymphoproliferative malignancies, while HTLV-III is cytopathic and associated with immunosuppression. HTLV-IV is, apparently, not directly highly pathogenic for man. An African origin for this family of viruses is suggested by recent isolations of similar (STLV-I and -III) viruses in Old World primates. Prospects for the possible development of a cross-reactive vaccine have increased since the demonstration of highly conserved DNA sequences in the envelope region of several divergent HTLV-III strains.