Isolation of STLV–I from Orangutan, a Great Ape Species in Southeast Asia, and Its Relation to Other HTLV–Is/STLV–Is

To study the evolutionary origin of human T–lymphotropic virus type I/simian T–lymphotropic virus type I (HTLV–I/STLV–I), we isolated and characterized STLV–I from orangutans (Pongo pygmaeus). Plasma samples from 3 out of 41 animals examined were reactive by particle agglutination and immunofluorescence, and one of these three was confirmed to be anti–HTLV–I antibody–positive by western blotting (WB). Cultured peripheral blood mononuclear cells from the WB–positive orangutan were reactive to anti–STLV–I–positive rhesus monkey plasma. The proviral long terminal repeat region was amplified by polymerase chain reaction and sequenced. A phylogenetic analysis indicated that orangutan STLV–I is related to the Melanesian group of HTLV–Is and other Asian STLV–Is, but the degree of divergence is considerable.