Identification in gelada baboons (Theropithecus gelada) of a distinct simian T-cell lymphotropic virus type 3 with a broad range of Western blot reactivity

Antibodies to simian T-cell lymphotropic virus (STLV) were found in serum or plasma from 12 of 23 (52.2%) gelada baboons (Theropithecus gelada) captive in US zoos. A variety of Western blot (WB) profiles was seen in the 12 seroreactive samples, including human T-cell lymphotropic virus (HTLV)-1-like (n = 5, 41.7%), HTLV-2-like (n = 1, 8.3%), HTLV-untypable (n = 4, 33.3%) and indeterminate (n = 2, 16.6%) profiles. Phylogenetic: analysis of tax or env sequences that had been PCR amplified from peripheral blood lymphocyte DNA available from nine seropositive geladas showed that four were infected with identical STLV-1s; these sequences clustered with STLV-1 from Celebes macaques and probably represent recent cross-species infections. The tax sequences from the five remaining geladas were also identical and clustered with STLV-3. Analysis of the complete STLV-3 genome (8917 bp) from one gelada, TGE-2117, revealed that it is unique, sharing only 62% similarity with HTLV-1/ATK and HTLV-2/Mo. STLV-3/TGE-2117 was closest genetically to STLV-3 from an Eritrean baboon (STLV-3/PH969, 95.6%) but more distant from STILV-3s from red-capped mangabeys from Cameroon and Nigeria (STLV-3/CTO-604, 87.7%, and STLV-3/CTO-NG409, 87.2%, respectively) and Senegalese baboons (STLV-3/ PPA-F3, 88.4%). The genetic relatedness of STLV-3/TGE-2117 to STILV-3 was confirmed by phylogenetic analysis of a concatenated gag-pol-env-tax sequence (6795 bp). An ancient origin of 73 628-109 809 years ago for STILV-3 was estimated by molecular clock analysis of third-codon positions of gag-pol-env-tax sequences. LTR sequences from five STLV-3-positive geladas were >99% identical and clustered with that from a Papio anubis x P. hamadryas hybrid Ethiopian baboon, suggesting a common source of STILV-3 in these sympatric animals. LTR sequences obtained 20 years apart from a mother-infant pair were identical, providing evidence of both mother-to-offspring transmission and a high genetic stability of STLV-3. Since STLV-3-infected primates show a range of HTLV-Iike WB profiles and have an ancient origin, further studies using STLV-3-specific testing are required to determine whether STILV-3 infects humans, especially in regions of Africa where STLV-3 is endemic.