489 The Second Exon of SIVmac239 Tat Contributes to Chronic Viral Replication and Disease Stephen M. Smith*1,2, Zachary Klase1, Sara Pentlicky1, Christine Neuveut3, Preston Marx4, K.T. Jeang3 1St Michael's Med Ctr, Newark, NJ; 2New Jersey Med Sch, Newark, NJ; 3Natl Inst of Allergy and Infectious Diseases, NIH, Bethesda, MD; and 4Tulane Univ Sch of Med, Covington, LA
Background: The Tat proteins of HIV/SIV are encoded by 2 exons. The role of the 2nd exon of HIV or SIV Tat has not been evaluated in vivo.
Methods: We constructed a clone (SIVtat1ex) of SIVmac239, which encodes for only the 1st exon of Tat, and studied it in vivo. Codons 96 and 97 were changed from GCA TCA to TGA TAA. Macaques were infected with SIVmac239 (SIVtat2ex) (2 animals; M154 and N361) or SIVtat1ex (4 animals; L840, L855, L882, and N200).
Results: Viral load peaks were equivalent (~108 copies/ml) in the 2 groups. In the post-acute period, the SIVtat2ex animals had high set point plasma viral loads. The viral loads in 2 SIVtat1ex animals (L882 and N200) remained near 106 copies/ml. The viral load values in the 2 other SIVtat1ex animals (L840 and L855) became undetectable. The CD4 counts of the SIVtat2ex animals steadily declined after infection; each died of AIDS by 7 months (mos) PI. In the 2 SIVtat1ex infected animals, with undetectable viremia, the CD4 counts rose after the first 4 wks. Both SIVtat1ex infected animals with high viremia had persistently low CD4 counts. L882 died of AIDS after 9 mos PI. The stop codons at the end of the 1st exon of tat in L882's virus opened at 2-3 months PI. The other (N200) remained well for > 2 yrs, but died of simian AIDS at 2.7 yrs PI; N200ís virus had intact stop codons throughout. At 14 mos PI, L855ís viral load increased from undetectable to > 106 copies/ml and its CD4 count fell substantially; L855ís virus contemporaneously opened. L840ís virus at 9 mos PI had opened. However, by 14 mos PI, L840ís virus reverted back to SIVtat1ex. L840 had persistently low level of viremia and a normal CD4 T-cell count. Only L840 developed a strong response to Tat exon 2 peptides. We also analyzed viral sequences from 3 laboratory workers (LW) infected with HXB2R, which has a premature stop codon in the second exon of Tat. Virus in one of the 3 LWs reverted and this reversion was associated with more rapid disease. In another LW, the majority virus was temporarily open, but was again replaced by virus with the original stop codon.
Conclusions: The 2nd exon of SIVmac Tat contributes to chronic virus replication and pathogenesis. Data from HXB2R infected LWs corroborates our animal study findings. Together, these data suggest that a vaccine, which induces a strong cellular response to Tat exon 2 epitopes, could select for less pathogenic HIV, which only encodes the first Tat exon.