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Session 10 Oral Abstract Session Late Breakers I Session Time: Monday, 2 pm - 4 pm Room 6E

2:30   LB3. Determination of Virus Burst Size in Vivo Using a Single-Cycle SIV in Rhesus Macaques H. Yuan Chen*1,2, M. Di Mascio3, A. Perelson3, A. Gettie1,4, D. Ho1,5, L. Zhang1, and 1>Aaron Diamond AIDS Res. Ctr., Rockefeller Univ., New York, NY; 2New York Univ., NY; 3Los Alamos Natl. Lab., NM; and 4Tulane Regional Primate Res. Ctrl, Covington, LA; and 5Rockefeller Univ., New York, NY Background: The burst size of HIV or SIV in vivo is one of few viral dynamic parameters that remain undetermined. Methods: To estimate this value, we conducted a series of experiments using a single-cycle virus to eliminate viral spread as a confounding factor in the analysis. A 415-bp deletion was created in env of SIVmac239, and single-cycle infectious virions were generated by pseudotyping with VSV envelope. 4 MamuA*01 rhesus macaques were utilized for the study. PBMC (1 - 4 x 107) were obtained from each monkey and stimulated for 2.5 days with SEB in vitro. These cells were then exposed to the single-cycle virus (MOI of 1-3) for 12-18 hours. Following extensive washes, virus-inoculated, autologous PBMC were re-infused back into each monkey intravenously. Blood was collected from each animal frequently during the first 3 days, and weekly thereafter. Results: Plasma viremia peaked (range 105 - 107/mL) in 1-2 days, and then declined rapidly in 2 phases. The mean decay rate of the first phase was ~0.88/day (t1/2 0.78 day), a value consistent with the death rate of productively infected lymphocytes determined by treatment studies in HIV-infected patients. The second phase, due to a significantly smaller component, decayed more slowly (~0.33/day, or t1/2 2.10 day). The viral burst size in vivo ranged from 4.0 x 104 to 7.0 x 104 virions/cell, as calculated by 2 independent mathematical methods. To determine the impact of immune responses on the burst size and the death rate of infected cells, these experiments were repeated in each of the 4 monkeys some weeks later when antibody seroconversion had already occurred (antibody titers of 50 - 1000 dilutions) together with the development of CTL responses as measured by tetramer staining (up to 1%). In each monkey, a substantially faster death rate (1.39/day) was observed for productively infected cells in the second experiment, although a consistent decrease in burst size could not be documented. Conclusions: In conclusion, we report for the first time an in vivo estimate for the viral burst size. This value, interpreted in conjunction with our prior understanding of the cell death rate and virion clearance rate, could be used to calculate the total number of productively infected cells in an infected person or animal when the plasma viral load is known.

©2002 9th Conference on Retroviruses and Opportunistic Infections