| Single-cycle SIV, incapable of initiating more than one round of infection, were constructed for use as a novel vaccine strategy against SIV as well as to determine the in vivo SIV burst size.; Two plasmids, one containing the full-length SIV genome with an engineered deletion in the env gene and a second containing a full-length env , were co-transfected into 293 cells to generate single-cycle SIV. In vitro experiments were performed to verify their retention of wild-type antigenic properties, infectious capability, and self-limited replicative nature.; Following three independent single-cycle SIV inoculations by ex vivo infection and intravenous re-infusion into rhesus macaques, humoral and CTL responses were detected, which increased with each inoculation. Compared to the immune responses elicited by macaques inoculated with replication-competent SIVmac239 or SIVmac251, the immune responses to the single-cycle SIV were not as potent. No sterilizing protection was observed in the vaccinated macaques following high-dose intravenous challenge with SIVmac251.; For in vivo burst size determinations, four MamuA*01 rhesus macaques were inoculated with single-cycle SIVDeltaenv/VSV-G by ex vivo infection and intravenous re-infusion. Plasma viremia peaked in 1 to 2.5 days and then declined rapidly in two phases. The mean decay rate of the first phase was ∼0.88 per day (t1/2 = 0.90 day), a value consistent with the death rate of productively infected lymphocytes as determined by treatment studies in HIV-infected patients. The second phase decayed more slowly at ∼0.37 per day (t1/2 = 2.50 day). Applying two independent mathematical methods, estimates of the in vivo SIV burst size ranged from 1.3 x 104 to 7.1 x 104 virions per cell. In a subsequent inoculation, comparable burst size estimates (1.5 x 104 to 2.1 x 105 virions per cell) were observed despite detectable levels of both humoral and CTL responses.; These calculations of the in vivo SIV burst size, together with previous determinations of the other viral dynamic parameters, permit a rough estimate of the number of productively infected cells in an infected individual when the plasma viral load is known. The use of single-cycle virions to model HIV-1 infection will enhance our overall understanding of HIV dynamics and pathogenesis. |
