Protection from HIV-1 Envelope-bearing Chimeric Simian Immunodeficiency Virus (SHIV) in Rhesus Macaques Infected with Attenuated SIV: Consequences of Challenge

Objectives: To determine whether prior infection with simian immunodeficiency virus (SIV)BK28 protects macaques from subsequent exposure to an HIV-1 envelope chimeric SIV (SHIV). Also, to determine the consequences of viral challenge on CD4 numbers and virus load on the current SIV infection.

Design And Methods: A total of 12 mature outbred Macacca mulatta were studied. Four naive controls and four previously infected with attenuated SIVBK28 were challenged with SHIV; four naive controls were not infected with SHIV. Sampling occurred twice monthly, and monthly thereafter. Changes in virus load, CD4 and CD8 populations were monitored. Highly sensitive and specific discriminative polymerase chain reaction (PCR) assays were used to distinguish between virus populations.

Results: SHIV readily infected challenged control animals, which developed a peak in virus load and a decline in CD4+ cell numbers. In controls, viral load declined and CD4 cell numbers rose to near normal levels after seroconversion. In contrast, in SIV-infected animals there was only a minor increase in viral load in only two out of four animals, 100-1000-fold lower than in naive animals. Interestingly, a decline in CD4 cells occurred in all four SIV-infected animals after SHIV challenge, which appeared more pronounced than in animals infected by SHIV alone. One SIV-infected animal which had low CD4 cell numbers at the time of SHIV challenge, developed a further decline in CD4 cells with a rising viral load. Discriminative PCR did not reveal SHIV in the challenged SIV animals. Interestingly the increase in viral load was due to SIV and not SHIV.

Conclusions: Broad protection of animals previously infected with live attenuated SIV was demonstrated with protection from subsequent infection with HIV-1 envelope-bearing chimeric SIV. Subsequent exposure in cases with low CD4 cell numbers reveal the possibility of activation of the vaccine strain with the possible risk of inducing disease progression.