Development and characterization of a new single cycle vaccine vector in the simian immunodeficiency virus model system.

We have developed a new single cycle lentiviral vector, SIVsmH4i-SC27.1, as a potential SIV/HIV-1 vaccine candidate. This viral vector is capable of expressing all of the SIV gene products but is limited to one round of infection. The vector was created by mutating 27 codons dispersed among the viral vif, env, and nef genes to block protein function, attenuate viral replication/infectivity, and reduce the ability of the virus to manipulate the host immune system. To complement the env and nef replication defects, SC27.1 was pseudotyped with the VSV G glycoprotein to allow particle entry. The vif mutation was complemented by producing particles from an APOBEC3G-negative cell line, and the Vif protein defect was validated by showing that the single cycle virus lost most of its infectivity when particles were produced in presence of APOBEC3G. To deal with the problem of an antibody response to the VSV G protein in a vaccination strategy, two additional serotypes of the VSV G protein were used to create pseudotyped virus particles, and we observed no cross-neutralization activity for two of the pseudotyped particles with a potent neutralizing antiserum to one of the VSV G proteins. We detected moderate inhibition of infectivity in normal human and macaque sera, especially to the New Jersey serotype of VSV G, but as a heat sensitive activity, presumably complement mediated. These particles can be used in a prime-boost strategy to determine if a single cycle lentiviral vaccine vector capable of expressing all of the viral gene products holds promise in inducing immunity and protection to an SIVsm challenge.