Synthetic peptides of envelope proteins of two different strains of simian AIDS retrovirus (SRV-1 and SRV-2) represent unique antigenic determinants for serum neutralizing antibodies.

There are at least three distinct serotypes of simian type D retrovirus (SRV) which exhibit extensive serological cross-reactivity, but no cross-reactivity exists at the level of serum neutralizing antibodies. Amino acid sequence analysis and hydrophobicity plots of SRV-1 and SRV-2 envelope proteins were compared in order to identify unique potential antigenic determinants to which respective neutralizing antibodies may be directed. Peptides representing residues 147-162 of SRV-1 and 96-102 of SRV-2 were synthesized and assessed for their immunoreactivity. Free peptide inhibition of strain-specific serum (rhesus) neutralizing antibodies to SRV-1 and SRV-2 was demonstrated using the SRV-1 147-162 peptide and the SRV-2 peptide, 96-102, respectively. Inhibition of serum neutralizing activity by these peptides was also strain-specific, showing no cross-inhibition. SRV-1 147-162 conjugated to a protein carrier and cross-linked to Sepharose beads specifically adsorbed neutralizing antibodies from SRV-1 immune rhesus sera. The antibodies eluted from the immunoadsorbent possessed SRV-1 neutralizing activity, but showed no effect on the infectivity of SRV-2. Peptide SRV-1 147-162 also exhibited the capacity to bind specifically with a mouse monoclonal antibody which neutralizes the infectivity of SRV-1. Mice immunized with a recombinant SRV-1 envelope protein or with whole, inactivated SRV-1 produced antibodies which bound the SRV-1 147-162 conjugate, but not the protein carrier itself. Mouse antibodies to the SRV-1 147-162 conjugate exhibited specific binding with both native SRV-1 and with recombinant SRV-1 envelope protein. These findings provide strong evidence that SRV-1 147-162 and SRV-2 96-102 constitute at least two unique antigenic determinants, or parts thereof, which participate in the strain-specific neutralizing antibody response. Moreover, the findings indicate that the SRV-1 neutralizing antibodies produced by monkeys and at least a certain population of neutralizing antibodies produced by mice recognize the same epitope of SRV-1.