Stimulation of neutralizing antibodies to human immunodeficiency virus type 1 in three strains of mice immunized with a 22 amino acid peptide of gp41 expressed on the surface of a plant virus

A plant virus, cowpea mosaic virus, expressing a 22 amino acid peptide 731–752 of the gp41 glycoprotein of human immunodeficency virus type 1 (HIV-1 IIIB), was shown previously to stimulate HIV-1 cross reactive neutralizing antibodies in adult C57BL6 mice. Here some parameters concerning the stimulation of HIV-1-specific neutralizing and ELISA antibody have been determined in adult C57BL6, C3HHe-mg and BALBc mice. Two injections per mouse of all CPMV-HIV/1 doses tested (100, 10 and 1 μg chimera which contained, respectively, 1700, 170 and 17 ng HIV peptide per injection) stimulated a strong serum neutralizing antibody response in all mice. One hundred micrograms or 10 μg CPMV-HIV/1 per injection gave 99% neutralization of HIV-1 IIIB in C8166 cells at a serum dilution of 1200, whereas sera from mice immunized with 1 μg per injection neutralized virus to 97%, 79% and 63% at a 1200 dilution of serum from C3HHe-mgC57BL6 and BALBc mice, respectively. Restimulation of these mice with the same immunogen dose marginally increased the neutralization titres. The longevity of the neutralizing antibody response increased as the immunogen dose decreased, and was dependent on the strain of mouse, in the order C57/BL6C3H/He-mgBALB/c. Re-immunization with a third injection improved the longevity of the antibody response. All mice immunized with 100 μg CPMV-HIV/1 responded with ELISA antibody to the gp41 peptide bound in solid phase. Ten micrograms stimulated ELISA antibody in some but not all mice, while mice immunized with 1 μg had no detectable ELISA antibody. This synthesis of ELISA antibody decreased ≥ 230-fold over the range of immunogen doses tested but, in the same mice, the neutralizing antibody response decreased only twofold, showing an unusual bias to production of the latter. Neutralizing antibodies were thus stimulated at a lower immunogen dose than ELISA antibodies. Antibody which was affinity purified using the free gp41 peptide gave a good ELISA titre but did not neutralize HIV-1, suggesting that the neutralizing antibody is recognizing a conformational epitope on the gp41 oligomer.