Serologic and biochemical analysis of latent a1 IgG

Immunization of rabbits with anti-allotype antibody (Ab) induces at least two populations of highly cross-reactive molecules. One of these populations bears the nominal VHa allotype of the producing rabbit and is designated the VHa-positive anti-IdX Ab. The other population lacks the expected nominal allotype and thus could represent an induced latent allotype-bearing Ig. To define better the putative latent allotypes, they were subjected to serologic, electron microscopic and biochemical analyses. The induced latent-like population was compared to nominal a1 and found to be indistinguishable in inhibition assays incorporating both rabbit anti-a1 Ab and mouse monoclonal anti-a1 Ab. In contrast, the latent-like Ig was less inhibitory than normal a1 Ig in assays with goat and guinea-pig anti-a1 Ab. The isolated anti-IdX population was less inhibitory than either nominal or latent a1 Ig in all assays. Immunoelectron microscopic analysis indicates that complexes composed of latent-like a1 molecules and Fab anti-a1 ab resemble allotype/anti-allotype (i.e. a1/anti-a1) complexes. Tryptic digests of the putative latent a1 H-chains reveals that these molecules share an a1-specific peptide with digests of nominal a1 H-chain. The peptides from both nominal and latent a1 IgG appear to have blocked N-terminal residues and have a similar though not identical amino acid composition. The composition of these peptides is correlated with the first nine amino acids of the nominal a1 H-chain. The data suggest that the induced latent a1-like molecules share the same major a1 epitope with nominal a1 but may differ in some subtle respects. The possible genetic bases for these observations are discussed.