Combining site specificities of rabbit antibodies to platelet-activating factor (PAF)

Anti-PAF sera from six different rabbits, immunized with C12- or C6-PAF as immunogen, were examined in hapten inhibition experiments in an attempt to define the fine structural recognition specificities of the antibody combining sites. Using a selection of naturally occurring lipids and PAF analogues, no significant cross-reactivity was observed with the lipids or with the inactive metabolite, lyso-PAF. Comparison of the structural specificity requirements of the antibodies from each rabbit showed some heterogeneity, with one antiserum demonstrating a different recognition specificity at position 1 on the glycerol backbone of the PAF molecule. A second rabbit antiserum showed a large degree of tolerance for analogues with increasing acyl chain length at position 2. In general, an ether group at position 1 and an acetyl at position 2 were required for inhibitory activity and a degree of tolerance was demonstrated at position 3, where the main structural requirement was for one or more methyl groups on the nitrogen atom of the phosphocholine moiety.