The specificity and affinity of polyclonal antibodies raised in rabbits against a hapten conjugated to rabbit serum albumin

Effect of different hapten-carrier conjugation ratios and molecular orientations on antibody affinity against a peptide antigen

Hapten-carrier conjugate immunization is an important tool in the generation of hapten-specific antibodies for analytical purposes and in the uncovering of basic vaccine-immunological mechanisms. The affinity of antibodies is known to play an important role for the resulting sensitivity of antibody-based assay systems and in deciding whether a vaccine-induced antibody response will be protective. With ovalbumin as a carrier protein and a peptide (7.2 NY) representing a 19 amino acid sequence from the E. coli-derived Verotoxin 2e as a model hapten we investigated whether it was possible to influence the affinity and titre of antibodies raised against the hapten using different conjugation ratios and orientations. The peptide was coupled to ovalbumin in four conjugation ratios and two molecular orientations - terminal and central - and the conjugates were verified by mass spectrometry. Mice were immunised ten times at two-weeks intervals with low doses of the eight conjugates. Blood samples collected between each immunisation were analysed by ELISA for specific antibody titres and relative affinities. With both types of conjugations, the anti-peptide antibody titres increased in response to increasing conjugation ratios, but central conjugation resulted in markedly higher titres than terminal conjugation. The overall anti-peptide antibody affinities reached approximately similar levels with both orientations, whereas a reversed proportionality was observed between conjugation ratio and antibody affinity for terminal conjugation. Thus, it appears that the molar ratio of a peptide and its carrier may affect the resulting antibody affinities, and that a conjugation ratio between a terminally conjugated peptide and its carrier approaching one will result in relatively high antibody affinities. Furthermore, the molecular orientation of the coupled peptide has a major effect on the anti-peptide antibody titres induced.

A novel subgroup of class I G-protein-coupled receptors

Based on structural similarities of an expressed sequence tag with the platelet-activating factor (PAF) receptor a cDNA clone encoding a novel G-protein-coupled receptor (GPCR), named GPR34, was isolated from a human fetal brain cDNA library. Genomic DNA analyses revealed the receptor to be encoded by an intronless single-copy gene at Xp11. 3-11.4. The predicted 381-amino-acid protein disclosed all structural features characteristic of a member of the class I GPCR family. Except an obvious sequence homology in transmembrane domain 6, no further similarities to the PAF receptor or any other known GPCR were found. The corresponding mouse receptor DNA was isolated from a genomic P1 library displaying a 90% amino acid identity compared to the human receptor. Phylogenetic studies showed that GPR34 is preserved among vertebrates, and the existence of GPR34 subtypes was demonstrated. The receptor mRNA is abundantly expressed in human and mouse tissues. In addition to the major 2-kb transcript, a 4-kb transcript was found only in mouse liver and testis. Expression of the human GPR34 in COS-7 cells followed by Western blot studies revealed specific bands of a highly glycosylated protein between 75 and 90 kDa. A number of potential ligands including phospholipids, leukotrienes, hydroxy-eicosatetraenoic acids, nucleotides and peptides were tested in functional assays. Since none of the applied substances led to significant changes in second messenger levels (cAMP and inositol phosphates), the natural ligand and coupling profile of this novel GPCR subgroup remains unknown.

Optimization and immunological characterization of a photochemically coupled lysergic acid diethylamide (LSD) immunogen

A photoreactive heterobifunctional linker was used to prepare an immunogen in which lysergic acid diethylamide was indirectly coupled to keyhole limpet hemocyanin at multiple sites on the drug. It was possible to attach approximately 35 drug molecules to each protein using approximately equal amounts of both species during the reaction. The presence of buffer components or water severely compromised reaction efficiency, as estimated from the molar substitution ratio. Factors such as excess linker, pH, irradiation of dry matrix in the absence of buffer, and the drug/protein ratio used during photolysis were shown to have pronounced effects on reaction efficiency. Structural insights regarding immunogen coupling were obtained by determining the specificities of antibodies which were raised against the immunogen. Cross-reactivity data indicated that haptenation of protein likely occurred at positions N1 and N6 of lysergic acid diethylamide, which is plausible given the electrophilicity of the photogenerated aryl nitrene.