Preparation of proteosome-based vaccines. Correlation of immunogenicity with physical characteristics

In order to facilitate the use of proteosome-based vaccines, we have identified and analyzed the parameters that affect their immunogenicity. As a model system we used synthetic peptides (LCF6) containing sequences from the immunodominant (NANP)n tandem repeat region of the P. falciparum circumsporozoite protein, hydrophobically complexed to multimeric protein preparations (proteosomes) of meningococcal outer membrane proteins (OMP), since we have previously shown that high levels of anti-(NANP)n IgG can be elicited in mice by use of this novel adjuvant system (Lowell et al., 1988a). We have now examined these preparations by velocity sedimentation and measured their ability to elicit an IgG response in mice. Velocity sedimentation of freshly mixed OMP and LCF6, without dialysis, produced a limited number of small complexes, whereas dialysis of the mixture for 4 d yielded heterogeneously sized complexes that became more homogeneous when the dialysis was carried out for 7 or 10 days. The most homogeneous of these peptide-proteosome complexes (those dialyzed for 10 days) induced substantial levels of anti-(NANP)n IgG in mice, and shorter periods of dialysis resulted in vaccines that induced proportionately lower titers. Analysis of a series of preparations with varying LCF6: OMP ratios (w/w) showed that the degree of peptide substitution of the proteosomes was inversely proportional to the rate of sedimentation of the complexes and that there exists an optimal degree of lipopeptide complexing to the proteosomes. Our results suggest that the parameters affecting the immunogenicity of the peptide-proteosome complexes are: (i) hapten density, and (ii) size of the complex. Furthermore, sedimentation analysis of peptide-proteosome immunogens may serve as a rapidly performed assay of immunogenic potency.