The class 3 OMP, a porin of Neisseria meningitidis, and the PI porin protein of N. gonorrhoeae have eight predicted surface exposed loops. The gene encoding this protein is different between different serotypes in regions corresponding to several of these loops. Protective antibodies to these porins appear to be raised to conformational not linear epitopes. In order to determine the protective epitopes, we have constructed synthetic cyclic peptides with lipid tails inserted into liposomes in an effort to mimic suspected conformational epitopes corresponding to surface exposed loops. Immunization of mice with peptide loops corresponding to loop 1 of serotypes 4 and 15 of N. meningitidis and to loops 1 and 5 of strain MS 11 N. gonorrhoeae have been completed. Characterization of the immune responses using ELISA, Western Blot analysis, and bactericidal assays is underway. Preliminary results indicate that a strong antibody response to the peptide is seen in some mice. Antibody to the MS 11 loop 1 peptide binds only to PI protein from strains with an identical loop 1 sequence suggesting that the antibodies are directed against the tip of the loop where the variable region is located. However, antibodies to the type 15 VR-1 meningococcal loop bind also to the type 4 porin, suggesting that these antibodies are directed against the lateral constant regions of the loop. Some antibody which binds to native outer membrane vescicles is also detected, and does not correlate with results of Western blots against the porin proteins. This suggests that much of the antibody may not bind to the native protein. Methods to obtain Westen blots of porin in their native trimers as well as functional assays are underway. These studies help to determine the relative importance of the class 2 or class 3 OMP in outer membrane protein vaccines for group B meningococcal disease and the potential protective nature of PI protein for gonococcal disease, and will expand our knowledge regarding the use of synthetic peptides as immunogens and as reagents in immunologic assays.
