Antigenic and molecular conservation of the gonococcal NspA protein.

A low-molecular-weight protein named NspA (neisserial surface protein A) was recently identified in the outer membrane of all Neisseria meningitidis strains tested. Antibodies directed against this protein were shown to protect mice against an experimental meningococcal infection. Hybridization experiments clearly demonstrated that the nspA gene was also present in the genomes of the 15 Neisseria gonorrhoeae strains tested. Cloning and sequencing of the nspA gene of N. gonorrhoeae B2 revealed an open reading frame of 525 nucleotides coding for a polypeptide of 174 amino acid residues, with a calculated molecular weight of 18,316 and a pI of 10.21. Comparison of the predicted amino acid sequence of the NspA polypeptides from the gonococcal strains B2 and FA1090, together with that of the meningococcal strain 608B, revealed an identity of 93%, suggesting that the NspA protein is highly conserved among pathogenic Neisseria strains. The level of identity rose to 98% when only the two gonococcal predicted NspA polypeptides were compared. To evaluate the level of antigenic conservation of the gonococcal NspA protein, monoclonal antibodies (MAbs) were generated. Four of the seven NspA-specific MAbs described in this report recognized their corresponding epitope in 100% of the 51 N. gonorrhoeae strains tested. Radioimmunobinding assays clearly indicated that the gonococcal NspA protein is exposed at the surface of intact cells.