Mapping of bactericidal epitopes on the P2 porin protein of nontypeable Haemophilus influenzae.

The P2 porin protein is the major outer membrane protein of nontypeable Haemophilus influenzae and is a potential target of a protective immune response. Nine monoclonal antibodies (MAbs) to P2 were developed by immunizing mice with nontypeable H. influenzae whole organisms. Each MAb reacted exclusively with the homologous strain in a whole-cell immunodot assay demonstrating exquisite strain specificity. All nine MAbs recognized abundantly expressed surface-exposed epitopes on the intact bacterium by immunofluorescence and immunoelectron microscopy. Each MAb was bactericidal to the homologous strain in an in vitro complement-mediated killing assay. Immunoblot assay of cyanogen bromide cleavage products of purified P2 indicated that MAb 5F2 recognized the 10-kDa fragment, and the other eight MAbs recognized the 32-kDa fragment. Competitive ELISAs confirmed that 5F2 recognized an epitope that is different from the other eight MAbs. To further localize epitopes, MAbs 5F2 and 6G3 were studied in protein footprinting by using reversed-phase high-performance liquid chromatography. Three potential epitope-containing peptides which were reactive in an enzyme-linked immunosorbent assay with both 5F2 and 6G3 were isolated. These peptides were identified by N-terminal amino acid sequence and localized to loops 5 and 8 of the proposed model for P2. Fusion proteins consisting of glutathione S-transferase fused with variable-length peptides from loops 5 and 8 were expressed in the pGEX-2T vector. Immunoblot assay of fusion peptides of loops 5 and 8 confirmed that 5F2 recognized an epitope within residues 338 to 354 of loop 8; 6G3 and the remaining MAbs recognized an epitope within residues 213 to 229 of loop 5. These studies indicate that nontypeable H. influenzae contains bactericidal epitopes which have been mapped to two different surface-exposed loops of the P2 molecule. These potentially protective epitopes are strain specific and abundantly expressed on the surface of the intact bacterium.