Mapping of a strain-specific bactericidal epitope to the surface-exposed loop 5 on the P2 porin protein of non-typeable Haemophilus influenzae.

The P2 protein is the major outer-membrane protein of non-typeable Haemophilus influenzae (NTHI) and shows extreme heterogeneity among strains. Based on the analysis of antigenic structure, the P2 protein consists of eight potentially surface-exposed loops. Previous studies of monoclonal antibodies (mABs) to a single strain of NTHI showed that P2 contains potentially immunodominant epitopes in loop 5 of the molecule. The goal of the current work is to test the hypothesis that strain-specific and potentially immunodominant epitopes are located in loop 5 of P2 in other strains of NTHI as well. Gene fragments which encode peptides of loop 5 of strains 2019 and 5657 were cloned into an expression vector and subjected to immunoassays with mABs which recognize surface-exposed, bactericidal, strain-specific epitopes. Each mAB recognized loop 5 of the P2 protein of the homologous strain. Analysis of mutant clones with minor amino acid changes showed a loss of reactivity with the mABs. These observations indicate that loop 5 of the P2 molecule contains strain-specific, abundantly expressed surface-exposed epitopes. This further supports the hypothesis that loop 5 is an immunodominant region of the P2 molecule.