Conserved regions of outer membrane proteins as vaccine antigens
Neary, John M
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Nontypeable Haemophilus influenzae (NTHI) is a small, gram-negative bacillus, which causes otitis media in children and lower respiratory infections in adults with chronic obstructive pulmonary disease (COPD). In both otitis media and COPD, patients routinely suffer recurrent episodes of disease. Factors such as health care costs, pain and suffering, and lost work time underscore the need for a vaccine against NTHI. The ability of NTHI to cause recurrent infections is partly attributable to antigenic variability in several surface-exposed loops of major outer membrane protein P2, which constitutes approximately one-half of the total outer membrane protein of the organism. Loop 5 is highly heterogeneous among strains and contains almost all of the epitopes to which an immune response is mounted when animals are immunized with the whole organism. Adults with COPD make new antibodies to strain-specific epitopes on P2 following infection by NTHI. Thus, immunity against NTHI is most often strain specific, leaving the patient vulnerable to reinfection by other strains. One approach to vaccine development for NTHI has been to study antigenically conserved outer membrane proteins as potential vaccine antigens. Considering the abundant expression of P2 on the bacterial surface, identification of a conserved, surface exposed region on the P2 molecule would be a significant step towards developing a vaccine against NTHI. Loop 6 of the P2 molecule has the potential to be such a vaccine antigen. In this study we evaluated the efficacy of the loop 6 region of P2 as a vaccine antigen. The degree of sequence conservation among NTHI was characterized by sequencing the loop 6 region of 108 strains from geographically and clinically diverse sources. Approximately 75% were conserved. Antibodies generated against conserved loop 6 sequences were reactive against multiple strains in western blots and flow cytometry assays. The ability of antibodies raised to conserved loop 6 sequences to kill multiple strains of NTHI was evaluated in bactericidal assays. These data support the concept of using conserved regions of outer membrane proteins as vaccine antigens. Development of a vaccine against NTHI could be beneficial in several ways. By the age of three years, approximately 80% of all children will have suffered at least one episode of otitis media, 15-40% of which are caused by NTHI. COPD manifests itself in periodic exacerbations caused by lower respiratory tract infections. An estimated 60% of exacerbations are caused by Haemophilus species. Preventing infections due to NTHI could have a significant impact on the morbidity, mortality, and health care costs incurred by COPD in adults and otitis media in children.