Abstract
Moraxella catarrhalis is a significant cause of respiratory tract infection against which a vaccine is sought. Several outer membrane proteins are currently under investigation as potential vaccine antigens, including the porin M35. We have previously shown that the third external loop of M35 was immunodominant over the remainder of the protein for antibody produced in mice against the refolded recombinant protein. However, as this loop is predicted to fold inside the porin channel we also predicted that it would not be accessible to these antibodies when M35 is expressed on the surface of the bacteria in its native conformation. This study investigated the functional activity of antibodies against M35 and those specific for the loop 3 region of M35 in vitro and in vivo. Antisera from mice immunized with M35 or the loop 3-deletion, M35loop3−, recombinant proteins were not bactericidal but did have enhanced opsonic activity, whereas antibodies raised against the loop 3 peptide were not opsoniszing indicating that the immunodominant loop 3 of M35 was not accessible to antibody as we had previously predicted. Mucosal immunization with M35, M35 that had an antigenically altered loop 3 [M35(ID78)] and M35loop3− enhanced the clearance of M. catarrhalis from the lungs of mice challenged with live M. catarrhalis. The in vivo clearance of bacteria in the mice with the M35-derived protein constructs correlated significantly (p < 0.001) with the opsonic activity assessed an in vitro opsonophagocytosis assay. This study has demonstrated that the immunodominant B-cell epitope to loop 3 of the M. catarrhalis outer membrane protein M35 is not associated with immune protection and that M35-specific antibodies are not bactericidal but are opsoniszing. The opsoniszing activity correlated with in vivo clearance of the bacteria suggesting that opsoniszing antibody may be a good correlate of immune protection.
Highlights
Moraxella catarrhalis is a Gram-negative bacterium that is often the causual organism for respiratory tract infections such as otitis media, sinusitis, and exacerbations of chronic obstructive pulmonary disease (COPD; Murphy, 1996; Karalus and Campagnari, 2000; Verduin et al, 2002)
Antibodies raised against M35 bound to the cell surface of M. catarrhalis as demonstrated by a small right-shift in the relative fluorescence compared to the control non-immune serum
Antibodies raised against L3DHFR and dihydrofolate reductase (DHFR) caused smaller right-shifts than those seen with the anti-M35 serum
Summary
Moraxella catarrhalis is a Gram-negative bacterium that is often the causual organism for respiratory tract infections such as otitis media, sinusitis, and exacerbations of chronic obstructive pulmonary disease (COPD; Murphy, 1996; Karalus and Campagnari, 2000; Verduin et al, 2002). On rare occasions this bacterium can cause more serious diseases such as meningitis and septicaemia (Meyer et al, 1995; Daoud et al, 1996). M35 contains an immunodominant epitope in the third external loop www.frontiersin.org
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