Recombinant PhpA protein, a unique histidine motif-containing protein from Streptococcus pneumoniae, protects mice against intranasal pneumococcal challenge.

The multivalent pneumococcal conjugate vaccine is effective against both systemic disease and otitis media caused by serotypes contained in the vaccine. However, serotypes not covered by the current conjugate vaccine may still cause pneumococcal disease. To address these serotypes and the remaining otitis media due to Streptococcus pneumoniae, we have been evaluating antigenically conserved proteins from S. pneumoniae as vaccine candidates. A previous report identified a 20-kDa protein with putative human complement C3-proteolytic activity. By utilizing the publicly released pneumococcal genomic sequences, we found the gene encoding the 20-kDa protein to be part of a putative open reading frame of approximately 2,400 bp. We recombinantly expressed a 79-kDa fragment (rPhpA-79) that contains a repeated HxxHxH motif and evaluated it for vaccine potential. The antibodies elicited by the purified rPhpA-79 protein were cross-reactive to proteins from multiple strains of S. pneumoniae and were against surface-exposed epitopes. Immunization with rPhpA-79 protein adjuvanted with monophosphoryl lipid A (for subcutaneous immunization) or a mutant cholera toxin, CT-E29H (for intranasal immunization), protected CBA/N mice against death and bacteremia, as well as reduced nasopharyngeal colonization, following intranasal challenge with a heterologous pneumococcal strain. In contrast, immunization with the 20-kDa portion of the PhpA protein did not protect mice. These results suggest that rPhpA-79 is a potential candidate for use as a vaccine against pneumococcal systemic disease and otitis media.

Evaluation of a 74-kDa transferrin-binding protein from Moraxella (Branhamella) catarrhalis as a vaccine candidate.

An outer membrane protein from Moraxella catarrhalis with a mass of 74-kDa was isolated and evaluated as a vaccine candidate. The 74-kDa protein binds transferrin, and appears to be related to the other proteins from the organism that are reported to bind transferrin. The 74-kDa protein possessed conserved epitopes exposed on the bacterial surface. This is based on the reactivity with whole bacterial cells as well as complement dependent bactericidal activity of sera from mice immunized with the isolated proteins from the O35E and TTA24 isolates. However, there was divergence in the degree of antibody cross-reactivity with the protein from one strain to another. This serotypic divergence was reflected in both the complement-dependent bactericidal activities of the antibodies elicited in mice and the capacity of immune mice to clear the bacteria in a murine pulmonary model. Antibodies affinity purified from human plasma lacked bactericidal activity even though they were reactive with all the tested isolates. The 74-kDa protein appears to be a good vaccine candidate, but more studies are needed to understand its antigenic variability and whether antibodies toward it are protective.