Immunization of mice with P6 of nontypeable Haemophilus influenzae: Kinetics of the antibody response and IgG subclasses

The kinetics of the anti-P6 antibody response was characterized in three strains of mice of different haplotypes (Balb/c; H-2(d), C3H/H; H-2(k), SJL/J; H-2(s)). Anti-P6 antibodies were measured on a weekly basis by enzyme- linked immunosorbent assay (ELISA). The primary response peaked 2 or 3 weeks after the initial injection with 40 μg of purified P6. The response remained at a plateau for 8-10 weeks. A maximum titer of 1:1,638,400 was attained and then steadily declined. To study the ability of P6 to generate a recall response, we opted to boost the vaccinated mice with a known subimmunogenic dose of live nontypeable Haemophilus influenzae (NTHI) bacteria. After the anti-P6 antibody titers in the primed animals had stayed at baseline levels for 2 weeks, the mice were injected intraperitoneally with 10⁸ cfu of NTHI in sterile saline. This challenge with live NTHI bacteria induced a very rapid and strong secondary antibody response in all mice. Finally, we demonstrated that these murine anti-P6 sera were 100% bactericidal against three strains of NTHI when tested in a complement dependant bactericidal assay.