Natural IgG antibodies in normal rabbit serum are involved in killing of the ompP2 mutant of Haemophilus parasuis SC096 strain via the classical complement pathway.

Serum resistance in Haemophilus parasuis strain SC096 has been shown to be dependent on expression of the outer membrane protein P2 (OmpP2) and loss of the ompP2 gene results in significantly greater sensitivity to both porcine and rabbit sera. However, the mechanism of complement activation by the serum sensitive ΔompP2 strain is unknown. In this study, the classical complement pathway is demonstrated to be the main pathway for killing the H. parasuis ΔompP2 strain, and not the mannan-binding lectin (MBL) or alternative pathway. In addition, absorption of antibodies against ΔompP2 strain or depletion of IgGs from serum inhibited serum killing activity, which could be restored by addition of heat-inactivated serum or purified IgGs. Western blot analysis indicated that the OmpP2 mutant could bind significantly more IgGs than the wild type strain SC096 when incubated with serum. Finally, IgGs in normal rabbit serum targeted to the OMPs, but not lipooligosaccharide (LOS) in the OmpP2 mutant strain were found to be involved in bacterial killing indicating that the bactericidal epitope(s) is in the outer membrane proteins.