Protection by meningococcal outer membrane protein PorA-specific antibodies and a serogroup B capsular polysaccharide-specific antibody in complement-sufficient and C6-deficient infant rats

Abstract

The relative contributions of antibody-induced complement-mediated bacterial lysis and antibody/complement-mediated phagocytosis to host immunity against meningococcal infections are currently unclear. Further, the in vivo effector functions of antibodies may vary depending on their specificity and Fc heavy-chain isotype. In this study, a mouse immunoglobulin G2a (mIgG2a) monoclonal antibody (MN12H2) to meningococcal outer membrane protein PorA (P1.16), its human IgG subclass derivatives (hIgG1 to hIgG4), and an mIgG2a monoclonal antibody (Nmb735) to serogroup B capsular polysaccharide (B-PS) were evaluated for passive protection against meningococcal serogroup B strain 44/76-SL (B:15:P1.7,16) in an infant rat infection model. Complement component C6-deficient (PVG/c-) rats were used to assess the importance of complement-mediated bacterial lysis for protection. The PorA-specific parental mIgG2a and the hIgG1 to hIgG3 derivatives all induced efficient bactericidal activity in vitro in the presence of human or infant rat complement and augmented bacterial clearance in complement-sufficient HsdBrlHan:WIST rats, while the hIgG4 was unable to do so. In C6-deficient PVG/c- rats, lacking complement-mediated bacterial lysis, the augmentation of bacterial clearance by PorA-specific mIgG2a and hIgG1 antibodies was impaired compared to that in the syngeneic complement-sufficient PVG/c+ rat strain. This was in contrast to the case for B-PS-specific mIgG2a, which conferred similar protective activity in both rat strains. These data suggest that while anti-B-PS antibody can provide protection in the infant rats without membrane attack complex formation, the protection afforded by anti-PorA antibody is more dependent on the activation of the whole complement pathway and subsequent bacterial lysis.

FIG. 1.

Development of bacteremia in complement-sufficient (HsdBrlHan:WIST) and C6-deficient (PVG/c⁻) infant rats as a function of bacterial challenge dose. The results are given as GM bacteremia level (CFU per milliliter of blood) in each group of six animals. Error bars indicate standard deviations.

FIG. 2.

Protective activity of P1.16 PorA-specific hIgG1 to hIgG4 isotypes in complement-sufficient HsdBrlHan:WIST infant rats. The results are given as GM bacteremia level (CFU per milliliter of blood) in each group of five or six animals. Error bars indicate upper 95% confidence levels. The different isotypes were tested in separate experiments. *, P < 0.05 compared to control animals not given antibody, LSD. **, P value obtained with one-way ANOVA.

FIG. 3.

Protective activities of monoclonal antibodies in complement-sufficient (HsdBrlHan:WIST and PVG/c⁺) and C6-deficient (PVG/c⁻) infant rats. a) Nmb735, B-PS-specific mouse IgG2a antibody; b) MN12H2, P1.16 PorA-specific mouse IgG2a antibody; c) hIgG1, P1.16 PorA-specific human IgG1 isotype derived from MN12H2. The results are given as GM bacteremia level (CFU per milliliter of blood) in each group of five or six animals. Error bars indicate upper 95% confidence levels. *, P < 0.05 compared to control animals not given antibody, LSD. **, P value obtained with one-way ANOVA.

FIG. 4.

Bactericidal activities of PorA-specific monoclonal antibodies in the presence HsdBrlHan:WIST or PVG/c⁺ infant rat complement. MN12H2, mouse IgG2a isotype; hIgG1, human IgG1 isotype derived from MN12H2.