Combined Roles of Human IgG Subclass, Alternative Complement Pathway Activation, and Epitope Density in the Bactericidal Activity of Antibodies to Meningococcal Factor H Binding Protein

Abstract

Meningococcal vaccines containing factor H binding protein (fHbp) are in clinical development. fHbp binds human fH, which enables the meningococcus to resist complement-mediated bacteriolysis. Previously, we found that chimeric human IgG1 mouse anti-fHbp monoclonal antibodies (MAbs) had human complement-mediated bactericidal activity only if the MAb inhibited fH binding. Since IgG subclasses differ in their ability to activate complement, we investigated the role of human IgG subclasses on antibody functional activity. We constructed chimeric MAbs in which three different murine fHbp-specific binding domains were each paired with human IgG1, IgG2, or IgG3. Against a wild-type group B isolate, all three IgG3 MAbs, irrespective of their ability to inhibit fH binding, had bactericidal activity that was >5-fold higher than the respective IgG1 MAbs, while the IgG2 MAbs had the least activity. Against a mutant with increased fHbp expression, the anti-fHbp MAbs elicited greater C4b deposition (classical pathway) and greater bactericidal activity than against the wild-type strain, and the IgG1 MAbs had similar or greater activity than the respective IgG3 MAbs. The bactericidal activity against both wild-type and mutant strains also was dependent, in part, on activation of the alternative complement pathway. Thus, at lower epitope density in the wild-type strain, the IgG3 anti-fHbp MAbs had the greatest bactericidal activity. At a higher epitope density in the mutant, the IgG1 MAbs had similar or greater bactericidal activity than the IgG3 MAbs, and the activity was less dependent on the inhibition of fH binding than at a lower epitope density.

Influence of IgG Subclass on Human Antimannan Antibody-Mediated Resistance to Hematogenously Disseminated Candidiasis in Mice

“…The ability of human IgG1, IgG2, and IgG3 to activate human complement has been well documented, and our data (Fig. 7) are in agreement with those of previous studies (49,50). In contrast, studies of the interaction of human antibody with the mouse complement system have been quite limited.…”

Binding of Complement Factor H to PorB3 and NspA Enhances Resistance of Neisseria meningitidis to Anti-Factor H Binding Protein Bactericidal Activity

“…After the binding of anti-FHbp antibodies, the Fc density on the bacterial surface may be too low to permit sufficient C3b deposition by the classical complement pathway alone for the formation of a membrane attack complex without alternative pathway amplification (27,39). Since FH downregulates the alternative pathway, the ability of anti-FHbp antibodies to block FH binding can be critical for eliciting anti-FHbp bacteriolysis (25,27,39,40). We investigated whether resistance to anti-FHbp bactericidal activity might be explained by binding of FH to ligands other than FHbp (16).…”

Inhibition of the Alternative Pathway of Nonhuman Infant Complement by Porin B2 Contributes to Virulence of Neisseria meningitidis in the Infant Rat Model

“…The AP acting in isolation is insufficient to kill meningococci (15, 19); however, the AP is required for maximal classical pathway-initiated killing (7, 19, 47). We investigated the role of PorB2 in mediating resistance to infant rat serum (all complement pathways functional) by comparing the isogenic strain pair in a bactericidal assay.…”

“…The AP is characterized by a positive-feedback loop that amplifies C3b deposition on microbial surfaces (5,6). The AP works in concert with the classical pathway and plays an important role in maximizing the killing activity of select antibodies, including those directed against the vaccine antigen, factor H binding protein (fHbp) (7). Factor H (fH), a host complement control protein, plays a key role in limiting unwanted activation of the AP (8,9) by assisting with factor I-mediated cleavage of C3b to iC3b (10,11) and by irreversibly dissociating the AP C3 convertase (C3bBb) (12,13).…”