Inhibition of the Classical Pathway of Complement by Meningococcal Capsular Polysaccharides

Abstract

Almost all invasive Neisseria meningitidis isolates express capsular polysaccharide. Antibody (Ab) is required for complement-dependent killing of meningococci. While alternative pathway evasion has received considerable attention, little is known about classical pathway (CP) inhibition by meningococci and forms the basis of this study. We engineered capsulated and unencapsulated isogenic mutant strains of groups A, B, C, W and Y meningococci to express similar amounts of the same factor H-binding protein (fHbp; a key component of group B meningococcal vaccines) molecule. Despite similar anti-fHbp mAb binding, significantly less C4b was deposited on all five encapsulated mutants compared to their unencapsulated counterparts (P<0.01), when purified C1 and C4 were used to deposit C4b. Reduced C4b deposition was the result of capsule-mediated inhibition of C1q engagement by Ab. C4b deposition correlated linearly with C1q engagement by anti-fHbp. While B, C, W and Y capsules limited CP-mediated killing by anti-fHbp, the unencapsulated group A mutant paradoxically was more resistant than its encapsulated counterpart. Strains varied considerably in their susceptibility to anti-fHbp and complement despite similar Ab binding, which may have implications for the activity of fHbp-based vaccines. Capsule also limited C4b deposition by anti-porin A mAbs. Capsule expression decreased binding of an anti-LOS IgM mAb (~1.2 to 2-fold reduction in fluorescence). Akin to observations with IgG, capsule also decreased IgM-mediated C4b deposition when IgM binding to the mutant strain pairs was normalized. In conclusion, we show that capsular polysaccharide, a critical meningococcal virulence factor, inhibits the CP of complement.

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

“…meningitidis downregulates the alternative complement pathway by redundant mechanisms, including certain capsular polysaccharides (40), NspA (17), FHbp (41), PorB2 (32), and LOS sialylation (22). Our data indicate that the binding of FH to both NspA and certain PorB3 amino acid sequence variants can confer resistance of serogroup B isolates to anti-FHbp 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).…”

Regulation of capsule inNeisseria meningitidis

“…Meningococcal CPSs can also inhibit the classical pathway of complement (Agarwal et al, 2014). All major meningococcal capsules (A/B/C/W/Y) interfere with the engagement of C1q by antibody, leading to decreased C4b deposition and inhibition of the classical pathway activation.…”

Contribution of murine IgG Fc regions to antibody binding to the capsule ofBurkholderia pseudomallei

“…2,4 Polysaccharide capsules are found on many pathogenic bacteria and fungi, and contribute to virulence by inhibiting complement activation and preventing phagocytosis. [4][5][6][7][8] They are high molecular weight antigens with repeating epitopes that are displayed on bacterial and fungal cell surfaces. [9][10][11][12] The B. pseudomallei CPS is comprised of an unbranched homopolymer of 1,3-linked 2-O-acetyl-6-deoxy-b-D-mannoheptopyranose residues.…”