Mechanisms of Resistance ofPorphyromonas gingivalisto Killing by Serum Complement

Abstract

The complement system plays an important role in the host defense against infection, and the formation of the terminal complement complex on the bacterial surface has been shown to be particularly important in killing of gram-negative bacteria. The gram-negative periodontal pathogen Porphyromonas gingivalis is resistant to complement killing, and possible mechanisms suggested for this resistance include protease production and capsule formation. In this study, P. gingivalis Arg-and Lys-gingipain deletion mutants and polysaccharide synthesis deletion mutants have been used to investigate these hypotheses. When Arg-and Lys-gingipain protease mutants were incubated in 20% normal human serum, deposition of complement components on the cell surface was significantly increased compared to that for the wild-type organism. However, despite the increased deposition, the protease mutants maintained resistance to killing and their viability was equal to that seen with heat-inactivated serum. Similar data were obtained when the wild-type organism was treated with gingipain protease inhibitors. K-antigen expression mutants were also resistant to killing. However, mutants which no longer synthesized a surface anionic polysaccharide (APS) (a phosphorylated branched mannan) were extremely sensitive to serum killing. These mutants lack the organized dense glycan surface layer present on the parent strain on the basis of electron microscopy. We conclude that the production of APS at the surface of P. gingivalis rather than Arg-and Lys-gingipain synthesis is the principal mechanism of serum resistance in P. gingivalis.

Biphasic Effect of Gingipains from Porphyromonas gingivalis on the Human Complement System

“…The results of this study are in contrast with other published results that showed that a P. gingivalis mutant lacking all three gingipains was extremely sensitive to lysis by complement (30). In direct contrast, a mutant lacking HRgpA and RgpB was reported to survive in NHS equally well as the wild type (20) or to survive at 28% as compared with 87% for the wild type (30). It is unclear at the moment what the reason is for these discrepancies.…”

“…Furthermore, we found that NHS preincubated with P. gingivalis wild-type strains also lost its bactericidal activity toward E. coli. A recent publication has shown that P. gingivalis lacking the gingipains HRgpA and RgpB or Kgp are indeed opsonized with larger amounts of C3b but that this does not lead to the lysis of bacteria (20). The authors suggest that lysis is prevented by the presence of anionic polysaccharides on a bacterial surface.…”

“…A bacterial mutant lacking HRgpA is less able to degrade C3 as compared with the wild type, which results in an increased phagocytosis by neutrophils (19). In contrast, a recent study showed that although bacterial strains lacking HRgpA and RgpB or Kgp were more efficiently opsonized by C3b and the membrane attack complex than the parental wild-type strain, they were not lysed (20).…”

Subversion of Innate Immunity by Periodontopathic Bacteria via Exploitation of Complement Receptor-3

“…Interestingly, P. gingivalis is very resistant to killing by complement; this is attributable to the ability of its gingipain proteases to degrade C3 and C5 and thereby prevent deposition of C3b on the bacterial cell surface, which moreover contains a complement-resistant anionic polysaccharide (Popadiak et al, 2007;Slaney et al, 2006). Intriguingly, degradation of C5 by P. gingivalis leads to generation of a biologically active C5a-like fragment (Wingrove et al, 1992).…”

Importance of heterogeneity in Porhyromonas gingivalis lipopolysaccharide lipid A in tissue specific inflammatory signalling

“…These loci are, therefore, probably involved in the biosynthesis of A-LPS [60,61]. Furthermore, the mutants were susceptible to killing by the complement system, suggesting that A-LPS has a role in serum resistance to P. gingivalis [61,62]. Also, in the non-pigmented P. gingivalis strain HG66, which lacks A-LPS but possesses O-LPS, a nonsense mutation in the wbpB gene was detected, and Wbp pathway gene mutants were found to be A-LPS deficient [60].…”