The molecular basis for the resistance of serogroup B
Neisseria meningitidis to the bactericidal activity of normal human sera (NHS) was examined with a NHS-resistant, invasive serogroup B meningococcal isolate and genetically and structurally defined capsule-, lipooligosaccharide (LOS)-, and sialylation-altered mutants of the wild-type strain. Expression of the (α2→8)-linked polysialic acid serogroup B capsule was essential for meningococcal resistance to NHS. The very NHS-sensitive phenotype of acapsular mutants (99.9 to 100% killed in 10, 25, and 50% NHS) was not rescued by complete LOS sialylation or changes in LOS structure. However, expression of the capsule was necessary but not sufficient for a fully NHS-resistant phenotype. In an encapsulated background, loss of LOS sialylation by interrupting the α2,3 sialyltransferase gene,
lst, increased sensitivity to 50% NHS. In contrast, replacement of the lacto-
N-neotetraose α-chain (Galβ1-4GlcNAcβ1-3Galβ1-4Glc) with glucose extensions (Glc
N) in a
galE mutant resulted in a strain resistant to killing by 50% NHS at all time points. Encapsulated meningococci expressing a Hep
2(GlcNAc)→KDO
2→lipid A LOS without an α-chain demonstrated enhanced sensitivity to 50% NHS (98% killed at 30 min) mediated through the antibody-dependent classical complement pathway. Encapsulated LOS mutants expressing truncated Hep
2→KDO
2→lipid A and KDO
2→lipid A structures were also sensitive to 50% NHS (98 to 100% killed at 30 min) but, unlike the wild-type strain and mutants with larger oligosaccharide structures, they were killed by hypogammaglobulinemic sera. These data indicate that encapsulation is essential but that the LOS structure contributes to the ability of serogroup B
N. meningitidis to resist the bactericidal activity of NHS.Serogroup B
Neisseria meningitidis (the meningococcus) is an obligate human pathogen and remains a leading cause of fulminant septicemia and meningitis. In addition to sporadic outbreaks, large epidemics of serogroup B meningococcal disease continue to occur in many parts of the world, including South America, the United States Pacific Northwest, Western Europe, and New Zealand (
4,
22). After penetrating upper respiratory tract mucosal surfaces,
N. meningitidis must survive and multiply in the bloodstream to cause sepsis, meningitis, and other manifestations of invasive meningococcal disease. A major mechanism inhibiting or preventing the multiplication of meningococci in the blood is the complement-mediated bactericidal activity of human sera (
17,
39). The importance of this activity in the prevention of systemic meningococcal disease is reinforced by host factors that alter bactericidal activity and increase the risk for development of invasive disease. These factors include the absence of bactericidal antibodies against meningococci (
17,
18,
45), deficiencies in the complement cascade (
13), and the presence of blocking immunoglobulin A antibodies that inhibit the bactericidal activity of human sera (
19). The bactericidal activity of human sera against meningococci is also used as a surrogate marker for assessing meningococcal vaccine efficacy.Meningococci have evolved mechanisms that protect them from the bactericidal activity of human sera. Invasive serogroup B meningococcal strains recovered from blood and cerebrospinal fluid often resist being killed by human sera (
48). The molecular basis for resistance has been attributed to the expression by this organism of an (α2→8)-linked polysialic acid capsule and a short-chained lipooligosaccharide (LOS) with terminal sialic acid residues (
23,
34,
35). Meningococci isolated from the bloodstream in invasive disease, in contrast to nasopharyngeal isolates, are heavily encapsulated (
9) and express the L3,7,9 LOS immunotypes (
28). These immunotypes have a lacto-
N-neotetraose originating from HepI of the inner core, which may be terminally sialylated (
34,
62). However, the experimental data defining the precise contributions of the capsule, LOS sialylation, and LOS structure to the ability of serogroup B meningococci to resist the bactericidal activity of human sera is conflicting (
11,
15,
20,
21,
27,
37,
63–
65).LOS epitopes are immunogenic in infants and children and induce protective bactericidal antibodies in convalescent sera (
10,
12). These bactericidal LOS antibodies appear to be directed at conserved low-molecular-weight LOS epitopes (
10,
12). LOS is also a component of new serogroup B outer membrane vesicle (OMV) vaccines and is proposed as a basis for other new meningococcal vaccines (
1–
3,
50). Although changes in the structure of LOS are known to influence the amount and epitopes of bactericidal and other functional antibodies elicited by OMV vaccines (
2), the precise LOS structure(s) to include in these and other LOS-containing meningococcal vaccines is uncertain.To help understand the basis for meningococcal survival following mucosal invasion and to facilitate development of meningococcal vaccines which may contain LOS, we created a series of genetically and structurally defined capsule-, sialylation-, and LOS-altered mutants of the serogroup B meningococcal strain NMB. We used these mutants to study the contributions of the capsule, LOS sialylation, and changes in LOS structure to meningococcal resistance to the bactericidal activity of normal human sera (NHS).
相似文献