Enhanced Factor H Binding to Sialylated Gonococci Is Restricted to the Sialylated Lacto-N-Neotetraose Lipooligosaccharide Species: Implications for Serum Resistance and Evidence for a Bifunctional Lipooligosaccharide Sialyltransferase in Gonococci

Abstract

We isolated serologically identical (by serovar determination and porin variable region [VR] typing) strains of Neisseria gonorrhoeae from an infected male and two of his monogamous female sex partners. One strain (termed 398078) expressed the L1 (Gal␣1 3 3Gal␤1 3 4Glc␤1 3 4HepI) lipooligosaccharide (LOS) structure exclusively; the other (termed 398079) expressed the lacto-N-neotetraose (LNT; Gal␤1 3 4GlcNAc␤1 3 3Gal␤1 3 4Glc␤1 3 4HepI) LOS structure. The strain from the male index case expressed both glycoforms and exhibited both immunotypes. Nuclear magnetic resonance analysis revealed that sialic acid linked to the terminal Gal of L1 LOS via an ␣2 3 6 linkage and, as expected, to the terminal Gal of LNT LOS via an ␣2 3 3 linkage. Insertional inactivation of the sialyltransferase gene (known to sialylate LNT LOS) abrogated both L1 LOS sialylation and LNT LOS sialylation, suggesting a bifunctional nature of this enzyme in gonococci. Akin to our previous observations, sialylation of the LNT LOS of strain 398079 enhanced the binding of the complement regulatory molecule, factor H. Rather surprisingly, factor H did not bind to sialylated strain 398078. LOS sialylation conferred the LNT LOS-bearing strain complete (100%) resistance to killing by even 50% nonimmune normal human serum (NHS), whereas sialylation of L1 LOS conferred resistance only to 10% NHS. The ability of gonococcal sialylated LNT to bind factor H confers high-level serum resistance, which is not seen with sialylated L1 LOS. Thus, serum resistance mediated by sialylation of gonococcal L1 and LNT LOS occurs by different mechanisms, and specificity of factor H binding to sialylated gonococci is restricted to the LNT LOS species.The lacto-N-neotetraose (LNT) lipooligosaccharide (LOS) of Neisseria gonorrhoeae can become sialylated when grown in medium supplemented with 5Ј-cytidinemonophospho-Nacetylneuraminic acid (CMP-NANA) (20). LOS sialylation renders strains that are otherwise susceptible to complementmediated killing, resistant to killing by nonimmune normal human serum (NHS) (26, 31). One explanation is the ability of gonococci bearing the sialylated LNT LOS to bind factor H (37), an important fluid-phase regulatory protein of the alternative pathway of complement (9,29,53,56). Gonococcal LOS undergoes significant phase variability in vivo, which may result in a shift of expression away from the "conventional" (terminal lactosamine of LNT) sialylation site (3, 12). LOS phase variation that results in the loss of the ability of an otherwise serum-sensitive gonococcal strain to sialylate its LOS may render such a strain highly susceptible to complement-mediated killing and therefore may be disadvantageous to an organism, necessitating a redundant strategy to evade killing by complement. The terminal Gal on meningococcal L1 LOS (Gal␣1 3 3Gal␤1 3 4Glc␤1 3 4HepI) can also be sialylated (51). The HepI hexose substitution of L1 LOS is structurally identical to the P K blood group antigen (19) and is commonly found in the LOS of Haemophilus and Moraxella ...

Factor H as a regulator of the classical pathway activation

“…2007). Factor H can bind to sialylated lipooligosaccharides (LOS) of Neisseria gonorrhoeae, but not of N. meningitidis (Gulati et al 2005;Schneider et al 2006). The protein por1A of gonococci, a 33 kDa protein of meningococci, and protein YadA of Yersinia are known factor H binders (Kraiczy and Würzner 2006;Gulati et al 2005;Schneider et al 2006).…”

“…Factor H can bind to sialylated lipooligosaccharides (LOS) of Neisseria gonorrhoeae, but not of N. meningitidis (Gulati et al 2005;Schneider et al 2006). The protein por1A of gonococci, a 33 kDa protein of meningococci, and protein YadA of Yersinia are known factor H binders (Kraiczy and Würzner 2006;Gulati et al 2005;Schneider et al 2006). There are no previous reports of direct binding of isolated factor H to E. coli, although binding of factor H during complement activation in serum has been observed (Kubens et al 1989;Maruvada et al 2008).…”

Outsmarting the host: bacteria modulating the immune response

“…Some bacterial bind factor H to inactive C3b and the complement cascade. N. gonorrhea uses a lipooligosaccharide to bind to factor H, rendering the bacterium resistant to complement killing [10,11]. This will inhibit the activity of C3 and block the downstream activation of the MAC.…”

Structure and Mechanism of the Lipooligosaccharide Sialyltransferase from Neisseria meningitidis

“…For encapsulated Neisseria meningococci, the capsular polysaccharide plays a dominant role in effecting virulence (10,11), and LOS also provides a second line of defense in ensuring its ability to survive in the host (12)(13)(14). The critical role of the Neisseria LOS in human pathogenesis has been demonstrated with the acapsular Neisseria gonococci, for which infection is initiated by molecular mimicry of host glycosphingolipid by the antigenically similar LOS on the extracellular surface of the bacteria (9,15), effectively evading the killing cascade of the host immune response (16). This camouflage mechanism is dependent on the addition of a terminal sialic acid to the LOS that enhances binding of the host regulatory protein Factor H to the bacterial surface, conferring resistance to complementmediated killing (2,16).…”

“…the globotriaose-and lacto-N-neotetraose-containing LOS acceptors) and flexibility in catalyzing either ␣2,3 or ␣2,6 glycosidic bond formation (9). In N. meningitidis expressing the L1-type LOS and some Neisseria gonorrhoeae strains that express an acceptor LOS with a globotriaose terminus, NST catalyzes ␣2,6 glycosidic bond formation between the sialic acid and the terminal galactosyl residue on the acceptor (16,19). In N. meningitidis serotypes L2, -3, -5, -7, and -9 and some N. gonorrhoeae strains, NST catalyzes ␣2,3 linkage formation from sialic acid to the lacto-N-neotetraose moiety on LOS (9).…”