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J. Exp. Med.,
Volume 187, Number 5, March 2, 1998 743-752
By


From * The Maxwell Finland Laboratory for Infectious Diseases, Boston Medical Center, Boston,
Massachusetts 02118; and the Factor H (fH), a key alternative complement pathway regulator, is a cofactor for factor I-mediated cleavage of C3b. fH consists of 20 short consensus repeat (SCR) domains. Sialic acid binding domains have previously been localized to fH SCRs 6-10 and 13. To examine fH binding
on a sialylated microbial surface, we grew Neisseria gonorrhoeae in the presence of 5
University of Texas Health Sciences Center, Tyler, Texas 75708
-cytidinemonophospho-N-acetylneuraminic acid, which sialylates lipooligosaccharide and converts
to serum resistance gonococci previously sensitive to nonimmune serum killing. fH domains
necessary for binding sialylated gonococci were determined by incubating organisms with recombinant human fH (rH) and nine mutant rH molecules (deletions spanning the entire fH
molecule). rH and all mutant rH molecules that contained SCRs 16-20 bound to the sialylated
strain; no mutant molecule bound to serum-sensitive nonsialylated organisms. Sialic acid was
demonstrated to be the fH target by flow cytometry that showed a fourfold increase in fH
binding that was reversed by neuraminidase-mediated cleavage of sialic acid off gonococci.
Functional specificity of fH was confirmed by decreased total C3 binding and almost complete
conversion to iC3b on sialylated gonococci. Sialic acid can therefore bind fH uniquely through
SCRs 16-20. This blocks complement pathway activation for N. gonorrhoeae at the level of C3.
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