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Sanjay Ram^*, Ajay K. Sharma, Scott D. Simpson^*, Sunita Gulati^*, Daniel P. McQuillen^*, Michael K. Pangburn, and Peter A. Rice^*

From ^* The Maxwell Finland Laboratory for Infectious Diseases, Boston Medical Center, Boston, Massachusetts 02118; and the University of Texas Health Sciences Center, Tyler, Texas 75708

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'-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.

This work was supported by National Institutes of Health grant AI-32725.

Address correspondence to Sanjay Ram, The Maxwell Finland Laboratory for Infectious Diseases, Boston Medical Center, Rm. 215, 774 Albany Street, Boston, MA 02118. Phone: 617-534-5282; Fax: 617-534-5280; E-mail: sram{at}bu.edu

Ajay K. Sharma's present address is Department of Neuro-oncology, M.D. Anderson Cancer Center, Houston, TX 77030.

¹ Abbreviations used in this paper: CMP-NANA, 5'-cytidinemonophospho- N-acetylneuraminic acid; fH, factor H; HIS, heat-inactivated serum; LOS, lipooligosaccharide; NHS, normal human serum; rH, recombinant fH; SCR, short consensus repeat; SR, serum-resistant; SS, serum-sensitive.

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