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2-Macroglobulin1
Sections of
* Infectious Diseases and
Hematology-Oncology, Evans Biomedical Research Center, Boston University Medical Center, Boston, MA 02118; and
Department of Medical Microbiology and Immunology, University of Aarhus, Aarhus, Denmark
We examined complement activation by Neisseria
gonorrhoeae via the mannan-binding lectin (MBL) pathway in
normal human serum. Maximal binding of MBL complexed with
MBL-associated serine proteases (MASPs) to N.
gonorrhoeae was achieved at a concentration of 0.3 µg/ml.
Preopsonization with MBL-MASP at concentrations as low as 0.03 µg/ml
resulted in 
60% killing of otherwise fully serum-resistant
gonococci. However, MBL-depleted serum (MBLdS) reconstituted with
MBL-MASP before incubation with organisms (postopsonization) failed to
kill at a 100-fold higher concentration. Preopsonized organisms showed
a 1.5-fold increase in C4, a 2.5-fold increase in C3b, and an
25-fold increase in factor Bb binding; enhanced C3b and factor Bb
binding was classical pathway dependent. Preopsonization of bacteria
with a mixture of pure C1-inhibitor and/or
2-macroglobulin added together with MBL-MASP, all at
physiologic concentrations before adding MBLdS, totally reversed
killing in 10% reconstituted serum. Reconstitution of MBLdS with
supraphysiologic (24 µg/ml) concentrations of MBL-MASP partially
overcame the effects of inhibitors (57% killing in 10% reconstituted
serum). We also examined the effect of sialylation of gonococcal
lipooligosaccharide (LOS) on MBL function. Partial sialylation of LOS
did not decrease MBL or C4 binding but did decrease C3b binding by 50%
and resulted in 80% survival in 10% serum (lacking bacteria-specific
Abs) even when sialylated organisms were preopsonized with MBL. Full
sialylation of LOS abolished MBL, C4, and C3b binding, resulting in
100% survival. Our studies indicate that MBL does not participate in
complement activation on N. gonorrhoeae in the presence
of "complete" serum that contains C1-inhibitor and
2-macroglobulin.
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