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A series of isogenic mutants lacking either the O1 (O-:K66) or K66 (O1:K-) antigens or both (O-:K-), some of which had additional defects in their LPS core polysaccharide was used to examine the interaction between polymorphonuclear leucocytes (PMNLs) and K. pneumoniae serotype O1:K66. In the absence of serum complement, only a O-:K- strain with a deep rough LPS chemotype elicited a PMNL-dependent chemiluminescent (CL) response. However, following opsonization of the non-capsulated strains by complement, the largest CL response was to the O1:K- mutant. This mutant also activated and bound more complement C3 than any of the other encapsulated or non-capsulated strains examined. Despite the surface exposure of smooth and rough LPS in the encapsulated parent and mutant strains, the K66 antigen reduced the binding of C3 and prevented PMNL activation. Both anti-LPS and anti-K66 antibodies, however, stimulated a PMNL-dependent CL response to the K66 bearing strains. |
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