Serotype b-specific polysaccharide antigen (SPA) of Actinobacillus actinomycetemcomitans Y4 consists of D-fucose and L-rhamnose. To clarify the role of SPA in phagocytosis of the organism by human polymorphonuclear leukocytes (PMNs), monoclonal antibodies (MAbs) against SPA and SPA-defective mutants, which were constructed by inserting the transposon Tn916 into strain Y4, were used in a chemiluminescence (CL) assay and a phagocytic killing assay. The CL responses of human PMNs to strain Y4 were very low, and the organism was not killed by PMNs. In contrast, SPA-defective mutants induced strong CL responses. The addition of immunoglobulin G MAbs against Y4 SPA enhanced significantly both the CL responses to strain Y4 and the killing of the organism in the presence of complement. The CL responses to SPA-defective mutants were little affected by the addition of these MAbs. We conclude that SPA of A. actinomycetemcomitans plays an important role in the resistance to host defenses by PMNs
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