The purpose of this study was to examine avidity of immunoglobulin G (IgG) antibody for surface antigens of Porphyromonas gingivalis in sera from patients with adult periodontitis. The antigens used were whole cell antigens, lipopolysaccharides (LPS), and fimbriae of non-invasive P. gingivalis ATCC 33277 and invasive 16-1. Serum IgG titers for the P. gingivalis antigens were measured by enzyme-linked immunosorbent assay (ELISA) before and after periodontal initial preparation. IgG avidity was measured by diethylamine dissociation ELISA. IgG titers for the whole cell antigens of 16-1, LPS, and the fimbria antigens from both P. gingivalis strains were significantly higher in the patient group than those in the control group, whereas values for avidities were significantly lower in patient sera. Although we found a statistically significant decrease in the IgG titers of patients following initial preparation, avidities against the fimbria antigen of invasive 16-1 strain increased. The present study showed that IgG antibodies elicited in patients were different in their ability to respond to invasive P. gingivalis 16-1 and to non-invasive 33277. The patient sera with high IgG titers demonstrated low values for avidity, suggesting that IgG responses in patients play a limited role in colonization inhibition or elimination of P. gingivalis. The data indicate that periodontally healthy individuals may have highly functional antibodies which may protect against P. gingivalis colonization. Our findings suggest that the ability to produce functional antibodies in the patient group is lower than that in the periodontally healthy group, but the functional antibodies can be induced by the initial preparation.
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