Increased titre and avidity of IgG antibodies to Porphyromonas gingivalis whole cells and a cell surface protein in subjects with adult periodontitis

The titre and avidity of IgG antibodies to Porphyromonas gingivalis whole cells and a 47 kDa cell surface protein were determined in serum samples taken from 20 subjects with adult periodontitis and 20 controls, matched for age, gender, ethnic origin and oral hygiene status. Antibody titres were measured by ELISA and antibody avidity was determined by a chaotrope-dissociation ELISA. Avidity was defined as the molarity of chaotrope required to reduce absorbance by 50% (ID50). The mean IgG antibody log titre to whole cells (8.29 vs. 6.92; p <0.01) and to the 47 kDa antigen (7.61 vs. 6.77; p <0.05) were higher in cases than in controls. Mean IgG antibody avidity to whole cells (4.59 vs. 2.47; p <0.001) and to the surface protein (2.54 vs. 1.67; p <0.001) were also higher in cases than in controls. In cases, IgG antibody titre was highly correlated with avidity for both whole cells (r=0.878; p=<0.001) and the 47 kDa protein (r=0.683; p <0.001). There was a weaker positive correlation between the titre and the avidity of antibody to whole cells (r=0.591; p <0.01) in the control population but antibody titre and avidity for the 47 kDa sonicate antigen were not correlated in the controls (r=0.104). We conclude that many patients with adult periodontitis have effective humoral immunity to P. gingivalis. However, in up to half the patients with adult periodontitis, antibody titres and avidities were low and similar to control values, indicating either susceptibility due to poor host response or that disease is not associated with this particular pathogen.