RECOGNITION OF ANTIGENIC EPITOPES IN LIPOPOLYSACCHARIDE AND PROTEIN FROM ACTINOBACILLUS-ACTINOMYCETEMCOMITANS BY SERUM ANTIBODIES IN UNTREATED RAPIDLY PROGRESSIVE PERIODONTITIS PATIENTS

Actinobacillus actinomycetemcomitans has been associated with early-on set periodontitis, including the localized juvenile and rapidly progre ssive forms. The immunodominant antigens of A. actinomycetemcomitans r ecognized by rapidly progressive periodontitis patients remain unident ified. Sera from 22 patients with rapidly progressive periodontitis an d 20 periodontally normal subjects were tested by enzyme-linked immuno sorbent assay (ELISA) for immunoglobulin G antibodies to whole-cell so nicate, protein, purified lipopolysaccharide and lipopolysaccharide fr actions of A. actinomycetemcomitans. The median titers of rapidly prog ressive periodontitis patients and control subjects to whole-cell soni cate were 25.0 and 14.5 ELISA units, respectively (not significantly d ifferent). Binding of antibody from patient sera occurred to both the lipopolysaccharide and the protein fractions, with greater binding to lipopolysaccharide than to protein. We show for the first time that pa tient sera contain antibodies that bind specifically to antigenic epit opes in lipid A and in the core carbohydrate of lipopolysaccharide tha t were previously considered to be inaccessible and unavailable, as we ll as to epitopes in the O side chains. Sera manifesting antibody tite rs 2-fold or greater than the median titer for control sera were judge d to be seropositive. More patients were seropositive for lipid A than for any of the other antigen preparations studied, and the median tit er for patient sera to lipid A but to none of the other purified lipop olysaccharide fractions was significantly elevated relative to control values. Of 22 patients, 10 were seropositive to whole-cell sonicate, 7 to protein, 8 to lipopolysaccharide, 7 to the high-molecular-weight lipopolysaccharide-polysaccharide fraction rich in O side chains, and 16 to lipid A. The core carbohydrate did not adhere to the test plate surface, and this precluded ELISA measurements. However, when the core carbohydrate was used in the ELISA inhibition assay, it reduced antib ody binding to lipopolysaccharide-coated plates by up to 45%, thereby demonstrating antibody binding to core carbohydrate. The core carbohyd rate fraction from the Re mutant of Salmonella minnesota known to cont ain no O-side chains also inhibited binding of specific antibody to pl ates coated with A. actinomycetemcomitans lipopolysaccharide. Overall, there was extreme variation in responses among patients to the variou s antigen preparations, with no single pattern dominating. Lipopolysac charide and its components appear to be the immunodominant epitopes, s ince most rapidly progressive periodontitis patients are seropositive for lipopolysaccharide and/or its components and they have higher tite rs relative to those for proteins.