Serum immunoglobulin G responses to various Actinobacillus actinomycetemcomitans serotypes in a young ethnographically heterogeneous periodontitis patient group

Sera from young patients with periodontal diseases have been shown to often contain highly elevated antibody levels to Actinobacillus actinomycetemcomitans, in particular serotype b. Such responses were reportedly predominated by antibodies of the immunoglobulin G2 (IgG2) subclass. The aim of this study was to investigate an ethnically diverse group of 14 early-onset periodontitis and 15 rapidly progressive periodontitis patients for the occurrence of elevated antibody titers against the five known A. actinomycetemcomitans serotypes, and to compare the patient's IgG subclass response profiles. Enzyme-linked immunosorbent assays were used to measure both total IgG and subclass specific IgG titers. Twenty-four subjects had markedly elevated total IgG levels against at least one serotype. The frequencies of high responses against serotypes a, b, c, d and e were 7, 11, 6, 4, and 4, respectively. Elevated antibody responses were predominated by IgG2, regardless of the serotype to which the response was directed. The serotype specificity of the host responses was further investigated by competitive binding studies with serotype-specific monoclonal antibodies, Twelve sera were found to contain antibodies capable to strongly inhibit the binding of monoclonal antibodies against a single serotype; four other sera had antibodies against epitopes of two, and one serum against those of three serotypes. The findings document broad serotype diversity in an ethnically heterogeneous group of patients and indicate that strong antibody responses to A. actinomycetemcomitans are predominated by IgG2 regardless of the serotype of the infective agent.