. . Volume 70 • Number 7 730 Background: Six serotypes of Porphyromonas gingivalis have recently been described. We sought to test the hypothesis that serotype specific carbohydrates from these strains are important antigens that elicit potent immune responses. Methods: Serum concentrations of IgG reactive with P. gingivalis serotypes K1-K6 were determined for 28 adult (AP) and 28 general- ized early-onset (G-EOP) periodontitis patients previously deter- mined to be seropositive for a broken cell preparation of P. gingivalis. To confirm relationships suggested for K1, K2, and K6 in the analysis of initial data, the study population was increased to 133. Results: Frequency of seropositivity for the 6 serotypes ranged from 26 to 54% of subjects. IgG concentrations ranged from 0 to 453 μg/ml with many subjects seropositive to more than one serotype. Concentrations for the subset of patients who was seropositive were high (mean responses ranged from 20 to 105 μg/ml for the 6 serotypes). Significant correlations between seropositivity to serotypes K1 and K5 as well as between K5 and K6 were found. Conclusions: We examined the relationship of diagnosis, race, gender, smoking, probing depth, attachment loss, and antibody reaction with the P. gingivalis serotypes by analysis of variance. Initial findings suggested potential relationships between diagnosis, smoking, race, gender, and antibody reactive with serotypes K1, K2, and K6. A significant relationship did exist between smoking and decreased antibody reactive with P. gingivalis serotype K2. No other relationships were substantiated. We also examined the IgG subclass distribution and found that responses were almost exclu- sively IgG2. These data support the concept that antibody responses to all 6 serotypes are common in both AP and G-EOP and that these K serotype carbohydrates elicit potent IgG2 responses. J Periodontol 1999; 70: 730-735. KEY WORDS Antibodies; antigens, K; IgG; periodontitis; serotype; Porphyromonas gingivalis. P gingivalis is a microorganism fre- quently found in periodontitis lesions. 1-5 Data also indicate that it is a significant risk factor for periodonti- tis. 6 We recently examined IgG antibody reactive with P. gingivalis and found that 30 to 45% of adult and early-onset peri- odontitis patients were seropositive, while only 5% of healthy controls were seroposi- tive. 7 Interestingly, antibody reactive with P. gingivalis is associated with decreased attachment loss in early-onset periodonti- tis suggesting it may play an important role in host defense. 8 Laine et al. 9 and van Winkelhoff et al. 10 have recently defined 6 P. gingivalis serotypes (K1-K6) by use of rabbit antisera. They identified the serotype-defining antigens as capsular polysaccharide. 9,10 Subsequently, they found the prevalence of the P. gingivalis K serotypes ranged from 1.1 to 23% in periodontitis patients. 11 Studies with Actinobacillus actinomycetemcomitans indicate that it is important to define the serotype when studying immune re- sponses in a given individual as the serotype specific carbohydrate is the dominant antigen. 12,13 Further, the response to this antigen is affected by smoking, associated with better periodon- tal health, related to diagnosis, and is higher in black than in white subjects. 12-17 The objectives of this study were to determine if the serotype specific antigens elicit potent immune responses and to examine the relationship of IgG concen- trations reactive with the 6 P. gingivalis K Antibody Reactive With Porphyromonas gingivalis Serotypes K1-6 in Adult and Generalized Early-Onset Periodontitis Joseph V. Califano, * Robert E. Schifferle, † John C. Gunsolley, ‡ Al M. Best, * Harvey A. Schenkein, * and John G. Tew * * Clinical Research Center for Periodontal Diseases, School of Dentistry, Virginia Commonwealth University, Richmond, VA. † Departments of Periodontology and Oral Biology, State University of New York, Buffalo, NY. ‡ Currently, Department of Periodontology, University of Maryland, Baltimore, MD; previously Virginia Commonwealth University, Richmond, VA.