Recognition of the carbohydrate modifications to the RgpA protease of Porphyromonas gingivalis by periodontal patient serum lgG

Periodontal infections by Porphyromonas gingivalis are associated with a sustained systemic IgG antibody response and elevations in local antibody synthesis to this organism. One of the targets of this response is a protease, RgpA(cat), which is an important virulence determinant of this organism. Recently, we demonstrated that this molecule is glycosylated and that the glycan chains are immunologically related to P. gingivalis lipopolysaccharide (LPS) (Curtis et al., Infect Immun 1999;62:3816-3823). In the present study, we examined the role of these glycan additions in the immune recognition of RgpA(cat) by sera from adult periodontal patients (n = 25). Serum IgG antibody levels to P. gingivalis W50, RgpA(cat) and LPS and to recombinant RgpA were determined by enzyme-linked immunosorbant assay (ELISA). No correlation was observed between the antibody levels to RgpA(cat) from P. gingivalis and the recombinant form of this enzyme expressed in Escherichia coli. However, a strong association was found between the recognition of LPS and the wild-type enzyme (R = 0.8926; p = 0.0005). Incorporation of LPS into the ELISA led to a significant reduction (mean 25%; range 0.8-43%, SD = 15; p < 0.05) in the recognition of RgpA(cat), but had no effect on the recognition of control antigens. Deglycosylation of RgpA(cat) led to the abolition of immune recognition by patient serum IgG, which suggests that the glycan additions to this molecule are the principal targets of the immune response. Therefore, glycosylation of the RgpA(cat) protease may play an important role in immune evasion by shielding the primary structure from immune recognition.