Autocitrullination of Porphyromonas gingivalis proteins and Rheumatoid Arthritis patient sera

Studies suggest an association between periodontitis and rheumatoid arthritis (RA).  A postulated causal mechanism is that the periodontopathic bacterium Porphyromonas gingivalis (Pg)  expresses  peptidyl arginine deiminase which causes citrullination of arginine peptides.   RA patients produce autoantibodies to citrullinated peptides.  Anti cyclic-citrullinated protein antibodies (ACPA) are present in early RA and are highly specific for the disease.  Patients with RA may also have elevated serum antibodies to Pg.  However, this might be because ACPA in RA patients cross react with auto-citrullinated Pg proteins in Pg-ELISA plates.  Therefore the aim of this study was to demonstrate the presence of autocitrullinated Pg proteins and to investigate the hypothesis that ACPA in RA patients reacts with these auto-citrullinated proteins of Pg.

Methods:

Pg whole cell antigens were separated by SDS PAGE and the presence of citrullinated epitopes investigated by Western blotting.  Known ACPA-positive and -negative RA sera was tested by Western blotting to determine the binding pattern of RA sera with Pg, and to test if cross reactivity of RA patient ACPAs with the citrullinated peptides occurred. 

Results:

One main citrullinated protein band was detected within the Pg antigen on Western Blots using control ACPA.  Sera from 24 patients with RA consistently bound to the citrullinated Pg band on Western Blots irrespective of whether they were reportedly ACPA +ve ( n= 13)or ACPA –ve  (n = 11).  RA sera also bound to a citrullinated protein control extract.  Control sera from patients with periodontitis bound to multiple Pg antigens, but did not bind to the citrullinated Pg antigen band or the citrullinated control extract.

Conclusions:

Sera from patients with RA bound to an autocitrullinated protein of Pg.  Further studies are required to determine the nature of this antigen particularly in those patients who were reportedly ACPA negative.