Arthritis is an autoimmune disease in which the immune system attacks and degrades the connective tissue of joints. Antibodies against modified amino acids, arginine converted to citrulline, and proteins commonly found in joints, mediate the arthritis disease process. The development of arthritis mimics the development of gluten intolerance, celiac, in which another enzyme, transglutaminase ( tissue transglutaminase, tTG or TG2) modifies the major gluten protein, gliadin, and antibodies are produced against both modified gliadin and TG2 autoantigen.
Arthritis of Joints Is Like Coeliac of Intestines; Autoantibodies to Protein Modifying Enzymes
In other articles, I outlined the pathology of gluten intolerance:
- The major protein of wheat gluten, gliadin, contains long stretches of glutamines.
- An intestinal enzyme, TG2, converts the glutamines to glutamates by deamination.
- As TG2 works it binds to gliadin.
- In celiac, the TG2-gliadin complexes are internalized and fragmented to stimulate antibody production against both TG2 and gliadin.
- I think that the internalization and processing for antibody stimulation is dependent on the basic triplet found in TG2.
Arthritis Is Mediated by Autoantibodies to Peptidylarginine Deiminase and Citrullinated Proteins
Parallel to the celiac example, in some forms of arthritis, antibodies are produced against an enzyme that modifies proteins. In arthritis, the enzyme involved, peptidylarginine deiminase (PAD) removes the terminal nitrogen from arginine (deimination) to produce citrullinated proteins. Antibodies are produced to both PAD and citrullinated proteins.
PAD Also Has a Triplet of Basic Amino Acids for Internalization
I of course wondered if PAD had the same triplet of basic amino acids, e.g. RRK, that I had found on all other autoantigens and allergens. Examining the sequence of human PAD in the NCBI sequence databases and comparing to other sequences, I found the basic triplet near the carboxy terminus. The same or an alternative basic triplet was found in PADs from other mammals.
Autoantigens and Predicted Basic Triplets of Amino Acids Reveal the Cause of Arthritis
Arthritis is an inflammatory disease. That means that without inflammation, arthritis cannot start and if inflammation is inhibited, arthritis cannot progress. It is likely that arthritis is the result of chronic inflammation plus a precipitating event, such as joint injury or joint infection. Alternatively, in a manner similar to Hashimoto’s thyroiditis, in which celiac produces anti-TG2 antibodies that attack the TG2 also produced in the thyroid gland, arthritis may be produced by autoantibodies stimulated in the inflammation of other tissues and spreading to the joints. Celiac is also a risk factor for arthritis. Trauma-based inflammation of a joint can also result in migration of Clamydia pneumonia (Cpn)-infected macrophages to the site of inflammation. Cpn could contribute to joint inflammation and promote immunological presentation of autoantigens and autoantibody production.
Stenberg P, Roth B, Wollheim FA. Peptidylarginine deiminases and the pathogenesis of rheumatoid arthritis: a reflection of the involvement of transglutaminase in coeliac disease. Eur J Intern Med. 2009 Dec;20(8):749-55. Epub 2009 Sep 19.