Could rheumatoid arthritis result from an abnormal T cell response towards lubricin/superficial zone protein?

The lack of some suppressor T cells (including TCD4+CD25+(high) positively selected first in thymic medulla) specific to a restricted set of autoantigens may be the common link between all patterns of rheumatoid arthritis. In other words, instead of a 'peak' of TCD4+ effector T cells common to all patients with rheumatoid arthritis (which has so far never been demonstrated), a 'hole' in TCD4+CD25+(high) responses towards a limited set of autoantigens responsible for the normal maintenance of tolerance within the joints may be shared by many patients with rheumatoid arthritis. The hallmark of this disorder is the involvement of tissues subjected to friction stress bathed in a lubricating fluid (rheumatoid nodules and bursae, tendinous sheaths, pleura, pericardium, sclera, and joints covered by hyaline cartilage). Consequently, autoantigens shared by all forementioned places may be better candidates than autoantigens restricted to the hyaline cartilage (like collagen II). Tenosynovitis, bursitis and rheumatoid nodules can herald rheumatoid arthritis, and rheumatoid pericarditis is very frequent at the histological level. Lubricin and superficial zone protein (SZP), which are closely related products of the megacaryocyte stimulating factor (MSF) gene, are among the best candidate autoantigens for such a positive selection of suppressor T cells. Lubricin is responsible for most of the lubricating properties of synovial fluid, and SZP (expressed by the superficial articular chondrocytes from diarthrodial cartilages and lining cells of synovial villi) also shares lubricating and cytoprotective properties. Moreover, the expression of lubricin is very probable in pericardium and pleura, and can be induced by friction stress. Although this mucinous glycoprotein may already share close similarities at the antigenic levels with mucins previously demonstrated in Hassall's corpuscles of the thymus, evidence for the ectopic expression of lubricin/SZP within normal human thymus may further support this hypothesis. The prenatal positive selection within the thymus of a functional pool of TCD4+CD25+(high) clones specific for most peripheral tissues is critical (at least in mice) for the quality of tolerance for the rest of the organism's lifespan. Therefore, a poor expression of lubricin/SZP early in life within the human thymus may also favour a lack of suppressor T cells specific to tissues bathed with synovial fluid, i.e. the onset of rheumatoid arthritis later on in life. As studies of human thymus long before the onset of rheumatoid arthritis are hampered for obvious reasons, studies of murine thymus could be a first step. In as much as the human counterpart of lubricin is expressed in the thymic medulla of mice, the generation of knocked-out mice for its expression within the thymus could be one of the best models to test the above hypothesis. The stimulation of TCD4+CD25+(high) clones specific for immunodominant epitopes from the joints/synovial fluid (belonging perhaps to lubricin or SZP) could help restore a normal balance between effector T cells and suppressor T cells in rheumatoid arthritis patients.