Because of the vast variety of antigens that are potential targets of NAbs, a combination of a minimum number of autoantibodies that reflects the state of the natural autoimmunity in humans is difficult to define.29 Researchers have focused on different antigens, initially as a means to explore and classify the variety of the target antigens recognized by NAbs and then focused on some due to their link with pathological conditions. Herein, we propose three antigens, namely TNP, G-actin and F(ab’)2 fragments, as representative surrogate target antigens for the estimation of the natural autoimmunity status. Anti-TNP antibodies have been already proposed and used by others13 as a surrogate measure for the estimation of the natural autoimmunity. Like 2,4-dinitrophenol (DNP), TNP is a synthetic molecule that is normally not present in the environment and, thus individuals are not normally exposed to it. This hapten is considered to bear a 3D structure that is commonly found in various autoantigens, such as DNA and cytoskeleton proteins.30,31 Thus, anti-TNP antibodies serve as an index of polyreactivity and, therefore, of natural autoimmunity.
Anti-actin autoantibodies were selected based on the fact that NAbs have been shown to recognize highly conserved autoantigens.32 Actually, actin is a vital and abundant component of the cytoskeleton, heavily conserved through evolution, with minor differences among the species, while autoantibodies to actin and other cytoskeletal proteins have been found in sera taken from cord blood and healthy children. Particularly, its globular form (G-actin) is a frequent target of NAbs both at polyclonal (in healthy sera) and monoclonal level (among human paraproteins and mouse NAbs).33, 34
Antibodies against F(ab’)2 fragments are the third measure we used for the estimation of the natural autoimmunity based on their implication in immunoregulatory mechanisms.35 Anti-F(ab’)2 autoantibodies possibly contribute to the termination of successful antibody responses and/or the control of autoreactive B-cell. This is achieved by their binding to BCR via the Fab region, that is followed by crosslinking of the BCR and the inhibitory FcγRIIb receptor on the surface of B cells via the Fc region of the anti-F(ab')2 autoantibody. Cross-linking the BCR and FcγRIIb can lead to apoptosis of the B cell resulting in an immunosuppressive effect.36, 37 Studies of the anti-F(ab’)2 autoantibodies in a variety of diseases, including autoimmune disorders, such as SLE, multiple sclerosis, nephritis, and others, support the experimental evidence regarding their immunoregulatory function.
Briefly, our study concluded that patients with oligo-JIA present with levels of serum IgA NAbs against all the three antigens studied that are higher than those observed in age-matched healthy controls. Moreover, disease activity as well as anterior uveitis were proved as independent factors affecting serum levels of IgM anti-TNP NAbs. Indeed, IgM anti-TNP NAbs levels in patients with inactive disease and/or anterior uveitis were higher than those of patients with active disease and/or without uveitis, the last of them presenting levels similar to those of normal controls.
During the last decade, several studies uncover intestinal dysbiosis as a factor contributing to the pathogenesis of JIA subtypes, including oligo-JIA.38, 39 On the other side, systematically produced and locally secreted IgA NAbs seem to play a critical, despite incompletely understood,40 role in the gut microbiome homeostasis.41 Thus, differences in the concentration of IgA NAbs between oligo-JIA and controls observed in our study may reflect disturbances of gut microbiome characterizing this subtype of JIA.
Our finding that patients with active disease present with lower IgM NAbs levels than those with inactive is in accordance with their contribution to the pathogenesis of autoimmune diseases.8, 9 More interesting, however, is our result that anterior uveitis affects independently the concentration of circulating IgM NAbs. In parallel with previous studies showing that in addition to ANA, serum antibodies anti-H3 histone and serum antibodies against ocular antigens are also associated with an increased risk of uveitis occurrence, 42,43 this result signifies the possible involvement of natural autoimmunity in the as yet unclarified pathogenesis of this specific uveitis entity.
Sex differences in IgM NAbs observed in our study are in accordance with the results of previous researchers indicating that these differences could be attributed to the fact that nutritional or other environmental exposures may vary by sex with potential long-term implications for immune-mediated disease.44
In conclusion, our results provide additional evidence that disturbances in natural autoimmunity may contribute to the as yet unclarified pathogenesis of oligo-JIA. It has been also shown that the three measured NAbs represent a panel satisfactorily describing the state of natural autoimmunity. Moreover, their independent association with disease activity and the presence of anterior uveitis indicates that it is worthy to be studied in appropriately designed epidemiological studies in order to elucidate their possible use as prognostic biomarkers.