Physicochemical characteristics of human IgG Fc fragments that expose regulatory rheumatoid factor neoepitopes and may show promise as antirheumatic agents

Regulatory Rheumatoid Factor is Specific to PD-1 and Uses PD-1 Pathway to Control CD4 T Lymphocytes

BACKGROUND

We have earlier discovered a new factor of autoimmunity downregulation, called regulatory rheumatoid factor (regRF). Being anti-idiotypic antibodies, regRF restricts the expansion of CD4+ T lymphocytes to the idiotype of which it is specific, according to the negative feedback principle. It has been shown that only activated CD4+ T lymphocytes are the target of regRF. However, it is still not clear the way regRF distinguishes activated cells from naive ones. RegRF molecules, apart from individual paratopes specific to unique sequences of B- and T-cell receptors, have a shared paratope. We assume that regRF by means of a shared paratope recognizes one of the surface activation molecules of CD4+ T lymphocytes and initiates the cell death. Programmed death-1 (PD-1) has been tested as a potential receptor of the shared regRF paratope and transmitter of the negative regRF signal into activated CD4+ T lymphocytes.

METHODS

The specificity of the shared regRF paratope to PD-1 was determined by ELISA. T cell activation was performed with immobilized anti-CD3ε antibodies. Flow cytometry was used to study the effect of regRF on PD-1+CD4+ lymphocytes.

RESULTS

We found that regRF binds to PD-1. IgG Fc fragments carrying epitopes specific to the shared paratope of regRF compete with PD-1 for binding to regRF. It follows that regRF recognizes specifically PD-1 by means of a shared paratope. RegRF-containing serum reduced the number of PD-1+CD4+ lymphocytes in proportion to their increase resulting from the action of anti-CD3ε antibodies.

CONCLUSION

RegRF uses PD-1 pathway to control activated CD4+ T lymphocytes.

Fourier-transform infrared spectroscopy of human IgG Fc fragments, which are a promising drug for the treatment of autoimmune diseases

IgG Fc fragments that expose regulatory rheumatoid factor epitopes (regRF epitopes) have emerged as a promising immunosuppressive drug. Immunization of rats with such Fc fragments reduced symptoms of experimental autoimmune diseases. The immunosuppressive effect of Fc fragments is based on stimulating the production of regRF, which kills activated CD4 T lymphocytes. The formation of regRF epitopes on Fc fragments was previously shown to be associated with a reduction in disulfide bonds in the fragments' hinge region. However, the structure of Fc fragments that bear regRF epitopes remained largely unclear. Infrared spectra were compared for lyophilized Fc fragments displaying regRF epitopes and Fc fragments without such epitopes. FTIR spectroscopy found no differences in the amide I, amide II, and amide III bands, indicating that there are no distinctive features in the secondary structure of Fc fragments bearing regRF epitopes. The distinctive feature of Fc fragments bearing regRF epitopes, irrespective of whether the free SH groups in the hinge were preserved or lost after lyophilization, is the presence of a band or a fine structure in the region containing the bending vibrations of the SH groups. Furthermore, the Fc fragments with regRF epitopes differ from those without in that they have a band in the absorption region of aromatic amino acid rings. Taken together, these facts suggest that the appearance of regRF epitopes results from changes in the tertiary structure of the hinge and the domains that occur when the hinge is reduced, and they also indicate that these conformational changes are resistant to subsequent changes in the state of cysteine residues in the hinge. Bands in the regions of aromatic amino acids and the bending vibrations of SH groups are markers of the presence of regRF epitopes on IgG Fc fragments. FTIR spectroscopy can be used to detect these epitopes.

Suppression of experimental autoimmune encephalomyelitis by IgG Fc fragments bearing regRF epitopes

Previously we identified a rheumatoid factor, the production of which provides rats with resistance to experimental autoimmune diseases. It has been named regulatory rheumatoid factor (regRF). Immunization with conformers of IgG Fc fragments carrying epitopes specific to regRF reduces rat collagen-induced arthritis. The aim of this study was to determine whether IgG Fc fragments bearing regRF epitopes suppress experimental autoimmune encephalomyelitis, and to evaluate the potential of a strategy of stimulating production of regRF to treat multiple sclerosis. Two days after myelin basic protein injection, rats were immunized with Fc fragments exhibiting regRF epitopes, as well as with Fc fragments without those epitopes. The effect of Fc immunization on clinical signs of EAE and immunological parameters was evaluated. Stimulation of regRF production by IgG Fc fragments bearing regRF epitopes diminished EAE symptoms in rats, while immunization with Fc fragments without those epitopes worsened EAE. The improvement of EAE symptoms in rats treated with Fc fragments bearing regRF epitopes was associated with regRF production and with the relatively low number of blood CD4 T lymphocytes during disease development. In experiments involving immunizing intact rats and lymph node mononuclear cell cultures, Fc fragments bearing regRF epitopes decreased the CD4 T lymphocyte population indirectly, via regRF production. RegRF is a promising biotarget in MS, and Fc fragments bearing regRF epitopes are a potential therapeutic agent for MS.

Comparison of the specificity of rheumatoid factor detected by latex fixation with that of regulatory rheumatoid factor

Background

Rheumatoid factor (RF), originally defined as pathological autoantibodies to IgG that are detected in rheumatoid arthritis, turned out to be multi‐specific antibodies, some of which exhibit immunoregulatory properties. Recently, we identified a RF, the production of which confers resistance to experimental autoimmune diseases and is associated with the remission of autoimmune diseases. To differentiate the RF, we discovered from the one associated with rheumatic disease onset or progression and to reflect its immunoregulatory properties, we named it regulatory rheumatoid factor (regRF). Immunization with conformers of Fc fragments that expose regRF neoepitopes reduces collagen‐induced arthritis in rats. Certain information about the specificity of classical RF and regRF indicates that these populations may be one and the same. Therefore, the aim of this study was to determine whether there is a difference between the classical RF and regRF.

Methods

Classical RF was measured in diseased blood by the latex fixation method, and regRF was detected by the agglutination of human IgG‐loaded tanned erythrocytes. Competitive analysis was used to determine the specificity of rheumatoid factors.

Results

It was found that regRF and pathology‐associated RF constitute different antibody populations. Pathology‐associated RF is specific for lyophilized IgG. RegRF does not interact with IgG. RegRF is specific to conformers of IgG Fc fragments that have a reduced hinge. In latex‐positive rheumatoid arthritis sera, regRF may be present in addition to pathology‐associated RF. The latex fixation method detects both rheumatoid factor populations.

Conclusion

RegRF and classical pathology‐associated RF have different specificity.