Can Antinuclear Antibodies Have a Pathogenic Role in Systemic Sclerosis?

Abstract

Systemic sclerosis (SSc) is a connective tissue disease characterized by extensive fibrosis of the skin and internal organs, associated with vasculopathy and autoimmune features. Antinuclear antibodies (ANA) are found in almost all SSc patients and constitute strong diagnosis and prognosis biomarkers. However, it remains unclear whether ANA are simple bystanders or if they can have a role in the pathophysiology of the disease. One might think that the nuclear nature of their targets prevents any accessibility to autoantibodies. Nevertheless, recent data suggest that ANA could be pathogenic or at least contribute to the perennation of the disease. We review here first the indirect clues of the contribution of ANA to SSc: they are associated to the disease subtypes, they may precede disease onset, their titer correlates with disease activity and severity, there is an association between molecular subsets, and some patients can respond to B-cell targeting therapy. Then, we describe in a second part the mechanisms of ANA production in SSc from individual genetic background to post-transcriptional modifications of neoantigens. Finally, we elaborate on the potential mechanisms of pathogenicity: ANA could be pathogenic through immune-complex-mediated mechanisms; other processes potentially involve molecular mimicry and ANA penetration into the target cell, with a focus on anti-topoisomerase-I antibodies, which are the most probable candidate to play a role in the pathophysiology of SSc. Finally, we outline some technical and conceptual ways to improve our understanding in this field.

Keywords: antibodies; antinuclear antibodies (ANA); biomarkers; pathogenic antibody; systemic sclerosis.

Figure 1

Main antigenic targets of ANA in SSc. Anti-topoisomerase-I autoantibodies (Aab) are specific to systemic sclerosis (SSc). Topoisomerase-I is involved in the DNA relaxation and can be found in the nucleus but also the cytoplasm. There are several centromeric proteins, but three can be targeted by SSc Aab: CENP-A (17 kDa), CENP-B (80 kDa), and CENP-C (140 kDa). CENP-B is the main antigen target, and ACA can recognize other epitopes. Anti-U3 RNP or anti-fibrillarin Aab recognize a 34-kDa protein in the U3-RNP complex in the nucleolus. U3 RNP has a role in the transcription of ribosomal RNA. Pm/Scl is in the nucleolus and is a complex of 11–16 proteins; the 75 and 100 kDa proteins are the main targets of these Aab. Although their precise role remains to be elucidated, they are thought to be involved in the ribosome synthesis. RNA polymerase III is involved in the DNA transcription of RNA and is in the nucleus. Anti-SS-A Aab are directed against two proteins of different molecular masses, namely, 52 and 60 kDa, and those proteins are part of different macromolecular complexes. Ro60 is part of a ribonucleic complex involved in the regulation and transcription of mRNA and Ro52 (also known as TRIM21) and plays a role in the immune response by acting as a cytosolic sensor during viral infection. SS-B or La protein is a protein involved in the maturation of transcripted RNA by RNA polymerase III. The anti-SS-B Aab is always associated with anti-SS-A Aab. Those ANA (SS-A and SS-B) can be found in the nucleus but also in the cytoplasm.

Figure 2

ANA generation in SSc. Release of neoantigens could lead in predisposed individuals to immunization and antibodies generation. These neoantigens may raise from apoptotic blebs. Their release could be favorized by environmental factors like silica or solvents. Neoantigens can undergo post-translational modification (PTM) or modifications by tumorigenesis (RNA polymerase 3). Then, epitope spreading mechanisms lead to immunization to both modified or normal antigens, which generate antibodies. Finally, molecular mimicry mechanisms with viral agents like Epstein Barr virus are also described.

Figure 3

ANA and antigen interaction at cell surface. Neoantigens, such as topoisomerase-I (TOPO-I), are released by apoptotic blebs. TOPO-I bind to CCR7 or proteoglycans at the membrane and are recognized by anti-topoisomerase-I (ATA). This binding recruits ATA at the FB membrane and promotes inflammatory and profibrotic pathways. The recruitment of more ATA activates monocytes and could promote autoantibody-dependent cell-mediated cytotoxicity mechanisms.

Figure 4

Immune complexes theory. Antinuclear antibodies (ANA) interact with their neoantigens released by apoptotic blebs and form immune complexes (IC). These IC interact with different Toll-like receptors (TLR) according to ANA subtypes, both on fibroblast (FB) and endothelial cells. FB are then activated into myofibroblasts and secrete extracellular matrix and proinflammatory and profibrotic cytokines. Endothelial cells engage pathways according to ANA subtype (like NF-κB and SAPK/JNK) and secrete cytokines that activate FB.

Figure 5

Endocytosis/cell penetration of ANA. Endocytosis of antinuclear antibodies (ANA) can be achieved via FcγR-dependent or FcγR-independent (for example, caveolin pathways) mechanisms. Endocytosis independent of FcγR could be carried out through binding to calreticulin at the membrane and then myosin in the cytoplasm. After reaching the cytoplasm or the nucleus, ANA may interact with its antigen and induce cell modifications.