Cross-recognition between histones and La/SSB may account for anti-DNA reactivity in SLE patients

Antibodies to La/SSB are detected in sera of patients with primary Sjogren's syndrome (pSS) and systemic lupus erythematosus (SLE). The vast majority of anti-La/SSB positive sera contain antibodies directed towards a linear B-cell epitope of La/SSB spanning the sequence 349–364aa (pep349–364). The aim of this study was to evaluate the fluctuation of antibody levels to major B-cell epitopes of La/SSB over time and investigate for their possible crossreactions. Sequential sera from 15 SLE and 15 pSS patients, followed from 3 to 10 years were obtained. All patients with SLE were positive for anti-Ro/SSA, anti-La/SSB and anti-dsDNA antibodies and patients with pSS were positive for anti-Ro/SSA and anti-La/SSB antibodies. Sera from 30 patients with SLE without anti-La/SSB antibodies and 30 healthy individuals served as disease and negative control respectivelly. All sera tested for the presence of anti-pep349–364 antibodies, using a specific ELISA. Specific anti-pep349–364 IgG was purified from sera of SLE patients and evaluated for cross reactivity against dsDNA and histones. In all SLE sera the levels of anti-pep349–364 antibodies varied in time and fluctuated in parallel with anti dsDNA antibodies. Anti-pep349–364 IgG purified from 7 SLE patients. Five out of 7 were found to react with calf thymus DNA in ELISA. All purified (7/7) anti-pep349–364 IgG preparations reacted with histone H1 and failed to produce a positive immunofluorescence pattern in Crithidia luciliae anti-dsDNA assay which lacks histones. Competative inhibition experiments demonstrated that histone H1 could inhibit completely the binding of anti-pep349–364 IgG to pep349–364 while pep349–364 inhibited by 70% the binding of anti-pep349–364 IgG to histone H1. These findings indicate that a subgroup of SLE patients possess cross-reacting anti-histone H1 antibodies and anti-pep349–364 antibodies, which can be faulty considered as anti-dsDNA reactivity in regular ELISA techniques

T cell help is required to induce idiotypic–anti-idiotypic autoantibody network after immunization with complementary epitope 289–308aa of La/SSB autoantigen in non-autoimmune mice

Immunotherapies against autoimmune diseases have been of limited success. Preventive vaccines could be developed on the basis to abrogate unwanted immune responses to defined autodeterminants. In this study it is shown that immunization of BALB/c mice with two linear T and B cell epitopes of the human La/SSB autoantigen (spanning the regions 289–308aa and 349–364aa) and their complementary forms specified by the complementary mRNA, results in characteristic B and T cell responses. Mice immunized with the 289–308aa epitope or its complementary peptide elicited specific antibodies against both epitopes. In contrast, mice immunized with the 349–364aa epitope or its complementary peptide mounted antibody titres against the immunizing peptide only. According to these data, the 289–308aa epitope and its complementary form were capable to generate an idiotypic–anti-idiotypic response, which were cross-regulated. Peptide-specific T cell proliferation and cytokine production in vitro revealed the induction of a two-stage T helper response (Th1→Th2 type) after immunization with either the epitope 289–308 or its complementary peptide. IgG1 was the predominant subclass after immunization with the two forms of epitopes 289–308 and 349–364, while a response of the IgG2b > IgG2a was obtained after the immunization with the complementary form of 349–364 epitope reflecting the TH2/TH1 polarization, respectively. Our data suggest that the complementary peptides of two immunodominant epitopes of human LaSSB can mimic the autoantibodies against these epitopes and establish an active idiotypic–anti-idiotypic network