Cross-reactivity of human IgG anti-F(ab')2 antibody with DNA and other nuclear antigens

Natural autoimmunity in oligoarticular juvenile idiopathic arthritis

BACKGROUND

Oligoarticular juvenile idiopathic arthritis (oligo-JIA) is considered as an antigen-driven lymphocyte-mediated autoimmune disease. Natural antibodies (NAbs) are pre-immune antibodies produced in the absence of exogenous antigen stimulation, participating in both, innate and adaptive immunity. Considering their major immunoregulatory role in homeostasis and autoimmune pathogenesis, we designed this study to further elucidate their role in oligo-JIA pathogenesis.

METHODS

Seventy children with persistent oligo-JIA and 20 healthy matched controls were enrolled in the study. Serum IgM and IgA antibodies against human G-actin, human IgG F(ab΄)2 fragments and the hapten TriNitroPhenol (TNP) as well as the total concentration of serum IgM and IgA were measured by in-house enzyme-immunoassays. Kolmogorov-Smirnov normality test, Kruskal-Wallis H and Mann-Whitney tests were used to assess data distribution, and significant differences of non-parametric data between groups of the study. Backward regression analysis was used to analyze the effect of multiple factors (age, gender, disease activity, anti-nuclear antibody positivity, presence of uveitis) on continuous dependent variables (activities and activity/ concentration ratios of IgM and IgA NAbs).

RESULTS

The ratios of IgA anti-TNP, anti-actin and anti-F(ab΄)₂ levels to total serum IgA concentration were found to be significantly increased in patients with oligo-JIA compared to healthy subjects. Significantly elevated levels of IgM anti-TNP antibodies were also found in children with inactive oligo-JIA compared to those of children with active disease and of healthy controls. In the presence of anterior uveitis, IgM anti-TNP levels were significantly higher than in patients without uveitis or in healthy controls. Backward regression analysis revealed that the disease activity and the presence of anterior uveitis independently affect IgM anti-TNP levels.

CONCLUSUIONS

Our findings are in accordance with the hypothesis that NAbs contribute to the pathogenesis of autoimmune diseases and provide additional evidence that disturbances in natural autoimmunity may contribute to the as yet unclarified pathogenesis of oligo-JIA.

Fine binding characteristics of human autoantibodies—partial molecular characterization

The fine binding characteristics of three well-characterized human autoantibodies B3, RH14 (anti-DNA) and UK4 (anti-cardiolipin) in their IgG and cloned Fab formats, were investigated. Although in severe combined immunodeficiency (SCID) mice B3 and RH14 both induce proteinuria, only RH14 induces early features of lupus nephritis, whereas UK4 exhibits lupus anticoagulant activity. RH14 exhibited up to 10 fold higher binding to DNA compared to that shown by B3 or UK4 and involved significant electrostatic and phosphate group interactions. Only RH14 exhibited strong anti-Sm cross-reactivity residing on the C-terminus of the antigen as determined by the use of 76 overlapping 15mer peptides. Chain shuffling experiments indicate that anti-Sm/RNP and anti-Jo-1 activities of B3 and UK4 co-exist on one of the two chains (light, B3; heavy, UK4). The present study provides evidence that a human anti-DNA antibody can also be an anti-ENA antibody. Furthermore, the anti-DNA antibodies also exhibited cross-reactivity against glutathione-S-transferase and DNA polymerase PolIV of bacterial origin. This is the first demonstration of the presence of such cross-reactivities on lupus anti-DNA antibodies. We now demonstrate that subsets of sera from the patients with lupus, recognise these antigens. This observation may in some cases provide a mechanism for the common expression of a variety of autoantibodies observed in systemic lupus erythematosus (SLE).

Characterization and V(H) sequences of human monoclonal anti-F(ab')(2) autoantibodies from normals and Sjögren's syndrome patients

To investigate the genetic background of anti-F(ab')(2) autoantibodies and the mechanism behind their production we have analyzed 10 human monoclonal antibodies directed against IgG F(ab')(2) and IgG Fab. They were all derived from peripheral blood by the EBV/hybridoma technique. Eight were from three healthy individuals and two from two patients with primary Sjögren's syndrome (pSS). They react with epitopes on distinct regions of IgG, including epitopes present on or near the hinge of IgG, epitopes on the Fd gamma, and an antigenic determinant(s) present on lambda light chains. These determinants are either exposed on the intact IgG molecule or revealed following pepsin or papain digestion. The V(H) germline gene repertoire used is diverse and with considerable overlap with that used by rheumatoid factors (RF). The two IgG antibodies from normals are extensively mutated (13 and 24 mutations/V(H)), but with a replacement to silent mutation ratio in the CDR(H)1 + 2 of only 3.7. The IgM antibodies from normals are also heavily mutated (mean 10 mutations/V(H)). This suggests that anti-F(ab')(2) from normals are generated by an antigen-driven somatic hypermutation mechanism. In contrast, the two IgM antibodies from pSS are virtually unmutated in both V(H) and V(L). Together with published data of pSS RF and anti-Ro 52-kDa sequences (1-3), this suggests that there is an expanded population of naïve B cells with autoantibody specificities in the peripheral blood of pSS patients.

Urinary loss of immunoglobulin G anti-F(ab)2 and anti-DNA antibody in systemic lupus erythematosus nephritis

The objective of this study was to determine whether the low levels of serum immunoglobulin G (IgG) anti-F(ab)2 seen in some patients with active systemic lupus erythematosus (SLE) were directly related to the deposition of antibody with this specificity in the kidney or alternatively to the urinary loss of IgG anti-F(ab)2. Serum Levels of IgG anti-F(ab)2, anti-tetanus toxoid, and anti-ds DNA antibody were measured in parallel with urinary excretion of these same 3 antibodies in 28 patients with SLE nephritis and in 28 control patients with other forms of chronic kidney disease. Low levels of both serum IgG anti-F(ab)2 or anti-tetanus antibody appeared to correlate with increased levels of urinary loss of these same antibodies in some patients with SLE and in control subjects with kidney disease. However, urinary loss could not account for low serum levels of either IgG antibody in many subjects. Quantitative 24-hour urinary losses of IgG anti-F(ab)2 and anti-DNA were much higher in patients with SLE than in control subjects with kidney disease (P < .05), whereas amounts of IgG urinary loss of anti-tetanus were similar in patients with SLE and in control subjects. In nearly 1 third of SLE nephritis patients, 13% to 53% of total excreted urinary IgG showed anti-DNA enzyme-linked-immunosorbent assay reactivity. Urinary IgG in many patients with SLE showed both anti-DNA and anti-F(ab)2 reactivity, but dual anti-DNA/F(ab)2 specificity was more pronounced in affinity-isolated serum IgG anti-DNA or anti-F(ab)2 than in excreted urinary IgG molecules. The affinity of urinary IgG for either DNA or F(ab)2 was much lower than the same antibody activities measured either in serum or in kidney biopsy eluates. When the relative affinity of anti-DNA antibody in serum, urine, and kidney biopsy eluate was measured in parallel, the highest affinity antibody was found in kidney biopsy eluates, followed by serum antibody with urine antibody affinity showing the lowest values. These findings suggest a relative concentration of the highest affinity, doubly reactive IgG anti-DNA/F(ab)2 in SLE kidney tissues during SLE nephritis and implicate this process as an important factor in ongoing tissue damage.