Anti-DNA antibodies and lupus nephritis: the complexity of crossreactivity

Future Perspectives on Pathogenesis of Lupus Nephritis: Facts, Problems, and Potential Causal Therapy Modalities

Divergent incommensurable models have been developed to explain the pathogenesis of lupus nephritis. Most contemporary models favor a central role for anti-chromatin antibodies. How they exert their pathogenic effect has, however, endorsed conflicts that at least for now preclude insight into definitive pathogenic pathways. The following paradigms are contemporarily in conflict with each other: i) the impact of anti-double-stranded DNA (dsDNA) antibodies that cross-react with inherent renal antigens, ii) the impact of anti-dsDNA antibodies targeting exposed chromatin in glomeruli, and iii) the impact of relative antibody avidity for dsDNA, chromatin fragments, or cross-reacting antigens. Aside from these three themes, the pathogenic role of T cells in lupus nephritis is not clear. These different models should be tested through a collaboration between scientists belonging to the different paradigms. If it turns out that there are different pathogenic pathways in lupus nephritis, the emerging pathogenic mechanism(s) may be encountered with new individual causal therapy modalities. Today, therapy is still unspecific and far from interfering with the cause(s) of the disorder. This review attempts to describe what we know about processes that may cause lupus nephritis and how such basic processes may be affected if we can specifically interrupt them. Secondary inflammatory mechanisms, cytokine signatures, activation of complement, and other contributors to inflammation will not be discussed herein; rather, the events that trigger these factors will be discussed.

Detection of Antibodies to dsDNA: An Overview of Laboratory Assays

This brief review is focused on different methodologies available for detection of anti-dsDNA antibodies with respect to the best adequacy between biological results of laboratory and clinical significance. A large array of assays has been developed for the measurement of anti-dsDNA. New assays continually introduced have reflected not only technical innovations to avoid difficulties of some assays, but even more with hope to correlate better with systemic lupus erythematosus (SLE), particularly with its clinical course and exacerbations. Finally, with the development of micro arrays technology new insights into the pathophysiology of autoimmune disease processes should be revealed.

Predominant role for activation-induced cytidine deaminase in generating IgG anti-nucleosomal antibodies of murine SLE

Serum IgG anti-nuclear antibodies (ANA) directed to complexes of DNA and histones are a hallmark of systemic lupus erythematosus (SLE) and reflect a failure in lymphocyte self-tolerance. A prior study utilizing spontaneously autoimmune B6.Nba2 mice deficient in terminal deoxynucleotidyl transferase (TdT) and with heterozygous deficiencies in Jh and Igk loci underscored the importance of somatic hypermutation (SHM) as a major generator of SLE-associated ANA. This interpretation had to be qualified because of severely limited opportunities for receptor editing and restricted VHCDR3 diversity. Therefore, we performed the converse study using mice that carried functional Tdt genes and wild type Jh and Igk loci but that could not undergo SHM. Analyses of ANA and ANA-producing hybridomas from B6.Nba2 Aicda(-/-) mice revealed that few animals produced high titers of the prototypical ANA directed to complexes of histones and DNA, that this response was delayed and that those cells that did produce such antibody exhibited limited clonal expansion, unusual Jk use and only infrequent dual receptor expression. This, together with the additional finding of an intrinsic propensity for SHM to generate Arg codons selectively in CDRs, reinforce the view that most IgG autoimmune clones producing prototypical anti-nucleosome antibodies in wild type mice are created by SHM.

Antinuclear antibodies with nucleosome-restricted specificity for targeted delivery of chemotherapeutic agents

Circulating antinuclear autoantibodies (ANAs) are well known to accompany various pathological conditions and can be artificially induced by immunization. Research and clinical data permit us to hypothesize a definite connection between cancer and ANAs. Based on the available data, my group's research suggested that exogenous ANAs may be used as anticancer therapeutics. Among these ANAs, nucleosome-specific ANAs may be particularly useful. Advances in cancer immunotherapy with monoclonal antibodies re-emphasized the role of humoral immunity in neoplasia control. The development of a universal antibody targeting diverse cancers is of clear importance. We showed that certain natural ANAs recognize the surface of numerous tumor cells but not normal cells via cell surface-bound nucleosomes originating from the apoptotically dying neighboring tumor cells, mediate antibody-dependent cellular cytotoxicity of tumor cells in vitro and inhibit the development of murine tumor in syngeneic mice. A single monoclonal antinuclear nucleosome-specific autoantibody, mAb 2C5, specifically recognizes multiple unrelated human tumor cell lines and accumulates at a high tumor-to-normal cell ratio in various human tumors in nude mice. Immunotherapy with mAb 2C5 resulted in significant suppression of the growth of several human tumors. In addition, mAb 2C5, when used in subtherapeutic quantities, can serve as a highly efficient specific ligand to target various drug- or diagnostic agent-loaded pharmaceutical nanocarriers, such as liposomes and polymeric micelles, to various tumors. Here, the data (accumulated predominantly in our laboratory over several years) on mAb 2C5-mediated tumor targeting of chemotherapeutic agents is reviewed.

Serum and monoclonal immunoglobulin E antibodies from NC/Nga mice with severe atopic-like dermatitis recognize an auto-antigen, histone H3

BACKGROUND

NC/Nga mice are known to show a spontaneous outbreak of atopic-like dermatitis accompanied by a marked elevation in serum IgE levels when reared in a conventional environment. The specific effects of such a strong serum IgE response on the development of the dermatitis and specific antigens recognized by the IgE antibodies are still uncertain.

OBJECTIVE AND METHODS

To characterize the IgE of NC/Nga mice, we established IgE-secreting hybridoma clones from spleen cells of NC/Nga mice spontaneously developing dermatitis and identified variable-region genes and specific antigens of the IgE monoclonal antibodies (mAbs). Serum polyclonal IgE, as well as IgG1 and IgG2a, specific for the identified antigen were also analysed.

RESULTS

Four IgE-producing hybridoma clones were established. Variable-region nucleotide sequences of the IgE mAbs showed that these clones did not necessarily share common germline gene segments (V, D or J) for each variable region, and several somatic mutations had occurred in the V gene segments. Through antigen screening, histone H3 was identified to be an auto-antigen recognized by three of the four IgE mAbs. Serum IgE as well as IgG1 specific for histone H3 were almost undetectable in 6-week-old mice, but rapidly increased by 10-12 weeks of age. This age-dependent increase in the serum anti-histone H3 IgE was roughly in parallel with the onset of dermatitis, and slightly preceding total IgE elevation. The serum-specific IgE level correlated well with a dermatitis-severity score of each mouse at 12-16 weeks of age, and weakly with the severity of ear erosion of each mouse over 28 weeks of age. Furthermore, immunologically detectable histone-H3 antigens were observed in skin tissue sections from the dermatitis sites.

CONCLUSION

In NC/Nga mice, anti-histone H3 auto-antibodies may contribute, at least in part, to the considerably elevated serum IgE and might play some roles in the development and exacerbation of dermatitis.

Abetimus sodium: a new therapy for delaying the time to, and reducing the incidence of, renal flare and/or major systemic lupus erythematosus flares in patients with systemic lupus erythematosus who have a history of renal disease

The safety and efficacy of abetimus sodium (abetimus) has been evaluated in 13 controlled and uncontrolled clinical trials involving > 800 patients or subjects. The two pivotal trials enrolled a total of 547 patients with systemic lupus erythematosus (SLE) who had a history of lupus nephritis. Evidence of clinical effectiveness of abetimus comes from analyses of data from patients with SLE and high-affinity antibodies to the DNA epitope of abetimus at baseline; a retrospective subgroup in the pivotal Phase II/III LJP-394-90-05 trial (90-05 trial) and the intent-to-treat population in the Phase III LJP-394-90-09 trial (90-09 trial). These studies enrolled SLE patients who had experienced prior renal manifestations of their disease and had elevated anti-dsDNA antibodies at baseline by Farr assay. Both were long-term studies, with a mean duration of treatment participation of 371 days in 90-05 and 310 days in 90-09 for the population of patients with high-affinity antibodies at baseline. The 90-05 and 90-09 studies, as well as all other clinical studies of abetimus, consistently showed that treatment with abetimus resulted in durable and persistent reductions in anti-dsDNA antibodies in SLE patients. Treatment with abetimus was associated with statistically significant decreases in anti-dsDNA antibody levels from baseline compared with placebo in both the 90-09 and 90-05 trials. Positive trends were noted for the incidence of renal flares and major SLE flares in patients treated with abetimus. Abetimus appeared to be well tolerated for treatment periods of < or = 22 months.

Relationship between anti-double-stranded DNA antibodies and exacerbation of renal disease in patients with systemic lupus erythematosus

OBJECTIVE

To examine the relationship between changes in anti-double-stranded DNA (anti-dsDNA) antibody levels and the risk of renal flare in patients with systemic lupus erythematosus (SLE), using data from 2 randomized, controlled trials.

METHODS

Analyses were based on 487 patients with SLE and a history of lupus nephritis who had an anti-dsDNA antibody titer >/=15 IU/ml at baseline, as measured by Farr assay. Results are presented for the combined population of patients, the placebo arms, and the drug treatment arms in which a dsDNA-based bioconjugate (abetimus sodium; LJP 394) was used.

RESULTS

Changes in anti-dsDNA antibody levels were inversely correlated with changes in the C3 level (P < 0.0001 in both trials). Cox proportional hazards regression models showed that changes in anti-dsDNA antibody levels correlated with the risk of renal flare. The models predicted that a point estimate of a 50% reduction in anti-dsDNA antibody levels is associated with a 52% reduction (95% confidence interval [95% CI] 26-68%, nominal P = 0.0007) and a 53% reduction (95% CI 33-69%, nominal P < 0.0001) in the risk of renal flare in the 2 trials, respectively. In the 2 trials, the incidence of renal flare was lower in patients with sustained reductions in anti-dsDNA antibodies (3.0% and 4.1%, respectively) than in patients with stable or increasing antibody levels (21.3% and 20.3%, respectively).

CONCLUSION

Changes in anti-dsDNA antibody levels were directly correlated with the risk of renal flare and inversely correlated with changes in the C3 level. Reducing anti-dsDNA antibody levels may represent a therapeutic objective in SLE patients with lupus nephritis, because it is associated with a reduced risk of renal flare.

Anti-cardiolipin/beta-2 glycoprotein activities co-exist on human anti-DNA antibody light chains

We have recently shown that the human anti-DNA antibodies B3 and 33H11 also bind cardiolipin and that the anti-autoantigen activity resides predominantly on their lambda light chains. We now show that the two auto-antibodies possess strong reactivity to the plasma-protein 2-Glycoprotein I (beta2-GPI) also. Utilizing chain shuffling experiments involving an unrelated anti-p185 antibody 4D5 with insignificant reactivity to cardiolipin or to beta2-GPI, we now demonstrate that hybrid Fabs with constituent light chain, but not the heavy chain, of B3 or 33H11, exhibit anti-cardiolipin activity. Furthermore, the constructs possessing the auto-antibody-derived light chain also exhibited significant reactivity to beta2-GPI. The results suggest that anti-DNA, anti-cardiolipin and anti-beta2-GPI activities co-exist on the light chains of the antibodies studied and, importantly, these activities could be transferred to antibody constructs by their light chains alone. Computer-generated models of the three-dimensional structures of the auto-antibodies and their hybrids, suggest predominant interaction of their light chains with domain IV of beta2-GPI.

Chromatin Specificity of Anti-Double-Stranded DNA Antibodies and a Role for Arg Residues in the Third Complementarity-Determining Region of the Heavy Chain

A spontaneous, autoreactive autoantibody called SN5-18 (IgG2b, kappa) binds to a complex of H2A/H2B/dsDNA in chromatin, but erroneously appears to bind dsDNA when the Ab is used in a form that is not highly purified. Because of this finding, we evaluated the antigenic specificity of a prototypic anti-dsDNA Ab, 3H9/Vkappa4, now used widely in transgenic studies of tolerance and autoimmunity. We found that the purified mAb 3H9/Vkappa4 binds chromatin and specifically a complex of H2A/H2B/dsDNA, but not dsDNA in solid phase or in solution. When used in the form of culture supernatant or as a standard protein G preparation, mAb 3H9/Vkappa4 appears to bind dsDNA, apparently due to nuclear proteins in the preparation that assemble on target DNA. Because of the reported role of V(H)CDR3 Arg residues in dsDNA binding and the near identity of the SN5-18 sequence to other dsDNA-specific Ab, we tested the contributions of two V(H)CDR3 Arg residues in SN5-18 to chromatin specificity. We found that both these Arg residues at positions 104 and 106 were required for detectable chromatin binding. These results indicate a role for V(H)CDR3 Arg residues in chromatin specificity of lupus-derived autoantibodies. Further, they provide an explanation for a possible discrepancy in the form of tolerance observed in different anti-DNA Ig transgene models.

Use of a novel elution regimen reveals the dominance of polyreactive antinuclear autoantibodies in lupus kidneys

OBJECTIVE

The autoantibody specificities that dominate the deposits in lupus kidneys remain unclear. Reasoning that previously utilized elution buffers such as acidic glycine and ammonium thiocyanate may not have been maximally effective in eluting all Ig deposits from the kidneys, this study was conducted to experiment with a stronger dissociating agent, urea-glycine.

METHODS

Seven antinuclear antibody-positive, nephritic female (SWR x NZB)F(1) (SNF1) lupus mice were selected for the elution study. Deposited Ig was eluted from their kidneys using 3 different elution buffers: 0.15M glycine-HCl buffer, 1.3M ammonia thiocyanate/0.15M glycine-HCl buffer, and 5M urea/0.15M glycine-HCl buffer. All eluates were tested for specificity against a variety of nuclear and glomerular antigens.

RESULTS

Compared with conventional elution buffers, the urea-based regimen eluted severalfold more IgG and IgM antinuclear antibodies from the kidneys of nephritic SNF1 lupus mice. IgG anti-double-stranded DNA (anti-dsDNA) antibodies were not only the most prevalent species in these renal deposits, they were also heavily enriched in the kidneys, relative to the corresponding serum levels. A substantial fraction of the anti-single-stranded DNA and antihistone/DNA (but not antihistone) reactivity in these eluates was due to cross-reactive anti-dsDNA antibodies. No reactivity with SSA, SSB, Sm, RNP, Jo-1, Scl-70, or ribosomal P antigens could be demonstrated in these eluates. Importantly, the urea-glycine eluates differed from the conventional eluates in having significantly greater reactivity to glomerular substrate and laminin.

CONCLUSION

This novel urea-based elution provides further support for the dominance of antibodies in lupus kidneys, with strong polyreactivity to DNA and glomerular substrate.

Molecular expression systems for anti-DNA antibodies--2

Antibodies to double-stranded DNA are the best-known serological markers of systemic lupus erythematosus, and are closely associated with its renal pathogenesis. How these antibodies recognize DNA is not fully understood. An understanding of the relationship between the functional attributes of an antibody with the three-dimensional structure of its antigen-combining site would allow an insight into the rules that dictate auto-antibody-nucleic acid interaction and consequent pathogenicity of the autoantibody. Data from such studies could assist the development of novel drugs as an approach to specific therapies that can inhibit or disrupt protein-nucleic acid interactions. A full understanding of the binding specificities can be achieved only by experimental determination of detailed three-dimensional structure of these antibodies alone, and of their complexes with specific DNA antigens. A prerequisite of such a study is the ability to produce multimilligram quantities of the antibody protein. However, these antibodies are particularly difficult to express, probably due to their DNA-binding activity. This review attempts to focus on the recent developments on the over-expression of anti-DNA antibody fragments in heterologous cell expression systems and their purification to homogeneity that would in turn allow their structural studies via crystallization.

RP105-lacking B cells from lupus patients are responsible for the production of immunoglobulins and autoantibodies

OBJECTIVE

We previously reported that B cells lacking the RP105 molecule, which proved to be highly activated B cells, are increased in the peripheral blood of patients with systemic lupus erythematosus (SLE). In the present study, we attempted to determine whether RP105-negative B cells obtained from SLE patients would be capable of producing autoantibodies as well as immunoglobulins.

METHODS

RP105-positive and RP105-negative B cells, sorted by cell sorter, were cultured for 5 days without stimulation, or were stimulated with Staphylococcus aureus Cowan 1 strain (SAC) or recombinant interleukin-6 (IL-6). For the assay of autoantibodies, RP105-positive and RP105-negative B cells were cultured separately for 10 days with anti-CD3 antibody-stimulated T cells. The production of immunoglobulins and autoantibodies was determined by enzyme-linked immunosorbent assay.

RESULTS

We demonstrated that RP105-negative B cells, but not RP105-positive B cells, obtained from SLE patients could spontaneously produce IgG and IgM in vitro until day 5. SAC and IL-6 enhanced production of IgG and IgM by RP105-negative B cells but failed to induce such production by RP105-positive B cells. The latter cells, however, when cocultured with activated T cells in the presence of IL-10, produced IgG, although the amount was very small compared with that produced by RP105-negative B cells. Most important, under these conditions, anti-double-stranded DNA antibodies were produced only by the RP105-negative B cells obtained from SLE patients.

CONCLUSION

These data indicate that RP105-negative B cells, constituting a subset of B cells in SLE patients, are highly activated and may be responsible for the production of autoantibodies as well as polyclonal immunoglobulins.

Cluster analysis of antinuclear autoantibodies in the prognosis of SLE nephropathy: are anti-extractable nuclear antibodies protective?

To investigate the possible role of anti-ENA autoantibodies in the pathogenesis of SLE nephropathy, we performed a cross sectional clustering study of 91 SLE patients using 75 clinical and laboratory variables examining the presence of anti-dsDNA and ENA autoantibodies by ELISA and Western blot. We applied principal component, hierarchical cluster, multiple correspondence and logistical regression analysis. Two polar forms of SLE nephropathy and five clinical groups were identified: group 1 without overt nephropathy (n = 37), group 2 with nephropathy and only proteinuria (n = 19), group 3 nephropathy and only hematuria (n = 11), group 4 with hematuria and proteinuria (n = 14) and group 5 on renal failure (n = 10). When analyzed individually, levels of anti-dsDNA and single anti-ENA antibodies did not allow us to differentiate between renal and non-renal groups. However, when the anti-ENA autoantibodies were analyzed as a cluster, a high predictive value for clinical nephropathy was obtained. Thus, the absence of ENA antibodies (ENA ve or Venezuelan cluster) increased eleven-fold the odds ratio to develop SLE nephropathy. We suggested that the ENA ve cluster may predict development of the most severe forms of renal lupus while the ENA Sm/RNP and the ENA Ro/La/Sm/RNP clusters could be associated with the absence and the most benign form of SLE nephropathy. It must be interesting to apply similar cluster methodology in an SLE population with different ethnic background.

Molecular cloning and expression of the Fabs of human autoantibodies in Escherichia coli. Determination of the heavy or light chain contribution to the anti-DNA/-cardiolipin activity of the Fab

The Fabs of three human autoantibodies (B3/33H11, anti-DNA; UK4, anti-phospholipid) and six related hybrids have been cloned, expressed in Escherichia coli, and purified to homogeneity. SDS-polyacrylamide gel electrophoresis and Western blot analysis of the recombinant Fab demonstrated the purified Fab to be of correct size and in assembled form. Protein expression levels of up to 5-9 mg per liter of culture were achievable. A sensitive and reliable comparative anti-DNA enzyme-linked immunosorbent assay, involving a defined biotinylated 35-mer oligonucleotide in its single- or double-stranded form, is also described. Crithidia assay and anti-DNA or anti-cardiolipin antibody enzyme-linked immunosorbent assay analyses demonstrated convincing binding of the recombinant Fab proteins to DNA/cardiolipin, confirming the expression of functional molecule. The comparative DNA/cardiolipin binding analyses of the nine Fabs revealed that the anti-DNA (light, B3/33H11) or anti-cardiolipin (heavy, UK4) activity lies predominantly on one of the two chains. However, a compatible partner chain is necessary for optimum antigen binding activity of the antibody.

Interaction of dual-specific autoantibodies with dsDNA and a synthetic dimer peptide simulating the hinge region of IgG2a molecules

Anti-dsDNA autoantibodies and immune complex formation are major factors in SLE pathogenesis. Understanding stable immune complex formation is critical in deciphering mechanisms of autoimmune pathogenesis. Previous studies identified a subpopulation of murine lupus monoclonal autoantibodies that exhibited dual specificity (anti-DNA and anti-IgG2a hinge) and formed stable immune complexes [J. Mol. Rec. 10(1997)225]. Two monoclonal autoantibodies, BV 17-45 and BV 16-13, were extensively studied because of their dual specificity. To quantitatively assess the role of each specificity in the formation of stable immune complexes, studies were performed to determine binding affinities for various sized dsDNA fragments (21, 43, 84, and 114 bp) and the covalent dimer of a nine amino acid hinge peptide. Results characterizing BV 17-45 showed that the affinity for dsDNA directly correlated with increased dsDNA size. Results with BV 16-13 revealed a generally lower affinity for the various dsDNA fragments. Binding inhibition studies, using a covalently linked dimer of a nine amino acid synthetic hinge peptide as an inhibitor of the antibody-43 bp dsDNA interaction, yielded relative affinities for the anti-hinge activity. Binding affinities for the synthetic hinge specificity were lower than affinities measured for the anti-dsDNA activity. Collectively, the binding and inhibition studies provided insight into the correlation between dual specificity and avid immune complex formation. A model was proposed based on the concept that large dsDNA fragments caused localization of the dual-specific antibodies through the anti-dsDNA activity, thereby facilitating subsequent binding and cross-linkage via the anti-hinge specificity. These synergistic interactions resulted in the formation of avid immune complexes.

Histone-containing immune complexes are to a large extent responsible for anti-dsDNA reactivity in the Farr assay of active SLE patients

Increased titres of anti-dsDNA antibodies, especially if of high avidity, are associated with renal exacerbations in patients with systemic lupus erythematosus (SLE). One of the most reliable assays to measure anti-dsDNA antibodies, the Farr assay, is believed to detect preferentially high avidity antibodies. Purified non-complexed monoclonal antibodies (mAbs) against nucleosomes, obtained from mice with SLE, are not reactive in the Farr assay, but can become so once complexed to nucleosomes. These Farr-positive, nucleosome containing, immune complexes were also able to bind in vivo to the glomerular basement membrane (GBM), predominantly via heparan sulphate (HS). To evaluate whether in SLE patients the same kind of immune complexes are responsible for Farr reactivity, IgG from serum or plasma was isolated under dissociating and physiological conditions. We observed that after purification under dissociating conditions, Farr reactivity was significantly decreased (P<0.0001) in contrast to reactivity with histones and two 'control' antigens: Epstein Barr Virus (EBV) and Ro/SS-A. Reactivity with nucleosomes also decreased after purification, although to a lesser extent. Plasma purified under physiological conditions showed no decrease in Farr reactivity. The importance of histones for the generation of immune complexes is supported by the two following observations. Firstly, the presence of histones could be demonstrated in serum and plasma of SLE patients but not in serum of healthy controls or in IgG preparations purified under dissociating conditions. Secondly, Farr reactivity of purified IgG preparations could be restored by addition of purified histones. From these studies we conclude that histones containing immune complexes are responsible for a large part of the Farr reactivity in active SLE, and are therefore indirectly implicated in the pathogenesis of lupus nephritis.

Undifferentiated connective tissue disease: natural history and evolution into definite CTD assessed in 84 patients initially diagnosed as early UCTD

Connective tissue diseases (CTDs) are chronic multisystemic inflammatory disorders whose indicative signs or symptoms have a high sensitivity but poor specificity in predicting the evolution into a given CTD. We have analysed 84 consecutive patients initially diagnosed as having an early undifferentiated CTD (early UCTD) with the aim of verifying the evolution into one definite CTD and of evaluating the predictive value of clinical and laboratory parameters. During a 5-year study period, 33 patients developed signs of a full-blown CTD; the highest probability of evolution was in the first 48 months after the onset. Multivariate analysis allowed us to select those variables correlating with evolution into a particular CTD, such as sclerodactyly and oesophageal dysfunction for systemic sclerosis, xerostomia and anti-nuclear antibodies (SS-A pattern) for Sjögren's syndrome, and fever and anti-DNA antibodies for systemic lupus erythematosus. Furthermore, we assessed the prevalence of various clinical and laboratory manifestations, complications and prognosis of those patients diagnosed after a 5-year disease duration period as having a UCTD. In our series, major organs such as the kidney or heart seem to be spared, whereas we detected a relatively high prevalence of endocrine disease of autoimmune origin.

Down-regulation of surface antigens recognized by systemic lupus erythematosus antibodies on embryonal cells following differentiation and exposure to corticosteroids

We have previously suggested that anti-DNA antibodies present in systemic lupus erythematosus patients can bind directly to tissues as a result of cross-reactivity with embryonal tissue-based antigens. Here we have analyzed the interaction between polyclonal and monoclonal mouse and human lupus autoantibodies and an embryonal cell line. We report that a murine embryonal stem cell line (ES) expresses a surface antigen which is recognized by mouse and human lupus autoantibodies. This surface antigen is down-regulated following maturation of the cells or incubation with corticosteroids. Adhesion molecules may serve as the target membrane antigen in ES cells since preincubation with these antibodies decreases the ability of ES cells to adhere to the plate.

Isolation and characterization of apoptotic nucleosomes, free and complexed with lupus autoantibody generated during hybridoma B-cell apoptosis

Increasing evidence suggests that immune complexes made of anti-nuclear antibodies bound to nucleosomes released from dead cells play an important role in the pathogenesis of lupus nephritis. However, the nature and composition of apoptotic nucleosomes still remain elusive. Since large amounts of nucleosomes are released from cells undergoing apoptosis in hybridoma cell cultures, we used hybridomas secreting anti-DNA and anti-nucleosome antibodies grown in protein-free medium to generate nucleosome/anti-DNA and /anti-nucleosome immune complexes, as well as an irrelevant antibody hybridoma to generate free, non-complexed apoptotic nucleosomes. Hybridoma supernatants were fractionated by size-exclusion gel chromatography and eluted fractions with a ratio of A260/A280 >1.2 were pooled and analysed for DNA and histone profiles by gel electrophoresis and immunoblotting. When run on a 'native' gel, 'intact' apoptotic nucleosomes, free or within anti-nucleosome immune complexes, showed a strikingly reduced size compared with 'standard' nucleosomes prepared in vitro by endonuclease digestion of cell nuclei. Nucleosomal DNA (extracted from either free or complexed apoptotic nucleosomes) appeared as a major band of 160-180 bp, and had the size of 'standard' mononucleosome DNA, suggesting degradation of the histone moiety of apoptotic nucleosomes. Histone immunoblotting revealed degradation of histones H3 and H4, which was dramatically enhanced when apoptotic nucleosomes were complexed with an anti-nucleosome antibody. Our results provide direct evidence for abnormal histone composition of apoptotic nucleosomes and suggest that the fine specificity of the complexing antibody has an influence on complexed nucleosome composition.

Induction of systemic lupus erythematosus-like disease in mice by immunization with heparan sulfate

Experimental systemic lupus erythematosus (SLE)-like disease was induced in BALB/c mice by immunization with heparan sulfate, the major glycosaminoglycan of glomerular basement membrane. Following booster injections with heparan sulfate (HS), high levels of anti-HS, anti-dsDNA, and anti-cardiolipin antibodies were detected in the sera of the immunized mice. An enzyme-linked immunospot (ELISPOT) assay indicted that IgG anti-HS and anti-dsDNA antibody-secreting cells were present in the kidneys and most likely contributed to antibody localization. Antibodies eluted from the kidneys of immunized mice were found to react strongly with HS and dsDNA when tested in vitro. The HS-immunized mice developed moderate to severe levels of proteinuria. Histologic examination of kidneys from HS-immunized mice revealed deposition of immunoglobulin in the kidneys. Our results describe the induction of SLE-like disease in normal mice following immunization with HS. This experimental model may be useful for understanding the immunologic basis for autoimmunity to HS.

Identification of endothelial cell membrane proteins that bind anti-DNA antibodies from patients with systemic lupus erythematosus by direct or indirect mechanisms

A subgroup of murine monoclonal anti-DNA antibodies bind to vascular endothelial cells either directly as a result of cross-reactivity, or indirectly through immunoglobulin-bound DNA and DNA-binding proteins on the endothelial cell membrane. To determine whether these mechanisms apply in human systemic lupus erythematosus (SLE), and to identify endothelial cell membrane protein(s) that bind human anti-DNA antibodies, we examined, by Western blotting, the binding of human polyclonal anti-DNA antibodies (PoAb) isolated from eight patients with SLE to human umbilical vein endothelial cell membrane proteins. PoAbs bind to endothelial membrane proteins with Mr 84,000 and 46,000, which correspond to the DNA-binding proteins previously reported. Such binding is diminished after removal of DNA by DNase treatment. In addition, PoAbs bind to membrane proteins with Mr 180, 000, 110,000, 68,000, 44,000, and 35,000-30,000. Such binding is unaffected by alterations in DNA concentration. Anti-dsDNA and anti-ssDNA PoAbs from individual patients exhibit identical binding patterns, as are PoAbs isolated during active disease or remission. The results show that human anti-DNA antibodies can bind to endothelial cells both indirectly via immunoglobulin-bound DNA, and directly due to cross-reactivity. These mechanisms of cellular binding by anti-DNA antibodies may depict patho-genetic steps in human SLE.

Production of anti-Ro/SS-A and anti-La/SS-B autoantibodies is closely coordinated in systemic lupus erythematosus and independent of anti-dsDNA production

Autoimmune diseases such as Sjögren's Syndrome and systemic lupus erythematosus (SLE) are characterized by serum autoantibodies against protein components of small cytoplasmic ribonucleoproteins (scRNPs). The origin and regulation of these anti-Ro/SS-A and anti-La/SS-B antibodies is not well understood. Regular co-occurrence of these two autoantibodies in humans together with murine studies on antibody responses against scRNPs after immunization suggest a role for scRNPs as common antigen. We sought additional evidence for this hypothesis in a longitudinal serological study of patients with SLE. Quantitative measurement of the antibody responses against Ro/SS-A and La/SS-B proteins and for comparison to dsDNA in 852 serum samples was performed. These samples were collected from nine patients during an average observation period of more than 10 years. A significant and strong correlation between the two anti-scRNP responses emerged during 90% of follow-up. In contrast, correlation of anti-scRNP with anti-dsDNA responses was remarkably absent in the same patients. Our results confirm the unique relationship between anti-Ro/SS-A and anti-La/ SS-B responses and could thus be interpreted as support for a model wherein induction and perpetuation of autoantibody production is dependent on scRNPs containing both proteins as antigen.

Histones interact with immunoglobulins and give them polyspecificity

The presence of antihistone antibodies was determined in different commercial immunoglobulin preparations. The ability of histones to perform the function of cofactor in interaction of immunoglobulin preparations and sera of healthy people with charged molecules such as DNA, cardiolipin and phosphatidylcholine, was determined. The addition of histones of immunoglobulin preparations or serum samples is accompanied by forming the stable complex, components of which interact with the charged molecules. The capacity of histones to play the role of cofactor is less evident in the case when they are preincubated with DNA or cardiolipin. The interaction of natural antibodies with histones can explain their polyspecificity in some cases.

Nephritogenic autoantibodies in lupus: current concepts and continuing controversies

In summary, we suggest that the following statements regarding lupus nephritis are best supported by the existing data. 1) Lupus nephritis is an immunologically complex disorder. Autoantibodies directed against multiple epitopes on chromatin, including but not limited to dsDNA, may contribute to nephritis. 2) The presence of charged residues within autoantibody heavy chain CDR regions, particularly CDR3, may be essential to the property of nephritogenicity. 3) Chromatin/antichromatin immune complexes (formed either in the circulation or in situ in the GBM) are likely the proximal cause of lupus nephritis. Cross-reactive autoantibodies or antibodies reacting directly to glomerular antigens are less likely to play a major pathogenic role. 4) The induction of lupus nephritis may relate to the propensity of chromatin or its components to bind to the GBM by virtue of the interactions of histones with type IV collagen and heparan-sulfated glycosaminoglycans. Nonetheless, as indicated above, there are numerous issues that remain to be addressed and clarified with respect to lupus nephritis. Insight into these issues is not only of theoretical interest, but may lead to new approaches to diagnostic testing and more specific therapies to replace currently use nonspecific immunosuppressive drugs, which have substantial toxicities.

Ligand recognition by anti-DNA autoantibodies. Affinity, specificity, and mode of binding

Understanding the molecular basis of DNA recognition by anti-DNA autoantibodies is a key element in defining the role of antibody.DNA complexes in the pathogenesis of the autoimmune disorder systemic lupus erythematosus. As part of our efforts to relate anti-DNA affinity and specificity to antibody structure, and ultimately to disease pathogenesis, we have generated a panel of eight anti-DNA mAbs from an autoimmune MRL MpJ-lpr/lpr mouse and have assessed the binding properties of these antibodies. We find that none of our anti-DNA mAbs bind to RNA and only one low-affinity mAb cross-reacts with non-DNA antigens, albeit weakly. None of the mAbs in our panel bind double-stranded DNA exclusively. Antibodies that recognize single-stranded DNA can be categorized into two groups based on their affinity and apparent mode of binding. One group possesses relatively high affinity for oligo(dT) and may recognize single-stranded DNA ligands by accommodating thymine bases in hydrophobic pockets on the antigen binding site. The second group binds more weakly, apparently recognizes single-stranded DNA nonspecifically, and in some cases also binds double-stranded DNA. Although different mechanisms are used for binding single- and double-stranded ligands, the mode of DNA recognition appears conserved within groups of antibodies.

Characterization of autoantibody activities in sera anti-DNA antibody and circulating immune complexes from 12 systemic lupus erythematosus patients

To examine autoantibodies present in patients with active systemic lupus erythematosus (SLE), sera, circulating immune complexes (CIC), and antibodies purified on DNA-immunoadsorbent were tested by enzyme immunoassay. A panel of self-antigens, including DNA, histones (HIS), glomerular basal membrane (GBM), thymus cell extract (TCE), actin (ACT), myosin (MS), and tubulin (TUB), was used to define their specificities. IgM antibodies against all antigens of the panel were detected in sera, CIC, and in antibodies eluted from the DNA-immunoadsorbent and demonstrated a large polyreactivity. IgG antibodies showed restricted activities against DNA, HIS, GBM, and TCE in sera and a large polyreactivity in CIC. Inhibition experiments were performed to assess their mono- or polyreactivities. Among the IgG autoantibody population recognizing DNA, two populations of IgG antibodies were detected in the sera and in the affinity purified anti-DNA: one recognizes DNA, HIS, and GBM, and the other binds to DNA and to cytoskeletal proteins. These autoantibody populations were found in CIC, which also often contained high amounts of IgG antibodies recognizing ACT and MS. A third population of IgG antibody that recognizes only TCE and could not be inhibited by DNA or other antigens was found in serum and CIC. Our data demonstrate the existence of several populations of autoantibody in serum and CIC of SLE patients: (1) IgM polyreactive autoantibodies, (2) IgG polyreactive autoantibodies recognizing DNA and cytoskeletal proteins, (3) IgG specific to DNA, which cross react with HIS and GBM, and (4) IgG specific to TCE antigens.

In vivo expression of a single viral DNA-binding protein generates systemic lupus erythematosus-related autoimmunity to double-stranded DNA and histones

Although the origin of autoimmune antibodies to double-stranded DNA is not known, the variable-region structures of such antibodies indicate that they are produced in response to antigen-selective stimulation. In accordance with this, results from experiments using artificial complexes of DNA and DNA-binding polypeptides for immunizations have indicated that DNA may induce these antibodies. Hence, the immunogenicity of DNA in vivo may depend upon other structures or processes that may render DNA immunogenic. We report that in vivo expression of a single DNA-binding protein, the polyoma virus T antigen, is sufficient to initiate production of anti-double-stranded DNA and anti-histone antibodies but not a panel of other autoantigens. Expression of a mutant, non-DNA-binding T antigen did result in strong production of antibodies to the T antigen, but only borderline levels of antibodies to DNA and no detectable antibodies to histones. Nonexpressing plasmid DNA containing the complete cDNA sequence for T antigen did not evoke such immune responses, indicating that DNA by itself is not immunogenic in vivo. The results represent a conceptual advance in understanding a potential molecular basis for initiation of autoimmunity in systemic lupus erythematosus.

Distributions of HLA class II alleles in autoantibody subsets among Norwegian patients with systemic lupus erythematosus

In order to find potential correlations between HLA class II alleles and anti-SS-A, -SS-B, -Sm and anti-snRNP responses among Norwegian patients with systemic lupus erythematosus (SLE), HLA-DRB1, -DRB3*0101, -DQA1 and -DQB1 alleles were determined by DNA typing 50 patients and 108 controls. HLA distributions were analysed in the following autoantibody subgroups: anti-SS-A with -SS-B, anti-SS-A without -SS-B, anti-snRNP without -Sm, anti-SS-A without -snRNP and anti-snRNP without -SS-A. The autoantibodies were detected by EIA (enzyme immunuassay). Patients with anti-SS-A and -SS-B had significantly increased frequencies of DRB1*03, DRB3*0101, DQA1*0501, DQB1*0201 (in linkage disequilibrium) versus controls and versus patients without anti-SS-A and -SS-B. No differences in HLA distribution were found when the group with anti-SS-A alone was compared to the group with anti-SS-A and concomitant -SS-B. Comparing the groups with and without anti-SS-A and -SS-B, the highest RR were found for the alleles DRB1*03, DRB3*0101, DQB1*0501, DQB1*0201 (in linkage disequilibrium) with RR: 16.8, 5.0, 19.6, 10.3, respectively, P < 0.05). RR for DQw2/DQw6 heterozygotes was 3.5 (Ns.), and RR for cases having DQ alpha molecules with glutamine in position 34 and DQ beta molecules with leucine in position 26 on both chains was 6.3 (P < 0.05). No HLA associations were observed in the group with anti-snRNP without concomitant -Sm or without concomitant -SS-A. These results show that production of anti-SS-A and -SS-B is associated to the HLA alleles DRB1*03, DRB3*0101, DQA1*0501, DQB1*0201, and that this haplotype shows stronger correlation to these responses than DQw2/DQw6 heterozygosity or HLA molecules having glutamine in position 34 (DQ alpha) and leucine in position 26 (DQ beta). The failure to observe any correlation with DRBI*15,16 (DR2) in the group with anti-SS-A alone may demonstrate ethnic differences concerning this response. The failure to identify any HLA associations for the anti-snRNP response may reflect the heterogeneity of the molecules that constitute this antigen.

No evidence for an independent role of anti-heparan sulphate reactivity apart from anti-DNA in lupus nephritis

The presence of anti-heparan sulphate (HS) reactivity in serum is closely related to the occurrence of nephritis in patients with systemic lupus erythematosus (SLE). Since patients with lupus nephritis in general also have high titres of anti-DNA antibodies, we wanted to clarify the relationship between anti-HS and anti-DNA reactivity in serum. Therefore, we studied longitudinally six patients with lupus nephritis who experienced 12 exacerbations of their disease, and five SLE patients without nephritis experiencing 10 periods of non-renal disease exacerbations. In addition, we tested single serum samples of another 24 patients obtained during a renal disease exacerbation and 22 sera of patients without nephritis. The sera of all patients were tested for anti-DNA (Farr assay) and anti-HS reactivity (ELISA). We confirmed that SLE patients during renal exacerbations have a significantly higher anti-HS reactivity than patients without nephritis (P < 0.003). In addition, patients with nephritis also had higher titres of anti-DNA antibodies during renal exacerbations than during non-renal exacerbations (P < 0.01). A correlation between anti-DNA and anti-HS reactivity was observed (r = 0.40, P < 0.02), which in itself explains the correlation between nephritis and anti-HS reactivity. Comparing sera from nephritis and non-nephritis patients matched for anti-DNA titre, we found no difference in anti-HS reactivity, and therefore must conclude that the anti-HS reactivity is a direct reflection of anti-DNA reactivity.

Serological markers of disease activity in systemic lupus erythematosus

When measured serially by Farr assay at a frequency of approximately once a month, changes in levels of anti-dsDNA appear to be a good predictor of clinical disease activity. Although the role of antibodies to the RNA component of snRNP awaits further studies, measurement of anti-UsnRNP antibody levels seems to be of limited value in monitoring lupus patients in clinical practice. The same holds for antibodies to SSA (Ro) and anti-histone antibodies. More recently described antibodies to C1q are probably useful in the follow-up of SLE patients suspected of proliferative renal involvement. The best alternative to measuring levels of the antibodies mentioned before is probably serial analysis of activation of the complement cascade. Levels of complement factors like C3, C4 and, functionally, CH50 remain a useful parameter for monitoring disease activity in SLE, although fluctuations in anti-dsDNA as measured by Farr assay seem superior with respect to sensitivity and specificity for an ensuing relapse. Despite the problems in sampling, measuring levels of activated split products of complement factors like C3a, C3d or C5a may prove to be a valuable tool in the follow-up of lupus patients. The involvement of the endothelial surface is illustrated by rising sVCAM-1 levels prior to relapses in SLE. Although one could expect that subsequent inflammation should be reflected by increased levels of inflammatory molecules like CRP and IL-6, the use of these molecules as predictors of lupus activity seems limited. Interferon-alpha as a direct reflector of the effector phase seems, however, rather promising in this respect and awaits longitudinal studies to analyse the possible relation with clinical disease activity and other serological parameters.

Autoantibodies to histone, DNA and nucleosome antigens in canine systemic lupus erythematosus

Dogs can develop systemic lupus erythematosus syndromes that are clinically similar to those seen in humans. In contrast, previous observations suggest differences in their autoantibody reactivity patterns against histones and DNA which are components of the nucleosome in chromatin. The objective of this study was to assess comprehensively the levels of autoantibodies against histone, DNA and nucleosome antigens in a population of lupus dogs. The specificities of antibodies in lupus and control dog sera were determined using IgM- and IgG-specific reagents in an ELISA against a variety of chromatin antigens. When compared with control sera, IgG antibodies to individual histones H1, H2A, H3 and H4 were significantly higher in the lupus group. In contrast, we did not detect IgG antibodies specific for H2B, H2A-H2B, DNA, H2A-H2B-DNA or nucleosome in lupus dogs. There was no significant increase in any of the IgM specificities tested. Therefore, the reactivity pattern to nucleosome antigens in canine lupus is restricted to IgG antibodies against individual histones H1, H2A, H3 and H4. This stands in contrast with human and murine lupus, where autoantibodies are directed against a wide variety of nucleosomal determinants, suggesting that unique mechanisms lead to the expansion of anti-histone antibody clones in canine lupus. The high incidence of glomerulonephritis in dog lupus suggests that anti-DNA antibodies are not required for the development of this complication, whereas IgG anti-histone antibodies may be relevant to its pathogenesis.

A new ELISA for the detection of anti-heparan sulfate reactivity, using photobiotinylated antigen

Autoantibodies reacting with a great variety of autoantigens are characteristic for the autoimmune disease systemic lupus erythematosus (SLE). Although reactivity with heparan sulfate (HS) in sera of patients with SLE is found in association with the occurrence of nephritis, the aetiological significance of this association is not clear. The assay which is generally used to measure anti-HS reactivity is subject to false-positive results, as a consequence of the binding of negatively charged moieties within immune complexes to the precoat employed (protamine sulfate). Therefore, we have developed a new ELISA in which photobiotinylated HS is efficiently and reproducibly bound to streptavidin-coated wells. We compared the new ELISA with the classical anti-HS ELISA by testing culture supernatants of 20 murine monoclonal antibodies (mAb) to DNA (containing free anti-DNA and anti-DNA/nucleosome immune complexes) and preparations of these mAb (containing only free anti-DNA), purified under dissociating conditions. In the classical anti-HS ELISA, 14 out of 20 of the culture supernatants reacted positively with HS; after purification no reactivity remained. The discrepancy must be due to anti-DNA/nucleosome immune complexes present in the culture supernatants. In the new ELISA only four out of 20 culture supernatants and one of the purified preparations reacted with HS. This latter reactivity is probably not specific, since this mAb also reacted with streptavidin alone. To find out whether there is a correlation between the occurrence of nephritis and anti-HS reactivity, measured in this new anti-HS ELISA, we tested sera of patients with a renal- or non-renal exacerbation of SLE in the newly developed anti-HS ELISA. We observed a correlation between anti-HS reactivity and nephritis.

Preferential binding of cross-reactive group A streptococcal anti-DNA monoclonal antibodies to decondensed human sperm DNA

Four murine anti-streptococcal monoclonal antibodies (mAbs) that cross-react with human DNA were evaluated by immunofluorescence flow cytometry for their reactivity with condensed and decondensed human sperm nuclei. All 4 anti-DNA mAbs reacted with condensed sperm nuclei. The reactivity of 3 of these mAbs with decondensed sperm nuclei was 13 to 177 times higher than that found with condensed nuclei. Under identical conditions, mAbs to cytoskeletal/cytocontractile proteins lacked reactivity with decondensed sperm nuclei. Binding of monoclonal anti-DNA antibodies to decondensed sperm nuclei was abolished by preincubation with double-stranded DNA. The preferential binding of anti-DNA antibodies to decondensed sperm DNA suggests the utility of decondensed sperm nuclei as the antigenic substrate for screening anti-DNA antibodies by flow cytometry.

Anti-nucleosome antibodies complexed to nucleosomal antigens show anti-DNA reactivity and bind to rat glomerular basement membrane in vivo

Histones can mediate the binding of DNA and anti-DNA to the glomerular basement membrane (GBM). In ELISA histone/DNA/anti-DNA complexes are able to bind to heparan sulfate (HS), an intrinsic constituent of the GBM. We questioned whether histone containing immune complexes are able to bind to the GBM, and if so, whether the ligand in the GBM is HS. Monoclonal antibodies (mAbs) complexed to nucleosomal antigens and noncomplexed mAbs were isolated from culture supernatants of four IgG anti-nuclear mAbs. All noncomplexed mAbs showed strong anti-nucleosome reactivity in ELISA. One of them showed in addition anti-DNA reactivity in noncomplexed form. The other three mAbs only showed anti-DNA reactivity when they were complexed to nucleosomal antigens. After renal perfusion a fine granular binding of complexed mAbs to the glomerular capillary wall and activation of complement was observed in immunofluorescence, whereas noncomplexed mAbs did not bind. Immuno-electron microscopy showed binding of complexes to the whole width of the GBM. When HS in the GBM was removed by renal heparinase perfusion the binding of complexed mAb decreased, but did not disappear completely. We conclude that anti-nucleosome mAbs, which do not bind DNA, become DNA reactive once complexed to nucleosomal antigens. These complexed mAbs can bind to the GBM. The binding ligand in the GBM is partly, but not solely, HS. Binding to the GBM of immune complexes containing nucleosomal material might be an important event in the pathogenesis of lupus nephritis.

Pathogenic anti-DNA idiotype-reactive IgG in intravenous immunoglobulin preparations

This study was addressed to explore the reactivity of natural anti-idiotypes from commercial lots of immunoglobulins to several idiotypes (Ids), usually expressed by anti-DNA molecules in lupus nephritis. Eleven intravenous immunoglobulin (IVIG) preparations and nine (three polyvalent and six hyper-immune) intramuscular IgG were investigated for specific content of anti-DNA, anti-F(ab')2 and antibodies reacting with several anti-DNA IgG Ids. Two samples (nos 6 and 11) showed high reactivity with allogeneic F(ab')2 and with F(ab')2 of myeloma proteins bearing the anti-DNA Id 3I+ and the 8.12+. Since both 3I and 8.12 Id markers are known to characterize pathogenic anti-DNA IgG in systemic lupus erythematosus (SLE), anti-Id antibodies to these markers were obtained by absorbing the IVIG samples nos 6 and 11 to Sepharose columns coupled with pooled F(ab')2 fragments of 3I(+)-F4(+)-8.12(+)-myeloma proteins. Inhibition experiments showed that anti-8.12 Id-eluted IgG induced a selective suppression of the DNA-reactive antibodies derived from patients with active lupus nephritis to their substrate, suggesting the involvement of 8.12+ molecules in the SLE glomerular damage. Since 8.12+ anti-DNA are nephritogenic antibodies, the occurrence of anti-8.12+ Id in commercial IVIG may be of potential therapeutic relevance in modulating the pathogenic SLE Id network. Previous variable results of IVIG treatment in SLE, such as resolution of proteinuria or worsening nephritis, could be related to variable enrichment of different lots of IVIG in suppressive anti-pathogenic Id antibodies.

The glomerular basement membrane in lupus nephritis

Systemic lupus erythematosus is a disease that is particularly suited for studies of glomerular basement membrane pathology. Classification of the renal pathology of lupus nephritis is usually based on light microscopic features, combined with immunofluorescence findings and electron microscopic alterations. Study of renal biopsy helps to distinguish potentially reversible and irreversible disease, and to estimate prognosis of patients with lupus nephritis. Moreover, studies of human disease, as well as the availability of animal models and in vitro cell culture systems employing biochemical and molecular biological studies of extracellular matrix, have led to a considerable increase in knowledge of the pathogenetic events underlying derangements of the glomerular basement membrane in lupus nephritis.

Inhibition of histone/anti-histone reactivity by histone-binding serum components; differential effect on anti-H1 versus anti-H2B antibodies

IgG fractions were purified on a protein G-agarose column from sera of both systemic lupus erythematosus (SLE) patients and healthy donors. All IgG fractions, after elution with 0.5 M acetic acid, reacted with histones in an anti-histone ELISA assay, and IgG anti-histone activity was in all instances higher in the IgG fraction than in the corresponding whole serum. This was shown to be due to the presence in serum of histone-binding components that inhibited IgG binding to histones. Both normal human and SLE patients' sera had these histone-binding components, and disparity between serum-positive and -negative anti-histone antibody (AHA) tests was not dependent on differences in the blocking capacity but on IgG antibody levels and avidity. Interaction of normal serum IgG fraction with all five histones was of low avidity, whereas interaction of IgG from AHA-positive SLE sera with both H1 and H2B had high avidity. Low-affinity antibodies to every histone fraction, but also high-affinity anti-H1 antibodies, were preferentially inhibited. Our data indicate that several serum protein components are inhibiting histone/anti-histone interaction and may play a protective role against both high-affinity anti-H1 antibodies present in SLE patients, and natural, low-affinity, anti-histone antibodies. As some acute phase proteins, notably C-reactive protein, bind to histones, it is conceivable that they play such a role. High-affinity anti-H2B antibodies, present in some SLE patients, and not inhibited by these serum components, may, on the other hand, participate in the pathogenesis of the disease.