Maternal Autoantibodies and Congenital Heart Block: No Evidence for the Existence of a Unique Heart Block-associated Anti-Ro/SS-A Autoantibody Profile

One of the rare examples of the transfer of autoimmune disease from mother to (unborn) child is the neonatal lupus syndrome. This syndrome comprises the development of fetal heart disease (congenital heart block) or neonatal skin rash and is specifically associated with maternal anti-Ro/SS-A autoantibodies. Previous studies have suggested that especially maternal autoantibody reactivity against the 52 kDa protein of the Ro/SS-A antigen and/or against the La/SS-B antigen is responsible for the development of congenital heart block (CHB).

To determine the CHB-associated antibody response in more detail, we analysed the presence of autoantibodies in sera from mothers of children with isolated heart block. All 14 mothers of children with congenital heart block were positive for anti-Ro/SS-A antibodies. Remarkably, their antibody profile, including recognition of different Ro/SS-A proteins and autoantibody levels against these proteins, did not differ from anti-Ro/SS-A positive mothers of healthy children. In contrast, all 8 anti-Ro/SS-A negative mothers had children with acquired heart block.

We conclude from our data that maternal anti-Ro/SS-A antibodies are essential for CHB but that fine analysis of this autoantibody response does not predict the occurrence of CHB.

Clinical laboratory assays for measuring anti-dsDNA antibodies. Where are we now?

It is over 40 years since anti-dsDNA antibodies were first described in patients with lupus. A wide variety of methods for detecting these immunoglobulins have been established. In this review, we consider the utility and practical problems that the main assays present. The clinical correlations between anti-dsDNA antibodies and clinical activity in patients with lupus is discussed.

A critical evaluation of enzyme immunoassays for detection of antinuclear autoantibodies of defined specificities. I. Precision, sensitivity, and specificity

OBJECTIVE

To determine the performance characteristics of enzyme-based immunoassay (EIA) kits for the detection of antinuclear and other autoantibodies of defined specificities.

METHODS

Nine manufacturers of EIA kits to detect antibodies of defined specificities participated in a study in which they received coded sera from the Centers for Disease Control and Prevention. These coded sera contained different dilutions of antibody of one specificity mixed with sera containing antibodies of other specificities. The manufacturers were asked to use their standard technology to determine antibody content and send the data to a committee of the International Union of Immunological Societies for analysis. The data were analyzed for sensitivity and specificity in the detection of anti-double-stranded DNA (anti-dsDNA), anti-single-stranded DNA, antihistone, anti-Sm, anti-U1 RNP, anti-SSA/Ro, anti-SSB/La, anti-Scl-70 (DNA topoisomerase I), anticentromere, and anti-Jo-1 antibodies. In addition, replicate samples were included in the coded sera to evaluate the precision of each EIA method.

RESULTS

Lack of sensitivity and specificity was most evident in the anti-dsDNA and anti-Sm kits, although 2 kits for anti-dsDNA achieved acceptable sensitivity and specificity. Generally, anti-SSA/Ro, anti-SSB/La, anti-Scl-70, anticentromere, and anti-Jo-1 kits performed well. Many false-positive results were obtained with a multiple myeloma serum containing cryoprecipitates, but multiple myeloma sera without cryoprecipitates presented no problem in the EIA system. Precision, based on evaluation of replicate samples, varied from very good to poor.

CONCLUSION

No single manufacturer was clearly superior to others in terms of their products' overall sensitivity, specificity, and precision. Areas that needed improvement were in kits for the detection of antibodies to dsDNA and to Sm antigen. Some EIA kits achieved good sensitivity and specificity. Individual manufacturers were informed of the performance of their respective kits so they could take measures to correct perceived deficiencies and thus improve the reliability of a group of important diagnostic assays used in the evaluation of systemic rheumatic diseases.

On the interaction between agalactosyl IgG and Fc gamma receptors

One of the serum abnormalities observed in autoimmune diseases such as rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE) is the occurrence of IgG that lacks the terminal galactose on asparagine-linked biantennary complex type oligosaccharides [Gal(0)-IgG] located in the CH2 domain. Additionally, IgG without glycosylation is known to be defective in several effector functions due to a reduced ability to bind to its specific receptors (Fc gamma R). It has thus been speculated that, by analogy with unglycosylated IgG, Gal(0)-IgG may also be functionally impaired or exert altered effector mechanisms. If this were true, Gal(0)-IgG could contribute to the phenotype of above-mentioned autoimmune diseases, like impaired immune complex clearance and defective down-regulation of activated B cells. Here, we show by three different methods that the interaction of Gal(0)-IgG and normally glycosylated IgG with the low-affinity Fc gamma RII (CD32) is indistinguishable with respect both to binding and receptor-mediated signalling. These data argue against a prominent role for Fc gamma R-dependent Gal(0)-IgG interactions in the etiology or pathogenesis of autoimmune diseases.

Functional analysis of synovial fluid and peripheral blood T cells from patients with rheumatoid arthritis

Rheumatoid arthritis is a T cell-mediated autoimmune disease. The lack of knowledge of the involved target antigens severely hampers research on relevant T cells in patients. Here we describe the functional analysis of freshly isolated T cells from the peripheral blood and the site of the lesion (synovial fluid or synovial membrane) of patients with rheumatoid arthritis. Healthy donors and osteoarthritis patients served as controls. Using various polyclonal stimuli, we analyzed CD4+ T cells with respect to proliferation and their ability to produce lymphokines. Our data show that lesion-derived CD4+ T cells of patients with rheumatoid arthritis are severely defective in proliferation and lymphokine (interleukin-2, interleukin-4, tumor necrosis factor-alpha, interferon-gamma) production. This activation defect was most pronounced at lower cell densities and was present in both synovial fluid derived and synovial membrane derived CD4+ T cells of all patients tested. No difference was found between responses of synovial fluid derived CD4+ T cells from osteoarthritis patients and those observed with peripheral blood derived T cells from all groups. The observed defect in lesion-derived CD4+ T cells from rheumatoid arthritis patients was not due to the effect of inflammatory factors in the synovial fluid because preincubation with synovial fluid could not induce a similar defect in control T cells. Together, our data show a rheumatoid arthritis specific, general defect in the activation of lesion-derived CD4+ T cells.

Detection of antibodies to DNA: a technical assessment

Antibodies to DNA can be found in the circulation of the majority of patients with Systemic Lupus Erythematosus (SLE). They are quite specific for this disease, which makes their detection an important diagnostic aid to the clinician. Fluctuations in the level of anti-dsDNA in an individual patient generally parallel the clinical state of that patient. Furthermore, the presence of anti-dsDNA may precede the diagnosis of SLE by more than a year. Four methods relevant for the measurement of anti-dsDNA antibodies are discussed in this paper: the ELISA, the indirect immunofluorescence test on Crithidia luciliae, the PEG assay, and the Farr assay. Each of these methods detects a part of the spectrum of anti-dsDNA antibodies present in the circulation of an individual patient. The ELISA is the most sensitive method, whereas the Farr assay is the most specific for SLE. However, with the latter method only antibodies of a relative high avidity for DNA are detected. Mild forms of SLE, where patients only have anti-dsDNA of a low avidity in their circulation, may easily be missed by this technique. Clinically, high avidity anti-dsDNA is related with the more frequent occurrence of nephritis, whereas low avidity anti-dsDNA antibodies are more often found in patients with central nervous system involvement.

Diagnostic value of antibodies against ribosomal phosphoproteins. A cross sectional and longitudinal study

Antibodies against ribosomal phosphoproteins (anti-P antibodies) are found in about 10% of patients with systemic lupus erythematosus (SLE). Using an ELISA with a synthetic peptide for screening and an immunoblotting technique as a confirmation test for detection of these antibodies, we found 16 positive patients among 946 sera sent to our reference laboratory for anti-DNA determination. All 12 patients on which we could obtain clinical data had clearcut SLE, fulfilling 6 ARA criteria on average. Skin symptoms were observed more frequently in anti-P positive than in anti-P negative patients with SLE. We also tested 56 sera from 54 patients with well defined SLE during monosymptomatic exacerbations and found an identical frequency of about 10% of anti-P antibodies in groups of patients with different types of exacerbation, including psychosis. A retrospective analysis of 9 patients followed 3-10 years showed that they all were already positive for anti-P at the onset of disease. In total these patients showed 15 major peaks in anti-P levels (defined as at least a 4-fold rise in titer in a period of 3 months). Five of the fluctuations in anti-P levels were paralleled by fluctuations in anti-DNA levels. In 6/15 cases, peaks of anti-P levels were accompanied by an exacerbation of SLE, but only 2 peaks of anti-P levels were already detectable before the onset of the exacerbation. In addition, 6 other exacerbations occurred in the absence of major anti-P rises in the preceding 3 months. Therefore, quantitative determinations of anti-P have at most a limited practical value in the followup of patients with SLE.

The specificity of the anti-dsDNA ELISA. A closer look

The anti-dsDNA ELISA is probably one of the most popular techniques for determining antibody reactivity towards dsDNA, since this assay system has proven high sensitivity and is easy to perform. An important difference from other ELISA systems is the use of an intermediate layer (e.g., protamine sulphate or poly-L-lysine) which has been found to be necessary in order to obtain sufficient coating of dsDNA to the plates. When a panel of monoclonal antibodies to DNA (n = 56), all reactive in this anti-dsDNA ELISA were tested on plates coated only with protamine sulphate (PS) a large number were positive, although with a lower reactivity than with DNA. Binding to protamine sulphate occurred via two mechanisms: (1) DNA/anti-DNA immune complexes, present in hybridoma culture supernatants and adherent to protamine sulphate and (2) some IgM antibodies appear to possess an intrinsic affinity for PS. The latter mechanism gives rise to a false positive reaction in the anti-dsDNA ELISA. It was found that 18% of the clones that were unreactive in any anti-dsDNA assay other than the anti-dsDNA ELISA were labelled 'anti-dsDNA' incorrectly. We therefore propose that antibody reactivity towards dsDNA in an ELISA system must be confirmed in other anti-dsDNA assays before such antibodies can be termed 'anti-dsDNA'.

Murine monoclonal antibodies to DNA. A comparison of MRL/lpr NZB/W and chronically graft-versus-host-diseased mice

Hybridomas producing monoclonal antibodies to DNA were prepared from NZB/W F1 (n = 20), MRL/lpr (n = 13), mice with a chronical graft versus-host-disease (GVHD) (n = 8) and polyclonally stimulated mice (n = 9). Screening was performed by means of an anti-DNA ELISA. Reaction patterns in four different anti-DNA assays (anti-DNA ELISA, indirect immunofluorescence on Crithidia luciliae, PEG assay and Farr assay) as well as avidity and cross-reactivity of these monoclonals were studied in relation to anti-DNA (sub)class and murine origin of the clones. It was found that monoclonal anti-DNA derived from mice with chronic GVHD did not differ from monoclonal anti-DNA derived from NZB/W F1 or MRL/lpr mice, with respect to isotype distribution, avidity towards DNA, cross-reactivity and assay behaviour in the anti-DNA assays mentioned before. In contrast, monoclonal anti-DNA obtained from polyclonally stimulated mice were all of the IgM isotype and displayed a stronger cross-reactive behaviour than the other three models. Altogether, these results exclude the possibility that anti-DNA in the GVHD mice originates from the non-specific pool of natural autoantibodies and further emphasize the relevance of chronic GVHD as a murine model of systemic lupus erythematosus.

A comparison of assays used for the detection of antibodies to DNA

In this paper we will briefly compare four assays in use in our institute for the measurement of antibodies to DNA: the anti-DNA ELISA, the PEG assay, the indirect immunofluorescence test on Crithidia luciliae and the Farr assay. Although with the use of sera from defined patients with systemic lupus erythematosus (SLE) quite good correlations were obtained between the various assays, these correlations were lost upon the use in routine screening of sera of undefined patients. It will be shown that the Farr assay has the highest specificity to systemic lupus erythematosus, whereas the ELISA and the PEG assay are the most sensitive methods. In its present form, however, the ELISA is not suited for the detection of IgM anti-DNA. Furthermore, this technique alone also detects DNA/-anti-DNA complexes present in sera or hybridoma culture supernatants.

Antibodies to DNA in patients with systemic lupus erythematosus. Their role in the diagnosis, the follow-up and the pathogenesis of the disease

In this paper an overview of the present knowledge on antibodies against DNA will be presented. Diagnostic, prognostic and pathogenic aspects of anti-DNA will be highlighted. Detection of antibodies to DNA in the circulation of a patient by an assay selective for high avidity anti-DNA is highly diagnostic for SLE. Anti-DNA of low avidity occurs in rheumatic diseases other than SLE as well, making detection of such antibodies of less diagnostic value. Furthermore, data will be presented that show that the anti-DNA ELISA in its present form is not suited as a diagnostic tool. Not only disease features of SLE vary from patient to patient, anti-DNA avidity does so too. A relationship between anti-DNA avidity and clinical features can be found: high avidity anti-DNA is more abundant in patients with nephritis, low avidity anti-DNA in patients with CNS involvement. Prognostically, a steady increase of the level of high avidity anti-DNA generally heralds an upcoming exacerbation in a patient. Furthermore, 85% of the patients who do not have SLE at the time (high avidity) anti-DNA is detected in their serum, will develop the disease within the next few years. It is noteworthy, that patients with only low avidity anti-DNA in their circulation develop a more mild form of SLE; the (low) avidity of their anti-DNA seldomly increases during the course of their disease. The relevance of anti-DNA to the pathogenesis of SLE still is a matter of debate. On the one hand, the association of parameters of anti-DNA that determine the size of the complexes formed with DNA is in favour of the classical hypothesis, which states that SLE is primarily an immune complex disease. On the other hand, recent data on crossreactions of anti-DNA with phospholipids, glycosaminoglycans and other (poly-negative) structures plead for a role of such crossreactivities in the pathogenesis of SLE.

Cloned human snRNP proteins B and B' differ only in their carboxy-terminal part

Antibodies to the snRNP proteins B' and B are frequently observed in autoimmune diseases. We analyzed different types of cDNAs encoding these proteins. One type of cDNA encoded a protein whose predicted mol. wt is 24.6 kd, whereas another type encoded a protein with a predicted mol wt of 23.7 kd. When translated in vitro from cDNA transcripts, the apparent mol. wts of these proteins on SDS-polyacrylamide gel were 29.5 and 28.5 kd respectively. The main difference between these two types of cDNAs proved to be the presence of an additional sequence of 146 nucleotides in the 3' part of the open reading frame of the clone encoding the shorter protein. This insert contains a termination codon in frame, at 9 nucleotides downstream from the 5' end of the insert. The additional sequence revealed at the 3' end a consensus sequence of vertebrates for intron-exon junctions. We demonstrated the presence of mRNAs corresponding with both types of cDNA in human cells. We hypothesize that the B' and B protein are derived from one pre-mRNA by alternative splicing, and show that they differ only at the carboxy terminus, where a proline rich motive is repeated once more in B'. A comparison with amino acid sequences of other cloned snRNP proteins is included.

Clinical significance of antibodies to double stranded DNA (dsDNA) for systemic lupus erythematosus (SLE)

In sera of patients with systemic lupus erythematosus (SLE) a wide variety of antibodies against nuclear antigens can be found, including antibodies to nucleic acids, histones and non-histone nuclear proteins. Among these, antibodies to double stranded DNA (dsDNA) appear to be mainly restricted to SLE. Yet, in daily practice one also finds patients that have antibodies to dsDNA during a long time ( greater than 5 years) but have not developed SLE. These anti-dsDNA positive non-SLE patients often fulfil several of the ARA criteria for SLE though one may not conclude that they form a specific clinical subset of SLE. It has often been tried to discriminate between clinical subsets of this heterogeneous disease by studying differences within the population of anti-dsDNA antibodies. Immunospecificity, complement-fixing ability, avidity, immunoglobulin (sub)class composition have all been the subject of different studies; yet, conclusions from these studies are often contradictory and more work will be necessary to elucidate this. The prognostic significance of anti-dsDNA levels in prospective studies has been proven valuable. A continuous increase in anti-dsDNA level correlates well with the appearance of an exacerbation of the disease. In other studies merely the amount of antibodies was found to be correlated with disease activity. Therapeutical consequences of these findings are still discutable. The role of anti-dsDNA antibodies in the pathogenesis of the disease is merely based on the above mentioned correlations and on the specificity of the antibodies to SLE. Questions regarding the etiology of SLE or the mere existence of anti-dsDNA antibodies in this kind of patients are unresolved.(ABSTRACT TRUNCATED AT 250 WORDS)

Complement-fixing antibodies to dsDNA detected by the immunofluorescence technique on Crithidia luciliae. A critical appraisal

Studies using an adapted immunofluorescence technique (IFT) on Crithidia luciliae to determine the complement fixing ability of antibodies to dsDNA in relation to disease manifestations, i.e., nephritis, have yielded conflicting results. To establish the relevance of these determinations, we studied sera containing antibodies to dsDNA from 64 patients with systemic lupus erythematosus (SLE), and found that anti-dsDNA of 52% of these sera had the ability to fix complement. SLE patients with nephritis demonstrated a much higher incidence of complement fixing anti-dsDNA (83%) than patients without nephritis (17%, p less than 0.01). On the other hand, patients with nephritis also had higher titers of anti-dsDNA (mean 1:400) than patients without nephritis (mean titer 1:75; p less than 0.01). A clearcut correlation between anti-dsDNA titer and complement fixing anti-dsDNA titer (p less than 0.01) was observed which obviously disturbs the correlation between nephritis and complement fixing anti-dsDNA. Comparing matched sera from patients with nephritis and patients without nephritis with the same antidsDNA titer, we found no difference in complement fixing anti-dsDNA. In the IFT used to measure complement fixing anti-dsDNA, incubation of the Crithidia slides with patients' serum was followed by an incubation with fresh normal serum which served as a source of complement. We observed that this incubation with fresh normal serum resulted in elution of anti-dsDNA antibodies from kinetoplast DNA. This elution was caused by IgG present in normal serum.

Detection of anti-dsDNA as a diagnostic tool: a prospective study in 441 non-systemic lupus erythematosus patients with anti-dsDNA antibody (anti-dsDNA)

The diagnostic significance of anti-double-stranded deoxyribonucleic acid (anti-dsDNA) determination was evaluated in a prospective manner from 1974 to 1982 in a group of 441 patients without systemic lupus erythematosus whose sera were found to contain antibodies to dsDNA on routine screening (Farr assay). Within one year 69% (304) of these patients fulfilled the preliminary American Rheumatism Association (ARA) criteria for systemic lupus erythematosus (SLE). Eighty-two of the remaining 137 patients were followed up for several years. At the end of the study 52% of these patients had also developed systemic lupus erythematosus. Patients who developed systemic lupus erythematosus were characterised by the occurrence of relatively high avidity anti-dsDNA in the circulation compared with patients who did not develop systemic lupus erythematosus. It can be concluded that about 85% of patients without systemic lupus erythematosus with anti-dsDNA in the circulation will develop SLE within a few years. Taking into account the relative avidity of anti-dsDNA, as determined by calculation of Farr/polyethylene glycol (PEG) ratios, we conclude that patients with relatively high avidity anti-dsDNA are more prone to develop systemic lupus erythematosus than patients with relatively low avidity anti-dsDNA.

Antinuclear antibody determination: the present state of diagnostic and clinical relevance

Determination of antinuclear antibodies (ANA) will gain in diagnostic significance if a specific type of ANA can be related to a defined clinical disorder. The past decade has brought us quite a lot of papers dedicated to this subject. Yet, with exception of the DNA/anti-DNA system, observed correlations have remained scarce or contradictory. Also, still little is known about the pathogenic role of ANA. Perhaps more recent approaches using biochemical technologies will provide us with highly purified nuclear antigens necessary to study possible correlations at a more sophisticated level.

Influence of pH on the detection of low- and high-avidity anti-dsDNA

In 2 radioimmunoassays in use to detect antibodies to dsDNA, the Farr assay and the PEG assay, we observed inhibitory effects of normal human serum (NHS) on the DNA binding by SLE sera. This was found to be due by the fact that, during incubation at 37 degrees C, CO2, introduced in the incubation mixture by the serum, evaporates from the mixture. This results in increase in pH to values well above pH 8.0, which in turn leads to a decreased DNA binding by antibody. When SLE sera are tested at low dilution, this phenomenon may lead to false negative results. Proper pH control, by the use of buffers with a greater buffering capacity than PBS, completely prevented the observed inhibitory effects. However, under these conditions NHS bound significant amounts of DNA in both assays. The non-specific DNA binding by NHS was found to be heat-stable, but could be eliminated either by aerosil treatment of the sera or by addition of dextran sulphate to the incubation mixture. Lipoproteins and, to a lesser extent, the complement component C1q appear responsible for this non-specific binding. To avoid false negative results with SLE sera as well as non-specific binding by NHS, we propose the use of stronger buffers in combination with added dextran sulphate to the incubation mixture in both the Farr assay and the PEG assay.

Measurement of low avidity anti-dsDNA by the Crithidia luciliae test and the PEG assay

With the immunofluorescence technique (IFT) using Crithidia luciliae as a substrate, 14,417 sera sent to our laboratory for routine anti-dsDNA determination, were screened for the presence of antibodies to dsDNA. The 1,260 sera that were found IFT positive were then assayed with the Farr radioimmunoassay, in which 3H-labelled PM2-DNA is used as antigen. Only 470 sera (37%) were found to be Farr positive. This discrepancy is, at least partially, caused by the fact that the Farr assay does not detect anti-DNA of low avidity, whereas the Crithidia-IFT does. Sixty-eight percent of the IFT-positive/Farr negative sera were found positive with the PEG assay, a radioimmunoassay that also employs double stranded PM2-DNA as antigen, and that also detects anti-dsDNA of low avidity. The IFT performed on IFT positive/Farr negative sera was found to be rather irreproducible. It was shown that this was due to local increases of the salt concentration resulting from the way the assay was performed. The problem could be overcome by careful control of the assay conditions, i.e. never letting Crithidia slides dry up after washing with PBS. In the PEG assay, these sera sometimes showed a DNA binding that decreased with time. It could be shown that this is caused by a parallel increase in pH during the incubation as a result of CO2 evaporation from the serum.

Specificity in systemic lupus erythematosus of antibodies to double-stranded DNA measured with the polyethylene glycol precipitation assay

Recently, a new radioimmunoassay--the polyethylene glycol (PEG) assay--was introduced to measure antibodies to double-stranded (ds) DNA. In this method, polyethylene glycol precipitation of formed 3H-DNA/antiDNA complexes is used instead of the ammonium sulfate precipitation used in the Farr assay. In contrast to the Farr assay, with which only high-avidity antibodies to dsDNA are detected, the PEG assay also reportedly measures anti-dsDNA of relatively low avidity. We studied whether this gain in antibody measurement results in loss of specificity for systemic lupus erythematosus. When the PEG assay was applied to a selected panel of 440 sera from patients with various well-defined autoimmune diseases and to a group of 197 normal human control sera, matched sex and age to the patients, the method was found to be fairly specific for systemic lupus erythematosus, although the sera from some patients with myasthenia gravis and some with autoimmune liver disease were also found positive. Screening of 352 additional serum specimens, sent to our laboratory for diagnostic reasons, revealed that, with the PEG assay, an extra population of relatively low-avidity antibodies to dsDNA--missed by the Farr assay--was detected. Upon clinical evaluation, we found that the patients in whom such antibodies were detected generally fulfilled a number of the preliminary criteria of the American Rheumatism Association for systemic lupus erythematosus, but that this diagnosis often was not made. We claim that the presence of low-avidity antiDNA characterizes a milder form of the disease in which patients often show only a single clinical feature of the disease. We conclude that results of the PEG assay add valuable diagnostic and clinical information to results obtained by the Farr assay.

Avidity of antibodies to dsDNA: comparison of IFT on Crithidia luciliae, Farr assay, and PEG assay

The Farr assay is thought to detect only antibodies to DNA of relative high avidity. This is due to the high salt concentration of the employed ammonium sulfate precipitation, which dissociates DNA-anti-DNA complexes of low avidity. A recently introduced method to detect anti-DNA, the PEG assay, circumvents these dissociating reaction conditions by using polyethylene glycol instead of ammonium sulfate to precipitate the complexes; we therefore thought to measure antibodies to DNA of low avidity as well. We tested this assumption in several ways. It was found that the PEG assay detects a population of antibodies to DNA that are missed by the Farr assay. Complexes made with these antibodies were salt labile and could readily be dissociated by means of excess DNA, whereas Farr-positive antibodies formed stable complexes with DNA. Avidity studies using the method described by Celada et al. indicated that the anti-DNA detected by the PEG assay but missed by the Farr assay was of relatively low avidity. An inverse correlation between avidity and slope of the binding curves in the PEG assay was observed. These results confirm the notion that the PEG assay detects antibodies to DNA of low avidity. The fact that the Farr assay does not measure these antibodies confers possible diagnostic importance upon the PEG assay.

Avidity of antibodies to dsDNA: comparison of IFT on Crithidia luciliae, Farr assay, and PEG assay

The Farr assay is thought to detect only antibodies to DNA of relative high avidity. This is due to the high salt concentration of the employed ammonium sulfate precipitation, which dissociates DNA-anti-DNA complexes of low avidity. A recently introduced method to detect anti-DNA, the PEG assay, circumvents these dissociating reaction conditions by using polyethylene glycol instead of ammonium sulfate to precipitate the complexes; we therefore thought to measure antibodies to DNA of low avidity as well. We tested this assumption in several ways. It was found that the PEG assay detects a population of antibodies to DNA that are missed by the Farr assay. Complexes made with these antibodies were salt labile and could readily be dissociated by means of excess DNA, whereas Farr-positive antibodies formed stable complexes with DNA. Avidity studies using the method described by Celada et al. indicated that the anti-DNA detected by the PEG assay but missed by the Farr assay was of relatively low avidity. An inverse correlation between avidity and slope of the binding curves in the PEG assay was observed. These results confirm the notion that the PEG assay detects antibodies to DNA of low avidity. The fact that the Farr assay does not measure these antibodies confers possible diagnostic importance upon the PEG assay.

The use of polyethylene glycol precipitation to detect low-avidity anti-DNA antibodies in systemic lupus erythematosus

With a recently introduced method for measurement of low-avidity anti-DNA, the polyethylene glycol (PEG) precipitation assay (Riley et al., 1979), high levels of DNA binding by normal human serum (NHS) were found when circular PM2-DNA was used as antigen. THe nature of this DNA binding was studied. The removal of low-density lipoproteins (LDL) from the serum, e.g., by Aerosil absorption, eliminated DNA binding by NHS. Purified LDL bound DNA to the same extent as NHS. Non-specific binding of NHS or LDL to DNA was prevented by adding dextran sulphate to the incubation mixture. Analysis on sucrose gradients showed that only large DNA-anti-DNA complexes were precipitated by 3.5% PEG. The PEG assay with dextran sulphate is a sensitive assay for low-avidity anti-DNA antibodies. It adds important information to results obtained with the Farr assay, which mainly detects antibodies of high avidity.