ELISA assays for IgM and IgG rheumatoid factors

Enzyme-linked immunosorbent assays (ELISA) for the detection of IgM RF and IgG RF are described. A quantitation step was introduced into the IgM RF assay allowing expression of results as mg/ml of IgM RF. The effect of pepsin digestion of test sera on the IgG RF assay was established and the possible advantages of including pepsin digestion of sera in the assay considered. The assays have been developed to allow the screening of large numbers of sera in as rapid a time as possible.     

Bicentric evaluation of Access Toxo immunoglobulin M (IgM) and IgG assays and IMx toxo IgM and IgG assays and comparison with Platelia Toxo IgM and IgG assays.

The recent Access immunoanalysis system (Sanofi Diagnostics Pasteur) for the serological diagnosis of toxoplasmosis was compared with the Abbott Toxo IMx EIA system, taking the Platelia Toxo immunoglobulin G (IgG) and Platelia Toxo IgM systems as references and using as confirmation methods an indirect fluorescence assay or a dye test for IgG and an immunosorbent agglutination assay (ISAGA) for IgM. A total of 1,461 serum samples were studied, of which 128 were collected from 42 recently seroconverted patients. Sensitivity and specificity rates of the Access system were 97.7 and 99.5%, respectively, for IgM and 98.6 and 100%, respectively, for IgG. Sensitivity and specificity rates of the Abbott IMx EIA system were 91 and 100%, respectively, for IgM and 92.5 and 100%, respectively, for IgG. The Access Toxo IgG and IgM EIA systems were found to be more sensitive than the Abbott Toxo IgG and IgM IMx EIA systems.     

The interference of IgM rheumatoid factor in enzyme-linked immunosorbent assays of rubella IgM and IgG antibodies

The interference of IgM rheumatoid factor (RF) in the indirect enzyme-linked immunosorbent assay (ELISA) for rubella IgM and IgG antibodies was evaluated quantitatively. In an ELISA for rubella IgM antibodies IgM RF produced false positive results in tests of sera with a rubella IgG concentration exceeding approximately 30 I.U./ml and an IgM RF concentration higher than 3.5 I.U./ml as determined by ELISA. Sera from 3 of 70 patients with recent rubella and sera from a similar proportion of blood donors contained IgM RF in a concentration exceeding this level.The false positive results were reproduced when an IgM RF preparation was added to sera containing rubella IgG. The rubella IgG values in ELISA, however, decreased after the addition of IgM RF . RF was absorbed by incubation of sera with a suspension of latex particles coated with aggregated human IgG. This method prevented the false positive results of the rubella IgM assay and increased the rubella IgM values of rubella convalescent sera. The activity in the rubella IgG assay also increased after absorption of RF. The interference of RF has been explained by a secondary binding of RF to rubella IgG-antigen complexes. The RF might subsequently be detected by the anti-IgM conjugate or compete with the anti-IgG conjugate.The common occurrence of IgM RF in concentrations sufficient to produce false positive results of the ELISA for specific IgM antibodies necessitates routine testing of IgM antibody positive sera for RF by a sensitive method and/or absorption of RF from such sera.     

Immunological properties of isolated IgG and IgM anti-gamma-globulins (rheumatoid factors)

Anti-gamma-globulins of the IgG and IgM classes have been isolated from sera of normal individuals and from patients with osteoarthritis, rheumatoid arthritis and juvenile rheumatoid arthritis. All of the isolated antibodies gave precipitin curves with heat-aggregated, reduced and alkylated gamma-globulin. IgM anti-gamma-globulins gave a positive latex fixation test at 4°C and 37°C while IgG anti-gamma-globulins generally gave a positive test only at 4°C. Anti-gamma-globulins from normals did not fix complement but IgG and IgM anti-gamma-globulins from rheumatoids fixed complement to a similar degree. This in vitro complement fixation could account at least in part for the diminished complement levels seen in many rheumatoid synovial effusions.     

Comparison of chemiluminescent immunoassay and ELISA for measles IgG and IgM

In the context of measles elimination, the identification of recent infections is important for clinical laboratories. Serological diagnosis is achieved by detecting specific IgG and IgM. Recently an automated chemiluminescent immunoassay (CLIA) (Liaison, DiaSorin, Italy) has been used to quantify the measles antibody. The aim of this study was to compare this assay with Enzygnost ELISA (Siemens, Germany), with final classification of discrepancies by indirect immunofluorescence (Euroimmun, Germany). For measles IgM, 204 sera were analyzed: 50 IgM‐positive, 104 IgM‐negative/IgG‐positive, and 50 from other viral infections (B19V, rubella, mumps, CMV, and EBV). For the measles IgG assay, 162 samples were tested: 106 were positive and 56 were negative. For measles IgM, the sensitivity and specificity of CLIA against ELISA were 94% (95% CI: 83.2–98.6) and 100% (95% CI: 97.1–100), respectively; the corrected figures after the final classification of discrepancies were 100% (95% CI: 91.0–100) and 99.4% (95% CI: 96.1–100), respectively. In relation to IgG, the sensitivity and specificity of CLIA against ELISA were, respectively, 97.2% (95% CI: 91.7–99.4) and 92.9% (95% CI: 82.5–97.7), and 95.5% (95% CI: 89.5–98.3) and 100% (95% CI: 91.8–100) after the final classification. CLIA showed excellent sensitivity and specificity in detecting measles IgG and IgM antibodies, eliminating the need to aliquot specimens before carrying out the assay.     

Comparison of PanBio Dengue Duo IgM and IgG capture ELISA and venture technologies dengue IgM and IgG dot blot.

BACKGROUND: A number of commercial ELISA for dengue diagnosis have recently become available, though direct comparison between these assays have not been published. OBJECTIVES: The Venture Technologies Dengue IgM and IgG Dot Blot assays and the PanBio Dengue Duo IgM and IgG Capture ELISA were compared. STUDY DESIGN: Paired sera from patients with dengue (n=20) and Japanese encephalitis (JE, n=10), and single sera from patients with typhoid (n=10), leptospirosis (n=10) and scrub typhus (n=10) were assayed according to the manufacturer's instructions. RESULTS: The Dot Blot IgM ELISA showed higher sensitivity than the PanBio IgM ELISA (100 vs. 95%), while the PanBio IgM ELISA showed higher specificity in JE (100 vs. 20%) and non-flavivirus infections (100 vs. 97%). Defining elevation of either IgM or IgG as a positive result, the Dot Blot and ELISA tests both showed 100% sensitivity in dengue infection, while the PanBio test showed superior specificity in JE (70 vs. 0%) and non-flavivirus infections (100 vs. 67%). CONCLUSIONS: Both assays are useful aids to the serological diagnosis of dengue infection. The clinical setting, user preference and local conditions will be important in determining which test is more appropriate.     

ELISA for human IgG and IgM anti-lipopolysaccharide antibodies with indirect standardization

A new attempt to standardize ELISAs for the quantitation of human IgM and IgG anti-lipopolysaccharide antibodies (anti-LPS), without the use of a specific standard material, is described. Sandwich ELISAs for total IgG and IgM were combined with indirect ELISAs for anti-LPS IgG and IgM antibodies on a 96-well microtest plate using identical assay conditions. The concentration of specific IgG or IgM anti-LPS was read on the respective standard curve for total IgG or IgM. The results were corrected for residual immunoreactivity remaining unbound in the wells after one sample incubation in the combined assays. The quantitative results of IgG anti-LPS correlated well with results obtained using an ELISA with direct standardization (r = 0.969). 28 mg/l of IgM anti-LPS was found as median value among 121 blood donors using the described ELISA principle. Binding studies demonstrated a lower apparent affinity of donor anti-LPS IgM than anti-IgG.     

The binding constants of IgM rheumatoid factors and their univalent fragments for native and aggregated human IgG;.

IgM rheumatoid factors (RF) were isolated from the sera of patients with rheumatoid arthritis and a serologically active Fabmicron RF fragment prepared by papain digestion. A radioimmunoassay was developed for the determination of interaction of 19S IgM RF and Fabmicron RF with human 7S IgG, heat-aggregated IgG, rabbit 7S IgG, and human pFc'. RF isolated under neutral conditions had a very low binding constant for human 7S IgG (of the order of 10(2) to 10(3) 1 mole-1) and a considerably higher value (ca. 10(5)) for the aggregated protein and monomeric rabbit IgG. RF obtained under acid conditions which dissociate the complexes with endogenous Ig, had a higher avidity for human IgG monomer as expected and also a comparable reactivity with rabbit IgG. Monovalent Fabmicron fragments of 'acid' RF had closely similar affinities for 7S and aggregated IgG suggesting that the enhanced binding with the aggregated protein is essentially dependent on its multivalency rather than the exposure of a new determinant lacking in the native molecule.     

IgG rheumatoid factors and anti-nuclear antibodies in rheumatoid vasculitis.

We studied the distribution and characteristics of circulating rheumatoid factors (RF) and anti-nuclear antibodies (ANA) in 30 rheumatoid arthritis (RA) patients who had polyarthritis alone (group I), 28 RA patients with polyarthritis and extra-articular disease (group II), 28 RA patients with systemic vasculitis (group III) and 60 healthy matched controls. IgG RF occurred more frequently and in higher serum titres in group III (100%) than RA patients in group I (40%), or in group II (18%) or in normal controls (5.8%). The serum titre of IgM RF was higher in vasculitis patients than in other RA patients. ANA were found in 74% of all RA patients and although the frequency did not differ in the three patient groups, the serum titre was significantly higher in the vasculitis group. Antibodies to extractable nuclear antigen were found only in group III (18.7%). Antibodies to histones were also more prevalent in group III than in the other RA groups. The serological abnormalities in rheumatoid vasculitis differed quantitatively as well as qualitatively from other RA patients.     

Spectrotypic analysis of IgM and IgA rheumatoid factors.

The spectrotypes of IgA and IgM rheumatoid factors (RF) were analysed in whole serum as well as immunoglobulin fractions and purified RF from patients with one of three autoimmune disorders. As predicted from the pI ranges of normal human serum IgM and IgA in agarose, IgM RF had near neutral pIs, whereas IgA RF showed more acidic (lower) pI values. Serum IgA and IgM RF from patients with rheumatoid arthritis or the sicca syndrome showed considerable charge heterogeneity whereas IgM RF from mixed cryoglobulinaemia or rheumatoid vasculitis patients showed monoclonal and oligoclonal banding patterns respectively. Clonotypic analysis was best achieved with isolated light and heavy chains from highly purified RF. The IgA RF spectrotypes from unmodified paired serum and saliva IgA were clearly different whereas after desialation, similar pI values and, in one case, similar spectrotypes were observed. These observations are compatible with the hypothesis that serum and saliva IgA RF are derived from similar clonal precursors. The methods used in these studies may also be of use in the analysis of IgA and IgM antibody diversity in a number of other situations.     

Determination of the affinity of monoclonal 19 S IgM rheumatoid factor for IgG by modified immunoassay (ELISA)

In rheumatoid arthritis (RA), the pathogenicity of IgM rheumatoid factor (RF), an autoantibody whose antigen is IgG, is still unclear although RF-IgG complexes appear to be important mediators of immune injury. The polyclonality of RF in RA makes it difficult to characterize certain qualitative properties such as specificity and affinity which may be very important in determining pathogenicity. Monoclonal IgM RF can be used to circumvent this problem. Monoclonal RF secreting cells can be produced via hybridizations with RA B lymphocytes fused with mouse or human myeloma cell lines. Another source of monoclonal RF is the sera of patients with Waldenstr?m's macroglobulinemia (WM). One particular WM IgM RF (Kas) was chosen for our experiments to measure affinities and specificities to eight different monoclonal IgGs (three IgG1s, three IgG3s, one IgG2, and one IgG4). 19 S IgM RF, a pentavalent molecule, was mildly reduced with DTT to make 7 S univalent fragments (7 S IgM RF). 7 S IgM RF was incubated with each of the different IgGs at several different concentrations. These mixtures were allowed to come to equilibrium. An aliquot was then used to determine the amount of free 7 S IgM RF by ELISA. By plotting the reciprocal of the fraction of bound RF versus the reciprocal of the concentration of free antigen at equilibrium, different affinities were determined. The results of these determinations compare favorably with published IgM RF affinities determined by more traditional methods. This method can also be used with proteolytic digest fragments of IgG and short synthetic peptides of the IgG molecule to better locate the antigen binding site. The technique may also help us to determine whether there are select clones of RSC producing RF with different affinities that could complex to a particular type of IgG which, in vivo, could produce greater inflammatory tissue damage. Furthermore, this methodology should be useful in the study of other autoimmune diseases characterized by pathogenic autoantibodies of differing affinities.     

Reverse ELISA for IgG and IgM antibodies to detect Lassa virus infections in Africa.

BACKGROUND: Anti-Lassa antibodies are detected by indirect immunofluorescence assay (IFA) or by enzyme-immunoassay (ELISA). Both methods have problems to detect low amounts of specific antibodies. OBJECTIVES: We report here highly sensitive and specific reverse ELISAs to detect Lassa virus IgG and IgM antibodies. Due to the reverse techniques, serum samples could be applied at dilutions of 1:10 without increasing non-specific background reactions. STUDY DESIGN: For IgM antibody detection microtiter plates were coated with anti-IgM antibodies and for IgG antibody detection with rheumatoid factor (RF) (Sachers M, Emmerich P, Mohr H, Schmitz H. Simple detection of antibodies to different viruses using rheumatoid factor and enzyme-labelled antigen (ELA). J Virol Methods 1985;10:99-110). In both assays a tissue culture antigen was used in combination with a labeled anti-Lassa monoclonal antibody (Hufert FT, Ludke W, Schmitz H. Epitope mapping of the Lassa virus nucleoprotein using monoclonal anti-nucleocapsid antibodies. Arch Virol 1989;106(3-4):201-12). RESULTS: The reverse ELISA turned out to detect virus-specific IgG and IgM antibody in all 20 samples of West African patients collected 2-8 weeks after onset of Lassa fever. Moreover, both IFA and reverse ELISA found IgG antibodies in 53 out of 643 samples of healthy West Africans (sensitivity of 100%). Six of the 643 samples were positive by reverse IgG ELISA only. Thus, the specificity compared to IIF was 99.0%, but it may be even higher, because compared to IFA the IgG ELISA was clearly more sensitive in detecting low antibody titers. CONCLUSIONS: In Ghana 3% seropositives were found by IFA, but 4% by the reverse ELISA. The reverse ELISAs can be performed with high sensitivity and specificity under field conditions in Africa.     

A solid-phase radioimmunoassay for IgG and IgM antigammaglobulin factor in rheumatoid arthritis sera.

Rheumatoid factors of IgG and IgM class were detected by means of a solid-phase radioimmunoassay using formalinized sheep erythrocytes sensitized with rabbit anti-sheep erythrocyte serum and 125I-labelled anti-human IgG or anti-human IgM antibody. 28 of 35 sera and all of 7 synovial fluids of patients with rheumatoid arthritis showed positive reaction for both IgG and IgM rheumatoid factor, though the activity of IgM factor was mostly higher than that of IgG factor. However, rheumatoid factor present in synovial fluids of 3 seronegative patients was mainly of IgG class. Of 14 sera of patient with other collagen diseases, 11 showed IgG and 6 showed IgM rheumatoid factor. The IgG rheumatoid factor seemed to be associated with autologous IgG to form macro-molecular complexes which could be dissociated into 7S molecules of IgG rheumatoid factor and IgG by acid treatment.     

Indirect immunoglobulin G (IgG) and IgM enzyme-linked immunosorbent assays (ELISAs) and IgM capture ELISA for detection of antibodies to lipopolysaccharide in adult typhoid fever patients in Pakistan.

Sera from 339 adult febrile patients in Pakistan were tested for antibodies to Salmonella typhi lipopolysaccharide by indirect immunoglobulin G (IgG) and IgM enzyme-linked immunosorbent assay (ELISA) and IgM capture ELISA. A total of 55 patients had S. typhi cultured from their blood, 20 had S. typhi cultured from their stool, 24 were blood or stool culture positive for S. paratyphi A, 41 were culture negative but clinically diagnosed as having enteric fever, 41 had gastrointestinal or urinary tract infections, 41 were clinically diagnosed as having malaria, 20 were smear-positive patients with malaria, 58 had respiratory infections, and the remaining 39 individuals were placed in a miscellaneous group who did not have Salmonella infection. The sensitivities of the indirect IgG ELISA, indirect IgM ELISA, and IgM capture ELISA determined with specimens obtained from the blood culture-positive patients with typhoid fever (positive controls) were 80, 64, and 62%, respectively. The specificities of the assays determined with sera from the patients with respiratory infections (negative controls) were 95, 95, and 97%, respectively. The percentage of smear-positive patients with malaria who were positive by these assays was lower than that in the negative control group. The percentages of individuals in the other patient categories who were positive by these tests were between those obtained with the positive and negative controls. Of the positive controls, 26 were positive by both IgM assays, 9 were IgM positive only by indirect ELISA, and 8 were IgM positive only by IgM capture ELISA. A total of 70% of the positive control patients who were tested for O agglutinins by the Widal tube agglutination assay were positive; however, 29% of the negative control patients were also positive. The indirect IgG ELISA was the single most effective test for the serodiagnosis of typhoid fever in this population.     

Interference in immunoenzymometric assays caused by IgM anti-mouse IgG antibodies

Serum samples from a female patient gave falsely elevated results in nine of ten immunoenzymometric assays (IEMAs) that used mouse monoclonal antibodies specific for human chorionic gonadotropin (n = 8), thyrotropin, or creatine kinase-MB isozyme. In contrast, normal results were obtained in five radioimmunoassays that used either mouse monoclonal antibodies or antisera specific for human chorionic gonadotropin (n = 3) or thyrotropin (n = 2). Incubation of her serum with IgG from different species or with F(ab)'2 from mouse IgG prior to IEMA showed that the interference was markedly inhibited by mouse IgG, indicating an antibody specific for the Fc portion of mouse IgG. The interfering activity was bound to and eluted from a column containing Protein A-Sepharose CL-4B. Fractionation of the eluted protein over another column containing Sephacryl S300 showed the activity was enriched in the first protein peak, which contained predominantly IgM. A model is proposed to explain how IgM anti-mouse IgG antibody selectively interferes in IEMAs that use mouse monoclonal antibodies.     

Comparison of five ELISA assays for IgG antibody against coxsackievirus B1

Enterovirus type and group specificities of five different IgG ELISA methods were compared, using neutralization titration tests as an indicator of the presence or absence of antibodies to coxsackie B (CB) viruses. One of the ELISA assays was a "standard" IgG assay, where the solid phase was coated directly with the purified virus, followed by incubations with human serum, biotinylated anti-human-IgG, streptavidin-peroxidase, and the substrate/chromogen. In a modified standard assay, blocking of common epitopes was attempted by incubating the CB1 virus antigen on the solid phase with a rabbit antiserum to CB5 before the human serum was added. In another modification the serum dilution buffer contained heat-denatured heterologous enteroviruses in an attempt to consume human antibodies reacting with common epitopes. In one assay the purified CB1 virus was captured by purified horse anti-CB1 IgG on the solid phase, before incubation with human serum. In the last of the five assays the serum specimen was incubated with CB1 virus (in the liquid phase) before the virus or virus-antibody complex was captured with purified horse anti-CB1-IgG. Reactions against common antigens dominated in the first three assays. The antigen-capture assay appeared to be at least predominantly type specific. Our data indicate that the liquid-phase assay may be type specific, but more studies are needed. The method of virus purification was critical for the type specificity of the antigen-capture and liquid-phase assays.     

Evaluation of six tests for circulating IgG complexes with special reference to IgM rheumatoid factors: analysis of systemic lupus erythematosus and rheumatoid arthritis series.

Six tests for circulating immune complexes (CIC) developed in four laboratories and representing four main principles [affinity of human platelets, Clq, of RF for aggregated IgG, and of conglutinin (Kg) for complex-bound C3] were evaluated on series of SLE and definite RA. All tests detected human model complexes in the presence of NHS and discriminated the patient series from the blood donor series, most powerful being the PIPA (platelet test). The high correlation between the RF-binding inhibition tests (RFbI) and the RF-latex test suggested interference due to intrinsic RFs. This received further support from experimental analyses in which RA sera mixed with varying doses of heat-aggregated IgG were assayed by the pRFbI test.     

Laboratory diagnosis of rheumatoid arthritis: a solid phase radioassay for IgG and IgM antiglobulins.

A technique suitable for the routine estimation of IgM and IgG antiglobulins has been devised. The assay involves the binding of antiglobulins to plastic tubes coated with rabbit immunoglobulin: the amount of antiglobulin bound is then determined by adding radiolabelled antihuman IgG or IgM. The conditions for the assay have been examined and optimal incubation times and amounts of reagents established. Verification of the antibody nature of antiglobulin activity has been obtained. Both IgG and IgM antiglobulins were raised in virtually all seropositive rheumatoid arthritics, and most seronegative patients gave elevated values for either IgM or IgG rheumatoid factors. The use of an anti-light chain reagent as a screening test for total antiglobulins was investigated. These tests should prove valuable in diagnosis and permit quantitative evaluation of research studies.     

IL-1ra ELISA: reduction and alkylation of synovial fluid eliminates interference by IgM rheumatoid factors

IL-1 and a specific receptor antagonist of IL-1, IL-1ra, may play important roles in the pathophysiology of rheumatoid arthritis and in other types of inflammatory synovitis. Measurement of IL-1ra in synovial fluids and in other body fluids may lead to a greater understanding of its possible activity as a modulator of the immune and inflammatory systems in vivo. Therefore, a modified sandwich ELISA was developed to measure IL-1ra protein concentration in synovial fluids. The antibodies used in this ELISA were polyclonal and derived from rabbits hyperimmunized with human recombinant IL-1ra. IgM rheumatoid factors within synovial fluid resulted in false elevation of determined IL-1ra by the sandwich ELISA through binding of the primary and secondary antibodies. Reduction and alkylation of synovial fluid samples before application to the ELISA plate eliminated the interference caused by > 2000 μg/ml IgM rheumatoid factor (latex agglutination titer of 1/5,120). This ELISA was specific for IL-1ra; there was no detection of IL-1, IL-1β, or lysozyme. The sensitivity of this ELISA was <200 pg/ml, making it a useful assay for the accurate measurement of synovial fluid IL-1ra protein concentration.