Immunosorbent Assay for Determination of Rubella Immunoglobulin M Using Monoclonal Antibodies

Transcription

1 JOURNAL OF CLINICAL MICROBIOLOGY, June 1987, p Vol. 25, No /87/ $02.00/0 Copyright C 1987, American Society for Microbiology Development and Evaluation of a Capture Enzyme-Linked Immunosorbent Assay for Determination of Rubella Immunoglobulin M Using Monoclonal Antibodies GIUSEPPE GERNA,l* MARCO ZANNINO,2 M. GRAZIA REVELLO,' ELEONORA PETRUZZELLI 2 AND MAURO DOVIS2 Virus Laboratory, Institute of Infectious Diseases, University of Pavia, and IRCCS Policlinico S. Matteo, Pavia,' and SORIN Biomedica, Saluggia,2 Italy Received 18 November 1986/Accepted 2 March 1987 A capture enzyme-linked immunosorbent assay (ELISA) for detection of virus-specific immunoglobulin M (IgM) antibody was developed which used a panel of labeled monoclonal antibodies to rubella virus hemagglutinin. The rapidity of the test system was increased by using, after 1-h incubation of the test serum, a second 1-h incubation of the serum with a mixture of viral antigen and labeled monoclonal antibody. The new assay was tested for specificity on 371 human sera from people without any recent contact with rubella virus; of these, 66 were sera selected from people with rheumatoid factor or IgM antibody to human cytomegalovirus, Epstein-Barr virus, or other viruses. In parallel, the new assay was performed on 191 sera from patients having recent contact with rubella virus. Results were compared with those obtained by an indirect ELISA method on IgM serum fractions, using purified rubella virus as a solid phase. Of the 371 sera tested for specificity, 5 (1.3%) gave false-positive results with indirect ELISA (1 rheumatoid factor, 2 heterophil antibody, and 2 human cytomegalovirus sera positive for IgM), and none were false-positive with the capture assay. Two sera from a patient with primary cytomegalovirus infection, which were positive for rubella IgM antibody with both methods and were initially interpreted as false-positive, were finally considered to be true-positive, since they were reactive only in the presence of IgM antibody and viral antigen. Of the 191 sera from 92 patients (84 patients with acute rubella, four newborns from mothers with rubella during pregnancy, and four vaccinees), 136 (71.2%) were found to be positive for IgM by indirect ELISA, and 128 (67.0%) were positive by capture ELISA; 12 sera drawn during the first 2 days of disease, or at least 40 days after onset (or after vaccination), were detected only by indirect ELISA, and 4 sera were detected only by capture ELISA. Thus, specificity and sensitivity, respectively, were 100 and 91.4% for capture ELISA and 98.6 and 97.1% for indirect ELISA. However, when the number of patients was considered, 86 were detected as IgM positive by indirect ELISA, and 87 were detected positive by capture ELISA. The overall agreement between the two assays was 96.2%. Capture ELISA using monoclonal antibody appears preferable over indirect ELISA on IgM serum fractions because of its higher specificity and shorter time for test performance; furthermore, there is no need for serum fractionation or virus purification for the capture ELISA. Diagnosis of acute rubella infection is done primarily by demonstrating seroconversion in paired sera or, less often, by virus isolation. However, when patients come to the physician's observation late after the onset of disease, it is necessary to look for virus-specific immunoglobulin M (IgM) to diagnose acute rubella infection. Until recently, IgM has preferably been determined by using immunoassays on IgM serum fractions obtained by sucrose density gradient centrifugation (1) or gel filtration (15), or on sera depleted of IgG by using protein A (12). These serum treatments, aimed to prevent false-positive results due to rheumatoid factor (RF) (18) or false-negative results due to high levels of virusspecific IgG (14, 15), have also been satisfactorily used in indirect radioimmunoassay (13) and enzyme-linked immunosorbent assay (ELISA) (23) systems for IgM determination, even though false-positive results were never completely avoided. Recently, a new approach (capture or reverse assay) has been introduced for viral IgM detection, involving an anti-human IgM as a solid phase, then incubation with test serum followed by labeled antigen (3, 19-22) or unlabeled viral antigen and labeled antiviral antibody (9, 11, 17). With the capture assay, previous treatment of sera has been * Corresponding author found to be unnecessary, since serum IgM is immunopurified in situ during the test serum incubation. In addition, the recent use of polyclonal F(ab')2 conjugate (10) or labeled monoclonal antibody (24) seems to have eliminated a few false-positive results due to the presence of IgM RFs reactive with aggregated rubella IgG present in polyclonal conjugates (11). The development and evaluation of a capture ELISA for rubella IgM determination are described in this report. The assay, based on the use of immune complexes of viral antigen and labeled monoclonal antibody, requires only two 1-h incubation steps after sensitization. MATERIALS AND METHODS Rubella virus cultivation, purification, and isolation. Rubella virus (HPV-77 strain) was cultivated on Vero cells (American Type Culture Collection, Rockville, Md.) and purified from infected cell culture medium by a reported procedure (8). Briefly, clarified infected cell culture medium was concentrated 100-fold in a hollow-fiber apparatus (UM-50 fibers; Amicon Corp., Danvers, Mass.). The concentrate was layered over a column of 25% (wt/wt) sucrose in 0.01 M Tris hydrochloride (ph 8.5) and centrifuged (80,000 x g, 4 h) onto a cushion of 50% (wt/wt) sucrose.

2 1034 GERNA ET AL. Virus was then centrifuged to equilibrium (90,000 x g, 40 h) in a 15-ml 25 to 50% (wt/wt) sucrose gradient. Fractions containing virus were identified by hemagglutination, combined, and banded in a 10-ml gradient of 10 to 30% (wt/wt) potassium tartrate containing 10% (vol/vol) glycerol. Fractions containing peak hemagglutination activity were pooled and stored at -80'C. Virus isolation and identification were performed as previously reported (7). Monoclonal antibody preparation, selection, and labeling. A panel of 10 monoclonal antibodies to rubella virus hemagglutinin was prepared as reported elsewhere (G. Gerna, M. G. Revello, M. Dovis, E. Petruzzelli, G. Achilli, E. Percivalle, and M. Torsellini, submitted for publication). Monoclonal antibodies were coupled to horseradish peroxidase by the periodate method (25). Characterization of monoclonal antibodies by competitive binding assays showed reactivity with five different epitopes of rubella virus hemagglutinin. Monoclonal antibodies of each of the five epitope classes were tested in the capture ELISA to select the most suitable ones. Capture ELISA. Polystyrene microtiter plates (M129A; Dynatech, Plochingen, Federal Republic of Germany) were coated with 100,ul per well (about 0.1 ptg) ofgoat anti-human IgM (IgG fraction, mu-chain specific; Cappel Laboratories, Cooper Biomedical, Malvern, Pa.) in 0.01 M sodium carbonate-bicarbonate buffer (ph 9.6). Alternatively, wells were coated with 100 >t1 (about 0.1,ug) of an affinity-purified goat anti-human IgM (Cappel Laboratories). After overnight incubation at room temperature, the microplates were washed with phosphate-buffered saline (PBS) (ph 7.2) containing 0.1% Tween 20, and coating was followed by postcoating for 1 h at room temperature, using 4% bovine serum albumin in PBS, and then incubation with test serum diluted in PBS- Tween 20 containing 1% fetal calf serum. After incubation for 1 or 2 h at 37 C, microplates were washed three times and then incubated for different times (see Results) at 37 C with 100 pl of a mixture of optimal dilutions of rubella antigen (10 x -concentrated infected cell culture medium) and the selected monoclonal antibody labeled with horseradish peroxidase. In some experiments, two sequential incubations were done with rubella antigen and labeled monoclonal antibody. After final washings, the substrate solution, which was prepared immediately before use by dissolving 3.4 mg of ortho-phenylenediamine in 10.0 ml of citrate phosphate buffer (0.05 M phosphate, 0.02 M citrate, ph 5.5) and then adding 40,u of 3% H202, was added and then incubated for 30 min at room temperature in the dark. The reaction was stopped with 100 tld per well of 4 N H2SO4, and the optical density of the reaction was measured spectrophotometrically (Titertek Multiskan; Flow Laboratories, Irvine, Scotland) at 492 nm. Triplicate wells of diluent control, negative, and 4 cutofff) and 20 arbitrary units (AU) positive control sera were included in each assay (see below). The 4-AU cutoff serum was selected as reported in the Results section. Test sera having an optical density (OD) higher than that of the cutoff serum were considered positive for presence of IgM. Indirect ELISA. The indirect ELISA, routinely used in our laboratory for rubella IgG determination, has been standardized for testing IgM serum fractions obtained by gel filtration (15). Briefly, each well ofpolystyrene microtiter plates ofthe same type used in the capture assay was coated overnight at room temperature with 0.025,ug of purified rubella virus in 100,ul of 0.01 M sodium carbonate-bicarbonate buffer (ph 9.6). The next day, microplates were washed with PBS- Tween 20 and then incubated with serial fourfold dilutions J. CLIN. MICROBIOL. (starting dilution, 1:50 in PBS-Tween 20 plus 1% fetal calf serum) of IgM fractions of test sera for 2 h at 37 C. Rubella IgM-negative and low- and high-positive control sera were included in each test run. After washings, wells were incubated for 1 h at 37 C with 100 pul of peroxidase-conjugated heavy-chain-specific goat IgG antibodies to human IgM (Cappel Laboratories). Finally, the substrate solution was added as in the capture assay, and the reaction product was read spectrophotometrically. The endpoint titer was considered the highest serum dilution giving an OD at least three times greater (0.30) than that of the mean of 20 negative serum controls (0.10). Patients and sera. The following groups of patients and sera were examined for rubella IgM antibody by both capture and indirect ELISA methods: (i) 168 sera from 84 patients with acute rubella; (ii) eight sera from four newborns from mothers with acute rubella during pregnancy; (iii) 15 sera from four rubella vaccinees; (iv) 140 rubella IgGnegative and 112 rubella IgG-positive sera from 252 healthy individuals; (v) 53 sera from healthy newborns; and (vi) 66 sera from 18 RF-positive subjects (18 sera), 12 heterophil antibody-positive patients (15 sera), 10 patients with primary human cytomegalovirus (HCMV) infection (19 sera), and a total of 14 subjects recovering from primary herpes simplex virus (4 sera), primary varicella-zoster virus (4 sera), measles (3 sera), or mumps (3 sera) infections. RESULTS Development of the capture ELISA. The assay was initially developed by using a pool of highly rubella IgM-positive sera diluted in a rubella-negative pool to obtain different concentrations of specific IgM antibody, expressed as AU. Negative and positive standard sera (containing 2 to 64 AU and previously tested by indirect ELISA on IgM serum fractions) were assayed in the new capture ELISA system, and results were expressed as a sampte/negative (S/N) ratio. The S/N ratio was obtained by dividing the OD of the standard serum by the OD of the negative control pool. Initial experiments for the development of the assay were performed on standard sera using three sequential incubations (2 h each at 37 C) of test serum, rubella antigen, and peroxidase-labeled monoclonal antibody. Preliminary results indicated that the cutoff value had to be selected in the range of 3.0 to 5.0 S/N ratios, corresponding to 2- to 8-AU standard sera. The optimal testing serum dilution was found to be 1:100 in PBS-Tween 20 plus 1% fetal calf serum. Since no significant difference in results was observed when purified rubella virus or concentrated infected cell culture medium was used, the crude antigen preparation was routinely employed at the dilution giving the highest S/N ratio. The labeled monoclonal antibody to be used in the assay was selected by titrating eight different peroxidase-labeled monoclonal antibodies (all reactive to the El polypeptide of rubella virus by Western blot analysis) with positive and negative standard sera. For each antibody, the dilution giving the highest S/N ratio was chosen as optimal. The four monoclonal antibodies giving an S/N ratio of -4.0 with the 4-AU standard serum were considerd for routine use in the test (Table 1). Of these four antibodies, those designated 7A6 and 6D2 were previously shown to belong to the epitope group A, and 3A4 and 8C1 belonged to the epitope group B (Gerna et al., submitted). Finally, 7A6 was the one chosen for routine use in the test because of its higher working dilution (1:9,000). Optimization of the assay. During the development of the assay, three incubation steps of 2 h at 37 C were consistently

3 VOL. 25, 1987 MONOCLONAL ANTIBODY-CAPTURE ELISA FOR RUBELLA IgM 1035 TABLE 1. Reactivity of peroxidase-labeled monoclonal antibodies to rubella virus hemagglutinin with standard human sera by capture ELISA Rubella IgM Reactivity (OD) of peroxidase-conjugated monoclonal antibodies: standard serum (AU) 7A6 6D2 2C3 3A4 3C3 8C1 4A6 3A2 High positive (64) Low positive (4) Negative (0) a Each antibody was used at its optimal working dilution. performed as reported above. Subsequent experiments were performed to verify the possibility of adding a mixture of antigen and conjugate to the solid phase. In addition, the mixture was prepared either 2 h in advance (procedure B) or just before use (procedure C). Results obtained with both procedures were compared with those obtained by adding the two reagents sequentially (procedure A). Both test sera and mixtures were incubated for 2 h at 37 C. Although the three-step procedure gave the highest S/N ratios, the twostep procedure appeared to discriminate adequately between low-positive and negative IgM results, while procedure C was somewhat superior to procedure B (Table 2). Consequently, rubella antigen and labeled monoclonal antibody were used for routine testing in a mixture prepared just before use. As to the incubation times, it was found that while the reaction for IgM capture from serum was rapid, reaching the equilibrium condition within 1 h, the reaction of captured virus-specific IgM with the antigen-labeled monoclonal antibody complex was slow and required several hours to reach equilibrium (Fig. 1). However, since a satisfactory S/N ratio (>3.0) was obtained with the 4-AU standard serum after 1-h incubation of the mixture, it was decided to use two incubation times of 1 h each to make the test more rapid. Specificity. The specificity of the assay was checked by testing 371 sera from 359 individuals (Table 3, groups 5 through 8), of which 66 (17.8%) could potentially interfere with the assay (group 8). The same sera had previously been tested by indirect ELISA on IgM serum fractions, and seven (one RF-positive, two heterophil antibody-positive, and four HCMV IgM-positive sera) were found to give an OD higher than the cutoff value (Table 4). In the capture ELISA, all but two of the same seven sera gave positive results with the 2-AU cutoff; only two sera, from a single patient, containing IgM to HCMV were positive with the 4-AU cutoff, and no serum was positive with the 8-AU standard. To verify whether the two sera that were still positive with the 4-AU TABLE 2. Comparison of three different procedures for the performance of the capture ELISA Rubella IgM S/N ratio by procedures: standard serum (AU) A B C a Procedure A, Three subsequent incubation steps, each of 2 h at 37 C, for test serum, antigen, and labeled monoclonal antibody. Procedure B, Two subsequent incubation steps, each of 2 h at 37 C, for test serum and a mixture of the antigen plus labeled monoclonal antibody, prepared 2 h before use. Procedure C, Two subsequent incubation steps, each of 2 h at 37 C, for test serum and the mixture of antigen plus labeled monoclonal antibody, prepared just before use. standard cutoff contained true rubella-specific IgM, they were repeatedly tested in the capture assay with the following modifications: (i) affinity-purified anti-human IgM; (ii) different lots of antigen, as well as purified rubella virus; (iii) no antigen; (iv) different labeled monoclonal antibodies to viral hemagglutinin; (v) sequential or simultaneous addition of viral antigen and labeled monoclonal antibody; and (vi) IgM serum fractions or protein A-Sepharose IgG-depleted sera (12). Only when the sera were tested in the capture system in the absence of rubella antigen did IgM positivity disappear. It must be noted that these two sera were drawn from the same patient (presenting fever, leukocytosis, mild hepatitis, and no rash) during the convalescence of a primary HCMV infection and contained high levels of HCMVspecific IgM, as well as antinuclear antibody of the IgM class. As a conclusion, the two sera were classified as rubella IgM positive. Thus, the specificity of the indirect ELISA was 98.6% (364 of 369 sera detected as negative), and the specificity of the capture ELISA was 100% (all 369 sera detected as negative). On the basis of the results reported above, it was decided to select as a final cutoff serum the standard containing 4 AU of IgM antibody. In addition, as a high rubella IgM-positive control, a standard serum containing 20 AU and giving an S/N ratio consistently over 5.0 was selected. Sensitivity. Sensitivity of the assay was tested on the o00 æoo hours INCUBATION OF ANTIGEN PLUS LABELED McAb MIXTURE FIG. 1. Kinetics of the second incubation step of the capture ELISA, with standard sera containing different concentrations of rubella-specific IgM antibody expressed as AU and the negative standard serum, using a mixture of rubella antigen plus peroxidaselabeled monoclonal antibody (McAb) to viral hemagglutinin.

4 1036 GERNA ET AL. J. CLIN. MICROBIOL. TABLE 3. Sensitivity and specificity of indirect and capture ELISA methods for rubella IgM determination No. of sera No. of sera positive by No. of sera with Patient group tested/no. of ELISA/no. of patients discordant resultsa patients Indirectb Capturec Indirect Capture 1, Patients with acute rubella 129/59 92/59 86/ , Patients with acute rubellae 39/25 35/23 34/ , Newbornsf 8/4 5/2 5/ , Rubella vaccinees 15/4 4/2 3/ , Rubella IgG negative 140/ , Rubella IgG positives 112/ , Newborns 53/53 0 O O O 8, Others RF positive 18/18 1/ Heterophile antibody positive 15/12 2/ Convalescent from primary HCMV 19/10 4h/3 2h/l 2 0 infection Convalescent from other viral infections' 14/ a Positive only by the indicated ELISA. b Cutoif value, 0.30 OD. c Cutoff value, OD given by the 4-AU control serum. d Virological diagnosis. e Clinical diagnosis. f From mothers with acute rubella infection during pregnancy. g Patients with no recent contact with rubella virus. h Two sequential serum samples from the same patient shown to be true rubella IgM positive. i Herpes simplex, varicella-zoster, measles, and mumps virus. following: (i) 129 acute- and convalescent-phase sera from 59 patients with acute rubella, virologically diagnosed on the basis of seroconversion, virus isolation, or both; (ii) 39 sera from 25 patients with clinically diagnosed acute rubella; (iii) eight sera from four newborns of mothers with rubella infection virologically diagnosed during pregnancy; and (iv) 15 sera from four vaccinees (Table 3, groups 1 through 4). Final test results were expressed by an IgM index, which was determined by dividing the OD of the test serum by the OD of the cutoff standard. Thus, all positive sera showed an IgM index of >1.0. Among the 84 patients with acute rubella examined, 75 showed at least one serum sample (taken between 4 and 55 days after onset) positive for rubella IgM antibody by both capture and indirect ELISA. Among eight of the remaining nine patients, for whom the onset of rubella could not be dated, seven showed IgM antibody by both methods, and in one, lacking IgM, seroconversion was observed in paired sera taken 4 months apart. In the last patient, lack of IgM antibody was due to the fact that the only available serum was drawn 2 days after onset of TABLE 4. Rubella IgM determination on seven selected sera potentially interfering with test results Rubella IgM antibody results Capture ELISA IgM Serum IgM. index with standard no. antibody to: OD by indirect serumb: 2 AU 4 AU 8 AU Fr 12 IgG (RF) EBVc Il EBV HCMV HCMV Il HCMV III HCMV a IgM serum fractions tested at 1:50 dilution; OD cutoff value = b IgM index obtained by dividing OD of test serum by OD of the relevant standard serum. C EBV, Epstein-Barr virus. infection, which was diagnosed by virus isolation. Among the four vaccinees tested, IgM antibody was detected in one only by capture ELISA and in other two by both methods, and one case was missed by both tests. In the group of four newborns from mothers with acute rubella during pregnancy, two showed presence of IgM antibody with both methods (virus was isolated from one patient), and the other two lacked IgM antibody concomitantly with negative virus isolation (Table 3). On the whole, of the 191 tested sera potentially containing rubella-specific IgM, 136 (71.2%) turned out to be positive by indirect ELISA, whereas 128 (67.0%) were positive by capture ELISA with the 4-AU cutoff serum (Table 5) (143 would have been positive with the 2-AU cutoff serum). Discordant results for patient groups 1 through 4 (Table 3) concerned 16 sera from 15 patients, of which 12 were positive only by indirect ELISA and 4 were positive only by capture ELISA. Thus, the total number of IgM-positive sera was 140 (124 by both assays, 12 only by indirect ELISA, and 4 only by capture ELISA), with a sensitivity of 97.1% for indirect ELISA and 91.4% for capture ELISA. Discordant sera were all drawn within the first 2 days or at least 40 days after onset of symptoms or vaccination. Similarly, all negative sera were drawn during the first 3 days or at least 50 days after onset. Among the acute-phase specimens, taken before day 4 after rash onset, 18 of 33 were IgM negative. Finally, rubella-specific IgM was detected in all serum specimens collected between 4 and 55 days after the onset. In conclusion, the overall agreement TABLE 5. Capture ELISA Comparative results of indirect and capture ELISA' No. of sera by indirect ELISA Negative Positive Total Negative Positive Total a Results of rubella IgM determination by indirect ELISA on IgM serum fractions and capture ELISA using labeled monoclonal antibodies on 191 sera from 92 patients with recent contact with rubella virus.

5 VOL. 25, 1987 MONOCLONAL ANTIBODY-CAPTURE ELISA FOR RUBELLA IgM 1037 between the two assays was 96.2%, with a concordance of positive and negative results in 541 of the 562 sera tested. The IgM index provided a rough indication of the amount of virus-specific IgM present in a serum sample. Determined on 146 sera from 75 patients with acute rubella at different times from onset, the IgM index was negative at the beginning of the exanthem and then became rapidly high (>4.0) at 7 days, persisting at this level until 21 days (Fig. 2). Subsequently, the index showed intermediate values at 45 days and borderfine values at 60 days after onset, becoming negative afterwards. DISCUSSION In developing an immunoassay for detection of virusspecific IgM, as we are for rubella, specificity must be considered a priority over sensitivity, since results of specific IgM determination are often decisive in advising or excluding an abortion. In some cases, rash is not clinically evident and seroconversion is not demonstrable, since the first serum sample is drawn late and already contains a high IgG antibody level. In such patients, the only tool now available for diagnosing acute rubella is IgM determination, often on a single serum sample. In evaluating the specificity of our assay, we selected a group of 371 human sera from 359 individuals who had not had any recent contact with rubella virus (of these, 66 were sera selected from 54 patients with RF or IgM to different viruses). Similarly, for sensitivity, a group of 191 sera from 92 patients having recent contacts of different types with rubella virus were tested. Thus, in the group of sera selected for specificity, any rubella IgM antibody-positive serum was considered potentially nonspecific or false. In parallel, in the group of sera selected for sensitivity, any positive IgM result was considered potentially true or specific. In our indirect ELISA assay, using highly purified rubella virus bound to the solid phase and IgM serum fractions, 7 of the 371 sera (1.9%) tested for specificity showed positive results. On the other hand, 136 of the 191 sera (71.2%) taken from patients who had contact with rubella virus were positive for IgM. With the capture assay using the 2-AU standard as a cutoff level, 143 of the 191 sera tested from patients with rubella resulted positive for virus-specific IgM. Of these, 15 became negative when the 4-AU serum was employed as a cutoff. However, this choice was suggested by the need of keeping in the negative zone one RF-positive and two heterophil antibody-positive sera giving presumptive false-positive results when the 2-AU standard serum was used as a cutoff. Nevertheless, in the capture assay with the cutoff at 4 AU, two sera from a single patient with primary HCMV infection, IgM-class antinuclear antibody, and no rubellalike rash, still gave positive results (IgM index slightly >1.0). These sera were also positive in the indirect ELISA on IgM serum fractions. The only way of obtaining negative IgM results in the capture assay with these two sera was to avoid the use of the antigen in the test. Thus, the rubella-specific IgM reactivity appeared to be mediated by rubella antigen, crude or purified. At this point, the IgM positivity could be interpreted as nonspecific, if mediated by an IgM RF reactive with the labeled antibody or with rubella-specific IgG present in both sera, to which viral antigen would attach and then bind the detector antibody (6). However, RF was not detected in the two sera, even by a very sensitive ELISA (26), and direct reactivity of sera with conjugate was excluded. In addition, even in the presence of very low levels of RF, the amount of rubella-specific IgG w Cà 3.0 z «2.0 -a oe l.o- i <1.0 cm f'. Ne NZ DAYS AFTER ONSET FIG. 2. Mean values (± standard deviation) of the rubella IgM index in 146 sera drawn from 75 patients with acute rubella infection at different times after onset. was not high enough to justify a possible interference with the assay by this mechanism (14). Furthermore, antinuclear antibody, which was actually present in both sera, has not been reported to interfere with the capture assay (10), even though antinuclear antibody-positive sera were recently shown to display a high nonspecific binding of immunoglobulins to high-binding-capacity solid phases (4). Thus, we considered that reactivity of the two sera with rubella antigen was specific and due either to the presence of rubella-specific IgM asconsequence a of a true rubella infection or to polyclonal activation of B lymphocytes in a patient with acute HCMV infection, a mechanism already suggested for patients with infectiousmononucleosis (16). The sensitivity of indirect ELISA in detecting rubellaspecific IgM in 191 sera from 92 patients having recent contact with rubella virus (84 patients with acute rubella, four vaccinees, and four newborns from mothers with rubella during pregnancy) was slightly superior to that of capture ELISA (136 versus 128 IgM-positive sera, respectively). However, 12 sera were found positive for IgM only by indirect ELISA, and 4 were positive only by capture ELISA. The specificity of the 16 discordant results was shown by the fact that the relevant sera were taken during the first 2 days of disease or at least 40 days after onset or vaccination. Of the 92 patients tested, 86 were detected as IgM positive by indirect ELISA, and 87 were positive by capture ELISA (in a single vaccinee IgM was detected only by capture ELISA). The five IgM-negative patients were: (i) two from the group with acute rubella, from whom a single serum sample was taken too early in one case and the convalescent-phase serum was taken too late in the other; (ii) two newborns that did not acquire rubella from the mother during pregnancy; and (iii) a vaccinee with IgM antibody undetectable by both methods. In capture ELISA, the IgM index provided a good indication of the time course of specific IgM antibody during the convalescent phase of le

6 1038 GERNA ET AL. rubella infection, showing a peak at 2 to 3 weeks after onset and disappearing after 60 days. In indirect ELISA, the IgM antibody titer grossly paralleled the IgM index. Although several capture ELISAs have been described in recent years (10, 11, 17), improvements achieved in the assay by using a labeled monoclonal antibody to the viral hemagglutinin are unequivocal, as already reported (5, 24). Previous false-positive results due to use of labeled polyclonal antibody in RF-positive sera (11) are avoided. In addition, monoclonal antibody allows the simultaneous incubation of antigen and conjugate, thus shortening the time required for test performance. Two other points must be mentioned in discussing lowpositive or equivocal rubella IgM results. First, in a limited number of patients, rubella IgM may last several months or years, usually at a relatively low titer (2). Secondly, all low-positive IgM results must be interpreted with caution and evaluated in association with all anamnestic and clinical data (9). Also, capture assays for IgM have already been reported to give a fair number of low-positive results of doubtful meaning (5). In conclusion, the data here presented show that the new capture ELISA for rubella IgM antibody determination displays a high level of specificity, which has been achieved at the cost of a slightly lower level of sensitivity. It must be emphasized that no test for rubella IgM is devoid of problems and that all IgM results must be considered with results of other tests and a full clinical history. However, because of its higher specificity, as well as no need for serum treatment and the shorter time required for test performance, the capture ELISA appears highly preferable to the indirect ELISA for rubella virus-specific IgM detection. ACKNOWLEDGMENTS This work was partially supported by the IRCCS Policlinico S.Matteo, Pavia, Ricerca Finalizzata "Tutela Materno-Infantile" 1984; by the Consiglio Nazionale delle Ricerche, Roma, grant no ; and by Progetto di Ricerca Tecnobiomedica no We thank Maria Mastrosimone for help in revision of the English. LITERATURE CITED 1. Best, J. M., J. E. Banatvala, and D. Watson Serum IgM and IgG responses in postnatally acquired rubella. Lancet ii: Best, J. M., S. J. Palmer, P. Morgan-Capner, and J. Hodgson A comparison of Rubazyme-M and MACRIA for the detection of rubella-specific IgM. J. Virol. Methods 8: Bonfanti, C., D. Meurman, and P. Halonen Detection of specific immunoglobulin M antibody to rubella virus by use of an enzyme-labeled antigen. J. Clin. Microbiol. 21: Cafruny, W. A., D. P. Heruth, M. J. Jaqua, and P. G. W. 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