Single Serum Dilution Research Articles

Abstract 3249: Simple, rapid, and robust bioluminescent cell-based assay for detecting neutralizing antibodies against AAV in serum

Abstract Adeno-associated viruses (AAV) are prevalent gene therapy vectors, delivering therapeutic transgenes to treat various diseases including cancer and rare monogenic diseases. Individuals with pre-existing immunity to AAVs are less likely to benefit from AAV-based therapies due to both, a reduction in cellular AAV uptake and activation of a cytotoxic T-cell response to transduced cells, decreasing efficacy or changing the safety profile. Measurement of serum neutralizing antibodies (NAbs) against AAV is essential to determine treatment eligibility and to identify clinical trial candidates. Here we introduce a cell-based assay using NanoLuc® luciferase to detect NAbs in serum samples. The superior brightness of NanoLuc® luciferase in our AAV reporter system results in the need of low concentrations of AAV and a short 24h assay time. The minimal AAV concentration in our assay aids in detecting low NAb titers in serum samples, reducing false negatives and enhancing decision-making accuracy. Testing 60 human serum samples against a range of AAV serotypes, we found that a large fraction of the population had pre-existing NAbs to at least one serotype, and of the pre-exposed individuals a majority displayed NAbs against multiple AAV serotypes. Interestingly, fewer samples were seropositive for AAV-DJ than for either parental serotype (AAV2, AAV8 or AAV9), highlighting the assays utility in the development of engineered capsids. Furthermore, the broad dynamic range of our assay facilitates serum sample categorization into four NAb tiers: negative, low, medium, and high using a single serum dilution. Neutralizing titers (ND50) which was determined by serial dilution of the serum samples strongly agreed with the assigned NAb tiers. Our study with 40 naïve mice further validated the assay's robustness, showing a clear correlation between NAb levels and injected AAV viral loads. Taken together, this rapid, highly sensitive, and reliable assay enabled by NanoLuc® AAV reporter technology precisely measures NAbs against AAVs in both human and animal serum. Further validation of the assay is required for future use in a clinical setting. Citation Format: Morten Seirup, Kirsten Wingate, Jeff Nelson, Kara Machleidt, Vanessa Ott, Trish Hoang, Marjeta Urh. Simple, rapid, and robust bioluminescent cell-based assay for detecting neutralizing antibodies against AAV in serum [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 3249.

Development of recombinant capsid protein-based single serum dilution ELISA for sero-detection of porcine circovirus type 2 infection in pigs.

Porcine Circovirus 2 (PCV2) is the major causative agent of postweaning multisystemic wasting syndrome (PMWS) of swine and is one of the reasons for severe economic loss in swine industry. In India, there is a considerable prevalence rate of PCV2 infection in pig population, PCV2d being the most prominent genotype. Proper sero-diagnosis and sero-surveillance of the disease is formulated as an effective control measure. In this study, a recombinant capsid protein-based single serum dilution indirect ELISA was developed for determination of antibody titre of the infected pigs. The capsid protein (Cap) of PCV2d was produced in Saccharomyces cerevisiae cells and the capsid protein was purified by affinity chromatography. This recombinant protein was used as a coating antigen to develop a cost effective, highly sensitive and specific single serum dilution ELISA. The in-house developed ELISA was optimized to be used in a 1:200 single serum dilution. The developed ELISA along with a commercial ELISA kit were compared with a sensitive immuno-peroxidase assay (IPMA) by receiver-operating characteristics (ROC) test. Our results showed that the developed single serum dilution ELISA had a higher sensitivity and specificity in comparison to the commercial ELISA. The area under the ROC curve (AUC) also confirmed that the developed ELISA had a better overall diagnostic performance in comparison to the commercial ELISA kit.

Titers of antibodies against ancestral SARS-CoV-2 correlate with levels of neutralizing antibodies to multiple variants

Diagnostic assays currently used to monitor the efficacy of COVID-19 vaccines measure levels of antibodies to the receptor-binding domain of ancestral SARS-CoV-2 (RBDwt). However, the predictive value for protection against new variants of concern (VOCs) has not been firmly established. Here, we used bead-based arrays and flow cytometry to measure binding of antibodies to spike proteins and receptor-binding domains (RBDs) from VOCs in 12,000 serum samples. Effects of sera on RBD-ACE2 interactions were measured as a proxy for neutralizing antibodies. The samples were obtained from healthy individuals or patients on immunosuppressive therapy who had received two to four doses of COVID-19 vaccines and from COVID-19 convalescents. The results show that anti-RBDwt titers correlate with the levels of binding- and neutralizing antibodies against the Alpha, Beta, Gamma, Delta, Epsilon and Omicron variants. The benefit of multiplexed analysis lies in the ability to measure a wide range of anti-RBD titers using a single dilution of serum for each assay. The reactivity patterns also yield an internal reference for neutralizing activity and binding antibody units per milliliter (BAU/ml). Results obtained with sera from vaccinated healthy individuals and patients confirmed and extended results from previous studies on time-dependent waning of antibody levels and effects of immunosuppressive agents. We conclude that anti-RBDwt titers correlate with levels of neutralizing antibodies against VOCs and propose that our method may be implemented to enhance the precision and throughput of immunomonitoring.

An eight-plex immunoassay for Group A streptococcus serology and vaccine development

Group A Streptococcus (GAS) is a major human pathogen responsible for superficial infections through to life-threatening invasive disease and the autoimmune sequelae acute rheumatic fever (ARF). Despite a significant global economic and health burden, there is no licensed vaccine available to prevent GAS disease. Several pre-clinical vaccines that target conserved GAS antigens are in development. Assays that measure antigen-specific antibodies are essential for vaccine research. The aim of this study was to develop a multiplex beadbased immunoassay that can detect and quantify antibody responses to multiple GAS antigen targets in small volume blood samples. This builds on our existing triplex assay comprised of antigens used in clinical serology for the diagnosis of ARF (SLO, DNase B and SpnA). Five additional conserved putative GAS vaccine antigens (Spy0843, SCPA, SpyCEP, SpyAD and the Group A carbohydrate), were coupled to spectrally unique beads to form an 8-plex antigen panel. After optimisation of the assay protocol, standard curves were generated, and assessments of assay specificity, precision and reproducibility were conducted. A broad range of antibody (IgG) titres were able to be quickly and accurately quantified from a single serum dilution. Assay utility was assessed using a panel of 62 clinical samples including serum from adults with GAS bacteraemia and children with ARF. Circulating IgG to all eight antigens was elevated in patients with GAS disease (n = 23) compared to age-matched controls (n = 39) (P < 0.05). The feasibility of using dried blood samples to quantify antigen-specific IgG was also demonstrated. In summary, a robust and reproducible 8-plex assay has been developed that simultaneously quantifies IgG antibodies to GAS vaccine and diagnostic antigens.

Predicting the presence and titre of rabies virus-neutralizing antibodies from low-volume serum samples in low-containment facilities.

Serology is a core component of the surveillance and management of viral zoonoses. Virus neutralization tests are a gold standard serological diagnostic, but requirements for large volumes of serum and high biosafety containment can limit widespread use. Here, focusing on Rabies lyssavirus, a globally important zoonosis, we developed a pseudotype micro-neutralization rapid fluorescent focus inhibition test (pmRFFIT) that overcomes these limitations. Specifically, we adapted an existing micro-neutralization test to use a green fluorescent protein-tagged murine leukaemia virus pseudotype in lieu of pathogenic rabies virus, reducing the need for specialized reagents for antigen detection and enabling use in low-containment laboratories. We further used statistical models to generate rapid, quantitative predictions of the probability and titre of rabies virus-neutralizing antibodies from microscopic imaging of neutralization outcomes. Using 47 serum samples from domestic dogs with neutralizing antibody titres estimated using the fluorescent antibody virus neutralization test (FAVN), pmRFFIT showed moderate sensitivity (78.79%) and high specificity (84.62%). Despite small conflicts, titre predictions were correlated across tests repeated on different dates both for dog samples (r=0.93) and in a second data set of sera from wild common vampire bats (r=0.72, N=41), indicating repeatability. Our test uses a starting volume of 3.5 µl of serum, estimates titres from a single dilution of serum rather than requiring multiple dilutions and end point titration, and may be adapted to target neutralizing antibodies against alternative lyssavirus species. The pmRFFIT enables high-throughput detection of rabies virus-neutralizing antibodies in low-biocontainment settings and is suited to studies in wild or captive animals where large serum volumes cannot be obtained.

Glycoprotein D peptide-based diagnostic approach for the detection of avian infectious laryngotracheitis antibodies

ABSTRACTInfectious laryngotracheitis (ILT) is a highly contagious respiratory disease of chickens, pheasants, and peafowl. It is caused by the alpha herpesvirus, infectious laryngotracheitis virus (ILTV). Glycoprotein D (gD) of ILTV is immunogenic and helps in its binding to the susceptible host cell receptor. In the present study, a recombinant gD protein was expressed in a prokaryotic system to develop a single serum dilution ELISA. In addition, two immunogenic peptides, corresponding to regions 77–89 and 317–328, were identified in gD protein. The peptides were synthesized using solid-phase peptide synthesis, purified using reversed-phase HPLC, and characterized using mass spectrometry. The peptides displayed a good titre and were found to be promising antigens to coat the ELISA plate to detect the ILTV antibodies in the serum sample. The developed ELISA showed 96.9% sensitivity, 87.5% specificity, and 95.3% accuracy as compared to OIE referenced standard indirect ILTV ELISA (whole viral coated). The assay may not differentiate vaccinated from infected birds when the flocks are administered with live attenuated vaccines. However, the assay could be useful to detect the disease condition in birds vaccinated with recombinant vaccine expressing glycoproteins other than gD. The developed ILTV single serum dilution ELISA could be an alternative to the existing diagnostics for the detection of ILTV antibodies.

Concanavalin A- Sandwich-ELISA for the detection of antibodies against Newcastle disease virus (NDV) in chicken

Concanavalin A (Con A), a lectin interacts with carbohydrate moieties of viruses and provide stable and sensitive detection when used as a capture agent. Indirect ELISA methods need purified Newcastle disease virus (NDV) or recombinant antigens for adsorption, whereas use of Con A as capture agent will enable the use of non-purified and non-concentrated virus as antigen replacing costly and time-consuming virus purification step. Con A based sandwich ELISA with non-purified NDV whole virus antigen with single serum dilution format has been developed in this study. The optimum concentrations of the capture agent, Con A and non-purified antigen preparations were determined by checker-board titration. Briefly, microplates were coated with predetermined optimum concentration of ConA (0.5 mg/ml; 50µg per well) and incubated for 18h at 4°C. After washing, allantoic fluid with Newcastle disease virus (NDV) LaSota (HA titre, 210) at a constant predetermined dilution (1:1; 50µl) was coated and incubated for 45 min at 37°C, followed by blocking with 2 % bovine serum albumin for 45 min at 37°C. The antigen coated plates were used in the detection of antibody titre against NDV in serum samples at single serum dilution of 1: 500. Then, wells were added with goat anti-chicken IgG horseradish peroxidase conjugate and incubated for 1h at 37°C, followed by addition of TMB substrate and the plates were read spectrophotometrically at 450 nm. ELISA antibody titres were determined by standard serial dilution of positive sera and endpoints were calculated by a subtraction method. By using positive negative threshold curve (PNT), intercept and slope of the standard curve were calculated. Total of 271 random chicken serum samples were analyzed for antibodies against NDV by Haemagglutination inhibition assay (HI), indirect ELISA and compared with the Con A- S- ELISA developed in this study. The Con A-S-ELISA showed a high coefficient of correlation (r=0.85, n=271, P less than 0.01) and an agreement of ê=0.99 with the commercially available Indirect-ELISA. The relative sensitivity and specificity were 98% and 85% respectively in comparison to HI test. Hence, the Con A-S-ELISA is a simple, easy and effective for monitoring serum antibody levels against NDV.

A single, continuous metric to define tiered serum neutralization potency against HIV.

HIV-1 Envelope (Env) variants are grouped into tiers by their neutralization-sensitivity phenotype. This helped to recognize that tier 1 neutralization responses can be elicited readily, but do not protect against new infections. Tier 3 viruses are the least sensitive to neutralization. Because most circulating viruses are tier 2, vaccines that elicit neutralization responses against them are needed. While tier classification is widely used for viruses, a way to rate serum or antibody neutralization responses in comparable terms is needed. Logistic regression of neutralization outcomes summarizes serum or antibody potency on a continuous, tier-like scale. It also tests significance of the neutralization score, to indicate cases where serum response does not depend on virus tiers. The method can standardize results from different virus panels, and could lead to high-throughput assays, which evaluate a single serum dilution, rather than a dilution series, for more efficient use of limited resources to screen samples from vaccinees.

Comparison of different enzyme-linked immunosorbent assay methods for avidity determination of antiphospholipid antibodies.

Avidity of antiphospholipid antibodies may be clinically useful as a valuable additional characteristic. The aim of this study was to compare several ELISA modifications with different chaotropic agents and calculation of avidity indices for the determination of anticardiolipin antibody (aCL) avidity. We examined 28 serum samples with positive IgG aCL by adapted ELISA using various concentrations of urea and sodium chloride as chaotropic agents and different dilution of sera. We tested these conditions of ELISA-a single diluted serum sample with fixed concentration of a chaotrope and a serially diluted serum in the constant concentration of a chaotropic agent. We demonstrated that ELISA method for avidity determination based on a single dilution of serum in the presence of fixed concentration of chaotrope is convenient for determination of IgG aCL antibody avidity. Concentrations 6 and 8mol/L of urea or 1 and 2mol/L of NaCl were suitable for sufficient dissociation of immune complexes during ELISA procedure. This way was in good agreement with more demanding procedures. Both urea and sodium chloride may be used as chaotropic agents. Reference values of avidity indices essential for interpretation of patients' results must be established individually for distinct assay conditions.

A microplate assay to measure classical and alternative complement activity.

We developed and validated a kinetic microplate hemolytic assay (HA) to quantify classical and alternative complement activity in a single dilution of human plasma or serum. The assay is based on monitoring hemolysis of sensitized sheep (or uncoated rabbit) red blood cells by means of a 96-well microplate reader. The activity of the calibrator was evaluated by reference to 200 healthy adults. The conversion of 50% hemolysis time into a percentage of activity was obtained using a calibration curve plotted daily. The linearity of the assay as well as interference (by hemolysis, bilrubinemia and lipemia) was assessed for classical pathway (CP). The within-day and the between-day precision was satisfactory regarding the performance of commercially available liposome immunoassay (LIA) and ELISA. Patients with hereditary or acquired complement deficiencies were detected (activity was measured <30%). We also provided a reference range obtained from 200 blood donors. The agreement of CP evaluated on samples from 48 patients was 94% with LIA and 87.5% with ELISA. The sensitivity of our assay was better than that of LIA, and the cost was lower than either LIA or ELISA. In addition, this assay was less time consuming than previously reported HAs. This assay allows the simultaneous measurement of 36 samples in duplicate per run of a 96-well plate. The use of a daily calibration curve allows standardization of the method and leads to good reproducibility. The same technique was also adapted for the quantification of alternative pathway (AP) activity.

Development of single dilution immunoassay to detect E2 protein specific classical swine fever virus antibody

Classical swine fever virus (CSFV) is the causative agent of a highly contagious disease in swine. The disease is endemic in different parts of the world and vaccination is the only way to protect pigs from CSFV infection. The virus surface protein E2 is the major immunogenic protein eliciting protective immunity against CSFV infection in swine. The whole virus antigen cannot differentiate CSFV from other pestiviruses as it cross reacts with border disease and bovine viral diarrhoea viruses. Commercial available ELISA is based on the whole CSFV particle and can lead to false positive results. Moreover, the available commercial ELISA is not cost effective. In the present study, a recombinant E2 protein based single serum dilution ELISA was developed which showed enhanced sensitivity, specificity and accuracy as compared to commercial CSFV detection ELISA. The recombinant E2 protein based ELISA could be an alternate to existing diagnostics against CSFV infection in pigs.

Recombinant phosphoprotein based single serum dilution ELISA for rapid serological detection of Newcastle disease virus

Newcastle disease virus (NDV) is the causative agent of a highly contagious disease in avian species. All strains of NDV belong to avian paramyxovirus serotype-1. The disease is endemic in different parts of the world and vaccination is the only way to protect birds from NDV infection. The virus non-structural phosphoprotein (P) is the second most abundant protein and a major modulator of viral replication. Although P protein shows lesser evolutionary divergence among NDV isolates, it is known to be highly divergent among different avian paramyxovirus serotypes. In the present study, a recombinant P protein based single serum dilution ELISA was developed which showed better sensitivity, specificity and accuracy as compared to conventional methods for NDV detection. The recombinant P protein based ELISA could be an alternative to existing diagnostics against NDV infection in chickens.

Recombinant nucleocapsid protein based single serum dilution ELISA for the detection of antibodies to infectious bronchitis virus in poultry

Avian infectious bronchitis is ubiquitous and highly contagious disease of poultry, with profound effect on commercial poultry production. For effective control of infectious bronchitis virus (IBV), quick and specific diagnosis is of utmost importance. In this study, the virus was isolated from clinical samples from India and the full length nucleocapsid (N) gene was amplified, cloned and expressed in a prokaryotic system. The purified recombinant N protein based single serum dilution enzyme linked immunosorbent assay (ELISA) was developed for IBV to measure specific antibody in the sera of chickens. A total of 310 chicken sera samples were tested using the commercial IDEXX kit along with the assay developed. A linear correlation was obtained between predicted antibody titres at a single working dilution of 1:100 and the corresponding serum titres observed as determined by the standard serial dilution method. Regression analysis was used to construct a standard curve from which an equation was derived which confirmed their correlation. The developed equation was then used to extrapolate predicated ELISA antibody titer from corrected absorbance readings of the single working dilution. The assay proved to be specific (95.8%) and sensitive (96.8%) when compared to the commercial IDEXX ELISA test.

Recombinant hexon antigen based single serum dilution ELISA for rapid serological profiling against fowl adenovirus-4 causing hydropericardium syndrome in chickens

A recombinant hexon antigen based single serum dilution enzyme linked immunosorbent assay (ELISA) was developed to measure the specific antibody in sera of chickens against Fowl adenovirus-4 (FAdV) causing Hydropericardium syndrome (HPS). An immunodominant partial hexon gene of 737bp was cloned into pRSET vector and expressed in Escherichia coli strain BL21 DE3 pLys S. Expression was tested by Western Blot test. The purified recombinant protein antigen was used in coating ELISA plate for FAdV-4 serology. A linear relationship was found between the predicted antibody titres at a single working dilution of 1:100 and the corresponding observed serum titres as determined by the standard serial dilution method. Regression analysis was used to determine a standard curve from which an equation was derived that allowed the demonstration of this correlation. The equation was then used to convert the corrected absorbance readings of the single working dilution directly into the predicted ELISA antibody titres. The assay proved to be sensitive, specific and accurate as compared to Q-AGID test. Recombinant antigen was also used in Dot ELISA. In an experimental vaccination of broiler chicken at 10 days old age, the geometric mean (GM) antibody titres as measured by ELISA ranged from 5.006±0.11log10 to 4.526±0.04log10 and by Dot ELISA titre were from 2.240±0.08log10 to 0.180±0.04log10 during 5th–8th weeks of age, results were compared with Q-AGID results. Field samples were collected randomly from breeder flocks, found to have antibody titre by both ELISA and Dot ELISA at 10th and only 75% samples were positive at 14th weeks of age. After revaccination at 16th weeks of age, all sera samples were found have considerably high antibody titre on 24th week but all samples were negative at 32nd weeks. Advantages of recombinant hexon antigen based ELISA and Dot ELISA in FAdV-4 serology have been discussed.

A duplex recombinant viral nucleoprotein microbead immunoassay for simultaneous detection of seroresponses to human respiratory syncytial virus and metapneumovirus infections

Serologic diagnosis of human respiratory syncytial virus (hRSV) and human metapneumovirus (hMPV) infections has been shown to complement virus detection methods in epidemiologic studies. Enzyme immunoassays (EIAs) using cultured virus lysate antigens are often used to diagnose infection by demonstration of a ≥4-fold rises in antibody titer between acute and convalescent serum pairs. In this study, hRSV and hMPV nucleocapsid (recN) proteins were expressed in a baculovirus system and their performance compared with virus culture lysate antigen in EIAs using paired serum specimens collected from symptomatic children. The recN proteins were also used to develop a duplex assay based on the Luminex microbead-based suspension array technology, where diagnostic rises in antibody levels could be determined simultaneously at a single serum dilution. Antibody levels measured by the recN and viral lysate EIAs correlated moderately (hRSV, r2=0.72; hMPV, r2=0.76); the recN EIAs identified correctly 35 of 37 (94.6%) and 48 of 50 (96%) serum pairs showing diagnostic antibody rises by viral lysate EIAs. Purified recN proteins were then coupled to microbeads and serum pairs were tested at a single dilution on a Luminex MAGPIX® analyzer. The duplex recN assay identified correctly 33 of 39 (85%) and 41 of 47 (86.7%) serum pairs showing diagnostic rises to hRSV and hMPV, respectively. The recN assay permits simultaneous testing for acute hRSV and hMPV infections and offers a platform for expanded multiplexing of other respiratory virus assays.

Survey of Human Antibody Responses to Influenza Virus Matrix Protein 2 by Use of a Sensitive Flow Cytometric Method

Conventional influenza vaccines are designed to elicit antibodies to strainspecific antigens, leaving a public health gap when novel viruses break out unexpectedly. As examples, strain-matched vaccine became available too late in the pandemic of 2009 to protect against the fall wave, and drift viruses are sometimes divergent enough to cause vaccine failure (eg, A/Sydney in 1997). Available vaccines are also inadequate to protect against various zoonotic strains, including avian influenza A(H5N1), influenza A(H7N7), and most recently influenza A(H7N9). For this reason, there is much interest in developing vaccines based on conserved influenza virus features that can provide protection regardless of strain. These are usually designed for influenza A virus, but such vaccines can also be made for influenza B virus. Universal influenza vaccines can induce immune protection dependent upon antibody or T-cell responses or both, and the target antigens explored have included nucleoprotein, matrix proteins (M1 and M2), the hemagglutinin (HA) stem, polymerase PB1, and other antigens, as reviewed previously [1]. M2, the focus of Zhong et al in the current issue of the Journal, has long been known as a target of antibodies that reduce viral replication and spread [2, 3]. Vaccines based on many forms of M2 (fusion proteins, M2 multiple antigenic peptides, peptide conjugates, and M2 expressed from viral vectors) provide protective immunity in animals [4–10], and some have been tested in humans. A clinical trial of recombinant M2 shows that this antigen is immunogenic when administered with adjuvant [11]. Human anti-M2 antibodies induced by natural infection have been reported, but data are scanty. Black et al reported that 6 of 17 pairs of acute-phase and convalescent-phase serum specimens showed increased anti-M2 activity by enzyme-linked immunosorbent assay (ELISA), while 12 of 17 convalescent-phase serum specimens demonstrated some signal by Western blot [12]. In a study by Feng et al, an increase in anti-M2 activity was found for 11 of 24 such serum pairs [13]. In that study, of the antibodies detectable by assay on cell surface tetrameric M2, only a minority also recognized M2e peptide. Thus, the majority of antibodies appeared to be conformational [13]. These antibodies may be biologically very important, so assays on native M2 are needed. However, measuring antibodies to native M2 has presented technical problems, with high background encountered in cell surface ELISA. To overcome these problems, Zhong et al developed a flow cytometric assay (M2-FCA) using a panel of 293FT transfected cell lines (M2-293FT) stably expressing full-length tetrameric forms of M2 from various viral strains [14]. With this sensitive assay, some mouse antibodies recognize strain-specific epitopes and some see epitopes cross-reactive among viral strains [14]. The present article uses the assay to analyze collections of human sera. Transfection efficiency and expression levels are normalized through the use of a positive human serum pool as a control. Unlike most serological assays in which a titer reflects the dilution at which a defined end point is reached, the unit of M2 antibody is defined for a single (1:40) dilution of serum. This allows large numbers of sera to be analyzed within a single run, but does not measure titers or other antibody properties revealed by dilution series. Results for healthy donors of different ages showed that antibodies to M2 were found in a higher percentage of, and at higher levels, in adults aged ≥40 years, compared with younger donors. If anti-M2 antibodies were present, they usually recognized both seasonal and swine-origin M2. Received 5 December 2013; accepted 5 December 2013. Correspondence: Suzanne Epstein, PhD, OCTGT, CBER, FDA, 1401 Rockville Pk, HFM-730, Rockville, MD 20852-1448 (suzanne.epstein@fda.hhs.gov). The Journal of Infectious Diseases Published by Oxford University Press on behalf of the Infectious Diseases Society of America 2013. This work is written by (a) US Government employee(s) and is in the public domain in the US. DOI: 10.1093/infdis/jit813

Development of an Enzyme-Linked Immunosorbent Assay Using a Recombinant LigA Fragment Comprising Repeat Domains 4 to 7.5 as an Antigen for Diagnosis of Equine Leptospirosis

Leptospira immunoglobulin (Ig)-like (Lig) proteins are a novel family of surface-associated proteins in which the N-terminal 630 amino acids are conserved. In this study, we truncated the LigA conserved region into 7 fragments comprising the 1st to 3rd (LigACon1-3), 4th to 7.5th (LigACon4-7.5), 4th (LigACon4), 4.5th to 5.5th (LigACon4.5-5.5), 5.5th to 6.5th (LigACon5.5-6.5), 4th to 5th (LigACon4-5), and 6th to 7.5th (LigACon6-7.5) repeat domains. All 7 recombinant Lig proteins were screened using a slot-shaped dot blot assay for the diagnosis of equine leptospirosis. Our results showed that LigACon4-7.5 is the best candidate diagnostic antigen in a slot-shaped dot blot assay. LigACon4-7.5 was further evaluated as an indirect enzyme-linked immunosorbent assay (ELISA) antigen for the detection of Leptospira antibodies in equine sera. This assay was evaluated with equine sera (n = 60) that were microscopic agglutination test (MAT) negative and sera (n = 220) that were MAT positive to the 5 serovars that most commonly cause equine leptospirosis. The indirect ELISA results showed that at a single serum dilution of 1:250, the sensitivity and specificity of ELISA were 80.0% and 87.2%, respectively, compared to those of MAT. In conclusion, an indirect ELISA was developed utilizing a recombinant LigA fragment comprising the 4th to 7.5th repeat domain (LigACon4-7.5) as a diagnostic antigen for equine leptospirosis. This ELISA was found to be sensitive and specific, and it yielded results that concurred with those of the standard MAT.

Recombinant UL30 antigen-based single serum dilution ELISA for detection of duck viral enteritis

A recombinant UL30 antigen-based single serum dilution enzyme linked immunosorbent assay (ELISA) was developed to measure specific antibody in the sera of ducks against duck enteritis virus (DEV). The partial UL30 gene of DEV was cloned, expressed, purified and tested for its diagnostic use by designing a single serum dilution enzyme linked immuno-sorbent assay (ELISA). A total of 226 duck sera samples were tested using the assay. A linear relationship was found between the predicted antibody titres at a single working dilution of 1:100 and the corresponding serum titres observed as determined by the standard serial dilution method. Regression analysis was used to determine a standard curve from which an equation was derived which demonstrated this correlation. The equation was then used to convert the corrected absorbance readings of the single working dilution directly into the predicted ELISA antibody titres. The assay proved to be specific, sensitive and accurate as compared to the virus neutralization test with a specificity, sensitivity and accuracy being 96%, 95% and 95% respectively.

Pitfalls in the diagnosis of congenital rubella syndrome in the first trimester of pregnancy

A 29-year-old woman, gravida 3 para 0, was referred at 19 weeks of gestation because of a maternal rubella infection during the first trimester of pregnancy. She consulted at 5 weeks for a rash, which, at that time, was considered to be urticaria. Her serological status for rubella was unknown at that time, and as rubella infection was not considered, serological investigations were not conducted. At 6 weeks, toxoplasma and rubella testings were performed as recommended by the French High Health Authority [Haute Autorité de Santé (HAS)]. HAS only recommends IgG testing for routine Rubella screening and both IgG and IgM testings for toxoplasma.1 Her results were negative for rubella-specific IgG and positive for toxoplasma-specific IgG [104 IU/mL (cutoff: 10.5 IU/mL)] and IgM [5.23 (cutoff: 1), Access 2, Beckman Coulter, Fullerton, CA, USA]. Because these findings suggested a recent toxoplasma infection, spiramycine was started pending the results of the IgG avidity test. The avidity index was high, 0.48 (cutoff: 0.30; Vidas bioMérieux, Marcy-l'Étoile, France), thus making postconceptional toxoplasma seroconversion very unlikely, and spiramycine was stopped. As recommended by HAS, all rubella seronegative patients have a subsequent rubella testing at around 20 weeks. Her second serology performed at 16 weeks indicated a seroconversion leading to IgM testing: rubella-specific IgG, 181.6 IU/mL (cutoff: 10 IU/mL; Architect Abbott, Wiesbaden Germany) and rubella-specific IgM, 1.48 (cutoff: 1.2; Vidas bioMérieux). Retrospective analysis of the serum collected at 6 weeks indicated that rubella-specific IgM was already positive at that time (3.15). The IgG avidity index was measured using our in-house urea wash method. This method is based on adding 6 M urea as a denaturing agent to the washing buffer to prevent the binding of low-avidity antibody to the antigen. Results are expressed as a ratio of absorbance of a single serum dilution with and without denaturing agent. An avidity index of <30% is considered as very low, between 30% and 70% as low, between 70% and 90% as moderate, and >90% as high.2 The IgG avidity index was 78% (moderate) at 16 weeks. Together with the analysis of IgM kinetics on both samples,2 the primary infection was dated at around 5 weeks, coinciding with the rash described by the patient. The ultrasound at 16 weeks was unremarkable. Amniocentesis at 18 weeks detected rubella viral RNA by RT-nested-PCR analysis3 indicating congenital rubella infection. The ultrasound scan at 19 weeks revealed intrauterine growth restriction, together with asymmetric bilateral microphthalmia (below the 5th percentile),4 opacity of the right lens (Figures 1 and 2), and an isolated hyperechogenic mitral focus. Because these signs indicated symptomatic fetal rubella infection, the couple asked for termination of pregnancy. The male fetus weighed 190 g (below the 5th percentile for 20 weeks). The pathology examination confirmed bilateral microphthalmia and the fetal infection with biopsies from the liver and the brain being positive for rubella viral RNA.

A single serum dilution enzyme-linked immunosorbent assay for determining anti-human papillomavirus (HPV) antibody titres in humans immunised with prophylactic HPV vaccines

Two types of prophylactic human papillomavirus (HPV) vaccines are currently available. However, there is no simple monitoring system for assessing acquired immunity that can cope simultaneously with large numbers of serum samples. Approximately 30% of women with normal cytology are known to be seropositive for HPV types 16 and 18 because of the high prevalence of these HPV types. Therefore, to be useful the monitoring system has to discriminate clearly between vaccine recipients and other serology groups. However, there has never been any focus on developing a method to satisfy this condition. In this study, we developed a high-throughput single-serum-dilution enzyme-linked immunoassay (ELISA) system for determining anti-HPV antibody titres following vaccination. We optimised the conditions for each ELISA step to increase its accuracy and precision and to avoid the high background of non-specific reactions that is a major problem for serology assays. The new ELISA system has superior linearity, accuracy and reproducibility. Moreover, it clearly discriminated between antibody levels in vaccine recipients and those in other serology groups such as individuals with normal cervical cytology and those with cervical cancer. Therefore, this single-serum-dilution ELISA should be very useful for assessing the acquired immunity of HPV vaccine recipients.