Immunosorbent assay based on recombinant hemagglutinin protein produced in a high-efficiency mammalian expression system for surveillance of measles immunity

Saliva as an alternative specimen for detection of Schmallenberg virus-specific antibodies in bovines

Background

Schmallenberg virus (SBV), discovered in continental Europe in late 2011, causes mild clinical signs in adult ruminants, including diarrhoea and reduced milk yield. However, fetal infection can lead to severe malformation in newborn offspring. Enzyme-linked immunosorbent assays (ELISA) are commercially available for detection of SBV-specific antibodies in bovine sera and milk. Here we describe the development and evaluation of an indirect ELISA based on a yeast derived recombinant SBV nucleocapsid protein (N) for the detection of SBV-specific antibodies in bovine saliva. Development of a non-invasive test to detect antibodies in individual bovine saliva samples could potentially provide a test suitable for calves and adult cattle. The aim of this study was to investigate the agreement between the levels of antibodies (IgG) measured in milk and sera, and the level of antibodies (IgG and IgA) in saliva, in comparison with the antibody levels detected in sera and milk with commercially available test.

Results

Serum, milk and saliva samples from 58 cows were collected from three dairy herds in Lithuania and tested for the presence of SBV-specific antibodies. The presence of IgG antibodies was tested in parallel serum and milk samples, while the presence of IgA and IgG antibodies was tested in saliva samples. The presence of SBV-specific IgG and IgA in saliva was tested using an indirect ELISA based on a yeast-derived recombinant N protein. The presence of SBV-specific IgG in milk and sera was tested in parallel using a commercial recombinant protein based test. The sensitivities of the newly developed tests were as follows: 96 % for the IgG serum assay and 94 % for the IgG milk assay and 85 % and 98 % for IgG and IgA in saliva tests, when compared with data generated by a commercial IgG assay.

Conclusions

Data from testing the saliva IgG and IgA and also the milk and serum IgG with indirect SBV-specific ELISAs showed close agreement with the commercial serum and milk IgG assay data. The level of IgG in saliva was notably lower in comparison to IgA. The newly developed method exhibits the potential to serve as an easily transferable tool for epidemiological studies.

Electronic supplementary material

The online version of this article (doi:10.1186/s12917-015-0552-0) contains supplementary material, which is available to authorized users.

Profiling of measles-specific humoral immunity in individuals following two doses of MMR vaccine using proteome microarrays

INTRODUCTION

Comprehensive evaluation of measles-specific humoral immunity after vaccination is important for determining new and/or additional correlates of vaccine immunogenicity and efficacy.

METHODS

We used a novel proteome microarray technology and statistical modeling to identify factors and models associated with measles-specific functional protective immunity in 150 measles vaccine recipients representing the extremes of neutralizing antibody response after two vaccine doses.

RESULTS

Our findings demonstrate a high seroprevalence of antibodies directed to the measles virus (MV) phosphoprotein (P), nucleoprotein (N), as well as antibodies to the large polymerase (L) protein (fragment 1234 to 1900 AA). Antibodies to these proteins, in addition to anti-F antibodies (and, to a lesser extent, anti-H antibodies), were correlated with neutralizing antibody titer and/or were associated with and predictive of neutralizing antibody response.

CONCLUSION

Our results identify antibodies to specific measles virus proteins and statistical models for monitoring and assessment of measles-specific functional protective immunity in vaccinated individuals.

Generation of recombinant Schmallenberg virus nucleocapsid protein in yeast and development of virus-specific monoclonal antibodies

Schmallenberg virus (SBV), discovered in continental Europe in late 2011, causes mild clinical signs in adult ruminants, including diarrhoea and reduced milk yield. However, fetal infection can lead to severe malformation in newborn offspring. To develop improved reagents for SBV serology, a high-level yeast expression system was employed to produce recombinant SBV nucleocapsid (N) protein. Recombinant SBV N protein was investigated as an antigen in SBV-specific IgG enzyme immunoassay and used for generation of monoclonal antibodies (MAbs). Yeast-expressed SBV N protein was reactive with anti-SBV IgG-positive cow serum specimens collected from different farms of Lithuania. After immunization of mice with recombinant SBV N protein, four MAbs were generated. The MAbs raised against recombinant SBV N protein reacted with native viral nucleocapsids in SBV-infected BHK cells by immunofluorescence assay. The reactivity of recombinant N protein with SBV-positive cow serum specimens and the ability of the MAbs to recognize virus-infected cells confirm the antigenic similarity between yeast-expressed SBV N protein and native viral nucleocapsids. Our study demonstrates that yeast expression system is suitable for high-level production of recombinant SBV N protein and provides the first evidence on the presence of SBV-specific antibodies in cow serum specimens collected in Lithuania.

The recombinant globular head domain of the measles virus hemagglutinin protein as a subunit vaccine against measles

Despite the availability of live attenuated measles virus (MV) vaccines, a large number of measles-associated deaths occur among infants in developing countries. The development of a measles subunit vaccine may circumvent the limitations associated with the current live attenuated vaccines and eventually contribute to global measles eradication. Therefore, the goal of this study was to test the feasibility of producing the recombinant globular head domain of the MV hemagglutinin (H) protein by stably transfected human cells and to examine the ability of this recombinant protein to elicit MV-specific immune responses. The recombinant protein was purified from the culture supernatant of stably transfected HEK293T cells secreting a tagged version of the protein. Two subcutaneous immunizations with the purified recombinant protein alone resulted in the production of MV-specific serum IgG and neutralizing antibodies in mice. Formulation of the protein with adjuvants (polyphosphazene or alum) further enhanced the humoral immune response and in addition resulted in the induction of cell-mediated immunity as measured by the production of MV H-specific interferon gamma (IFN-γ) and interleukin 5 (IL-5) by in vitro re-stimulated splenocytes. Furthermore, the inclusion of polyphosphazene into the vaccine formulation induced a mixed Th1/Th2-type immune response. In addition, the purified recombinant protein retained its immunogenicity even after storage at 37°C for 2 weeks.

Development of a novel efficient fluorescence-based plaque reduction microneutralization assay for measles virus immunity

The measurement of functional measles virus-specific neutralizing antibodies is of considerable interest for vaccine-related research. In this study, we developed and standardized a simple, rapid, highly sensitive, and reproducible fluorescence-based plaque reduction microneutralization (PRMN) assay with visual and automated readout, using a recombinant measles virus engineered to express enhanced green fluorescent protein. The assay is performed in micro format, requires less time to complete (2 versus 4 to 7 days), and is less labor-intensive and less costly than the classical plaque reduction neutralization (PRN) test, widely accepted as the "gold standard" in measles serology. Two available WHO international anti-measles virus standards and one in-house reference serum were used to develop and standardize the new assay. The mean PRMN values from repeated assays were found to be similar to those reported in the literature or assigned to the WHO standards by the classical PRN assay. For validation, we used three groups of low, moderate, and high measles virus vaccine responders' sera with moderate values of correlation in antibody levels (mIU/ml) between PRMN and the Dade Behring immunoglobulin G enzyme immunoassay (EIA). The PRMN assay was more sensitive at low antibody levels and more informative in terms of protection than this commercial EIA. In conclusion, we have developed and validated a sensitive and high-throughput measles virus-specific PRMN that can be readily used in large population-based measles studies.

Expression of biologically active measles virus hemagglutinin glycoprotein by a recombinant baculovirus

In this study, one of the measles virus membrane proteins, named hemagglutinin (H) which has a key role in tropism, receptor binding, hemagglutinating activity and also induction of protective immunity against viral infection, was expressed by the baculovirus expression system using specific plasmid (pDONR221) to produce entry clone. Measles Virus (AIK-C strain) genome was extracted from infected Vero cells. H gene was amplified by specific primers during RT-PCR reaction and inserted into the specific plasmid (pDONR221) using BP recombination reaction. Recombinant baculovirus harboring H gene was consequently constructed by LR reaction. Insect cells (Sf9) were infected with recombinant baculovirus. In order to increase viral titer, recombinant baculoviruses were passaged four times in Sf9 cells. Synthesis of H protein was verified by SDS-PAGE, western-blot and indirect immunoflourescene using goat polyclonal antibody against Measles Virus. The results showed that H protein was partially glycosylated, but it appeared to be active in hemagglutination assay.

Study of measles virus recombinant proteins and their immunobiological properties

Recombinant proteins rN (nucleocapsid) and rH/Nh (hemagglutinin) of the measles virus strain NovO/96 of genotype A were obtained. The immunobiological properties of the proteins were studied in the reaction with a panel of positive and negative sera. BALB/c mice were immunized with recombinant proteins and native antigen of the measles virus strain NovO/96 in order to obtain hyperimmune serum and its analysis using ELISA (enzyme-linked immunosorbent assay) and PRN (plaque reduction neutralization). The hyperimmune sera against recombinant proteins and native antigen of the measles virus strain NovO/96 were found to be highly active in ELISA. The antibodies against the proteins rN and rH/Nh were found to be capable of neutralizing the virus in titer 1 : 13.5 and 1 : 22.9, respectively. The neutralization titer of the antibodies generated against native antigen of the measles virus strain NovO/96 was 1 : 25.7.

Development of a new neutralization test for measles virus

Sero-epidemiological studies are required to identify populations susceptible to measles. The hemagglutination inhibition (HI) test is no longer sensitive enough to confirm immunity to measles, and at present the particle agglutination (PA) test and enzyme-linked immunosorbent assay (EIA) are employed. The most reliable method is the neutralization test (NT), but it is time-consuming and requires experience. To simplify the NT, a recombinant measles AIK-C virus expressing green fluorescence protein (GFP-MVAIK) was constructed and used as a challenge virus. Plaques and cytopathic effects were visualized under ultraviolet light and detected easily, and measuring the intensity of the fluorescence enabled a reduction in the time-consuming steps. Neutralizing antibody titers of a complete inhibition neutralization test were equivalent to those of a 90% plaque reduction neutralization test. Comparison of four methods, HI, PA, EIA and the complete inhibition neutralization test, showed that only the results of EIA correlated well with those of the complete inhibition neutralization test, but sera with borderline levels by EIA were sometimes negative by the complete inhibition neutralization assay.

Measles virus-specific antibody levels in individuals in Argentina who received a one-dose vaccine

In spite of active measles virus (MV) vaccination strategies, reemergence continues to occur, impairing global eradication programs. The immune status against measles was evaluated in 350 vaccinated healthy Argentine children and teenagers who received a single dose of the MV Schwarz strain Lirugen vaccine (Aventis Pasteur). Sera were assessed for immunoglobulin G (IgG) antibodies by a commercial enzyme immunoassay (EIA) (Enzygnost; Behring), an in-house EIA, and neutralization EIA. Results obtained with these methods showed a marked decline in IgG level with increasing age. At 1 to 4 years of age, 84% of children had IgG antibodies above 200 mIU/ml, conventionally accepted as protective levels, whereas only 32% of older children and teenagers had antibody levels exceeding 200 mIU/ml. Moreover, the MV IgG content in the teenage group was significantly lower than the IgG antibody level of the group of younger children (P < 0.0001). In contrast, screening for IgG antibody levels to inactivated tetanus vaccine showed that, on average, 80% of this population was fully protected and that this high level of protection remained through the teenage years. This study suggests that within this population a considerable proportion of individuals had low measles antibody levels that may be insufficient to protect against reinfections or clinical disease.

Experimental vaccines against measles in a world of changing epidemiology

Vaccination with the current live attenuated measles vaccine is one of the most successful and cost-effective medical interventions. However, as a result of persisting maternal antibodies and immaturity of the infant immune system, this vaccine is poorly immunogenic in children <9 months old. Immunity against the live vaccine is less robust than natural immunity and protection less durable. There may also be some concern about (vaccine) virus spread during the final stage of an eventual measles eradication program. Opinions may differ with respect to the potential threat that some of these concerns may be to the World Health Organisation goal of measles elimination, but there is a consensus that the development of new measles vaccines cannot wait. Candidate vaccines are based on viral or bacterial vectors expressing recombinant viral proteins, naked DNA, immune stimulating complexes or synthetic peptides mimicking neutralising epitopes. While some of these candidate vaccines have proven their efficacy in monkey studies, aerosol formulated live attenuated measles vaccine are evaluated in clinical trials.

Neutralizing B cell response in measles

Co-evolving mechanisms of immune clearance and of immune suppression are among the hallmarks of measles. B cells are major targets cells of measles virus (MV) infection. Virus interactions with B cells result both in immune suppression and a vigorous antibody response. Although antibodies fully protect against (re)infection, their importance during the disease and in the presence of a potent cellular response is less well understood. Specific serum IgM appears with onset of rash and confirms clinical diagnosis. After isotype switching, IgG1 develops and confers life-long protection. The most abundant antibodies are specific for the nucleoprotein, but neutralizing and protective antibodies are solely directed against the two surface glycoproteins, the hemagglutinin and the fusion protein. Major neutralizing epitopes have been mapped mainly on the hemagglutinin protein with monoclonal antibodies, producing an increasingly comprehensive map of functional domains.

Neutralizing immunogenicity of transgenic carrot (Daucus carota L.)-derived measles virus hemagglutinin

Although edible vaccines seem to be feasible, antigens of human pathogens have mostly been expressed in plants that are not attractive for human consumption (such as potatoes) unless they are cooked. Boiling may reduce the immunogenicity of many antigens. More recently, the technology to transform fruit and vegetable plants have become perfected. We transformed carrot plants with Agrobacterium tumefaciens to generate plants (which can be eaten raw) transgenic for an immunodominant antigen of the measles virus, a major pathogen in man. The hemagglutinin (H) glycoprotein is the principle target of neutralizing and protective antibodies against measles. Copy numbers of the H transgene were verified by Southern blot and specific transcription was confirmed by RT-PCR. The H protein was detected by western blot in the membrane fraction of transformed carrot plants. The recombinant protein seemed to have a 8% lower molecular weight than the viral protein. Although this suggests a different glycosylation pattern, proper folding of the transgenic protein was confirmed by conformational-dependent monoclonal antibodies. Immunization of mice with leaf or root extracts induced high titres of IgG1 and IgG2a antibodies that cross-reacted strongly with the measles virus and neutralized the virus in vitro. These results demonstrate that transgenic carrot plants can be used as an efficient expression system to produce highly immunogenic viral antigens. Our study may pave the way towards an edible vaccine against measles which could be complementary to the current live-attenuated vaccine.

Large-scale preparation of biologically active measles virus haemagglutinin expressed by attenuated vaccinia virus vectors

A procedure described here allows the efficient and rapid purification of histidine-tagged measles virus haemagglutinin that is synthesized under the control of powerful promoters (PSFJ1-10 and PSFJ2-16) of the highly attenuated vaccinia virus (VV) strain LC16mO. A single affinity chromatography step purifies recombinant haemagglutinin proteins from the lysates of cells infected with the recombinant VVs. The recovery and purity are both very high (a yield of 0.5-2.8 mg/10(8) cells and purity of >94-98%), indicating that this procedure is approximately 400 times more efficient than the conventional methods used to prepare haemagglutinin. The haemagglutinins are correctly transported to the cell surface and have haemadsorption activity. Moreover, the recombinant haemagglutinin proteins cooperate with the measles virus fusion protein to elicit cell fusion activity. In addition, the antibody titres against measles virus, as measured by enzyme-linked immunosorbent assay using the purified haemagglutinin as the capture antigen, correlated closely with neutralization test titres (R(2) = 0.84, p < 0.05), indicating the preservation of immunologically relevant antigenicity. Such recombinant haemagglutinin preparations will be useful in diagnostic tests that measure functional anti-measles immunity and investigate the biological functions and structure of the haemagglutinin.

Placental transfer and decay of maternally acquired antimeasles antibodies in Nigerian children

BACKGROUND

In developing countries vaccination against measles virus (MV) is generally administered at 9 months of age, although it is well-documented that protection of most infants by passively acquired maternal MV antibodies is waning before immunization is given. The purpose of this study was to investigate the decay of maternally derived MV antibodies in Nigerian infants as well as to compare a German and Nigerian cohort of paired mothers and newborns regarding the placental transfer efficiency of MV-specific IgG and total IgG antibodies.

METHODS

MV-specific IgG antibodies were measured with a commercially available MV-enzyme-linked immunosorbent assay, a recombinant hemagglutinin enzyme-linked immunosorbent assay as well as a neutralization assay. Total IgG values were determined with a standard immunoturbidimetric test.

RESULTS

Anti-MV IgG titers were twice as high in German newborns as in Nigerian newborns. An increased concentration of immunoglobulins transferred via the placenta was found only in the German cohort. High concentrations of total maternal IgG reduced the concentration of MV-specific as well as total IgG that crossed the placenta. Furthermore only 17% of the 4-month-old Nigerian infants were still protected against measles. Antibodies had a biologic half-life of 33 days and a biochemical half-life of 48 days.

CONCLUSIONS

Our findings demonstrate that the decay of passively acquired MV antibodies occurred even more rapidly than expected resulting in susceptibility to MV in most of the 4-month-old infants in Nigeria. Furthermore transfer of maternal anti-MV IgG and total IgG antibodies to the newborn was more efficient in the German cohort compared with the Nigerian group. These findings suggest the use of alternative vaccination strategies in developing countries to possibly reduce the window of susceptibility against measles.

New immunofluorescence assays for detection of Human herpesvirus 8-specific antibodies

Several assays have been developed for detection of immunoglobulin G antibodies to Human herpesvirus 8 (HHV-8), including immunofluorescence assays (IFAs) and enzyme-linked immunosorbent assays (ELISAs). However, the specificity and sensitivity of these assays are not completely defined due to the lack of a "gold standard." Although IFAs based on primary effusion lymphoma (PEL) cell lines are used widely, the assays can be confounded by nonspecific reactions against cellular components and potential cross-reaction with antibodies against other herpesviruses. To provide more reliable IFAs, we established recombinant Semliki Forest viruses (rSFVs) expressing the HHV-8-specific proteins ORF73 and K8.1 and used BHK-21 cells infected with these rSFVs for IFA (ORF73-IFA and K8.1-IFA). Expression of the HHV-8-specific proteins at very high levels by the rSFV system allowed easy scoring for IFA and thereby increased specificity. The rSFV system also allowed detection of antibodies against glycosylation-dependent epitopes of K8.1. Titers measured by rSFV-based IFAs and PEL-based IFAs correlated well (correlation coefficients of >0.9), and concordances of seroreactivities between rSFV-based and PEL-based IFAs were >97% (kappa > 0.93). K8.1-IFA was more sensitive than either ORF73-IFA or peptide ELISAs. Using PEL-based lytic IFA as a reference assay, the sensitivity and specificity of K8.1-IFA were estimated to be 94 and 100%, respectively. HHV-8 prevalences determined by K8.1-IFA among the human immunodeficiency virus (HIV)-positive (HIV(+)) Kaposi's sarcoma (KS) patients, HIV(+) KS(-) patients, and healthy controls were 100, 65, and 6.7%, respectively, which were consistent with prior reports. Therefore, our rSFV-based IFAs may provide a specific and sensitive method for use in epidemiology studies. In addition, they will provide a basis for further development of diagnostic tests for HHV-8 infection.

Evaluation of different measles IgG assays based on recombinant proteins using a panel of low-titre sera

During the WHO campaign to eradicate measles, accurate discrimination between immune and non-immune individuals will become increasingly important. Due to waning immunity in vaccinated populations, the performance of a measles IgG assay depends mainly on its ability to detect reliably seronegative individuals among many vaccinees with low antibody levels. New serological tests based on recombinant proteins detect only a fraction of the total measles virus (MV) specific antibodies. Therefore, several assays based on recombinant MV-haemagglutinin (ELISA and flow cytometry) or MV-fusion protein (flow cytometry) as well as neutralisatlon and haemagglutination test have been evaluated using a large panel of low-titre and negative sera. Since such an evaluation is highly dependent on threshold values for positivity, the receiver operating characteristic curve analysis was applied. The H-FACS and the H-ELISA showed the best performing characteristics (specificity: 97.4 and 96.1%, respectively; sensitivity: 88.1 and 89.6%, respectively) and may be an alternative to the neutralisation assay. The number of undefined/grey zone sera was significantly lower compared to a commercial whole virus-based ELISA and therefore fewer individuals would be vaccinated unnecessarily.

A simplified and standardized neutralization enzyme immunoassay for the quantification of measles neutralizing antibody

A simplified and standardized neutralization enzyme immunoassay (Nt-EIA) was developed to detect measles virus growth in Vero cells and to quantify measles neutralizing antibody. Heat-inactivated sera were diluted serially 4-fold and tested in duplicate. The 50% reduction point (50%RP) of virus growth was calculated using the Reed-Muench formula and the neutralizing antibody titre of test sera was converted into mIU/ml by comparing their 50%RP with that of the international standard serum. The optimal virus input and incubation time were found to be 50-100 plaque forming unit (PFU)/well and 64-72 h, respectively. The simplified Nt-EIA had a good reproducibility with only 3.7-4.2% of duplicate tests having a ratio > 4 in an evaluation of intra assay variation and the coefficients of variance were 2-9% in an evaluation of inter assay variation. In addition, the simplified Nt-EIA had a high sensitivity(98.6%), specificity (100%) and agreement (98.8%) in qualitative comparison with plaque reduction neutralization test (PRNT). In quantitative comparison, the correlation coefficient between Nt-EIA and PRNT was 0.83 without log transformation or 0.77 after log transformation and 90% of 61 positive sera had a ratio < 4 between antibody titre tested by the two methods. The simplified Nt-EIA is thus a suitable alternative to the PRNT for the quantification of measles neutralizing antibody.

Evaluation of hemagglutinin protein-specific immunoglobulin M for diagnosis of measles by an enzyme-linked immunosorbent assay based on recombinant protein produced in a high-efficiency mammalian expression system

Recombinant hemagglutinin (H) of the measles virus (MV) expressed in a mammalian high-expression system based on the Semliki Forest virus replicon was used in an enzyme-linked immunosorbent assay (ELISA) for the detection of specific immunoglobulin M (IgM) and IgG in patients with acute-phase measles. One hundred twelve serum specimens from 70 patients with measles were analyzed. Case definition was based on a commercial IgM ELISA that utilizes MV-infected cells (MV-ELISA) (Enzygnost; Behring Diagnostics); the clinical criteria of the Centers for Disease Control and Prevention (Atlanta, Ga.); and/or the increase in hemagglutinin test titers, neutralization test titers, and levels of MV-specific IgG whenever paired sera were available. The initial time courses of the IgM signal after the onset of rash are similar in the H- and MV-ELISAs. On days 0 to 19, both ELISAs detected IgM in 67 of 68 (98.5%) sera. Average maximal levels of IgM seem to persist, however, about 10 days longer in the MV-ELISA (up to day 25) than in the H-ELISA (day 15). From days 20 to 29 and 30 to 59, the H-ELISA detected only 64.3 (9 of 14) and 19.2% (5 of 26), respectively, of sera that were IgM positive by MV-ELISA. At least up to day 30, the performance of the H-ELISA seemed to be similar to that reported for commercial ELISAs based on whole MV. Our results demonstrate that MV H-specific IgM can be used to diagnose most measles cases from a single serum specimen collected within 19 days after the onset of rash and that the recombinant protein used in this study is suitable for this purpose.

A simplified immunoassay based on measles virus recombinant hemagglutinin protein for testing the immune status of vaccinees

Simplified tests based on recombinant antigens are considered to be important for monitoring immunity against measles virus (MV). The hemagglutinin protein (H) is the main target for neutralising and protective antibodies. We produced a recombinant MV-H protein, in a high-yield mammalian expression system based on the Semliki Forest virus replicon. The antigenicity of this recombinant protein was investigated with monoclonal antibodies and its suitability for measuring the immune status of vaccinees was tested in a large cohort by ELISA (H-ELISA). The results were evaluated against neutralisation (NT) and hemagglutination inhibition (HI) titers and MV-specific IgG measured in a commercial whole-virus based ELISA (MV-ELISA, Enzygnost). The H-ELISA correlated better with HI (r=0.78) and NT titers (r=0.80), than the MV-ELISA (HI, r=0.58; NT, r=0.59). In contrast to the MV-ELISA, the H-ELISA detected no false-positive sera (P < 0.02) and the number of false-negative sera was significantly lower in the H-ELISA than in the MV-ELISA (4/378 vs. 15/378; P < 0.025). The performance of the H-ELISA did not deteriorate significantly when, instead of background corrected net values, uncorrected raw O.D. values of the H-antigen were considered, or when early time points (30 min) were evaluated. These results demonstrate that the recombinant H-ELISA detects efficiently non-immune individuals among vaccinees, despite their relatively low MV-antibody levels. A simplified format with single value measurements did not result in loss of sensitivity or specificity and its performance compared favorably with commercial ELISAs based on whole virus.