Immunity status against poliomyelitis in Germany: determination of cut-off values in International Units

BACKGROUND

To prevent importations of wild polioviruses into a polio free region a high level of population immunity must be kept. Standard methodology for determination of polio antibodies is a feature aimed at obtaining consistent results. An International Standard Serum for polio antibodies exists, but no protective level in International Units is defined.

METHODS

A representative study was carried out in order to determine the serological status against poliomyelitis in Germany (n = 2564, age 18-79 years). Furthermore, sera from persons aged less than 18 years were included (n = 881). Microneutralization test has been used for determination of antibody levels. Results have been expressed in International Units.

RESULTS

The results of this study indicate that the cut-off level for polio antibodies is 0.075 IU/ml for Polio 1, 0.180 IU/ml for Polio 2 and 0.080 IU/ml for Polio 3. Neutralizing antibodies against poliovirus type 1, 2 and 3 were detected in 96.2%, 96.8% and 89.6% of samples, respectively.

CONCLUSIONS

Overall, this seroprevalence indicates a very high level of immunity of the general population. It must be kept after the switch of immunization strategy from attenuated to inactivated vaccine in Germany.

Comparison of four serological methods for the detection of diphtheria anti-toxin antibody

The aim of this study was to compare four serological methods for the detection of Corynebacterium diphtheriae IgG anti-toxin antibodies (IgG-DTAb) in human serum. One hundred serum samples were evaluated for C. diphtheriae IgG-DTAb by four different methods: passive haemagglutination (PHA), latex agglutination test (LA), toxoid enzyme-linked immunosorbent assay (Toxoid-ELISA), and toxin-binding inhibition enzyme-linked immunosorbent assay (ToBI-ELISA). As the external standardisation the neutralisation test for C. diphtheriae toxin in Vero cells (TN Vero) was used. For internal standardisation of IgG-DTAb titres, the WHO standard serum of human diphtheria antitoxin was used. The study revealed a poor correlation between the reference test and the PHA (r=0.34 Pearson's correlation coefficient), an acceptable correlation for the LA (r=0.74), a good correlation for the Toxoid-ELISA (r=0.81) and a very good correlation for ToBI-ELISA (r=0.93). The sensitivity measurements of PHA, LA, Toxoid-ELISA and ToBI-ELISA tests, were 14, 100, 94, 96% respectively and the corresponding specificity characteristics were 86, 76, 94, 90 respectively. Of the four evaluated methods, the ToBI-ELISA could be recommended for scientific and precise laboratory assays of diphtheria antibody levels in humans. For screening purposes the Toxoid-ELISA could be used, but the accuracy of antibody titres below 0.1 IU/ml, considered as the limits of protection, is questionable. Both tests offer very useful alternatives to the in vitro diphtheria toxin neutralisation test in Vero cells. Because of their unsatisfactory correlation and sensitivity as compared to the reference method, PHA and LA should be avoided and replaced by one of the two enzyme immunoassays.

RT-PCR and cell culture infectivity assay to detect enteroviruses during drinking water treatment processes

In this study, 62 water samples were collected from two water treatment plants (WTPs) in Suez Canal cities (Port Said and Ismaillia) and one plant in Cairo (Giza WTP) in addition to the beginning of the two Nile river branches (Rosetta and Damietta). Viruses were concentrated by adsorption-elution ethod sing 142 mm-diameter nitrocellulose membrane of 0.45 microm pore size and eluted with 3% beef extract at pH 9.5. The concentrated samples were inoculated for 3 successive passages in three cell culture types (Vero, BGM and RD). Enterovirus RNAs in CPE-induced samples were extracted by guanidinium thiocyanate/ phenol/chloroform and heat shock methods and detected by RT-PCR and neutralization test. The results showed that eight samples [14.5% (8/62)] contained enteroviruses most of them were polioviruses [87.5% (7/8)] and coxsackievirus type B2 [12.5% (1/8)]. The three cell cultures were of the same sensitivity to detect the isolated viruses. Also, RT-PCR followed by neutralization assay facilitates and accelerate the results. The guanidinium thiocyanate extraction method was more sensitive than heat shock method. The results turned our attention to review our technology of water treatment and disinfection step in addition to the selection of suitable intake for the drinking water treatment plants.

The micro-focus reduction neutralization test for determining dengue and Japanese encephalitis neutralizing antibodies in volunteers vaccinated against dengue

A micro-focus reduction neutralization test (mFRNT) was evaluated as an alternative test to the ordinary plaque reduction neutralization test (PRNT) for the determination of dengue virus and Japanese encephalitis virus neutralizing antibody responses in persons receiving dengue vaccine. The 2 tests were similar in terms of titres and ability to detect seroconversion. Although the neutralizing antibody titres obtained by mFRNT were slightly lower than those given by PRNT, the differences were less than two-fold, indicating than mFRNT was reliable. Reproducibility of mFRNT was confirmed by 10 replicate tests using the same control serum. Therefore, mFRNT may be useful in large-scale investigations of neutralizing antibody levels, for example, in young children forming part of an immunization programme; it can be performed quickly and is economical, requiring only a small volume of sera.

Evaluation of monoclonal antibodies to human immunodeficiency virus type 1 primary isolates by neutralization assays: performance criteria for selecting candidate antibodies for clinical trials. AIDS Clinical Trials Group Antibody Selection Working Group

A collaborative study was organized to identify monoclonal antibodies (MAbs) that may be broadly and potently neutralizing for a panel of human immunodeficiency virus type 1 (HIV-1) low-passaged adult and pediatric primary isolates in peripheral blood mononuclear cells. Five laboratories evaluated a coded panel of seven human MAbs to HIV-1 subtype B envelope V3, CD4 binding region, gp41, and other conformationally sensitive determinants. Each laboratory measured neutralizing activity of the MAbs against the laboratory isolate HIV(MN) and a panel of 9 subtype B primary isolates. Antibodies were classified as suitable candidates for future clinical studies if they could neutralize at least half of the 9 primary isolates at a concentration of < or = 25 microg/mL for 90% viral inhibition. The study identified three MAbs that met stated performance criteria: IgG1b12, 2G12, and 2F5. These results may provide a rationale for examining the clinical efficacy, either singly or in combination, of the three MAbs.

Use of dot-immunobinding assay for visual detection of rinderpest antibodies in vaccinated cattle

A dot-immunobinding (DIB) assay was used to detect rinderpest antibodies in cattle vaccinated with Kabete 'O' strain vaccine, using purified rinderpest virus. Of 120 serum samples from vaccinated and non-vaccinated animals, rinderpest antibodies were detected in 80%, 88.4% and 91.6% of samples at 2, 3 and 4 weeks postvaccination respectively. All the serum samples from non-vaccinated animals were negative. The DIB results had a good correlation with those of the micro neutralisation test. The technique is simple, easy to perform and suitable for routine use in detecting rinderpest antibodies following vaccination.

A survey for antibodies to equine arteritis virus in donkeys, mules and zebra using virus neutralisation (VN) and enzyme linked immunosorbent assay (ELISA)

A seroepidemiological survey of donkeys in South Africa (n = 4300) indicated a wide distribution and increasing prevalence of antibodies to equine arteritis virus (EAV). Donkey sera inhibited equine arteritis virus infection in virus neutralisation (VN) tests and in ELISA specifically bound to a recombinant antigen derived from the Bucyrus isolate of EAV. These results suggest that donkeys have been exposed to the same serotype of this virus as circulates among horses. A good correlation existed between EAV neutralising antibody titres and ELISA absorbance values (0.8631); the ELISA was sensitive and specific (99.2% and 80.3% respectively) for donkey sera when compared to the VN test and the recombinant ELISA antigen did not cross-react with sera positive for common African equine pathogens. VN+ ELISA+ donkeys were also found in Morocco and Zimbabwe and seropositive mules in both South Africa and Morocco. No seropositive zebra (n = 266) were detected from game reserves or zoos in 9 countries. The results confirm that in addition to horses and donkeys, mules are naturally infected with EAV.

Comparison of in vivo and in vitro methods for determining unitage of diphtheria antitoxin in adsorbed diphtheria-tetanus (DT) and diphtheria-tetanus-pertussis (DTP) vaccines

In view of the current efforts to find a reliable in vitro method which can suitably act as an alternative for determining the potency of the diphtheria component in a combined vaccine, we have analysed experimental batches by the method proposed by WHO [1] i.e. challenge method in guinea pigs. The same batches were also analysed by the alternative antibody induction method as suggested in the Indian Pharmacopoeia (I.P.) [2] which is similar to the old method suggested in the British Pharmacopoeia (B.P.) 1973. As per I.P. the initial part of raising the antibodies remains unaltered but the actual titration of diphtheria antitoxin from the immunised guinea pigs was performed by using the following in vitro methods: a) indirect haemagglutination test using human "O" red blood cells to coat diphtheria toxoid using chromic chloride as the coupling agent [3]; b) toxin neutralisation test using Vero cells [4]; c) a double diffusion technique in agar gel for titration of diphtheria antitoxin [5]. Our findings show clearly that the results of two in vivo methods i.e. Challenge Test, Alternative I.P. Method and the above-mentioned three in vitro methods are comparable and would certainly reduce the number of animals required by making a combination of in vivo and in vitro techniques to give us an assessment of the potency of the vaccine to be tested.

A comparative study of serological techniques for detection of West Nile virus antibodies

This study was conducted to compare different serological techniques namely enzyme-linked immunosorbent assay (ELISA), hemagglutination inhibition (HI), indirect immunofluorescence (IFT) and plaque reduction neutralization (PRNT), for detection of West Nile virus (WNV) antibodies in sera collected from inhabitants of a flooded village (Begiram, Minufiya Governorate). ELISA showed 45% while HI and IFT indicated 37.6 and 26.4% positive sera among the tested 178 sera taken from the flooded village, respectively. The results obtained by ELISA, HI and IFT of only 55 randomly chosen sera were compared with their PRNT results. ELISA was found to be more sensitive (83.8%) while IFT and HI were more specific (88.9% and 83.3%, respectively). The positive predictive values for the 3 tests were more than 80% while the negative predictive values were different for these tests: 66.7% for ELISA, 44.1% and 37% for HI and IFT, respectively. From this study it is concluded that for screening of population in an endemic area, two serological tests in combination can be used starting with ELISA (the more sensitive) followed by HI and/or IFT (the more specific) to exclude the false positive results.

Selection of European Union standard reference sera for use in the serological diagnosis of infectious bovine rhinotracheitis

A study, consisting of three trials, was undertaken in order to find standard reference sera for use in serological diagnostic procedures for infectious bovine rhinotracheitis (IBR) in the European Union (EU). A total of forty-nine laboratories participated in the trials, in which panels of positive, weak positive, and negative candidate sera were tested blind by neutralisation, indirect and blocking enzyme-linked immunosorbent assay, indirect immunofluorescence and passive haemagglutination. A serum which scored positive in all tests in all laboratories was selected as a strong positive standard serum. The chosen negative standard serum gave negative results in virtually all tests. A 1/36 dilution of a positive serum was chosen as the weak positive standard serum and only very sensitive techniques were able to score it positive. Three EU reference sera are now available for the standardisation of serological tests for IBR.

Comparative studies of the in vivo toxin neutralization and the in vitro Vero cell assay methods for use in potency testing of diphtheria component in combined vaccines/toxoids. 1: Standardization of a modified Vero cell assay for toxin-antitoxin titration of immunized guinea-pig sera

The in vivo Toxin Neutralization (TN) test is currently used as an antibody limit test for regulatory control testing of diphtheria potency in combined vaccines/toxoids. This test requires the use of guinea-pigs for estimating diphtheria antitoxin titre in immunized sera. The acceptability criteria (minimum requirement) for adsorbed pediatric vaccines/toxoids in the TN test are the production of at least 2.0 IU/ml of antitoxin in the serum pool of immunized animals. The feasibility of employing the in vitro Vero cell assay as an alternative limit test to the TN test was investigated. Parallel titration of serum samples from guinea-pigs immunized with DPT (ads) vaccine (pediatric use) showed that although the in vitro antitoxin titre paralleled the in vivo values, a direct correlation between the two methods could not be established. The mean in vitro titres were lower (in terms of IU/ml) than those obtained by the in vivo TN test. This could possibly be explained by the differences between reference serum which is a hyperimmune horse serum and the test sera which are from guinea-pigs which have been immunized only once. The in vitro/in vivo ratio of the antitoxin titre ranged from 0.1 to 0.24 IU/ml with a mean value of 0.17 (SD, 0.06). Hence, for pediatric vaccines, a tentative in vitro limit of 0.3 IU/ml, which would correspond to the currently required limit of 2.0 IU/ml in the in vivo TN test, could be accepted as the minimum requirement for the antitoxin titre in the Vero cell assay.

Comparability of poliovirus neutralizing antibody tests

A serology panel was used to assess the comparability of results of poliovirus neutralizing antibody assays. Five laboratories from three countries provided results with seven in-house methods. A high degree of within-laboratory reproducibility was seen for all laboratories and methods, with 94% of results for repeat titrations within plus or minus one twofold dilution. Considerable differences in sensitivity between methods were, however, observed. The period of serum/virus incubation prior to inoculation of cells was shown by one laboratory to have a 10-fold effect on titres, and another laboratory showed that use of Sabin or wild-type challenge virus significantly influenced results for types 1 and 3. Expression of results as relative potencies abolished these difference due to method. The difference in sensitivity between laboratories for end-point titres (if one very sensitive method was excluded) was around fourfold for types 1 and 2, and ninefold for type 3. This range increased to 15-fold for type 2; and 20-fold for types 1 and 3 if the most sensitive method was included. Expression of results relative to a standard considerably improved agreement with a residual variation of around fourfold. Therefore, expression of results from serological studies, including vaccine trials, in International Units (i.e. relative to the new International Standard) will assure that comparisons between studies can be made with confidence.

Using the virus challenge dose in the analysis of virus neutralization assays

We propose a new basis for adjusting the results of virus neutralization assays. These assays consist of two separate experiments performed in parallel: a virus titration experiment and a serum dilution assay. In the virus titration experiment, one estimates the amount of virus used in the assay (the virus challenge dose). In a typical virus neutralization assay, the virus challenge dose may range over an order of magnitude. In the serial dilution assay, one measures the serum neutralizing activity against a particular virus. Most standard methods use the virus titration results only to ensure that the overall experiment is acceptable; the specific results of the virus titration experiment are not used to adjust the estimate of serum neutralizing activity. Although adjustment based on calibration with reference sera could be done, this seldom occurs in practice. We use results from recent studies of the kinetics and stoichiometry of polio virus neutralization with monoclonal antibodies to develop a method to use results from the virus titration experiment to adjust the serum neutralizing activity directly. Our results also indicate that a simple ad hoc procedure can improve the accuracy of the estimated serum neutralizing activity.

Serological and virological studies of Japanese encephalitis in the Terai region of Nepal

In 1987 and 1990, serum samples were collected from people living in the two districts (Itahari and Chitwan) of the Terai region of Nepal. Antibodies against Japanese encephalitis (JE) virus in these sera were detected by the hemagglutination inhibition (HI) and neutralization (N) tests. By the HI test, 26 out of 172 (15.1%) sera from Chitwan and 15 out of 137 (10.9%) sera from Itahari showed positive titers. Higher positive rates were shown by the N test, where 46 out of 172 (26.7%) sera from Chitwan and 22 out of 137 (16.1%) sera from Itahari had antibodies against JE virus. A JE strain was isolated from a blood specimen of a pig raised in Kathmandu. When the nucleotide sequence of the pre-M region of the strain was compared to the same region of the other JE virus strains reported, the highest similarity was observed to the strains isolated in Nepal in 1985. These results suggest that the Terai region has been an epidemic area of JE.

Sensitivity of the standardized pseudorabies virus neutralization test varies with the test strain used

The effect of altering the strain of the test virus used in the standardized pseudorabies virus neutralization (VN) test on the sensitivity of the assay was evaluated. Comparative VN tests were performed using 4 different strains: the avirulent Bartha parental, the avirulent recombinant Bartha gIIIKa, the moderately virulent Shope (currently used for the VN test at the National Veterinary Services Laboratory, Ames, IA), and the highly virulent P2208 (Funkhauser). A radioimmunoassay and a Western immunoblotting technique were employed to verify the presence of anti-pseudorabies virus (PrV) antibodies in sera. Statistical analysis indicated that replacement of the Shope strain by the Bartha gIIIKa or the P2208 strain resulted in VN titers that were 4.23- and 2.00-fold higher, respectively. Despite these differences, specificity with regard to PrV diagnosis was unaltered. This apparent enhancement of the sensitivity of the PrV VN test would be beneficial for the serologic identification of PrV-infected animals during an eradication effort.

Evaluation of monoclonal antibodies to HIV-1 by neutralization and serological assays: an international collaboration. Collaborating Investigators

In a National Institutes of Health (NIH)/World Health Organization (WHO)-sponsored collaboration, 26 laboratories characterized a coded panel of monoclonal antibodies (MAb) to HIV-1 envelope protein. The MAb were evaluated by serological [radioimmunoprecipitation, immunoblot, enzyme-linked immunosorbent assay (ELISA) and peptide mapping] and neutralization assays. Although laboratories used diverse neutralization assays that vary considerably in sensitivity, qualitatively similar data were obtained. The MAb were classified into three neutralization specificities: type-specific for MN and SF2, type-specific for IIIB, and group-specific for MN, SF2, and IIIB. The group-specific MAb displayed much lower neutralizing titers than the type-specific MAb. The specificity of MAb for neutralization was greater than for serological recognition of gp120 protein or peptide epitopes. Some MAb that bound to the same or closely overlapping linear epitopes had very different neutralization properties. The distinction between serological recognition and neutralization may result from differences in affinity of the MAb or may indicate that MAb can neutralize by interactions at a site distinct from the antibody binding site.

[The standardization of the in-vitro tetanus toxin neutralization test on Chinese hamster ovary cells]

A test for the titration of B. pertussis toxin with antisera on Chinese hamster ovary (CHO) cells has been worked out. B. pertussis protective antigenic cell-free complex containing 48-54% of B. pertussis toxin has been used as antigen. The specificity of the effect of this complex on CHO cells has been confirmed in the toxicity neutralization test with antisera. CHO cells have been adapted to reagents and culture media made in the USSR. The titration of B. pertussis toxin and antisera on CHO cells did not require the use of highly purified antigen.

Design, development, and interpretation of HIV neutralization assays

In developing therapeutic reagents for the control of HIV infection, it is necessary to screen candidate products in vitro for their ability to reduce or neutralize viral infection. Although the current literature describes numerous neutralization assays, no universally accepted standards have been adopted. In this article, we briefly review the available neutralization assays and describe in detail the methods we have selected in our laboratory for the screening and characterization of reagents with potential anti-HIV properties. After evaluating many different technical protocols and experimental procedures, we have found the syncytium inhibition and syncytial focus assays to be particularly useful and have found p24 gag antigen production to be an excellent objective measure of HIV infection under a variety of conditions. These assays proved reproducible and sensitive and are suitable for use in the majority of laboratories.

A quantitative enzyme-linked immunosorbent assay for bovine herpesvirus type 1 (BHV-1) antibody

A quantitative enzyme-linked immunosorbent assay (ELISA) was developed to detect and measure antibody to bovine herpesvirus type 1 (BHV-1) in cattle sera. The optical density produced from a single dilution of test serum was compared with a standard curve and the results were read and printed out from a computer interfaced to a multichannel ELISA reader. The printed results were expressed in ELISA units. The ELISA results obtained on 370 cattle sera were compared with those of the serum neutralisation test (SNT). An agreement of 90.5% was obtained when reciprocal SNT titres equal to or greater than 4 and IgG ELISA units equal to or greater than 50 were taken as indicative of a specific reaction. Of the 370 sera, 35 gave discrepant results of which 21 were SNT positive/IgG ELISA negative and 14 were SNT negative/IgG ELISA positive. When the SNT positive sera negative in the IgG ELISA were tested in an IgM ELISA, 19 were found to be positive. Thus, when the IgG and IgM ELISA results were combined the overall agreement between the ELISA and SNT increased to 95.7%. The IgG ELISA had a sensitivity of 82.4% and specificity of 94.4% relative to the SNT, whereas the combined IgG and IgM ELISA results gave a sensitivity and specificity of 98.3% and 94.4% respectively. There was a good positive correlation between the two tests (r = 0.86).

The effect of the virus-serum incubation period upon vaccinia virus serum neutralization titers

Rabbit and human vaccinia virus immune sera were titrated by the plaque reduction serum neutralization (SN) method. Titers determined following a 2-h, 37 degree C virus-serum incubation period were not significantly different from those determined following an 18-h incubation period (2-h, 37 degree C incubation followed by a 16-h, 4 degree C period). However, control virus plaque counts decreased significantly after the longer incubation period. Numerous factors affecting SN titer estimation are reviewed. Standardization of SN test conditions may be useful in comparative potency evaluation of vaccinia-vectored recombinant DNA viral vaccines.

Three-day assay for human cytomegalovirus applicable to serum neutralization tests

A fluorescing cell assay (FCA) technique utilizing the indirect fluorescent-antibody method to measure human cytomegalovirus (CMV)-infected cells has been applied to the rapid determination of CMV-neutralizing antibody. Human sera with complement fixation titers to CMV of 1/32 or greater and fluorescein-conjugated rabbit anti-human globulin are the primary and secondary reagents in the fluorescent-antibody test. FCA measured in 3 days the same number of infectious units measured by plaque assay in 2 weeks. FCA and plaque assay yielded identical neutralizing antibody titers to CMV in 20 human sera.