A comparison of enzyme immunoassay and bioassay for the quantitative determination of antibodies to tetanus toxin

Low specificity of 2 tetanus rapid tests in Cambodia

OBJECTIVES

Rapid testing for tetanus on serum or blood allows for an immediate evaluation of individual protection against tetanus in developed countries, using a "single step" immunochromatographic technique using tetanus toxoid. The specificity of these tests, compared to the reference method for tetanus, mouse serum neutralization testing, has however never been assessed in these countries, due to the difficulty to perform serum neutralization titration in mice, because of animal testing bioethical regulations.

POPULATION AND METHODS

A collection of sera from adult volunteers in Cambodia, living in rural environment, was tested for tetanus antibodies by ELISA in France, and by mouse serum neutralization in Vietnam. This allowed estimating the sensitivity and specificity of 2 rapid tetanus tests, available on the market: TQS™ and Tetanotop™.

RESULTS

The sensitivity of these tests was adequate, compared to mice serum neutralization test, for a test threshold of 0.01 IU/mL, (100% for TQS™, 91% for Tetanotop™), but their specificity was very low (1% for TQS™ and 13% for Tetanotop™).

CONCLUSION

The results prove that these rapid tests for the assessment of individual protection against tetanus should not be used in the adult rural Cambodian population.

New equine antitoxins to botulinum neurotoxins serotypes A and B

Hyperimmune monovalent antitoxins to botulinum neurotoxin serotypes A and B have been produced by immunizing horses with newly developed formalin toxoids. After primary immunization, horses developed acceptable prophylactic antibody titers (1-5 IU/mL). Three horses received additional toxoid booster injections to induce hyperimmune antibody titers with antitoxin-A and antitoxin-B titers reaching peaks of approximately 2000 IU/mL and 150-625 IU/mL, respectively. Titers were quantified throughout the process by antigen-capture ELISA and by in-vivo neutralization. ELISA titers and neutralization titers correlated (R² ∼0.62-0.92), however, unique correlations between in-vitro and in-vivo titers were observed for each horse. Monovalent antitoxin pools were made by combining plasma that had been collected twice via plasmaphoresis several months after primary immunization. Neutralizing units were established for each pool relative to the current US and WHO reference standards. Titers were determined at the L(+)/10 and L(+)/40 toxin dose for Toxin types A and B, respectively, and U.S. and international units were assigned to each monovalent antitoxin. Avidity of the new Anti-A pool was equivalent to the WHO Anti-A reference at the L(+), L(+)/10 and L(+)/30 dose. Each monovalent plasma pool failed to cross-neutralize other botulinum neurotoxin serotypes indicating a high degree of specificity of each antitoxin for the toxin serotype used during immunization.

Antibodies to diphtheria, tetanus and pertussis in infants before and after immunization with DTP (Triple Antigen) vaccine

OBJECTIVE

To determine antibody levels to the Australian manufactured combined diphtheria, tetanus and pertussis (DTP) vaccine (Triple Antigen, CSL Ltd) in infants before and after their primary immunization course.

METHODOLOGY

Serosurvey (antibody prevalence study) in two groups: infants aged 5-9 weeks who had not received any immunizations (n = 25), and infants aged 7-10 months who had received two (n = 25) or three immunizations (n = 57) with DTP, sampled from infants attending the Royal Children's Hospital, Melbourne, either as inpatients or outpatients between February and April 1993. The immunization history for each infant was determined from hospital records, the parent-held child health record, or the local council or family doctor who immunized the infant.

RESULTS

Enzyme immunoassay (EIA) of antibodies to diphtheria and tetanus showed all infants to have adequate protective levels after two or three vaccinations (> or = 0.01 IU/mL). All subjects who had received all three DTP vaccinations had detectable antibody to at least one pertussis antigen. Antibodies to the pertussis antigens filamentous haemagglutinin and pertussigen (pertussis toxin) were comparable to levels determined for whole cell pertussis vaccines used elsewhere in the world. EIA-determined antibodies to pertussis agglutinogen type 2 and agglutinogen type 3 showed substantially higher geometric mean titres when results for pre-immunization and post-immunization subjects were compared.

CONCLUSIONS

These data show that the Australian manufactured DTP vaccine has immunogenic properties similar to those of vaccines used elsewhere, and that antibody concentrations following immunization are at levels consistent with efficacy.

In vitro potency testing of inactivated biologics: current situation in the E.E.C

A whole series of regulations are currently being implemented in the European Economic Community in the field of biologics that are complementing the European Pharmacopoeia and widening its field of application. The object of these new regulations is to ensure a very high level of safety and efficacy, but unfortunately, they tend towards the increased use of animals for testing purposes. However, for ethical, economic and practical reasons, the number of animals used must be reduced which makes the development of in vitro tests an important issue. In vitro testing is complicated since inactivated products are often adsorbed onto aluminum hydroxide or presented as an emulsion, and tests are easier to carry out prior to final blending operations. It is suggested that in-process controls can replace finished product controls when batch-to-batch consistency has been formerly validated, a quality assurance system has been adopted, and that system is regularly subjected to a suitable inspection procedure. The new inspection system applied in the United Kingdom should facilitate progress in this field and lead to a parametric release system. To avoid future problems, it is particularly important that efforts be made now for the harmonization of new techniques between Europe and the United States.

Detection of tetanus antitoxin using Eu(3+)-labeled anti-human immunoglobulin G monoclonal antibodies in a time-resolved fluorescence immunoassay

Tetanus antitoxin in human sera was detected with solid-phase immunoassays in microtitration modules coated with tetanus toxoid by using Eu(3+)-labeled anti-human monoclonal antibodies on the basis of an exactly calibrated antibody standard. The use of a time-resolved fluorescence immunoassay (TR-FIA) significantly improved the quantitative detection of tetanus antitoxin over that of the enzyme-linked immunosorbent assay (ELISA) technique because of its high sensitivity and its wide measurement range, detecting antibody levels between 0.001 and 12.5 IU/ml with a single serum dilution of 1:100. For the same purpose, two different serum dilutions (1:100 and 1:1,000) were needed in the ELISA technique. TR-FIA is reproducible and can be performed in 3.5 h. A study of 2,630 serum samples was undertaken to examine the age-dependent distribution of titer levels, indicating the decline of sufficient protection in patients older than 60 years. The wide measurement range of TR-FIA enabled fast examination of large numbers of serum samples without the need for repetition, with further sample dilution, as was often necessary in the ELISA procedure.

ELISA for the routine determination of antitoxic immunity to tetanus

Serum samples from 727 persons with different vaccination histories were assessed for tetanus antitoxin content in an enzyme linked immunosorbent assay (ELISA) and tested for tetanus toxin neutralization activity in mice in order to compare the results obtained by the two methods. Neutralizing antibody activities in sera from individuals previously completely vaccinated correlated well with results obtained by ELISA and the accuracy increased with increasing antitoxin concentration in serum. This correlation was observed in sera from persons vaccinated recently as well as in sera from persons vaccinated many years ago. In sera from persons with an incomplete vaccination history ELISA was found to be an unreliable tool for the prediction of in vivo results. Many of these sera had antitoxin levels by ELISA far above the in vivo values, probably due to the presence of non specific or low avidity antitoxin which is detected in ELISA. The lowest ELISA value reliably predictive of protective antibody activity in serum irrespective of vaccination history was found to be 0.16 IU/ml. It was concluded that ELISA is useful for larger population studies as an initial test, but sera with an antitoxin content below 0.16 IU/ml should also be assessed in a neutralization system.

An evaluation of the use of a pH indicator for the detection of beta-lactamase in enzyme immunoassay

The use of a mixed pH indicator containing bromocresol purple and bromothymol blue was evaluated for the detection of beta-lactamase activity in enzyme immunoassays (EIA) based on this enzyme. The EIA was found to correlate well with a bioassay for antibodies to tetanus toxoid in sheep sera. Results could be read spectrophotometrically at 450 nm or visually as a colour change from dark blue to yellow. The mixed pH indicator was found to have a number of desirable features including good stability, ease of preparation and a sharp colour change which makes the system suitable for visual determination of titration end points.

Immune response and reactions to various dose regimens for raising hyperimmune antisera in sheep

To determine the optimum procedure for raising hyperimmune sera to tetanus toxin, three adjuvants, four antigen preparations and two routes of administration in various combinations were investigated in sheep. Oil-in-water adjuvants alone or in combination with aluminum gels were superior to aluminium gels on their own. This disadvantage of aluminium gels was partially but not completely abrogated when the frequency of doses was increased to three per week. Intensity of local reaction was strongly correlated with immune response; the more immunogenic a dose, the more reactive. Reactivity of oily adjuvants could be lessened by use of a more suitable route of administration, thus oily adjuvants appeared suitable for use when administered by the intraperitoneal route even though moderate to severe reactions resulted from subcutaneous injections. Of other variables investigated, toxin did not confer any advantage over toxoid as an immunogen, purified toxoid was a significantly better immunogen than unpurified toxoid and two large bleeds (30% of total blood volume each) every six weeks rather than 20 ml test bleeds did not affect the titre of the hyperimmune serum produced.