Evaluation of the immune response to diphtheria and tetanus toxoids by the serological methods

Abstract

Scientific relevance. Currently, two types of methods are used to evaluate the potency of diphtheria and tetanus toxoids: the gold standard, which involves administering toxins to immunised animals, and serological methods, which involve quantifying protective antibodies in the serum of immunised animals. International validation studies of serological methods for assessing the potency of diphtheria and tetanus toxoids have resulted in revisions to the relevant chapters of the WHO Manual for Quality Control of Diphtheria, Tetanus and Pertussis Vaccines, as well as the European, Japanese, and several other pharmacopoeias. Consequently, some diphtheria and tetanus vaccine manufacturers have substituted the potency evaluation methods that require administering toxins to animals with alternative serological methods.Aim. The study aimed to assess the suitability, reproducibility, and feasibility of an alternative enzyme immunoassay method (ELISA) for assessing the potency of diphtheria and tetanus components of diphtheria, tetanus, and whole-cell pertussis (DTwP) vaccines and to determine the possibility of implementing this method in Russia using standards and reagents manufactured in the country.Materials and methods. The study used combined vaccines for diphtheria prophylaxis, diphtheria toxin, and reference vaccines. The potency of diphtheria and tetanus toxoids was determined in guinea pigs and mice by the pharmacopoeial lethal challenge method and an alternative ELISA method.Results. ELISA and lethal challenge methods demonstrated comparable results of potency determination: 230 IU/mL vs 264 IU/mL (diphtheria toxoid), 188 IU/mL vs 160 IU/mL (tetanus toxoid), respectively. As opposed to the lethal challenge, the serological method allows testing the efficacy of several antigens in the same animals. The study showed the possibility of using purified diphtheria and tetanus toxoids manufactured in Russia as coating antigens. The authors obtained comparable antibody titres for each animal, using plates coated with international standards and Russian-made diphtheria and tetanus toxoids.Conclusions. The authors demonstrated the possibility of using ELISA to determine the potency of diphtheria and tetanus toxoids in DTwP vaccines. Moreover, the study demonstrated the suitability of Russian purified diphtheria and tetanus toxoids as coating antigens. Researchers should continue working on enzyme immunoassay validation with a view to harmonising national and international methods for assessing the potency of diphtheria and tetanus toxoid vaccines, as these efforts will not only facilitate the registration of foreign vaccines in Russia but also accelerate the approval of Russian vaccines in other countries.