Selection of promising immunological tests for assessing the strength
of immunity against plague

Klyueva S.N., Bugorkova S.A., Kravtsov A.L., Kashtanova T.N., Kozhevnikov V.A.

Russian Anti-Plague Institute "Microbe" of Rospotrebnadzor, Saratov, Russia

Due to the lack of approved standards for assessing the level of post-vaccination immunity in people vaccinated against plague, the search for informative methods for characterizing the strength of specific anti-plague immunity in vaccinated individuals remains a relevant area of research.

The goal of this study is to select an optimal set of informative and readily available immunological tests for assessing the effectiveness of anti-plague vaccination by comparing changes in immunological parameters with protection indicators in a model of bubonic plague in guinea pigs.

Materials and methods. On days 3 and 21 after immunization with the Y. pestis EV NIIEG vaccine strain, blood was collected from guinea pigs, then on day 21 they were infected subcutaneously with a culture of the virulent Y. pestis strain 231(708) at a dose of 400 LD50 (2000 CFU/0.5 ml). The production of IFN-γ, IL-4, the titer of specific antibodies to the capsular antigen of the plague microbe (F1), the neutrophil injury index (NII), the phagocytic index (PI), and the phagocytic number (CN) were assessed. In the immunological tests used, ultrasonic disintegrates of the Y. pestis EV NIIEG strain and Y. pestis EV NIIEG pTurboGFP-B cultures grown at 28°С and 37°С (EVUD28, EVUD37 and EVGFP28, EVGFP37, respectively) were used as inducers of cell reactivity.

Results. A significant increase in IFN-γ production was observed by 2.8 times on day 3 and by 2.1 times on day 21 when EVUD37 was used as an inducer compared to samples treated with EVUD28 (p<0.05). On day 3, the PPN index increased by 1.8 times in samples with EVUD37 compared to samples with EVUD28. On day 21, the FI in experiments with EVGFP37 significantly exceeded the corresponding index in experiments with EVGFP28 (p<0.05). In both the experimental and control groups, the PF values in samples with EVGFP37 significantly exceeded the similar indices in samples with EVGFP28. The most informative immunological indicators were the values of IFN-γ and FI, which was confirmed by a strong direct correlation (from 0.92 to 0.99) with protective indicators (survival and life expectancy of animals).

Conclusion. According to our data, the most informative indicators of plague immunity are antigen-stimulated IFN-γ production and the level of granulocyte phagocytic activity. Therefore, these immunological tests can be effectively used to assess the immunological efficacy of plague vaccination in at-risk populations.