Selection of Criteria for Assessing Protective Immunity at Different Times of Experimental Tularemia in a Mouse Model

Abstract

It is known that the body's defense against infection by the intracellular bacterium Francisella tularensis is provided by the activation of the cellular and humoral immune response. However, their role in long-term protection (25 years and more) against virulent strains of F. tularensis is not well understood. The identification of clear criteria for assessing protective immunity to the tularemia causative agent at different times after vaccination will make it possible to more efficiently develop new genetically determined vaccine strains. The goal of our research was to select and assess immunological parameters reflecting the protective properties of the vaccine strain F. tularensis 15 NIIEG and its derivatives, F. tularensis 15/23-1∆recA and F. tularensis 15/ 23-1/sodB∆recA, in the long term after immunization. To assess the functional activity of T and B cells, flow cytometry was used.The assessment of the production of cytokines IFN-γ, IL-4, IL-10, IL-17A and titers of specific class G immunoglobulins to F. tularensis lipopolysaccharide (LPS)in blood serum was performed by ELISA on days 30, 60, 90 and 180 after immunization. Evaluation of the protective properties of vaccine preparations in the above-mentioned terms was carried out after subcutaneous infection with test-infecting virulent strains, Schu and 503 of tularensis and holarctica subspecies, respectively. It was shown that vaccination with the studied strains in 100% of cases protected from infection with the strain 503 of the holarctica subspecies, analogous to the vaccine strain. When infected with a virulent Schu strain of the hetrologous tularensis subspecies, a decrease in the effectiveness of protection was observed starting from 60 days after immunization. Evaluation of immunological parameters showed that at all studied periods after immunization, IgG antibodies to F. tularensis LPS were detected in the blood sera of immunized mice. In vitro experiments on stimulation of immune response in spleen lymphocytes of vaccinated mice to the F. tularensis antigen showed a significant increase in the level of secreted IFN-γ, a slight increase in secreted IL-10 and an enhanced expression of the CD69 molecule on the surface of T and B cells. Thus, the level of IFN-γ and the expression of the CD69 molecule on the surface of T and B cells in response to restimulation of lymphocytes of immune animals with tularemia antigen can serve as criteria for immune protection in experimental tularemia in a mouse model at different times after vaccination. Key words: vaccine strain, Fransicella tularensis, immunogenicity, protection, memory T cells, IgG, cellular immunity Funding - The work was supported by the Branch Program of the Russian Federal Service for Surveillance on Consumer Rights Protection and Human Wellbeing.