Evaluation of Humoral and Cellular Immune Responses to the Conjunctival Brucella abortus S19 Vaccine in Türkiye

Introduction: Brucellosis is a zoonotic infection caused by Brucella species, which have been identified in various hosts and are associated with significant economic losses. The control of bovine brucellosis in Türkiye has primarily relied on the administration of the Brucella abortus S19 vaccine via the conjunctival route. The present study investigates the humoral immune response induced by the administration to cattle of the vaccine produced in Türkiye, with the Rose Bengal Plate Test (RBPT), an enzyme-linked immunosorbent assay (ELISA), and a complement fixation test (CFT), and evaluates cellular immunity through flow cytometry methods. Materials and Methods: For the purpose of the study, blood samples were collected from 20 nonvaccinated calves aged 3–6 months housed at the Eastern Anatolian Agricultural Research Institute, and from 20 vaccinated calves on days 0, 14, and 28, and months 3, 6, 9, and 12 following vaccination. RBPT, iELISA, and CFT were used to evaluate humoral immunity, and the flow cytometric analysis for the evaluation of cellular immunity included the measurement of CD4 + , CD8 + and CD14 + cell populations, as well as CD4 + IFN-γ + , CD8 + IFN-γ + , and CD14 + IL-12 + levels. Results: The study revealed an increase in CD4 + and CD8 + T cell populations, along with elevated IFN-γ + and IL-12 + cytokine levels in vaccinated animals, which are critically important for the development of cellular immunity. Also observed was a very low rate of vaccine-associated antibody positivity. Conclusions: The study showed that conjunctival vaccination of calves with locally produced B. abortus S19 enhanced cellular immunity (CD4 + , CD8 + , CD14 + , IL-12, IFN-γ) but induced minimal detectable antibody responses.