Iraqi Journal of Veterinary Sciences, Vol. 36, Supplement I, 2022 (27-30) Proceedings of the 7th (1st International) Scientific Conference, College of Veterinary Medicine, University of Mosul 27 Iraqi Journal of Veterinary Sciences www.vetmedmosul.com Immune response strategies of Brucella melitensis and their antigens in rats B.S. Noomi , S.S. Ahmed , H.Y. Khalaf and N.A. Jafar Department of Microbiology, College of Veterinary Medicine, University of Tikrit, Tikrit, Iraq Article information Abstract Article history: Received August 1, 2022 Accepted November 1, 2022 Available online November 25, 2022 Brucella melitensis is an intracellular bacterium and is the main brucella species that cause abortion and placenta retention in sheep and goats. It has many mechanisms to evade the immune response. The current study aimed to investigate Brucella melitensis strategies for producing immune responses in rats after challenging the bacterium. For this purpose, live and killed Brucella melitensis REV1 strain was given to rats subcutaneously, and immunological markers like TLR2, TLR4, IFN- γ, and anti-brucella antibodies were determined. The results showed that the level of immunological markers like TLR2 and TLR4 did not significantly increase in rat groups inoculated with live Brucella melitensis, while it increased in the rats’ groups vaccinated with the sonicated Brucella melitensis; also, the results showed an increase in the level of IFN-γ and anti- brucella antibody titers in all animal groups. The study concluded that the inoculation with killed bacteria and REV1 could protect the animals against challenging doses, as seen when the groups were inoculated with challenge dose of the bacterium. Keywords: Brucella melitensis Immune response REV1 Correspondence: B.S. Noomi vetbashar@tu.edu.iq DOI: 10.33899/ijvs.2022.134990.2431, ©Authors, 2022, College of Veterinary Medicine, University of Mosul. This is an open access article under the CC BY 4.0 license (http://creativecommons.org/licenses/by/4.0/). Introduction Brucella is gram negative intracellular zoonotic bacteria, that causes many diseases to human and animals (1). Brucella melitensis is the main Brucella species that caused abortion and placenta retention in sheep and goats (2). Live attenuated and killed vaccines were used for the protection of animals and humans. REV1 vaccine which is prepared from live attenuated Brucella melitensis strain is the main vaccine types used for sheep and goat’s protective against brucellosis (3). The protective effects of the REV1 vaccine are due to the stimulation of T- helper and T-cytotoxic with their mediators (3,4). Both cellular and humoral immune responses were activated after Brucella infection or vaccination with a live and killed vaccine (5). Innate immunity plays an important role in host protection against Brucella by activation of neutrophil and NK cells. Both macrophages and dendritic cells play an important role as antigens presenting cells which consider the key to immune responses (6,7). Toll-like receptors (TLRs) recognized pathogen associated molecular patterns (PAMPs), which lead to the activation of dendritic cells and produce proinflammatory cytokines (8). They are 13 types of TLR in rats. Bacterial compounds such as LPS, flagella and lipopeptide will be recognized by TLR2, TLR4, TLR5, and TLR9 (9). Both TLR2 and TLR4 can recognized bacterial components and are considered triggers to adaptive immune response. Brucella can evade immune system mechanism by many pathways this activity prevents both TLR2 (outer membrane proteins) and TLR4 (lipopolysaccharide) from activation (10,11). Cellular immune responses against Brucella and other intracellular pathogen involve activation of CD4 and CD8 T-cells, host Protection occurs by Th1 mediated, interferon-γ (IFN-γ) which is an important cytokine of Th1 that stimulate of macrophages (12). The Aim of the study was to investigate the ability of live and