**1. Introduction**

Brucellosis is a worldwide zoonotic disease caused by facultative intracellular pathogen of the genus *Brucella* [1]. Bacteria of the genus *Brucella* infect a wide variety of land and aquatic mammals, including pigs, cattle, goats, sheep, dogs, dolphins, whales, seals, and desert wooden mice. Traditionally, the genus *Brucella* consisted of six recognized species, grouped according to their primary host preferences, i.e., *B. abortus*, bovine; *B. melitensis*, sheep and goats; *B. suis*, pigs; *B. ovis*, sheep; *B. kennels*, dogs; and *B. neotomae*, desert wood mice. Recent new species were isolated from humans (*B. inopinata*), aquatic mammals (*B. pinnipedialis* and *B. ceti*), and from a common rat (*B. microti*), raising the current number to 10 species of the genus [2]. Human brucellosis can be mainly caused by *Brucellaabortus* and *Brucella melitensis*, leading not only to cases of morbidity but also severe economic losses caused mainly by abortions and infertility in infected animals [3]. The natural infection by *Brucella* occurs mainly by the oral and nasal routes through consumption of raw milk and unpasteurized dairy products from infected animals, inhalation of aerosols containing the pathogen, contact with infected animals and their secretions, and by the habit of cattle to lick and smell newborn animals or even aborted fetuses. In addition, there is laboratory and occupational contamination, a ffecting researchers, farmers, slaughterhouse workers, butchers, and veterinary doctors (many cases of accidental self-inoculation of the vaccine against animal brucellosis), and there are forms (although very unlikely) of human transmission such as contamination of plants by feces and urine from infected animals, and breastfeeding [4–7].

Brucellosis is a systemic disease in which any organ or tissue of the body might be involved. A ffected individuals present nonspecific symptoms shared with several other diseases, which cause human brucellosis to present underestimated data of epidemiological distribution [8]. In humans, the main symptoms of the acute phase of the disease are undulating fever, headaches, fatigue, myalgia, and weight loss. In the chronic phase of the disease endocarditis, arthritis, osteomyelitis, and neurological complications can be observed [9]. In animals, brucellosis is a chronic infection that persists throughout life. In females, *Brucella* causes tropism through the bovine placental hormone, erythritol, leading to lesions in the uterine glands, while in males, the bacterium causes tropism through male hormones like testosterone, addressing the testicles. Thus, *Brucella* infection primarily affects the reproductive organs causing abortion and infertility [10].

*Brucella* spp. can resist death by neutrophils, and replicate within macrophages and dendritic cells, thus, maintaining a long lasting interaction with the host cells [11]. Therefore, innate immunity has developed important mechanisms for detecting and eliminating these bacteria. Toll-like receptors (TLRs) have already proven to be important in the control of *Brucella abortus* infection. The recognition of *B. abortus* molecules by TLR2 (external membrane proteins, Omp16 and Omp19), TLR4 (*Brucella* LPS and *Brucella* lumazine synthase), and TLR9 (*Brucella* DNA), activates intracellular signaling via MyD88, resulting in the activation of NF-κB, MAP kinases, and expression of pro-inflammatory cytokines [12–16]. TLR2 does not participate in the in vivo control of infection, contributing only to the production of pro-inflammatory cytokines [12,14]. However, TLR9 has played a prominent role in relation to in vivo and in vitro control of *B. abortus* infection [17]. In addition to the receptors mentioned above, *B. abortus* leads to activation of NLRP3 (through reactive mitochondrial oxygen species induced by bacteria) and AIM2 (recognition of bacterial DNA) inflammasomes, leading to activation of innate immunity and infection control [18]. The STING protein was also determined as an important adapter molecule required for resistance against this bacterium [19,20].

In the context of intestinal immunity, it is important to highlight the importance of these receptors in this microenvironment, since several of these innate immunity receptors such as the TLRs, NRLs, G protein-coupled receptors (GPCRs), and STING are also expressed in the intestinal mucosa, having an important function in the maintenance of host commensal microbiota and intestinal homeostasis [21–23]. Among these innate immunity receptors, the ST2 receptor and its ligand, cytokine IL-33 has been widely studied since its discovery [24]. The ST2 receptor of the IL-1 family, also called IL1rl1, tumorigenicity suppressor 2, growth stimulation expressed in gene 2 and serum stimulation 2, was classified as a receptor for IL-33 in 2005 [25,26].There are four isoforms encoded by the ST2 gene. The two most prominent isoforms include the ST2L transmembrane, which acts as a membrane receptor, responsible for binding the IL-33 and activating the signaling cascades to improve the functions of the cells that express this receptor, and the sST2, presented in a soluble form, which acts by sequestering the free IL-33, preventing its signaling. They are the consequence of a double system of promoters (sST2 proximal promoter and ST2L distal promoter) that results in the di fferential expression of mRNA. ST2L, like other IL-1 receptors, consists of an extracellular domain, transmembrane domain and cytoplasmic domain (Toll/interleukin-1 receptor (TIR)), while sST2 does not have the transmembrane and cytoplasmic domains and therefore exists as a soluble protein. In addition, alternative splicing results in the formation of ST2V and ST2LV. ST2V shares the same extracellular and transmembrane domain as ST2L, but is remarkable for its unique hydrophobic tail and is particularly enriched in the gastrointestinal tract. Finally, ST2LV notably does not have the ST2L transmembrane domain, but maintains the intracellular domain [27,28].The ST2 receptor is expressed in a wide variety of immune cells, such as conventional T cells, particularly regulatory T cells (T regs) [29], innate type 2 lymphoid cells (ILC2) [30], polarized macrophages M2 [31], eosinophils [32], basophils [33], neutrophils [33], NK cells [34], iNKT cells [34], and several other immune cells and their soluble isoform.sST2 can be produced spontaneously by the small intestine.

As an alarmine, IL-33 is one of the first molecules that "sounds the alarm" to indicate that there has been a violation of the primary defenses of the intestinal epithelium against pathogens and other threats [35]. IL-33 is produced by a variety of stromal cells and organ parenchyma, such as smooth muscle cells, fibroblasts, myofibroblasts, endothelial cells, glia cells, osteoblasts, adipocytes, and by di fferent cells of the immune system, such as macrophages, dendritic cells, and mast cells [36]. IL-33 acts on several cell types, including cells of hematopoietic origin and non-hematopoietic cells. The secretion of IL-33 has been described in monocyte lineage (THP-1 cells), in response to di fferent stimuli—bacterial infection, lipopolysaccharide (LPS) with aluminum adjuvant, and isolated LPS. [29,32]. In order to maintain the integrity of this mucosal barrier, the intestinal epithelium undergoes rapid and continuous self-renewal to replace the damaged cells. Activation of the IL-33/ST2 pathway in epithelial progenitor cells leads to inhibition of Notch signaling and results in di fferentiation of stem cells towards a line of secretory intestinal cells [37], resulting in the production of mucin, an important barrier mechanism of intestinal immunity, decreasing the interaction of the intestinal epithelium and pathogenic bacteria [38]. Moreover, the activation of this axis is important to recruit and activate innate immune cells, inducing Th1 or Th2 responses, according to the required immune response [39]. Although brucellosis is a worldwide zoonosis, the mechanisms involved during the course and establishment of the natural oral infection by *Brucella abortus* are still poorly studied. With regard to the process of invasion of *Brucella* through mucosal barriers, there are few studies on the mechanisms involved in the ability of this pathogen to interact with the epithelial cells of the gastrointestinal (GI) tract with the host microbiota, and also with the subsequent immune and homeostatic response in the gastrointestinal tract. The intraperitoneal infection pathway is the most commonly used in studies using the murine model. This route favors the immediate systemic dissemination of *Brucella* and its proliferation in lymphoid tissues, especially in the spleen. However, considering that the oral route is the main route of natural infection in humans and animals, there is a need to understand the mechanisms of the establishment of oral infection, so new therapeutic strategies can be developed in order to control this disease. Since, the IL-33/ST2 axis is positioned to interact with the main components of the intestine, which include epithelial cells in response to cell damage and a microbiome composed of commensal bacteria and immune mucosal cells [24], we investigated the role of the ST2 receptor in the immune response against *Brucella abortus* oral infection.
