Immunostimulatory effects triggered by Enterococcus faecalis CECT7121 probiotic strain involve activation of dendritic cells and interferon-gamma production

MATÍAS ALEJANDRO MOLINA; AILÉN MAGALÍ DÍAZ; CHRISTINA HESSE; WIEBKE GINTER; MARÍA VIRGINIA GENTILINI; GUILLERMO GABRIEL NUÑEZ; ANDREA MERCEDES CANELLADA; TIM DOMINIK SPARWASSER; LUCIANA BEROD; MARISA SILVIA CASTRO; MARCELA ALEJANDRA MANGH

PLOS ONE

PUBLIC LIBRARY SCIENCE

Lugar: San Francisco; Año: 2015

1932-6203

The gastrointestinal microbiota varies in a continuum range between mutualism and pathogenicity, as a consequence of both residential and ingested microorganisms [1]. This microbiota is essential to human health and imprints unique characteristics in each human being; it contributes to food digestion and the development and optimal functioning of the immune system. Moreover, an imbalance of these microorganisms has not only local but also systemic consequences, as observed in cases of intestinal inflammation and infection [1]. Therefore, maintenance of the correct balance and even an enhancement of the beneficial effects of the microbiota is an opportunity for treatments both in health and disease conditions. In the last years, the increased interest in the beneficial functions of the human microflora has resulted in the selection of specific species with putative health-promoting capacities. These selected microorganisms, recognized as probiotics, are defined as ?live microorganisms which when administered in adequate amounts confer a health benefit on the host? [2]. Clinical applications of probiotics include the prevention and treatment of gastrointestinal infections, inflammatory bowel diseases, allergies, and use as adjuvants in vaccination [1]. The mechanisms by which probiotics exert these beneficial effects are diverse and can be classified into three main categories: (1) those involving the influence on other microorganisms (especially pathogens); (2) those by which the barrier function played by the intestinal epithelium is enhanced; and (3) those involving the modulation of the immune system. While the first one is the best-studied effect, the immunomodulatory mechanisms are less understood and seem to be genus/strain specific [1]. Understanding how these microorganisms contribute to improve the health status is still a challenge, and a better knowledge of how probiotic bacteria interact with host cells is needed for their optimized application. Dendritic cells (DCs) include a heterogeneous population of cells whose central function is to present antigens. In contrast to other antigen-presenting cells, DCs have the unique capacity of stimulating primary immune responses. Besides, they can induce either immunogenic or tolerogenic immune responses depending upon the type of DC and the nature of the antigen encountered [3-5]. DCs recognize and respond to microbial structures via pattern-recognition receptors (PRRs) including Toll-like receptors (TLRs) and lectins, among others [6-7]. Recognition of pathogen-associated molecular patterns (PAMPs) by these receptors results in functional changes on DCs including up-regulation of their migratory capacity, expression of co-stimulatory molecules, cytokines production, etc. [8]. Different stimuli induce the production of specific cytokines scenarios that are responsible for the outcome of adequate immune responses in each case. Interleukin-(IL-)12-producing DCs are responsible for the shift of the adaptive immune response towards a T helper (Th) 1-profile, which favors the development of inflammatory, cell-mediated, cytotoxic immune responses. IL-6 and Tumor Necrosis Factor alpha (TNFα) are pro-inflammatory cytokines that have important effects in systemic inflammation. In contrast, IL-10 secreted by DCs has anti-inflammatory actions and is important in the generation of T regulatory cells. Despite all the available information about the understanding of DC general functions, the mechanisms by which DCs respond to probiotics in the intestine and select appropriate immune responses have been poorly studied [1]. Most microorganisms that are considered probiotics are generally selected from the Lactobacillus or Bifidobacterium genera. Other probiotics that belong to the Gram-positive Lactic Acid Bacteria include Enterococcus faecalis strains, and even Gram-negative microorganisms such as Escherichia coli Nissle 1917 or yeasts identified as Saccharomyces boulardii have also been demonstrated to confer beneficial effects to the host [1]. Enterococcus faecalis CECT7121 is an interesting candidate for being considered as a probiotic strain [9-12]. This microorganism has been recovered from a corn silage at an establishment located in the city of Tandil (Argentina). Dwellers of Tandil devote themselves to breeding and commercializing farm animals (especially cows). Cattle fed on this corn silage stand out among specimens of other facilities in the region, showing an optimum state of health and weight gain above the regional average. Employing several murine models, we have previously demonstrated that E. faecalis CECT7121 implants itself and persists in the gastrointestinal tract [9], and enhances and skews the profile of cytokines to the Th1 phenotype in situations such as vaccination, anti-tumoral immunity, and allergic reactions [10-12]. In these models, we have shown that the intragastric administration of E. faecalis CECT7121 can induce a stronger cellular anti-tetanic and anti-diphtheric response in BALB/c mice immunized with Diphtheria-Tetanus-Bordetella pertussis vaccine [10]. This probiotic is also able to reduce the mortality of animals challenged with a lethal dose of the murine LBC lymphoma cells, triggering mechanisms that include the generation of a Th1 response characterized by a high production of IL-12 and interferon-gamma (IFNγ) [11]. Finally, this bacterium prevents the establishment of an allergic response against ovalbumin, diminishing Immunoglobulin-E (IgE) specific titers as well as Th2 cytokines levels (i.e. IL-4, IL-5, and IL-13), and impairing the generation of active cutaneous anaphylactic reactions [12]. In the present work, we intended to elucidate the main mechanisms that are triggered by E. faecalis CECT7121 and that lead to the beneficial effects observed in mice. In this context, we analyzed the effects caused by the stimulation of DCs and the influence of this activation in the establishment of Th responses. In addition, we studied the role of regulatory cells in the beneficial effects conferred by E. faecalis CECT7121. Our results allowed us to conclude that the activation of DCs might be the most relevant consequence after inoculation of this probiotic, skewing the adaptive immune response towards the production of IFNγ. This work postulates E. faecalis CECT7121 as a novel adjuvant candidate for mucosal immunostimulation, in both oral and systemic vaccination strategies.