Abstract
Probiotics and probiotic-related nutritional interventions have been described to have beneficial effects on immune homeostasis and gut health. In previous studies, Lactobacillus rhamnosus GG (LGG) soluble mediators (LSM) have been demonstrated to exert beneficial effects in preclinical models of allergic sensitization, bacterial infection, and intestinal barrier function. In the context of allergic diseases, differentiation of dendritic cells (DCs) and their interactions with T cell populations are crucial for driving tolerogenic responses. In this study, we set out to evaluate whether these LSM can modulate DC maturation and have an impact on prompting protective and/or tolerogenic T cell responses. Monocytes were isolated from PBMC of healthy blood donors and cultured in the presence of GM-CSF, IL-4, and LSM or unconditioned bacterial culture medium control (UCM) during 6 days to induce DC differentiation. Subsequently, these DCs were matured in the presence of TNF-α for 1 day and analyzed for their phenotype and ability to induce autologous T cell activation and differentiation to model recall antigens. After 7 days of co-culture, T cells were analyzed for activation and differentiation by flow cytometry of intracellular cytokines (IFN-γ, IL-2, IL-10, and IL-17A), activation markers (CD25), and Foxp3+ expression. LSM did not alter DC numbers or maturation status. However, these DCs did show improved capacity to induce a T cell response as shown by increased IL-2 and IFN-γ producing T cell populations upon stimulation with recall antigens. These enhanced recall responses coincided with enhanced Foxp3+ expression that was not observed when T cells were cultured in the presence of UCM-treated DCs. By contrast, the number of activated T cells (determined by CD25 expression) was only slightly increased. In conclusion, this study reveals that LSM can influence adaptive immune responses as shown by the modulation of DC functionality. These mechanisms might contribute to previous observed effects in animal models in vivo. Altogether, these results suggest that LSM may provide an alternative to live probiotics in case life bacteria may not be used because of health conditions, although further clinical testing is needed.
Highlights
Dendritic cells (DCs) are sensitive to immunomodulatory effects of harmless and endogenous bacteria through pattern-recognition receptors such as caspase recruitment domain 15 and toll-like receptor (TLR)2 [1]
No significant differences could be detected in the number of cells expressing CD11c after 6 days of culture between cells incubated in the presence of LGG soluble mediators (LSM) compared with unconditioned bacterial culture medium (UCM)
dendritic cells (DCs) differentiated from monocytes at day 6 of culture in the presence of LSM showed a non-significant increase in CD86 and CD40 expression compared with DC diffe rentiated in UCM (Figure 1B)
Summary
Dendritic cells (DCs) are sensitive to immunomodulatory effects of harmless and endogenous (microbiota) bacteria through pattern-recognition receptors such as caspase recruitment domain 15 and toll-like receptor (TLR)2 [1]. In particular for Lactobacillus rhamnosus GG (LGG), culture supernatants were shown to protect intestinal epithelial cells from apoptosis, to promote their proliferation, and to attenuate alcohol or hypoxiainduced impaired epithelial cell resistance and permeability in vitro [5,6,7] These benefits have been linked to structural components or bioactive compounds, for example, immunomodu latory effects of the pili structure, stimulation of cell proliferation and protection from apoptosis induced by secreted proteins p40 and p75, and improved stress adaptability in the host by LGG exopolysaccharides [reviewed by Segers and Lebeer [8]]. These observations support our previous work with soluble mediators obtained from the late-exponential growth phase (LEG) of LGG on improved allergic airway inflammation in an ovalbumin-induced acute allergic airway inflammation mouse model and reduced local and systemic inflammation among neonatal rats in a model for short bowel syndrome [10, 11]
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