Background and Purpose: The gut immune system is daily exposed to inorganic particles used in the food industry as food additives or processing aids, such as silicon dioxide (SiO2), a common anticaking agent in manufacturing powdered foods (milk, infant formulae, instant soups), ingredients (spices, salt, flours) and supplements. In the gut, a state of immune unresponsiveness to innocuous dietary antigens such as food proteins (known as oral tolerance, OT) is established by the interaction of diverse immune cells to prevent food sensitivities, including allergy. In physiological conditions, the key event for inducing OT is the antigen uptake by intestinal dendritic cells (DCs) and their migration to mesenteric lymph nodes (MLN). In the MLN, DCs produce TGF-β and present antigen to naïve Tcells, mediating their conversion into Treg and Tr1 cells that secrete the anti-inflammatory cytokines IL-10 and TGF-β with suppressor function on immune responses. In a previous study using a mouse model of OT induction to the food antigen model ovalbumin, we showed that chronic oral exposure to food-grade (fg) SiO2 led to OT breakdown with a decreased production of IL-10 and TGF-β by MLN cells, and intestinal inflammation. Here we investigated ex vivo whether these fg-SiO2-mediated immunological defects could result from a direct interaction of SiO2 particles with immune cells in the MLN. Methods: MLN cells of untreated mice (C57BL/6) were exposed to fg-SiO2 (0, 6.25, 12.5, 25 and 50 µg/mL) for 24, 48 or 72h. Cell viability was assessed by labelling cells with propidium iodide. The production of IL-10, IFN-γ and TGF-β by fg-SiO2 exposed-MLN cells was evaluated by ELISA following unspecific simulation with phorbol 12-myristate 13-acetate (PMA) and ionomycin (PMA/iono) of all MLN cells, or anti-CD3/anti-CD28 stimulation specific of T cell subsets. The populations of MLN cells producing these cytokines were identified by flow cytometry. To assess the effects of fg-SiO2 on T cell proliferation, MLN cells were exposed for 3 days to concanavalin-A (a T-cell mitogen) in the presence of fg-SiO2, and the frequency of dead and proliferative T cells was determined by flow cytometry. All animal experiments were approved by the Local Animal Care and Use Committee. Results: Cell viability was not impacted after exposure to fg-SiO2 regardless of the dose tested. A decreased production of TGF-β was observed in anti-CD3/CD28 stimulated T cells after exposure to the highest dose of fg-SiO2 while TGF-β secretion was not altered after PMA/iono stimulation. In the presence of fg-SiO2, the production of the anti-inflammatory cytokine IL-10 was decreased whatever MLN cell stimulation, but the decrease was only significant in PMA/iono stimulated cells exposed at the highest doses of the food additive. The production of IFN-γ by MLN cells was not modulated in presence of fg-SiO2 regardless the stimulation used. Flow cytometry analysis showed that the immunological effects of fg-SiO2 were due to a decreased production of IL-10 and TGF-β by Treg cells (CD3+ CD4+ CD25+ FOXP3+ IL-10+) and DCs (CD45+ CD103+ CD11b+ TGF-β+), respectively. In proliferative condition, fg-SiO2 treatment of MLN cells evoked a dose-dependent decrease of T cell proliferation without reduction in T cell viability. This suggested that the fg-SiO2-induced failure in IL-10 and TGF-β secretion could be also partly due to an anti-proliferative effect of the food additive. Conclusions: Taken together with our previous study, these data show that the ability of the food additive SiO2 to block the establishment of OT is partly due to a direct interaction with MLN cells, reducing T cell proliferation and decreasing the secretion of the suppressive factors IL-10 and TGF-β by Treg cells and DCs. This raises the hypothesis that chronic exposure to SiO2 through the diet could act as an environmental trigger for loss of OT, possibly contributing to the development of food sensitivities. Our results should encourage epidemiological research on potential adverse effects related to the chronic intake of food additive SiO2, in particular, its association with food allergies and intolerances.