Combined Action of Human Commensal Bacteria and Amorphous Silica Nanoparticles on the Viability and Immune Responses of Dendritic Cells

Dendritic cells (DCs) regulate the host-microbe balance in the gut and skin, tissues likely exposed to nanoparticles (NPs) present in drugs, food, and cosmetics. We analyzed the viability and the activation of DCs incubated with extracellular media (EMs) obtained from cultures of commensal bacteria ( , ) or pathogenic bacteria ( , ) in the presence of amorphous silica nanoparticles (SiO NPs). EMs and NPs synergistically increased the levels of cytotoxicity and cytokine production, with different nanoparticle dose-response characteristics being found, depending on the bacterial species. and EMs plus NPs at nontoxic doses stimulated the secretion of interleukin-1β (IL-1β), IL-12, IL-10, and IL-6, while and EMs plus NPs at toxic doses stimulated the secretion of gamma interferon (IFN-γ), tumor necrosis factor alpha (TNF-α), IL-4, and IL-5. On the contrary, and EMs induced cytokines only when they were combined with NPs at toxic concentrations. The induction of maturation markers (CD86, CD80, CD83, intercellular adhesion molecule 1, and major histocompatibility complex class II) by commensal bacteria but not by pathogenic ones was improved in the presence of noncytotoxic SiO NP doses. DCs consistently supported the proliferation and differentiation of CD4 and CD8 T cells secreting IFN-γ and IL-17A. The synergistic induction of CD86 was due to nonprotein molecules present in the EMs from all bacteria tested. At variance with this finding, the synergistic induction of IL-1β was prevalently mediated by proteins in the case of EMs and by nonproteins in the case of EMs. A bacterial costimulus did not act on DCs after adsorption on SiO NPs but rather acted as an independent agonist. The inflammatory and immune actions of DCs stimulated by commensal bacterial agonists might be altered by the simultaneous exposure to engineered or environmental NPs.