Toll-like receptor-3-activated human mesenchymal stromal cells significantly prolong the survival and function of neutrophils

Bone marrow-derived mesenchymal stromal cells (BM-MSC) are stromal precursors endowed with extensive immunomodulative properties. In this study, we aimed to assess whether Toll-like receptor(TLR)3- and TLR4-activated BM-MSC influence human neutrophil responses under coculture conditions. We show that TLR3 triggering by poly(I:C) dramatically amplifies, in a more significant manner than TLR4 triggering by LPS, the antiapoptotic effects that resting BM-MSC constitutively exert on neutrophils under co-culture conditions, preserving a significant fraction of viable and functional neutrophils up to 72 hours. In addition, TLR3- and TLR4-activated BM-MSC enhance respiratory burst ability and CD11b expression by neutrophils. The coculture in absence of cell contact and the incubation of neutrophils in supernatants harvested from TLR3- and TLR4-activated BM-MSC yield comparable results in terms of increased survival and immunophenotypic changes, thus suggesting the involvement of endogenous soluble factors. Neutralizing experiments reveal that the biological effects exerted on neutrophils by TLR3-activated BM-MSC are mediated by the combined action of IL-6, IFN-β and GM-CSF, while those exerted by TLR4-activated BM-MSC mostly depend on GM-CSF. MSC isolated from thymus, spleen and subcutaneous adipose tissue behave similarly. Finally, the effects exerted by TLR3- or TLR4-stimulated BM-MSC on neutrophils are conserved even after the previous priming of BM-MSC with IFN-γ and TNF-α. Our data highlight a novel mechanism by which MSC sustain and amplify the functions of neutrophils in response to TLR3- and TLR4-triggering and may consequently contribute to inflammatory disorders.