Summary: Mesenchymal stromal cells (MSCs) sense and modulate inflammation and represent potential clinical treatment for immune disorders. However, many details of the bidirectional interaction of MSCs and the innate immune compartment are still unsolved. Here we describe an unconventional but functional interaction between pro-inflammatory classically activated macrophages (M1MΦ) and MSCs, with CD54 playing a central role. CD54 was upregulated and enriched specifically at the contact area between M1MФ and MSCs. Moreover, the specific interaction induced calcium signaling and increased the immunosuppressive capacities of MSCs dependent on CD54 mediation. Our data demonstrate that MSCs can detect an inflammatory microenvironment via a direct and physical interaction with innate immune cells. This finding opens different perspectives for MSC-based cell therapy. : Mesenchymal stromal cells (MSCs) are promising for cell-based therapy in inflammatory disorders by switching off the immune response. Varin and colleagues demonstrate that MSCs and inflammatory macrophages communicate via an unconventional but functional interaction that strongly increases the immunosuppressive capacities of MSCs. This new communication between the innate immune system and MSCs opens new perspectives for MSC-based cell therapy. Keywords: macrophages, bone marrow mesenchymal stromal cells, functional interaction, CD54, immunosuppression, indoleamine 2,3-dioxygenase, cell therapy