MSCs are sensors of the

MSCs are sensors of the microenvironment and acquire abilities depending on the inflammatory state of their environment (Bernardo and Fibbe, 2013). Previous studies linked the effect of macrophages on MSCs to a paracrine mechanism. Indeed, M1MФ, via cytokine secretion, inhibit the proliferation and migration of human MSCs and modify their cytokine secretion profile (Freytes et al., 2013). Moreover, MSCs express soluble factors such as IDO and adhesion molecules such as CD54, and become more immunosuppressive in inflammatory environments (Ren et al., 2010). However, we found that innate immune Axitinib Supplier modulated the MSC function in a contact-dependent manner. Indeed, direct contact with M1MФ, via CD54, was required to increase the immunosuppressive abilities of MSCs. M1MФ increased only CD54 expression on MSCs, whereas pro-inflammatory cytokines increase both CD54 and CD106 expression (Ren et al., 2010). The differential expression of CD54 and CD106 on MSCs after interaction with M1MФ confirms that the effect of M1MФ is contact dependent and is not linked to cytokine secretion. The importance of the MSC-macrophage physical crosstalk was previously highlighted in BM. In fact, a subpopulation of macrophages modulated the MSC phenotype to maintain a hematopoietic stem cell niche (Chow et al., 2011). In the secondary lymphoid organs, MSCs structure and organize lymphoid organs and also determine the future adaptive immune response by modulating hematopoietic cells. Moreover, a tight interaction between stromal cells and dendritic cells is associated with exchange of protein membranes (Roozendaal and Mebius, 2011). In this work, we clearly demonstrated that pro-inflammatory M1MΦ closely interact with stromal cells and modify their relationship with adaptive immune cells. An important finding of our study is that an “unconventional synapse” is formed between macrophages and MSCs associated with polarization of CD54 at the contact area. The “conventional” immunological synapse between two immune cells is a polarized, highly organized molecular structure that plays an essential role in communication between immune cells. As in our system, CD54 plays an important role in the formation of functional immune synapses. CD54, also known as intercellular adhesion molecule 1, is a highly glycosylated immunoglobulin superfamily member that binds the leukocyte integrins “leukocyte function antigen 1” (LFA-1) or CD54 itself. CD54 is constitutively expressed on leukocytes, epithelial and resting endothelial cells, or MSCs, and is upregulated in response to inflammatory mediators (Roebuck and Finnegan, 1999). In our system, inhibition of LFA-1 on the macrophage surface did not modify the MSC function (data not shown); therefore, MSCs could interact with macrophages via a homotypic CD54-CD54 interaction independent of LFA-1. CD54 has a crucial role in conventional immunological synapse formation by stabilizing the cell-to-cell conjugate and sustaining signaling. Indeed, at the contact between T cells and antigen-presenting cells, the immunological synapse is formed by the concentration of the T cell receptors and major histocompatibility II complexes in the center of the structure, whereas adhesion receptors such as CD54 segregate to form an external ring, which stabilizes the structure and leads to a strong cell interaction that optimizes intracellular communication (Grakoui et al., 1999; Stinchcombe et al., 2001). Our data show greater expression and enrichment of CD54 in the contact area between MSCs and M1MΦ. However, key events in the immunological synapse are the orientation of the cytoskeleton toward the target cell and the polarization of several organelles such as the centrosome, Golgi apparatus, and ER. In our model, the microtubule-organizing center as well as the actin cytoskeleton did not polarize toward MSCs or macrophages during cell interaction. However, the M1MФ-MSC interaction induced calcium signaling in both cells, which confirms the idea of an “unconventional” but functional synapse between stromal and innate immune cells. Moreover, intracellular and extracellular communication between MSCs and macrophages were highlighted by organelle exchange such as mitochondrial and exosome transfer between both cells (Ekstrom et al., 2013; Phinney et al., 2015).