The inflammatory response is driven by signals that recruit and elicit immune cells to areas of tissue damage or infection. The concept of a mononuclear phagocyte system postulates that monocytes circulating in the bloodstream are recruited to inflamed tissues where they give rise to macrophages. A recent publication demonstrated that the large increase in the macrophages observed during infection was the result of the multiplication of these cells rather than the recruitment of blood monocytes. We demonstrated previously that B-1 cells undergo differentiation to acquire a mononuclear phagocyte phenotype in vitro (B-1CDP), and we propose that B-1 cells could be an alternative origin for peritoneal macrophages. A number of recent studies that describe the phagocytic and microbicidal activity of B-1 cells in vitro and in vivo support this hypothesis. Based on these findings, we further investigated the differentiation of B-1 cells into phagocytes in vivo in response to LPS-induced inflammation. Therefore, we investigated the role of B-1 cells in the composition of the peritoneal macrophage population after LPS stimulation using osteopetrotic mice, BALB/ Xid mice and the depletion of monocytes/macrophages by clodronate treatment. We show that peritoneal macrophages appear in op/op (−/−) mice after LPS stimulation and exhibit the same Ig gene rearrangement (VH11) that is often found in B-1 cells. These results strongly suggest that op/op (−/−) peritoneal “macrophages” are B-1CDP. Similarly, the LPS-induced increase in the macrophage population was observed even following monocyte/macrophage depletion by clodronate. After monocyte/macrophage depletion by clodronate, LPS-elicited macrophages were observed in BALB/ Xid mice only following the transfer of B-1 cells. Based on these data, we confirmed that B-1 cell differentiation into phagocytes also occurs in vivo . In conclusion, the results strongly suggest that B-1 cell derived phagocytes are a component of the LPS-elicited peritoneal macrophage population.

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The potent immunomodulatory activities displayed by mesenchymal stromal cells (MSCs) have motivated their application in hundreds of clinical trials to date. In some countries, they have subsequently been approved for the treatment of immune disorders such as Crohn's disease and graft-versus-host disease. Increasing evidence suggests that their main mechanism of action in vivo relies on paracrine signaling and extracellular vesicles. Mesenchymal stromal cell-derived extracellular vesicles (MSC-EVs) play a prominent role in intercellular communication by allowing the horizontal transfer of microRNAs, mRNAs, proteins, lipids and other bioactive molecules between MSCs and their targets. However, despite the considerable momentum gained by MSC-EV research, the precise mechanism by which MSC-EVs interact with the immune system is still debated. Available evidence is highly context-dependent and fragmentary, with a limited number of reports trying to link their efficacy to specific active components shuttled within them. In this concise review, currently available evidence on the molecular mechanisms underlying the effects of MSC-EV cargo on the immune system is analyzed. Studies that pinpoint specific MSC-EV-borne mediators of immunomodulation are highlighted, with a focus on the signaling events triggered by MSC-EVs in target immune cells. Reports that study the effects of preconditioning or “licensing” in MSC-EV-mediated immunomodulation are also presented. The need for further studies that dissect the mechanisms of MSC-EV cargo in the adaptive immune system is emphasized. Finally, the major challenges that need to be addressed to harness the full potential of these signaling vehicles are discussed, with the ultimate goal of effectively translating MSC-EV treatments into the clinic.

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Mesenchymal stromal cells (MSC) may exert their functions by the release of extracellular vesicles (EV). Our aim was to analyze changes induced in CD34 + cells after the incorporation of MSC-EV. MSC-EV were characterized by flow cytometry (FC), Western blot, electron microscopy, and nanoparticle tracking analysis. EV incorporation into CD34 + cells was confirmed by FC and confocal microscopy, and then reverse transcription polymerase chain reaction and arrays were performed in modified CD34 + cells. Apoptosis and cell cycle were also evaluated by FC, phosphorylation of signal activator of transcription 5 (STAT5) by WES Simple, and clonal growth by clonogenic assays. Human engraftment was analyzed 4 weeks after CD34 + cell transplantation in nonobese diabetic/severe combined immunodeficient mice. Our results showed that MSC-EV incorporation induced a downregulation of proapoptotic genes, an overexpression of genes involved in colony formation, and an activation of the Janus kinase (JAK)-STAT pathway in CD34 + cells. A significant decrease in apoptosis and an increased CD44 expression were confirmed by FC, and increased levels of phospho-STAT5 were confirmed by WES Simple in CD34 + cells with MSC-EV. In addition, these cells displayed a higher colony-forming unit granulocyte/macrophage clonogenic potential. Finally, the in vivo bone marrow lodging ability of human CD34 + cells with MSC-EV was significantly increased in the injected femurs. In summary, the incorporation of MSC-EV induces genomic and functional changes in CD34 + cells, increasing their clonogenic capacity and their bone marrow lodging ability. STEM CELLS 2019;37:1357-1368 SIGNIFICANCE STATEMENTIn the current study, the authors validate for the first time that preincubating human CD34 + cells with extracellular vesicles derived from human mesenchymal stromal cells not only modifies the gene expression of the recipient cells (inducing a downregulation of proapoptotic genes and overexpression of genes involved in colony formation and JAK-STAT pathway) but also significantly increases their in vitro clonogenic ability and, most importantly, increases their 4-week bone marrow lodging ability in vivo in a standard xenotransplantation model. This strategy could potentially be exploited to increase the hematopoietic engraftment in the clinical setting.