In the past decade, extracellular vesicles (EVs) have been recognized as potent vehicles of intercellular communication, both in prokaryotes and eukaryotes. This is due to their capacity to transfer proteins, lipids and nucleic acids, thereby influencing various physiological and pathological functions of both recipient and parent cells. While intensive investigation has targeted the role of EVs in different pathological processes, for example, in cancer and autoimmune diseases, the EV-mediated maintenance of homeostasis and the regulation of physiological functions have remained less explored. Here, we provide a comprehensive overview of the current understanding of the physiological roles of EVs, which has been written by crowd-sourcing, drawing on the unique EV expertise of academia-based scientists, clinicians and industry based in 27 European countries, the United States and Australia. This review is intended to be of relevance to both researchers already working on EV biology and to newcomers who will encounter this universal cell biological system. Therefore, here we address the molecular contents and functions of EVs in various tissues and body fluids from cell systems to organs. We also review the physiological mechanisms of EVs in bacteria, lower eukaryotes and plants to highlight the functional uniformity of this emerging communication system.

Extracellular vesicles (EVs), such as exosomes and microvesicles, are released by different cell types and participate in physiological and pathophysiological processes. EVs mediate intercellular communication as cell-derived extracellular signalling organelles that transmit specific information from their cell of origin to their target cells. As a result of these properties, EVs of defined cell types may serve as novel tools for various therapeutic approaches, including (a) anti-tumour therapy, (b) pathogen vaccination, (c) immune-modulatory and regenerative therapies and (d) drug delivery. The translation of EVs into clinical therapies requires the categorization of EV-based therapeutics in compliance with existing regulatory frameworks. As the classification defines subsequent requirements for manufacturing, quality control and clinical investigation, it is of major importance to define whether EVs are considered the active drug components or primarily serve as drug delivery vehicles. For an effective and particularly safe translation of EV-based therapies into clinical practice, a high level of cooperation between researchers, clinicians and competent authorities is essential. In this position statement, basic and clinical scientists, as members of the International Society for Extracellular Vesicles (ISEV) and of the European Cooperation in Science and Technology (COST) program of the European Union, namely European Network on Microvesicles and Exosomes in Health and Disease (ME-HaD), summarize recent developments and the current knowledge of EV-based therapies. Aspects of safety and regulatory requirements that must be considered for pharmaceutical manufacturing and clinical application are highlighted. Production and quality control processes are discussed. Strategies to promote the therapeutic application of EVs in future clinical studies are addressed.

Summary Stem cell regulation and hierarchical organization of human skeletal progenitors remain largely unexplored. Here, we report the isolation of a self-renewing and multipotent human skeletal stem cell (hSSC) that generates progenitors of bone, cartilage, and stroma, but not fat. Self-renewing and multipotent hSSCs are present in fetal and adult bones and can also be derived from BMP2-treated human adipose stroma (B-HAS) and induced pluripotent stem cells (iPSCs). Gene expression analysis of individual hSSCs reveals overall similarity between hSSCs obtained from different sources and partially explains skewed differentiation towards cartilage in fetal and iPSC-derived hSSCs. hSSCs undergo local expansion in response to acute skeletal injury. In addition, hSSC-derived stroma can maintain human hematopoietic stem cells (hHSCs) in serum-free culture conditions. Finally, we combine gene expression and epigenetic data of mouse skeletal stem cells (mSSCs) and hSSCs to identify evolutionarily conserved and divergent pathways driving SSC-mediated skeletogenesis.

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Mesenchymal stromal cells (MSCs) are promising candidates for novel cell therapeutic applications. For clinical scale manufacturing, human factors from serum or platelets have been suggested as alternatives to fetal bovine serum (FBS). We have previously shown that pooled human serum (HS) and thrombin-activated platelet releasate in plasma (tPRP) support the expansion of adipose tissuederived MSCs. Contradictory results with bone marrow (BM)-derived MSCs have initiated a comprehensive comparison of HS, tPRP, and pooled human platelet lysate (pHPL) and FBS in terms of their impact on MSC isolation, expansion, differentiation, and immunomodulatory activity. In addition to conventional Ficoll density gradient centrifugation, depletion of lineage marker expressing cells (RosetteSep) and CD271 1 sorting were used for BM-MSC enrichment. Cells were cultured in medium containing either 10% FBS, HS, tPRP, or pHPL. Colony-forming units and cumulative population doublings were determined, and MSCs were maximally expanded. Although both HS and tPRP comparable to FBS supported isolation and expansion, pHPL significantly accelerated BM-MSC proliferation to yield clinically relevant numbers within the first two passages. MSC quality and functionality including cell surface marker expression, adipogenic and osteogenic differentiation, and immunosuppressive action were similar in MSCs from all culture conditions. Importantly, spontaneous cell transformation was not observed in any of the culture conditions. Telomerase activity was not detected in any of the cultures at any passage. In contrast to previous data from adipose tissue-derived MSCs, pHPL was found to be the most suitable FBS substitute in clinical scale

Replacing FBS with HPL prevents bovine prion, viral, and zoonose contamination of the stem cell product. This new efficient FBS-free two-step procedure for clinical-scale MSC propagation may represent a major step toward challenging new stem cell therapies.

Key Points Epigenetics and in vivo behavior can distinguish MSCs from different sources. BM-derived MSCs form a hematopoietic niche via a vascularized cartilage intermediate.

Key Points VCAM-1/VLA-4 triggers reciprocal NF-κB activation in leukemia and stromal cells and mediates cross-talk between leukemia and stromal cells. VCAM-1/VLA-4 and NF-κB signaling plays a pivotal role in the development of leukemia chemoresistance.