Cells communicate with each other through secreting and releasing proteins and vesicles. Many cells can migrate. In this study, we report the discovery of migracytosis, a cell migration-dependent mechanism for releasing cellular contents, and migrasomes, the vesicular structures that mediate migracytosis. As migrating cells move, they leave long tubular strands, called retraction fibers, behind them. Large vesicles, which contain numerous smaller vesicles, grow on the tips and intersections of retraction fibers. These fibers, which connect the vesicles with the main cell body, eventually break, and the vesicles are released into the extracellular space or directly taken up by surrounding cells. Since the formation of these vesicles is migration-dependent, we named them “migrasomes”. We also found that cytosolic contents can be transported into migrasomes and released from the cell through migrasomes. We named this migration-dependent release mechanism “migracytosis”.

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The promise of cell therapy for repair and restoration of damaged tissues or organs relies on administration of large dose of cells whose healing benefits are still limited and sometimes irreproducible due to uncontrollable cell loss and death at lesion sites. Using a large amount of therapeutic cells increases the costs for cell processing and the risks of side effects. Optimal cell delivery strategies are therefore in urgent need to enhance the specificity, efficacy, and reproducibility of cell therapy leading to minimized cell dosage and side effects. Here, we addressed this unmet need by developing injectable 3D microscale cellular niches (microniches) based on biodegradable gelatin microcryogels (GMs). The microniches are constituted by in vitro priming human adipose-derived mesenchymal stem cells (hMSCs) seeded within GMs resulting in tissue-like ensembles with enriched extracellular matrices and enhanced cell-cell interactions. The primed 3D microniches facilitated cell protection from mechanical insults during injection and in vivo cell retention, survival, and ultimate therapeutic functions in treatment of critical limb ischemia (CLI) in mouse models compared with free cell-based therapy. In particular, 3D microniche-based therapy with 10 5 hMSCs realized better ischemic limb salvage than treatment with 10 6 freeinjected hMSCs, the minimum dosage with therapeutic effects for treating CLI in literature. To the best of our knowledge, this is the first convincing demonstration of injectable and primed cell delivery strategy realizing superior therapeutic efficacy for treating CLI with the lowest cell dosage in mouse models. This study offers a widely applicable cell delivery platform technology to boost the healing power of cell regenerative therapy.C ell-based regenerative therapy holds great promise for repair and restoration of damaged tissues or organs with numerous clinical trials and preclinical animal testing reported for treating complex diseases (1). Common route of cell administration for clinical cell therapy is based on either systematic administration (e.g., i.v. infusion), relying on cells homing to the lesion sites (2), or direct injection of cells into the damaged tissues (3). However, therapeutic benefits of the administered cells are still limited and sometimes irreproducible due to cell loss and cell death (4). Taking cell therapy for ischemic heart diseases as an example, only ∼5% of mesenchymal stem cells (MSCs) survived after being transplanted into an infarcted porcine heart (5). Mechanical damage during injection, high rate of cell loss and leakage to surrounding tissues, cell death due to lack of appropriate cell-cell and cell-matrix interactions in the ischemic and inflammatory lesion tissues could all contribute to poor cell retention, survival, functionality, and reproducibility of the treatment (6, 7).A rational solution to enhance the therapeutic efficacy and reproducibility of cell therapy is to administer a large dose of cells to ensure sufficient number of functional cells ...

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