EV products obtained from iPSC-derived MSCs show batch-to-batch variations in their ability to modulate allogeneic immune responses in vitro.

Mesenchymal stromal cells (MSCs) have demonstrated therapeutic potential in diverse clinical settings, largely due to their ability to produce extracellular vesicles (EVs). These EVs play a pivotal role in modulating immune responses, transforming pro-inflammatory cues into regulatory signals that foster a pro-regenerative milieu. Our previous studies identified the variability in the immunomodulatory effects of EVs sourced from primary human bone marrow MSCs as a consistent challenge. Given the limited proliferation of primary MSCs, protocols were advanced to derive MSCs from GMP-compliant induced pluripotent stem cells (iPSCs), producing iPSC-derived MSCs (iMSCs) that satisfied rigorous MSC criteria and exhibited enhanced expansion potential. Intriguingly, even though obtained iMSCs contained the potential to release immunomodulatory active EVs, the iMSC-EV products displayed batch-to-batch functional inconsistencies, mirroring those from bone marrow counterparts. We also discerned variances in EV-specific protein profiles among independent iMSC-EV preparations. Our results underscore that while iMSCs present an expansive growth advantage, they do not overcome the persistent challenge of functional variability of resulting MSC-EV products. Once more, our findings accentuate the crucial need for batch-to-batch functional testing, ensuring discrimination of effective and ineffective MSC-EV products for considered downstream applications.

Cell-to-cell spread inhibiting antibodies constitute a correlate of protection against herpes simplex virus type 1 reactivations: A retrospective study.

Background: Herpes simplex viruses (HSV) cause ubiquitous human infections. For vaccine development, knowledge concerning correlates of protection is essential. Therefore, we investigated (I) if humans are in principle capable producing cell-to-cell spread inhibiting antibodies against HSV and (II) whether this capacity is associated with a reduced HSV-1 reactivation risk. Methods: We established a high-throughput HSV-1-ΔgE-GFP reporter virus-based assay and evaluated 2,496 human plasma samples for HSV-1 glycoprotein E (gE) independent cell-to-cell spread inhibiting antibodies. Subsequently, we conducted a retrospective survey among the blood donors to analyze the correlation between the presence of cell-to-cell spread inhibiting antibodies in plasma and the frequency of HSV reactivations. Results: In total, 128 of the 2,496 blood donors (5.1%) exhibited high levels of HSV-1 gE independent cell-to-cell spread inhibiting antibodies in the plasma. None of the 147 HSV-1 seronegative plasmas exhibited partial or complete cell-to-cell spread inhibition, demonstrating the specificity of our assay. Individuals with cell-to-cell spread inhibiting antibodies showed a significantly lower frequency of HSV reactivations compared to subjects without sufficient levels of such antibodies. Conclusion: This study contains two important findings: (I) upon natural HSV infection, some humans produce cell-to-cell spread inhibiting antibodies and (II) such antibodies correlate with protection against recurrent HSV-1. Moreover, these elite neutralizers may provide promising material for immunoglobulin therapy and information for the design of a protective vaccine against HSV-1.

Qualification of a multidonor mixed lymphocyte reaction assay for the functional characterization of immunomodulatory extracellular vesicles.

BACKGROUND AIMS: Extracellular vesicles (EVs), including exosomes and microvesicles, are released by almost all cells and found in all body fluids. Unknown proportions of EVs transmit specific information from their cells of origin to specific target cells and are key mediators in intercellular communication processes. Depending on their origin, EVs can modulate immune responses, either acting as pro- or anti-inflammatory. With the aim to analyze the immunomodulating activities of EV preparations, especially those from mesenchymal stromal cells (MSCs) in vitro, a multi-donor mixed lymphocyte reaction (mdMLR) assay was established and stressed for its reproducibility. METHODS: To this end, human peripheral blood-derived mononuclear cells (PBMCs) of 12 different healthy donors were pooled warranting mutual allogeneic cross-reactivity, even following an optimized freezing and thawing procedure. After thawing, mixed PBMCs were cultured for 5 days in the absence or presence of EVs to be tested. Reflecting allogeneic reactions, in the absence of EVs, pooled PBMCs form characteristic satellite colonies whose appearance can be modulated by EVs. More quantifiable, the strength of the allogenic reaction is reflected by the content of activated CD4 and CD8 T cells being recognized by means of their CD25 and CD54 expression. RESULTS: Of note, connected to the use of primary cells, independent multi-donor PBMC pools differed in their capability to activate their cultured T cells. Thus, throughout the study, only pooled PBMC batches were used whose activated T-cell contents exceeded 25% of the total T-cell population at culture day 5 and whose contents were reproducibly reduced in the presence of immunomodulatory active MSC-EVs. T-cell activation-suppressing effects of the MSC-EV preparations tested were in all cases accompanied by the impact on monocytes. In the presence of immunomodulatory active MSC-EVs, more monocytes were harvested from mdMLR cultures than in their absence. Furthermore, in the absence of immunomodulatory EVs, most monocytes appeared as non-classical (CD14+CD16+) monocytes, whereas immunomodulatory active MSC-EVs promoted the appearance of classical (CD14++CD16-) and intermediate (CD14++CD16+) monocyte subpopulations. CONCLUSIONS: Overall, the obtained results qualify the mdMLR assay as a robust experimental tool for the evaluation of immunomodulatory potentials of given MSC-EV samples. However, further assay development is required to develop and qualify an authority-acceptable potency assay for clinically applicable MSC-EV products.

Independent human mesenchymal stromal cell-derived extracellular vesicle preparations differentially attenuate symptoms in an advanced murine graft-versus-host disease model.

BACKGROUND AIMS: Extracellular vesicles (EVs) harvested from conditioned media of human mesenchymal stromal cells (MSCs) suppress acute inflammation in various disease models and promote regeneration of damaged tissues. After successful treatment of a patient with acute steroid-refractory graft-versus-host disease (GVHD) using EVs prepared from conditioned media of human bone marrow-derived MSCs, this study focused on improving the MSC-EV production for clinical application. METHODS: Independent MSC-EV preparations all produced according to a standardized procedure revealed broad immunomodulatory differences. Only a proportion of the MSC-EV products applied effectively modulated immune responses in a multi-donor mixed lymphocyte reaction (mdMLR) assay. To explore the relevance of such differences in vivo, at first a mouse GVHD model was optimized. RESULTS: The functional testing of selected MSC-EV preparations demonstrated that MSC-EV preparations revealing immunomodulatory capabilities in the mdMLR assay also effectively suppress GVHD symptoms in this model. In contrast, MSC-EV preparations, lacking such in vitro activities, also failed to modulate GVHD symptoms in vivo. Searching for differences of the active and inactive MSC-EV preparations, no concrete proteins or miRNAs were identified that could serve as surrogate markers. CONCLUSIONS: Standardized MSC-EV production strategies may not be sufficient to warrant manufacturing of MSC-EV products with reproducible qualities. Consequently, given this functional heterogeneity, every individual MSC-EV preparation considered for the clinical application should be evaluated for its therapeutic potency before administration to patients. Here, upon comparing immunomodulating capabilities of independent MSC-EV preparations in vivo and in vitro, we found that the mdMLR assay was qualified for such analyses.

CD73 activity of mesenchymal stromal cell-derived extracellular vesicle preparations is detergent-resistant and does not correlate with immunomodulatory capabilities.

BACKGROUND AIMS: Extracellular vesicles (EVs) derived from human mesenchymal stromal cells (MSCs) show immunomodulatory activity in different assays both in vitro and in vivo. In previous work, the authors compared the immunomodulatory potential of independent MSC-EV preparations in a multi-donor mixed lymphocyte reaction (mdMLR) assay and an optimized steroid-refractory acute graft-versus-host disease (aGVHD) mouse model. The authors observed that only a proportion of the MSC-EV preparations showed immunomodulatory capabilities and demonstrated that only MSC-EV preparations with mdMLR immunomodulating activities were able to suppress aGVHD symptoms in vivo and vice versa. Since the mdMLR assay is complex and depends on primary human cells of different donors, the authors sought to establish an assay that is much easier to standardize and fulfills the requirements for becoming qualified as a potency assay. METHODS: The bona fide MSC antigen CD73 possesses ecto-5'-nucleotidase activity that cleaves pro-inflammatory extracellular adenosine monophosphate into anti-inflammatory adenosine and free phosphate. To test whether the ecto-5'-nucleotidase activity of the MSC-EV preparations reflected their immunomodulatory potential, the authors adopted an enzymatic assay that monitors the ecto-5'-nucleotidase activity of CD73 in a quantitative manner and compared the activity of well-characterized MSC-EV preparations containing or lacking mdMLR immunomodulatory activity. RESULTS: The authors showed that the ecto-5'-nucleotidase activity of the MSC-EV preparations did not correlate with their ability to modulate T-cell responses in the mdMLR assay and thus with their potency in improving disease symptomatology in the optimized mouse aGVHD model. Furthermore, the ecto-5'-nucleotidase activity was resistant to EV-destroying detergent treatment. CONCLUSIONS: Ecto-5'-nucleotidase activity neither reflects the potency of the authors' MSC-EV preparations nor provides any information about the integrity of the respective EVs. Thus, ecto-5'-nucleotidase enzyme activity is not indicative for the immunomodulatory potency of the authors' MSC-EV products. The development of appropriate potency assays for MSC-EV products remains challenging.

Release of VAMP5-positive extracellular vesicles by retinal Müller glia in vivo.

Cell-cell interactions in the central nervous system are based on the release of molecules mediating signal exchange and providing structural and trophic support through vesicular exocytosis and the formation of extracellular vesicles. The specific mechanisms employed by each cell type in the brain are incompletely understood. Here, we explored the means of communication used by Müller cells, a type of radial glial cells in the retina, which forms part of the central nervous system. Using immunohistochemical, electron microscopic, and molecular analyses, we provide evidence for the release of distinct extracellular vesicles from endfeet and microvilli of retinal Müller cells in adult mice in vivo. We identify VAMP5 as a Müller cell-specific SNARE component that is part of extracellular vesicles and responsive to ischemia, and we reveal differences between the secretomes of immunoaffinity-purified Müller cells and neurons in vitro. Our findings suggest extracellular vesicle-based communication as an important mediator of cellular interactions in the retina.

Anti-Inflammatory Mesenchymal Stromal Cell-Derived Extracellular Vesicles Improve Pathology in Niemann-Pick Type C Disease.

Niemann-Pick type C (NPC) disease is a rare neurovisceral lipid storage disease with progressive neurodegeneration, leading to premature death. The disease is caused by loss-of-function mutations either in the NPC1 or NPC2 gene which results in lipid accumulation in the late endosomes and lysosomes. The involved disease mechanisms are still incompletely understood, making the design of a rational treatment very difficult. Since the disease is characterized by peripheral inflammation and neuroinflammation and it is shown that extracellular vesicles (EVs) obtained from mesenchymal stromal cells (MSCs) provide immunomodulatory capacities, we tested the potential of MSC-EV preparations to alter NPC1 disease pathology. Here, we show that the administration of an MSC-EV preparation with in vitro and in vivo confirmed immune modulatory capabilities is able to reduce the inflammatory state of peripheral organs and different brain regions of NPC1-diseased mice almost to normal levels. Moreover, a reduction of foamy cells in different peripheral organs was observed upon MSC-EV treatment of NPC1-/- mice. Lastly, the treatment was able to decrease microgliosis and astrogliosis, typical features of NPC1 patients that lead to neurodegeneration. Altogether, our results reveal the therapeutic potential of MSC-EVs as treatment for the genetic neurovisceral lipid storage disease NPC, thereby counteracting both central and peripheral features.

Single Extracellular Vesicle Analysis Performed by Imaging Flow Cytometry and Nanoparticle Tracking Analysis Evaluate the Accuracy of Urinary Extracellular Vesicle Preparation Techniques Differently.

Small extracellular vesicles isolated from urine (uEVs) are increasingly recognized as potential biomarkers. Meanwhile, different uEV preparation strategies exist. Conventionally, the performance of EV preparation methods is evaluated by single particle quantification, Western blot, and electron microscopy. Recently, we introduced imaging flow cytometry (IFCM) as a next-generation single EV analysis technology. Here, we analyzed uEV samples obtained with different preparation procedures using nanoparticle tracking analysis (NTA), semiquantitative Western blot, and IFCM. IFCM analyses demonstrated that urine contains a predominant CD9+ sEV population, which exceeds CD63+ and CD81+ sEV populations. Furthermore, we demonstrated that the storage temperature of urine samples negatively affects the recovery of CD9+ sEVs. Although overall reduced, the highest CD9+ sEV recovery was obtained from urine samples stored at -80 °C and the lowest from those stored at -20 °C. Upon comparing the yield of the different uEV preparations, incongruencies between NTA and IFCM data became apparent. Results obtained by both NTA and IFCM were consistent with Western blot analyses for EV marker proteins; however, NTA results correlated with the amount of the impurity marker uromodulin. Despite demonstrating that the combination of ultrafiltration and size exclusion chromatography appears as a reliable uEV preparation technique, our data challenge the soundness of traditional NTA for the evaluation of different EV preparation methods.

Small extracellular vesicles obtained from hypoxic mesenchymal stromal cells have unique characteristics that promote cerebral angiogenesis, brain remodeling and neurological recovery after focal cerebral ischemia in mice.

Obtained from the right cell-type, mesenchymal stromal cell (MSC)-derived small extracellular vesicles (sEVs) promote stroke recovery. Within this process, microvascular remodeling plays a central role. Herein, we evaluated the effects of MSC-sEVs on the proliferation, migration, and tube formation of human cerebral microvascular endothelial cells (hCMEC/D3) in vitro and on post-ischemic angiogenesis, brain remodeling and neurological recovery after middle cerebral artery occlusion (MCAO) in mice. In vitro, sEVs obtained from hypoxic (1% O2), but not 'normoxic' (21% O2) MSCs dose-dependently promoted endothelial proliferation, migration, and tube formation and increased post-ischemic endothelial survival. sEVs from hypoxic MSCs regulated a distinct set of miRNAs in hCMEC/D3 cells previously linked to angiogenesis, three being upregulated (miR-126-3p, miR-140-5p, let-7c-5p) and three downregulated (miR-186-5p, miR-370-3p, miR-409-3p). LC/MS-MS revealed 52 proteins differentially abundant in sEVs from hypoxic and 'normoxic' MSCs. 19 proteins were enriched (among them proteins involved in extracellular matrix-receptor interaction, focal adhesion, leukocyte transendothelial migration, protein digestion, and absorption), and 33 proteins reduced (among them proteins associated with metabolic pathways, extracellular matrix-receptor interaction, focal adhesion, and actin cytoskeleton) in hypoxic MSC-sEVs. Post-MCAO, sEVs from hypoxic MSCs increased microvascular length and branching point density in previously ischemic tissue assessed by 3D light sheet microscopy over up to 56 days, reduced delayed neuronal degeneration and brain atrophy, and enhanced neurological recovery. sEV-induced angiogenesis in vivo depended on the presence of polymorphonuclear neutrophils. In neutrophil-depleted mice, MSC-sEVs did not influence microvascular remodeling. sEVs from hypoxic MSCs have distinct angiogenic properties. Hypoxic preconditioning enhances the restorative effects of MSC-sEVs.

Scaled Isolation of Mesenchymal Stem/Stromal Cell-Derived Extracellular Vesicles.

Mesenchymal stem/stromal cells (MSCs) provide therapeutic effects in many diseases. Contrary to initial hypotheses, they act in a paracrine rather than a cellular manner. To this end, extracellular vesicles (EVs) have been found to mediate the therapeutic effects, even when harvested from MSC-conditioned cell culture supernatants. Lacking self-replicating activity and being so small that MSC-EV preparations can be sterilized by filtration, EVs provide several advantages as therapeutic agents over cellular therapeutics. At present, methods allowing EV preparation from larger volumes are scarce and regularly require special equipment. We have developed a polyethylene glycol-based precipitation protocol allowing extraction of EVs from several liters of conditioned medium. MSC-EVs prepared with this method have been successfully applied to a human graft-versus-host disease patient and to several animal models. Although the method comes with its own limitations, it is extremely helpful for the initial evaluation of EV-based therapeutic approaches. Here, we introduce the technique in detail and discuss all critical steps. © 2020 The Authors. Basic Protocol 1: Preparation of MSC-conditioned medium for scaled MSC-EV production Basic Protocol 2: PEG precipitation OF MSC-EV from MSC-conditioned medium.

Mesenchymal Stromal Cell-Derived Small Extracellular Vesicles Induce Ischemic Neuroprotection by Modulating Leukocytes and Specifically Neutrophils.

Background and Purpose- Small extracellular vesicles (sEVs) obtained from mesenchymal stromal cells (MSCs) were shown to induce neurological recovery after focal cerebral ischemia in rodents and to reverse postischemic lymphopenia in peripheral blood. Since peripheral blood cells, especially polymorphonuclear neutrophils (PMNs), contribute to ischemic brain injury, we analyzed brain leukocyte responses to sEVs and investigated the role of PMNs in sEV-induced neuroprotection. Methods- Male C57Bl6/j mice were exposed to transient intraluminal middle cerebral artery occlusion. After reperfusion, vehicle or sEVs prepared from conditioned media of MSCs raised from bone marrow samples of 3 randomly selected healthy human donors were intravenously administered. sEVs obtained from normoxic and hypoxic MSCs were applied. PMNs were depleted in vehicle and MSC-sEV-treated mice. Neurological deficits, ischemic injury, blood-brain barrier integrity, peripheral blood leukocyte responses, and brain leukocyte infiltration were evaluated over 72 hours. Results- sEV preparations of all 3 donors collected from normoxic MSCs significantly reduced neurological deficits. Preparations of 2 of these donors significantly decreased infarct volume and neuronal injury. sEV-induced neuroprotection was consistently associated with a decreased brain infiltration of leukocytes, namely of PMNs, monocytes/macrophages, and lymphocytes. sEVs obtained from hypoxic MSCs (1% O2) had similar effects on neurological deficits and ischemic injury as MSC-sEVs obtained under regular conditions (21% O2) but also reduced serum IgG extravasation-a marker of blood-brain barrier permeability. PMN depletion mimicked the effects of MSC-sEVs on neurological recovery, ischemic injury, and brain PMN, monocyte, and lymphocyte counts. Combined MSC-sEV administration and PMN depletion did not have any effects superior to PMN depletion in any of the readouts examined. Conclusions- Leukocytes and specifically PMNs contribute to MSC-sEV-induced ischemic neuroprotection. Individual MSC-sEV preparations may differ in their neuroprotective activities. Potency assays are urgently needed to identify their therapeutic efficacy before clinical application. Visual Overview- An online visual overview is available for this article.

Individual Immune-Modulatory Capabilities of MSC-Derived Extracellular Vesicle (EV) Preparations and Recipient-Dependent Responsiveness.

Treatment with extracellular vesicles (EVs) derived from mesenchymal stem/stromal cells (MSCs) have been suggested as novel therapeutic option in acute inflammation-associated disorders due to their immune-modulatory capacities. As we have previously observed differences in the cytokine profile of independent MSC-EV preparations, functional differences of MSC-EV preparations have to be considered. To evaluate the immune-modulatory capabilities of specific MSC-EV preparations, reliable assays are required to characterize the functionality of MSC-EV preparations prior to administration to a patient. To this end, we established an in vitro assay evaluating the immune-modulatory capacities of MSC-EV preparations. Here, we compared the efficacy of four independent MSC-EV preparations to modulate the induction of T cell differentiation and cytokine production after phorbol 12-myristate 13-acetate (PMA)/Ionomycin stimulation of peripheral blood mononuclear cells (PBMC) derived from six healthy donors. Flow cytometric analyses revealed that the four MSC-EV preparations differentially modulate the expression of surface markers, such as CD45RA, on CD4+ and CD8+ T cells, resulting in shifts in the frequencies of effector and effector memory T cells. Moreover, cytokine profile in T cell subsets was affected in a MSC-EV-specific manner exclusively in CD8+ naïve T cells. Strikingly, hierarchical clustering revealed that the T cell response towards the MSC-EV preparations largely varied among the different PBMC donors. Thus, besides defining functional activity of MSC-EV preparations, it will be crucial to test whether patients intended for treatment with MSC-EV preparations are in principal competent to respond to the envisioned MSC-EV therapy.