Avoiding false positive antigen detection by flow cytometry on blood cell derived microparticles: the importance of an appropriate negative control

Extracellular vesicles from chronic lymphocytic leukemia cells promote leukemia aggressiveness by inducing the differentiation of monocytes into nurse-like cells via an RNA-dependent mechanism

Chronic lymphocytic leukemia (CLL) cells receive several stimuli from surrounding cells, such as B-cell receptor (BCR) stimulation, and can manipulate their microenvironment via extracellular vesicle (EV) release. Here, we investigated the small RNA content (microRNA and YRNA) of CLL-EVs from leukemic cells cultured with/without BCR stimulation. We highlight an increase of miR-155-5p, miR-146a-5p, and miR-132-3p in EVs and in cells after BCR stimulation (p < 0.05, n = 25). CLL-EVs were preferentially internalized by monocytes (p = 0.0019, n = 6) and able to deliver microRNAs and the hY4 RNA. Furthermore, BCR CLL-EV induced modifications in monocytes (shape change, microRNA and gene expression, secretome) suggesting nurse-like cell (NLC) differentiation, the tumor-associated macrophages of CLL. Functionally, monocytes treated with BCR CLL-EVs protect CLL cells from spontaneous apoptosis by pro-survival cytokine production and induce their migration as well as the migration of other immune cells. We finally reported by transfection experiments that hY4 is able to induce the expression of CCL24, a key gene in M2 macrophage differentiation. In conclusion, we showed that BCR stimulation modifies the small RNA content of CLL-EVs and that the addition of leukemic EVs to monocytes leads to monocyte differentiation into NLCs establishing a protective microenvironment that supports leukemic cell survival.

Standardized procedure to measure the size distribution of extracellular vesicles together with other particles in biofluids with microfluidic resistive pulse sensing

The particle size distribution (PSD) of extracellular vesicles (EVs) and other submicron particles in biofluids is commonly measured by nanoparticle tracking analysis (NTA) and tunable resistive pulse sensing (TRPS). A new technique for measuring the PSD is microfluidic resistive pulse sensing (MRPS). Because specific guidelines for measuring EVs together with other particles in biofluids with MRPS are lacking, we developed an operating procedure to reproducibly measure the PSD. The PSDs of particles in human plasma, conditioned medium of PC3 prostate cancer cell line (PC3 CM), and human urine were measured with MRPS (nCS1, Spectradyne LLC) to investigate: (i) the optimal diluent that reduces the interfacial tension of the sample while keeping EVs intact, (ii) the lower limit of detection (LoD) of particle size, (iii) the reproducibility of the PSD, (iv) the optimal dilution for measuring the PSD, and (v) the agreement in measured concentration between microfluidic cartridges with overlapping detection ranges. We found that the optimal diluent is 0.1% bovine serum albumin (w/v) in Dulbecco’s phosphate-buffered saline. Based on the shape of the PSD, which is expected to follow a power-law function within the full detection range, we obtained a lower LoD of 75 nm for plasma and PC3 CM and 65 nm for urine. Normalized PSDs are reproducible (R² > 0.950) at dilutions between 10–100x for plasma, 5–20x for PC3 CM, and 2–4x for urine. Furthermore, sample dilution does not impact the dilution-corrected concentration when the microfluidic cartridges are operated within their specified concentration ranges. PSDs from microfluidic cartridges with overlapping detection ranges agreed well (R² > 0.936) and when combined the overall PSD spanned 5 orders of magnitude of measured concentration. Based on these findings, we have developed operating guidelines to reproducibly measure the PSD of EVs together with other particles in biofluids with MRPS.

Comparison of extracellular vesicle isolation and storage methods using high-sensitivity flow cytometry

Extracellular vesicles (EVs) are of interest for a wide variety of biomedical applications. A major limitation for the clinical use of EVs is the lack of standardized methods for the fast and reproducible separation and subsequent detection of EV subpopulations from biofluids, as well as their storage. To advance this application area, fluorescence-based characterization technologies with single-EV resolution, such as high-sensitivity flow cytometry (HS-FCM), are powerful to allow assessment of EV fractionation methods and storage conditions. Furthermore, the use of HS-FCM and fluorescent labeling of EV subsets is expanding due to the potential of high-throughput, multiplex analysis, but requires further method development to enhance the reproducibility of measurements. In this study, we have applied HS-FCM measurements next to standard EV characterization techniques, including nanoparticle tracking analysis, to compare the yield and purity of EV fractions obtained from lipopolysaccharide-stimulated monocytic THP-1 cells by two EV isolation methods, differential centrifugation followed by ultracentrifugation and the exoEasy membrane affinity spin column purification. We observed differences in EV yield and purity. In addition, we have investigated the influence of EV storage at 4°C or -80°C for up to one month on the EV concentration and the stability of EV-associated fluorescent labels. The concentration of the in vitro cell derived EV fractions was shown to remain stable under the tested storage conditions, however, the fluorescence intensity of labeled EV stored at 4°C started to decline within one day.

Microvesicles in Cancer: Small Size, Large Potential

Extracellular vesicles (EV) are secreted by all cell types in a tumor and its microenvironment (TME), playing an essential role in intercellular communication and the establishment of a TME favorable for tumor invasion and metastasis. They encompass a variety of vesicle populations, among them the well-known endosomal-derived small exosomes (Exo), but also larger vesicles (diameter > 100 nm) that are shed directly from the plasma membrane, the so-called microvesicles (MV). Increasing evidence suggests that MV, although biologically different, share the tumor-promoting features of Exo in the TME. Due to their larger size, they can be readily harvested from patients' blood and characterized by routine methods such as conventional flow cytometry, exploiting the plethora of molecules expressed on their surface. In this review, we summarize the current knowledge about the biology and the composition of MV, as well as their role within the TME. We highlight not only the challenges and potential of MV as novel biomarkers for cancer, but also discuss their possible use for therapeutic intervention.

Extracellular vesicles and coagulation in blood from healthy humans revisited

Background: In 2001, we studied the presence and coagulant properties of “microparticles” in the blood of healthy humans. Since then, multiple improvements in detection, isolation and functional characterization of the now called “extracellular vesicles” (EVs) have been made, and shortcomings were identified. Aim: To revisit the presence and function of EVs in blood from healthy humans. Methods: Blood was collected from 20 healthy donors. EV-containing plasma was prepared according to new guidelines, and plasma was diluted to prevent swarm detection. Single EVs were measured by flow cytometry with known sensitivity of fluorescence and light scatter. The haemostatic properties of EVs were measured by thrombin-, fibrin-, and plasmin generation. Plasma concentrations of thrombin-antithrombin complexes and prothrombin fragment 1 + 2 were measured to assess the coagulation status in vivo. Results: Compared to 2001, the total concentrations of detected EVs increased from 190- to 264-fold. In contrast to 2001, however, EVs are non-coagulant which we show can be attributed to improvements in blood collection and plasma preparation. No relation is present between the plasma concentrations of EVs and either TAT or F1 + 2. Finally, we show that EVs support plasmin generation. Discussion: Improvements in blood collection, plasma preparation and detection of EVs reveal that results from earlier studies have to be interpreted with care. Compared to 2001, higher concentrations of EVs are detected in blood of healthy humans which promote fibrinolysis rather than coagulation.

Perivascular dendritic cells elicit anaphylaxis by relaying allergens to mast cells via microvesicles

Anaphylactic reactions are triggered when allergens enter the blood circulation and activate immunoglobulin E (IgE)-sensitized mast cells (MCs), causing systemic discharge of prestored proinflammatory mediators. As MCs are extravascular, how they perceive circulating allergens remains a conundrum. Here, we describe the existence of a CD301b⁺ perivascular dendritic cell (DC) subset that continuously samples blood and relays antigens to neighboring MCs, which vigorously degranulate and trigger anaphylaxis. DC antigen transfer involves the active discharge of surface-associated antigens on 0.5- to 1.0-micrometer microvesicles (MVs) generated by vacuolar protein sorting 4 (VPS4). Antigen sharing by DCs is not limited to MCs, as neighboring DCs also acquire antigen-bearing MVs. This capacity of DCs to distribute antigen-bearing MVs to various immune cells in the perivascular space potentiates inflammatory and immune responses to blood-borne antigens.

Optimisation of imaging flow cytometry for the analysis of single extracellular vesicles by using fluorescence-tagged vesicles as biological reference material

Extracellular vesicles (EVs) mediate targeted cellular interactions in normal and pathophysiological conditions and are increasingly recognised as potential biomarkers, therapeutic agents and drug delivery vehicles. Based on their size and biogenesis, EVs are classified as exosomes, microvesicles and apoptotic bodies. Due to overlapping size ranges and the lack of specific markers, these classes cannot yet be distinguished experimentally. Currently, it is a major challenge in the field to define robust and sensitive technological platforms being suitable to resolve EV heterogeneity, especially for small EVs (sEVs) with diameters below 200 nm, i.e. smaller microvesicles and exosomes. Most conventional flow cytometers are not suitable for the detection of particles being smaller than 300 nm, and the poor availability of defined reference materials hampers the validation of sEV analysis protocols. Following initial reports that imaging flow cytometry (IFCM) can be used for the characterisation of larger EVs, we aimed to investigate its usability for the characterisation of sEVs. This study set out to identify optimal sample preparation and instrument settings that would demonstrate the utility of this technology for the detection of single sEVs. By using CD63eGFP-labelled sEVs as a biological reference material, we were able to define and optimise IFCM acquisition and analysis parameters on an Amnis ImageStreamX MkII instrument for the detection of single sEVs. In addition, using antibody-labelling approaches, we show that IFCM facilitates robust detection of different EV and sEV subpopulations in isolated EVs, as well as unprocessed EV-containing samples. Our results indicate that fluorescently labelled sEVs as biological reference material are highly useful for the optimisation of fluorescence-based methods for sEV analysis. Finally, we propose that IFCM will help to significantly increase our ability to assess EV heterogeneity in a rigorous and reproducible manner, and facilitate the identification of specific subsets of sEVs as useful biomarkers in various diseases.

Systematic Methodological Evaluation of a Multiplex Bead-Based Flow Cytometry Assay for Detection of Extracellular Vesicle Surface Signatures

Extracellular vesicles (EVs) can be harvested from cell culture supernatants and from all body fluids. EVs can be conceptually classified based on their size and biogenesis as exosomes and microvesicles. Nowadays, it is however commonly accepted in the field that there is a much higher degree of heterogeneity within these two subgroups than previously thought. For instance, the surface marker profile of EVs is likely dependent on the cell source, the cell’s activation status, and multiple other parameters. Within recent years, several new methods and assays to study EV heterogeneity in terms of surface markers have been described; most of them are being based on flow cytometry. Unfortunately, such methods generally require dedicated instrumentation, are time-consuming and demand extensive operator expertise for sample preparation, acquisition, and data analysis. In this study, we have systematically evaluated and explored the use of a multiplex bead-based flow cytometric assay which is compatible with most standard flow cytometers and facilitates a robust semi-quantitative detection of 37 different potential EV surface markers in one sample simultaneously. First, assay variability, sample stability over time, and dynamic range were assessed together with the limitations of this assay in terms of EV input quantity required for detection of differently abundant surface markers. Next, the potential effects of EV origin, sample preparation, and quality of the EV sample on the assay were evaluated. The findings indicate that this multiplex bead-based assay is generally suitable to detect, quantify, and compare EV surface signatures in various sample types, including unprocessed cell culture supernatants, cell culture-derived EVs isolated by different methods, and biological fluids. Furthermore, the use and limitations of this assay to assess heterogeneities in EV surface signatures was explored by combining different sets of detection antibodies in EV samples derived from different cell lines and subsets of rare cells. Taken together, this validated multiplex bead-based flow cytometric assay allows robust, sensitive, and reproducible detection of EV surface marker expression in various sample types in a semi-quantitative way and will be highly valuable for many researchers in the EV field in different experimental contexts.

Saline is a more appropriate solution for microvesicles for flow cytometric analyses

Microvesicles (MVs) are carriers of molecular and oncogenic signatures present in subsets of tumor cells and tumor-associated stroma, and a focus of cancer research. Although methods to detect MVs are mature, we were concerned that the buffer used could lead to false results when quantitating MVs by flow cytometry. In this work,we detected MVs by flow cytometry withthree different solutions: water, saline, and phosphate-buffered saline (PBS). The results demonstrated that PBS, when reacted with annexin V binding buffer, produced nano-sized vesicles even when there were no MVs in the sample. No similar events occurred in the saline and water groups (P < 0.01). Annexin V positive rate increased significantly when PBS was used as the buffer, compared to saline and water. These false negative results were also observed when we quantified some markers of MVs such as CD3 and CD19. A probable explanation for these findings is the production of insoluble Ca(H2PO4)2 or Ca3PO4 from calcium in the binding buffer and phosphate in PBS. Thus, considering the osmotic pressure of water, we suggest that saline is a more suitable buffer when counting MVs by flow cytometry.

Ovine Leukocyte Microparticles Generated by the CentriMag Ventricular Assist Device In Vitro

Ventricular assist devices (VADs) are a life-saving form of mechanical circulatory support in heart failure patients. However, VADs have not yet reached their full potential due to the associated side effects (thrombosis, bleeding, infection) related to the activation and damage of blood cells and proteins caused by mechanical stress and foreign materials. Studies of the effects of VADs on leukocytes are limited, yet leukocyte activation and damage including microparticle generation can influence both thrombosis and infection rates. Therefore, the aim was to develop a multicolor flow cytometry assessment of leukocyte microparticles (LMPs) using ovine blood and the CentriMag VAD as a model for shear stress. Ovine blood was pumped for 6 h in the CentriMag and regular samples analyzed for hemolysis, complete blood counts and LMP by flow cytometry during three different pump operating conditions (low flow, standard, high speed). The high speed condition caused significant increases in plasma-free hemoglobin; decreases in total leukocytes, granulocytes, monocytes, and platelets; increases in CD45⁺ LMPs as well as two novel LMP populations: CD11b^bright /HLA-DR^- and CD11b^dull /HLA-DR⁺ , both of which were CD14^- /CD21^- . CD11b^bright /HLA-DR^- LMPs appeared to respond to an increase in shear magnitude whereas the CD11b^dull /HLA-DR⁺ LMPs significantly increased in all pumping conditions. We propose that these two populations are released from granulocytes and T cells, respectively, but further research is needed to better characterize these two populations.

Platelet subpopulations remain despite strong dual agonist stimulation and can be characterised using a novel six-colour flow cytometry protocol

It is recognised that platelets respond differently to activation, where a subpopulation of platelets adopt a procoagulant phenotype while others are aggregatory. However, it has not been thoroughly tested whether these subpopulations will remain in maximally activated samples, or if they are merely a result of different platelet sensitivities to agonist activation. Here platelets were activated with gradually increasing concentrations of thrombin and/or the GPVI agonist cross-linked collagen-related peptide (CRP-XL). Platelet activation was investigated using a novel six-colour flow cytometry protocol evaluating exposure of phosphatidylserine, active conformation of the fibrinogen receptor α_IIbβ₃, α-granule and lysosomal release (P-selectin and LAMP-1 exposure), mitochondrial membrane integrity and platelet fragmentation. Upon activation by CRP-XL or thrombin+CRP-XL, platelets formed three differently sized subpopulations. Normal-sized platelets showed high exposure of aggregatory active α_IIbβ₃ and intact mitochondria, while the smaller platelets and platelet fragments showed high exposure of procoagulant phosphatidylserine. The distribution of platelets between the differently sized subpopulations remained stable despite high agonist concentrations. All three were still present after 30 and 60 min of activation, showing that all platelets will not have the same characteristics even after maximal stimulation. This suggests that platelet subpopulations with distinct activation patterns exist within the total platelet population.

Extracellular vesicles of bone marrow stromal cells rescue chronic lymphocytic leukemia B cells from apoptosis, enhance their migration and induce gene expression modifications

Interactions between chronic lymphocytic leukemia (CLL) B cells and the bone marrow (BM) microenvironment play a major function in the physiopathology of CLL. Extracellular vesicles (EVs), which are composed of exosomes and microparticles, play an important role in cell communication. However, little is known about their role in CLL / microenvironment interactions. In the present study, EVs purified by ultracentrifugation from BM mesenchymal stromal cell (BM-MSC) cultures were added to CLL B cells. After their integration into CLL B cells, we observed a decrease of leukemic cell spontaneous apoptosis and an increase in their chemoresistance to several drugs, including fludarabine, ibrutinib, idelalisib and venetoclax after 24 hours. Spontaneous (P=0.0078) and stromal cell-derived factor 1α -induced migration capacities of CLL B cells were also enhanced (P=0.0020). A microarray study highlighted 805 differentially expressed genes between leukemic cells cultured with or without EVs. Of these, genes involved in the B-cell receptor pathway such as CCL3/4, EGR1/2/3, and MYC were increased. Interestingly, this signature presents important overlaps with other microenvironment stimuli such as B-cell receptor stimulation, CLL/nurse-like cells co-culture or those provided by a lymph node microenvironment. Finally, we showed that EVs from MSCs of leukemic patients also rescue leukemic cells from spontaneous or drug-induced apoptosis. However, they induce a higher migration and also a stronger gene modification compared to EVs of healthy MSCs. In conclusion, we show that EVs play a crucial role in CLL B cells/BM microenvironment communication.

Review: the Multiple Roles of Monocytic Microparticles

Monocytic microparticles (mMP) are microparticles derived from human monocytes either under in vivo or in vitro conditions. The size of mMP is between 0.1 and 1.0 μm. Apart from the size range, mMPs are also identified based on phosphatidylserine and CD14 expression on their surface, though this is not always the case. Monocytic MP are critical players in inflammation, endothelial cell function, and blood coagulation. They exhibit dual function by either helping the progression of such conditions or limiting it, depending on certain factors. Furthermore, the numbers of mMP are elevated in some autoimmune diseases, infectious diseases, and metabolic disorders. However, it is unknown whether mMP play an active role in these diseases or are simply biomarkers. The mechanism of mMP modulation is yet to be identified. In this review, we highlight the mechanism of mMP formation and the roles that they play in inflammation, blood coagulation, and different disease settings.

A direct-imaging cryo-EM study of shedding extracellular vesicles from leukemic monocytes

The human leukemia monocytic cell line (THP-1) is known to shed extracellular vesicles (EVs) under various stimulations. We studied the effects of two types of common stimulation types, lipopolysaccharide (LPS) and starvation conditions by high resolution cryogenic electron microscopy, namely, cryo-SEM and cryo-TEM. Cryo-SEM data of cells undergoing EV blebbing and shedding is presented here for the first time. The high-resolution images show good agreement with models describing the membrane processes of shedding. Cells that underwent a 48-h starvation treatment exhibited differing morphological features, including shrunken nucleus and elongated membrane protrusions. LPS treated cells, however, showed extensive blebbing originating from the cell membrane, in good agreement with the sizes of EVs imaged by cryo-TEM. EVs isolated from both types of stimulations were measured by nanoparticle tracking analysis (NanoSight), by which LPS-EVs samples exhibited higher concentration and smaller mean diameter, as compared to starvation-EVs. Our results suggest a difference in the effects of the two stimulation types on the shedding process and possibly on the type of EVs shed. Our unique methodologies provide an important and innovative outlook of the shedding process and on its products, paving the way to further discoveries in this developing field of research, in which much is still unknown.

Differential detergent sensitivity of extracellular vesicle subpopulations

This work shows for the first time that exosomes are more resistant to detergents than microvesicles and apoptotic bodies.