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PMC
Results
C. albicans induces vesicle release from human blood monocytes
Human monocytes directly recognize C. albicans and react in multiple ways to the fungus. They take up fungal cells by phagocytosis; release DNA traps9, similar to neutrophils, to immobilize the fungus; and secrete toxic reactive oxygen species. As monocytes also produce vesicles to communicate with other cells10, we addressed the question whether C. albicans induces vesicle release in monocytes. Human blood monocytes were isolated from buffy coats by magnetic sorting of CDpositive cells (~95% purity), and incubated with complement-pre-opsonized C. albicans on a coverslip. After 1h of incubation, the cells were fixed onto a microscopy slide, and the monocytes were monitored for the presence of vesicles using the previously described vesicle marker tetraspanin (CD63)11. Monocytes alone without C. albicans showed several vesicles, which predominantly surrounded the nucleus (Fig. 1a). When monocytes were incubated with C. albicans, vesicle formation substantially increased. Again, vesicles surrounded the nucleus, but were also found extracellularly, indicating vesicle release. Vesicles formed in response to C. albicans are referred to from here on as opsonized Candida-induced monocytic extracellular vesicles (MEVsCa) to distinguish them from the monocytic extracellular vesicles (MEVs) that are spontaneously produced in the absence of C. albicans.
a Vesicle formation (arrow) increases in C. albicans-infected monocytes using confocal laser-scanning microscopy (CLSM). Staining red: CD63, green: CD14, and orange: nucleic acids after 1h of co-incubation. Bars: 5µm. Representative data of five independent experiments are shown. b Release of EVs from the infected monocytes visualized by live cell imaging using CLSM. After phagocytosis (23min) of C. albicans, the EVs (42min) (arrows) are released. Staining green: CD14 and cyan: nucleic acids. Bars: 10µm. Representative data of three independent experiments are shown. c Infected monocytes in an ex vivo whole-blood model show early (20min) release of MEVsCa (arrow) by live cell imaging (CLSM). Staining red: CD63, green: CD14, and orange: nucleic acids. Bars: 10µm. Representative data of five independent experiments are shown. d Scanning electron microscopy (SEM) confirms the presence of MEVs and MEVsCa released by untreated and infected monocytes (arrows) respectively. Bars: 1µm. Representative data of three independent experiments are shown.
To follow vesicle formation in real time, CD14+ monocytes in the presence of opsonized C. albicans were tracked by live cell imaging in culture dishes using nucleic acid stainingSytox Orange, which does not penetrate living cells but can penetrate extracellular vesicles. Live cell imaging revealed phagocytosis of C. albicans by monocytes within minutes and generation of nucleic acid-containing vesicles. Release of vesicles was observed after ~min (Fig. 1b). Vesicle generation and release from monocytes in presence of C. albicans was captured in real time using dynamic light-scattering microscopy (DLSM) (Supplementary Video 1), confirming fast release of generated vesicle. To track vesicle generation by monocytes under more physiologic conditions, live cell imaging of monocytes was performed in an ex vivo whole-blood model system. Whole blood was infected with C. albicans, and monocytes were stained with anti-CD14 and vesicles were stained with anti-CD Vesicle generation was seen within 10min after infection, and increased after 20min (Fig. 1c). Released vesicles were also confirmed by scanning electron microscopy (SEM) (Fig. 1d). In summary, monocytes treated with opsonized C. albicans released EVs within 1h after infection. In all subsequent experiments, C. albicans infection was performed for 1h, unless otherwise indicated.
MEVsCa are double-layered vesicles
For detailed characterization, MEVsCa generated by isolated human blood monocytes in response to opsonized C. albicans were isolated using a polymer precipitation method. These vesicles were analyzed for their number and size by measuring the Brownian movement of vesicles in suspension using DLSM (Fig. 2a). The number of MEVsCa harvested from C. albicans-infected monocytes (5×105) was about ten times higher than the number of MEVs harvested from the same number of uninfected monocytes (5×105) (Fig. 2b). Thus, monocytes release substantially more vesicles upon infection with the fungus. Five major populations of MEVs were identified, with sizes ranging from 50 to nm, and three major populations of MEVsCa were observed, with sizes ranging from 50 to nm (Fig. 2c). For further characterization, cryogenic electron microscopy (Cryo-EM) and freeze-fracture electron microscopy (FFEM) were performed on MEVsCa. Cryo-EM verified the presence of nm vesicles, and showed a double-layered membrane (Fig. 2d). Under FFEM, which revealed fractured concave and convex vesicles, the inner and outer membranes became visible (Fig. 2e).
a Tracking of EVs by dynamic light-scattering microscopy (DLSM). EVs were isolated from 5×105 uninfected or opsonized C. albicans-infected monocytes (MEVs or MEVsCa, respectively) by polymer precipitation. Representative data of three independent experiments (three donors) are shown. b MEVsCa are significantly increased compared with MEVs (data are presented as mean values+/ SD, p=, unpaired two-tailed t test, n=3 different donors). EVs isolated from same number of infected or uninfected monocytes were counted by DLSM. c Size distribution of MEVs and MEVsCa as determined by DLSM using NanoSight NTA software. Graphs were generated by overlaying the size distribution of MEVs and MEVsCa from n=3 donors. d The double-membrane structure and round shape of the MEVsCa are visible by cryogenic electron microscopy (Cryo-EM). Bars: nm. e MEVsCa structure and shape is confirmed by freeze-fracture electron microscopy (FFEM). Bars: nm. f MEVsCa show significantly higher protein content compared with MEVs (p, unpaired two-tailed t test, n=3 different donors). Proteins from MEVs and MEVsCa (each from 1×107 monocytes) were detected by label-free LC-MS/MS-based proteomics. g Most of the proteins enriched in MEVsCa compared with MEVs are extracellular exosome-related proteins (Gene Ontology (GO) enrichment analysis). h Complement pathway proteins were significantly increased in MEVsCa (Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis). i Heatmap of complement and coagulation proteins upregulated in MEVCas. Comparisons in gi rely on protein content of MEVsCa from three different donors. j Diagram of an MEVsCa showing the presence of receptors, marker proteins, and cytokines detected in the current study. k TGF-²1 (data are presented as mean values+/ SD, p=, unpaired two-tailed t test, n=3 donors), l but not IL-6, is significantly increased in MEVsCa compared with MEVs (data are presented as mean values+/ SD, p=, unpaired two-tailed t test, n=3 different donors). Cytokines from MEVs and MEVsCa (each isolated from monocytes (5×105)) were determined by ELISA.
In addition, the composition of MEVs and MEVsCa was determined using label-free liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based proteomics. Proteins were analyzed from vesicles derived from the same number of infected and control monocytes (1×107) after tryptic digest. MEVsCa showed a significant increased level of proteins compared with MEVs, while 29 proteins were significantly decreased in abundance (Fig. 2f). The significantly increased proteins were further categorized using Gene Ontology enrichment analysis for biological process, molecular function, and cellular compartment. Most () of the proteins with higher abundance in MEVsCa were extracellular exosome-related proteins (Fig. 2g). Heat shock proteins, along with histone fragments commonly found in vesicles12, were found in MEVs and MEVsCa. The presence of CD14 in both MEVs and MEVsCa confirmed their monocytic origin. In addition, myeloid lineage marker such as CD11b, complement receptor type I (CR1), and Toll-like receptor 2 (TLR2) were increased in MEVsCa by 2-, 8-, and fold, respectively (Fig. 2j). In addition, the tetraspanin CD9 was also identified on MEVsCa, together with commonly found annexins. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis showed a high abundance of physiologically important complement pathway proteins (Fig. 2h, i, j). As cytokines are important regulators of the immune system and are difficult to detect by mass spectroscopy13, cytokine levels in MEVs and MEVsCa were determined by sandwich ELISA. Substantial amounts of transforming growth factor-²1 (TGF-²1) were detected in MEVsCa derived from infected monocytes but not in MEVs derived from the same number of uninfected monocytes (Fig. 2k). By contrast, levels of IL-6 (Fig. 2l), IL, and IL-1² (not shown) were very low or were not detected. To exclude extracellular proteins in vesicle polymer precipitates, the results were confirmed also with EVs isolated by ultracentrifugation and size-exclusion chromatography (Supplementary Fig. 1). Furthermore, vesicle markers CD9 as well as HSP90 were identified on TGF-²1-transporting vesicles (70nm) by size-exclusion chromatography, which in addition, characterized these vesicles as exosomes. Presence of few TGF-²1-transporting vesicles without vesicle markers is explained by simultaneous use of different antibodies.
MEVsCa transport TGF-²1
To verify the presence of TGF-²1 in MEVsCa, monocytes were incubated with C. albicans for 1h on cover slips, and cells were fixed and stained with an antibody against TGF-²1. C. albicans-infected monocytes but not uninfected monocytes showed significant production of MEVsCa of nm in size by confocal laser-scanning microscopy (CLSM) (Fig. 3a, a,b;b; Supplementary Fig. 2a). To visualize MEVsCa in more detail, TGF-²1 was labeled with immunogold, and the vesicles were analyzed by SEM. MEVsCa but not MEVs showed gold labeling on the surface, demonstrating the presence of TGF-²1 on the outer membrane. The size of MEVs and MEVsCa was ~nm (Fig. 3c), which was in the same range as the size measured by DLSM (Fig. 2c).
a MEVsCa transporting TGF-²1 (arrows) are frequently observed by CLSM when monocytes are incubated with C. albicans. Representative data of n=4 experiments (four donors) are shown. Staining red: TGF-²1. Bars: 10µm. b Expression of TGF-²1 in MEVsCa compared with MEVs. TGF-²1-positive EVs were counted using Image J software (data are presented as mean values+/ SD, p=, unpaired two-tailed t test, n=4 different donors). c TGF-²1 (arrow) is detected on the membranes of MEVsCa but not MEVs by immunogold labeling of TGF-²1 using SEM. Bars: nm. d Monocytes generate (15min) and release (45min) MEVsCa during whole-blood infection. MEVsCa were tracked by live cell imaging using CLSM. Staining red: TGF-²1, green: CD14, and orange: nucleic acids. Bars: 10µm. Representative data of three experiments (three donors) are shown. e TGF-²1-transporting vesicle tracking in a whole-blood ex vivo infection model by live cell imaging using CLSM. Staining red: TGF-²1. Bars: 10µm. Representative data of n=5 experiments (five donors) are shown. f Blood infected with C. albicans shows a significant increase of TGF-²1-transporting vesicles. TGF-²1-positive EVs were counted using Image J software (data are presented as mean values+/ SD, p=, unpaired two-tailed t test, n=5 donors). g High frequency of TGF-²1-transporting vesicles in liver tissue of C. albicans-infected mice (24h). Staining red: TGF-²1 and orange: nucleic acids. Bars: 10µm. Representative immunohistochemistry of n=3 different experiments (three donors). h TGF-²1 and i Rho A-labeled vesicles are counted using Image J (data are presented as mean values+/ SD, p, unpaired two-tailed t test, n=3 donors, five images for TGF-²1 and six images for Rho A). j High content of TGF-²1-transporting vesicles in the blood of C. albicans-infected mice (24h) (data are presented as mean values+/ SD, p=, unpaired two-tailed t test, n=6 uninfected and n=14 infected mice). Blood vesicles were isolated, and TGF-²1 was detected by ELISA.
To observe MEVsCa under physiological conditions, MEVsCa were also tracked ex vivo in C. albicans-infected whole blood using live cell imaging and CLSM. Monocytes were tracked over time with anti-CD14 labeling, and vesicles were tracked with anti-TGF-²1 labeling. Shortly after infection (15min), the monocytes in the blood began to form TGF-²1-transporting vesicles intracellularly, which can be seen as red dots within the monocytes (Fig. 3d). When C. albicans started forming hyphae (45min later), TGF-²1-transporting vesicles from the same monocytes were detected extracellularly (Fig. 3d). After 1h of infection, significantly increased numbers of TGF-²1-transporting vesicles (BEVsCa) were detected in the blood, in contrast to uninfected blood (Fig. 3e, e,ff).
To confirm the formation of TGF-²1-transporting vesicles in vivo, mice were infected with C. albicans and killed 1 day later, and extensively perfused liver tissue was stained for TGF-²1. TGF-²1-transporting vesicles were abundant in the liver tissue of infected, but not control, mice (Fig. 3g, g,h).h). Similarly, the vesicle marker RhoA was abundant (Supplementary Fig. 2b; Fig. 3i). To further confirm the generation of TGF-²1-transporting vesicles, blood was collected from the mice, vesicles were isolated, and the TGF-²1 content of the vesicles was analyzed by ELISA. The results confirmed a significant (p=, unpaired two-tailed t test, n=614) increase in TGF-²1-transporting vesicles in the blood of infected mice compared with blood from untreated control mice (Fig. 3j).
TGF-²1-transporting vesicles were also purified by positive selection and analysis revealed four different vesicle populations ranging from 30 to nm, with predominant population residing under nm (Supplementary Fig. 2c, d). CD9 transporting vesicles were isolated from selected TGF-²1-transporting vesicles resulting in a fraction of CD9 plus TGF-²1-transporting vesicles (Supplementary Fig. 2eg). These results confirm the presence of TGF-²1 on vesicles.
To understand the release of TGF-²1-transporting vesicles, infected (30min) and control monocytes were fixed and stained for intracellular TGF-²1. Infected monocytes showed substantially lower concentrations of intracellular TGF-²1 compared with control monocytes, suggesting that TGF-²1 is released on vesicles from infected monocytes. Furthermore, comparative qPCR revealed no upregulation of TGFB1 transcription in infected monocytes (Supplementary Fig. 2hj).
Fungal ²-glucan interaction with CR3 induces MEVsCa release
To determine how TGF-²1-transporting MEVsCa are generated, we focused on CR3, as CR3 has been described as a key recognition receptor for pathogens14. Therefore, CR3 was blocked with the anti-cholesterol drug simvastatin15. When CR3-blocked monocytes were infected with opsonized C. albicans, induction of TGF-²1-transporting vesicles was significantly reduced (p=, unpaired two-tailed t test, n=3) (Fig. 4a). To further verify the involvement of the CR3 receptor in this process, CR3 receptor subunit CD11b was knocked out in monocytic THP-1 cells using the clustered regularly interspersed short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (CAS9) system. CD11b knockout (KO) THP-1 cells lost expression of CR3, as shown by flow cytometry and western blot analysis (Fig. 4b).
a TGF-²1 is significantly upregulated in MEVsCa compared with MEVs, but not when CR3 was blocked (data are presented as mean values+/ SD, p=, unpaired two-tailed t test, n=3 donors). MEVs were isolated from 1-h-infected monocytes (5×105), and TGF-²1 was detected by ELISA. b CD11b knockout (KO) THP-1 cells generated via CRISPR-CAS9 lacks CD11b expression by both western blot and flow cytometry (see uncropped WB and gating strategy in Supplementary Fig. 7a, c). c TGF-²1 significantly increases in wild-type THPderived TVsCa, but not EVs from CD11b KO THP-1 cells. EVs were isolated from uninfected or C. albicans-infected (1h) wild-type or CD11b KO THP-1 cells (TVs or TVsCa, respectively) (each 5×105) (data are presented as mean values+/ SD, p=, unpaired two-tailed t test, p=, one-way ANOVA, n=3 donors). d TGF-²1 significantly increases in ex vivo blood cell infection-derived vesicles (BCEVsCa) compared with control blood cell-derived vesicles (BCEVs) in wild-type mice (C57BL/6), but not CR3-deficient (CD11b KO) mice (BS4-Itgamtm1Myd/J). In all cases, vesicles were isolated from 1-h-infected blood cells (1×107) (data are presented as mean values+/ SD, p=, unpaired two-tailed t test, p=, one-way ANOVA, n=3 donors). e MEVsCa from wild-type but not from CR3-deficient mice show increased TGF-²1 concentration compared to MEVs (data are presented as mean values+/ SD, p=, unpaired two-tailed t test, p=, one-way ANOVA, n=5 wild-type mice and n=3 or CR3-deficient mice). Monocytes were generated from mouse bone marrow-derived stem cells. MEVs and MEVsCa (1h infection) were each isolated from 5×105 monocytes. f CR3 (CD11b/ CD18) harbors two binding domains: the I-domain (binds iC3b) and the lectin-like site (LLS) domain (binds soluble ²-glucan (s²G)) (modified from OBrien et al.18). g MEVsSc-s²G and MEVsCa-s²G (induced for 1h by s²G from S. cerevisiae and C. albicans, respectively) but not by iC3b show increased TGF-²1 concentrations compared with MEVs. EVs were isolated from activated monocytes (5×105) (data are presented as mean values+/ SD, p=, p=, p=, unpaired two-tailed t test, n=3 donors). h MEVs or MEVsCa-s²G tracking by DLSM. Data are representative of four independent experiments. i MEVsCa-s²G significantly increase compared with MEVs (data are presented as mean values+/ SD, p=, unpaired two-tailed t test, n=5 donors). j Size distributions of EVs determined with NanoSight NTA software. EVs were isolated from 5×105 induced or control monocytes. Graphs were generated by overlaying the size distribution of MEVs and MEVsCa-s²G derived from five donors. k s²G from C. albicans binds to CR3 on monocytes as seen by proximity ligation assay (PLA). PLA staining red: s²G /CD11b complexes, blue: DNA. Bars: 10µm. Representative data of n=5 experiments (five donors).
Like human blood monocytes, THP-1 cells expressing CR3 released TGF-²1-transporting vesicles in response to opsonized C. albicans. However, CD11b KO THP-1 cells completely failed to release TGF-²1-containing vesicles (Fig. 4c).
In addition, CR3 dependence was investigated in an ex vivo system using whole blood from wild-type and CR3-deficient (CD11b KO) mice (Supplementary Fig. 3a, b). Peripheral blood cells (1×107) from mice of each genotype were infected with C. albicans for 1h, and vesicles were subsequently isolated and assayed for TGF-²1. Unlike blood cells from wild-type mice, cells from CD11b KO mice produced small amounts of TGF-²1-transporting vesicles (Fig. 4d). To connect these results to monocytes, mouse monocytes were generated from isolated bone marrow cells from both wild-type and CD11b KO mice (Supplementary Fig. 3ad), and MEVs and MEVsCa were isolated. No significant difference in TGF-²1 content was observed between MEVs and MEVsCa derived from CD11b KO mouse monocytes, but MEVsCa generated from wild-type monocytes showed a significant 46-fold increase in TGF-²1 levels compared with MEVs (p=, unpaired two-tailed t test, n=5) (Fig. 4e).
Having shown the central role of CR3 in the production of TGF-²1-transporting vesicles by C. albicans-infected monocytes, we were interested in identifying the ligand of CR3 responsible for this effect. As iC3b is deposited onto C. albicans due to opsonization, iC3b was used to induce TGF-²1 vesicle release by binding to the so-called I-domain of CR3 (Fig. 4f). Human blood monocytes were stimulated for 1h with iC3b, and vesicles were isolated and assayed for their TGF-²1 content by ELISA. No significant difference was observed between MEVs and vesicles induced with iC3b, indicating that iC3b is not relevant in TGF-²1-containing vesicle release. As soluble ²-glucan is commonly expressed on the fungus16,17 and has been previously described to bind to the lectin-like site (LLS) of CR318, purified soluble ²-glucan from Saccharomyces cerevisiae was used to induce vesicles (MEVsSc-s²G) in human monocytes. Similarly, soluble ²-glucan was extracted/enriched from C. albicans and used to induce vesicles (MEVsCa-s²G). Both types of monocyte-derived vesicle showed significant TGF-²1 content (p=, p=, unpaired two-tailed t test, n=3) (Fig. 4g). Similar to induction with whole C. albicans cells, the number of vesicles released from monocytes increased by about tenfold upon stimulation with soluble ²-glucan from C. albicans (Fig. 4i). MEVsCa-s²G were composed of six major vesicle populations, with sizes ranging from 50nm to nm (Fig. 4j). To determine whether ²-glucan interacted directly with CR3, a proximity ligation assay (PLA) was performed. In the presence of C. albicans, soluble ²-glucan on the C. albicans surface interacted with CD11b on the monocyte surface (Fig. 4k). When enriched soluble ²-glucan was used instead of whole C. albicans cells in the same PLA, the interaction between soluble ²-glucan and CD11b was confirmed. No fluorescence was detected from CR3-expressing monocytes alone in the absence of soluble ²-glucan (Fig. 4k). iC3b also bound to CD11b on the monocyte (Supplementary Fig. 3e), and both iC3b and soluble ²-glucan-induced reactive oxygen species (ROS) formation in monocytes upon binding (Supplementary Fig. 3f). Label-free LC-MS/MS-based proteomics was also used to determine the composition of ²-glucan-induced vesicles. MEVsCa-s²G showed a significant increase of proteins compared with MEVs, while the level of 13 proteins were significantly decreased (Fig. 5a). According to a Gene Ontology enrichment analysis for cellular compartments, again most of the identified proteins () were extracellular exosome-related proteins, similarly to the proteins identified in whole C. albicans-induced vesicles (Fig. 5b). KEGG pathway enrichment analysis also showed a high abundance of complement pathway proteins (Supplementary Fig. 4a). In addition, out of commonly reported vesicle markers (ExoCarta)19, 68 were upregulated in both type of vesicles (MEVsCa and MEVsCa-s²G; Fig. 4c). These data confirm that soluble ²-glucan is a component of C. albicans that induces TGF-²1-transporting vesicles.
a MEVsCa-s²G show significantly increased protein concentrations compared with MEVs. MEVs and MEVsCa-s²G were isolated from 1×107 monocytes, and proteins were detected using label-free LC-MS/MS-based proteomics (p, unpaired two-tailed t test, n=3 different donors). b Most of the proteins in MEVsCa-s²G are extracellular exosome-related as determined by GO enrichment analysis for cellular compartments. Comparison was performed between protein contents of MEVsCa-s²G and MEVs from n=3 donors. c MEVsCa and MEVsCa-s²G show upregulation of 68 vesicle marker proteins (ExoCarta) compared with MEVs. Protein contents of MEVsCa and MEVsCa-s²G were compared with that of MEVs from n=3 donors. d TGF-²1 increases in MEVsAB compared with MEVs. EVs were isolated after 1h of co-incubated monocytes (5×105) with opsonized apoptotic bodies (data are presented as mean values+/ SD, p=, unpaired two-tailed t test, n=3 donors). e TGF-²1 significantly increases in ex vivo blood cell-derived vesicles induced by apoptotic bodies (BCEVsAB) compared with control blood cell-derived vesicles (BCEVs), but not when CR3 was blocked. Vesicles were isolated from 1h co-incubated blood cells (1×107) (data are presented as mean values+/ SD, p=, p=, unpaired two-tailed t test, n=3 donors). f TGF-²1 significantly increases in ex vivo blood cell apoptotic body-induced vesicles (BCEVsAB) compared to control blood cell-derived vesicles (BCEVs) in wild-type mice, but not in CD11b KO mice. In all cases, vesicles were isolated from 1h induced blood cells (1×107) (data are presented as mean values+/ SD, p=, unpaired two-tailed t test, n=3 donors). g TGF-²1 on MEVsCa but not MEVs binds to TGF-²RII on macrophages by proximity ligation assay (PLA). Cells were incubated with EVs for 30min, and PLA assay was performed to detect TGF-²1/TGF-²RII complexes by CLSM. PLA staining red: TGF-²1/TGF-²RII complexes, blue: DNA. Bars: 10µm. A representative PLA of three experiments (n=3 donors) is shown. h IL-6 is induced in LPS-stimulated macrophages in the presence of MEVs, but not MEVsCa. Blocking of TGF-²1 on MEVsCa did not reduce IL-6 production. M1 macrophages were differentiated from human blood monocytes, and IL-6 was measured by ELISA (data are presented as mean values+/ SD, p=, p=, p=, unpaired two-tailed t test, n=3 donors).
Apoptotic bodies induce release of TGF-²1-transporting MEVs
TGF-²1 has been described primarily as an anti-inflammatory cytokine20, and targeted disruption of the mouse Tgfbr1 gene results in several inflammatory diseases21. To determine whether C. albicans exploits a physiological regulatory mechanism to dampen the immune response to the fungus, we aimed to identify a physiological condition where TGF-²1-transporting vesicles are released by monocytes in the absence of an infection. As apoptosis is characterized as a process of cell clearance without inflammation, we wondered whether human apoptotic bodies induce similar TGF-²1-transporting vesicles in monocytes. Apoptotic bodies were generated from human umbilical vein endothelial cells (HUVECs), isolated, opsonized in complement-active human serum, and incubated for 1h with human blood monocytes. The vesicles induced in response to opsonized apoptotic bodies (MEVsAB) were isolated and assayed for TGF-²1 by ELISA. Apoptotic bodies induced significantly higher amounts of TGF-²1-transporting vesicles (MEVsAB) compared with MEVs (p=, unpaired two-tailed t test, n=3) (Fig. 5d). This effect was also analyzed in the ex vivo blood system. Apoptotic body-induced blood cell vesicles (BCEVsAB) carried about twice as much TGF-²1 compared with blood cell vesicles (BCEV) induced in the absence of apoptotic bodies (Fig. 5e). To investigate whether CR3 is involved in this process, apoptotic cells were incubated in whole blood in the presence of simvastatin, which blocks CR3 activation. Under these conditions, the amount of TGF-²1 in BCEVsAB was significantly reduced (p=, unpaired two-tailed t test, n=3) and comparable with the levels in BCEVs derived in the absence of apoptotic cells (Fig. 5e).
In addition, CR3 dependence was investigated in an ex vivo system using whole blood from wild-type and CD11b KO mice. Peripheral blood cells (1×107) from mice of each genotype were incubated with opsonized apoptotic bodies generated from whole blood of respective mouse for 1h, and vesicles were subsequently isolated and assayed for TGF-²1. No significant difference in TGF-²1 content was observed between BCEVs and BCEVsAB derived from CD11b KO mouse monocytes, but BCEVsAB generated from wild-type monocytes revealed a significant increase in TGF-²1 levels compared with BCEVs (p=, unpaired two-tailed t test, n=3) (Fig. 5f). Thus, TGF-²1-containing vesicles are released by monocytes in a CR3-dependent manner in response to apoptotic bodies, and this pathway is also used by C. albicans.
MEVsCa downregulate IL-6 expression in human macrophages
Detection of increased abundance of endocytosis proteins in MEVsCa and MEVsCa-s²G compared with MEVs (Supplementary Fig. 4b, c) suggested uptake by and interaction of these EVs with other cells. As discussed above, TGF-²1 acts predominantly as an anti-inflammatory cytokine. To assess the functional effects of TGF-²1-transporting vesicles, human blood monocytes were differentiated into macrophages and subsequently incubated with TGF-²1-transporting vesicles. To inhibit inflammation, TGF-²1 on the vesicles interacts with TGF-²RII on macrophages. Therefore, binding of TGF-²1-transporting vesicles to this receptor was evaluated by PLA. Co-incubation revealed complex formation between TGF-²1 on the vesicle and TGF-²RII on the macrophage, in contrast to MEVs, which did not show any interaction with TGF-²RII in the PLA (Fig. 5g).
As TGF-²1-transporting vesicles were expected to dampen the inflammatory response after binding to the TGF-²RII, macrophages were first incubated with lipopolysaccharide (LPS) to induce production of the inflammatory cytokine IL-622. TGF-²1-transporting vesicles significantly reduced IL-6 production by macrophages (p=, unpaired two-tailed t test, n=3), as measured by ELISA (Fig. 5h). When TGF-²1 on vesicles was blocked with TGF-²1-neutralizing antibodies, LPS-induced IL-6 was not reduced (Fig. 5h). This result confirmed the anti-inflammatory role of TGF-²1 vesicles.
MEVsCa reduce inflammation via the SMAD7 pathway
To determine whether TGF-²1-transporting vesicles regulate IL-6 also in systemic C. albicans infection, human whole blood was infected with C. albicans for 4h, and IL-6 transcription was assayed in whole blood cells by qPCR. No changes of IL-6 transcription was detected after infection and with TGF-²1-neutralizing antibodies (Fig. 6a). To further follow the function of TGF-²1-transporting vesicles in systemic C. albicans infection, interaction of these vesicles with human blood monocytes was assessed by PLA. Incubation of monocytes with TGF-²1-transporting vesicles for 15min revealed the formation of complexes between TGF-²1 on vesicles and TGF-²RII on monocytes. Vesicles from uninfected monocytes did not show any fluorescent signal (Fig. 6b). This result demonstrates that TGF-²1-transporting vesicles generated in response to C. albicans bind to monocytes via TGF-²RII. Tracking these vesicles with SEM confirmed binding of multiple vesicles also to the C. albicans surface (Fig. 6c). When TGF-²1 was labeled with immunogold, TGF-²1-transporting vesicles were observed, particularly on C. albicans hyphae (Fig. 6d). To determine the effect of TGF-²1-containing vesicles in C. albicans infection, human whole blood was infected with C. albicans for 4h, and then blood cells were lysed, and proteins were separated by SDS-PAGE and immunoblotted for markers of TGF-²1 pathway activation, including phosphorylated SMAD2/3 and SMAD7. C. albicans infection resulted in strong induction of SMAD2/3 phosphorylation (Fig. 6e). However, when TGF-²1 was blocked during C. albicans whole blood infection with a neutralizing antibody, SMAD2/3 phosphorylation was inhibited (Fig. 6e). As SMAD2/3 phosphorylation leads to the upregulation of SMAD7, which subsequently signals through NF-ºB23, we also determined the expression of SMAD7 by western blot analysis. SMAD7 was substantially upregulated during C. albicans infection, and was reduced in the presence of an anti-TGF-²1 antibody (Fig. 6e).
a No change in IL6 transcription occurs in whole-blood ex vivo infection with C. albicans, also after blocking TGF-²1 (data are presented as mean values+/ SD, p=, unpaired two-tailed t test, n=3 donors). b TGF-²1 on MEVsCa binds to TGF-²RII on monocytes. No such interaction was observed with MEVs. Cells were incubated with EVs for 30min, and TGF-²1/TGF-²RII complexes detected by PLA using CLSM. PLA staining red: TGF-²1/TGF-²RII complexes, blue: DNA. Bars: 10µm. c MEVsCa and d TGF-²1 adhere to C. albicans and its hyphae. Monocytes were infected with opsonized C. albicans for 1h, and immunogold-labeled TGF-²1 visualized by SEM. e Phosphorylated SMAD2/3 and SMAD7 increase in whole blood infected with C. albicans ex vivo (1h), but not when TGF-²1 was neutralized. Cells were lysed and intracellular proteins were for total and phosphorylated SMAD2/3 and SMAD7 detected by Western blot. (see uncropped WB in Supplementary Fig. 7d) The experiments in b, c, d, e are representatives of each n=3 experiments. f Illumina-based RNA-seq reveals significantly upregulated genes in C. albicans-infected monocytes (1h) compared with control monocytes. g RNA-seq analysis shows upregulation of SMAD2, NFKBIA, and SMAD7. For f, g RNA from four different donors were pooled before sequencing. h TGF-²1 inhibits IL-1² synthesis in renal fibrosis. TGF-²1 binds to TGF-²RII, and induces the phosphorylation of SMAD2/3. Phosphorylated SMAD2/3 in combination with SMAD4 upregulate SMAD7, which in turn upregulates IºB±. IºB± inhibits the IL-1²-positive feedback loop by inhibiting phosphorylation of NFºB54. i Little upregulation of IL1B expression is observed in whole blood infected with C. albicans ex vivo, but strong upregulation of IL1B occurs when TGF-²1 or TGF-²RI was blocked during ex vivo infection (data are presented as mean values+/ SD, p=, p=, unpaired two-tailed t test, n=3 donors). jIL1b expression is not upregulated in whole blood from wild-type mice (C57BL/6) infected with C. albicans, but in blood from CR3-deficient (CD11b KO) mice (BS4-Itgamtm1Myd/J) (data are presented as mean values+/ SD, p=, p=, unpaired two-tailed t test, n=4 donors). After 4h whole blood infection, cells were lysed, and RNA was isolated and subjected to comparative qPCR.
RNA-seq of infected and control monocytes confirmed these results, as infected monocytes showed significant upregulation of SMAD7 and NFKBIA as well as SMAD2 (Fig. 6f, g). Induction of SMAD7 and IºB± is also reported to inhibit IL-1² and TNF-± in renal inflammation by blocking NF-ºB-mediated transcription (Fig. 6h)24. A low amount of IL1B production was detected early during whole blood infection with C. albicans (4h), and blocking of TGF-²1 significantly increased IL1B synthesis (Fig. 6i). Similarly, a large amount of IL1B was produced upon selective blocking of the TGF-²1 receptor-like kinase ALK5 and its relatives ALK4 and ALK7 with SB (Fig. 6i). Also whole-blood infection with C. albicans (4h) resulted in a minor increase of soluble TGF-²1 (Supplementary Fig. 5a). This demonstrates a reduction of the pro-inflammatory response in early infection.
The regulating effect of TGF-²1-transporting vesicles was subsequently confirmed in C. albicans-infected whole mouse blood. The release of TGF-²1-transporting vesicles was seen in wild-type mouse blood in early infection (Fig. 4d) with no significant upregulation of IL1b transcription (Fig. 6j). In contrast, no TGF-²1-transporting vesicles were released in infected CD11b KO blood cells (Fig. 4d), but high expression of IL1b transcription (Fig. 6j).
MEVsCa act anti-inflammatory on endothelial cells
To verify the presence of TGF-²1-transporting vesicles in vivo, liver tissue sections from C. albicans-infected mice (24h) were screened for TGF-²1 expression by CLSM. The sections were stained with an anti-TGF-²1 primary antibody and a fluorescently labeled secondary antibody together with the nucleic acid dye Sytox Orange. Strong TGF-²1 staining was identified along, and in endothelial cells of blood vessels of infected mice, but little or no staining was observed in tissues from uninfected mice (Fig. 7a). As blood vesicles easily come in contact with endothelial cells, these cells were subjected to further studies. First, BCEVs and BCEVsCa were isolated by polymer precipitation. The total amount of BCEVsCa was significantly higher (about 4 fold) compared with BCEV (Fig. 7c). BCEVsCa size ranged from 40 to nm (Fig. 7d). To confirm the presence of TGF-²1-transporting vesicles during whole-blood infection, the membrane and cytosol fractions of the vesicles were separated by lysis of the vesicles and subsequent centrifugation. Supernatants and membrane fractions were separated and immunoblotted using an anti-TGF-²1 antibody. BCEVsCa but not BCEVs showed the presence of TGF-²1 in the membrane fraction (Fig. 7e). To understand the effect of TGF-²1-transporting vesicles on the blood vessel endothelium in systemic candidiasis, the interaction of the vesicles with human endothelial cells was assessed. First, the interaction of BCEVsCa with HUVECs was assayed by PLA. After 30min of incubation, multiple complexes between TGF-²1 on isolated vesicles and TGF-²RII on the HUVEC surface were detected. Vesicles from uninfected blood cells, however, did only generate low or no signals (Fig. 7f, f,g).g). Following incubation of vesicles with HUVECs for 6h, the effect of TGF-²1-transporting vesicles on the HUVEC phenotype was determined by qPCR. HUVECs treated with TGF-²1-transporting vesicles revealed an upregulation of several anti-inflammatory cytokines, including TGFB1 and IL4 (Fig. 7h; Supplementary Fig. 6). Increased production of TGF-²1 in endothelial cells was confirmed by immunofluorescence using LSM and measuring intracellular TGF-²1 by ELISA. HUVECs showed significantly more intracellular TGF-²1 when they were incubated with TGF-²1-containing blood cell vesicles from C. albicans
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