Author Response: FGF2-FGFR1 Signaling Regulates Release of Leukemia-Protective Exosomes from Bone Marrow Stromal Cells

Article Figures and data Abstract eLife digest Introduction Results Discussion Materials and methods Data availability References Decision letter Author response Article and author information Metrics Abstract Protective signaling from the leukemia microenvironment leads to leukemia cell persistence, development of resistance, and disease relapse. Here, we demonstrate that fibroblast growth factor 2 (FGF2) from bone marrow stromal cells is secreted in exosomes, which are subsequently endocytosed by leukemia cells, and protect leukemia cells from tyrosine kinase inhibitors (TKIs). Expression of FGF2 and its receptor, FGFR1, are both increased in a subset of stromal cell lines and primary AML stroma; and increased FGF2/FGFR1 signaling is associated with increased exosome secretion. FGFR inhibition (or gene silencing) interrupts stromal autocrine growth and significantly decreases secretion of FGF2-containing exosomes, resulting in less stromal protection of leukemia cells. Likewise, Fgf2 -/- mice transplanted with retroviral BCR-ABL leukemia survive significantly longer than their +/+ counterparts when treated with TKI. Thus, inhibition of FGFR can modulate stromal function, reduce exosome secretion, and may be a therapeutic option to overcome resistance to TKIs. Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter). https://doi.org/10.7554/eLife.40033.001 eLife digest Leukemias are cancers of white blood cells. The cells grow and divide rapidly, often because of mutations in proteins called kinases. Since the kinase mutations do not occur in healthy cells, they provide a good target for anti-leukemia drugs. Several such kinase inhibitors are effective at treating leukemia patients. However, most leukemia cells develop ways to resist the effects of the kinase inhibitors over time, leading to relapses of the disease. One way that leukemia cells resist kinase inhibitors is by taking advantage of signals coming from supportive cells, known as stromal cells, in the bone marrow. When patients are treated with kinase inhibitors, the bone marrow stromal cells produce more of a signaling protein called FGF2. The leukemia cells then use FGF2 to survive the effects of the kinase inhibitors. It was not clear how the FGF2 signal reaches the leukemia cells from the bone marrow stromal cells. Now, using biochemical techniques, Javidi-Sharifi, Martinez et al. show that bone marrow stromal cells package FGF2 into small compartments called exosomes. The stromal cells release the exosomes into the bone marrow, and the leukemia cells then engulf and internalize the exosomes. Leukemia cells that had taken up FGF2 in this way were better able to survive kinase inhibitor treatment than leukemia cells that had not. Javidi-Sharifi, Martinez et al. also observed that FGF2 also affects the bone marrow stromal cells themselves, causing them to grow faster, produce more FGF2 and release more exosomes. Blocking the effects of FGF2 on the stromal cells slowed their growth and caused fewer exosomes to be released. In addition, mice whose bone marrow stromal cells could not produce FGF2 survived leukemia for longer than mice whose stromal cells provided protective FGF2 in exosomes to leukemia cells. This suggests that taking advantage of drugs that prevent bone marrow stromal cells from releasing FGF2 in exosomes might improve treatments for leukemia. Further research will be needed to confirm whether this strategy would be effective in humans. https://doi.org/10.7554/eLife.40033.002 Introduction TKIs have revolutionized the treatment of CML and have shown promise in AML, however development of resistance remains a problem. In CML, resistance develops in a minority of patients, and is most often caused by resistance mutations. However, some patients still develop resistance in the absence of known resistance mutations. In contrast, development of resistance in AML is the norm. Inhibitors of mutated FLT3, which is present in about 30% of AML patients, are initially quite efficacious (Smith et al., 2012). However, resistance to FLT3 kinase inhibitors in AML typically develops within a few months. In some cases, resistance is cell-intrinsic and due to secondary mutations in the activating loop of FLT3 that prevent drug binding (Weisberg et al., 2009), however, resistance still develops in the absence of these mutations. Within the marrow microenvironment, leukemia cell survival can be mediated by extrinsic ligands that activate alternative survival pathways (Smith et al., 2017; Ghiaur and Levis, 2017; Wilson et al., 2012) and over time can lead to development of intrinsic resistance mutations (Wilson et al., 2012; Traer et al., 2012). Bone marrow stromal cells provide a supportive structure and secrete cytokines that contribute to the normal hematopoietic stem cell niche, but can also protect leukemic cells from therapy (Colmone et al., 2008; Ayala et al., 2009). Initial studies into the mechanisms of resistance utilized normal marrow stroma (Manshouri et al., 2011), but the stroma can be altered by leukemia, in a manner similar to development of cancer associated fibroblasts in solid tumors (Paggetti et al., 2015; Huang et al., 2015; Huan et al., 2015). We found that fibroblast growth factor 2 (FGF2) expression is increased in marrow stromal cells during tyrosine kinase inhibitor (TKI) therapy and protects leukemia cells (Ware et al., 2013; Traer et al., 2014; Javidi-Sharifi et al., 2015; Traer et al., 2016). FGF2 has also been shown to be essential for stress hematopoiesis after chemotherapy (Itkin et al., 2012; Zhao et al., 2012), suggesting that leukemia cells can hijack a normal marrow stress response for their own survival. Despite its important roles in physiology and pathology, several aspects of FGF2 biology remain poorly understood. FGF2 does not have a signal peptide and is not secreted through the canonical secretory pathway. Alternative mechanisms for secretion have been proposed, but how FGF2 is conveyed between two cells remains unclear (Steringer et al., 2015; Zacherl et al., 2015). Additionally, while recombinant FGF2 directly stimulates myeloid colony formation (Berardi et al., 1995), there are also reports suggesting that FGF2 can indirectly regulate hematopoiesis by stimulating stromal cells to produce cytokines (Avraham et al., 1994). We discovered that FGF2 is largely secreted in extracellular vesicles (ECVs) and exosomes from bone marrow stromal cells. ECVs are able to protect leukemia cells from the effects of TKI therapy. Furthermore, autocrine FGF2-FGFR1 activation in marrow stromal cells increases the secretion of FGF2-laden exosomes, indicating that exosome secretion is regulated in part by FGF2-FGFR1 signaling. Inhibition of FGFR1 can reverse this protective stroma-leukemia interaction and restore leukemia cell TKI sensitivity in the marrow niche. Results Stromal cell ECVs protect leukemia cells from TKI therapy The human stromal cell line HS-5 expresses abundant FGF2, in addition to other soluble cytokines such as IL-5, IL-8 and HGF (Roecklein and Torok-Storb, 1995), and conditioned media (CM) from HS-5 is highly protective of leukemia cell lines. HS-5 CM was ultracentrifuged at 100,000 g to separate soluble proteins (supernatant, S100) from ECVs and larger macromolecules (pellet, P100). We compared the protective effect of unfractionated CM, S100, and P100 fractions on the viability of two leukemia cell lines: MOLM14 (FLT3 ITD+ AML) and K562 (CML), in the presence of their respective TKIs, quizartinib (AC220, a highly selective and potent inhibitor [Zarrinkar et al., 2009]) and imatinib (Figure 1A and B). The protective capacity of the S100 fraction was less than unfractionated CM, and protection was enriched in the concentrated P100 ECV fraction (Figure 1), indicating that a substantial protective component of HS-5 CM is mediated by ECVs. A more extensive profiling of protection is also shown in Figure 1—figure supplement 1. Figure 1 with 3 supplements see all Download asset Open asset Extracellular vesicles (ECVs) secreted by HS-5 cells are internalized by MOLM14 and K562 cells and protect from treatment with AC220 or imatinib, respectively. HS-5 conditioned media (CM) was collected and separated by ultracentrifugation at 100,000 g into a supernatant (S100) and pellet (P100) fraction containing ECVs. These fractions were incubated with (A) K562 cells ± 1 μM imatinib, or (B) MOLM14 cells ± 10 nM AC220, and viability measured by MTS assay after 48 hr. Values were normalized to respective untreated condition. All wells were plated in triplicate and error bars indicate standard deviation. RPMI is the media control. p values are indicated by *<0.05, **<0.005, and ***=0.0007. (C), MOLM14 and K562 cells were stained with DiO (green) tracer, washed, and immobilized on Poly-D-Lysine coated chamber slides. HS-5 P100 fraction was stained with DiI (red) tracer and added to the cells for a 24 hr incubation. Slides were stained with DAPI (blue) and imaged by confocal fluorescent microscopy. A movie of the z-stack images is included in Supplemental data. https://doi.org/10.7554/eLife.40033.003 To determine if ECVs produced by HS-5 cells are internalized by K562 and MOLM14 leukemia cells, K562 and MOLM14 cells were stained with a green lipophilic tracer (DiO) and incubated with HS-5 ECVs stained with a red lipophilic tracer (DiI). Analysis by confocal microscopy showed that ECVs are indeed internalized by leukemia cells, although the exact mechanism of internalization is still under investigation (Figure 1C and Figure 1—video 1 and 2). FGF2 is contained in stromal cell ECVs and exosomes FGF2 is highly expressed in the HS-5 stromal cell line but the related HS-27 expresses little FGF2 (Figure 2A; Traer et al., 2016). We analyzed FGF2 in S100 and P100 fractions of both HS-5 and HS-27 by immunoblot (Figure 2B). Little FGF2 was detected in the soluble protein fraction (S100), but FGF2 was enriched in ECVs (P100). Washing the ultracentrifuge tube with detergent liberated even more FGF2 (detergent wash P100), due to ECVs adhering to the ultracentrifuge tube. To compare FGF2 to other soluble cytokines, HS-5 CM was ultracentrifuged into S100 and ECVs, cytokines quantified by Luminex multiplex assay, and normalized to unfractionated CM (Figure 2C). Pelleted ECVs were resuspended in 10% of the original CM volume, and the P100 bars in Figure 2B thus represent a 10-fold enrichment, although as shown in Figure 2B not all ECVs can be liberated from the ultracentrifuge tube. FGF2 was uniquely enriched in ECVs, whereas soluble cytokines such as stem cell factor, interleukin (IL)−6, IL-8, etc. were found primarily in the S100 fraction. Figure 2 Download asset Open asset FGF2 is enriched in exosomes from HS-5 bone marrow stromal cells. (A) Immunoblot of FGFR1, FGF2 and actin in HS-5 and HS-27 whole cell lysates. (B) HS-5 and HS-27 CM were ultracentrifuged at 100,000 g for 2 hr at four degrees C. CM, soluble protein (S100), and ECV (P100) fractions were collected and analyzed by immunoblot, using 10, 50, and 100 ng/ml recombinant FGF2 for comparison. The ultracentrifuge tube was also washed with detergent to remove adherent ECVs and material (detergent wash P100). (C) HS-5 CM, S100 and P100 fractions (concentrated ~10 fold compared to HS-5 CM) were solubilized in 0.1% NP-40 and analyzed by cytokine multiplex ELISA (Luminex). The S100 and P100 fractions were normalized to CM. (D) The HS-5 P100 fraction (starting material, or SM) was further fractionated on a sucrose step-gradient. Sucrose layer interfaces (0–7.5%, 7.5–15%, 15–30%, 30–45%, and 45%-pellet) were collected, lysed and analyzed by immunoblot with antibodies against the exosomal marker CD9, FGF2, and cytoplasmic marker actin. (E) HS-5 and HS-27 ECVs (P100), recombinant FGF2, and HS-5 cells were exposed to proteinase K and analyzed by immunoblot. (F) HS-5 exosomes were isolated by sucrose step-gradient (see panel D) and then exposed to proteinase K with or without detergent (0.1% Triton X-100, used to dissolve the lipid membrane). Samples were subjected to immunoblot analysis using antibodies against tsg101, CD9 and FGF2. The * indicates degraded FGF2 after partial proteinase K digestion. https://doi.org/10.7554/eLife.40033.007 HS-5 ECVs were further separated into microvesicles, exosomes, and insoluble extracellular matrix proteins (ECM) using a sucrose step-gradient to separate by density. FGF2 and cell compartment-specific molecular markers were probed by immunoblot (Figure 2D). FGF2 was most highly enriched in the 15–30% sucrose interface, which also contained the exosome-specific marker CD9. To determine if FGF2 was bound to the outside of ECVS, or contained within ECVs, proteinase K was used to digest proteins not enclosed by lipid membrane. Recombinant FGF2, HS-5 or HS-27 ECVs, and intact HS-5 cells were incubated with proteinase K and probed for FGF2 by immunoblot (Figure 2E). Recombinant FGF2 was completely degraded by proteinase K (* indicates degraded fragments) but intact FGF2 was detected in both HS-5 ECVs and control HS-5 cells. We repeated this experiment using purified HS-5 exosomes and again observed that a fraction of FGF2 was protected from digestion (Figure 2F). Addition of 0.1% Triton X-100 disrupted the lipid membrane and resulted in complete digestion of all protein. We found a similar digestion pattern with the exosomal transmembrane proteins CD9 and tsg101. We conclude that FGF2 is contained within ECVs and exosomes, however we cannot exclude that FGF2 may also be on the surface since partial FGF2 degradation was noted in intact HS-5 cells, ECVs and purified exosomes (Figure 2E–F). HS-5 stromal cells overproduce ECVs Since HS-5 CM is more protective than HS-27 CM (Manshouri et al., 2011; Traer et al., 2016; Weisberg et al., 2008), we suspected that ECVs may be more numerous in HS-5 CM. We chose several orthogonal methods to quantify vesicles in CM. First, we used nanoparticle tracking analysis to quantify and compare HS-5 and HS-27 ECVs (Figure 3A). In parallel, we employed the Virocyt Virus Counter, a flow cytometry-based technique developed to detect viruses, which also works well to quantify ECVs (Figure 3B). As a gold standard, negative stain transmission electron microscopy of purified HS-5 and HS-27 exosomes was also used to image and quantify exosomes by counting (30–100 nm diameter with cup-shape appearance characteristic for exosomes, Figure 3C). Finally, we used sucrose step-gradient fractionation of HS-5 and HS-27 ECVs to compare cell compartment and exosome-specific markers by immunoblot (Figure 3D). Exosomes layer primarily at the 15–30% sucrose interface as indicated by exosomal markers CD9 and tsg-101, and are increased in HS-5 cells compared to HS-27. Interestingly the receptor for FGF2, FGFR1, was also found to localize preferentially with HS-5 exosomes. With all methods, we consistently observed greater than two-fold excess of vesicles produced by HS-5 compared to HS-27 cells (see Figure 3—figure supplement 1 for additional data). Markers of nucleus (lamin A/C), endoplasmic reticulum (calreticulin) and mitochondria (Bcl-XL) were located in the 45–60% interface containing larger microvesicles and apoptotic bodies. Figure 3 with 1 supplement see all Download asset Open asset HS-5 cells secrete more exosomes than HS-27 cells. Equal numbers of HS-5 and HS-27 cells were plated in RPMI with exosome-depleted FBS for 24 hr. The ECVs were pelleted by ultracentrifugation at 100,000 g for 2 hr at four degrees C and resuspended in PBS. ECVs were quantified by (A) Nanosight, a nanovesicle tracking analysis, (B) Virocyt Virus Counter, a proprietary flow cytometry using fluorescent dyes that stain both nucleic acid and protein, or (C) transmission electron microscopy. (D) HS-5 and HS-27 exosomes were collected by sucrose-step gradient and analyzed by transmission electron microscopy. Vesicles were quantified by counting in three 2 × 2 μm areas per sample. All experiments were done in triplicate, error bars represent standard deviation, p values are indicated by *<0.05, **<0.005. HS-5 and HS-27 ECVs (P100) were obtained by ultracentrifugation (starting material, or SM), and the exosome fraction was further purified by a sucrose step-gradient. Sucrose layer interfaces (0–7.5%, 7.5–15%, 15–30%, 30–45%, and 45%-pellet) were collected, lysed and analyzed by immunoblot. Blots were probed with antibodies against exosomal markers CD9 and tsg101; cell compartment markers: fibronectin, lamin A/C, BCL-XL; as well as FGFR1 and FGF2. The lanes with highest enrichment for CD9 and tsg-101, indicating exosomes, are marked below. https://doi.org/10.7554/eLife.40033.008 FGF2-FGFR1 signaling promotes stromal growth and paracrine protection of leukemia FGF2 is an autocrine signaling protein for stroma, but recombinant FGF2 also mediates paracrine protection of leukemia cells (Traer et al., 2016; Traer et al., 2014). Thus there are two potential mechanisms by which FGFR inhibition can attenuate protection of leukemia cells in the marrow microenvironment: (1) FGFR inhibitors block FGF2-mediated paracrine protection at the leukemia cells; and/or (2) FGFR inhibitors interrupt stromal FGF2-FGFR1 autocrine signaling to reduce secretion of protective FGF2-containing exosomes. To compare the relative effect of FGFR inhibition on autocrine and paracrine signaling, HS-5 cells were pre-treated with the FGFR inhibitor PD173074 (Mohammadi et al., 1998; Trudel et al., 2004) for one week prior to collection of CM. CM collected from HS-5 cells pre-treated with PD173074 was significantly less protective than CM from an equal number of untreated HS-5 cells (Figure 4A), providing evidence that interruption of FGF2-FGFR1 signaling affects subsequent protection of leukemia cells. In contrast, addition of PD173074 to untreated HS-5 CM only modestly attenuated protection of MOLM14 cells. We found similar results with K562 cells exposed to imatinib (Figure 4—figure supplement 1). Purified ECVs from HS-5 CM, which are enriched in FGF2, were more sensitive to FGFR inhibition (Figure 1—figure supplement 1), however pre-treatment of HS-5 cells with PD173074 still had the greatest absolute reduction in protection. These results indicate that FGFR inhibitors overcome protection of leukemia cells primarily by directly altering secretion of FGF2-expressing stromal cells, making them significantly less protective. Figure 4 with 3 supplements see all Download asset Open asset FGF2 is an autocrine growth factor in bone marrow stromal cells, and FGFR inhibition attenuates growth. (A) HS-5 cells were cultured in media ± 250 nM PD173074 for one week and then equal numbers of cells were replated for comparison. After adhesion, the cells cultured in PD173074 were washed and fresh media added to collect CM. MOLM14 cells were resuspended in media, untreated HS-5 CM, and PD pre-treated HS-5 CM and treated with ± 10 nM AC220 and ± 250 nM PD173074. Viability was measured by MTS assay after 72 hr and values were normalized to the relevant UT control. Error bars represent standard deviation, p values are indicated by *<0.05, **<0.005, and ***=0.0007. (B) HS-5 and HS-27 cells were plated in triplicate on 96 well plates in a gradient of FGFR inhibitor PD173074. Proliferation was measured using MTS reagent after 72 hr. Error bars indicate standard deviation. (C) HS-5 and HS-27 cells were incubated media ± 250 nM PD173074 (PD). The number of viable cells was measured with Guava ViaCount every 3 days over a 15 day period. Fresh media and PD173074 was added every 3 days. (D) HS-5 and HS-27 cells were incubated in media ± 1 µM PD173074 for 1 week. Brightfield microscopy images were obtained using a 10X objective. (E) HS-5 cells were incubated in 4-well glass chamber slides in media ± 250 nM PD173074 (PD). Cells were stained with lipophilic tracer DiI for 24 hr, fixed, then nuclei stained with DAPI. Immunofluorescent images were analyzed with CellProfiler software to determine cell size (μm [Weisberg et al., 2009]) and number of cells for each size range was binned and graphically displayed. PD173074 had no effect on HS-27 growth, morphology or size, consistent with an on-target FGFR effect. (F) Ex vivo cultured primary bone marrow stromal cells from a series of leukemia patients (n = 42) were lysed for RNA extraction and cDNA synthesis. Taqman qPCR analysis was performed using FGFR1, FGFR2, FGFR3, FGFR4, and FGF2 Taqman primer assays and expression plotted (n = 42 for each except FGFR4 which is n = 41 due to failed PCR for one sample). (G) FGFR1 and FGF2 qPCR values (2^-ΔCT) were plotted against each other. There were 9 AML patients with FLT3 ITD (most newly diagnosed) and these patients are indicated with red dots. Linear regression produced a line fit with r2 = 0.5683 and slope significantly non-zero with p<0.0001. https://doi.org/10.7554/eLife.40033.010 To further evaluate the effects of FGFR inhibition in stromal cells, HS-5 cells were evaluated for viability, morphology, and growth using HS-27 cells as comparison (low FGF2). HS-5 or HS-27 cells had little reduction in cell viability after 72 hr treatment with PD173074 (Figure 4B), however HS-5 growth slowed dramatically over 15 days (Figure 4C). HS-5 cells exposed to PD173074 changed morphology and became less refractile, larger, and more adherent (Figure 4D). Cell size was quantified using CellProfiler software and PD173074 significantly increased HS-5 cell size (Figure 4E). To evaluate FGF2 and FGFR1 expression in primary leukemia stroma, bone marrow aspirates from a series of leukemia patients were cultured ex vivo and FGF2 and FGFR1-4 expression quantified by RT-PCR (Figure 4F). FGFR1 and FGF2 transcripts were the most highly expressed in primary stroma, and there was a strong positive correlation between FGFR1 and FGF2 expression (Figure 4G, r2 = 0.5683 and p<0.0001 on nonparametric correlation). This indicates that FGF2 and FGFR1 expression are coordinately regulated in primary marrow stromal cells consistent with activation of an FGF2-FGFR1 autocrine loop. There were nine stromal cultures from AML patients with FLT3 ITD (indicated with red dots), but most of them were newly diagnosed, and based upon our previous data we would not expect increased expression of FGF216. Similar to our observations in cell lines described above, we also detected FGFR1 and FGF2 in ECVs derived from primary marrow stromal cultures (Figure 4—figure supplement 2). However, primary marrow stromal cells grow slowly and produce smaller amounts of ECVs, so we were unable to evaluate the effect of FGFR inhibitors on cell morphology, growth, and ECV production with primary marrow stromal cells. Additional characterization of primary stromal cultures is contained in Figure 4—figure supplement 3. FGFR inhibition decreases stromal cell production of exosomes Since FGFR inhibition attenuates HS-5 growth and morphology, we hypothesized that it might also reduce secretion of ECVs. HS-5 cells exposed to graded concentrations of PD173074 and BGJ-398 had a dose-dependent decrease in ECVs measured by Virocyt Virus Counter (Figure 5A,B). Notably, there was a significant decrease in vesicle number as early as 6 hr after drug exposure (Figure 5—figure supplement 1), suggesting that FGFR inhibition directly affects vesicle production or release. ECVs were also collected from HS-5 and HS-27 cells exposed to PD173074 and analyzed by immunoblot. PD173074 reduced the exosome markers tsg101 and CD9 (and FGF2) but had no effect on ECV production from HS-27 cells (Figure 5C, similar results with BGJ398 shown in Figure 5—figure supplement 1). Scanning electron microscopy of HS-5 cells revealed abundant budding membrane, whereas the surface of PD173074-exposed cells was smoother, implicating a change in membrane dynamics (Figure 5—figure supplement 2). To evaluate exosome secretion specifically, sucrose step-gradient fractionation was performed on ECVs from untreated and PD173074 treated HS-5 cells. PD173074 reduced exosomal markers CD9, tsg101, and FGF2 in the expected 15–30% interface fraction (Figure 5D). Figure 5 with 2 supplements see all Download asset Open asset FGFR inhibition decreases exosome production in FGF2-expressing stroma. HS-5 cells were exposed to a gradient of the FGFR inhibitors (A) PD173074 and (B) BGJ-398 for 48 hr prior to collecting CM. ECVs were pelleted by ultracentrifugation at 100,000 g and quantified by Virocyt Virus Counter. Error bars indicate standard deviation and p values are indicated by *<0.05. (C) HS-5 and HS-27 cells were incubated in media ± 1 µM PD173074 for 72 hr prior to collecting ECVs. ECVs were analyzed by immunoblot for FGF2. The exosome markers CD9 and tsg101 are also shown. (D) HS-5 cells were plated in media ± 1 µM PD173074 for 72 hr. P100 fractions were obtained by ultracentrifugation, and further fractionated on a sucrose step-gradient. The interfaces (0–7.5%, 7.5–15%, 15–30%, 30–45%, and 45%-pellet) were collected, lysed and processed by immunoblot with antibodies against the exosomal markers CD9 and tsg101 as well as FGFR1 and FGF2. https://doi.org/10.7554/eLife.40033.014 Genetic knock-down of FGFR1 or FGF2 attenuates exosome production To confirm that decreased exosome secretion is specific for FGFR1 inhibition, HS-5 cells were stably transfected with either a GFP-expressing lentivirus control vector (GIPZ), or doxycycline-induced shRNA targeting FGFR1. FGFR1 silencing led to a significant reduction in ECVs (Figure 6C). Similar results were obtained with siRNA targeting FGFR1 (Figure 6—figure supplement 1). siRNA and shRNA constructs targeting FGF2 did not achieve reliable silencing of FGF2. HS-5 CRISPR/Cas9 knockout of FGFR1 and FGF2 in HS-5 cells were generated, however genetic silencing prevented continued growth. Multiple attempts to make stable deleted cell lines were unsuccessful, likely due to the importance of FGF2-FGFR1 signaling for HS-5 self-renewal and growth (Bianchi et al., 2003; Coutu et al., 2011; Zhou et al., 1998). That being said, ECVs collected shortly after CRISPR/CAS9 treatment, which resulted in partial silencing of FGF2 or FGFR1, both demonstrated decreased ECVs by immunoblot and reduced protection of MOLM14 cells (Figure 6—figure supplement 2). To test the role of FGF2 in ECV production in primary cells, equal numbers of murine stromal cells from Fgf2 +/+ and -/- mice (Fgf2tm1Doe [Zhou et al., 1998]) were treated with PD173074 and ECVs quantified by Virocyt (Figure 6D). Fgf2 +/+ stromal cells secreted significantly more ECVs than -/-, and PD173074 only reduced ECV secretion in +/+ stroma. ECVs from Fgf2 +/+ and -/- mice were also analyzed by immunoblot with similar reduction in ECV proteins from Fgf2 -/- stroma (Figure 6E). Figure 6 with 2 supplements see all Download asset Open asset Genetic silencing of FGFR1 or deletion of FGF2 attenuates exosome secretion. A doxycycline-inducible lentiviral shRNA targeting FGFR1 was used to create a stable HS-5 cell line. The cells were then treated with doxycycline to induce FGFR1 silencing and compared to a GIPZ lentiviral control. (A) Silencing of FGFR1 expression is shown by immunoblot of cell lysates. ECVs from doxycycline-treated cells were analyzed by (B) immunoblot or (C) Virocyt Virus Counter. *p<0.05. (D) Bone marrow was isolated from Fgf2 +/+ and -/- mice and cultured ex vivo to grow adherent marrow stroma. Equal numbers of cells were then plated, CM collected for 72 hr, and then ultracentrifuged to collect ECVs. The ECVs were quantified by Virocyt. *p<0.05. (E) Equal number of cultured marrow cells from Fgf2 +/+ and -/- mice were plated and then ECVs collected by ultracentrifugation and analyzed by immunoblot. https://doi.org/10.7554/eLife.40033.017 Fgf2 -/- stroma produces fewer exosomes and is less protective of BCR-ABL leukemia To test the role of stromal Fgf2 in an in vivo leukemia model, bone marrow from Fgf2 +/+ mice was retrovirally transfected with BCR-ABL containing GFP as a marker (Traer et al., 2012) and used to transplant lethally irradiated FGF2 +/+ and -/- mice. This induces a very aggressive disease in mice that is more akin to AML than CML, and TKIs are only effective for a limited duration. Mice were treated with the ABL inhibitor nilotinib 75 mg/kg/day by oral gavage starting on day 14 post-transplant. Mice that were found to have aplastic marrow (unsuccessful transplantation) were excluded from analysis since their death was not related to leukemia (four mice in the Fgf2 +/+ untreated group, one mouse in the Fgf2 -/- untreated group, two mice in the Fgf2 +/+ nilotinib group, and two mice in the Fgf2 -/- nilotinib group). The survival curves of the remaining mice are shown in Figure 7A. The cohorts of untreated Fgf2 +/+ and -/- both died rapidly from disease, as expected. Nilotinib significantly increased survival of Fgf2 +/+ and -/- mice compared to untreated mice, but the survival of the nilotinib-treated Fgf2 -/- was also significantly longer than their Fgf2 +/+ counterparts. To ensure equal engraftment of disease in both backgrounds, the blood and bone marrow was analyzed for GFP and found to be similar in both Fgf2 +/+ and -/- mice at time of death (Figure 7B and Figure 7—figure supplement 1), suggesting that nilotinib was more effective at attenuating disease progression of BCR-ABL leukemia cells in an Fgf2 -/- microenvironment. To directly evaluate the protective effect of ECVs on leukemia progenitor cells, ECVs were isolated from equal numbers of +/+ and -/- primary marrow stro