An early increase in the level of endothelial progenitor cells (EPCs) in the systemic circulation occurs in patients with septic infection/sepsis. The significance and underlying mechanisms of this response remain unclear. This study investigated the bone marrow EPC response in adult mice with septic infection induced by intravenous injection (i.v.) of Escherichia coli. For in vitro experiments, sorted marrow stem/progenitor cells (SPCs) including lineage(lin)^-stem cell growth factor receptor(c-kit)⁺stem cell antigen-1(Sca-1)^-, lin^-c-kit⁺, and lin^- cells were cultured with or without lipopolysaccharides (LPS) and recombinant murine vascular endothelial growth factor (VEGF) in the absence and presence of anti-Sca-1 crosslinking antibodies. In a separate set of experiments, marrow lin^-c-kit⁺ cells from green fluorescence protein (GFP)⁺ mice, i.v. challenged with heat-inactivated E. coli or saline for 24 h were subcutaneously implanted in Matrigel plugs for 5 weeks. Marrow lin^-c-kit⁺ cells from Sca-1 knockout (KO) mice challenged with heat-inactivated E. coli for 24 h were cultured in the Matrigel medium for 8 weeks. The marrow pool of EPCs bearing the lin^-c-kit⁺Sca-1⁺VEGF receptor 2 (VEGFR2)⁺ (LKS VEGFR2⁺) and LKS CD133⁺VEGFR2⁺ surface markers expanded rapidly following septic infection, which was supported by both proliferative activation and phenotypic conversion of marrow stem/progenitor cells. An increase in marrow EPCs and their reprogramming for enhancing angiogenic activity correlated with cell-marked upregulation of Sca-1 expression. Sca-1 coupled with ras-related C3 botulinum toxin substrate 2 (Rac2) in signaling the marrow EPC response. Septic infection caused a substantial increase in plasma levels of IFN-γ, VEGF, G-CSF, and SDF-1. The early increase in circulating EPCs was accompanied by their active homing and incorporation into pulmonary microvasculature. These results demonstrate that the marrow EPC response is a critical component of the host defense system. Sca-1 signaling plays a pivotal role in the regulation of EPC response in mice with septic infection.

Animals

C57BL/6 and BALB/c mice (6-8 weeks old, both sexes) were purchased from Charles River Laboratories (Wilmington, MA) and Taconic Biosciences, Inc. (Germantown, NY). Breeding pairs of C57BL/6-Tg (UBC-GFP)30Scha/j mice were purchased from the Jackson Laboratory (Bar Harbor, ME). C57BL/6-Tg (UBC-GFP)30Scha/j mice and Sca-1^-/- mice (Sca-1 knockout or Sca-1 KO considered congenic on the C57BL/6 background, originally transferred from Dr. William L. Stanford’s group at University of Toronto, Toronto, Canada) were bred under specific pathogen-free conditions in the Animal Care Facilities of Northeast Ohio Medical University, Michigan State University, and Louisiana State University Health Sciences Center, respectively. These mice were used for experiments at the age of 6-8 weeks old. All animals were housed in specific pathogen-free facilities with a 12-hour light/dark cycle. Approvals from the Institutional Animal Care and Use Committees in adherence with National Institutes of Health guidelines were obtained before the initiation of experiments.

Septic infection was induced in mice as described previously with minor modifications (24-26). Briefly, an intravenous injection (i.v. via the jugular or penile vein) of live or heat-inactivated Escherichia coli (E. coli strain E11775 from the American Type Culture Collection, Rockville, MD, ~1 x 10⁷ to ~1 x 10⁸ CFUs in 100 μl of pyrogen-free saline/mouse). Control mice were injected with an equal volume of saline. In a subset of experiments, septic infection was induced in mice by i.v. ~1 x 10⁸ CFUs of E. coli in 50 μl of saline/mouse. 5-bromo-2-deoxyuridine (BrdU, 1 mg in 100 μl of phosphate-buffered saline, BD Bioscience, San Jose, CA) was i.v. administered simultaneously to each mouse. Animals were sacrificed at scheduled time points as indicated in each figure legend.

At the time of sacrifice, a heparinized blood sample was obtained by cardiac puncture. White blood cells (WBCs) were quantified under a light microscope with a hemocytometer. Plasma samples were collected after centrifugation of blood samples at 500 g for 10 min. Peripheral blood mononuclear cells (PBMCs) were isolated using lymphocyte density gradient separation media 1.086 (Cedarlane, Burlington, ON, Canada). Both femurs and tibias were collected. Bone marrow cells (BMCs) were flushed out from these bones with a total volume of 2 ml RPMI-1640 medium (Life Technologies, Grand Island, NY) containing 2% bovine serum albumin (BSA, Sigma-Aldrich, St. Louis, MO) through a 23-gauge needle. BMCs were filtered through a 70-micron nylon mesh (Sefar America Inc. Kansas City, MO). Contaminating erythrocytes in BMC samples were lysed with RBC lysis solution (Qiagen Sciences, Germantown, MD). Nucleated BMCs were washed with RPMI-1640 medium containing 2% BSA and then quantified under a light microscope with a hemocytometer.

To generate green fluorescence protein (GFP) expressing bone marrow chimeric mice, recipient wildtype C57BL/6 mice received whole body irradiation at 900 rads/mouse and then were i.v. injected (via the jugular vein) with nucleated bone marrow cells (5 x 10⁶/mouse) from donor C57BL/6-Tg (UBC-GFP)30Scha/j mice under isoflurane anesthesia. Antibiotics (neomycin 1.1 g/liter of water and Polymyxin 1 million unit/liter of water) in drinking water were given to recipient mice 1 week before and 1 week after the bone marrow transplantation. Between 6 to 8 weeks post bone marrow transplantation, a blood sample was obtained from the tail vein of each mouse under isoflurane anesthesia for assessing hematologic reconstitution by counting GFP⁺ cells under an Olympus IX81 time-lapse deconvolution fluorescent/phase contrast microscope (Olympus America Inc. Melville, NY). In this set of experiments, GFP⁺ white blood cells (WBCs) comprised 92±1% of total WBCs in peripheral blood samples at the time of assessment. Septic infection (i.v. 1 x 10⁸ CFUs of E. coli in 100 µl saline/mouse) was induced in GFP bone marrow chimeric mice 12 weeks after bone marrow transplantation to collect lung samples at designated time points for morphological assessment of pulmonary recruitment of marrow cells.

In another set of experiments, C57BL/6-Tg (UBC-GFP)30Scha/j mice received i.v. challenge with either heat-inactivated (60°C for 60 min) E. coli (1 x 10⁸ CFUs in 100 µl saline/mouse) or an equal volume of saline for 24 h. Marrow lin^-ckit⁺ cells are sorted and seeded in ice-cold HC Matrigel (BD Bioscience) containing 50 ng/ml recombinant murine VEGF (BioLegend, San Diego, CA) at 1 x 10⁵ cells/ml. Matrigel (0.5 ml of Matrigel containing 50,000 cells/pouch) was implanted subcutaneously into each recipient wildtype C57BL/6 mouse for 5 weeks. Implanted Matrigel plugs were then collected for analyzing vasculogenic activity in donor GFP⁺ marrow lin^-ckit⁺ cells.

Preparation of bacteria         

For each experiment, a frozen stock culture of E. coli was added to tryptic soy broth and incubated for 18 h at 37°C in an orbital shaker. Bacteria were collected and washed twice with phosphate-buffered Saline (PBS, Thermo Scientific, Frederic, MD). Suspension of bacteria in saline at appropriate concentrations was prepared based on its optical density at 600 nm. Actual numbers of viable bacteria were verified by standard plate counts of the bacterial suspensions on MacConkey agar plates following overnight incubation at 37°C.

In vitro culture of marrow SPCs

Sorted bone marrow lin^-ckit⁺Sca1^-, lin^-ckit⁺, and lin^- cells from naïve mice were cultured in StemSpan Serum-Free Expansion Medium (StemCell Technologies, Vancouver, BC, Canada) containing 10% of mouse plasma, 100 units-100 µg/ml of penicillin-Streptomycin (Thermo Fisher Scientific, Inc., Rockford, IL) with or without lipopolysaccharides (LPS, E. coli 0111:B4, 20-50 ng/ml, Sigma-Aldrich) and recombinant murine VEGF (50-250 ng/ml, BioLegend) in the absence and presence of anti-Sca-1 crosslinking antibodies (25 µg/ml of each clone D7 and E13, BD Biosciences) for 24 h. In one set of cultures, BrdU (10 µM, BD Biosciences) was added to the culture system during the last 4 h of culture.

Determination of late EPC colony-forming activity in marrow SPCs

Sca-1 KO and wildtype C57BL/6 mice were i.v. challenged with heated inactivated E. coli (1 x 10⁸ CFU in 100 µl saline/mouse) or an equal volume of saline for 24 h. Marrow lin^-ckit⁺ cells were sorted from each animal and cultured in 24-well plates. Each well contained 510 µl of Matrigel culture mixture [3 x 10⁴ cells in 510 µl of 2/3 Matrigel-1/3 endothelial progenitor outgrowth cell (EPOC) medium (BioChain Institute Inc., Newark, CA) mixture containing 25 ng recombinant murine VEGF (BioLegend)] overlaid with 200 µl of EPOC medium containing penicillin-Streptomycin (100 units-100 µg/ml). Cell cultures were maintained at 37°C in an atmosphere of 5% CO₂. The overlaid EPOC medium was changed every 2-3 days during the cultural period of 8 weeks. Late EPC colonies formed in each well were counted under the Olympus IX81 time-lapse deconvolution fluorescent/phase contrast microscope (Olympus America Inc.) at the end of the culture.

Flow cytometry and cell sorting

Cell phenotype, intracellular expression of specificity protein 1 (SP1) as well as cell BrdU incorporation were determined with flow cytometry as previously described (24-27). Briefly, nucleated BMCs and PBMCs suspended in RPMI-1640 containing 2% BSA (1 x 10⁶ cells in 100 μl medium) were added with a mixed panel of biotinylated anti-mouse lineage markers [10 µg/mL of each antibody against CD3e (clone 145-2C11), CD45R/B220 (clone RA3-6B2), CD11b (Mac-1, clone M1/70), TER-119 (clone TER-119)] and granulocyte differentiation antigen 1 (Gr-1 or Ly-6G/Ly-6C, clone RB6-8C5), or isotype control antibodies (clones A19-3, R35-95, A95-1) (BD Biosciences). Following incubation for 20 min at 4°C, flourochrome-conjugated streptavidin, anti-mouse c-kit or CD117 (clone 2B8), anti-mouse Sca-1 or Ly-6A/E (clone D7), anti-mouse VEGFR2 (CD309, Clone Avas 12α1) (BD Biosciences) anti-mouse CD133 (clone 13A4, eBiosciences, San Diego, CA) or the matched isotype control antibodies were added into the incubation system at the final concentration of 10 µg/mL for each agent. Samples were further incubated in the dark for 20 min at 4°C. Antibody-stained cells were then washed with cold PBS containing 2% BSA. For measuring cell BrdU incorporation, cells were further processed using a BD BrdU Flow Kit (BD Biosciences) with the procedure provided by the manufacturer. For measuring cell expression of cyclin D1 and SP1, cells were further processed to make both cell membrane and nuclear membrane permeable for antibody using the procedure (without the step of DNA digestion with DNase) provided by BD BrdU Flow Kit (BD Biosciences). Permeabilized cells were incubated with 10 µg/mL of fluorochrome-conjugated anti-human/mouse cyclin D1 antibody (Clone DCS-6, Thermo Fisher Scientific, Inc.) or anti-human/mouse SP1 antibody (clone E-3, Santa Cruz Biotechnology, Inc.) in the dark for 20 min at room temperature. To cell samples incubated with anti-human/mouse SP1 antibody,  flourochrome-conjugated anti-mouse IgG2a, (10 µg/mL, clone R-1915, BD Biosciences) was then added. The cells were further incubated in the dark for 20 min at room temperature. The background staining control samples were incubated with the fluorochrome-conjugated 2nd antibody only. At the end of the staining procedure, cells were washed with the washing buffer provided with the BD BrdU Flow Kit (BD Biosciences) and then suspended in 0.5 ml of PBS containing 1% paraformaldehyde. Analysis of cell phenotypes, expression of SP1, and BrdU incorporation as well as sorting of selected cell subpopulations were performed on a FACSAria Fusion flow cytometer with FACSDiva software (Becton Dickinson, San Jose, CA). Cell populations of interest were gated based on their marker or marker combinations. Depending on the cell types analyzed, the number of cells acquired in each sample was in the range of 200,000 to 300,000.

Co-immunoprecipitation and mass spectrometry

Nucleated BMCs were isolated from Sca-1 KO and wildtype C57BL/6 mice following 24 h systemic E. coli infection. Isolated BMCs were lysed with a lysing buffer (10 mM Tris-HCl buffer containing 1% Triton X-100, 5 mM EDTA, 50 mM NaCl, 30 mM sodium pyrophosphate, 2 mM sodium orthovanadate, 1 mM PMSF, 50 mM sodium fluoride, 5 mg/ml aprotinin, 5 mg/ml pepstatin, and 5 mg/ml leupeptin, pH 7.6) for 30 min. After removing debris in cell lysates by centrifugation at 10,000 g for 5 min at 4°C, soluble protein samples in the supernatants were collected. Co-immunoprecipitation of Sca-1-associated proteins was performed using rat anti-mouse Sca-1 monoclonal antibody (clone E13-161.7, BD Biosciences) and Pierce® Co-Immunoprecipitation (Co-IP) Kit (Thermo Scientific, Frederic, MD). Sca-1 co-immunoprecipitated protein fractions were resolved using the 12% SDS-PAGE ready gel (Bio-Rad Laboratories, Hercules, CA). Recovery of the specific 21 KDa bands identified with Sca-1 pulldown in the SDS-PAGE gel was processed according to the typical in-gel digestion. Tryptic peptides were analyzed by Thermo Scientific Orbitrap Elite mass spectrometer coupled with Easy nano-LC system (Waltham, MA).  The protein matches were identified using Thermo Proteome Discoverer 1.4 with the Mascot search program (Matrix Science, London, UK) against the Mouse of SwissProt database.  A Percolator was used for the validation of peptide matches.

Immunohistochemistry and morphological analysis

Sorted marrow lin^-ckit⁺ cells from Sca-1 KO and wildtype C57BL/6 mice following 24 h systemic E. coli infection were fixed with 8% paraformaldehyde in PBS for 30 min. After washing 3 times with PBS containing 1% BSA, fixed cells were smeared onto poly-L-lysine-coated glass slides (Sigma-Aldrich) followed by a brief air dry. Fixed cells on slides were blocked with 10% goat serum (BioLegend) for 1 hour. After cell surface staining of Sca-1 with fluorochrome-conjugated rat anti-mouse Sca-1 (clone D7, BD Biosciences) for 1 h, the cell smears were washed 3 times with PBS containing 1% BSA. Cells were permeabilized with 0.5% Tween-20 (Sigma-Aldrich) in PBS for 10 min. After washing 3 times with 0.1% Tween-20, cells on slides were sequentially re-blocked with 10% goat serum, stained with fluorochrome-conjugated rabbit polyclonal antibody against mouse ras-related C3 botulinum toxin substrate 2 (Rac2, Bioss Inc., Woburn, MA), and eventually mounted with coverslips using the mounting medium containing 4′,6-diamidino-2-phenylindole (DAPI, Abcam, Cambridge, United Kingdom) for imaging analysis under the Olympus IX81 time-lapse deconvolution fluorescent/phase contrast microscope (Olympus America Inc.).

Subcutaneously implanted Matrigel plug samples collected from mice were fixed with 10% paraformaldehyde in PBS for 24 h. After washing 3 times with PBS, the fixed samples were processed using Leica ASP300S Fully Enclosed Tissue Processor (Leica Biosystems, Deer Park, IL) and paraffin-embedded with Leica Histore Arcadia H and C (Leica Biosystems). Microtome sections (5-10 µM) of paraffin-embedded tissue blocks were prepared using Leica RM2235 (Leica Biosystems). Sections were mounted on Superfrost Plus Microscope Slides (Fisher Healthcare, Pittsburgh, PA) and sequentially subjected to steps of deparaffinization, hydration, antigen retrieval, and blocking with 0.1% BSA. Slides were stained with fluorochrome-conjugated rabbit polyclone antibody against mouse von Willebrand factor (vWF, Bioss Inc.) Stained slides were mounted with coverslips using the Vectashield Antifade Mounting Medium with DAPI (Vector Laboratories, Inc. Newark, CA). Lung Tissue samples collected from mice were fixed with 4% paraformaldehyde at 4°C for 24 h. After washing 3 times in PBS, lung tissue samples were sequentially placed into 15% sucrose solution in PBS for 2 h at 4°C and 30% sucrose solution in PBS for 24 h at 4°C. Tissue samples were then frozen with optimal cutting temperature compound (OCT) (Sakura Tissue-Tek, Torrance, CA). Cryostat sections (10 µM) of OCT-embedded tissue were prepared using Leica CM1950 (Leica Biosystems). Sections were mounted on Superfrost Plus Microscope Slides (Fisher Healthcare), permeabilized with 0.2% Triton (Trition X-100 Sigma-Aldrich), and blocked with Normal Serum Block (Biolegend). Slides were stained with fluorochrome-conjugated Lycopresision Esculentum Lectin (isolectin B4, Thermo Fisher Scientific Inc.). Stained slides were mounted with coverslips using Vectashield Antifade Mounting Medium with DAPI (Vector Laboratories, Inc.) for imaging analysis under an Olympus FV1000 IX81 Spectral Confocal Microscope (Olympus America Inc.).

Real-time RT-PCR

Total RNA samples were prepared from marrow lin^-c-kit⁺Sca-1^- and lin^-c-kit⁺ cells with RNeasy Plus Mini Kit (Qiagen, Valencia, CA). Real-time RT-PCR analysis of mRNA expression by cells was performed as reported previously (24). Each RNA sample was subjected to 2-step real-time RT-PCR using iScriptTM Reverse Transcription Supermix kit and SsoFastTM EvaGreen® Supermix kit (Bio-Rad Laboratories), respectively, on the CFX96TM Real-Time System (Bio-Rad Laboratories). The amplification primer pairs were: 

Sca-1:            Forward           5’-GTTTGCTGATTCTTCTTGTGGCCC

                      Reverse           5’-ACTGCTGCCTCCTGAGTAACAC

VEGFR2:       Forward           5’-ATCCAGATGAGGGCAAGTTTAG

                       Reverse           5’-GCATACCCACTGACTGTGATAG

18SrRNA:       Forward           5’-ATTCGAACGTCTGCCCTATAA

                       Reverse           5’-GTCACCCGTGGTCACCATG

These sets of primers were designed using Primer Express software (Life Technologies Co, Carlsbad, CA). The expression of Sca-1 and VEGFR2 mRNA was determined by normalizing the cycle threshold (CT) number of their mRNA with that of 18S rRNA in each sample. Changes in specific gene mRNA expression by cells from groups with different treatments are expressed as fold alterations over the baseline expression by cells from the corresponding control group.

ELISA determination

Lung tissue homogenate (10%) was prepared by homogenizing lung tissue in PBS containing cOmplete Tablets Mini protease inhibitor cocktail (1 tablet/10 ml of PBS, Sigma-Aldrich) with a TH-115 homogenizer (OMNI International, Kennesaw GA). After centrifugation at 10000 g for 10 min, the supernatant of each tissue homogenate sample was collated. VEGFR2, SDF-1, and IFN-γ concentrations in plasma and lung tissue homogenate samples were determined with Quantikine Mouse VEGF Immunoassay kit, Mouse CXCL12/SDF-1 Immunoassay Kit, and Mouse IFN-γ Immunoassay Kit (R&D Systems, Minneapolis, MN), respectively. G-CSF levels in plasma and lung tissue homogenate samples were determined with the Mouse CSF3 (G-CSF) ELISA Kit (Thermo Scientific). Protein content in lung tissue homogenate samples was determined using the BCA Protein Assay Kit (Thermo Fisher Scientific, Inc.) with the protocol provided by the manufacturer.

Statistical analysis

Data were presented as mean±SEM. The sample size is indicated in each figure legend. Statistical analysis was conducted using Student’s t-test, one-way ANOVA followed by Student-Newman-Keuls test, liner correlation, and Chi-square test. Difference with statistical significance is accepted at p < 0.05.