Human pulmonary alveolar epithelial cells type I were obtained from AcceGen Biotechnology (Cat.# ABC-TC3770) and cultured following the manufacturer’s guidelines.
Twenty-four lots of culture expanded human hUC-MSCs were utilized in this study, isolated from human umbilical cord tissue from normal, healthy births, voluntarily donated with a fully executed informed consent form. Sixteen of those lots were obtained from a biotechnology company that manufactures hUC-MSCs for use in clinical trials (Medistem Panama, City of Knowledge of the Republic of Panama). The cell lots were passage 5 and were received frozen in dry shippers and stored at -150°C until studied. Manufacturing methodology is described in detail in publications of clinical trials that used these hUC-MSCs for treatment.33,34
For the other eight lots, umbilical cords were obtained from a tissue acquisition company licensed by the American Association of Tissue Banks (AATB) in the United States. The isolation, selection and culture were performed by Aidan Research and Consulting LLC for research purposes only. The isolation process was done using the Umbilical Cord Dissociation Kit, human (Miltenyi Biotec 130-105-737) following the manufacturer’s guidelines. When colony-forming units reached 70% of confluency, hUC-MSCs were selected using the MSC Phenotyping Kit, human (Miltenyi Biotec 130-125-285) and >95% positive cells for CD73/CD105 were sorted using SH800 cell sorter (Sony Biotech). Cells were expanded through passage 5 and used for the measurements reported here.
All cell lots used in this study met release criteria, namely: 75% viability and >95% positive for CD90, CD73, CD105 cell surface markers as determined by flow cytometry. These 24 lots were all used as samples for the subsequent experiments, and one of the Aidan Research and Consulting lots was transfected with an ACE-2 plasmid and used as a positive control for ACE-2 measurements.
For transfection, 6,000 hUC-MSCs per cm2 were plated in 100cm2 cell culture dishes. Once they reached 70% of confluency, 3µg of ACE2 plasmid was transfected using Lipofectamine™ Stem Transfection Reagent (Sino Biological HG10108-ACR) following the manufacturer’s instructions. Twenty-four hours after transfection cells were either fixated using Image-iT™ Fixation/Permeabilization Kit (Thermo Fisher Scientific #R37602) for imaging or lysed using RIPA buffer (Thermo Fisher Scientific #89901) with 1X protease and phosphatase inhibitor (Thermo Fisher Scientific 78444) for Western Blot analysis.
- Protein preparation and Western blot
Human lung homogenates were purchased from the OriGene tissue bank (CP565585, CP565542, CP565577, CP565443 and CP565542). Whole protein was obtained by sonication of a pellet of harvested hUC-MSCs and quantified using the Pierce™ Rapid Gold BCA Protein Assay Kit (Thermo Fisher Scientific #A53226). 25 µg of protein was added to 4x LDS loading buffer and incubated at 50°C for 5 minutes. SDS-PAGE was performed with Criterion TGX Stain-free 4-20% Gel (Bio-Rad #5678093) and transferred to a PVDF membrane using the iBlot™ 2 Gel Transfer Device (Thermo Fisher Scientific #IB21001). Membranes were blocked for 1 hour in Pierce Clear Milk Blocking Buffer (Thermo Fisher Scientific # 37587) at room temperature and incubated overnight at 4°C with 1:500 Rabbit anti-ACE2 (Invitrogen, MA5-32307), 1:500 primary antibody TMPRSS2 made in rabbit (Abcam # ab92323), and 1 hour at room temperature with 1:10,000 Mouse anti-GAPDH (Millipore, MAB374). Membranes were incubated in secondary antibodies, 1:5,000 Alexa Fluor 680 goat anti-rabbit (Life Technologies, A21076) and 1:5,000 Alexa Fluor 488 donkey anti-mouse (Invitrogen: A21202), for one hour at room temperature. Detection of relevant proteins and images were taken using iBright FL1500 Imaging System (Invitrogen). For relative quantification, the volume intensity of the bands was obtained using iBright software. The relative expression was calculated by dividing the values to GAPDH.
- Quantitative real-time Polymerase chain reaction (qPCR)
Total RNA was isolated from cells using the Trizol™ Plus RNA Purification Kit (Invitrogen [Thermo Fisher Scientific]) and DNA was removed using the TURBO DNA-free Kit (Invitrogen) from all hUC-MSC lots. Additionally, RNA isolated from human lung tissues (OriGene Technologies; cat.#: CR559346, CR559185, CR560789, CR562469 and CR561266) was included as a positive control (n=5). All RNA extractions were then quantified using a Varioskan LUX™ (Thermo Fisher), and their integrity was checked using a 1% E-Gel™ EX Agarose Gel (Thermo Fisher Scientific). Subsequently, 200 ng of purified RNA was reverse-transcribed to cDNA using the iScript™ cDNA Synthesis Kit (Biorad) following the manufacturer’s protocol. Then, 20 ng of cDNA was amplified by qPCR using the TaqMan™ Fast Advanced Master Mix along with TaqMan™ Gene Expression Assays for ACE2 (Hs01085333_m1), TMPRSS2 (Hs01120965_m1), and for the reference gene PPIA (Hs99999904_m1). All qPCR reactions were performed in triplicates on a QuantStudio™ 3 Real-Time PCR System (Applied Biosystems). Raw cycle thresholds values were calculated using the QuantStudio™ Design and Analysis Software v.1.5.1 using automatic baseline settings and a threshold of 0.3. The relative expression of genes of interest was normalized to the expression of PPIA. A Mann-Whitney Rank Sum Test was used to calculate statistically significant differences between expression of ACE2 and TMPRSS2 in human lung RNA, human pulmonary alveolar type I cells, and UC-MSCs.
Cells were plated on 12-well glass-bottomed MatTek plates (P12G-1.5-14-F) and fixed using an Image-iT™ Fixation/Permeabilization Kit (Thermo Fisher Scientific #R37602) following the manufacturer’s instructions. Primary antibody ACE2 anti-rabbit (Invitrogen: MA5-32307), in a dilution 1:50, was added to the indicated wells and left overnight at 4°C. The primary antibody TMPRSS2 made in rabbit (Abcam # ab92323), in a dilution 1:50 was used and incubated at room temperature for 2 hours. Secondary antibodies Alexa Fluor 488 donkey anti-mouse (Invitrogen A21202) and Cyanine3 goat anti-rabbit (Invitrogen: A10520), in a dilution 1:50, were added to the wells and incubated for one hour at room temperature. Coverslips were mounted onto the slides using Prolong DAPI (Invitrogen P36935) and photographs were taken using a Lionheart FX automated microscope (BioTek).
Single cell suspensions of 24 hUC-MSC lots and one alveolar cell line were stained with primary antibody TMPRSS2 made in rabbit (Abcam, ab92323) and with a secondary PE-Cy5.5 goat anti-rabbit antibody (Invitrogen, L42018). Afterwards, the cells were stained with a conjugated FITC anti-human ACE2 antibody made in rabbit (LSBio, LS-C458088). Cells from 5 Medistem lots were stained with the TMPRSS2 secondary antibody to show Non-Specific-Binding (NSB). The cells were resuspended in 300 µL of sorting buffer (0.05% FBS in PBS) and analyzed using the flow cytometry functions on a Sony SH800S Cell Sorter.
SigmaPlot 12.5 (Systat Software) was used for all statistical analyses. Means and standard errors of the means were calculated for all relevant quantities. One-way ANOVA (Mann-Whitney Rank Sum Test) was used for all comparisons. A p-value ≤ 0.05 was taken as the level for statistical significance.