We demonstrate how to identify and isolate 6 subsets of myeloid progenitors from murine bone marrow using a combination of magnetic and fluorescence sorting (MACS and FACS). This protocol can be used for in vitro culture assays (methylcellulose or liquid cultures), in vivo adoptive transfer experiments, and RNA/protein analyses.
This method could be helpful for hematologists and immunologists studying the production and function of myeloid cells, including neutrophils, monocytes, and dendritic cells. The main advantage of this technique is that it enables more precise identification of myeloid progenitor subsets than previous strategies. To enrich bone marrow cells for progenitors by magnetic-activated cell sorting, or MACS, pellet the cells by centrifugation and resuspend the pellet in 40 microliters of MACS staining buffer per one times 10 to the seven bone marrow cells.
To deplete differentiated lineage positive cells, add 10 microliters of biotin antibody cocktail per one times 10 to the seven cells and to mix by pipetting for a 10 minute incubation at four degrees Celsius. At the end of the incubation, add 30 microliters of staining buffer per one times 10 to the seven cells with mixing, followed by 20 microliters of anti-biotin microbeads per one times 10 to the seven cells. After 15 minutes at four degrees Celsius, wash the cells with one milliliter of staining buffer per one times 10 to the seven cells and resuspend the pellet in 500 microliters of fresh staining buffer for up to one times 10 to the eight cells.
Next, set up the automated magnetic separator according to the manufacturer's instructions and place the tube of labeled cells into the separator. Begin the negative selection program to collect the negative fraction which contains the progenitor enriched lineage negative cells. Pellet the lineage negative cells by centrifugation and resuspend the pellet, which is now white, in two milliliters of staining buffer per mouse for counting.
To isolate the myeloid progenitor cells by FACS, add one times 10 to the fifth lineage negative cells to each of eight control microcentrifuge tubes for voltage selection and color compensation and add the rest of the cell sample to a ninth microcentrifuge tube for staining with all seven of the surface marker antibodies of interest for progenitor identification and sorting. Pellet the cells by centrifugation and resuspend the control pellets in 100 microliters of FACS staining buffer and the experimental sample pellet in 100 microliters of staining buffer per five times 10 to the six cells. Next, add anti-CD16/CD32 antibody to the Fc gamma receptor single stain tube and the sample tube.
Vortex gently and incubate the samples for 10 minutes at four degrees Celsius. At the end of the incubation, add the other antibodies to the appropriate corresponding single stain tubes and to the sample tube and after gentle vortexing, incubate the samples for 15 minutes at four degrees Celsius. At the end of the incubation, add 900 microliters of staining buffer to the control tubes and one milliliter of staining buffer per one times 10 to the seven cells to the sample tube for centrifugation.
Resuspend the control pellets in 200 microliters of fresh staining buffer per tube and the experimental pellet in 500 microliters per 2.5 times 10 to the seven cells and transfer the labeled cells to individual corresponding five milliliter FACS tubes. Then set up the flow cytometer according to standard protocols and use the unstained and single stained controls to set the voltages and the color compensations. To identify and isolate the myeloid progenitor cells, load the experimental cell sample onto the cytometer and create a forward scatter area versus side scatter area plot, gating to exclude the debris and dead cells.
Create a forward scatter height versus forward scatter width plot of the live cells and gate to exclude the doublets. Repeat this process with a side scatter height versus side scatter width plot of the forward scatter singlets and gate to exclude the doublets. Create a Sca-1 versus c-Kit plot and gate to select the c-Kit positive Sca-1 negative progenitors.
Then create a CD-34 versus FC gamma receptor plot and gate to select the mixed common myeloid progenitor and mixed granulocyte monocyte progenitor populations. It's important to gate the mixed common myeloid progenitor and mixed granulocyte monocyte progenitor populations as precisely as possible. It can be helpful to use a pseudo-color density plot or a contour plot for more accurate gating.
Use the cells in the mixed common myeloid progenitor gate to create a CD115 versus Flt3 plot and gate to select the common myeloid progenitor Flt3 positive CD115 low cells, the common myeloid progenitor Flt3 negative CD115 low cells, and the Flt3 positive CD115 high monocyte dendritic cell progenitors. Use the cells in the mixed granulocyte monocyte progenitor gate to create a Ly6C versus FC Gamma receptor plot and gate to select the Ly6C negative cells and the Ly6C positive cells. Then create a CD115 versus Flt3 plot for each gated mixed granulocyte monocyte progenitor cell subpopulation and gate the Ly6C negative Flt3 negative CD115 low granulocyte monocyte progenitors, the Ly6C positive Flt3 negative CD115 low granulocyte progenitors, and the Ly6C positive Flt3 negative CD115 high monocyte progenitors.
Max lineage depletion is efficient at depleting differentiated cells and for enriching for the c-Kit positive progenitor cells. The lineage negative fraction does still contain some c-Kit negative cells, but these cells will be eliminated in subsequent flow cytometry gating steps. Progenitor yields after FACS sorting can vary depending on the sorter settings used, but it should be possible to obtain between one to four times 10 to the fourth cells per fraction from each mouse with a post-sort purity of greater than 95%for each fraction.
Good staining is critical for clean separation of the populations. You may need to titrate the antibodies or choose different fluorophores to ensure optimal separation. This protocol can be used to identify progenitor cells to assess differences in bone marrow composition between samples, to isolate progenitors for molecular analysis or to assess their ability to produce myeloid cells.
