This article describes a flow cytometry-based method for analyzing bone marrow microenvironmental populations, such as the endothelial and mesenchymal stromal cells, from MDS and leukemia murine models (Figure 1). Figure 2 depicts the gating strategy for detection of populations of interest, beginning with the selection of cells (P1) in the digested and CD45/Ter119 depleted fraction through forward and side scatter profile. Example gating of cells in a leukemia sample are shown in P1 (Figure 2A). Singlets are selected and doublets are excluded from this analysis, P2 (Figure 2B). Figure 2C shows gates to select propidium iodine negative live cells, P3. To focus on non-hematopoietic stromal populations, cells that are CD45-/Ter119-, P4 (APC, Y-axis) vs. SSC-A are selected (Figure 2D). This initial gating strategy is common for all samples to be analyzed.
The leukemia murine model is used to illustrate gating for endothelial cells in Figures 2E-G. Figure 2E depicts CD31 positive (Pe-Cy7, Y-axis) vs. SSC-A cells in a non-cancer control mouse as well as a leukemic mouse, gated through P4. Cells positively labeled with CD31 are endothelial cells, P5. In Figure 2F, gated through P5, arteriolar endothelial cells are identified as CD45-Ter119-CD31+Sca1+, P6, and sinusoidal endothelial cells are identified as CD45-Ter119-CD31+Sca1-, P7 (BV421, Y-axis) vs. SSC-A.
Although this analysis focusses on non-leukemic bone marrow microenvironmental cells, it is also helpful to determine tumor burden. This can be done with a small fraction of undigested/non-depleted sample prior to initiating the experiment. In this experimental control (dashed lines) Figure 2G, P8 represents tumor burden in the sample and P9 represents the non-cancerous cells.
The MDS murine model is used to illustrate analysis of mesenchymal stromal cells in Figure 2H-J. Figure 2H depicts CD31 negative (Pe-Cy7, Y-axis) vs. SSC-A cells, gated through P4. Mesenchymal stromal cell populations in a wild-type control mouse and MDS mouse are shown in Figure 2I. Gated through P10, mesenchymal stromal cells are identified as CD45-Ter119-CD31-CD51+CD140a+, P113 (Pe, Y-axis; Pe-Cy5, X-axis). The experimental control (dashed lines), Figure 2J shows examples of single-color gating controls, CD51 single stain and CD140a single stain used to gate for the MSCs shown in the experimental samples in Figure 2I.
This data indicates that arteriolar endothelial cells significantly expand in the AML microenvironment, with a concomitant loss in the sinusoidal endothelial populations (Figure 2F), consistent with earlier studies using patient derived xenografts in immunodeficient mice28. It is likely that the small expansion in the mesenchymal stromal cells seen in the NHD13 mice at 8 weeks of age (Figure 2I) may increase at 16-20 weeks, when these mice start to display characteristics of MDS25. Although only the leukemia model is used to illustrate the endothelial cell population and the MDS murine model is used to illustrate the mesenchymal stromal cell populations, similar staining and gating strategies can be used to analyze the different microenvironmental populations in either of these models, or indeed, in any genetically engineered murine model of interest.

Figure 1: Isolation of bone marrow stromal cells. Schematic shows the process of isolating non-hematopoietic bone marrow stromal cells from control and leukemic mice. Briefly, bone spicules and bone marrow are digested separately and then pooled. The CD45-Ter119- population is enriched by magnetic sorting, and stained with antibody panels against populations of interest, such as mesenchymal stromal cells and endothelial cells. The cells are then analyzed by flow cytometry. Please click here to view a larger version of this figure.

Figure 2: Gating strategy for non-hematopoietic bone marrow and stromal cells. (A-D) Flow cytometry gating strategy used to select digested, CD45-/Ter119-, non-hematopoietic populations (P4) in both the leukemia and MDS murine model. (E) Leukemia bone marrow endothelial cells (P5, CD31+) can be sub-divided as (F) Sca1+ arteriolar (P6) or Sca1- sinusoidal endothelial cells (P7). (G) Leukemia engraftment in an AML sample where P8 represents engraftment while P9 represents non-cancer cells. Dotted lines indicate that this plot is an experimental control. (H) Gated through P4, MDS bone marrow CD31- cells, P10 (I) Mesenchymal stromal cells (CD51+/CD140a+, P11). (J) Example single color gating controls, CD51 (left) and CD140a (right) used to determine the gates for mesenchymal stromal cells in (I). Please click here to view a larger version of this figure.
| Solution | Reagent | Concentration | Amount to add |
| FACS buffer | HBSS | 10x | 100 mL |
| EDTA | 0.5 M | 4 mL |
| Fetal Bovine Serum | - | 50 mL |
| Water | - | 848 mL |
| Bone marrow digestion mixture | HBSS | 1x | 2 mL |
| DNase 1 | 1 µg/mL in 1x DPBS | 20 µL |
| Dispase II Powder | - | 4 mg |
| Collagenase Type IV | - | 2 mg |
| Bone spicule digestion mixture | DPBS | 1x | 320 mL |
| Collagenase Type 1 | - | 1 g |
| Fetal Bovine Serum | - | 80 mL |
| MACs buffer | BSA | 66 g/mol | 5 g |
| DPBS | 10x | 100 mL |
| EDTA | 0.5 M | 4 mL |
| Water | - | 896 L |
Table 1: Composition of solutions and buffers used in the present study.