Materials and Reagents

IMDM (powder or liquid)
Horse serum (HS) (may require batch testing if low cell viability)
Foetal bovine serum (FBS)
L-Glutamine
Penicillin/Streptomycin (P/S)
SCF-conditioned medium (SCF-CM) (see Notes)
Recombinant human erythropoietin (e.g. epoietin alpha, Amgen) (obtained through hospital pharmacy under appropriate local guidelines)
Recombinant mouse interleukin-3 (IL-3) (e.g. Pepro Tech, catalog number: 213-13 )
Recombinant mouse granulocyte-monocyte colony-stimulating factor (GM-CSF) (e.g. Pepro Tech, catalog number: 315-03 )
All-Trans Retinoic Acid (ATRA) (e.g. Sigma-Aldrich, catalog number: 302-79-4 , reconstitute in ethanol)
Culture supplements and antibiotics (L-Glutamine and Penicillin/Streptomycin)
Monoclonal anti-mouse antibodies for flow cytometry
1. C-kit/CD117 (clone 2B8) (e.g. PE-Cy7, eBioscience, catalog number: 25-1171 ) (suggested use at 1:100 dilution)
2. CD34 (clone RAM34) (e.g. Alexa-Fluor 647, eBioscience, catalog number: 51-0341 ) (suggested use at 1:100 dilution)
3. Mac-1/CD11b (clone M1/70) (e.g. PE, eBioscience, catalog number: 12-0112 ) (suggested use at 1:100 dilution)
4. Gr1/Ly6C (clone RB6-8C5) (e.g. FITC, eBioscience, catalog number: 11-5931 ) (suggested use at 1:100 dilution)
5. Sca-1/Ly6A/E (clone D7) (e.g. Pacific blue, BioLegend, catalog number: 122520 ) (suggested use at 1:50 dilution)
  Note: Stain on ice, in culture medium, for 20 min; wash with 10-20x volume of medium; pellet cells at 400 x g for 5 min. Re-suspend in 300-500 μl of medium for FACS analysis.
Trypan blue

Equipment

T175 tissue culture flasks with filtered cap (for production of conditioned medium)
T75 or T25 tissue culture flasks with filtered cap, or 6-well plates
Note: For EML cultures, the size of the tissue culture vial is determined by the cell number seeded at the cell densities indicated in the protocol.
Cell culture incubator

Procedure

Maintenance culture conditions
1. EML cells maintenance culture conditions are IMDM with 5% HS, 2 mM L-glutamine and 1x penicillin/streptomycin (P/S) and 10-15% of SCF-conditioned medium (SCF-CM) obtained from BHK-MKL cells.
  Note: The original cultivars of EML cells were grown in medium supplemented with rat SCF at a final concentration of 200 ng/ml (Tsai et al., 1994).
2. Cells should be seeded at 2 x 10⁴ cells/ml and split to the initial dilution when a cell density of 10⁵-2 x 10⁵/ml (absolute maximum) is reached. This typically means splitting the cultures every 2 or sometimes 3 days. Cultures die rapidly at higher cell densities.
  Note: The dead cell fraction in maintenance cultures, as judged by Trypan blue exclusion, should not exceed 5-8% (10% is acceptable if not indicative of a culture exhaustion trend.).
3. Cultures can be maintained for at least 20 passages post-thaw without any significant changes in biology.
Erythroid differentiation conditions
1. Use EML cells in logarithmic growth phase from a maintenance culture, and plate them at a seeding concentration of 2 x 10⁴ cells/ml in fresh maintenance culture medium + recombinant human erythropoietin (EPO) at a final concentration of 10 U/ml.
2. EML erythroid differentiation cultures are very sensitive to cell crowding, so opt for increasing culture volumes rather than initial cell densities if high numbers of cells are required.
3. Culture for 2 days. After this initial period, wash cells 2-3x in IMDM with 5% HS + 2 mM L-glutamine and 1x P/S, and re-seed at 2 x 10⁴ cells/ml in IMDM, 5% HS, 2 mM L-glutamine and 1x P/S, with 2.5% SCF-CM and 10 U/ml EPO.
4. Differentiate cells for a total of 7 days (so, 5 additional days), with fresh EPO (10 U/ml) added on day 5 to the existing culture.
5. It is expected to observe an average 20-fold expansion in cell numbers during the culture period, with cell viability decreasing gradually as cells differentiate, to a minimum of 40-50% at day 7.
6. The extent of cell differentiation can be determined by flow cytometry staining with C-Kit, Sca1 and CD34, with accumulation of differentiated Sca1^loCD34-C-kit^lo/- cells (referred to as Kit^-, see Figure 1 for representative plot) (Pina et al., 2012). It is expected that 20-30% of the culture become Kit^- by day 6 of differentiation.
  
  Figure 1. Erythroid differentiation of EML cells. A representative plot of flow cytometry analysis at day 4 is shown; c-kit^-CD34^- cells are Sca1^lo/-. In this instance, cultures were seeded at day 0 with a population of cells with culture-reconstituting (or self-renewal) potential (Pina et al., 2012).
7. When erythroid differentiation cultures are seeded with committed erythroid cells (e.g. sorted Sca1^loCD34^- cells cultured as per B1 above) (Pina et al., 2012), 60-80% of the culture are Kit^- by day 4 of differentiation, reflecting faster differentiation kinetics from committed cells.
8. Not unexpectedly, cultures seeded with erythroid-committed cells are less proliferative, with a 10-fold expansion in cell numbers expected by day 4, after which the cultures plateau and become less viable.
Neutrophil differentiation conditions
1. Use EML cells in logarithmic growth phase from a maintenance culture, and plate them at a seeding concentration of 2 x 10⁴ cells/ml in fresh maintenance culture medium + 10 ng/ml of recombinant mouse IL-3 and 10 μM ATRA.
2. Viability is less affected by cell crowding than in erythroid differentiation, and it is thus possible to seed cultures at higher densities (0.5-1 x 10⁵) if high cell numbers are required.
3. After 2 days, wash cells 2-3x in IMDM with 5% HS + 2 mM L-glutamine and 1x P/S, and re-seed at 2 x 10⁴ cells/ml in IMDM, 5% HS, 2 mM L-glutamine and 1x P/S, with 2.5% SCF-CM + 10 ng/ml of recombinant mouse GM-CSF and 10 μM ATRA.
4. Keep the culture growing for a total of 7 days (so, 5 additional days), with fresh GM-CSF (10 ng/ml) added on day 5 to the existing culture.
5. It is expected to observe an average 10-fold expansion in cell numbers during the culture period, with viabilities close to 80% or higher throughout the duration of the culture.
6. The extent of cell differentiation can be determined by flow cytometry after staining with Gr-1 and Mac-1 antibodies (Pina et al., 2012), with accumulation of granulocytic Gr1⁺Mac-1⁺ cells to an expected proportion of 40-50% by days 4-5 of culture (see Figure 2 for representative plot). Proportions can drop thereafter, although absolute numbers of differentiated cells continue to increase.
  
  Figure 2. Neutrophil differentiation of EML cells. A representative plot of flow cytometry analysis at day 6 is shown.

Notes

SCF-conditioned medium (SCF-CM)
1. One possible source are BHK cells expressing the murine kit ligand protein (MKL, or SCF).
2. Conditioned medium can be produced in T175 flasks with a total production volume of 60 ml/flask.
3. Culture the cells in IMDM with 10% FBS, 2 mM L-Glutamine and 1x Penicillin/Streptomycin until they reach 80% confluence.
4. At this stage replace with fresh medium and culture for an additional 48 h, at which point the supernatant is collected, floating cells removed by pelletting at 400 x g, 5 min, and the medium filtered through a 0.2 μm mesh.
5. Conditioned medium can be aliquoted and stored for several months at -20 °C without loss of activity.

Acknowledgments

The protocol herein described was originally reported in Pina et al. (2012). Its development and implementation was financially supported by the Medical Research Council of the United Kingdom, Leukaemia and Lymphoma Research, EuroSyStem and STEMEXPAND.

References

Orford, K., Kharchenko, P., Lai, W., Dao, M. C., Worhunsky, D. J., Ferro, A., Janzen, V., Park, P. J. and Scadden, D. T. (2008). Differential H3K4 methylation identifies developmentally poised hematopoietic genes. Dev Cell 14(5): 798-809.
Pina, C., Fugazza, C., Tipping, A. J., Brown, J., Soneji, S., Teles, J., Peterson, C. and Enver, T. (2012). Inferring rules of lineage commitment in haematopoiesis. Nat Cell Biol 14(3): 287-294.
Tsai, S., Bartelmez, S., Sitnicka, E. and Collins, S. (1994). Lymphohematopoietic progenitors immortalized by a retroviral vector harboring a dominant-negative retinoic acid receptor can recapitulate lymphoid, myeloid, and erythroid development. Genes Dev 8(23): 2831-2841.
Ye, Z. J., Kluger, Y., Lian, Z. and Weissman, S. M. (2005). Two types of precursor cells in a multipotential hematopoietic cell line. Proc Natl Acad Sci U S A 102(51): 18461-18466.