Cell density during differentiation can alter the phenotype of bone marrow-derived macrophages

BACKGROUND: Bone marrow-derived macrophages (BMDMs) are widely used primary cells for studying macrophage function. However, despite numerous protocols that are currently available, lack of a notable consensus on generating BMDMs may obscure the reliability in comparing findings from different studies or laboratories. FINDINGS: In this study, we addressed the effect of cell density on the resulting macrophage population. With reference to previously published methods, bone marrow cells from wild type C57BL/6 mice were plated at either 4 × 10(5) cells or 5 × 10(6) cells per 10 cm and cultured in 20% L-cell conditioned media for 7 days, after which they were analyzed for cell surface markers, production of proinflammatory cytokines, and responsiveness to polarizing signals. Reproducibly, cells plated at lower density gave a pure population of CD11b(+)F4/80(+) macrophages (97.28 ± 0.52%) with majority being Ly-6C(-)Ly-6G(-) and c-Fms(+), while those plated at higher density produced less CD11b(+)F4/80(+) cells and a considerably higher proportion of CD11b(+)F4/80(+)CD11c(+) (68.72 ± 2.52%) and Ly-6C(-)Ly-6G(+) (71.10 ± 0.90%) cells. BMDMs derived from higher plating density also secreted less proinflammatory cytokines such as IL-6, IL-12 and TNF-α and were less phagocytic, and had a different pattern of expression for M1- and M2-related genes upon LPS or IL-4 stimulation. CONCLUSIONS: Overall, our findings indicate that altering cell density during BMDM differentiation can give rise to distinct macrophage populations that could vary the outcome of a functional study.