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Bone marrow-derived macrophages (BMDMs) are cells derived from stem cells in vitro that can differentiate into mature macrophages in the presence of growth factors and other signaling molecules. Differentiation requires M-CSF (macrophage colony-stimulating factor), also known as CSF-1, a growth factor that is responsible for the proliferation and commitment of myeloid progenitors into monocytes (which then mature into macrophages). Macrophages have a wide variety of functions in the body including phagocytosis of foreign invaders and other cellular debris, cytokine release to trigger an immune response, and antigen presentation. This makes studying macrophages critical but requires a large, homogenous population. BMDMs are playing an increasingly important role in making macrophage research possible and financially feasible.

Production
In order to produce BMDMs, mesenchymal stem cells are removed from tibia or femur of mice. Since BMDMs are derived from bone marrow, derived cells are healthy and naïve (or unactivated), regardless of the condition of donor mice. The femoral or tibia bone marrow cells are incubated with CSF-1. Without CSF-1, the cells enter a quiescent state but can reinitiate DNA synthesis if stimulated later. Mature macrophages and fibroblasts are removed since they might carry unwanted growth factors. IL-3 and hemopoietin 1, two potentiating growth factors, are often added to increase yield and promote rapid terminal differentiation. Exogenous media containing growth factors and other serums must also be added to make the cells continually viable. Full growth and differentiation take approximately 5-8 days. Millions of BMDMs can be derived from one mouse; cells are then frozen for years and can be thawed and respond to a variety of stimuli such as LPS, IFNy, PAMPs, signal MAPs, NF-κB, and IRF3. These signals induce translation of genes that produce cytokines that determine if macrophages are M1 (pro-inflammatory) or M2 (anti-inflammatory). If BMDMs are not frozen, they become less viable with age since CSF-1 concentration decreases.

Proliferation of BMDMs can also be inhibited by a number of reagents. For example, growth and differentiation is dependent on CSF-1 and a functional CSF-1 receptor, a member of the tyrosine kinase family, on stem cells. Interferons can cause a down regulation of CSF-1 receptor. Additionally, without stimuli like LPS to induce macrophage maturation to M1 or M2, mice accumulate immature macrophages which are less helpful to the body.

Schema of in vitro BMDM production