A Comprehensive Guide to Macrophage Differentiation, Polarization, and Identification

Overview of Macrophages: Origin, Roles, and Plasticity

Macrophages are a type of white blood cell whose life cycle begins in the bone marrow. After differentiating from monocytes, they are released into the bloodstream and subsequently migrate into tissues, where they further differentiate into tissue-resident macrophage populations, such as microglia in the brain and Kupffer cells in the liver.

The primary function of macrophages is the phagocytosis of bacteria, viruses, and other pathogens. They also eliminate tumor cells and apoptotic or necrotic cells, thereby acting as professional “scavengers” within the body. In addition, macrophages participate in immune responses by presenting antigens, secreting chemokines and cytokines, and activating other immune cells. Beyond immunity, macrophages are involved in tissue repair and play a role in the survival and rejection of transplanted organs^[1].

However, macrophages can also act as accomplices in disease. In certain contexts, they promote tumor growth and metastasis, support the persistence of pathogens, or drive tissue damage and chronic inflammatory responses.

(1) Macrophages recognize and migrate toward foreign bacteria and engulf them. (2) Phagosomes form around internalized bacteria and fuse with lysosomes, leading to the release of digestive enzymes. (3) Bacteria are degraded by phagolysosomal enzymes, and undigested remnants are expelled from the cell by exocytosis.

Depending on the local microenvironment, macrophages can be polarized into classically activated type 1 (M1) or alternatively activated type 2 (M2) phenotypes. M1 macrophages are induced by pro-inflammatory stimuli such as IFN-γ, TNF-α, lipopolysaccharide (LPS), and GM-CSF. They secrete pro-inflammatory cytokines, nitric oxide, and reactive oxygen species (ROS), thereby promoting inflammatory responses and pathogen clearance.

In contrast, M2 macrophages are activated by anti-inflammatory signals and contribute to tissue repair and fibrosis. These cells are typically induced by anti-inflammatory factors that contribute to tissue repair, immunomodulation, and fibrosis^[1][2].

Macrophages are broadly classified into M1 and M2 subtypes, with M2 macrophages further divided into M2a, M2b, M2c, and M2d phenotypes. M1 macrophages primarily mediate pro-inflammatory responses and pathogen clearance, whereas M2 macrophages promote anti-inflammatory processes and tumor progression. Specifically, M2a macrophages are associated with tissue repair, M2b with immune regulation, M2c with phagocytosis and resolution of inflammation, and M2d with tumor angiogenesis.

Experimental Induction of Macrophage Differentiation and Polarization

In laboratory settings, macrophage differentiation is commonly initiated from peripheral blood mononuclear cells (PBMCs) or from human acute monocytic leukemia cells (THP-1). Differentiation into mature macrophages is achieved by treatment with specific differentiation factors.

PBMCs can be isolated from whole blood samples using density gradient centrifugation.

● Fresh anticoagulated blood samples were mixed with an equal volume of density gradient medium (e.g., Ficoll-Hypaque-1077 or Lymphoprep).
● Samples were centrifuged at low speed (400 × g, 30 min), resulting in stratification of cell populations by density. PBMCs were in an opaque intermediate layer between plasma and erythrocytes.
● The mid-layer cell suspension was carefully aspirated and washed repeatedly with PBS (4°C, 250 × g for 10 min) until a clear supernatant was obtained^[3].

Following PBMC isolation, monocytes can be enriched and differentiated into macrophages through supplementation with growth factors. Moreover, in practice, direct culture of PBMCs is often employed, as monocytes selectively adhere to culture surfaces while non-adherent lymphocytes are gradually removed. The procedure below outlines a representative protocol for inducing macrophage differentiation from PBMCs.

1. Isolated PBMCs are resuspended in 10 mL RPMI 1640 medium supplemented with penicillin, streptomycin, 2 mM glutamine, and 15% fetal calf serum (FCS). Cell density is adjusted to 1 × 10⁷ cells/mL.
2. Add 10 mL of cell suspension to a Petri dish without removing non-adherent cells.
3. Cells are incubated at 37°C in a humidified atmosphere containing 5% CO₂. Fresh RPMI/15% FCS medium is added on days 2 and 4.
4. By day 6, cells have differentiated into macrophages and exhibit firm adherence.
5. The culture dish is placed on ice, and 10 mL of cold PBS is added. After incubation for 30 min, adherent cells are gently detached using a cell scraper.
6. Cells are centrifuged at 250 × g for 5 min and resuspended in RPMI 1640 containing 5% FCS, then replated.
7. The medium is replaced with fresh RPMI 1640 supplemented with 10% FCS.
8. Add 10 ng/mL LPS and 5 U/mL human recombinant IFN-γ (to induce M1 polarization) or 20 ng/mL human recombinant IL-4 (to induce M2 polarization) and incubate for 24 h at 37°C in 5% CO₂.
9. The supernatant is removed, and cells are maintained in fresh RPMI 1640 containing 5% FCS.

In addition to primary monocyte-derived macrophages, THP-1 cells can be directly induced to differentiate into macrophage-like cells ^[5][6].

1. THP-1 cells were cultured in RPMI 1640 medium containing 10% fetal bovine serum (FBS), 2 × 10^-3 M L-glutamine, 1% antibiotic-antifungal solution, and 50 × 10^-6 M 2-mercaptoethanol.
2. Cells were maintained at 37°C in a humidified incubator with 5% CO₂.
3. As a suspension cell line, THP-1 cells require relatively high cell density to maintain optimal viability. Routine maintenance is typically performed using a 1:2 medium exchange, with minimal centrifugation and the use of high-quality serum.

1. THP-1 cells were seeded into multiwell plates at a density of 5 × 10⁵ cells/mL and treated with phorbol 12-myristate 13-acetate (PMA; 50 – 200 ng/mL) for 24 h to induce differentiation.

After 24 h of PMA treatment, cells transition from suspension to adherent growth. Under high magnification, cells display increased cytoplasmic granularity, a flattened and irregular morphology, and the formation of pseudopodia, consistent with macrophage-like characteristics.

1. M0 (resting) macrophages: After 24 h of PMA treatment, adherent cells are washed twice with serum-free medium and cultured in PMA-free medium for an additional 24 h to obtain M0 macrophages.
2. M1 macrophages: PMA-treated cells are co-cultured with 100 ng/mL LPS and 20 ng/mL IFN-γ for 24 h to generate classically activated M1 macrophages.
3. M2 macrophages: PMA-treated cells are co-cultured with 20 ng/mL IL-4 and 20 ng/mL IL-13 for 24 h to generate alternatively activated M2 macrophages.

Identification of M1/M2 Macrophage Phenotypes

Accurate characterization of macrophage subtypes is critical for elucidating their physiological functions, disease-associated roles, and responses to pharmacological interventions. A multidimensional identification strategy incorporating morphological features, surface marker expression, and secreted factors enables precise assessment of macrophage functional states.

M1 macrophages typically exhibit a flattened morphology, dispersed cellular distribution, and numerous pseudopodia, features that facilitate pathogen recognition and phagocytosis. Their cytoplasm contains abundant lysosomes and digestive enzymes that support antimicrobial activity.

In contrast, M2 macrophages display a more rounded morphology with fewer surface protrusions. Their cytoplasm is enriched in phagosomes and other organelles associated with the clearance of apoptotic cells and the promotion of tissue regeneration.

(A) Polarization of macrophages into M1 and M2 phenotypes in response to LPS/IFN-γ and IL-4/IL-13, respectively, and their corresponding morphological features under light microscopy. (B) Left: comparative morphology of M0, M1, and M2 macrophages based on cell thickness and adherence. Right: nuclear (blue) and actin (green) staining of M0, M1, and M2 macrophages visualized by microscopy.

M1 macrophages secrete pro-inflammatory cytokines, including TNF-α, IL-1β, IL-6, and IL-12, as well as high levels of ROS and nitrogen species.

M2 macrophages predominantly secrete anti-inflammatory mediators such as IL-10, CCL18, and CCL22. They also express characteristic surface receptors, including the mannose receptor CD206 (MRC1), scavenger receptor CD163, dectin-1, and DC-SIGN.

Macrophage differentiation and polarization represent fundamental processes in immunological research and provide powerful tools for studying disease mechanisms, drug screening, and therapeutic development. With continued methodological refinement, these in vitro approaches enable deeper investigation of macrophage roles in inflammation, infection, and cancer, thereby offering new insights and potential targets for immunotherapy and precision medicine.