To date, five activated macrophage phenotypes have been identified, including M1 macrophages, M2 macrophages, CD169+ macrophages, TCR+ macrophages, and tumor-associated macrophages (TAMs); however, M1 and M2 macrophages remain the most commonly used in research. M1 and M2 macrophages are distinguished based on their functional characteristics and levels of inflammatory cytokine secretion.

M1 macrophages are pro-inflammatory cytokine-producing macrophages, also known as classical activated macrophages. ( Classical activated macrophages, M1 ). They express high levels of major histocompatibility complex class II molecules and co-stimulatory molecules CD80 and CD86. Upon stimulation with IFN-γ, LPS, and other factors, quiescent macrophages ( M0 type ) Can be induced to polarize into M1 macrophages, which secrete large amounts of pro-inflammatory cytokines, including IL-1, IL-6, TNF-α, iNOS, high levels of IL-12, and lower levels of IL-10, thereby primarily promoting the initiation and progression of inflammation, exerting bactericidal and phagocytic functions. At the same time, they produce chemokines that attract Th1 cells via chemotaxis, thus enhancing a robust Th1 immune response. Moreover, M1 macrophages can enhance antigen presentation by upregulating the expression of MHC class II and the co-stimulatory molecules CD80 and CD86 on their cell surface.

M2 macrophages are also known as alternatively activated macrophages. ( Alternatively activated macrophages, M2 ), which is primarily activated by the inflammatory cytokine IL-4 and exerts its effects mainly by secreting anti-inflammatory cytokines such as IL-10 to suppress M1 macrophages, playing a central role in responses to parasitic infections, tissue repair, angiogenesis, wound healing, and allergic diseases. M2 macrophages can release large amounts of cytokines including VEGF, Arg-1, PDGF, TGF-β, and IL-10; at the same time, they secrete chemokines such as CCL17, CCL18, CCL22, and CCL24, which recruit Th2 cells, basophils, and eosinophils, and produce various matrix metalloproteinases (such as MMP-2, MMP-7, MMP-9, CCL18, and CCL22), thereby promoting cancer cell metastasis. Type II macrophages, in turn, comprise three subtypes: M2a, M2b, and M2c. M2a and M2b exert immunomodulatory functions and promote type II immune responses, whereas M2c inhibits immune responses and facilitates tissue remodeling.

Differentiation of Type I and Type II Macrophages

In vitro, humans Both type I and type II macrophages can be derived from CD14+ monocytes under the induction of different cytokines. . Add to the complete culture medium Recombinant human GM-CSF Type I macrophages can be obtained, while the addition of Human M-CSF Type II macrophages can be obtained. After 6 days of culture, most type I macrophages adhere to the culture dish in a fried-egg morphology, whereas type II macrophages exhibit a long spindle-shaped morphology. Flow cytometric analysis of surface molecules on the macrophages revealed that both types express CD14, intercellular adhesion molecule-1 (CD54), CD11b, and CD11c, with no significant differences between them. The expression of immunoglobulin-binding receptors CD23/FcεRII and CD64/FcγRI was also indistinguishable; however, the levels of CD16/FcγRIII and CD32/FcγRII were higher in type II macrophages than in type I macrophages. In contrast, CD163, a member of the cysteine-rich scavenger receptor family, is expressed exclusively in type II macrophages.

Cytokines Secreted by Type I and Type II Macrophages

In vitro, Classical macrophages are activated by interferon-γ or by the combination of interferon-γ with bacterial products such as bacterial lipopolysaccharide (LPS). These macrophages secrete large amounts of IL-12 and IL-23, but only low levels of IL-10. Moreover, these activated macrophages efficiently produce effector molecules such as iNOS and pro-inflammatory cytokines including TNF, IL-6, IL-12, and IL-15, thereby functioning as both inducer and effector cells in the Th1 response and effectively clearing pathogens. Non-classical macrophages include M2a, M2b, and M2c, which are activated upon stimulation by IL-4 or IL-13, immune complexes, and IL-10, respectively. Type II macrophages, across all their subtypes, are characterized by the secretion of low levels of IL-12 and IL-23 and generally exhibit high expression of scavenger receptors, such as CD163, the mannose receptor, and the galactose receptor. Although arginine metabolism is highly active in Type I macrophages, leading to substantial production of nitric oxide metabolites, the arginine metabolic pathway is predominantly found in M2a and M2c cells, where it generates ornithine and proline. Activated Type II macrophages do not produce pro-inflammatory cytokines such as TNF, IL-6, IL-12, and IL-15; instead, they primarily secrete IL-10. In addition, Type II macrophages also secrete IL-8, monocyte chemoattractant protein-1 (MCP-1), interferon-inducible protein 10 (IP-10), and macrophage inflammatory protein-1 (MIP-1), among others.

Chemokines Secreted by Type I and Type II Macrophages

Type I and type II macrophages also secrete distinct chemokines. Macrophages activated by bacterial lipopolysaccharide (LPS) induce NF-κB–dependent transcription of proinflammatory chemokines, including CXCL1, CXCL2, CXCL3, CXCL5, CXCL8, CXCL9, CXCL10, as well as CCL2, CCL3, CCL4, CCL5, CCL11, and CCL17. In addition, LPS and interferon-γ both stimulate macrophages to express CXCL10, CXCL9, and CCL5; LPS-mediated induction of these chemokines is mediated through activation of the transcription factor interferon regulatory factor-3 (IRF-3), which in turn leads to production of type I interferon and subsequent activation of STAT1 (signal transducer and activator of transcription 1). Signals induced by type II macrophages generally inhibit the production of chemokines by type I macrophages. For example, IL-4 and IL-10 suppress the LPS- and interferon-γ–dependent production of CXCL10, CCL5, and CXCL9. The inhibitory effect of IL-10 on LPS-activated macrophages depends on two mechanisms: a STAT3–dependent mechanism and an inhibition of NF-κB activation. Moreover, IL-10 can directly block the expression of CXCL10 and CXCL9 by suppressing STAT1 phosphorylation. However, the signaling mediated by type II macrophages does not merely inhibit the production of proinflammatory chemokines by type I macrophages; it also induces the production of chemokines associated with Th2 responses. For instance, in M2a macrophages, IL-4 and IL-13 selectively induce the production of CCL24, CCL17, and CCL22. In addition, IL-4 also induces the production of CCL2; analysis of target genes in mice reveals that CCL2 is a Th2-associated chemokine. The expression of CCL18 is likewise induced by Th2-related cytokines such as IL-4, IL-13, and IL-10, whereas signals mediated by interferon-γ inhibit its production; thus, CCL18 is also produced by type II macrophages.

Differences in the Functions of Type I and Type II Macrophages

1. Phagocytic function

Both type I and type II macrophages are capable of phagocytosing latex particles; however, FcγR–mediated phagocytosis—such as the phagocytosis of sensitized red blood cells—differs substantially between the two types, with type II macrophages exhibiting markedly greater phagocytic capacity. Approximately 98% of type II macrophages can phagocytose sensitized red blood cells, whereas only about 30% of type I macrophages possess this capability.

2. Macrophages and Insulin Resistance

Insulin resistance arises from alterations in the function of insulin-target cells and the accumulation of pro-inflammatory macrophages that secrete inflammatory cytokines. At the molecular level, the polarization of macrophages from the M2 phenotype to the M1 phenotype promotes the development of insulin resistance. In a lean state, macrophages express F4/80 and CD11b but do not express CD11c or exhibit only weak expression of inflammatory markers; they also secrete anti-inflammatory cytokines such as IL-10 and factors that enhance insulin sensitivity. In contrast, during obesity, adipocyte metabolism undergoes a shift, leading to increased lipolysis and the release of pro-inflammatory free fatty acids, which in turn recruit and activate M1 macrophages. These M1 macrophages secrete pro-inflammatory cytokines that reduce insulin sensitivity and also release chemokines that recruit additional macrophages. Importantly, in the context of insulin resistance, M1 macrophages are not the sole players; M2 macrophages also contribute to the pathophysiology. The negative feedback loop underscores the opposing roles of M1 and M2 macrophages: M2 macrophages secrete IL-10, which exerts potent anti-inflammatory effects.

3. Macrophages and HIV Infection

Research findings indicate that mononuclear macrophages are also primary target cells for HIV infection and serve as the major reservoir for the virus in the later stages of infection. Studies on the disease have shown that antiretroviral therapy can markedly suppress HIV replication in infected T cells and reduce viremia; however, this treatment is ineffective against HIV infection of mononuclear macrophages residing in lymphoreticular endothelial tissues, and these infected macrophages are capable of producing viral particles within the human body. There are differences in the susceptibility of the two types of macrophages to HIV infection. During the viral life cycle, activation of the HIV 5′ long terminal repeat (LTR) promoter is a critical step.

4. Macrophages and Tumors

Macrophage surveillance plays a critical role in inhibiting the growth of mutant cells. Classically activated macrophages exert a substantial influence during the early stages of tumorigenesis; the cytokines and other bioactive molecules they produce can induce DNA damage and genomic mutations, thereby driving malignant transformation of host cells. As tumors progress, the tumor microenvironment exerts profound effects on macrophages, giving rise to a distinct subset known as tumor-associated macrophages (TAMs). Recent studies have demonstrated that, within the tumor microenvironment, TAMs are predominantly activated via a MyD88-dependent NF-κB signaling pathway. TAMs suppress the host immune response against tumor-derived antigens and also inhibit the activation of neighboring macrophages. Moreover, emerging evidence indicates that TAMs promote angiogenesis and lymphangiogenesis, thereby facilitating tumor growth, metastasis, and malignant progression. Upon activation, these macrophages secrete pro-angiogenic and pro-lymphangiogenic factors, including acidic fibroblast growth factor (aFGF/FGF1), basic fibroblast growth factor (bFGF/FGF2), vascular endothelial growth factor (VEGF), transforming growth factor-α, and insulin-like growth factor, among others. Concurrently, TAMs produce proteases that cleave and release molecules bound to heparan sulfate—such as proteoglycans and fibrin fragments—which in turn inhibit matrix metalloproteinases (MMPs 1, 2, 3, 9, and 12), plasminogen activators, urokinase-type plasminogen activator, and their receptors, all of which are key mediators of tumor-induced angiogenesis and lymphangiogenesis. Notably, these tumor-associated macrophages exhibit robust secretion of IL-10 while producing little or no IL-12 and virtually no TNF-α; moreover, they may suppress the antigen-presenting function of dendritic cells. Consequently, this subset displays characteristics of non-classical macrophages and exerts a pro-tumorigenic effect by promoting tumor growth.

Macrophages perform a variety of critical functions in vivo, including immune defense, regulation of inflammatory responses, and tissue repair. They have broad clinical applications: