Neutrophils not only participate in antimicrobial defense and tissue injury responses but also infiltrate the tumor microenvironment and acquire highly heterogeneous functional states. They can contribute to antitumor immunity while also promoting tumor progression through immunosuppression, angiogenesis, extracellular matrix remodeling, and neutrophil extracellular trap formation, making neutrophils important regulatory targets in tumor immunology.

Keywords: neutrophils; tumor-associated neutrophils; TANs; PMN-MDSCs; CXCR2; NETs; PAD4; tumor immunity

1 Overview of Neutrophils in the Tumor Microenvironment

1.1 Basic Functions of Neutrophils

Neutrophils are among the most abundant leukocytes in peripheral blood and perform chemotaxis, phagocytosis, degranulation, reactive oxygen species production, and neutrophil extracellular trap formation. Mature neutrophils are released from the bone marrow into peripheral blood and migrate toward sites of inflammation or tissue injury in response to chemokines such as CXCL8, CXCL1, CXCL2, and CXCL5. Tumor-derived chemokines, growth factors, and metabolic signals can continuously alter neutrophil production, mobilization, recruitment, and functional states. Neutrophils that enter tumor tissues are generally referred to as tumor-associated neutrophils (TANs).

1.2 Recruitment of Neutrophils to Tumor Tissues

(1) CXCL–CXCR1/2 Chemotactic Axis

Tumor cells, cancer-associated fibroblasts, and myeloid cells can secrete ELR⁺ CXC chemokines such as CXCL1, CXCL2, CXCL5, and CXCL8. CXCR1 and CXCR2 participate in granulocyte chemotaxis, with CXCR2 representing an important receptor in research on the recruitment of TANs and polymorphonuclear myeloid-derived suppressor cells (PMN-MDSCs).

(2) G-CSF and Granulopoiesis

Tumor-derived G-CSF can promote bone marrow granulopoiesis and neutrophil mobilization, thereby increasing the number of circulating granulocytes. Persistent tumor-associated inflammation may also promote the expansion of immature granulocytes and generate myeloid cell populations with immunosuppressive activity.

(3) Local Inflammatory and Hypoxic Signals

Hypoxia, necrosis, and inflammatory mediators can enhance chemokine expression within tumor tissues and alter neutrophil survival, activation, and metabolic states, promoting the persistent accumulation of TANs in the tumor microenvironment.

1.3 Functional Heterogeneity of TANs

Early studies frequently classified TANs into antitumor “N1” and protumor “N2” phenotypes. Type I interferon-associated signaling tends to support antitumor states, whereas signals such as TGF-β can promote immunosuppressive phenotypes. Single-cell sequencing and functional studies have demonstrated that neutrophils at different maturation stages, tissue locations, and stimulation conditions can develop distinct transcriptional and functional characteristics. The N1/N2 framework is therefore more appropriate as a simplified representation of functional polarization than as a strict classification of neutrophil subtypes.

2 Antitumor and Protumor Functions of Neutrophils

2.1 Antitumor Effects

(1) Direct Cytotoxicity

Activated neutrophils can release ROS, myeloperoxidase (MPO), neutrophil elastase (NE), and other granule proteins. Under specific conditions, these effector molecules can damage tumor cell membranes, proteins, and nucleic acids and promote tumor cell death.

(2) Antibody-Dependent Tumor Cell Clearance

Neutrophils express multiple Fc receptors that recognize antibody-coated tumor cells and participate in antibody-dependent cellular cytotoxicity and antibody-dependent phagocytosis-like processes. These effects are influenced by Fc receptor type, antibody structure, tumor antigen density, and the local immune environment.

Figure 1. Mechanisms of Neutrophil-Mediated Tumor Cell Clearance

(3) Coordination of Antitumor Immunity

Certain neutrophil states can produce proinflammatory mediators and regulate the functions of antigen-presenting cells, CD8⁺ T cells, and NK cells, thereby contributing to local antitumor immune responses.

2.2 Immunosuppressive Effects

The tumor microenvironment can induce subsets of neutrophils to acquire pronounced immunosuppressive functions. These cells may reduce T-cell proliferation and effector activity through ROS, arginine metabolism, lipid mediators, and other inhibitory signals. Pathological granulocytes with strong immunosuppressive activity may be classified as PMN-MDSCs. Because PMN-MDSCs and conventional neutrophils substantially overlap in morphology and certain surface markers, their identification requires integration of the pathological context, phenotype, and functional evidence of immunosuppression.

2.3 Angiogenesis and Extracellular Matrix Remodeling

TANs can release MMP9, NE, VEGF, and other inflammatory or proteolytic factors, promoting extracellular matrix degradation, vascular basement membrane remodeling, and angiogenesis. Protease-mediated matrix alterations can also modify tumor cell migration routes and intercellular adhesion, thereby creating tissue conditions that support tumor invasion and metastasis.

3 NET-Mediated Regulation of Tumors

3.1 Formation of Neutrophil Extracellular Traps

Neutrophil extracellular traps (NETs) are primarily composed of decondensed chromatin decorated with MPO, NE, Cathepsin G, and other granule proteins. Some NET-forming pathways involve ROS production, NE and MPO translocation, and PAD4-mediated histone citrullination. NET release is highly dependent on the stimulus and underlying mechanism, and not all NET-forming processes require the same molecular pathways.

3.2 NETs and Tumor Metastasis

(1) Capture of Circulating Tumor Cells

The DNA–protein meshwork formed by NETs can capture circulating tumor cells and promote their adhesion to vascular endothelial cells and distant tissues. Some tumor cells can sense extracellular DNA through molecules such as CCDC25, enhancing migration and metastasis-associated behavior.

(2) Formation of the Premetastatic Microenvironment

Tumor-associated inflammation can induce neutrophil accumulation and NET formation in distant organs and alter the local extracellular matrix, vascular permeability, and immune environment, thereby creating conditions that support the adhesion and colonization of metastatic cells.

(3) Reactivation of Dormant Tumor Cells

Inflammation-induced NET-associated proteases can remodel the extracellular matrix and alter the adhesive and signaling environment surrounding dormant tumor cells, allowing some dormant cells to re-enter the cell cycle and contribute to tumor recurrence.

3.3 NETs and Immune Evasion

NETs can form physical and biochemical barriers around tumor cells, restrict the access of CD8⁺ T cells and NK cells to tumor cells, and influence lymphocyte effector functions through NET-associated proteins and inflammatory signals. Treatment-induced tissue injury and inflammation may also enhance NET formation, allowing NETs to contribute to treatment adaptation and immunosuppression in certain tumors.

3.4 NETs and Cancer-Associated Thrombosis

DNA, histones, and granule proteins within NETs can promote platelet adhesion and coagulation and provide a structural scaffold for thrombus formation. Tumor-associated hypercoagulability can simultaneously enhance neutrophil activation, allowing NET formation to contribute to the initiation and progression of cancer-associated thrombosis.

4 Neutrophils and Tumor Treatment Responses

4.1 Neutrophil-to-Lymphocyte Ratio

The neutrophil-to-lymphocyte ratio (NLR) reflects systemic myeloid inflammation and lymphocyte status and has been associated with prognosis and treatment responses in multiple tumor types. NLR is also influenced by infection, medication, tissue injury, and other inflammatory conditions and cannot directly represent the abundance or functional state of TANs within tumor tissues.

4.2 Resistance to Immune Checkpoint Inhibitors

Extensive recruitment of CXCR2⁺ granulocytes and PMN-MDSCs into tumors can restrict CD8⁺ T-cell infiltration and establish an immunosuppressive environment. NETs may further promote spatial exclusion and functional suppression of lymphocytes, thereby reducing the sensitivity of certain tumors to PD-1/PD-L1 pathway blockade.

4.3 Neutrophil Remodeling After Chemotherapy and Radiotherapy

Chemotherapy, radiotherapy, and certain targeted therapies can alter the expression of tumor-derived chemokines and inflammatory mediators, thereby affecting neutrophil recruitment, activation, and NET formation. Neutrophils after treatment may participate in tissue repair but may also acquire immunosuppressive or prometastatic functions. These effects are jointly regulated by tumor type, treatment modality, and the local microenvironment.

5 Major Strategies for Targeting Tumor-Associated Neutrophils

5.1 Blockade of the CXCL–CXCR1/2 Recruitment Axis

Chemokines such as CXCL1, CXCL2, CXCL5, and CXCL8 promote the recruitment of neutrophils and PMN-MDSCs through CXCR1/2. CXCR1/2-associated antagonists such as SB225002, AZD5069, Reparixin, and SX-682 can reduce granulocyte chemotaxis. CXCR2 blockade is currently an important strategy for interfering with the recruitment of tumor-associated neutrophils.

5.2 Intervention in NET Formation and Structure

(1) PAD4 Inhibition

PAD4 participates in histone citrullination and chromatin decondensation during some forms of NET formation.

PAD4 inhibitors such as GSK484 can be used to investigate PAD4-dependent NET formation and its effects on tumor-associated inflammation, metastasis, and immune regulation.

(2) Neutrophil Elastase Inhibition

NE participates in granule protein release, extracellular matrix remodeling, and certain NET-forming processes. NE inhibitors such as Sivelestat can be used to investigate the regulation of NET formation, tissue remodeling, and tumor invasion by neutrophil-derived proteases.

(3) DNase I-Mediated NET Degradation

DNase I hydrolyzes the extracellular DNA scaffold of NETs and disrupts their meshwork structure. It is primarily used to investigate the functions of established NETs in tumor cell adhesion, migration, metastasis, and immune regulation.

5.3 Functional Reprogramming of Neutrophils

Direct depletion of neutrophils may simultaneously impair host antimicrobial defense and certain antitumor functions. Factors such as TGF-β, type I interferons, and the tumor metabolic environment can regulate TAN functional states. Remodeling the differentiation or activation programs of protumor neutrophils therefore represents an intervention strategy distinct from direct cell depletion.

5.4 Combination with Immunotherapy

Blocking CXCR1/2-mediated granulocyte recruitment, reducing PMN-MDSC accumulation, or lowering the NET burden may remodel the tumor myeloid immune environment and improve conditions for T-cell entry into tumor tissues. Combinations of CXCR1/2 inhibition with PD-1/PD-L1 pathway blockade have entered studies in multiple solid tumors.

6 Research Progress in Neutrophil-Targeted Strategies

6.1 SX-682

(1) Dual CXCR1/2 Blockade

SX-682 is a CXCR1/2 inhibitor that primarily restricts CXCR1/2-dependent recruitment of granulocytes and PMN-MDSCs, reduces myeloid immunosuppression within tumors, and improves conditions for effector T-cell infiltration.

(2) Combination Studies with Immunotherapy

SX-682 in combination with immune checkpoint inhibitors such as Pembrolizumab has entered early clinical studies in metastatic melanoma and other solid tumors. These studies mainly evaluate whether blockade of myeloid-cell recruitment can overcome primary or acquired resistance to immunotherapy.

6.2 AZD5069

(1) Blockade of CXCR2-Mediated Recruitment

AZD5069 inhibits CXCR2 and reduces the migration of CXCR2⁺ granulocytes into tumor tissues. In a study of metastatic castration-resistant prostate cancer, AZD5069 combined with Enzalutamide reduced circulating and tumor-associated myeloid cells, with signals of antitumor activity observed in some patients.

(2) Combination Treatment Studies

Studies involving AZD5069 have primarily examined the contribution of CXCR2⁺ myeloid cells to treatment resistance and evaluated the feasibility of combining granulocyte recruitment blockade with androgen receptor pathway inhibition.

6.3 GSK484

(1) PAD4 Inhibition

GSK484 is a reversible PAD4 inhibitor that reduces PAD4-mediated histone citrullination and is used to investigate PAD4-dependent chromatin decondensation and NET formation.

(2) NET-Targeted Research

PAD4 inhibition has been used to reduce NET formation in multiple preclinical tumor models and to evaluate NET-associated tumor metastasis, inflammation, and immunosuppression. Because NET-forming mechanisms vary among different stimuli, PAD4 inhibition primarily targets PAD4-dependent NET processes.

6.4 DNase I

(1) Degradation of NET Structures

DNase I disrupts the NET meshwork by degrading its extracellular DNA scaffold. In breast cancer, colorectal cancer, and other tumor models, DNase I has been used to investigate the effects of NET structures on tumor cell capture, migration, distant colonization, and immune regulation.

(2) Combination Intervention Studies

Reducing the NET burden within tumor tissues or metastatic microenvironments can be used to evaluate the potential effects of combining NET degradation with immune checkpoint blockade, chemotherapy, or other treatments. DNase I-mediated NET targeting currently remains primarily at the mechanistic and preclinical research stages.

7 Products

7.1 Products for Neutrophil Recruitment and Chemotactic Signaling Research

7.2 Products for NET Formation and Neutrophil Effector Research

7.3 Products for Neutrophil Isolation, Phenotyping, and Functional Detection

Neutrophils exhibit substantial functional plasticity in tumors, and their activities are jointly regulated by maturation state, chemotactic signaling, NET formation, and the immune microenvironment. The CXCR1/2 recruitment axis, PMN-MDSC-mediated immunosuppression, and NETs have become important research directions connecting tumor-associated inflammation with metastasis and treatment resistance.

References

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[2] Jaillon S, Ponzetta A, Di Mitri D, et al. Neutrophil diversity and plasticity in tumour progression and therapy. Nat Rev Cancer. 2020;20:485–503.

[3] Coffelt SB, Wellenstein MD, de Visser KE. Neutrophils in cancer: neutral no more. Nat Rev Cancer. 2016;16:431–446.

[4] Albrengues J, Shields MA, Ng D, et al. Neutrophil extracellular traps produced during inflammation awaken dormant cancer cells in mice. Science. 2018;361:eaao4227.

[5] Yang L, Liu Q, Zhang X, et al. DNA of neutrophil extracellular traps promotes cancer metastasis via CCDC25. Nature. 2020;583:133–138.

[6] Guo C, Sharp A, Gurel B, et al. Targeting myeloid chemotaxis to reverse prostate cancer therapy resistance. Nature. 2023;623:1053–1061.

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