Neutrophils in cancer: At the crucial crossroads of anti-tumor and pro-tumor.

Jie He and Chunxiang Li designed and conceived the study. Wenpeng Cai and Tao Fan wrote the manuscript draft and achieved the visualization. Ziqin Deng, Yixiao Liu, and Chu Xiao revised the manuscript. All authors read and approved the final manuscript.

Not applicable.

The tumor microenvironment (TME) is a highly structured ecosystem composed of innate immune cells (such as neutrophils and macrophages), stromal cells (such as fibroblasts and pericytes), adaptive cells (T cells and B cells), extracellular matrix (ECM), and soluble factors [ 1 , 2 ]. Cellular and non‐cellular components within the TME interact and play a profound role in tumor biology, including tumorigenesis, tumor proliferation, tumor metastasis, and tumor angiogenesis [ 3 ]. The type and abundance of tumor‐infiltrating immune cells have emerged as biomarkers of the response to cancer therapy [ 4 ], particularly neutrophils show a remarkable propensity to infiltrate tumor tissue, constituting a major proportion of the infiltrating immune cells in several tumor types [ 5 , 6 ]. Studies have established that neutrophils play an important role in the initiation and development of cancer, and considerable efforts are being made to elucidate their potential as therapeutic targets [ 7 , 8 ]. A comprehensive assessment of the complex interactions between neutrophils and tumor cells may be critical for addressing the current challenges of conventional immunotherapy and facilitating the development of next‐generation immunotherapies. Neutrophils are the most abundant myeloid cells in human peripheral blood, accounting for 50%‐70% of human circulating leukocytes [ 9 , 10 , 11 , 12 ]. As the first line of defense against invading pathogens, they play a key role in immune responses to various diseases, including cancer [ 13 , 14 ]. However, neutrophils are fragile, terminally differentiated, and non‐proliferative, with a short half‐life of approximately 6‐8 hours. These biological characteristics have made neutrophil studies particularly challenging. Most single‐cell RNA sequencing (scRNA‐seq) approaches fail to achieve high‐throughput profiling of neutrophils [ 15 ], and large‐scale systematic studies remain limited. Consequently, the functional diversity of neutrophils in cancer has not been explored as extensively as that of other myeloid cells [ 16 , 17 ]. In recent years, significant progress has been made in reassessing neutrophil biology due to technical advances such as high‐dimensional transcriptomic and epigenomic approaches, as well as single‐cell resolution studies. These innovations have enabled researchers to appreciate the previously unappreciated heterogeneity, diversity, and plasticity of neutrophils. For example, a recent study demonstrated that circulating neutrophils can survive for nearly 6 days [ 18 , 19 ]. The traditional view that neutrophils are specialized cells that perform only a limited set of functions in immune defense has been revolutionized. In addition, both anti‐tumor and pro‐tumor functions of neutrophils have been described in cancer, with both direct and indirect effects on tumor cells as well as other immune content in the TME [ 20 , 21 , 22 ]. Targeting neutrophils as an innovative cancer therapy strategy has shown promising results in both preclinical and clinical trials, attracting increasing research attention [ 23 ]. In this review, we explored the mechanisms by which tumor‐derived signals regulate neutrophil production and recruitment, discussed recent advances in understanding the role of neutrophils in cancer development and treatment, and analyzed the correlation of neutrophils with cancer prognosis. We also summarized the limitations of current studies on neutrophils and suggested future research directions that could enhance the manipulation of neutrophils for therapeutic purposes in cancer patients. These insights from collaborative studies could deepen our understanding of neutrophil biology in cancer, refine neutrophil‐targeted therapeutic strategies, and contribute to advancements in cancer diagnosis, treatment, and prognosis monitoring studies.

Although many promising therapeutic strategies and small molecules have been developed, several challenges remain in targeting neutrophils, particularly issues related to specificity. Selective targeting of neutrophils is difficult due to their close association with other myeloid lineages, such as monocytes and macrophages. In fact, most approaches aimed at targeting neutrophils are likely to affect other myeloid cell types as well [ 302 , 303 ]. Given the critical roles of these myeloid cells, targeting neutrophil strategies may inadvertently impact bone homeostasis and immune defense. Several studies have described approaches to increase the selectivity of targeting neutrophils. For instance, one study aimed to selectively inhibit Bruton's tyrosine kinase (BTK) in neutrophils by conjugating BTK siRNA to an anti‐neutrophil monoclonal antibody fragment. Intranasal instillation of the antibody‐siRNA conjugates attenuated experimental acute lung injury in mice, highlighting the potential of the approach [ 304 ]. Another recent report suggests that cell‐targeted enhancement can be achieved by using conjugates of ROS amplifiers with ligands binding to neutrophil receptors (such as the myeloid differentiation marker Ly6G), or by designing reagents activated by neutrophil function (such as phagocytic activity or enzymatic activity of NE) [ 305 ]. Furthermore, given the role of neutrophils in antimicrobial host defense, there is a general concern that therapeutic inhibition of neutrophils could trigger severe infectious complications [ 306 ]. Targeting specific neutrophil subsets may offer a strategy to selectively inhibit their anti‐inflammatory functions without compromising their role in antimicrobial defense. Notably, several studies have reported contrasting clinical outcomes, indicating that neutrophil depletion may accelerate tumor growth in immunocompetent mice [ 307 ]. This phenomenon could be attributed to the hybrid properties of neutrophils. As previously mentioned, APC‐like hybrid neutrophils have been identified in GBM and early‐stage lung cancer [ 104 , 106 ]. In fact, the polarization of TANs into APCs is indispensable to tumor suppression; when researchers eliminated hybrid neutrophil precursors through irradiation, the remaining canonical TANs instead promoted prognostically unfavorable necrosis. While neutrophil depletion accelerated GBM tumor growth in immunocompetent mice, the opposite was observed in T cell‐depleted mice. These findings are likely because, while hybrid TANs act as APCs in T cell‐enriched tumors, their stimulatory effect on stem‐like glioblastoma stem cells (GSCs) predominates in T cell‐impoverished contexts [ 104 , 106 ]. Thus, further work is needed to determine how these findings translate in cancer patients who have fewer and functionally impaired T cells than healthy adults.

Neutrophils exhibit significant heterogeneity and diversity, enabling them to play distinct roles in tumor progression. However, further research is required to develop effective therapeutic strategies that maximize their potential while mitigating their pro‐tumor effects. The combination of neutrophil‐targeted therapy with conventional anticancer therapies may represent a more effective, safer, and promising strategy for the treatment of cancer. Although significant progress has been made in elucidating the biology, functions, and heterogeneity of neutrophils, several critical questions remain unanswered. Addressing these gaps through well‐designed studies may uncover novel roles of neutrophils in tumor biology and facilitate the development of innovative therapeutic strategies. First, it is essential to identify surface markers that distinguish different neutrophil subtypes. Accurate characterization of neutrophil phenotypes is crucial for understanding their specific roles and mechanisms within the TME Second, the phenotypic plasticity of neutrophils at each step of their differentiation trajectory needs to be characterized in detail. As previously mentioned, neutrophils exhibit immunostimulatory properties in the early stages of cancer, promoting the proliferation and activation of various immune cells to suppress tumor growth. However, as tumor progression advances, neutrophils acquire immunosuppressive functions. Therefore, precise investigations are required to determine exactly when TANs pass the “point of change” and cause their immunosuppressive function, to identify the defining biological features of this state, and to explore whether there are better reversal therapies. Additionally, while the neutrophil‐to‐lymphocyte ratio (NLR) has been evaluated as a prognostic indicator, it remains unclear whether targeted interventions on NLR could improve clinical outcomes. Furthermore, current neutrophil‐targeting studies have primarily been conducted in mouse models using human circulating neutrophils as samples; thus, whether circulating neutrophils exhibit the same characteristics as TANs and whether these findings can be directly translated to human applications need to be further validated. Lastly, although various neutrophil‐targeted therapies have been proposed and extensively investigated in preclinical studies, their successful translation into clinical trials remains limited due to challenges in mitigating their immunosuppressive effects. Therefore, future research should prioritize strategies to control the adverse effects of neutrophil‐targeted therapies, thereby facilitating their clinical applicability.

Neutrophils are short‐lived, terminally differentiated cells that require continuous replenishment from bone marrow precursors to maintain their homeostasis in peripheral blood. In healthy adults, the bone marrow produces 1∼2 × 10 11 granulocytes daily, of which neutrophils constitute the predominant population, accounting for approximately 95% [ 24 ]. Neutrophil generation and maturation occur within the bone marrow, which devotes approximately two‐thirds of its space to the production of granulocytes and monocytes under steady‐state conditions [ 25 ]. The classical view implies that neutrophils originate from hematopoietic stem cells (HSCs), which differentiate into common myeloid progenitors (CMPs) and subsequently into granulocyte‐monocyte progenitors (GMPs) [ 25 , 26 , 27 , 28 ]. The GMPs differentiate into mature neutrophils through the following sequence: myeloblast → promyelocytes → myelocytes → metamyelocytes → band neutrophils → mature neutrophils [ 26 , 29 ] (Figure  1 ). The transition from myeloblast to promyelocyte is characterized by the first appearance of primary granules. Secondary granules form during the myelocyte to metamyelocyte transition, and tertiary granules emerge during the shift from band cell to segmented cell stage [ 26 ]. These granules compartmentalize a variety of antimicrobial factors and enzymes essential for neutrophil function [ 30 ]. The origin, differentiation, and recruitment of TANs. Neutrophils originate from a common progenitor cell and differentiation into distinct phenotypic TANs in the TME. Abbreviations: TAN, tumor‐associated neutrophil; MDSC, myeloid‐derived suppressor cell. A variety of biological mediators, including transcription factors (such as early growth response factor 1 [EGR‐1] and signal transducer and activator of transcription 3 [STAT3]), proteins (such as neutrophil elastase [NE] and granulocyte colony‐stimulating factor [G‐CSF]), and receptors (such as the G‐CSF receptor [G‐CSFR]), are involved in the regulation of neutrophil development [ 31 , 32 ]. Among these, G‐CSF serves as the principal regulator of neutrophil generation and differentiation, which promotes the commitment of progenitor cells to the myeloid lineage, stimulates the formation and activation of neutrophil precursors, reduces their transit time through the compartment, and facilitates the release of mature neutrophils [ 33 ]. Its receptor, G‐CSFR, is expressed throughout the myeloid lineage, from the early stem and progenitor cells to fully differentiated neutrophils. Deficiency in either G‐CSF or G‐CSFR results in severe neutropenia [ 34 , 35 ]. The transcription factor RAR‐related orphan receptor gamma1 (RORC1/ROR γ) is a recently identified regulator of myelopoiesis in tumor‐bearing mice, and its expression can be induced by G‐CSF [ 36 ]. Other mediators, such as granulocyte‐macrophage colony‐stimulating factor (GM‐CSF) [ 37 ], and the pro‐inflammatory cytokines interleukin‐4 (IL‐4) [ 38 ], IL‐6 [ 39 ], IL‐17 [ 40 ], and IL‐23 [ 41 ], are also involved in regulating the development of neutrophils, particularly during inflammatory responses. However, the single knockouts of these molecules did not significantly alter neutrophil levels in vivo, suggesting a high degree of redundancy in this regulatory network [ 42 , 43 ]. In addition, homeostatic regulation of neutrophil production is thought to match neutrophil elimination, thereby maintaining a relatively constant number of neutrophils in the blood. Phagocytosis of apoptotic neutrophils has been shown to regulate granulopoiesis by suppressing the secretion of IL‐23 and IL‐17 by phagocytes [ 44 , 45 ]. As neutrophils mature in the bone marrow, they increase deformability, enhance motility, and ultimately migrate into the bloodstream through trans‐endothelial pores [ 46 ]. The trafficking of neutrophils from bone marrow to peripheral blood is tightly regulated, with circulating mature neutrophils constituting only 1%‐2% of all neutrophils under homeostatic conditions [ 47 ]. CXC‐chemokine receptor 4 (CXCR4) and CXCR2 play a crucial role in the retention and release of neutrophils [ 48 ]. Increased CXCR4 signaling results in a deficiency of neutrophils in circulation by promoting the accumulation of mature neutrophils in the bone marrow, while deletion of CXCR4 causes a shift in the pool of mature neutrophils from bone marrow to circulation [ 49 , 50 ]. Bone marrow stromal cell expression of CXC‐chemokine ligand 12 (CXCL12) mediates the retention of CXCR4 + immature neutrophils [ 51 , 52 ]. Aging neutrophils upregulate the CXCR4 expression, driving their homing back to the bone marrow and subsequent elimination by macrophages [ 53 ]. CXCR2 is another cytokine receptor expressed on myeloid cells, and its ligands, CXCL1 and CXCL2, are expressed by bone marrow endothelial cells. Secretion of CXCL1 and CXCL2 by endothelial cells contributes to the release of neutrophils into the circulation via CXCR2 signaling. Deletion of CXCR2 causes a myelokathexis phenotype with the retention of mature neutrophils in the bone marrow [ 54 ]. Notably, double deletion of both CXCR2 and CXCR4 results in a CXCR4 deletion‐like phenotype, namely an inability to retain neutrophils in the bone marrow, and CXCR2 stimulation does not induce additional neutrophil release from the bone marrow in the absence of CXCR4, demonstrating the fundamental role of CXCR4 signaling in the homing of neutrophils in the bone marrow [ 54 ]. In addition, G‐CSF downregulates CXCL12 production, and CXCR4 expression on myeloid cells and blocks CXCR4‐mediated cell signals, thereby promoting neutrophil release from the bone marrow [ 52 ]. Numerous immunohistochemical studies have shown a significant increase in neutrophil abundance in tumor tissue compared to healthy tissue. Altered hematopoiesis (such as neutrophilia) has been observed in almost all established tumor models, suggesting that the recruitment of tumor‐infiltrating neutrophils could be regulated by tumor‐derived factors [ 55 , 56 , 57 ]. Recruitment of neutrophils is a tightly coordinated process that involves the rolling and adhesion of neutrophils on activated endothelium, haptotaxis along a chemotactic gradient, and directed transmigration across the vasculature into the surrounding tissue [ 58 ]. Several molecules present in the TME act as potent neutrophil chemoattractants and activators, the most potent and studied being the CXC chemokine subfamily [ 59 ]. Neutrophils express high levels of the chemokine receptors CXCR1 and CXCR2, which play a critical role in their recruitment to the TME, where CXC chemokine ligands are expressed by tumor cells and other tumor‐infiltrating cells [ 60 ]. In addition to chemokines, inflammatory cytokines (for example, IL‐1 [ 61 ], IL‐2 [ 62 ], and tumor necrosis factor [TNF] [ 63 ]) have been implicated in neutrophil mobilization and recruitment in cancer. In particular, these cytokines are part of an inflammatory circuit that leads to the production of G‐CSF and the subsequent formation and mobilization of neutrophils from the bone marrow [ 63 , 64 ]. Moreover, these pro‐inflammatory cytokines can further upregulate the expression of CXC chemokines [ 65 ]. Tumor‐derived oxysterols and the complement component anaphylatoxin C5a have also been demonstrated to play significant roles in neutrophil recruitment [ 66 ]. Other cellular components in the TME also serve as additional regulators for neutrophil recruitment. For example, activated T cells can produce interferon‐beta (IFN‐β), which impedes the migration of circulating neutrophils to tumors by suppressing CXCR2 ligand expression [ 67 ]. Another mechanism modulating neutrophil recruitment involves modifying their lifespan. Tumor‐derived cytokines such as IL‐1β and G‐CSF can significantly increase the survival of neutrophils, thereby increasing the accumulation of neutrophils in the TME [ 68 ]. Additionally, migration inhibitory factor, a macrophage migration inhibitor derived from tumors and identified in various malignancies, has been shown to inhibit neutrophil apoptosis, resulting in the substantial accumulation of neutrophils [ 69 , 70 ]. Autophagy, which occurs in cancer cells due to chronic hypoxia and inflammation, also contributes to cancer cell survival under stressful conditions. In hepatocellular carcinoma (HCC), infiltrating neutrophils showed enhanced autophagy and sustained survival through activation of Erk1/2, p38, and NF‐κB signaling pathways [ 71 ]. IFN‐β regulates apoptosis of tumor‐infiltrating neutrophils by modulating both extrinsic and intrinsic apoptosis pathways. In tumor‐bearing IFN‐β(‐/‐) mice, the lifespan of neutrophils is remarkably prolonged compared to wild‐type controls [ 72 ].

NLR evaluation has been identified as a reliable prognostic factor for survival across various tumor types, including HCC [ 216 ], breast cancer (BC) [ 217 ], and CRC [ 218 ], highlighting the potential value of incorporating NLR into well‐established prognostic scores for clinical decision‐making [ 219 , 220 ]. In a meta‐analysis in 2014, Templeton and colleagues explored the correlation between NLR and overall survival (OS) in data from over 40,000 tumor patients and 100 studies involving different solid tumors. Investigators found that a high NLR (defined as ≥4) was associated with poor OS in several solid tumors, including pancreatic cancer, CRC, HCC, pancreatic adenocarcinoma (PAAD), and other malignancies. Particularly, the prognostic effect of a high NLR was the strongest in patients with CRC, PAAD, mesothelioma, and RCC [ 219 ]. Furthermore, NLR has been identified as the only significant parameter for predicting early post‐surgical complications in patients undergoing hepatectomy for colorectal metastasis [ 221 ]. Finally, a high abundance of circulating neutrophils has been recognized as a contributing factor to cancer‐associated thrombosis [ 222 ]. In a 2015 meta‐analysis, researchers analyzed gene signatures from over 18,000 cancer biopsy samples and found that infiltrating neutrophils were present in the majority of the cancer samples examined. They also identified that a high ratio of infiltrating neutrophils was the strongest prognostic predictor of poor OS across all cellular populations and cancer types [ 9 , 223 ]. Other studies have similarly reported a correlation between the presence of infiltrating neutrophils and poor prognosis, these studies involved early‐stage melanoma, head and neck cancer, HCC, and other malignancies [ 224 , 225 ]. These studies also demonstrated that abundance infiltrating neutrophils was independently associated with poor OS, recurrence‐free survival (RFS), and disease‐specific survival outcomes [ 226 ]. Furthermore, a positive correlation has been observed between a high abundance of intratumoral neutrophils and cancer pT status, pM status, as well as clinical stage [ 227 ]. Recently, several studies have also proposed a strong correlation between the abundance of infiltrating neutrophils and decreased hemoglobin levels, as well as elevated creatinine concentrations [ 228 ]. Notably, although the majority of studies have shown that high levels of infiltrating neutrophils are associated with poor prognosis in tumor patients, some conflicting results suggest that infiltrating neutrophils may sometimes serve as a favorable prognostic factor [ 8 , 229 ]. This discrepancy may arise from the use of different markers. For example, the frequency of high CD66 + neutrophils is positively correlated with the malignancy of CRC, while MPO + neutrophils exhibit an inverse correlation and serve as a favorable prognostic factor [ 227 ]. These discrepancies highlight the urgent need for precise characterization of the heterogeneous populations of neutrophils. Identifying robust surface markers that can distinguish different neutrophil phenotypes may be crucial for accurately elucidating the roles and mechanisms of TANs, as the current marker system is inadequate [ 230 ]. Furthermore, definitive strategies for detecting neutrophil phenotypes using other techniques such as flow cytometry and PCR remain to be developed.

Tumorigenesis involves a series of critical events, including oncogenic mutations in progenitor cells and the establishment of an inflammatory environment [ 110 ]. Several genotoxic DNA substances released by neutrophils contribute to increased mutational load. The first evidence of neutrophils’ pro‐carcinogenic effect was associated with the release of ROS, which has been shown to cause oxidative DNA damage in epithelial cells, thereby promoting oncogenic transformation [ 111 ] (Figure  2A ). Notably, a study by Wculek et al. [ 112 ] highlighted that neutrophils just amplify DNA damage induced by genotoxic substances through ROS and thus promote tumor transformation when exposed to carcinogens, but do not directly drive tissue damage or initiate carcinogenesis. In recent years, novel mechanisms that do not depend on ROS have been identified. For example, activated neutrophils can release microparticles armed with proinflammatory microRNAs (miRNAs), such as miR‐23a and miR‐155. These miRNAs promote the accumulation of DNA double‐strand breaks and increase genomic instability by inducing lamin B1‐dependent replication fork collapse and inhibiting homologous recombination [ 113 , 114 ]. In an in vitro co‐culture model of neutrophils and tumor cells, activated neutrophils induce an hMSH2‐dependent G2/M phase arrest and replication errors in target cells, increasing cancer risk [ 115 ]. Depletion of circulating neutrophils effectively reduces oxidative DNA damage and suppresses carcinogenesis [ 115 , 116 , 117 ]. Mechanisms of TANs‐mediated pro‐tumor effects. (A) TANs promote carcinogenesis by inducing oxidative damage to DNA. (B) TANs promote tumor progression by secreting various pro‐tumor factors. (C) Feedback of tumor cells and neutrophils further promotes tumor progression and the recruitment of TANs. (D) TANs suppress the immune response. (E) TANs induce tumor‐promoting inflammatory responses. (F) TANs stimulate tumor metastasis through multiple mechanisms. Abbreviations: MPO, myeloperoxidase; PGE2, prostaglandin E2; IL‐1RA, interleukin‐1 receptor antagonist; PDG, platelet‐derived growth factor; PGE, prostaglandin E; HGF, hepatocyte growth factor; EGF, epidermal Growth Factor; NE, neutrophil elastase; NETs, neutrophil extracellular traps; IL‐1, interleukin‐1; IL‐17, interleukin‐17; TGF‐beta, transforming growth factor beta; PD‐L1, programmed cell death ligand 1; TNF, tumor necrosis factor; G‐CSF, granulocyte colony‐stimulating factor; VEGF, vascular endothelial growth factor; HMGB1, high mobility group box‐1 protein. Inflammation is closely linked to tumorigenesis. Neutrophils are the major immune cells involved in forming the inflammatory milieu, playing a key role in bridging the inflammatory microenvironment and inflammation‐associated tumor initiation. A typical example of neutrophil‐mediated chronic inflammation‐associated cancer is colorectal cancer (CRC) [ 118 ]. Erdman et al. [ 119 ] used recombinase‐activating gene‐2‐deficient (Rag2 −/− ) mice to simulate chronic inflammatory bowel disease and demonstrated that neutrophils accumulate in the colon infection site, releasing inducible nitric oxide synthase (iNOS) and nitric oxide (NO) through increased production of TNF‐ɑ. This cascade of events leads to intestinal inflammation and promotes carcinogenesis. In an invasive intestinal adenocarcinoma model, CXCR2 deficiency markedly suppressed inflammation‐driven tumorigenesis in both the skin and intestine, as well as spontaneous adenocarcinoma formation [ 120 ]. In a model of melanoma zebrafish larval, inflammation induced by elevated prostaglandin E2 stimulated the formation of local melanoma in a neutrophil‐dependent manner. Depletion of neutrophils using anti‐Ly6G antibodies inhibited melanoma formation in chemically induced and spontaneous models [ 121 ]. Non‐alcoholic steatohepatitis (NASH), a progressive inflammatory fatty liver disease, is a known high‐risk factor for HCC [ 122 ]. The formation of NETs, a process known as “NETosis”, was found to be elevated in the serum of NASH patients. Their formation was suggested to be associated with the pro‐tumorigenic inflammatory milieu in NASH as well as the progression of HCC development [ 123 ]. Neutrophils produce and secrete various substances, such as matrix metalloproteinase‐9 (MMP‐9), which promotes tumor cell proliferation (Figure  2B ) [ 124 , 125 ]. MMP‐9 degrades the ECM, thereby releasing vascular endothelial growth factor (VEGF), which contributes to tumor growth and angiogenesis [ 126 ]. Additionally, NE can enter the endosomal compartment of tumor cells, where it degrades insulin receptor substrate‐1 (IRS‐1), facilitating the interaction between phosphatidylinositol 3‐kinase (PI3K) and the potent mitogen platelet‐derived growth factor receptor (PDGFR), thereby accelerating tumor cell proliferation [ 127 ]. Direct cell‐to‐cell contact between neutrophils and tumor cells further enhances NE activity and promotes the release of other inflammatory mediators (such as leukotrienes) which in turn recruit additional neutrophils (Figure  2B ) [ 117 ]. Neutrophil α‐defensins also promote tumor cell proliferation in a MAP kinase‐dependent manner, and this induced proliferative effect can be blocked by anti‐defensin antibodies [ 128 ]. In HCC, neutrophils secrete bone morphogenetic protein‐2 (BMP‐2) and TGF‐β2, which enhance stem cell‐like properties. These neutrophil‐induced stem‐like HCC cells further recruit TANs and increase HCC proliferation by exhibiting elevated NF‐κB signaling activity and increased CXCL5 expression, thereby forming a positive feedback loop [ 129 ] (Figure  2C ). Furthermore, it has been reported that neutrophils antagonize cellular senescence in cancer by secreting interleukin‐1 receptor antagonists, thus sustaining tumor growth [ 130 ] (Figure  2B ). In addition to directly promoting tumor growth through the release of biological factors, neutrophils also influence other infiltrating cell types within the TME, thereby indirectly supporting tumor progression (Figure  2D ). In HCC, neutrophils recruit macrophages and regulatory T cells (Tregs), which contribute to cancer cell proliferation and resistance to sorafenib [ 131 ]. In the context of GC, GM‐CSF secreted by cancer cells activates neutrophils and induces the expression of programmed death‐ligand 1 (PD‐L1) on neutrophils. The activated PD‐L1 + neutrophils, in turn, suppress T cell proliferation and IFN‐γ secretion, thereby promoting GC progression and contributing to poor survival outcomes [ 132 ]. Additionally, the cytokine IL‐17C, expressed by epithelial cells, recruits neutrophils to inflamed tissues, further enhancing tumor cell growth [ 133 ] (Figure  2E ). NK cells regulate neutrophil phenotype and function through an IFN‐γ‐dependent mechanism. In NK cell‐depleted mice, neutrophils adopt a tumor‐promoting phenotype characterized by increased VEGF‐A expression, promoting tumor growth and angiogenesis [ 134 ]. Recent studies have identified several novel mechanisms by which neutrophils contribute to tumor progression. For example, Yazdani et al. [ 135 ] reported that NETs enhance mitochondrial biogenesis in tumor cells by activating Toll‐like receptor 4, thereby providing additional energy to support accelerated tumor growth. Neutrophils also influence tumor growth by modulating hormone receptor expression, such as increasing androgen receptor expression, which promotes proliferation of the renal cell carcinoma (RCC) [ 136 ]. Additionally, neutrophils can induce tumor cell ferroptosis by transferring myeloperoxidase‐containing granules, with neutrophil‐induced ferroptosis promoting tumor necrosis in GBM progression [ 137 ]. Notably, tumor necrosis is associated with greater tumor aggressiveness and poorer prognosis in GBM [ 137 , 138 ]. Furthermore, induction of ferroptosis can convert non‐suppressive neutrophils to immune‐suppressive neutrophils, causing immune evasion in cancer [ 139 ]. Tumor metastasis is the leading cause of patient mortality and involves a complex pathological process, including tumor cell invasion, intravasation, extravasation, dormancy awakening, and persistent growth [ 140 , 141 ]. Neutrophils have been shown to play a regulatory role in each step of the metastatic cascade. (Figure  2F ). Neutrophils play a critical role in promoting tumor cell invasion. During the early stages of tumor metastasis, neutrophils secrete a variety of proteases, including MMP‐9, serine proteases, NE, and cathepsin G. These enzymes degrade the ECM encapsulation, thereby promoting the invasive behavior of cancer cells toward adjacent organs [ 142 , 143 , 144 ]. Neutrophil‐derived tissue inhibitor of metalloproteinase‐1 (TIMP‐1) induces epithelial‐mesenchymal transition (EMT) in endothelial cells, which allows target cells to lose tight cell junctions and acquire mesenchymal characteristics, enabling tumor cells to acquire increased motility, aggressiveness, and ECM reconstruction [ 145 , 146 ]. Moreover, EMT in tumor cells further amplifies TIMP‐1 secretion by neutrophils [ 147 , 148 ]. Neutrophils also promote tumor cell invasion by regulating EMT and JAK2/STAT3 signaling through chemerin, an effective chemoattractant protein [ 40 , 149 ]. A recent study has reported that NETs augment the invasiveness of tumor cells and induce EMT by activating the TGF‐β signaling pathway [ 150 ]. Neutrophils facilitate tumor cell intravasation [ 151 ]. NETs can adhere to endothelial cells, inducing morphological alterations in the endothelial cells by disrupting cell‐cell junctions and increasing vascular permeability. This formation of gaps between endothelial cells induced by NETs contributes to the intravasation of tumor cells [ 152 ]. Additionally, the bloodstream is a hostile environment for circulating tumor cells (CTCs), which have to overcome various obstacles, including physical challenges and immune attacks. However, neutrophils contribute to CTCs survival [ 153 , 154 ]. In the bloodstream, neutrophils and CTCs form clusters known as CTC‐neutrophil clusters, which consist of hundreds of cells that contribute to CTCs overcoming the fluidic challenges posed by blood flow. These CTC‐neutrophil clusters exhibit several‐fold increased metastatic potential compared to individual CTCs [ 155 , 156 ]. Beyond surviving in the bloodstream, neutrophils also promote CTCs adhesion to the endothelium and facilitate their extravasation from the vascular system into surrounding tissues [ 157 ]. Furthermore, the neutrophil‐derived biological factors, including high mobility group box protein 1, IL‐1β, and MMPs, significantly enhance the extravasation of disseminated tumor cells [ 158 , 159 ]. Neutrophils play a key role in the formation of a pre‐metastatic niche [ 160 ]. In these niches, neutrophils produce and release multiple factors, such as TIMP‐1, which contribute to the establishment of an immunosuppressive microenvironment that supports tumor cell survival and metastasis [ 161 ]. Moreover, neutrophils promote the release of lipid transport protein 2 in a pre‐metastatic niche, which induces EMT and promotes colonization and metastatic growth of tumor cells [ 162 , 163 ]. Tumor cells enter a dormant state when they metastasize to distant sites, where they cease proliferating and evade immune surveillance until awakened by special trigger mechanisms [ 164 ]. Neutrophils are involved in reactivating dormant tumor cells, particularly NETs release is necessary for this awakening process. Two NET‐associated proteases, NE and MMP‐9, contribute to cleaving laminin. The remodeled laminin then promotes the proliferation of dormant cancer cells by activating the integrin α3β1‐integrin signaling pathway [ 124 ]. Angiogenesis is a hallmark of malignant tumors, regulated by the interactions between infiltrating cells and cytokines in the TME, and neutrophils are the switch that mediates initial angiogenesis [ 165 , 166 ]. Neutrophils release various pro‐angiogenic factors, such as VEGF, H 2 O 2 , and prokineticin 2 (Bv8), which stimulate vascular proliferation [ 167 , 168 , 169 ]. In particular, the secretion of Bv8 by neutrophils has been identified as a key indication of tumor angiogenesis [ 170 ]. Additionally, neutrophils secrete various proteases, including MMP‐9, which contribute to revascularization. MMP‐9, a protease that induces the angiogenic switch critical for tumor progression, is involved in VEGF activation, the induction of chronic angiogenesis, and early tumor growth [ 171 , 172 , 173 , 174 ]. Numerous studies have reported that primary tumors, including prostate cancer, actively recruit infiltrating MMP‐9 positive neutrophils to stimulate angiogenesis, while MMP‐9 deficiency significantly reduces microvascular density in the TME [ 175 ]. TIMPs regulate the activity of MMP‐9, which is usually released together with MMP‐9 by forming complexes to block MMP‐9 function [ 176 , 177 ]. Interestingly, human neutrophils uniquely release TIMP‐free MMP‐9, which serves as a potent catalytic stimulator of angiogenesis. This enables neutrophil‐derived MMP‐9 to rapidly hydrolyze the ECM and release sequestered growth factors, thereby promoting angiogenesis [ 178 ]. Additionally, neutrophil‐derived chemokines have been reported to promote endothelial cell proliferation and differentiation [ 179 ]. Chemokines can also stimulate the rapid degranulation of MMP‐9, and this degranulated MMP‐9 produces a more potent chemokine by hydrolyzing the amino terminus of CXCL8. This creates a positive feedback loop, further stimulating angiogenesis by recruiting additional neutrophils to the tumor site [ 180 ]. Furthermore, neutrophils contribute to angiogenesis through direct cell‐to‐cell interactions with endothelial cells. In an in vitro angiogenesis model, co‐culture of endothelial cells with neutrophils significantly increased angiogenesis compared to endothelial cells alone. This effect was inhibited by blocking ICAM‐1 or E‐selectin, suggesting that the mechanism of stimulation of angiogenesis by neutrophils may involve the adherence of neutrophils to endothelial cells via E‐selectin and ICAM‐1 [ 181 ]. The crosstalk between neutrophils and tumor cells has been underestimated. Indeed, the role of neutrophils in regulating host defense and natural immunity extends far beyond their conventional role as individual phagocytes [ 182 ] (Figure  2D ). For example, neutrophils strongly induce apoptosis in CD8 + T cells and impair their anti‐tumor functions through TNF‐α and NO‐dependent mechanisms, thereby promoting a tumor‐supporting environment [ 183 ]. Arginine is an amino acid that promotes T cell proliferation and sustains T cell function [ 184 ]. Arginase‐1 (ARG‐1) released by neutrophils can degrade extracellular arginine. This degradation inhibits T cell proliferation and responses, creating an immunosuppressive environment contributing to tumor growth [ 185 , 186 , 187 ]. Neutrophils also release TGF‐β, which inhibits the recruitment of DCs [ 188 ]. DCs play a crucial role in host immunity, serving as a bridge between innate and adaptive immune responses. TGF‐β also inhibits IL‐12 production by DCs. IL‐12 is a pivotal cytokine responsible for the activation of T cell function [ 189 ]. Additionally, it has been found that TGF‐β promotes the expansion of CD4 + CD25 + Foxp3 + regulatory T cells, a subset of cells that can inhibit immune cytotoxicity and induce immunosuppression [ 190 , 191 ]. Neutrophils not only interact with immune cells, but also affect and are influenced by non‐immune cells within the TME. For example, during advanced CRC progression, tumor stem‐like cell‐derived exosomal RNAs induce the expression of IL‐1β through a pattern recognition‐NF‐κB signaling axis to sustain neutrophil survival. Depletion of neutrophils using a Ly6G‐specific antibody attenuates the tumorigenicity of cancer stem cells [ 192 ]. In obese patients with pancreatic cancer, neutrophils recruited by adipocyte‐secreted IL‐1β induce enhanced activation of pancreatic stellate cells (PSCs). Activated PSCs, in turn, secrete IL‐1β, which further recruits neutrophils. Inactivation of PSCs inhibits IL‐1β expression and reduces neutrophil recruitment. This crosstalk among PSCs, neutrophils, and adipocytes mediated by IL‐1β exacerbates desmoplasia and leads to poorer treatment outcomes [ 193 ]. Cancer‐associated fibroblasts (CAFs) are major sources of chemokines that recruit granulocytes to tumors. CAFs can induce NETs formation, which supports tumorigenesis. These tumor‐induced NETs are driven by a ROS‐mediated pathway that is dependent on CAF‐derived amyloid β [ 194 , 195 ].

Although the primary outcome of the interaction between neutrophils and tumor cells tends to be pro‐tumor, there is evidence suggesting that neutrophils also play a critical role in combating cancer and contribute to anti‐tumor processes, especially in the early stages of tumor development [ 196 , 197 ]. Neutrophils, as key effectors in the fight against cancer, constitute a significant proportion of leukocytes circulating leukocytes. They can exert anti‐tumor roles through various mechanisms, including direct cytotoxicity, the activation of antitumor adaptive immunity, and the induction of antibody‐dependent cellular cytotoxicity (ADCC) (Figure  3 ). Mechanisms of TANs‐mediated anti‐tumor effects. (A) TANs exert direct cytostatic and cytotoxic functions. (B) TANs participate in the activation of innate and adaptive immunity. Abbreviations: IL‐17, interleukin‐17; MPO, Myeloperoxidase; TNF, tumor necrosis factor; ROS, reactive oxygen species; NET, neutrophil extracellular trap; ADCC, antibody‐dependent cellular cytotoxicity; HGF, hepatocyte growth factor; IFN‐γ, interferon‐gamma. The direct cytotoxicity of neutrophils has been extensively demonstrated both in vitro and in vivo, although this tumor‐killing activity is relatively limited (Figure  3A ). A variety of small molecular substances produced by activated neutrophils, including ROS, membrane‐perforating agents, and proteases, contribute to the elimination of tumor cells [ 198 , 199 , 200 , 201 ]. Angiostatin, secreted by neutrophils, inhibits tumor angiogenesis by facilitating the degradation of vascular endothelial cells and suppressing neutrophil migration mediated by CXCL chemokine families [ 202 , 203 ]. Neutrophils also mediate tumor cell apoptosis through the Fas/Fas ligand (FasL) pathway, an effect that can be attenuated by treatment with a Fas receptor antagonist or by knockout of the Fas gene. Fas and FasL are members of the TNF receptor and TNF family, respectively, and the ligation of Fas to FasL activates a caspase cascade that mediates cell cycle arrest [ 204 ]. In addition, NETs, composed of MPO, NE, defensins, and DNA histone protease complex, have been reported to exhibit cytotoxic effects on tumor cells. MPO within NETs can kill tumor cells and inhibit tumor growth. NE can cleave plasminogen, releasing the anti‐angiogenic factor and degrading VEGF. Defensins, another NET component, can induce tumor cell lysis. Histones in NETs can destroy epithelial cells and blood vessels that support tumor growth. DNA extruded from NETs can sequester tumor cells and inhibit their proliferative and metastatic abilities [ 205 , 206 , 207 ]. Neutrophils also mediate antibody‐dependent cellular cytotoxicity (ADCC), an immune mechanism in which effector cells expressing Fc receptors (FcR) kill target cells by binding to the Fc region of monoclonal antibodies conjugated to the surface of these cells [ 208 ]. In addition to their direct cytotoxic effects, neutrophils play an important role in initiating and regulating adaptive immune responses. Tumor‐infiltrating neutrophils participate in sophisticated bidirectional interactions with NK cells, DCs, lymphocytes, and other immune cells (Figure  3B ) [ 209 , 210 ]. For instance, TLR‐stimulated neutrophils release soluble mediators that attract and activate NK cells in vitro, and neutrophils‐conditioned NK cells display enhanced cytotoxicity and cytokine production. Moreover, neutrophils‐conditioned NK cells promote the maturation of monocyte‐derived DCs, which, in turn, facilitate T cell proliferation and IFN‐γ production [ 211 ]. IFN‐γ are prerequisite factors contributing immature neutrophils into APC‐like hybrid neutrophils. The APC‐like hybrid neutrophils express CD86 and HLA‐DR, further potentiating T cell anti‐tumor responses [ 212 ]. Neutrophils also interact with macrophages to stimulate IL‐12 secretion, which facilitates the polarization of unconventional T cells. These unconventional T cells can produce IFN‐γ, contributing to anti‐tumor immunity in various cancers [ 211 ]. Chemokines and pro‐inflammatory cytokines released by neutrophils, such as CCL3, CCL9, CXCL10, TNF‐α, and IL‐1, recruit and activate T cells, as well as enhance the cytotoxicity of T cells, exerting a tumor suppressor role [ 213 ]. Furthermore, IL‐17‐producing γδ T cells (γδ17 T cells) have been reported to be associated with tumor growth and metastasis. Neutrophils can inhibit the production of γδ17 T cell‐derived IL‐17 through the neutrophil/ROS/γδ17 axis, thereby exerting anti‐tumor effects [ 214 , 215 ]. Additionally, neutrophils enhance anti‐tumor immunity by increasing the responsiveness of CD8 + T cells to the T cell receptor complex, which has been shown to improve survival in patients with CRC [ 215 ]. Overall, there is growing evidence supporting that neutrophils play a sophisticated role in orchestrating the immune response to tumors.

We declare no competing interests.

Increasing evidence suggests neutrophils exhibit heterogeneous phenotypes and functional plasticity in human health and diseases, including cancer. Shaul et al. [ 73 ] first proposed the N1/N2 paradigm for tumor‐associated neutrophils (TANs), categorizing them based on their activation states in cancer. N1 phenotypes are short‐lived, highly cytotoxic mature cells with proinflammatory and immunostimulatory activity [ 74 , 75 ]. In contrast, N2 phenotypes are long‐lived, low‐cytotoxic immature cells with pro‐angiogenic, prometastatic, and immunosuppressive activity [ 74 , 76 ]. A variety of cytokine signals, epigenetic modifications, and other microenvironment factors can modify neutrophils into distinct phenotypic and functional subpopulations [ 77 , 78 ]. Tumor‐derived transforming growth factor‐β (TGF‐β), produced by the tumor cells and other cells within the TME, is a key modulator of neutrophil polarization [ 79 , 80 ]. TGF‐β can induce polarization and gathering of N2 TANs at the tumor site [ 76 ]. Blockade of the TGF‐β receptor results in the accumulation of N1 TANs, which enhances pro‐inflammatory responses, including activation of cytotoxic CD8 + T cells, increased expression of pro‐inflammatory cytokines, and upregulation of the costimulatory molecule ICAM‐1 [ 76 ]. Beyond TGF‐β, neutrophil polarization is also promoted by cytokines such as interferon‐gamma (IFN‐γ) and tumor necrosis factor‐alpha (TNF‐ɑ). Priming neutrophils with IFN‐γ and TNF‐ɑ in vitro can convert N2 TANs into N1 TANs [ 81 ]. In addition to tumor cells, other cells in the TME also influence neutrophil polarization. For example, in gastric cancer (GC), IL‐6 produced by mesenchymal stem cells can induce N2 polarization of neutrophils via the IL‐6/STAT3/ERK signaling axis [ 82 ]. Recent studies have reported that angiotensin‐converting enzyme inhibitors and angiotensin II type I receptor antagonists can directly induce the N1 phenotype polarization or indirectly enhance N1 polarization by inhibiting TGF‐β activation [ 83 ]. Another study has shown that hypoxia is a potent determinant of TANs phenotype. After the removal of hypoxia, neutrophil recruitment to the tumor significantly decreased, but the recruited neutrophils exhibited enhanced tumor cell killing ability, and their capacity to promote tumor cell proliferation was reduced [ 84 ]. In the study proposing the N1/N2 paradigm, N1 neutrophils were characterized by hypersegmented nuclei, and N2 neutrophils were identified by banded or ring‐like nuclei [ 76 ]. Nuclear morphology is a well‐established feature of neutrophil differentiation [ 85 ]. However, it remains unclear whether the so‐called N1/N2 neutrophils represent simply mature and immature cells, or if they reflect distinct polarized states. This uncertainty leaves the relationship between polarization and maturation unresolved. Overall, the binary N1/N2 classification system likely oversimplifies neutrophil polarization, and neutrophil polarization probably exists as a spectrum of activation states rather than two extremes. Notably, the simplistic dichotomy of neutrophils in cancer may not fully capture the complexity of the immune landscape. Neutrophils are capable of responding to a range of inflammatory stimuli and adapting to diverse microenvironments, with distinct phenotypic variations observed across different inflammatory contexts [ 86 ]. Several neutrophil subsets with immunosuppression functions have been identified in cancer and have been classified using different terms, such as myeloid‐derived suppressor cells (MDSCs) [ 87 , 88 , 89 ]. These subpopulations are primarily defined by their functional phenotype. However, our current understanding of neutrophil subtypes remains incomplete and controversial, largely due to the lack of specific molecular markers and the lack of standardized study approaches. Whether these subsets represent distinct populations or reflect a spectrum of phenotypic states is still unclear. MDSCs refer to a heterogeneous group of myeloid cells that are absent in healthy individuals but emerge in cancer and pathological conditions. These cells are closely related to neutrophils and monocytes and play a significant role in tumor progression [ 90 , 91 ]. Two major subsets of MDSCs have been identified: polymorphonuclear MDSCs (PMN‐MDSCs) and monocytic MDSCs (M‐MDSCs). Distinguishing TANs from PMN‐MDSCs remains challenging due to the overlap of their phenotypic and functional characteristics [ 90 ]. In humans, mature neutrophils are identified by cell surface markers of CD14 − CD15 + CD66b + CD16 + , while PMN‐MDSCs are primarily characterized by CD14 − CD15 + CD66b + CD16 + CD11b + CD33 + HLA − DR − markers [ 92 , 93 ]. Although both g‐MDSCs and TANs contribute to tumor growth, transcriptomic analysis by Fridlender et al. [ 80 ] has revealed that g‐MDSCs and TANs are significantly distinct. Specifically, TANs exhibit impaired cytoskeletal organization, antibacterial function, granule protein production, and respiratory burst at the mRNA level compared to g‐MDSCs, which may be related to the development of the immobilization phenotype after TANs infiltrate tumors [ 80 ]. Recently, scRNA‐seq associated technologies have emerged as powerful tools to define the emergence of MDSCs and to compare differences between MDSC populations [ 94 , 95 ]. For example, Veglia et al. [ 96 ], using single‐cell RNA‐seq, cell mass spectrometry, flow cytometry, and functional analysis, described three populations of polymorphonuclear neutrophils (PMNs) in tumor‐bearing mice: classical PMNs, PMN‐MDSCs, and activated PMN‐MDSCs with potent immune suppressive activity. Meanwhile, they found that PMN‐MDSCs gradually replaced PMNs during tumor progression and, through trajectory analysis, demonstrated that classic PMNs can differentiate into either PMN‐MDSCs or activated PMN‐MDSCs via different pathways. However, PMN‐MDSCs cannot differentiate into activated PMN‐MDSCs [ 96 ]. Additionally, a recent transcriptomic study evaluated PMNs from healthy individuals and cancer patients, as well as PMN‐MDSCs exclusively from cancer patients. Notably, no tumor samples were included in this study, as these two cell types cannot be distinguished in tissues based on surface marker expression. Instead, PMN/PMN‐MDSCs were isolated from the peripheral blood by density centrifugation (HDN/LDN) and further purified with CD15 bead selection. This study was particularly significant as it allowed for the comparative analysis of two samples from the same patient. The results revealed that PMN‐MDSCs from cancer patients exhibited a more distinct gene expression profile compared to PMNs from the same patient or healthy individuals [ 97 ]. For instance, an elevated endoplasmic reticulum stress response was observed in PMN‐MDSCs, along with high expression of lectin‐like oxidized low‐density lipoprotein receptor‐1 (LOX‐1), a marker that had not previously been extensively studied in neutrophils. The gene expression profiles of LOX‐1 + PMNs closely correlated with those of PMN‐MDSCs and were associated with enhanced T‐cell suppression [ 97 ]. Furthermore, a subsequent study found that LOX‐1 + CD15 +  PMN‐MDSCs were significantly elevated in patients with HCC. These cells demonstrated the capacity to suppress T‐cell proliferation and interferon‐gamma (IFN‐γ) production while exhibiting increased arginase and reactive oxygen species (ROS) levels [ 98 ]. Additionally, LOX‐1 + PMN‐MDSC staining within tumor tissue correlated with their levels in peripheral blood [ 98 ], underscoring the potential relevance of LOX‐1 as a biomarker. However, it remains critical to functionally compare LOX‐1 + and LOX‐1 − TANs within tissue samples to better understand their respective roles. Condamine et al. [ 97 ] proposed that LOX‐1 is not merely associated with the PMN‐MDSC phenotype but is also integral to its function. However, the study did not establish whether LOX‐1 directly contributes to the increased immunosuppressive activity of these cells. While LOX‐1 appears to be a promising candidate, its expression was limited to only one‐third of PMN‐MDSCs and, notably, is absent in murine models, posing challenges for further mechanistic studies in preclinical models. Neutrophils can also be categorized into distinct subpopulations based on their density gradient upon isolation and centrifugation [ 99 , 100 ]. Normal or high‐density neutrophils (HDNs) are typically associated with anti‐tumor activity, while low‐density neutrophils (LDNs) are thought to expand in malignancy and exhibit characteristics of MDSCs, contributing to pro‐tumor effects. Interestingly, similar to TANs, HDNs can transition into LDNs in response to environmental factors [ 101 , 102 , 103 ]. Overall, the MDSC or N1/N2 nomenclature may be overly simplistic and self‐limiting when it comes to neutrophil polarization. Assigning a specific label to a cell or group of cells based on a single function, such as immunity activity, implies that these cells are dedicated to one purpose and incapable of performing other roles. In fact, neutrophils are highly dynamic and adaptable, capable of executing a range of functions—sometimes even opposing ones—simultaneously depending on the inflammatory environment they encounter. Over the past decade, there has been growing interest in the interactions between neutrophils and other myeloid cells within the TME. Hybrid properties of neutrophils have been described in a variety of tumors [ 104 , 105 ]. For example, Singhal et al. [ 104 ] describe a subset of TANs in early‐stage human lung cancer that exhibit hybrid phenotypic and functional characteristics of both neutrophils and antigen‐presenting cells (APCs). These APC‐like hybrid TANs, derived from CD11b + CD15 hi CD10 − CD16 low immature progenitors, outperform classical TANs in their ability to induce and stimulate anti‐tumor T cell responses. They are capable of cross‐presenting antigens amplifying anti‐tumor T cell responses [ 104 ]. Lad et al. [ 105 ] further confirmed the presence of APC‐like hybrid TANs in glioblastoma (GBM), showing that these APC‐like hybrid TANs activate T cells in an MHC II‐dependent manner. Additionally, studies have shown that both immature and mature neutrophils, when cultured with selected cytokines, can revert their functional maturation program and acquire features typical of dendritic cells (DCs) [ 106 ]. These hybrid cells display dual properties of neutrophils and DCs, expressing markers of both neutrophils (Ly6G, CXCR2, and 7/4) and DCs (CD11c, MHC II, CD80, and CD86) [ 106 ]. While retaining the intrinsic functions of neutrophils (such as the ability to engulf exogenous material, extrude neutrophil extracellular traps (NETs), and kill bacteria via cathelicidin production), they also acquire key characteristics typically reserved for DCs, including dendritic morphology, probing motility, podosome formation, and the acquisition of molecular features of DCs [ 107 ]. These molecular changes include neo‐expression of the DC‐associated surface molecules cluster of differentiation 1a (CD1a), CD1b, CD1c, human leukocyte antigen (HLA)‐DR, HLA‐DQ, CD80, CD86, CD40, CD54, and CD5, and downregulation of CD15 and CD65 [ 106 ]. Importantly, these neutrophil‐derived DCs are highly efficient at stimulating T cells—over 10,000 times more efficient than freshly isolated monocytes at presenting soluble antigens to autologous T cells, whereas freshly isolated neutrophils lack this ability [ 108 , 109 ]. Overall, these findings highlight the existence of a specialized TANs subset in human cancer.

TANs have been proposed as promising targets for innovative cancer therapies. Various neutrophil‐based strategies have been explored to treat cancer (Figure  4 ), targeting different stages of neutrophil development, including maturation, activation, release into circulation, recruitment in tumors, and their functional roles [ 231 , 232 ]. Several prospective preclinical and clinical studies are currently evaluating the safety and therapeutic efficacy of neutrophil‐targeted or ‐related cancer therapies (Table  1 ). Neutrophil‐targeted tumor therapy strategies. (A) Inhibition of chemokines and their receptors, such as G‐CSF and CXCR1/2, can effectively suppress the recruitment of neutrophils. (B) Transform tumor‐promoting N2 TANs into anti‐tumor N1 TANs. (C) Inhibition of neutrophil‐derived factors, such as VEGF, can effectively suppress tumor progression. (D) Inhibition of PD‐1/PD‐L1 for reversing the immunosuppressive effect mediated by TANs. Abbreviations: G‐CSF, granulocyte colony‐stimulating factor; TGF‐beta, transforming growth factor beta; VEGF, vascular endothelial growth factor; GM‐CSF, granulocyte‐macrophage colony‐stimulating factor; TNF, tumor necrosis factor; IFN‐gamma, interferon‐gamma; PD‐1, programmed cell death protein‐1; PD‐L1, programmed cell death ligand 1. Overview of clinical trials for neutrophil‐targeted cancer therapies. A high abundance of infiltrating neutrophils is associated with poor tumor prognosis, and the trafficking of neutrophils from the bone marrow into circulation is tightly regulated, particularly by the CXCLs/CXCR2 and CXCLs/CXCR4 signaling pathways [ 233 , 234 , 235 ]. Inhibition of neutrophil accumulation in tumors by blocking the CXCLs/CXCR axis has shown promise as a potent cancer therapy (Figure  4A ) [ 236 , 237 , 238 , 239 ]. In lung cancer and pancreatic cancer models, genetic or pharmacologic inhibition of CXCR2 significantly reduced neutrophil infiltration, suppressed primary tumor growth, and inhibited cancer metastasis [ 240 ]. In head and neck cancer models, inhibition of MDSC trafficking with SX‐682, a CXCR1/2 inhibitor, enhanced NK cell‐based immunotherapy [ 241 ]. In colon cancer and melanoma models, oral administration of SCH‐527123, another small molecule CXCR1/2 antagonist, reduced tumor cell proliferation, metastasis, and angiogenesis, while promoting apoptosis of malignant cells [ 242 , 243 ]. In the phase I clinical trial ( NCT02001974 ), reparixin, a CXCR1/CXCR2 inhibitor, was evaluated for safety and pharmacokinetics in BC patients. When combined with weekly paclitaxel, reparixin was well‐tolerated, showing a 30% response rate in the enrolled patient population [ 244 ]. However, a subsequent randomized placebo‐controlled phase II trial of reparixin ( NCT02370238 ) showed suboptimal clinical efficacy, with the primary purpose of prolonged progress‐free survival (PFS) not met in BC [ 245 ]. CXCR4, another important regulator of neutrophil recruitment, is highly expressed on neutrophils and plays a crucial role in early tumor metastasis events by controlling neutrophil motility. CXCR4‐deficient neutrophils fail to interact with tumor cells, thereby impairing early metastatic events [ 246 , 247 , 248 ]. CXCR4 antagonists, such as TN14003 and AMD3100, have been used in experimental and clinical studies to enhance cancer therapy. TN14003 significantly reduced tumor growth and metastasis in various human xenograft models by inhibiting neutrophil migration in a dose‐dependent manner and has also been explored as a diagnostic tool to identify CXCR4 receptor‐positive tumor cells [ 249 , 250 ]. In a xenograft mouse model of human pancreatic cancer, AMD3100 sensitized cancer cells to docetaxel, and the combination of docetaxel with AMD3100 exhibited a stronger anti‐tumor effect than docetaxel alone [ 251 ]. Ulocuplumab, an anti‐CXCR4 monoclonal antibody, has shown anti‐tumor effects in multiple hematologic malignancy models [ 252 ]. A phase Ib/II trial of ulocuplumab, alone or in combination with lenalidomide or bortezomib plus dexamethasone, in relapsed/refractory multiple myeloma showed that blocking the CXCLs/CXCR4 axis with ulocuplumab was safe and resulted in a high response rate even in heavily pretreated patients, leading to prolonged PFS of over 22 months in this relapsed/refractory population, while 14.9 months in the elotuzumab plus lenalidomide and dexamethasone arm [ 253 ]. Additionally, CCR5 inhibitors have been reported to exert anti‐tumor effects by blocking the release of immature neutrophils from the bone marrow and then blocking the recruitment of these immature cells to tumor sites [ 254 ]. In clinical trial NCT03274804 , the CCR5 inhibitor maraviroc is being investigated for the treatment of metastatic CRC. The combination therapy with pembrolizumab and maraviroc has shown feasibility and beneficial toxicity patterns. Furthermore, improved responses to subsequent salvage chemotherapy were observed [ 255 ]. Another therapeutic strategy is to target the CD47‐SIRPα signaling axis with anti‐CD47 or anti‐SIRPα antibodies. More and more evidences show that blocking this pathway can limit neutrophil migration to tumor site and trigger macrophage‐mediated phagocytosis of tumor cells [ 256 , 257 ]. Hu5F9‐G4 (5F9), a humanized IgG4 antibody targeting CD47, has completed safety, pharmacokinetic and pharmacodynamic evaluation, showing well tolerance. In a phase I trial, 5F9 produced objective responses in patients with solid tumors. However, its optimal therapeutic impact is likely to be achieved in combination with other anticancer therapies [ 258 ]. Ongoing combination studies are evaluating 5F9 combined with rituximab in B‐cell lymphoma, with cetuximab in CRC, with azacitidine in treatment‐naive AML and myelodysplastic syndrome, and with the PD‐L1 checkpoint inhibitor avelumab in ovarian cancer patients ( NCT02953509 , NCT02953782 , NCT03248479 , and NCT03558139 ). Additionally, 5F9 has received Fast Track designation from the FDA for further development in DLBCL and follicular lymphoma [ 258 , 259 ]. In addition, preclinical studies have shown that neutrophil depletion using an anti‐Ly6G approach with selectivity properties may have a therapeutic effect [ 260 ]. However, Ly6G‐based strategies have also been found to deplete other immune cells, such as monocytes and subsets of CD8 + T cells, which limits the selectivity of the treatment and may delay pathogen clearance [ 261 ]. Another fundamental strategy in neutrophil‐targeted therapy is promoting the differentiation of neutrophils toward the N1 phenotype while inhibiting the differentiation toward the N2 phenotype (Figure  4B ). TGF‐β is a key modulator of neutrophil polarization. Inhibition of TGFβ signaling has been shown to induce antitumor manifestations in TANs [ 76 , 262 ]. Several small‐molecule inhibitors targeting the TGF‐β signaling pathway have been developed [ 263 , 264 ]. NIS793, a pioneering human anti‐TGF‐β monoclonal antibody, has demonstrated the capacity to block the TGF‐β pathway in tumor cells and modulate the TME, thereby reversing immunosuppression. Currently, NIS793 is undergoing a series of preclinical trials worldwide, with indications including PAAD, CRC, and lung cancer [ 265 ]. Fresolimumab, an antibody capable of neutralizing all human isoforms of TGF‐β, has shown an acceptable safety profile and preliminary evidence of anti‐tumor activity in various cancer patients in phase I/II trials [ 266 , 267 ]. Galunisertib, a small molecule inhibitor of the TGF‐β receptor, has demonstrated antitumor activity and the ability to resensitize drug‐tolerant cells to anticancer therapeutics in animal models [ 268 , 269 ]. It has been proposed as a strategy to enhance the efficacy of immune checkpoint inhibitors and is being tested in clinical trials both as a monotherapy and in combination with other treatments [ 270 , 271 ]. Galunisertib combinations with chemotherapy or immunotherapy are being evaluated and have shown acceptable safety, as well as prolonged OS outcome in various cancer types, such as GBM ( NCT01582269 and NCT01220271 ), pancreatic cancer ( NCT01373164 ), and HCC ( NCT01246986 and NCT02423343 ) [ 272 , 273 , 274 ]. Furthermore, the complex myeloid plasticity and hybrid properties of neutrophils have also been reported to influence therapeutic strategies and clinical outcomes [ 105 , 275 ], and these unique neutrophil hybrid populations provide a potential therapeutic target. For example, Ye et al. [ 275 ] identified a distinct cluster of cancer cells in PCa bone metastases that expressed myeloid cell markers and exhibited enhanced proliferative rate and metastatic capacity. In an in vitro study, they compared the docetaxel sensitivity of myeloid‐like tumor hybrid cells and parental RM1 cells. Docetaxel is the primary chemotherapeutic agent used for patients with metastatic castration‐resistant PCa [ 276 ]. Results indicate that the dose‐response curve showed that hybrid cells were less sensitive to docetaxel than parental RM1 cells. To further evaluate the docetaxel sensitivity of hybrid cells in vivo, tumor‐bearing mice were treated with docetaxel following subcutaneous injection of tumor cells. Consistent with the in vitro findings, docetaxel treatment significantly inhibited tumor growth in the RM1 group, whereas no such inhibition was observed in the hybrid cell group [ 275 ]. These results suggest that myeloid‐like tumor hybrid cells exhibit reduced sensitivity to docetaxel in vivo, implicating cell fusion as a potential mechanism underlying drug resistance in bone metastases. Interestingly, compared with parental RM1 cells, hybrid cells demonstrated greater inhibition of cell proliferation and colony formation after radiotherapy [ 275 ]. Collectively, these findings indicate that spontaneous cell fusion can generate myeloid‐like tumor hybrid cells that contribute to the progression of bone metastasis. Given their unique characteristics, these hybrid cells may serve as a novel therapeutic target for tumor metastasis. As previously mentioned, ARG‐1 produced by neutrophils suppresses T cell proliferation. In a preclinical mouse model, depletion of ARG‐1 by treatment with arginase inhibitors reversed this immune suppression [ 185 , 277 ]. Bioengineered arginase also exhibits anti‐tumor effects by inducing cell cycle arrest and apoptosis in hepatocellular and pancreatic carcinoma models. The bioengineered human ARG‐1 showed additive anti‐tumor effects when combined with anti‐PD‐1 or anti‐PD‐L1 in a preclinical model [ 278 , 279 , 280 ]. The combination of arginase inhibitors or engineered arginase with immunotherapies is currently being tested in clinical and has shown synergistic therapeutic efficacy [ 281 , 282 , 283 ]. Another targetable feature that contributes to immunosuppressive activity is considered the anaphylatoxin C5a receptor (C5aR). Anaphylatoxin C5a, a potent immune mediator generated after complement activation, induces endothelial cell chemotaxis and angiogenesis, and also contributes to the immunosuppressive microenvironment required for tumor growth [ 284 ]. Increased expression of C5aR is found on neutrophils. Pharmacological blockade of C5a‐receptor interactions has shown promising results in many preclinical studies [ 285 , 286 ]. The anti‐C5aR antibody, IPH5401, in combination with PD‐L1 blockade has been tested in Phase I clinical trials for the treatment of selected advanced solid tumors ( NCT03665129 ). In addition, neutralization of neutrophil‐derived angiogenic factors, including VEGF‐A, Bv8, and S100A4, with antibodies has been shown to inhibit angiogenesis and reduce resistance to anti‐VEGF therapy (Figure  4C ) [ 287 , 288 , 289 ]. Cancer immunotherapy, particularly targeting the crosstalk of PD‐1/PD‐L1, has emerged as one of the most clinically effective strategies for cancer treatment. It has also proven effective in targeting neutrophils [ 290 ]. PD‐L1 + neutrophils have been identified in various tumors, including HCC, squamous cell carcinoma, and gastric carcinoma, and have been shown to possess prognostic significance [ 226 , 291 , 292 ]. Soluble factors secreted by the TME, such as IFN‐γ, TNF‐α, and GM‐CSF, can induce the expression of PD‐L1 on neutrophils through multiple pathways, including the JAK/STAT3 and IL6/STAT3 pathway [ 292 ]. PD‐L1 + neutrophils have been shown to suppress adaptive immunity by inhibiting T cell function and promote tumor development (Figure  4D ) [ 291 , 292 ]. In addition to affecting adaptive immune cells, PD‐L1 + neutrophils interact with PD‐1 + NK cells and dampen their antitumor immune activity, which can be restored by PD‐L1 inhibitors [ 293 ]. In pre‐clinical mouse models of pancreatic ductal adenocarcinoma, suppression of neutrophils by lorlatinib was observed to attenuate tumor growth and enhance the efficacy of immune checkpoint blockade therapy [ 294 ]. Recently, cell‐mediated drug delivery technologies have garnered significant attention due to their potent therapeutic specificity and efficacy, particularly neutrophil‐mediated anti‐tumor drug delivery systems [ 295 , 296 ]. Neutrophils are the most abundant cells in circulation and have the ability to penetrate both the blood‐brain barrier (BBB) and the blood‐brain tumor barrier (BBTB) [ 297 ]. GBM is characterized by a high mortality rate, short life span, and high tendency to relapse. One key factor contributing to poor prognosis is the presence of BBB and BBTB, which prevents the delivery of chemotherapy drugs, such as paclitaxel, thereby limiting their anti‐cancer effects [ 298 , 299 ]. In neutrophil‐based delivery vehicles, paclitaxel is encapsulated in cationic liposomes and internalized by neutrophils, which can effectively deliver PTX to the brain. This neutrophil‐mediated delivery system efficiently slows the recurrent growth of tumors and significantly improves survival rates [ 297 ]. In patients with melanoma lung metastasis, a neutrophil‐mediated tumor‐targeting delivery system, utilizing oncolytic bacteria combined with ICB, has also shown promising results [ 300 ]. Moreover, Zhu et al. [ 301 ] designed a glucose oxidase‐loaded bovine serum albumin nanoparticle, which will be specifically delivered to ectopic lesions in a neutrophil‐dependent manner. This system depletes glucose and induces apoptosis in the ectopic lesions, showing excellent anti‐endometriosis effects when administered in inflammatory phases.