More friend than foe: the emerging role of neutrophils in tissue repair (2025)

In recent years it has become apparent that resolution of inflammation is an active process in which neutrophils play a seminal role. In most injuries, which are usually traumatic in nature, there is resolution involving controlled neutrophil activation and deactivation, while inflammation that involves an “unnatural stimulus,” e.g., high-fat diets, alcohol, and drugs like acetaminophen or bleomycin, will lead to inadvertent and perpetual neutrophil activation and injury. In addition, ischemia/reperfusion associated with transplantation, or even MI and strokes, may also cause neutrophils to injure self. However, we would argue that as we have not evolved molecular mechanisms to deal with these perturbations, neutrophils have difficulty discerning and responding appropriately to these insults, resulting in their induction of maladaptive effector functions. Conversely, in trauma and other injuries, the neutrophil evolved a proper response to ensure survival, and absence of neutrophils might delay proper healing.

Neutrophils contribute to tissue repair via multiple mechanisms, and the response is often time-dependent (Figure 2 and ref. 45). As professional phagocytes, neutrophils are involved in clearing necrotic tissue and cellular debris. It is essential for tissue repair that cellular remnants are removed to prevent persistent proinflammatory signaling. It is unclear whether neutrophils also remove injured cells, but this would explain the many studies that show decreased area of injury if neutrophils are depleted before experimentation. The majority of neutrophil-depleting studies address acute injury without measuring repair that occurs days or weeks later, leaving the impression that neutrophils simply induce more injury. However, neutrophils contribute to resolution of inflammation and repair by releasing a plethora of presynthesized mediators such as growth factors and proangiogenic factors (63). A case in point is MMP-9, which is produced by neutrophils in great abundance and is capable of degrading DAMPs such as HMGB1 and HSP90, thus dampening the recruitment of additional inflammatory cells (64).

Angiogenesis is essential for healing, and new vasculature delivers oxygen and nutrients that facilitate tissue regrowth (65). Growing evidence suggests that neutrophils promote angiogenesis following injury to help in repair. Indeed, a subset of proangiogenic CXCR4^hiVEGFR⁺CD49d⁺ neutrophils was recently described in humans and mice (66, 67). In a model of avascular pancreatic islet transplantation, the neutrophils recruited by VEGF-A expression in pancreatic islet cells expressed high levels of MMP-9 (67), a potent activator of VEGF activity. MMP-9–deficient mice showed impaired revascularization (67). In a model of toxic corneal injury, neutrophils invading the cornea expressed high levels of VEGF (68). Neutrophil depletion inhibited corneal angiogenesis and reduced protein levels of VEGF and the proinflammatory cytokines MIP-1α and MIP-2. It has been proposed that neutrophils make the tunnels or sleeves that allow for revascularization into tissue in injured organs (69). Notably, the proangiogenic role of neutrophils extends further to other physiologic and pathophysiologic settings. A recent study identified a role for neutrophils in maintaining normal pregnancy and placental development by inducing proangiogenic T cells (70). Proangiogenic neutrophils were shown to promote tumor growth, perhaps through angiogenesis (71).

Intriguingly, all the toxic substances made by neutrophils, including NETs, oxidants, and proteases, may serve important healing functions. For example, neutrophil-mediated resolution was recently described in gout, a neutrophil-predominant joint inflammation. Surprisingly, this study showed that aggregation of NETs promoted resolution of inflammation by degrading cytokines and chemokines, thus disrupting the recruitment of additional inflammatory cells (ref. 72 and Figure 2). NETosis-deficient mice showed increased chronic inflammation, and adoptive transfer of in vitro aggregated NETs reduced this phenotype by degrading proinflammatory cytokines through NET-bound proteases (72). The idea of antiinflammatory NET functions warrants further validation, but is an intriguing concept. Much like NETs, oxidants can also have dual functions, and while their inflammatory properties are well documented, ROS can also suppress inflammation independent of their role in NETosis (52). In fact, even proteases have been shown to be both potently inflammatory and to play key roles in revascularization and healing (69).

Some investigators have attempted to explain the dichotomy of beneficial and detrimental roles of neutrophils with different neutrophil subsets, but the existence of neutrophil subsets and whether they reflect differential adaptation to local environments are unclear. Indeed, it was recently postulated that neutrophils can obtain different polarization states in the tumor microenvironment, leading to the terms “N1” (antitumorigenic) and “N2” (protumorigenic) (73), analogous to the M1/M2 concept in macrophages. Transition between N1 and N2 neutrophils was TGF-β–mediated, and transcriptomic analysis confirmed N1 and N2 neutrophils to be separate populations (74). Temporal neutrophil polarization was also recently shown following MI, with N2 neutrophils increasing during the healing phase (75). These studies discuss the possible existence of specialized repair neutrophils; however, lineage-tracing studies are needed to clarify the existence of bona fide neutrophil subsets (76).

It is quite intriguing that neutrophils in all organs have evolved to provide healing properties despite vast differences in architecture and specific tissue cell types. Nevertheless, below, we summarize the role of neutrophils as repair cells in various organs (Figure 3).

Neutrophils in lung repair. Early evidence indicated that neutrophil α-defensins induce lung epithelial cell proliferation in vitro (77), and a recent study by Blázquez-Prieto et al. identified a role for neutrophils in lung repair via MMP-9 activity (78). The authors noted increased lung damage but reduced levels of MMP-9 in neutrophil-depleted mice with ventilator-associated lung injury. Similar findings were made in the bronchoalveolar lavage (BAL) fluid of neutropenic patients. Therapeutic administration of MMP-9 significantly reduced tissue damage in acute lung injury (78). NADPH oxidase, a membrane-bound enzyme complex associated with intracellular membranes of phagosomes, known for its antimicrobial function through the production of ROS (79, 80), was recently discovered to also contribute to the attenuation of lung inflammation (81). NADPH oxidase–deficient mice developed progressive inflammation, augmented NF-κB activation, and elevated proinflammatory cytokines upon intratracheal zymosan or LPS challenge (81). In a subsequent study, the authors showed that NADPH oxidase deficiency led to increased neutrophil recruitment to the lungs following injury and that the NADPH oxidase’s protective effect required activation of nuclear factor erythroid 2–related factor 2 (Nrf2), a redox-sensitive antiinflammatory transcription factor (82). Zemans et al. recently demonstrated that neutrophil transmigration across lung epithelial cells, in vivo in mice and in vitro using human neutrophils, triggered repair of lung epithelium via β-catenin signaling (83). Mice treated with intratracheal LPS showed activation of β-catenin signaling in type II pneumocytes, mediated by elastase-induced cleavage of E-cadherin, thus promoting epithelial repair (83). Further elaborating on these findings, Paris et al. demonstrated decreased re-epithelialization and increased alveolar protein concentration in neutrophil-depleted or G-CSF–deficient mice in a model of acute lung injury mimicking ARDS (84). Unbiased proteomic analysis of BAL fluid revealed differential expression of MMP-2, MMP-9, and Fgf1, indicating that neutrophils could positively alter pathways related to epithelial regeneration (84). Neudecker et al. recently demonstrated another mechanism by which neutrophils promote tissue repair, elegantly showing that intercellular transfer of microRNAs (specifically miRNA-223) from neutrophils to pulmonary epithelial cells attenuated lung damage by repressing PARP-1 (85).

Neutrophils in central and peripheral nervous system repair. Regeneration after injury to the peripheral or central nervous system requires the immune system (86). In a model of spinal cord injury (SCI), Stirling et al. demonstrated a beneficial role for neutrophils (87) wherein neutrophil depletion delayed recovery, impaired wound healing, and reduced astrocyte reactivity (87). Moreover, secretory leukocyte protease inhibitor (SLPI), which is secreted by neutrophils and astrocytes in the spinal cord, was shown to be beneficial in SCI (88). Similarly, in a model of optic nerve injury, neutrophils produced high levels of the atypical growth factor oncomodulin, thereby contributing to optic nerve repair (89). Neutrophil depletion or an oncomodulin antagonist prevented optic nerve regeneration (89). It was recently shown that neutrophils play a crucial part in removal of debris following peripheral nerve injury (90). Neutrophil depletion significantly impaired clearance of nerve debris, demonstrating an important role for neutrophil-mediated phagocytosis as an initiating step of tissue repair (90).

Neutrophils in cutaneous wound healing. Neutrophils also play a crucial role in cutaneous wound repair, and neutropenic patients often suffer from impaired wound healing (44). Neutrophils are the first immune cells to arrive at cutaneous wounds in an effort to sterilize the injury (44). Liu et al. recently demonstrated a critical role for neutrophils in wound repair, as FPR1/2-deficient mice showed reduced neutrophil recruitment to wounds, which exhibited delayed healing (91). In a previous study, neutrophil depletion delayed wound healing in older mice but not young mice, an effect that was reversed in older mice upon G-CSF injections (92). In another study, MMP-8–deficient mice displayed delayed healing and reduced neutrophil infiltration at early stages but persistent inflammation at later stages (93). Mechanistically, TGF-β signaling and neutrophil apoptosis were impaired in MMP-8–deficient mice (93). Moreover, human neutrophils that migrated to skin wounds showed transcriptional differences and upregulated levels of cytokines and chemokines that promote angiogenesis and keratinocyte and fibroblast proliferation compared with circulating neutrophils (94). This could perhaps be related to the transmigration into a new environment. Cutaneous neutrophils also produce SLPI, and SLPI-deficient mice showed increased levels of inflammation and elastase as well as delayed wound healing (95). Chronic wounds have also been associated with increased protease levels, reduced neutrophil apoptosis, and neutrophil persistence, indicating that signals from the wound environment likely contribute to the fate of neutrophils (44). Interestingly, in diabetes, neutrophils were primed to undergo NETosis impairing normal wound healing in a high-glucose environment (96).

Neutrophils in cardiovascular repair. MI is accompanied by a pronounced neutrophilic infiltration, and the prevailing view has been that neutrophils augment cardiac damage (97). However, an elegant study recently demonstrated that neutrophil depletion led to impaired cardiac function, increased fibrosis, and progressive heart failure following MI (98). Mechanistically, the authors found altered macrophage polarization states in neutrophil-depleted mice. Furthermore, neutrophil gelatinase–associated lipocalin (NGAL) seemed to increase the capacity of cardiac macrophages to engulf apoptotic cells, and treatment with recombinant NGAL was able to restore this phenotype in neutrophil-depleted mice (98). A previous study identified that the cytokine oncostatin M (OSM), produced by neutrophils and macrophages, induced dedifferentiating cardiomyocytes to release regenerating islet-derived protein 3β (REG3β), which then modulated the degree of macrophage accumulation in the heart to fine-tune wound healing (99). Furthermore, neutrophil-borne cathelicidin (mouse CRAMP, human LL-37) was recently discovered to mediate arterial healing by promoting re-endothelialization after acute injury in mice and patients (100).

Neutrophils in bone fracture healing. The role of neutrophils in bone fracture healing is well recognized (101–103). Fracture healing is characterized by an inflammatory phase, a repair phase, and ultimately a remodeling phase (104). The initial inflammatory response starts with a fracture hematoma that serves as a scaffold for immune cells and progenitor cells (104). Neutrophils are found in abundance in the fracture hematoma, arriving within minutes after fracture, and comprise both mature neutrophils from the circulation and immature neutrophils from bone marrow (102). G-CSF applied locally to fractures was associated with a significant improvement in healing and angiogenesis (105). In a mouse model of bone fractures, the authors noted that depletion of neutrophils led to impaired healing after fracture (106). Furthermore, they observed increased levels of inflammatory cytokines as well as altered monocyte and macrophage recruitment, indicating a crucial role for neutrophils in initiating the repair process. Along those lines, Bastian et al. recently discovered neutrophils in the fracture hematoma of human patients (107). These neutrophils costained with fibronectin, and hence this group proposed a mechanism of “emergency ECM production” by infiltrating neutrophils promoting early fracture healing (107).

Neutrophils in liver repair. The liver is an organ with a remarkable regenerative capacity and thus is often used to scrutinize tissue repair experimentally. Numerous studies suggest neutrophil involvement in the pathogenesis of acute and chronic liver diseases (108–110). A fully healing model of thermal liver injury demonstrated that neutrophils, guided by an intravascular chemokine gradient, were the first cells to arrive at the injury site, where they infiltrated the necrotic area (ref. 31 and Figure 1). Interestingly, neutrophil depletion resulted in increased cellular debris, delayed revascularization, and ultimately delayed healing (69). Intravital imaging revealed that neutrophils contributed to healing by phagocytosing debris and promoting vascular regrowth in part by making new sleeves for blood vessels to grow into (69). Along the same lines, a recent study identified a dual role for neutrophils in acetaminophen-induced acute liver injury, with neutrophil-mediated injury amplification early on, but protective effects during the repair phase, as depletion increased liver damage (111). The early phase may reflect killing and/or removal of injured cells that presents as increased injury but is required for repair. A recent study by Saijou et al. (112) investigated the role of neutrophils in chronic liver injury. Using a mouse deficient in tribbles pseudokinase (Trib1), which increases neutrophil counts, the authors found reduced liver fibrosis, increased intrahepatic neutrophils, and increased MMP-8 and MMP-9 levels. Neutrophil infusion diminished fibrosis, while neutrophil depletion increased fibrosis (112). As a result, the authors concluded that neutrophils suppressed fibrosis in chronic liver injury potentially by promoting fibrolysis through the production of metalloproteases (112). Along the same lines, in chronic cholestatic liver injury, neutrophil depletion decreased MMP-8 levels and decreased collagen degradation (113). Clearly, in liver, neutrophils are very important in regulating regeneration versus fibrosis.

All in all, it is evident that regardless of the tissue, neutrophils contribute to healing. Some of the mechanisms, such as MMP delivery, appear to be nonspecific and occur in all organs, while in certain situations, including healing of the optic nerve, or regrowth of lung epithelial cells, there are clear specific effector functions.