ABSTRACT

INTRODUCTION

Neutrophils play a crucial role in host defense in urinary tract infection and bacterial pyelonephritis by degrading virulence factors and identifying and eradicating pathogens, but on the other hand contribute to tissue damage in several non-infectious forms of kidney disease, e.g. ischemic injury and glomerulonephritis. Their migration from the blood into the injured tissue occurs in a tightly regulated cascade via adhesion molecules and circulating factors such as cytokines and chemokines. Once recruited to the site of tissue injury, neutrophils contribute to inflammation by releasing pro-inflammatory mediators such as reactive oxygen species (ROS) and neutrophil extracellular traps (NETs), and subsequently can promote further organ damage. While there is no doubt that the antimicrobial effector functions of neutrophils are lifesaving, e.g. in the context of immediate host defense, it has become evident that neutrophils can act as a key player in numerous inflammatory disease contexts including kidney disease.

Neutrophils are classically viewed as short-lived, terminally differentiated phagocytic cells with a rather unspecific mode of action. It was thought that neutrophils originate from hematopoietic stem cells through common bone marrow–derived progenitors, are pro-inflammatory immune cells and have an inability to proliferate, with unidirectional trafficking. This classical concept has been challenged by emerging evidence over recent years describing neutrophils as heterogenous population [1] that originate not only from the bone marrow but also the spleen, with the ability to rapidly adapt to the changing environment [2], to reversely migrate into other tissues [3], to communicate with other cell types and to promote even tissue repair [4]. Recent work revealed that whereas neutrophils indeed exhibit a short half-life in the circulation and some tissues, the life span differs between tissues, suggesting an adaptation of neutrophils to the microenvironment [5].

This review highlights the thus far underestimated phenotypic heterogeneity and functional diversity of neutrophils. We provide mechanistic evidence for their emerging role as effector cells during homeostasis but also during infectious and non-infectious diseases with a particular emphasis on kidney disease. Finally, we discuss how these new insights can be used to target neutrophils therapeutically.

NEUTROPHIL ORIGIN, DIFFERENTIATION AND AGING

Neutrophils are thought to originate from hematopoietic stem cells through common myeloid progenitors, granulo-monocytic progenitors and recently identified unipotent neutrophil progenitors (pro-neutrophils 1 and pro-neutrophils 2) [6, 7]. In recent decades, the classical concept that neutrophils are a homogenous terminally differentiated population has been challenged. Recent evidence points toward a phenotypic and functional heterogeneity of neutrophils under homeostatic and pathogenic conditions [8]. Neutrophils are continuously generated in the bone marrow. In humans, neutrophils mature in the bone marrow from committed myeloid precursors that, through subsequent differentiation stages (here defined as “immature neutrophils”), differentiate into segmented mature neutrophils (Fig. 1) [9]. In mice, three neutrophil subsets have been identified within the bone marrow: a committed proliferative neutrophil precursor (pre-neutrophils), which differentiates into non-proliferating immature neutrophils and mature neutrophils under the control of transcription factors [mainly PU.1 and CCAAT/enhancer-binding protein (C/EBP)α–ζ] [7, 10].

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Figure 1:

Neutrophil populations in homeostatic conditions. In mice, neutrophils develop from myeloid precursors within the bone marrow and are comprised of three distinct subpopulations, namely pre-neutrophil (proliferative neutrophils precursor), immature and segmented mature neutrophils. Under resting conditions, only terminally differentiated neutrophils are released into the blood and recovered as normal-density neutrophils (NDN). Evidence suggests that different neutrophil subpopulations, defined by the indicated markers, are present in the blood under homeostatic conditions. Neutrophils reside in pools within the lung vasculature, spleen and liver, but also transit and reside in other organs such as muscle, skin, lymph nodes and intestine. In the mouse spleen, three neutrophil subpopulations localized in the red pulp or marginal zone (MZ) participate in emergency granulopoiesis (Ly6G^int) and pneumococcal clearance (Ly6G^high), and can regulate antibody production and activation in MZ B cells (termed as B cell helper neutrophils, NB_H). In humans, neutrophils mature in the bone marrow and subsequently differentiate from immature into mature neutrophils. As observed in mice, based on differential surface marker expression, circulating human neutrophils exhibit heterogeneity that associates with distinct effector activities. Under homeostatic conditions, these mature neutrophils can marginate in tissues including spleen (as N_BH), lung and liver, but the phenotypes and functions of these cells remain poorly defined. This figure was generated with BioRender software. OLFM4, glycoprotein olfactomedin 4; TCRαβ, T cell receptor αβ; VEGFR1, vascular endothelial growth factor receptor 1.

The pool of neutrophils is maintained by a fine balance between granulopoiesis, neutrophil release from the bone marrow into the circulation and ultimately their return/homing to the bone marrow for final clearance by resident macrophages. Humans and mice differ in their numbers of circulating neutrophils. In humans, 50%–70% of circulating leukocytes are neutrophils, whereas only 10%–25% are in mice. Evidence suggests that different neutrophil subpopulations exist in the human and mouse blood depending on the course of their lifespan (approximately 5.4 days in humans and 12.5 h in mice). For example, 45%–65% of circulating human neutrophils are CD177⁺ and 20%–25% of neutrophils express the glycoprotein olfactomedin 4 (OLFM4) [8]. Moreover, as shown by flow cytometry analysis performed at different times of the day, the proportion of CXCR4⁻CD62L⁺ young and CXCR4⁺CD62L⁻ aged neutrophil populations among circulating neutrophils from healthy individuals has been shown to be regulated by circadian oscillations [11].

Healthy tissues were traditionally considered to be devoid of neutrophils in the absence of inflammatory insults. However, under physiological conditions, neutrophils can be found in the lung, liver and spleen [8], but the phenotypes and functions of these cells remain poorly defined. For example, a human splenic neutrophil population that resides within the splenic marginal zone has been identified as B-cell helper neutrophils (NB_H cells) with an ability to promote B-cell proliferation and antibody production [12]. Further research is needed to gain a better understanding of the phenotypic and functional features of neutrophil subpopulations in the circulation and tissues of healthy individuals.

The phenotypic and functional diversity of neutrophils change and progressively deteriorate with increased life span and aging (reviewed in [13, 14]). While the number of neutropihls remain unchanged with increasing age, neutrophils in the elderly tend to exhibit dysfunctional phagocytic and migratory abilities [15]. Inaccurate migration of aged neutrophils via increased activity of phosphorylated phosphatidylinositol 3-kinase can even cause damage to normal tissues [16]. The reduced phagocytic and bactericidal activity of neutrophils in the elderly is associated with increased intracellular calcium concentrations and reduced hexose uptake [15]. Moreover, aged neutrophils are less able to form NETs and to produce ROS. Thus, age-related changes in neutrophils are associated with immunosenescence, inflammaging (chronic inflammation), gut dysbiosis, increased susceptibility to infection and impaired vaccine response in older adults [17], and represent risk factors for several pathologies including cancer, cardiovascular disease (CVD) and kidney disease. Aging and chronic kidney disease (CKD) are interconnected as kidney function declines with age and kidney disease is more common in the elderly. Both share similar features such as cellular senescence, oxidative stress, DNA damage, mitochondrial dysfunction, telomere shortening and Wnt/β-catenin signaling (reviewed in [18]) that contribute to age-related neutrophil dysfunction.

NEUTROPHIL MIGRATION FROM THE CIRCULATION INTO TISSUES AND VICE VERSA

Neutrophil recruitment from the circulation into the infected or injured tissue during an inflammatory response follows a well-defined cascade of adhesion and activation events including rolling, slow rolling, adhesion, adhesion strengthening, crawling and transmigration (Fig. 2). This cascade is tightly regulated by a broad range of receptors and their respective ligands, including selectins, β₂ integrins and chemokine receptors (reviewed in [19]). Efficient neutrophil recruitment is of the utmost importance, since inadequate recruitment results in either impaired host response that might lead to recurrent and life-threatening infections or to hyperinflammation and subsequent tissue damage [20]. Although leukocyte rolling and adhesion has been known since the 19th century, the involved signaling pathways are still not fully understood.

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Figure 2:

Neutrophil recruitment cascade from the vasculature to the tissue. Tethering and slow rolling are selectin-dependent, while β₂ integrin interactions mediate slow rolling, adhesion strengthening, crawling and transmigration. Following rolling, neutrophils are activated by chemokines lining the luminal part of the endothelium. Neutrophil activation induces conformational changes in β₂ surface integrins. Following a chemokine gradient, neutrophils crawl along the endothelium, guiding them to preferential sites of transmigration such as inflamed tissues either through a paracellular (between endothelial cells) or transcellular (through endothelial cells) mechanism. Activated neutrophils have several different fates mediated by factors such as chemokines, cytokines, ROS, MVs, and lipid mediators. These neutrophil fates include: degranulation, where neutrophil granules release antibacterial proteins such as defensins, lactoferrin, cathepsins and lysozyme, into either phagosomes or the extracellular space to kill pathogens; phagocytosis, where neutrophils engulf microorganisms into phagosomes to kill the pathogens using granules or various other mechanisms such as NADPH oxygenase-dependent mechanisms including ROS; and NET formation, which can eliminate extracellular microorganisms via PAD4 and the release of DNA, NE, MPO, histones and S1000A8/A9 proteins to immobilize pathogens and thereby to prevent spreading and facilitating phagocytosis of trapped microorganisms or direct killing of pathogens via antimicrobial histones and proteases. Neutrophils also die via apoptosis and are recognized by other phagocytes including macrophages by “eat-me signals” such as phosphatidylserine on their plasma membrane. Extravasated neutrophils can also migrate from the inflamed tissue back into circulation, a mechanisms called reverse transmigration [156]. This figure was generated with BioRender software. NADPH, nicotinamide adenine dinucleotide phosphate; MMP, matrix metalloproteinase; MVs, microvesicles; NE, neutrophil elastase.

Neutrophil recruitment highly depends on the activation of β₂ integrins [lymphocyte function-associated antigen 1 (LFA-1, CD18/CD11a), macrophage-1 antigen (MAC-1, CD18/CD11b)] during rolling and adhesion induced by selectin-, chemokine- and toll-like receptor (TLR)-mediated signaling [21–25]. Regulation of β₂ integrins and subsequent linkage to the actin cytoskeleton occurs via their intracellular domains supported by various adapter molecules including talin 1 and kindlin3 [26], but also new regulators have been identified recently, among them coronin-1A, hematopoietic progenitor kinase 1, integrin-linked kinase and the ion channel K_V1.3 [27–30]. Furthermore, new molecular players such as myosin-9, mammalian STE20-like protein kinase 1 and unconventional myosin-IXb have been identified to be important during neutrophil migration, transmigration and interstitial migration, respectively [31–33]. The central role of src family kinases (SFKs) in neutrophil recruitment and migration has been known for years [34, 35]. In a recent multidimensional study, neutrophils were screened for their migratory behavior and linked to their biochemical properties in order to identify non-pathogenic and pathogenic neutrophil states. Here, the SFK member tyrosine-protein kinase Fgr was identified as a central regulator of the pathogenic state [36], further highlighting a role of SFKs in neutrophils. Neutrophil recruitment also depends on tissue-specific recruitment cues [2]. In the kidney, neutrophil recruitment within the glomerulus occurs without neutrophil rolling, but is dependent on selectins and β₂ integrins [37], whereas “classical” selectin-dependent rolling and downstream integrin activation occur in the kidney cortex [38, 39].

It is thought that most neutrophils die in the injured and inflamed tissue, and are cleared by macrophages. Experimental evidence now suggests that migrated neutrophils can re-enter the vasculature through downregulation of the junctional adhesion protein JAM-C during inflammation [40], a process referred to as reverse neutrophil migration (Fig. 2) [3, 41, 42]. So far, reverse-migrating neutrophils (1%–2% of peripheral blood neutrophils) have been reported in patients with rheumatoid arthritis (RA) [43]. Notably, these reverse-migrated neutrophils were more resistant to apoptosis [43]. Explanations for this phenomenon of reverse neutrophil migration could be that it preserves neutrophils when they are not needed for host defense, or neutrophils re-entering the circulation could disseminate systemic inflammation or even chronic inflammation in other organs and contribute to tissue damage. Support for the contribution of reverse neutrophil transmigration to organ damage comes from recent experimental evidence [44, 45].

NEUTROPHIL CROSSTALK WITH PLATELETS AND OTHER IMMUNE AND NON-IMMUNE CELL SUBSETS

Although neutrophil interactions are crucial during the inflammatory response, additionally, neutrophils contribute to their own homeostasis through their communication with niche cells in the bone marrow. The effector functions of neutrophils in several inflammatory diseases occur through their mutual interactions with other cell types, either by physical contact or through the release of cytokines, chemokines or granule proteins. Neutrophil communication with endothelial cells has been widely studied in the context of CVDs, where they increase endothelial permeability, contributing to the recruitment of other immune cells such as monocytes. Furthermore, the communication of neutrophils with endothelial cells in inflammatory conditions is usually concomitant with the interactions of further cell types. For example, in CVD, the communication between platelets and neutrophils is crucial to the combination of inflammation and thrombosis. Similarly, in acute kidney injury (AKI) and CKD, the interactions between neutrophils and platelets facilitate thrombosis through the formation of NETs. These NETs create a platform that contributes to platelet activation, which in turn accumulates and favors thrombosis, e.g. cholesterol crystal embolism [46]. However, this interaction is not always detrimental and its outcome depends on the tissue microenvironment, the nature of the disease and the inflammatory stimuli. In the case of bacterial pneumonia, the depletion of platelets promoted the accumulation of neutrophils in the lung, aggravating lung injury [47]. Hence, the fine tuning of these interactions is essential to avoid the perpetuation of inflammation or cardiovascular damage. Neutrophils remarkably impact the pathogenicity of sickle cell disease through their promiscuous interactions with red blood cells, platelets and endothelial cells, which initiate vaso-occlusion, through regulating adhesion molecules [48].

Neutrophils interact closely with monocytes and macrophages, starting in the bone marrow where macrophages influence granulopoiesis via granulocyte colony-stimulating factor release [49] and modulate neutrophil retention by CXCL12/CXCR4 [50] or by direct interaction [51] (Fig. 3). Macrophages induce neutrophil recruitment to the site of inflammation through the secretion of chemoattractants such as CXCL1/CXCL2 and MCP-1 [52, 53]. Apoptotic neutrophils are cleared by macrophages promoting the production of interleukin-4 (IL-4), which in turn reduces neutrophil trafficking, activation and effector functions [54]. Conversely, neutrophils promote monocyte and macrophage recruitment by secreting LL37 and azurocidin [55]. The release of azurocidin and lactoferrin supports a proinflammatory (M1-like) macrophage phenotype, while neutrophil-generated ROS and gelatinase-associated lipocalin triggers a reparative (M2-like) macrophage phenotype [56, 57]. Enhanced NETosis and granule proteins further enhance the phagocytosis of pathogens by macrophages [58, 59].

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Figure 3:

Crosstalk of neutrophils with other cells. Platelets: neutrophil interactions with platelets occurs by direct adhesion through PSGL-1, integrin α_Lβ₂, CD40 on neutrophils and P-selectin, CD40L, ICAM-2 and CD40L on platelets. The release of C5a, C3a, HMGB1 and S100A8/A9 proteins by activated neutrophils promotes megakaryocyte proliferation and platelet production. Platelet secretion of extracellular MVs containing chemokines and the release of chemokines from platelet granules activate neutrophils and promote NET formation. Vice versa, NET proteins can activate the coagulation cascade in platelets. Endothelial cell: L-selectin mediates neutrophil activation of endothelial cells with E-selectin, P-selectin, Tissue Factor and ICAM-1 via MMP, C5a, S100A8/A9 and ROS. Endothelial cells recruit activated neutrophils via Integrin αMβ2 after undergoing NET release–related damage. Monocyte/macrophage: monocytes can extend the lifespan of neutrophils and promote neutrophil recruitment through the secretion of GM-CSF, TNF-α and G-CSF. Recruitment of monocytes is mediated by granule proteins (PR3, LL-37, CTSG, azurocidin and HNP1–3) released from neutrophils. Lactoferrin, azurocidin, S100A8/A9, HNP1–3, LL-37 and NETs induce M1-like macrophage polarization, while for example Anx1 promotes M2-like macrophage polarization. T and B cell: neutrophils can act as APCs to promote T-cell differentiation into effector T cells through MHC molecules but also inhibit T cell proliferation. Activated neutrophils express intracellular and extracellular BAFF, which binds to the BCMA receptor on B cells and promotes B-cell maturation, survival and differentiation into plasma cells [61]. Tubular epithelial cells: kidney injury causes tubular epithelial cell necrosis and subsequent release of pro-inflammatory mediators that indirectly trigger neutrophil activation and induce NET formation and NETosis. The induced inflammatory response (release of NETs) contributes to further tubular cell necrosis in a vicious cycle known as necroinflammation. This figure was generated with BioRender software. Anx1, annexin 1; APCs, antigen presenting cells; BAFF, B cell activating factor; BCMA, B cell maturation antigen; CD40L, CD40 ligand; CTSG, cathepsin G; C3a and C5a, complement component 3a and 5a; HNP1–3, human neutrophil peptides 1–3; HMGB1, high mobility group box 1; ICAM, intercellular adhesion molecule; LL-37, cathelicidin; MHC, major histocompatibility complex; MMP, matrix metalloproteinase; MVs, microvesicles; NE, neutrophil elastase; PSGL-1, P-selectin granulocyte ligand 1; PR3, proteinase 3.

Neutrophils interact not only with innate immune cells but also with the adaptive immune system (reviewed in [51]). For example, in the marginal zone of human spleens neutrophils acquire B cell–stimulating characteristics through the production of chemokines and cytokines including B-cell activating factor as well as the pattern recognition receptor pentraxin 3 (see Fig. 3), thus stimulating immunoglobulin production by nearby B cells [60–62]. The functions of T cells can be either positively or negatively modulated by neutrophils [63–65]. However, it remains unclear whether the capacity to influence T-cell functions is exerted by pre‐existing neutrophils, that under specific circumstances acquire immunomodulatory properties, or by discrete neutrophil populations that emerge under physiopathological conditions and become specialized in either promoting or inhibiting T‐cell responses [51].

Neutrophils indirectly interact with non-immune cells such as tubular epithelial cells in the kidney. Upon AKI, necrotic tubular epithelial cells release danger-associated molecular patterns (DAMPs) including cytokines and histones, which in turn promote neutrophil infiltration in the kidney and subsequent peptidyl argine deiminase 4 (PAD4)-mediated NETs formation and NETosis [66]. Such NETs contribute to further tubular epithelial cell death and DAMPs release in a vicious cycle known as necroinflammation (see Fig. 3) [67].

NEUTROPHILS IN INFECTIOUS DISEASES

Infections (e.g. COVID-19, sepsis, pneumonia, hepatitis, herpes zoster and tuberculosis) affecting millions of people each year are associated with high mortality rates. In patients with kidney disease, infectious complications are the second most common cause of death [17]. Complications of severe infections can range from thrombosis, acute respiratory distress syndrome and arterial hypotension to organ damage including the kidney [68]. Certain infections can also trigger vasculitis through immune complex deposition and dysregulated complement activation, such as anti-neutrophil cytoplasmic antibodies (ANCA)-associated vasculitis (AAV), hepatitis B virus–associated polyarteritis nodosa and hepatitis C virus–related cryoglobulinaemia, but conversely, infections are also very common in patients with vasculitis (reviewed in [69, 70]). Myeloid cells are known to contribute to the immunopathology of infection, with neutrophils being implicated in both hyperinflammation and immunosuppression through well-defined mechanisms including oxidative burst, degranulation, phagocytosis and NET formation. Interestingly, lymph-borne hematopoietic stem cells are shown to originate from the bone marrow and can proliferate to myeloid cells within the kidney when implanted under the kidney capsule in response to TLR pathogens, suggesting a role for neutrophil granulopoiesis during infection [71, 72].

Recent evidence points toward a heterogeneity and functional diversity of neutrophils. Of note, four clusters of immature neutrophils—termed IL1R2⁺, PADI4⁺, MPO⁺ and MKI67⁺CYP1B1⁺ neutrophils—were identified in the blood of people with sepsis [73]. Such neutrophils display transcriptional features of so-called granulocytic myeloid-derived suppressor cells, and they are able to inhibit the proliferation and activation of CD4⁺ T cells via prostaglandin E2 [73]. Additionally, low-density neutrophils (LDN) are also high in numbers in patients with severe COVID-19 during the acute phase, where they display enhanced phagocytic capacity, spontaneous NET formation and elevated cytokine production [68]. A subset of immunosuppressive CD16^brightCD62L^dim neutrophils was identified in patients with severe COVID-19 that secondarily developed pulmonary embolism. In convalescent individuals, LDNs decrease in numbers, while CD16^low neutrophils remain expanded over the disease course [74].

Studies have illustrated a phenomenon called neutrophil swarming, a rapid accumulation of neutrophils mediated by intercellular neutrophil communication and sensing of pathogen and damage signals at sites of infection or injury (reviewed in [75, 76]). These swarming neutrophils produce chemoattractants such as CXCL2 and lipid mediator leukotriene B4 that are positively regulated by calcium, complement, Bruton's tyrosine kinase [35] and integrins, while NADPH oxidase 2 rather negatively regulates neutrophil swarming [77]. Whether neutrophil swarming also occurs in kidney disease is currently unknown. It is possible that neutrophil swarming might contribute to granulomatous interstitial nephritis similar to that observed in chronic granulomatous lung disease [78].

Of note, some bacterial pathogens have even developed mechanisms to escape neutrophil control, leading to chronic infections in the host associated with complications such as tissue damage or eventually death. One such example is Mycobacterium tuberculosis. Recent insights suggest a beneficial role of neutrophils for the initial M. tuberculosis clearance but if this fails, neutrophils are detrimental due to their excessive recruitment, neutrophil necrosis and tissue damage [79].

In high-risk patients where neutrophil immunity to host defense is impaired, clinical outcomes are worse. Diseases that interfere with the immune system and contribute to neutrophil dysfunction include cancer, cardiovascular disease, diabetes, inflammatory disorders and kidney disease. Thus, patients with kidney disease are more susceptible to infections due to the secondary immunodeficiency related to kidney disease (SIDKD) (reviewed in [17]). Neutrophil dysfunctions include decreased phagocytic capability to clear pathogens, reduced respiratory burst, abolished selectin-induced slow leukocyte rolling and transmigration [80, 81], and increased neutrophil apoptosis but diminished NET release and NETosis [17, 82] (see Table 1). Further studies are needed to unravel the diverse functional phenotypes of neutrophils during infections but also during SIDKD.

Table 1:

Mechanistic features of neutrophils in health and disease.

Physiological states	Mechanisms of action	Ref
Health
Homeostasis	CXCR4 expression in circulating neutrophils from healthy subjects correlates with neutrophil migration and aging. CXCR4−CD62L+ young and CXCR4+CD62L− aged neutrophils are regulated by circadian oscillation	[11, 145]
	CD177 is a glycoprotein on the plasma membrane and can serve as receptor of membrane-bound proteinase 3). CD177+ neutrophils in the blood of healthy individuals can promote the interaction of neutrophils with endothelial cells and their transmigration into the tissue via PECAM-1 or β2 integrins	[1, 146]
	Human OLFM4+ and OLFM4– neutrophils do not display major functional differences other than a differential ability to produce NETs in vitro. OLFM4+ neutrophils can migrate and increase in numbers during inflammation relative to healthy conditions	[147]
	Other neutrophil subpopulations are present in healthy individuals such as neutrophils expressing the T-cell receptor αβ or proangiogenic CD49+CXCR4+ vascular endothelial growth factor (VEGFR1)+	[148, 149]
Disease
Infection	Increased numbers of LDNs (CD66b+CD16intCD44lowCD11bint) in patients with COVID-19 during the acute phase	[68]
	Contribution of neutrophils to hyperinflammation via oxidative burst (ROS production), degranulation, phagocytosis and NET formation	[73]
	Identification of four immature neutrophil subsets (IL1R2+, PADI4+, MPO+ and MKI67+CYP1B1+) in the blood of patients with sepsis	[73]
	Neutrophils contribute to immunosuppression and act as granulocytic myeloid-derived suppressor cells (PMN-MDSCs, CD16brightCD62Ldim neutrophils)	[74]
Non-infectious diseases
Kidney cancer	TGF-β, G-CSF and GM-CSF drive neutrophils toward a pro-tumor phenotype, while type I IFNs facilitate anti-tumor activity of these cells	[89, 91, 92]
Autoimmune diseases	In AAV: ANCAs bind to self-antigens such as MPO and proteinase 3, and trigger neutrophil activation and the release of ANCAs, inflammatory cytokines (e.g. IL-6), ROS and lytic enzymes (e.g. histones, neutrophil elastase, MMPs), and NET formation	[150]
	In SLE: three subpopulations of neutrophils exist in the blood of SLE patients, LDNs (CD11bbright, CD15+, CD16+CD33+CD14neg-loCD66bhighCD10+), mature high-density neutrophils (CD11b+CD15+CD16+CD66b+CD10+) and PMN-MDSCs (CD11b+CD14–CD15+CD66b+). LDNs release pro-inflammatory cytokines (TNF, IFN-α, IL-18), have impaired phagocytosis, enhanced NET formation (NETosis) via release of mitochondrial ROS, GSDMD oligomerization-dependent mitochondrial DNA and IL-17 release	[151, 152]
	Neutrophils are involved in disrupting endothelial permeability through the secretion of granule proteins like azurocidin, proteinase-3 and α-defensin
Cardiovascular disease	Neutrophils mediate monocyte recruitment and endothelial adhesion through secretion of cathepsin G, cathelicidin and α-defensin-platelet-derived CCL5 complexes	[111, 153–155]
	Neutrophil recruitment occurs via β2 integrin activation, and arrest and crawling via E-selectin, ITAM and SLP-76/ADAP
Acute and chronic kidney disease	Contribution of neutrophils to necroinflammation through release of pro-inflammatory mediators (ROS, MPO, neutrophil elastase and histones), NET formation and RIPK1–RIPK3-MLKL-dependent necroptosis	[38]
	Granulopoiesis: two distinct LDG subsets, CD14lowCD16+CD15+ (mature neutrophil-like phenotype with increased size and granularity, higher expression of CD10, CD35) and CD14−CD16−CD15+ (immature neutrophil-like phenotype, derived from the bone marrow, higher expression of CD68 and DEF3) were identified in CKD patients on dialysis	[136]

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ADAP, adhesion and degranulation-promoting adaptor protein; DEF3, defensing 3; GSDMD, gasdermin D; ITAM, immunoreceptor tyrosine-based activation motif; MMP, matrix metalloproteinase; SLP-76, SH2-domain-containing leukocyte protein of 76 kDa.

NEUTROPHILS IN NON-INFECTIOUS DISEASES

SUMMARY AND THERAPEUTIC PRINCIPLES

Taken together, neutrophils have recently been recognized as cells with multifaceted roles in various biological contexts. They participate in acute host defense and represent the first line of innate immunity in the context of infection, but also contribute to a variety of acute and chronic inflammatory diseases. In cancer, neutrophils display heterogeneous roles: while their abundance in the blood of cancer patients often correlates with poor prognosis, early-stage neutrophil presence can have immunostimulatory anti-tumor effects. Furthermore, they are also involved in the pathogenesis of a variety of autoimmune diseases, contributing to systemic inflammation, augmentation of adaptive immunity and subsequent tissue damage.

Given their emerging roles in a wide range of diseases, therapeutic targeting of neutrophil functions represents a promising treatment approach in the areas of kidney disease, oncology, immunology, infectiology and even cardiology. Indeed, simple removal of neutrophils from the circulation by monocyte- or granulocyte apheresis is able to ameliorate certain inflammatory diseases like psoriasis [138], although these relatively crude approaches have been abandoned in favor of more sophisticated therapies. Although few existing drugs are specifically aimed at neutrophils, several drugs already target neutrophils either directly or indirectly, including corticosteroids, dapsone, cytokine-inhibiting antibodies such as anti-TNF-α and anti-IL-17A antibodies, immunomodulatory treatments targeting type I IFNs, or neutrophil protease inhibitors (e.g. sivelestat, alvestat) [139]. Complement inhibitors including avacopan are clinically used for the management of patients with AAV to block C5a-mediated neutrophil activation [140], and are promising approaches also for patients with immunoglobulin A nephropathy [141] and other forms of kidney disease. Of note, so far specific therapies based on specifically inhibiting neutrophil β₂ integrins have failed clinical trials [142]. Based on our current knowledge, more specific treatment approaches are warranted including AKI and CKD, for example drugs targeting NETosis (e.g. PAD4 inhibitor Cl-amidine, GSK199, necrostatin 1) [143] and NETs [144], neutrophilic ROS production, neutrophil adhesion and migration (e.g. 5-lipoxygenase inhibitors) as well as neutrophil survival pathways, to specifically address pathology mediated by neutrophils while preserving the function of other immune cells.

Contributor Information

Daniela Maier-Begandt, Institute of Cardiovascular Physiology and Pathophysiology, Walter Brendel Center for Experimental Medicine Biomedical Center (BMC), Ludwig-Maximilians-Universität München, Munich, Germany.

Noelia Alonso-Gonzalez, Institute of Immunology, Universität of Münster, Münster, Germany.

Luisa Klotz, Department of Neurology with Institute for Translational Neurology, University Hospital Münster, Münster, Germany.

Luise Erpenbeck, Department of Dermatology, University Hospital Münster, Münster, Germany.

Jadwiga Jablonska, Department of Otorhinolaryngology, University Hospital Essen, University Duisburg-Essen, Essen, Germany. German Cancer Consortium (DKTK) partner site Düsseldorf/Essen, Essen, Germany.

Roland Immler, Institute of Cardiovascular Physiology and Pathophysiology, Walter Brendel Center for Experimental Medicine Biomedical Center (BMC), Ludwig-Maximilians-Universität München, Munich, Germany.

Anja Hasenberg, Institute of Experimental Immunology and Imaging, University Hospital Essen, University Duisburg-Essen, Essen, Germany.

Tonina T Mueller, Department of Medicine I, Ludwig-Maximilians-University Hospital, Ludwig-Maximilians-University Munich, Munich, Germany.

Andrea Herrero-Cervera, Institute for Experimental Pathology, Center for Molecular Biology of Inflammation, Universität of Münster, Münster, Germany.

Irene Aranda-Pardos, Institute of Immunology, Universität of Münster, Münster, Germany.

Kailey Flora, Renal Division, Department of Medicine IV, Ludwig-Maximilians-University Hospital, Ludwig-Maximilians-University Munich, Munich, Germany.

Alexander Zarbock, Department of Anaesthesiology, Intensive Care and Pain Medicine, University Hospital Münster, Münster, Germany.

Sven Brandau, Department of Otorhinolaryngology, University Hospital Essen, University Duisburg-Essen, Essen, Germany.

Christian Schulz, Department of Medicine I, Ludwig-Maximilians-University Hospital, Ludwig-Maximilians-University Munich, Munich, Germany.

Oliver Soehnlein, Institute for Experimental Pathology, Center for Molecular Biology of Inflammation, Universität of Münster, Münster, Germany.

Stefanie Steiger, Renal Division, Department of Medicine IV, Ludwig-Maximilians-University Hospital, Ludwig-Maximilians-University Munich, Munich, Germany.

REFERENCES