Neutrophils contain numerous effector proteins involved in host defense, including MPO, NE, Cathepsin G, PR3, BPI, Azurocidin, Lactoferrin, and Lysozyme. In human neutrophils, HNPs are also prominent α-defensin effectors. These proteins cooperate in pathogen clearance, whereas their persistent or excessive release can promote inflammation and tissue damage.

Keywords: neutrophil effector proteins; myeloperoxidase; neutrophil elastase; Cathepsin G; proteinase 3; defensin; lactoferrin; BPI; degranulation; innate immunity

1 Overview of Neutrophil Effector Proteins

1.1 Research Scope of Neutrophil Effector Proteins

“Neutrophil effector proteins” do not constitute a structurally or functionally uniform protein family. Instead, the term broadly encompasses proteins expressed, stored, or released by neutrophils that directly participate in host defense or inflammatory regulation. The complete neutrophil proteome also includes membrane receptors, cytoplasmic signaling proteins, cytoskeletal proteins, metabolic enzymes, and components of oxidase systems.

In anti-infective and inflammatory research, particular attention is given to proteins with direct effector functions, including MPO, NE, Cathepsin G, PR3, BPI, Azurocidin, Lactoferrin, and Lysozyme; in human neutrophils, HNPs are also major α-defensin effectors.

1.2 Granule Storage and Graded Release

Many classical neutrophil effector proteins are pre-stored in cytoplasmic granules. Azurophilic granules mainly contain MPO, NE, Cathepsin G, PR3, BPI, and Azurocidin; in human neutrophils they are also rich in HNPs. Specific granules contain Lactoferrin, NGAL, and related proteins, whereas gelatinase granules are represented by extracellular matrix-remodeling proteins such as MMP9. Lysozyme is distributed across multiple granule subsets.

2 Major Neutrophil Effector Proteins

2.1 Myeloperoxidase

Myeloperoxidase (MPO) is a representative enzyme in azurophilic granules. After a pathogen is phagocytosed, the NOX2/NADPH oxidase system initiates the respiratory burst and generates H₂O₂. MPO uses H₂O₂ and halide ions to generate highly oxidizing products, of which hypochlorous acid (HOCl) is one of the most representative. HOCl can damage microbial membrane lipids, proteins, and other biological macromolecules.

MPO released into the extracellular environment also retains oxidative activity. Localized and controlled MPO reactions contribute to pathogen clearance, whereas persistent or excessive activation may modify host proteins, lipids, and extracellular matrix components. MPO protein abundance and MPO enzymatic activity reflect different levels of biological information and should therefore be evaluated separately.

2.2 Neutrophil Serine Proteases

(1) Neutrophil Elastase

Neutrophil elastase (NE/ELANE) hydrolyzes a broad range of microbial and host proteins. Within phagosomes, NE participates in the degradation of pathogen-derived proteins. Extracellular NE can act on elastin, other extracellular matrix components, cell-surface proteins, and certain inflammatory mediators.

NE activity is controlled by endogenous antiprotease systems such as α1-antitrypsin. When NE release exceeds the local antiprotease capacity, it can promote extracellular matrix degradation and tissue-barrier damage.

(2) Cathepsin G

Cathepsin G (CTSG) is another important neutrophil serine protease. It participates in the elimination of certain bacteria and fungi and can hydrolyze host proteins and process selected cytokines, chemokines, and cell-surface molecules.

Its biological effects depend closely on enzymatic activity, release location, and the local protease-inhibitory environment. Cathepsin G contributes to antimicrobial defense but may also influence inflammation and tissue remodeling when persistently released into the extracellular environment.

(3) Proteinase 3

Proteinase 3 (PR3), together with NE and Cathepsin G, constitutes the principal neutrophil serine protease system. PR3 participates in protein-substrate degradation and the processing of inflammation-associated proteins.

A proportion of PR3 can localize to the neutrophil surface. PR3 is also an important autoantigen targeted by PR3-ANCA, making it a key protein connecting neutrophil effector functions with ANCA-associated vasculitis.

2.3 HNPs, BPI, and Azurocidin

(1) Human Neutrophil Peptides

Human neutrophil peptides (HNPs) belong to the α-defensin family and mainly include HNP-1, HNP-2, HNP-3, and HNP-4. HNPs are cationic and amphipathic molecules that interact with negatively charged microbial surfaces and alter membrane permeability.

HNP-1, HNP-2, and HNP-3 are highly homologous, whereas HNP-4 differs more substantially in sequence and certain functional properties. In addition to their direct antimicrobial effects, extracellular HNPs can influence immune-cell recruitment and inflammatory responses.

(2) Bactericidal/Permeability-Increasing Protein

Bactericidal/permeability-increasing protein (BPI) is an important neutrophil effector protein directed against Gram-negative bacteria. It binds lipopolysaccharide (LPS) with high affinity and acts on the bacterial outer membrane, altering membrane stability and permeability.

BPI can also reduce certain LPS-associated biological activities, thereby combining direct antimicrobial action with endotoxin-regulatory effects.

(3) Azurocidin

Azurocidin, also known as CAP37, is structurally related to the neutrophil serine protease family but lacks typical effective proteolytic catalytic activity. Its principal functions include cationic antimicrobial activity and regulation of monocyte recruitment, endothelial responses, and vascular permeability.

Azurocidin therefore occupies a functional position distinct from those of catalytically active serine proteases such as NE, Cathepsin G, and PR3.

2.4 Lactoferrin and Lysozyme

(1) Lactoferrin

Lactoferrin is a representative protein in specific granules and possesses a high iron-binding capacity. Because the growth of many microorganisms depends on iron availability in the host environment, Lactoferrin can restrict microbial proliferation by reducing locally available iron.

Lactoferrin can also act on microbial surfaces and influence host immune responses, making it an important component of neutrophil-mediated nutritional immunity.

(2) Lysozyme

Lysozyme hydrolyzes the β-1,4 glycosidic bond between N-acetylmuramic acid and N-acetylglucosamine in bacterial peptidoglycan, thereby weakening bacterial cell-wall integrity.

Within neutrophil phagosomes, Lysozyme can cooperate with MPO-mediated oxidative killing, membrane-disruptive effects of α-defensins such as HNPs and BPI, and proteolysis mediated by granule proteases.

2.5 NGAL and MMP9

(1) NGAL

Neutrophil gelatinase-associated lipocalin (NGAL), also known as LCN2, is an important specific-granule protein. It binds selected bacterial siderophores and restricts siderophore-mediated iron acquisition, thereby contributing to nutritional immunity.

NGAL can also be produced by renal and other tissue cells. Increased NGAL in blood, urine, or tissue therefore cannot independently serve as specific evidence of neutrophil degranulation.

(2) MMP9

Matrix metalloproteinase-9 (MMP9) is a representative gelatinase-granule protein. It primarily participates in extracellular matrix and basement-membrane remodeling, facilitating neutrophil migration across tissue barriers.

MMP9 is not specific to neutrophils and can also be produced by macrophages, tumor cells, and other cell types. Determining its tissue source therefore requires integration with cell-specific markers and spatial localization.

3 Cooperative Antimicrobial Mechanisms of Neutrophil Effector Proteins

3.1 Multimechanistic Killing Within Phagosomes

After a pathogen is engulfed by a neutrophil, multiple granule populations fuse with the phagosome. Oxidative killing, proteolysis, membrane disruption, and cell-wall degradation are consequently concentrated within the same local compartment.

Disruption of microbial membranes and cell walls allows oxidants and proteases to gain improved access to internal targets. These antimicrobial mechanisms reinforce one another, enabling neutrophils to rapidly establish a high-intensity, multitarget environment for pathogen clearance.

3.2 Degranulation Extends Extracellular Effects

Following stimulation by microbial components, chemotactic factors, or inflammatory mediators, neutrophils can release effector proteins into the extracellular environment through fusion of granules with the plasma membrane.

The contents of readily mobilized granules contribute to local antimicrobial defense and tissue migration. As stimulation intensifies, highly active proteins such as NE, MPO, and Cathepsin G may also be released in substantial quantities. Moderate degranulation helps control pathogens that have not been phagocytosed, whereas persistent extracellular protease and oxidase activity can redirect damage from pathogens toward host cells and extracellular matrix components.

3.3 NETs and Granule Proteins

Neutrophil extracellular traps (NETs) primarily consist of extracellular chromatin and can carry MPO, NE, and other granule proteins. The DNA network can capture certain pathogens and concentrate antimicrobial effector molecules locally.

NET formation is distinct from conventional degranulation. Increased extracellular MPO or NE alone does not demonstrate NET formation because both proteins can also enter the extracellular environment through granule release. NET studies generally require additional evidence involving extracellular DNA, granule protein–DNA complexes, or microscopic colocalization.

4 Dysregulation of Neutrophil Effector Proteins in Disease

4.1 Infection and Excessive Inflammation

During the early phase of infection, rapid neutrophil recruitment establishes a coordinated anti-infective response involving oxidative killing, proteolysis, membrane damage, cell-wall degradation, and iron restriction.

In severe infection or septic inflammation, widespread neutrophil activation may be accompanied by extensive degranulation and NET formation. High concentrations of proteases, MPO, and other effector molecules entering tissues and the circulation can further increase endothelial damage, microvascular inflammation, and organ dysfunction.

4.2 Chronic Airway Inflammation and Protease Imbalance

Chronic obstructive pulmonary disease, bronchiectasis, and related disorders are frequently accompanied by persistent neutrophil infiltration. Continuous release of proteases such as NE and Cathepsin G can promote extracellular matrix degradation and epithelial-barrier damage, while MPO can increase local oxidative stress.

When the protease–antiprotease balance remains disrupted, effector proteins that normally contribute to pathogen clearance can continuously act on host tissues, promoting airway structural changes and sustaining chronic inflammation.

4.3 MPO, PR3, and ANCA-Associated Vasculitis

MPO and PR3 are important physiological neutrophil effector proteins and major autoantigens in ANCA-associated vasculitis. Autoantibodies against MPO or PR3 can promote further activation of primed neutrophils, leading to ROS production, degranulation, and inflammatory vascular injury.

MPO and PR3 are therefore important research targets connecting neutrophil granule biology and effector functions with autoimmune small-vessel vasculitis.

4.4 Extended Functions in the Tumor Microenvironment

Some tumors exhibit substantial neutrophil infiltration accompanied by changes in NE, MPO, MMP9, and NET-associated proteins. These molecules may alter the extracellular matrix, vascular barriers, and local inflammatory environment.

Their effects are strongly dependent on tumor type and microenvironmental context. The level of a single protein is therefore insufficient to determine whether neutrophils exert tumor-promoting or antitumor activity.

5 Products

5.1 Representative Human Neutrophil Proteins

5.2 Products for MPO, NE, and Cathepsin G Detection and Functional Research

5.3 Products for HNP and α-Defensin Research

5.4 Products for Lactoferrin, NGAL, and Related Protein Research

5.5 Tools for Human Neutrophil Isolation, Oxidative Function, and NET Research

Neutrophil effector proteins form a coordinated antimicrobial defense system through oxidative killing, proteolysis, membrane disruption, cell-wall degradation, and nutrient restriction. Their controlled release contributes to pathogen clearance, whereas persistent or excessive release can drive inflammation and tissue injury.

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[3] Eosinophilic Leukemia Detection Tests