Increased respiratory burst in myeloperoxidase-deficient monocytes

Studies of the respiratory burst in myeloperoxidase (MPO) deficient monocytes were undertaken to assess the physiologic consequence of the absence of MPO in these cells. As previously demonstrated with neutrophils, MPO-deficient monocytes had a greater initial rate, duration, and total superoxide production in response to phagocytosis of zymosan than did normal monocytes. Introduction of purified eosinophil peroxidase (EPO) into the phagosome by binding the enzyme to the surface of the zymosan particles changed the hypermetabolic characteristics of superoxide production in MPO-deficient cells to more closely resemble normal cells, but had no effect on superoxide generation by the normal monocytes. Further, inactivation of the bound EPO before ingestion restored the supranormal respiratory burst by the MPO-deficient cells. Iodination by MPO-deficient monocytes was significantly depressed as compared to normal monocytes following the ingestion of zymosan (1.9 versus 10.1 nmole I-/10(7) monocytes/30 min; p less than 0.01). In contrast, iodination was markedly augmented in MPO-deficient cells compared to normal cells after ingestion of zymosan coated with EPO (208 versus 70 nmole I-/10(7) monocytes/30 min; p less than 0.005), presumably reflecting the greater amounts of hydrogen peroxide formed by MPO-deficient cells. There were no differences in the levels of endogenous scavengers of reactive oxygen products (catalase, superoxide dismutase, glutathione peroxidase and reductase, and total glutathione) in MPO-deficient and normal monocytes that would account for the enhanced respiratory burst of MPO-deficient cells. These findings support a role for peroxidase in the termination of the respiratory burst of monocytes.     

Increased degranulation of human myeloperoxidase-deficient polymorphonuclear leucocytes

Myeloperoxidase (MPO)-deficient neutrophils (PMN) released considerably more beta-glucuronidase, lysozyme and vitamin B12-binding activities, when exposed to opsonized zymosan (STZ), than the normal counterpart. Release of the soluble enzyme lactate dehydrogenase was not appreciably changed over the incubation time with particles in either cell type. MPO-deficient PMN and normal PMN ingested STZ particles at a similar rate at early times, but thereafter phagocytosis by MPO-deficient PMN was significantly higher than that by normal PMN. The difference in degranulation between the two cell types greatly exceeded the difference in ingestion and was evident already at early phagocytosis times when no difference in phagocytosis was observed; this suggested that the higher degranulation in MPO-deficient PMN was at least in part independent of the increased ingestion. This was confirmed by experiments with the soluble stimulant N-formyl-L-norleucyl-L-leucyl-phenylalanine (FNLLP). MPO-deficient PMN and normal PMN exhibited a comparable respiratory burst when exposed to FNLLP plus cytochalasin B, but the defective cells released more azurophilic and specific granule markers than normal PMN. These results indicate that MPO-deficient PMN degranulate more than normal PMN and suggest a role for MPO in the regulation of degranulation.     

Normal and deficient neutrophils can cooperate to damage Aspergillus fumigatus hyphae

Using a metabolic test of hyphal viability, the interaction between neutrophils and Aspergillus hyphae was investigated over a broad range of hyphae-to-neutrophil ratios. Normal neutrophils were found to damage hyphae whereas neutrophils from patients with both chronic granulomatous disease (CGD) and myeloperoxidase (MPO) deficiency did not. Further, both azide and catalase + superoxide dismutase inhibited the ability of normal neutrophils to damage hyphae, suggesting that this damage is mediated by products of the respiratory burst and by the MPO-halide system. Also, mixtures of small numbers of normal neutrophils with larger numbers of CGD neutrophils (range, 1:5 to 1:15) damaged hyphae more efficiently than either population of cells alone. Further, mixtures of CGD and MPO-deficient neutrophils, neither of which alone could efficiently damage hyphae, were able to damage the hyphae almost as well as a comparable number of normal neutrophils. These data demonstrate that intact neutrophils can cooperate to synergistically damage Aspergillus hyphae, possibly by extracellular mixing of hydrogen peroxide and MPO.     

Iron deficiency and neutrophil function: different rates of correction of the depressions in oxidative burst and myeloperoxidase activity after iron treatment

The polymorphonuclear granulocyte (PMN) kills ingested bacteria by mechanisms that include myeloperoxidase (MPO) and a sudden increase in oxygen consumption (the oxidative burst), both of which are iron dependent. The magnitude of the oxidative burst and activity of MPO were determined in PMNs during the progression of iron deficiency (ID) and following its treatment in rats. As ID developed, the oxidative burst after zymosan activation was less depressed than the activity of MPO. There was no change in the oxidative burst after activation with phorbol myristate acetate (PMA) or in the generation of superoxide (O2- ) by NADPH oxidase-containing particles from PMNs. Following iron treatment, impairment of the oxidative burst after zymosan activation was corrected after 1 day. In contrast, the deficit in MPO activity was not corrected until 7 days after initiation of iron treatment. The pattern of recovery in MPO activity after iron treatment corresponded to the prolonged period of maturation of the PMN primary granule since the formation of primary granules, which contain MPO, takes place only in the early, mitotic stages of maturation. The tendency of the PMN to maintain the oxidative burst allows the cell to preserve its capacity for bacterial killing during the progression of iron deficiency.     

Viricidal effect of stimulated human mononuclear phagocytes on human immunodeficiency virus type 1.

Human monocytes stimulated with phorbol 12-myristate 13-acetate or opsonized zymosan in vitro were viricidal to human immunodeficiency virus type 1 (HIV-1) as measured by the inability of the virus to replicate in CEM cells. Monocytes, when stimulated, release myeloperoxidase (MPO) and produce H2O2; MPO reacts with H2O2 and chloride to form hypochlorous acid, a known microbicidal agent. The viricidal activity of stimulated monocytes was inhibited by the peroxidase inhibitor azide, implicating MPO, and by catalase but not heated catalase or superoxide dismutase, implicating H2O2. Stimulated monocytes from patients with chronic granulomatous disease (CGD) or hereditary MPO deficiency were not viricidal to HIV-1 unless they were supplemented with the H2O2-generating enzyme glucose oxidase or MPO, respectively. The viricidal activity of stimulated, glucose oxidase-supplemented CGD monocytes and MPO-supplemented MPO-deficient monocytes, like that of normal stimulated monocytes, was inhibited by azide and catalase. Monocytesmaintained in culture differentiate into macrophages with loss of MPO and decreased H2O2 production. The viricidal activity of 3- to 9-day monocyte-derived macrophages was decreased unless MPO was added, whereas the loss of viricidal activity by 12-day-old monocyte-derived macrophages was not reversed by MPO unless the cells were pretreated with gamma-interferon. These findings suggest that stimulated monocytes can be viricidal to HIV-1 through the release of the MPO/H2O2/chloride system and that the decreased viricidal activity on differentiation to macrophages results initially from the loss of MPO and, with more prolonged culture, also from a decreased respiratory burst that can be overcome by gamma-interferon.     

Myeloperoxidase mediates neutrophil activation by association with CD11b/CD18 integrins

Recruitment and activation of polymorphonuclear neutrophils (PMNs) reflects a primary immunological response to invading pathogens and has also emerged as a hallmark of vascular inflammation. One of the principal enzymes released upon PMN activation is myeloperoxidase (MPO), a heme protein that not only generates cytotoxic oxidants but also impacts deleteriously on nitric oxide-dependent signaling cascades within the vasculature. Because MPO also associates with the membrane of PMN, we evaluated whether MPO could also function as an autocrine modulator of PMN activation. The extent of PMN membrane-associated MPO was elevated in patients with acute inflammatory vascular disease compared with healthy individuals. Isolated PMNs bound free MPO by a CD11b/CD18 integrin-dependent mechanism. PMNs exposed to MPO were characterized by increased tyrosine phosphorylation and p38 mitogen-activated protein kinase activation. Also, nuclear translocation of NFkappaBin PMN was enhanced after incubation with MPO, as was surface expression of CD11b. Binding of PMN to MPO-coated fibronectin surfaces amplified PMN degranulation, as evidenced by increased release of MPO and elastase. MPO also augmented PMN-dependent superoxide (O(2)(*-)) production, which was prevented by anti-CD11b antibodies, but not MPO inhibitors. Collectively, these results reveal that binding of MPO to CD11b/CD18 integrins stimulates PMN signaling pathways to induce PMN activation in a mechanism independent of MPO catalytic activity. These cytokine-like properties of MPO thus represent an additional dimension of the proinflammatory actions of MPO in vascular disease.     

Myeloperoxidase-deficient polymorphonuclear leucocytes. (II) Longitudinal study in acute myeloid leukaemia, untreated, in remission and in relapse

Myeloperoxidase (MPO) activity of blood granulocytes was estimated in 96 cases of acute myeloid leukaemia (AML), in 35 patients obtaining complete remission and in 14 of these patients during later relapse. As the pretreatment value of MPO activity was the same in patients who died before obtaining remission as in patients obtaining complete remission, determination of MPO-deficient PMN has no prognostic value with respect to the probability of obtaining complete remission. While half of the untreated patients had increased numbers of MPO-deficient PMN in the blood, all patients in complete remission had normal MPO activity (P = 0.0002). A normalization of the MPO activity after induction therapy, therefore suggests remission. Positive correlations could be demonstrated between an initially abnormal MPO activity and abnormal activity at relapse as well as between an initially abnormal MPO activity and normal activity at relapse (P = 0.0004). It is concluded that determination of the % of MPO-deficient PMN in the blood, may be a useful indicator of complete remission in AML, and in serial determinations during the remission phase also an indicator of threatening relapse.     

Hydrogen peroxide production, chemiluminescence, and the respiratory burst of fertilization: interrelated events in early sea urchin development.

After fertilization of the sea urchin, Strongyl-ocentrotus purpuratus, a crosslinked fertilization membrane is formed; the crosslinks (dityrosine residues) are synthesized in a reaction catalyzed by an ovoperoxidase that is released from the cortical granules during fertilization. The substrate for ovoperoxidase activity, hydrogen peroxide, is generated by the egg coincident with the "respiratory burst" that follows parthenogenetic activation by the divalent ionophore A23187 or fertilization. This burst of oxygen consumption may be almost quantitatively accounted for by hydrogen peroxide evolution, as measured by the peroxidase-catalyzed quenching of scopoletin fluorescence. Neither the burst of oxygen consumption nor hydrogen peroxide production occurs when the inhibitor of cortical granule discharge, procaine, is present at fertilization.Fertilization or parthenogenetic activation with A23187 also is associated with a burst of light emission. This chemiluminescence is inhibited in vivo by inhibitors of the ovoperoxidase, such as 3-amino-1,2,4-triazole, phenylhydrazine, sulfite, or azide. A crude ovoperoxidase preparation catalyzes hydrogen peroxide-dependent chemiluminescence that is similarly inhibited. Thus, the bursts of oxygen uptake, peroxide production, and chemiluminescence appear to be several manifestations of the peroxidative system released at fertilization. This system may additionally be responsible for spermicidal activity and thus may act as a component of the block to polyspermy.     

Myeloperoxidase-deficient polymorphonuclear leucocytes. (I) Incidence in untreated myeloid leukaemia, lymphoid leukaemia and normal humans

Neutrophil myeloperoxidase (MPO) activity was analysed semi-quantitatively both by (i) MPO-scoring of polymorphonuclear leucocytes (PMN) and (ii) counting the MPO-deficient PMN (PMN lacking MPO) in 164 subjects (60 cases of leukaemia and 104 normal humans). The scoring method showed that 10 out of 21 (48%) cases of acute myeloid leukaemia (AML), 2 out of 10 (20%) cases of chronic myeloid leukaemia (CML), 0 out of 29 cases of lymphoid leukaemia (ALL + CLL), and 1 out of 104 normal humans had decreased MPO scores. These figures correlated well with the more simple counting of PMN lacking MPO in the same groups: 8 out of 21 (37%) cases of AML, 6 out of 10 (60%) cases of CML and 0 out of 29 cases of lymphoid leukaemia showing more than 4% PMN lacking MPO. In cases of otherwise unclassifiable acute leukaemia, a decreased MPO score and an increased number of MPO-deficient PMN suggests the diagnosis of AML and not ALL. Counting the number of PMN lacking MPO was found to be a time-saving and even more reliable method than the semiquantitative scoring of MPO activity in PMN.     

Myeloperoxidase-Deficient Polymorphonuclear Leucocytes

Neutrophil myeloperoxidase (MPO) activity was analysed semi-quantitatively both by (i) MPO-scoring of polymorphonuclear leucocytes (PMN) and (ii) counting the MPO-deficient PMN (PMN lacking MPO) in 164 subjects (60 cases of leukaemia and 104 normal humans). The scoring method showed that 10 out of 21 (48%) cases of acute myeloid leukaemia (AML), 2 out of 10 (20 %) cases of chronic myeloid leukaemia (CML), 0 out of 29 cases of lymphoid leukaemia (ALL + CLL), and 1 out of 104 normal humans had decreased MPO scores. These figures correlated well with the more simple counting of PMN lacking MPO in the same groups: 8 out of 21 (37 %) cases of AML, 6 out of 10 (60 %) cases of CML and 0 out of 29 cases of lymphoid leukaemia showing more than 4 % PMN lacking MPO. In cases of otherwise unclassifiable acute leukaemia, a decreased MPO score and an increased number of MPO-deficient PMN suggests the diagnosis of AML and not ALL. Counting the number of PMN lacking MPO was found to be a time-saving and even more reliable method than the semiquantitative scoring of MPO activity in PMN.     

Myeloperoxidase-deficient polymorphonuclear leucocytes (VI): Relation to cytogenetic abnormalities in primary myelodysplastic syndromes

Relations between cytogenetic status, FAB-classification and an abnormal subpopulation of myeloperoxidase (MPO)-deficient polymorphonuclears (PMN) in 45 patients with myelodysplastic syndrome (MDS) are reported. Clonal abnormalities were demonstrated in 85% of the patients, with a lower incidence in the RA+ group (refractory anaemia with ring sideroblasts) compared to the others (p = 0.004). In 12 patients a spontaneous progression in cytogenetic aberrations occurred and in 7 of these (60%) a simultaneous progression in FAB-subtype was seen. The appearance of MPO-deficient PMNs was observed in 6 of these patients (55%). A progression in FAB-subtype was noted in further 4 patients and 2 additional patients developed MPO-deficient PMNs. Only one sufficient cytogenetic investigation was available in these patients. Thus 100% of the fully studied patients showed progression in cytogenetic abnormalities when a progression in FAB-subtype or a development of MPO-deficient PMNs was seen. 3 (49%) of the 8 patients developing MPO-deficient PMNs too showed a progression in FAB-subtype. Although no significant correlation to specific categories of structural aberrations or abnormalities in specific chromosome pairs could be demonstrated, clonal cytogenetic aberrations seem to be involved when the disease progresses and when MPO-deficient PMN develop.     

CNI—1493对重症急性胰腺炎大鼠外周血多形核粒细胞功能的影响

目的：探讨p38MAPK信号传导通路阻断剂（CNI－1493）对大鼠重症急性胰腺炎（severe acute pancreatitis SAP)时外周血多形核粒细胞（polymorphonuclear,PMN)功能影响，方法：以胰胆管逆行注射5％牛磺胆酸钠建立SD大鼠SAP模型，将54只SD大鼠随机分为3组：假手术组（SO,n=18)；SAP组（SAPn=18）；CNI－1493治疗组（CNI，n＝18），术后3h,6h,12h取血，用密度梯度法分离PMN，用流式细胞仪测定呼吸爆发功能，并测定PMN释放髓过氧化物酶（MPO）的变化情况。结果：SAP组PMN呼吸爆发亢进，MPO释放明显增加，在各时间点上CNI－1493都能抑制PMN的功能亢进，减少MPO的释放，结论：CNI－1493可以明显抑制SAP时PMN的病理性功能亢进，是治疗SAP的重要机制之一，提示可能具有临床应用的前景。     

Polytrauma induces increased expression of pyruvate kinase in neutrophils

Polytrauma (PT) leads to systemic activation of polymorphonuclear neutrophils (PMNs). Organ damage commonly found in these patients is ascribed to respiratory bursts of activated PMNs. With the use of sodium dodecyl sulfate-polyacrylamide gel electrophoresis, PMN extracts from PT patients were found to contain a clear protein band not seen in control PMNs from healthy volunteers. This band was identified by amino acid sequencing and Western blotting as pyruvate kinase (PK). Enzymatic assays revealed a 600-fold increase in PK activity in PMNs of PT patients, with the highest levels occurring between the fifth and seventh posttraumatic day. In lymphocytes, no such increase was detectable. As PK is a major regulatory enzyme in glycolysis, glucose-dependent lactate production in PMNs from PT patients was assayed. These cells showed a higher glycolytic lactate production than controls. It was additionally demonstrated that acute activation of respiratory burst activity depends mainly on breakdown of glucose to lactate via the pentose-phosphate pathway and glycolysis. In PMNs from PT patients, this glucose-dependent respiratory burst activity was more than twofold higher than in controls. The increase in expression and activity of PK in PMNs from PT patients may contribute to the high glucose-dependent respiratory burst activity seen in these cells.     

Influence of drugs that block calcium channels on the microbicidal function of human neutrophils

The central role of calcium ions in cell physiology prompted us to examine the hypothesis that pharmacological concentrations of calcium channel-blocking drugs might affect human neutrophil (PMN) functions. The capacity of PMNs suspended in verapamil hydrochloride for killing Pseudomonas aeruginosa during two hour incubations was significantly impaired (P less than .05). Several observations suggested that this drug effect was the result of altered calcium metabolism: exposure to verapamil decreased the uptake of 45Ca++ by PMNs subsequently exposed to the calcium ionophore A23187; verapamil did not impair PMN function in the absence of extracellular calcium; and the addition of A23187 concomitantly with (but not following) verapamil prevented PMN dysfunction. In addition, nifedipine, a structurally dissimilar calcium channel-blocking drug, also impaired the bactericidal activity of PMNs against Pseudomonas aeruginosa (P less than .02). Further studies revealed that treatment with verapamil did not affect PMN phagocytosis, but significantly impaired the PMN respiratory burst (as shown by superoxide anion generation assay; P less than .05). We conclude that PMNs exposed to pharmacological concentrations of calcium channel-blocking drugs exhibit a reduced capacity to kill bacteria.     

Polymorphonuclear neutrophil function in systemic sclerosis.

In vitro functions of polymorphonuclear (PMN) neutrophils were studied in 20 patients with progressive systemic sclerosis (PSS). An increase in the basal chemiluminescence (CL) activity of peripheral blood PMNs was found, suggesting that these cells had been preactivated in vivo. Patients with more extensive skin disease or signs of disease progression tended to have higher basal CL values. Active oxygen products during the respiratory burst may increase the extent of inflammatory and fibrotic processes and could be involved in the endothelial injury in PSS. The stimulatory capacity of CL response was normal in our study. No alterations were found in the opsonised yeast phagocytic activity of granulocytes when compared with control values. The binding of erythrocyte-antibody particles was found also to be normal. A depressed chemotactic activity of PMN cells against zymosan activated serum was also shown. The cause of the decreased chemotaxis of PMNs remains to be elucidated.     

Neutrophils from patients after burn injury express a deficiency of the oxidase components p47-phox and p67-phox

Infection is a major cause of morbidity and mortality in patients after thermal injury. This predisposition to infections is related, in part, to abnormal polymorphonuclear leukocyte (PMN) function and a diminished respiratory burst. To evaluate the biochemical basis for the defective respiratory burst after major burns, the status of the oxidase enzyme system and its components was investigated. PMNs were isolated from 24 patients with 12% to 62% burns. Oxidase activity of intact PMNs, measured as superoxide anion (O2-) generation or oxygen consumption, was decreased in burn compared with healthy controls. Subcellular fractions from patient PMNs generated less O2- in the sodium dodecyl sulfate cell-free system, and this was related to a diminished contribution by cytosol but not by plasma membrane. Subsequently, cytosol was separated with CM-Sepharose, yielding two fractions; one contained the p47-phox and p67-phox (47/67 mix) and the other contained the remaining cytosolic components (run through [RT]). Although the contribution to oxidase activity made by RT from patient cytosol was similar to that of control, the activity of p47/67 mix from PMNs of burn patients was deficient. Quantitative assays using an immunoautoradiographic technique showed a consistent, but significant decrease in both p47-phox and p67-phox. The addition of purified or human recombinant p47-phox but not p67-phox corrected the diminished oxidase activity of cytosol from burn patients. Thus, decreased respiratory burst activity found in PMNs from individuals with thermal injury was associated with a specific, quantitative deficiency of p47- phox.     

Chronic oxidative stress reduces the respiratory burst response of neutrophils from beta-thalassaemia patients

Beta-thalassaemia patients are susceptible to infections by mechanisms that are not fully understood. Polymorphonuclear neutrophils (PMN) destroy microbes by producing a burst of reactive oxygen species (ROS) (respiratory burst) in response to bacterial components, as well as to phorbol-myristate-acetate (PMA). In the present study, we compared ROS generation by normal and beta-thalassaemia PMN and assessed their response to PMA. Blood cells were subjected to gelatin separation, staining with dichlorofluorescin-diacetate and flow cytometry. At basal level, the fluorescence (mean fluorescence channel) of normal and thalassaemia PMN were 12.7 +/- 4.5 and 95.6 +/- 19.8 respectively; it changed to 283.4 +/- 72.5 and 39.5 +/- 14.3, respectively, upon PMA stimulation, indicating that thalassaemia PMN have a higher basal ROS but a reduced response to PMA. When normal PMN were treated with the oxidants hydrogen peroxide and butyl-hydroxyperoxide, as well as iron and haemin, which are elevated in thalassaemia, their basal ROS increased 5-22-fold, but the PMA response was abolished. Treating thalassaemic PMN with antioxidants (N-acetyl-L-cysteine or vitamins C and E) reduced their basal ROS but enhanced their PMA response. Our findings indicate that chronically stressed PMN, e.g. in thalassaemia, have reduced capacity to elicit a respiratory burst, which may compromise their antibacterial capacity, and imply prophylactic treatment with antioxidants for recurrent infections.     

Granulocyte transmigration through the endothelium is regulated by the oxidase activity of vascular adhesion protein-1 (VAP-1)

Polymorphonuclear leukocytes (PMNs) migrate from the blood into areas of inflammation by binding to the endothelial cells of blood vessels via adhesion molecules. Vascular adhesion protein-1 (VAP-1) is one of the molecules mediating leukocyte-endothelial cell interactions. It is also an endothelial cell-surface enzyme (amine oxidase) that produces reactive oxygen species during the catalytic reaction. To study the role of the enzymatic activity of VAP-1 in PMN extravasation, we used an enzymatically inactive VAP-1 mutant, specific amine oxidase inhibitors (including a novel small molecule compound), and anti-VAP-1 antibodies in several flow-dependent models. The enzyme inhibitors diminished PMN rolling on and transmigration through human endothelial cells under conditions of laminar shear stress in vitro. Notably, the enzyme inactivating point mutation abolished the capacity of VAP-1 to mediate transmigration. Moreover, the new VAP-1 inhibitor effectively prevented the extravasation of PMNs in an animal model of inflammation. These data show that the oxidase activity of VAP-1 controls PMN exit from the blood during the relatively poorly understood transmigration step.     

Increased polymorphonuclear leukocyte respiratory burst function in type 2 diabetes

The predisposition to infection and chronic inflammation in diabetes may in part be related to the effects of hyperglycemia or other metabolic abnormality on polymorphonuclear leukocytes (PMN). We evaluated oxidative respiratory burst activity (superoxide production) in non-stimulated and stimulated PMN from 70 stable type 2 Hispanic diabetic patients, as compared to 70 healthy Hispanic individuals without diabetes. The influences of protein kinase C (PKC) inhibitors and certain antibiotics on superoxide production were examined. Both resting and stimulated (PMA, zymosan) PMN from diabetic individuals produced more superoxide than PMN from controls. Inhibitors of PKC, a possible mediator of the augmented respiratory burst activity, decreased superoxide production in all (resting and stimulated) diabetic and control PMN. Azithromycin, which is markedly concentrated by PMN, profoundly inhibited superoxide generation in all groups of diabetic and control cells. PMN from Hispanic diabetic patients produced greater quantities of superoxide than non-diabetic controls. This increased oxidative respiratory burst activity may predispose to infection and chronic inflammation in diabetes. PKC inhibitors and azithromycin inhibited this respiratory burst response. The possible role of PKC (especially PKC beta) as the mediator of this augmented respiratory burst response requires further evaluation, and may lead to therapeutic studies with appropriate inhibitors.     

Myeloperoxidase attracts neutrophils by physical forces

Recruitment of polymorphonuclear neutrophils (PMNs) remains a paramount prerequisite in innate immune defense and a critical cofounder in inflammatory vascular disease. Neutrophil recruitment comprises a cascade of concerted events allowing for capture, adhesion and extravasation of the leukocyte. Whereas PMN rolling, binding, and diapedesis are well characterized, receptor-mediated processes, mechanisms attenuating the electrostatic repulsion between the negatively charged glycocalyx of leukocyte and endothelium remain poorly understood. We provide evidence for myeloperoxidase (MPO), an abundant PMN-derived heme protein, facilitating PMN recruitment by its positive surface charge. In vitro, MPO evoked highly directed PMN motility, which was solely dependent on electrostatic interactions with the leukocyte's surface. In vivo, PMN recruitment was shown to be MPO-dependent in a model of hepatic ischemia and reperfusion, upon intraportal delivery of MPO and in the cremaster muscle exposed to local inflammation or to intraarterial MPO application. Given MPO's affinity to both the endothelial and the leukocyte's surface, MPO evolves as a mediator of PMN recruitment because of its positive surface charge. This electrostatic MPO effect not only displays a so far unrecognized, catalysis-independent function of the enzyme, but also highlights a principal mechanism of PMN attraction driven by physical forces.