Utilization of liposomes for correction of the metabolic and bactericidal deficiencies in chronic granulomatous disease

Redox reactions and microbial killing in the neutrophil phagosome

SIGNIFICANCE

When neutrophils kill microorganisms, they ingest them into phagosomes and bombard them with a burst of reactive oxygen species.

RECENT ADVANCES

This review focuses on what oxidants are produced and how they kill. The neutrophil NADPH oxidase is activated and shuttles electrons from NADPH in the cytoplasm to oxygen in the phagosomal lumen. Superoxide is generated in the narrow space between the ingested organism and the phagosomal membrane and kinetic modeling indicates that it reaches a concentration of around 20 μM. Degranulation leads to a very high protein concentration with up to millimolar myeloperoxidase (MPO). MPO has many substrates, but its main phagosomal reactions should be to dismutate superoxide and, provided adequate chloride, catalyze efficient conversion of hydrogen peroxide to hypochlorous acid (HOCl). Studies with specific probes have shown that HOCl is produced in the phagosome and reacts with ingested bacteria. The amount generated should be high enough to kill. However, much of the HOCl reacts with phagosomal proteins. Generation of chloramines may contribute to killing, but the full consequences of this are not yet clear.

CRITICAL ISSUES

Isolated neutrophils kill most of the ingested microorganisms rapidly by an MPO-dependent mechanism that is almost certainly due to HOCl. However, individuals with MPO deficiency rarely have problems with infection. A possible explanation is that HOCl provides a frontline response that kills most of the microorganisms, with survivors killed by nonoxidative processes. The latter may deal adequately with low-level infection but with high exposure, more efficient HOCl-dependent killing is required.

FUTURE DIRECTIONS

Better quantification of HOCl and other oxidants in the phagosome should clarify their roles in antimicrobial action.

Myeloperoxidase: a front-line defender against phagocytosed microorganisms

Successful immune defense requires integration of multiple effector systems to match the diverse virulence properties that members of the microbial world might express as they initiate and promote infection. Human neutrophils--the first cellular responders to invading microbes--exert most of their antimicrobial activity in phagosomes, specialized membrane-bound intracellular compartments formed by ingestion of microorganisms. The toxins generated de novo by the phagocyte NADPH oxidase and delivered by fusion of neutrophil granules with nascent phagosomes create conditions that kill and degrade ingested microbes. Antimicrobial activity reflects multiple and complex synergies among the phagosomal contents, and optimal action relies on oxidants generated in the presence of MPO. The absence of life-threatening infectious complications in individuals with MPO deficiency is frequently offered as evidence that the MPO oxidant system is ancillary rather than essential for neutrophil-mediated antimicrobial activity. However, that argument fails to consider observations from humans and KO mice that demonstrate that microbial killing by MPO-deficient cells is less efficient than that of normal neutrophils. We present evidence in support of MPO as a major arm of oxidative killing by neutrophils and propose that the essential contribution of MPO to normal innate host defense is manifest only when exposure to pathogens overwhelms the capacity of other host defense mechanisms.

How human neutrophils kill and degrade microbes: an integrated view

Neutrophils constitute the dominant cell in the circulation that mediates the earliest innate immune human responses to infection. The morbidity and mortality from infection rise dramatically in patients with quantitative or qualitative neutrophil defects, providing clinical confirmation of the important role of normal neutrophils for human health. Neutrophil-dependent anti-microbial activity against ingested microbes represents the collaboration of multiple agents, including those prefabricated during granulocyte development in the bone marrow and those generated de novo following neutrophil activation. Furthermore, neutrophils cooperate with extracellular agents as well as other immune cells to optimally kill and degrade invading microbes. This brief review focuses attention on two examples of the integrated nature of neutrophil-mediated anti-microbial action within the phagosome. The importance and complexity of myeloperoxidase-mediated events illustrate a collaboration of anti-microbial responses that are endogenous to the neutrophil, whereas the synergy between the phagocyte NADPH (nicotinamide adenine dinucleotide phosphate) oxidase and plasma-derived group IIA phospholipase A(2) exemplifies the collective effects of the neutrophil with an exogenous factor to achieve degradation of ingested staphylococci.

Liver-homing of purified glucose oxidase: a novel in vivo model of physiological hepatic oxidative stress (H2O2)

BACKGROUND/AIMS

Reactive oxygen species (ROS), such as H2O2, are implicated in normal and pathological liver function. However, due to the lack of suitable in vivo models of ROS generation the (patho) physiological relevance of H2O2 remains controversial.

METHODS

We established a novel model of sustained hepatic H2O2 release using intravenous administration of purified Aspergillus niger glucose oxidase (GOX) in rats.

RESULTS

GOX is rapidly cleared from the blood stream and almost exclusively localizes to Kupffer cells. GOX maintained its ability to generate H2O2 over 24h. While sublethal GOX doses induced hepatocellular necrosis, our morphological and functional studies show that lower levels of GOX which generate H2O2 comparable to release by inflammatory cells are non-toxic and do not induce histological inflammation. However, these non-toxic H2O2 levels increased oxidized glutathione and induced heme oxygenase 1 in the liver. In addition, several hepatocyte transporters were downregulated, while no decrease of bile formation, a sensitive marker of liver function, was observed.

CONCLUSIONS

Our in vivo model allows to study the effects of extracellular H2O2 in the liver as is released by inflammatory cells. Thus analysis of the role of this major ROS in the absence of confounding inflammatory cofactors will be possible.

How neutrophils kill microbes

Neutrophils provide the first line of defense of the innate immune system by phagocytosing, killing, and digesting bacteria and fungi. Killing was previously believed to be accomplished by oxygen free radicals and other reactive oxygen species generated by the NADPH oxidase, and by oxidized halides produced by myeloperoxidase. We now know this is incorrect. The oxidase pumps electrons into the phagocytic vacuole, thereby inducing a charge across the membrane that must be compensated. The movement of compensating ions produces conditions in the vacuole conducive to microbial killing and digestion by enzymes released into the vacuole from the cytoplasmic granules.

Myeloperoxidase: friend and foe

Neutrophilic polymorphonuclear leukocytes (neutrophils) are highly specialized for their primary function, the phagocytosis and destruction of microorganisms. When coated with opsonins (generally complement and/or antibody), microorganisms bind to specific receptors on the surface of the phagocyte and invagination of the cell membrane occurs with the incorporation of the microorganism into an intracellular phagosome. There follows a burst of oxygen consumption, and much, if not all, of the extra oxygen consumed is converted to highly reactive oxygen species. In addition, the cytoplasmic granules discharge their contents into the phagosome, and death of the ingested microorganism soon follows. Among the antimicrobial systems formed in the phagosome is one consisting of myeloperoxidase (MPO), released into the phagosome during the degranulation process, hydrogen peroxide (H2O2), formed by the respiratory burst and a halide, particularly chloride. The initial product of the MPO-H2O2-chloride system is hypochlorous acid, and subsequent formation of chlorine, chloramines, hydroxyl radicals, singlet oxygen, and ozone has been proposed. These same toxic agents can be released to the outside of the cell, where they may attack normal tissue and thus contribute to the pathogenesis of disease. This review will consider the potential sources of H2O2 for the MPO-H2O2-halide system; the toxic products of the MPO system; the evidence for MPO involvement in the microbicidal activity of neutrophils; the involvement of MPO-independent antimicrobial systems; and the role of the MPO system in tissue injury. It is concluded that the MPO system plays an important role in the microbicidal activity of phagocytes.

Reconstitution of bactericidal activity in chronic granulomatous disease cells by glucose-oxidase-containing liposomes

Chronic granulomatous disease (CGD) is an inherited primary immunodeficiency characterized by phagocytes devoid of a functioning nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. The failure of CGD phagocytes to produce reactive oxygen species (ROS) results in a marked increase in the susceptibility of affected patients to life-threatening bacterial and fungal infections. This study investigated whether loading of CGD phagocytes with glucose oxidase (GO)-containing liposomes (GOLs) could restore cellular production of bactericidal ROS (eg, H2O2 and HOCl) in vitro. Results indicate that GO encapsulated in liposomes enabled NADPH oxidase-deficient phagocytes to use H2O2 for the production of highly bactericidal HOCl. The intracellular colocalization of bacteria and liposomes (or liposome-derived ferritin) was demonstrated by confocal laser microscopy and electron microscopy. After uptake of GOLs (approximately 0.2 U/mL at 1 mM total lipid concentration, size approximately 180 nm), CGD granulocytes produced HOCl levels comparable to those of normal phagocytes. Remarkably, after treatment with GOLs, CGD phagocytes killed Staphylococcus aureus as efficiently as normal granulocytes. Moreover, treated cells retained sufficient motility toward chemotactic stimuli as measured by chemotaxis assay. Side effects were evaluated by measuring the H2O2 concentrations and the production of methemoglobin in whole blood. These studies revealed that H2O2 produced by GOLs was degraded immediately by the antioxidative capacity of whole blood. Elevated methemoglobin levels were observed only after application of extremely high amounts of GOLs (2 U/mL). In summary, the application of negatively charged GOLs might provide a novel effective approach in the treatment of patients with CGD at high risk for life-threatening infections.

Free radicals in medicine. II. Involvement in human disease

This review explores evidence that free radicals might be involved in various human disease processes. Such involvement is difficult to prove because direct evidence is often lacking and is based on animal models of the disease process. Evidence for free radical involvement includes demonstrating abnormal free radical production in the disease, finding that deliberately applying free radical-producing systems into the cellular locus responsible for the disease reproduces its manifestations, and showing that free radical scavengers control facets of the disease process. Confirmation of free radical involvement in a particular disease may have clinical relevance, inasmuch as clinically applicable techniques are currently being developed to remove free radicals from cellular sites where they are injurious and, in other situations such as chemotherapy, techniques or drugs that produce free radicals are available to destroy harmful cells.

Differential breakdown of phylogenetically diverse ribosomal RNA's inserted via liposomes into mammalian cells

Liposomes were used to deliver ribosomal RNA's from the different organisms into cultivated mouse plasmacytoma cells. Ribosomal RNA from Escherichia coli was degraded intracellularly within 1 hour, whereas mouse and yeast ribosomal RNA's were degraded more slowly. This indicates that cells can discriminated between different ribosomal RNA's.

Current view of neutrophil dysfunction: an integrated clinical perspective

Normal neutrophil function is dependent on the integration of chemotaxis, phagocytosis, degranulation and oxidative metabolism. The availability of in vitro assays for the separate quantitative evaluation of each function has permitted the definition of specific congenital and acquired neutrophil abnormalities, which are associated with defective host resistance. The appreciation of complex and often adverse effects of certain systemic diseases and drugs on neutrophil function as well as the use of new approaches to therapy suggest the importance of assessing the role of the neutrophil in states of impaired host defense.

Neutrophil dysfunction associated with states of chronic and recurrent infection

Infants, children, and young adults who suffer chronic and recurrent bacterial or fungal infection despite adequate numbers of circulating granulocytes and normal or elevated levels of immunoglobulins should be suspected of having fundamental defects in granulocyte functioning. This article considers clinical disorders for which there is evidence for associated defects of polymorphonuclear leukocytes.