Microbicidal mechanisms of phagocytes

L-Arginine and tetrahydrobiopterin supported nitric oxide production is crucial for the microbicidal activity of neutrophils

Recent report from this lab has shown role of Rac2 in the translocation of inducible nitric oxide synthase (iNOS) to the phagosomal compartment of polymorphonuclear leukocytes (PMNs) following phagocytosis of beads. This study was undertaken to further assess the status and role of tetrahydrobiopterin (BH4), a redox-sensitive cofactor, L-arginine, and the substrate of nitric oxide synthase (NOS) in sustained nitric oxide (˙NO) production in killing of phagocytosed microbes (Escherichia coli) by human PMNs. Time-dependent study revealed consistent NO and reactive oxygen species (ROS) production in the PMNs following phagocytosis of beads. In addition, levels of L-arginine and BH4 were maintained or increased simultaneously to support the enzymatic activity of NOS in the bead activated PMNs. Moreover, translocation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (NOX) subunits along with iNOS was reconfirmed in the isolated phagosomes. We demonstrate that increase in the level of NO was supported by L-arginine and BH4 to kill E. coli, by using PMNs from NOS2-/- mice, human PMNs treated with biopterin inhibitor, N-acetyl serotonin (NAS), or by suspending human PMNs in L-arginine deficient medium. Altogether, this study demonstrates that following phagocytosis, sustained. NO production in the PMNs was well-maintained by redox sensitive cofactor, BH4 and substrate, and L-arginine to enable microbial killing. Further results suggest NO production in the human PMNs, along with ROS and myeloperoxidase (MPO) is important to execute antimicrobial activity.

The Pathophysiology of Phagocytes

No survey of clinically important immunological phenomena would be complete without consideration of the functions of phagocytic cells. They play a pivotal role in the immune response by kiling microbes, by presenting antigens to lymphocytes and by serving as supportive, accessory cells to lymphocytes, at least partly by releasing soluble factors. The phagocytes of the body, professional and non professional, consist of two specialized groups of cells: granulocytes, which can be mobilized rapidly and which reach inflamed sites quickly and in large numbers, and which are highly efficient at dealing with many types of injury and infection but which have no capacity for differentiation and live only a short time; and the mononuclear phagocyte system consisting partly of motile cells which respond initially more slowly than neutrophils but which can differentiate in sites of inflammation into cells which are more efficient in various functions than the cells from which they originated. Many mononuclear phagocytes are fixed cells located in tissues where they act as trays or filters for material circulating through the tissue. Phagocytes, which usually function as the primary defender in infections, have also been implicated as effector cells in several conditions characterized by a destructive inflammatory response.

Interference of antibacterial agents with phagocyte functions: immunomodulation or "immuno-fairy tales"?

Professional phagocytes (polymorphonuclear neutrophils and monocytes/macrophages) are a main component of the immune system. These cells are involved in both host defenses and various pathological settings characterized by excessive inflammation. Accordingly, they are key targets for immunomodulatory drugs, among which antibacterial agents are promising candidates. The basic and historical concepts of immunomodulation will first be briefly reviewed. Phagocyte complexity will then be unravelled (at least in terms of what we know about the origin, subsets, ambivalent roles, functional capacities, and transductional pathways of this cell and how to explore them). The core subject of this review will be the many possible interactions between antibacterial agents and phagocytes, classified according to demonstrated or potential clinical relevance (e.g., neutropenia, intracellular accumulation, and modulation of bacterial virulence). A detailed review of direct in vitro effects will be provided for the various antibacterial drug families, followed by a discussion of the clinical relevance of these effects in two particular settings: immune deficiency and inflammatory diseases. The prophylactic and therapeutic use of immunomodulatory antibiotics will be considered before conclusions are drawn about the emerging (optimistic) vision of future therapeutic prospects to deal with largely unknown new diseases and new pathogens by using new agents, new techniques, and a better understanding of the phagocyte in particular and the immune system in general.

Interference of antibacterial agents with phagocyte functions: immunomodulation or "immuno-fairy tales"?

Professional phagocytes (polymorphonuclear neutrophils and monocytes/macrophages) are a main component of the immune system. These cells are involved in both host defenses and various pathological settings characterized by excessive inflammation. Accordingly, they are key targets for immunomodulatory drugs, among which antibacterial agents are promising candidates. The basic and historical concepts of immunomodulation will first be briefly reviewed. Phagocyte complexity will then be unravelled (at least in terms of what we know about the origin, subsets, ambivalent roles, functional capacities, and transductional pathways of this cell and how to explore them). The core subject of this review will be the many possible interactions between antibacterial agents and phagocytes, classified according to demonstrated or potential clinical relevance (e.g., neutropenia, intracellular accumulation, and modulation of bacterial virulence). A detailed review of direct in vitro effects will be provided for the various antibacterial drug families, followed by a discussion of the clinical relevance of these effects in two particular settings: immune deficiency and inflammatory diseases. The prophylactic and therapeutic use of immunomodulatory antibiotics will be considered before conclusions are drawn about the emerging (optimistic) vision of future therapeutic prospects to deal with largely unknown new diseases and new pathogens by using new agents, new techniques, and a better understanding of the phagocyte in particular and the immune system in general.

Neutrophil chloramines: missing links between innate and acquired immunity

Neutrophils are the major cellular component of the acute inflammatory response. By contrast, macrophages are the major cellular component in most chronic immunological responses, and act as key regulators of the specific acquired response. Here, Janusz Marcinkiewicz examines recent data indicating that chloramines, the neutrophil-specific products of the myeloperoxidase--hydrogen-peroxide--halide system, may provide a bridge between the afferent branches of the innate and acquired immune response.

Nitric oxide and antimicrobial activity of reactive oxygen intermediates

It is well documented that nitric oxide contributes to the bactericidal activities of phagocytes. Murine activated neutrophils and macrophages produce both reactive oxygen intermediates (ROI) and reactive nitrogen intermediates (RNI). However, only neutrophils in the presence of myeloperoxidase, produce an antimicrobial agent, hypochlorous acid (HOCl). Complex interactions of RNI (nitric oxide) with other antimicrobial agents of phagocytes are likely to exist, but these have not been clearly demonstrated. In this study, we treated bacteria (Escherichia coli) with the NO donor, S-nitrosoglutathione (GSNO) and hydrogen peroxide (H2O2) or HOCl. We found that exposure to H2O2 of the bacteria tested resulted in minimal toxicity. However the killing activities of H2O2 were potentiated by GSNO. On the contrary, the NO-donor completely abolished the bactericidal activity of HOCl. Our results indicate that NO-donating drugs in non toxic concentrations used for experimental purposes may strongly affect the cytotoxic activity of neutrophils and macrophages. We suggest that the similar interactions may exist at sites of inflammation.

Human polymorphonuclear leucocytes stimulated by tumour necrosis factor-alpha show increased adherence to extracellular matrix proteins which is mediated via the CD11b/18 complex

The present study demonstrates that tumour necrosis factor (TNF) and FMLP, but not IL-1 or IL-8, enhanced the adherence of polymorphonuclear neutrophil (PMN) to fibronectin, an extracellular matrix protein. The adherence induced by FMLP was very rapid, within 5 min while the induction of adherence by TNF was much slower, reaching maximum at 60 min. TNF also enhanced an adhesion of PMN to other extracellular matrix proteins, such as laminin, collagen IV and gelatin II, but not to human serum albumin. Anti-CD18 MoAb completely inhibited the binding of TNF-stimulated PMN to fibronectin and partially inhibited the binding to laminin. Further investigation showed that adhesion of TNF-stimulated PMN to fibronectin and laminin was inhibited by anti-CD11b MoAb and to a lesser extent by CD11a MoAb. In contrast to TNF-stimulated PMN the binding of unstimulated PMN to fibronectin and laminin was only inhibited by anti-CD11a MoAb. Anti-CD11c had no effect on PMN adherence. These results suggest that unstimulated PMN adhere to extracellular proteins through the CD11a/18, while TNF-stimulated PMN adhere through the CD11b/18. These results suggest that TNF secreted at the site of inflammation may enhance the interaction of PMN with the extravascular environment through the CD11b/18 complex.

Effect of a human immunodeficiency virus protease inhibitor on human monocyte function

Human immunodeficiency virus (HIV) is the cause of acquired immunodeficiency syndrome (AIDS). Encoded by the HIV genome are several precursor proteins that undergo proteolytic cleavage to yield functional proteins. The env precursor protein is cleaved by a cellular protease. The gag precursor protein of HIV (p55), however, is cleaved by a virally encoded aspartate protease (HIV Protease). Cleavage of p55 is required for viral maturation and infectivity. There are also several host cell aspartate proteases that serve important homeostatic functions. Cathepsins D and E are lysosomal aspartate proteases which are believed to play an important role in macrophage function, and it has been suggested that inhibition of these enzymes by an HIV protease inhibitor may exacerbate immunosuppression in AIDS patients. We have studied the effect of SK&F 107461 (a hydroxyethylene dipeptide isostere inhibitor of HIV protease), on various host defense functions of human monocytes. Pepstatin A (an inhibitor of most aspartate proteases) and leupeptin (an inhibitor of serine and cysteine proteases) were included as controls. Although less potent than the prototypic aspartate protease inhibitor pepstatin, SK&F 107461 inhibited partially purified cathepsin D in vitro. However, in cell-based assays, SK&F 107461 had no effect on the degradation of hemoglobin, antigen processing of the protein antigen streptokinase, or secretion of 17-kD IL-1 beta by monocytes at concentrations which inhibit maturation of intracellular virus in HIV infected monocytes. Furthermore, SK&F 107461 had no effect on constitutive candidacidal activity. In contrast, leupeptin and pepstatin A partially inhibited accessory cell function of monocytes in the proliferative response to the recall antigen streptokinase. In addition, leupeptin partially inhibited degradation of hemoglobin.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction and intracellular killing of Candida albicans blastospores by human polymorphonuclear leucocytes, monocytes and monocyte-derived macrophages in aerobic and anaerobic conditions

Polymorphonuclear leucocytes (PMN), monocytes and monocyte-derived macrophages were capable of interacting with opsonized C. albicans in both aerobic and anaerobic conditions. Superoxide anion release by these cells was inhibited in anaerobic conditions while lysozyme release and phagocytosis were equally efficient in both aerobic and anaerobic conditions. All cell types tested were capable of intracellular killing of C. albicans and this appeared to be maximum at 6 h for monocytes and macrophages and 24 h for PMN. Monocytes killed the lowest number of organisms, 1 x 10(6), and the killing was similar for aerobic and anaerobic conditions. In contrast, PMN and macrophages demonstrated greater killing of C. albicans in aerobic conditions compared with anaerobic conditions; PMN killed 1.9 x 10(6) organisms and macrophages 3 x 10(6) when incubated anaerobically. Inhibitors of oxygen metabolism decreased intracellular killing of C. albicans by macrophages and PMN in aerobic but not anaerobic conditions. The oxygen reaction products involved in the killing of C. albicans appeared to be different however: macrophage killing was decreased by superoxide anion and hydrogen peroxide inhibitors. PMN killing was decreased by superoxide anion, hydrogen peroxide, hypochlorous acid and hydroxyl radical inhibitors. The present study shows that although monocytes, macrophages and PMN function similarly in their interaction with C. albicans, they appear to use different oxygen reactive products for the intracellular killing of C. albicans.

Effects of anaerobiosis and aerobiosis on interactions of human polymorphonuclear leukocytes with the dental plaque bacteria Streptococcus mutans, Capnocytophaga ochracea, and Bacteroides gingivalis

Human polymorphonuclear leukocytes (PMN) were able to generate and release superoxide anions upon stimulation of Streptococcus mutans, Bacteroides gingivalis, and Capnocytophaga ochracea when incubated aerobically but not when incubated anaerobically. Lysozyme release and phagocytosis by PMN were independent of oxygen, and no difference between PMN incubated aerobically or anaerobically was observed (PMN stimulated by B. gingivalis released 7.6% total lysozyme when aerobic and 6.9% when anaerobic). There were variations in lysozyme release and phagocytosis for the three organisms, particularly for phagocytosis. B. gingivalis and C. ochracea yielded lower phagocytosis values than those for S. mutans, e.g., at 1 h 67% of the initial inoculum of S. mutans was phagocytosed (versus only 40% for B. gingivalis). Transmission electron microscopy showed that both S. mutans and B. gingivalis were internalized into classical phagolysosomes. In contrast, C. ochracea showed two forms of internalization; C. ochracea either formed a classical phagolysosome or was tightly bound in the cytoplasm with no surrounding cell membrane. Intracellular killing of S. mutans and C. ochracea was unaffected by anaerobiosis, but killing of C. ochracea was much lower than that of S. mutans (1 x 10(7) to 2 x 10(7) bacteria killed compared with 5.1 x 10(7) bacteria killed at 6 h). In contrast, a greater number of B. gingivalis was killed in the presence of oxygen (5.3 x 10(7) bacteria were killed when aerobically incubated and 1.9 x 10(7) bacteria were killed when anaerobically incubated). These results suggest that the ability to survive anaerobically may enable some bacteria to evade PMN killing; however, abnormal phagocytosis may represent a more efficient way to evade both oxygen-dependent and -independent killing mechanisms, leading to enhanced virulence of the organism.