P. H. Nibbering

Antibacterial Activity of Human Neutrophil Defensins in Experimental Infections in Mice Is Accompanied by Increased Leukocyte Accumulation

Neutrophil defensins (or human neutrophil peptides-HNP) are major constituents of the azurophilic granules of human neutrophils and have been shown to display broad-spectrum antimicrobial activity. Other activities of these defensins, which are released from stimulated neutrophils, include cytotoxic, stimulatory, and chemotactic activities toward a variety of target cells. We studied the potential use of HNP-1 for antibacterial therapy of experimental bacterial infections in mice. In experimental peritoneal Klebsiella pneumoniae infections in mice, HNP-1 injection was shown to markedly reduce bacterial numbers in the infected peritoneal cavity 24 h after infection. This antibacterial effect was found to be associated with an increased influx of macrophages, granulocytes, and lymphocytes into the peritoneal cavity. These leukocytes appeared to be a requirement for the antibacterial effect, since in leukocytopenic mice administration of HNP-1 did not display antibacterial activity. HNP-1 treatment also reduced bacterial numbers in experimental K. pneumoniae or Staphylococcus aureus thigh muscle infections. In this model, radiolabeled HNP-1 was found to accumulate at the site of infection, whereas most of the injected HNP-1 was rapidly removed from the circulation via renal excretion. These results demonstrate that neutrophil defensins display marked in vivo antibacterial activity in experimental infections in mice and that this activity appears to be mediated, at least in part, by local leukocyte accumulation.

Effects of low molecular constituents from Aloe vera gel on oxidative metabolism and cytotoxic and bactericidal activities of human neutrophils

In traditional South-East Asian medicine the therapeutic value of the parenchymous leaf-gel of Aloe vera for inflammatory-based diseases is well-reputed. The aim of this study is to investigate at which level gel-constituents exert their activity. We show here that low -Mr constituents of an aqueous gel-extract inhibit the release of reactive oxygen species (ROS) by PMA-stimulated human PMN. The compounds inhibit the ROS-dependent extracellular effects of PMN such as lysis of red blood cells. The capacity of the PMN to phagocytose and kill micro-organisms at the intracellular level is not affected. The inhibitory activity of the low-Mr compounds is most pronounced in the PMA-induced ROS production, but is significantly antagonized by the Ca-ionophore A23187. It is shown that the inhibitory effect of the low-Mr compounds is the indirect result of the diminished availability of intracellular free Ca-ions.

Ingestion of pulmonary surfactant by human monocytes inhibits their antibacterial functions

Alveolar macrophages are believed to play a critical role in the maintenance of the integrity of the lungs by both ingesting potentially harmful agents, such as allergens and infectious microorganisms, and producing a number of specific mediators, such as reactive oxygen intermediates, arachidonic acid metabolites, chemotactic factors, and cytokines (1). These macrophages reside in a microenvironment which is rich in surfactant and they also contain this material (2, 3). Surfactant, comprising about 90% lipid, 8% protein, and 2% carbohydrates, is produced by type 2 epithelial alveolar cells; its main function is to reduce surface tension in the alveoli (4). Despite their strategic position outside the epithelial lining of the air spaces, alveolar macrophages are limited in their bactericidal activities compared to other phagocytes (5–7).

Macrophage activation by recombinant cytokines

Activated macrophages are considered to be the major effector cells in the host defense against infections with facultative intracellular pathogens. These macrophages differ from resident cells in many respects, such as a changed morphology, expression of cell-surface receptors, enhanced production of reactive oxygen intermediates (ROI) and nitrogen intermediates (RNI), increased microbicidal activity and enhanced release of cytokines, such as interleukins (IL)-1, 6 and 8, granulocyte-macrophage colony-stimulating factor (GM-CSF), and tumor necrosis factor α (TNF-α) (1, 2) . In general, activated macrophages are considered to phagocytose and kill microorganisms more efficiently than resident macrophages. In vivo, macrophage activation occurs mainly via interaction between T lymphocytes and macrophages and is mediated by cytokines.

The role of calcium and protein kinase C in the intracellular killing of bacteria by human monocytes

Optimal intracellular killing of bacteria by human granulocytes and monocytes requires extracellular stimulation by immunoglobulins and complements factors (1–3). Interactions between these serum proteins and their corresponding receptors on the surface of phagocytes initiates intracellular signalling which determines the functional activities of these cells under specific conditions (4).

Effect of interferon-γ on the development of inflammatory lesions in the liver of mice during infection with Listeria monocytogenes

Immunity against infections with facultative intracellular bacteria, such as Listeria monocytogenes, is characterized by activation of macrophages and the formation of inflammatory lesions in infected organs (1). The initial phase of host defense is marked by an influx of granulocytes followed by monocyte infiltration to sites where L. monocytogenes have accumulated. Although both types of phagocyte initially kill substantial numbers of the bacteria in the tissues (2, 3) the number of remaining bacteria increases gradually until cell-mediated immunity is fully developed. Cell-mediated immunity is characterized by the appearance of numberous Listeria-specific T lymphocytes and the activation of macrophages which result in a sharp drop in the number of bacteria in the infected tissue.

Defective intracellular killing of micro-organisms by murine alveolar macrophages

Peritoneal and alveolar macrophages differ in phenotype, endocytic activities, and oxidative metabolism.

Signal transduction in resident and activated macrophages

During an infection with (facultative) intracellular bacteria, macrophages become activated, displaying multiple biochemical changes which include the synthesis and secretion of various biochemical effector molecules (1, 2). These changes mediate the acquisition of several functional characteristics, such as an enhanced bactericidal activity (3–6), the ability to inhibit the intracellular proliferation of various protozoa (7–9), and the destruction of tumor cells (10, 11). The features most widely used as criteria for macrophage activation are: the ability to inhibit intracellular proliferation of protozoa (7–9); the enhanced expression of Ia antigen (10, 12); and the increased production of reactive oxygen intermediates (ROI) (13). In an earlier study we showed that the level of reactive nitrogen intermediates (RNI), measured as NO - 2 production, in murine peritoneal macrophages correlates with these classical criteria for macrophage activation (14).

Interaction between activated macrophages and Mycobacterium fortuitum

Cell-mediated immunity against mycobacteria has been known for more than 100 years (1). This type of immunity is the result of cooperation between at least two types of cells: T lymphocytes and macrophages. The specificity of the immunity lies in the interaction between T cells and the mycobacterial antigens presented by macrophages and dendritic cells. CD4+ as well as CD8+ T cells produce a number of cytokines, e.g. IFN-γ, GM-CSF, IL-2, IL-4 and IL-6, which can activate macrophages and thus enhance their antimy-cobacterial functions (2–6). These activated macrophages are thought to be the ultimate effector cells of cellular immunity against mycobacteria. Other types of cell, such as CD8+ T cells (7), NK cells (8) and (activated) granulocytes (9), also play an important role in the defense against mycobacterial infections as well.

Activation of murine peritoneal macrophages by Salmonella typhimurium

The results of these studies indicate that the interaction between activated macrophages and S. typhimurium depends on the characteristics of the micro-organism and the kind of activation.