The timing of exposure of mononuclear phagocytes to recombinant interferon gamma and recombinant tumor necrosis factor alpha alters interactions with Nocardia asteroides

Intracellular behavior of Nocardia seriolae and its apoptotic effect on RAW264.7 macrophages

Nocardia seriolae, an intracellular gram-positive pathogen, is prone to infecting immunocompromised and surface-damaged fish, causing serious losses to the aquaculture industry. Although a previous study has demonstrated that N. seriolae infects macrophages, the persistence of this bacterium in macrophages has not been well characterized. To address this gap, we used the macrophage cell line RAW264.7, to investigate the interactions between N. seriolae and macrophages and deciphered the intracellular survival mechanism of N. seriolae. Confocal and light microscopy revealed that N. seriolae entered macrophages 2 hours post-inoculation (hpi), were phagocytosed by macrophages at 4-8 hpi, and induced the formation of multinucleated macrophages by severe fusion at 12 hpi. Flow cytometry, evaluation of mitochondrial membrane potential, release of lactate dehydrogenase, and observation of the ultrastructure of macrophages revealed that apoptosis was induced in the early infection stage and inhibited in the middle and later periods of infection. Additionally, the expression of Bcl-2, Bax, Cyto-C, Caspase-3, Capase-8, and Caspase-9 was induced at 4 hpi, and then decreased at 6-8 hpi, illustrating that N. seriolae infection induces the activation of extrinsic and intrinsic apoptotic pathways in macrophages, followed by the inhibition of apoptosis to survive inside the cells. Furthermore, N. seriolae inhibits the production of reactive oxygen species and releases large amounts of nitric oxide, which persists in macrophages during infection. The present study provides the first comprehensive insight into the intracellular behavior of N. seriolae and its apoptotic effect on macrophages and may be important for understanding the pathogenicity of fish nocardiosis.

Treatment of disseminated nocardiosis: a host-pathogen approach with adjuvant interferon gamma

Disseminated nocardiosis is a rare, life-threatening disease. Particularly at risk are immunocompromised patients, highlighting the crucial role of host factors. Conventional intensive antibiotic treatment has improved survival rates, but the overall prognosis of patients with disseminated nocardiosis remains unsatisfactory. In this Grand Round, we present a case of severe nocardiosis that did not respond to standard therapy. The patient's condition deteriorated when antibiotic therapy was given alone and improved substantially only after coadministration of interferon gamma. We review the literature relevant to adjuvant interferon gamma therapy of nocardiosis and discuss its potential harms and benefits. Overall, we consider such treatment as beneficial and low risk if the patient is followed-up closely. We conclude that clinicians should consider this regimen in refractory cases of severe Nocardia infection.

The role of cytokines and their signaling pathways in the regulation of immunity to Toxoplasma gondii

The development of a strong cellular immune response is critical for the control of the intracellular pathogen Toxoplasma gondii. This occurs by activation of a complex, integrated immune response, which utilizes cells of the innate and adaptive immune systems. In the last two decades there have been major advances in our understanding of the role of cytokines in the initiation and maintenance of protective immunity to T. gondii, and IFN-gamma has been identified as the major mediator of resistance to this pathogen. This article provides an overview of the biology of toxoplasmosis and focuses on the pivotal role of cytokines and their signaling pathways during infection. It also addresses the role of cytokines in modulating other immune functions that are critical in determining the balance between a protective and a pathological immune response.

Increase of gammadelta T lymphocytes in murine lungs occurs during recovery from pulmonary infection by Nocardia asteroides

Previous studies have demonstrated that gammadelta T lymphocytes are important for host resistance to pulmonary infection of the murine lung by log-phase cells of Nocardia asteroides. To study the role of gammadelta T cells in nocardial interactions in the murine lung, C57BL/6J wild type and C57BL/6J-Tcrd (gammadelta T-cell knockout mice) were infected intranasally with log-phase cells of N. asteroides GUH-2. At 3, 5, and 7 days after infection, the gammadelta T cells were quantified by multiparameter flow cytometry. At the same time, Gram and hematoxylin-eosin stains of paraffin sections were performed to monitor the host responses. The data showed that gammadelta T lymphocytes increased significantly within the lungs after intranasal infection, and the peak of this cellular increase occurred at 5 days. Furthermore, at this time, greater than 50% of the CD3 T-cell receptor (TCR)-positive (CD3+) cells were gammadelta TCR positive. Histological examination clearly showed divergent inflammatory responses in the lungs of wild-type mice compared to gammadelta T-cell knockout mice. The C57BL/6J-Tcrd mice were less capable of clearing the organism, and the polymorphonuclear leukocyte response lasted longer than in wild-type C57BL/6J mice. These results showed that gammadelta T cells were actively involved in modulating the innate host responses to murine pulmonary infection by N. asteroides.

Differences in the interactions of Nocardia asteroides with macrophage, endothelial, and astrocytoma cell lines

An in vitro model for studying host cell interactions with Nocardia asteroides was developed. Thus, macrophage cell lines J774A.1 and P388D1, pulmonary artery endothelium cell line CPAE, rat glial tumor cell line C6, and human astrocytoma cell lines CCF-STTG1 and U-373 MG were infected with either log- or stationary-phase cells of N. asteroides GUH-2, and the host cell-nocardia interactions were determined by light microscopy and electron microscopy. Polyclonal antinocardial antibody did not enhance uptake of nocardiae by any of these cell lines; however, log-phase cells of GUH-2 infected a higher percentage of J774A.1 and P388D1 than did stationary-phase organisms. When cells infected with stationary-phase GUH-2 were incubated for 6 h, filaments developed, which indicated that nocardial growth had occurred. In J774A.1 and P388D1, only 31 to 57% of the total stationary-phase coccobacillary cells that were phagocytized formed filaments within 6 h. This indicated that there was some inhibition of growth of the phagocytized nocardiae within these macrophage cell lines; however, the nocardiae grew within the endothelial (> 87% filaments) and astrocytoma (100% filaments) cell lines. Microfilament inhibitor cytochalasin B inhibited uptake of GUH-2 by macrophages and other cell lines, except that there was no effect on uptake of nocardial cells by astrocytoma cell line U-373 MG. Scanning and transmission electron microscopy showed phagocytosis of GUH-2 by the different cell lines. In cytochalasin B-treated cells, nocardiae were shown to penetrate through the cell surface and become internalized in a manner distinct from typical phagocytosis, suggesting that filamentous forms of this organism have a phagocytosis-independent invasion factor. The extent of this cytochalasin-resistant cellular penetration by the nocardiae differed in the different cell lines.

Nocardia species: host-parasite relationships

The nocardiae are bacteria belonging to the aerobic actinomycetes. They are an important part of the normal soil microflora worldwide. The type species, Nocardia asteroides, and N. brasiliensis, N. farcinica, N. otitidiscaviarum, N. nova, and N. transvalensis cause a variety of diseases in both normal and immunocompromised humans and animals. The mechanisms of pathogenesis are complex, not fully understood, and include the capacity to evade or neutralize the myriad microbicidal activities of the host. The relative virulence of N. asteroides correlates with the ability to inhibit phagosome-lysosome fusion in phagocytes; to neutralize phagosomal acidification; to detoxify the microbicidal products of oxidative metabolism; to modify phagocyte function; to grow within phagocytic cells; and to attach to, penetrate, and grow within host cells. Both activated macrophages and immunologically specific T lymphocytes constitute the major mechanisms for host resistance to nocardial infection, whereas B lymphocytes and humoral immunity do not appear to be as important in protecting the host. Thus, the nocardiae are facultative intracellular pathogens that can persist within the host, probably in a cryptic form (L-form), for life. Silent invasion of brain cells by some Nocardia strains can induce neurodegeneration in experimental animals; however, the role of nocardiae in neurodegenerative diseases in humans needs to be investigated.

Interactions of Nocardia asteroides with murine glia cells in culture

Astrocytes and microglia were obtained from brains of newborn mice and infected with Nocardia asteroides. Scanning electron micrographs showed nocardiae adhering to the astrocyte cell surface and entering the cytoplasm. After 6 h of incubation the intracellular nocardiae appeared as filaments, demonstrating that growth was occurring. In contrast, the microglia phagocytized nocardiae, but after 6 h the presence of coccoid cells indicated that nocardial growth was inhibited.

Ultrastructural analysis of growth of Nocardia asteroides during invasion of the murine brain

BALB/c mice were infected with 10(6) CFU of log-phase cells of Nocardia asteroides GUH-2 by tail vein injection (at this lethal inoculum dose, approximately 800 to 1,000 CFU becomes deposited in the brain). At 24 h after infection, the ultrastructural interactions of the nocardiae during growth within the murine brain were investigated. The nocardiae grew perivascularly in the pons, substantia nigra, hypothalamus, and thalamus portions of the brain, where they were either within or associated with most brain cell types. There appeared to be a propensity for growth within the soma of neurons and their axonal extensions. The nocardial cells were surrounded by 1 to 30 layers of membrane, and the innermost membrane was usually tightly adherent to the cell wall. This compartmentalization of nocardiae within brain cells could contribute to their failure to induce an inflammatory response or a cytopathic effect. Nevertheless, the bacteria were able to obtain adequate nutrients from the host to grow within the brain. The nocardiae were not completely inert, since some of the brain cells showed signs of degeneration. The myelin sheaths of axons were the most strongly affected, and there was evidence of demyelinization and axonal degeneration. Nocardiae growing within brain cells were phagocytized by compact, dense cells that were probably microglia. There was no ultrastructural evidence that the nocardiae were damaged by these phagocytes 24 h after infection; nevertheless, these cells may be important for the elimination of nocardiae from the brain during a nonlethal infection.

Killing of Coccidioides immitis by human peripheral blood mononuclear cells

The ability of human peripheral blood mononuclear cells (MNL) obtained from healthy donors to kill the fungus Coccidioides immitis was examined in vitro with an assay that uses a single fungal particle per well. MNL killed 25.0% +/- 3.5% of a coccidioidal arthroconidial target, compared with the 4.7% +/- 2.9% killed by polymorphonuclear leukocytes obtained from the same donors (P = 0.012). Arthroconidial killing by MNL was not dependent on donor delayed dermal hypersensitivity to spherulin. Killing of another fungal target, Candida glabrata, was not significantly different between MNL and polymorphonuclear leukocytes (P = 0.783). Depletion of monocytes from MNL with Sephadex G-10 resulted in a significant reduction in arthroconidial killing (21.4% +/- 13.6% versus 2.4% +/- 3.4%; P = 0.025), while enrichment of monocytes by Percoll density gradient centrifugation or plastic adherence resulted in significantly increased arthroconidial killing compared with that by MNL (P = 0.005 and 0.001, respectively). Killing of 96-h spherules by MNL was 7.3% +/- 3.1%, significantly less than the 21.4% +/- 2.8% killing of arthroconidia in the same experiments (P = 0.016). Incubation of MNL with human recombinant gamma interferon or tumor necrosis factor alpha did not result in increased MNL killing of coccidioidal arthroconidia under various conditions. These results suggest that MNL have an inherent ability to kill coccidioidal arthroconidia in vitro which is not dependent on prior host exposure to C. immitis. This activity appears to reside in peripheral blood monocytes.