Inhibition of growth of Mycobacterium avium in murine and human mononuclear phagocytes by migration inhibitory factor

Retracted science and the retraction index

Articles may be retracted when their findings are no longer considered trustworthy due to scientific misconduct or error, they plagiarize previously published work, or they are found to violate ethical guidelines. Using a novel measure that we call the "retraction index," we found that the frequency of retraction varies among journals and shows a strong correlation with the journal impact factor. Although retractions are relatively rare, the retraction process is essential for correcting the literature and maintaining trust in the scientific process.

Temporal effect of tumor necrosis factor alpha on murine macrophages infected with Mycobacterium avium

Members of the Mycobacterium avium complex are a family of bacteria that persist within macrophages in the face of an immune response. Elimination of these organisms is likely due to cytokine-induced macrophage activation. Because macrophage activation by tumor necrosis factor alpha (TNF-alpha) appears critical for killing of intracellular M. avium, early downregulation of TNF-alpha levels in infected macrophages has been suggested as a survival mechanism for virulent strains of M. avium. We examined the relationship between TNF-alpha and growth of M. avium strains of differing virulence, as measured by their ability to grow in murine bone marrow-derived macrophages. When exogenous TNF-alpha was added immediately following macrophage infection, significant growth inhibition of virulent M. avium strains was observed. If TNF-alpha addition was delayed by 24 h or more, growth inhibition was abrogated. To determine if early downregulation of TNF-alpha levels could explain the differential growth of virulent and avirulent strains, levels of TNF-alpha and prostaglandin E2 (PGE2), which has been shown to suppress TNF-alpha production in uninfected macrophages, were quantified over time. Upregulation of both TNF-alpha and PGE2, as measured by enzyme-linked immunosorbent assay, was evident by 6 h postinfection, indicating that the ability of M. avium to replicate in macrophages was not directly correlated with early downregulation of TNF-alpha production. However, TNF-alpha bioactivity, as measured by cytotoxicity, was significantly decreased in virulent M. avium strains at all time periods examined. Treatment of infected macrophages with gamma interferon immediately after infection resulted in significantly increased levels of nitric oxide but did not affect the growth of virulent M. avium strains. These results suggest that while significant levels of TNF-alpha are present in supernatants from all M. avium strains, levels of biologically active TNF-alpha are significantly reduced in supernatants from virulent M. avium strains. Preliminary results suggest that upregulation of the soluble p75 TNF receptor may be one mechanism by which TNF-alpha bioactivity reduction occurs.

Mycobacterium-containing phagosomes are accessible to early endosomes and reflect a transitional state in normal phagosome biogenesis

The success of Mycobacterium as a pathogen hinges on its ability to modulate its intracellular environment. Mycobacterium avium reside in vacuoles with limited proteolytic activity, maintain cathepsin D in an immature form and remain accessible to internalized transferrin. Artificial acidification of isolated phagosomes facilitated processing of cathepsin D, demonstrating that pH alone limits proteolysis in these vacuoles. Moreover, analysis of IgG-bead phagosomes at early time points during their formation indicates that these phagosomes also acquire LAMP 1 and cathepsin D prior to the accumulation of proton-ATPases, and are transiently accessible to sorting endosomes. This suggests that the anomolous distribution of endosomal proteins in M. avium-containing vacuoles results from their arrested differentiation in an early transitional stage through which all phagosomes pass.

Immunobiology of Mycobacterium avium infection

Infection caused by organisms of the Mycobacterium avium complex is diagnosed in 50% to 60% of AIDS patients with the advanced stage of disease. Mycobacterium avium is an environmental bacterium that gains access to the host through both the gastrointestinal tract and the respiratory tract. After crossing the mucosal barrier Mycobacterium avium disseminates, infecting chiefly mononuclear phagocytes of the reticuloendothelial system. A number of cells of the immune system such as CD4+ T cells, natural killer cells and macrophages have been shown to be involved in the host response to Mycobacterium avium. The interaction between Mycobacterium avium and macrophages results in the production of immune-suppressive cytokines that inhibit the effector function of TH1 subtype CD4+ T cells, natural killer cells and macrophages, possibly allowing survival of Mycobacterium avium. Some cytokines such as tumor necrosis factor alpha and granulocyte-macrophage colony-stimulating factor have been shown to induce mycobacteriostatic activity and mycobactericidal activity in infected macrophages. Over the next few years, much new information will certainly be gleaned about host-pathogen interactions, which will lead to a better understanding of the disease and possibly to the design of new forms of therapy.

MIF protein are theta-class glutathione S-transferase homologs

MIF proteins are mammalian polypeptides of approximately 13,000 molecular weight. This class includes human macrophage migration inhibitory factor (MIF), a rat liver protein that has glutathione S-transferase (GST) activity (TRANSMIF), and the mouse delayed early response gene 6 (DER6) protein. MIF proteins were previously linked to GSTs by demonstrating transferase activity and observing N-terminal sequence homology with a mu-class GST (Blocki, F.A., Schlievert, P.M., & Wackett, L.P., 1992, Nature 360, 269-270). In this study, MIF proteins are shown to be structurally related to the theta class of GSTs. This is established in three ways. First, unique primary sequence patterns are developed for each of the GST gene classes. The patterns identify the three MIF proteins as theta-like transferase homologs. Second, pattern analysis indicates that GST members of the theta class contain a serine residue in place of the N-terminal tyrosine that is implicated in glutathione deprotonation and activation in GSTs of known structure (Liu, S., et al., 1992, J. Biol. Chem. 267, 4296-4299). The MIF proteins contain a threonine at this position. Third, polyclonal antibodies raised against recombinant human MIF cross-react on Western blots with rat theta GST but not with alpha and mu GSTs. That MIF proteins have glutathione-binding ability may provide a common structural key toward understanding the varied functions of this widely distributed emerging gene family. Because theta is thought to be the most ancient evolutionary GST class, MIF proteins may have diverged early in evolution but retained a glutathione-binding domain.

Effector mechanisms involved in cytokine-mediated bacteriostasis of Mycobacterium avium infections in murine macrophages

In this study we found that addition of a range of doses of interferon-gamma (IFN-gamma), tumour necrosis factor-alpha (TNF-alpha), or granulocyte-macrophage colony stimulating factor (GM-CSF) to cultures of bone marrow-derived murine macrophages infected with the 25291 strain of Mycobacterium avium gave rise to varying degrees of bacteriostasis. In contrast, similar treatment with interleukin-4 (IL-4) or IL-6 had no effect. However, when similar experiments with the former set of cytokines were performed using a panel of M. avium isolates, substantial isolate-to-isolate variation was observed. In cultures containing IFN-gamma, synthesis of substantial levels of reactive nitrogen intermediates was observed; however, neither these materials, nor reactive oxygen intermediates, were found to be responsible for observed bacteriostasis. In further experiments, in which the culture medium was supplemented with various concentrations of a weak acid or a weak base in order to influence the pH of macrophage intracellular compartments, it was found that the presence of the weak acid augmented the activity of IFN-gamma, whilst the weak base counteracted this effect. These data support the hypothesis, therefore, that the bacteriostatic effect of IFN-gamma against the growth of M. avium, rather than depending on reactive radical production, is mediated through acidification of the infected phagosome, perhaps through activation of proton pumps in the phagosomal membrane.

Immunity to mycobacteria

Recent progress in the field of immunity to mycobacteria has centered on T cell subset responses and the cytokines these cells secrete. In addition, there has been steady progress in identifying and characterizing several classes of major mycobacterial proteins; included amongst these are the secreted/export proteins of Mycobacterium tuberculosis, which several laboratories now believe may represent the key protective immunity-inducing antigens of the bacillus.