Quorum-sensing in Gram-negative bacteria.

It has become increasingly and widely recognised that bacteria do not exist as solitary cells, but are colonial organisms that exploit elaborate systems of intercellular communication to facilitate their adaptation to changing environmental conditions. The languages by which bacteria communicate take the form of chemical signals, excreted from the cells, which can elicit profound physiological changes. Many types of signalling molecules, which regulate diverse phenotypes across distant genera, have been described. The most common signalling molecules found in Gram-negative bacteria are N-acyl derivatives of homoserine lactone (acyl HSLs). Modulation of the physiological processes controlled by acyl HSLs (and, indeed, many of the non-acyl HSL-mediated systems) occurs in a cell density- and growth phase-dependent manner. Therefore, the term 'quorum-sensing' has been coined to describe this ability of bacteria to monitor cell density before expressing a phenotype. In this paper, we review the current state of research concerning acyl HSL-mediated quorum-sensing. We also describe two non-acyl HSL-based systems utilised by the phytopathogens Ralstonia solanacearum and Xanthomonas campestris.

Inhibition of interferon-gamma by an interferon-gamma receptor immunoadhesin.

Interferon-gamma (IFN-gamma) is an important cytokine which regulates inflammatory and immune response mechanisms. IFN-gamma enhances the presentation and recognition of antigens by inducing the expression of major histocompatibility complex (MHC) proteins, by activating effector T cells and mononuclear phagocytes, and by modulating immunoglobulin production and class selection in B cells. Inappropriate production of IFN-gamma has been implicated in the pathogenesis of several autoimmune and inflammatory diseases and in graft rejection. Here, we describe a recombinant inhibitor of IFN-gamma, termed murine IFN-gamma receptor immunoadhesin (mIFN-gamma R-IgG). We constructed this immunoadhesin by linking the extracellular portion of the mouse IFN-gamma R to the hinge and Fc region of an IgG1 heavy chain. Murine IFN-gamma R-IgG is secreted by transfected cells as a disulphide-bonded homodimer which binds IFN-gamma bivalently, with high affinity and in a species-specific manner. In vitro, mIFN-gamma R-IgG can block mIFN-gamma-induced antiviral activity and expression of the class I MHC antigen H-2Kk in cultured cells. In vivo, mIFN-gamma R-IgG can block the function of endogenous mIFN-gamma in mouse models of infection with Listeria monocytogenes and of contact sensitivity. These results show that mIFN-gamma R-IgG is an effective and specific inhibitor of mIFN-gamma both in vitro and in vivo. Thus, in general, IFN-gamma receptor immunoadhesins may be useful for investigating the biological functions of IFN-gamma as well as for preventing deleterious effects of IFN-gamma in human disease.

Molecular cloning and expression of the murine RANTES cytokine: structural and functional conservation between mouse and man.

The infiltration and activation of monocytes is a hallmark of chronic inflammation, including that associated with a variety of disease states such as rheumatoid arthritis, atherosclerosis, and various autoimmune conditions. Recently, a family of small molecular mass proteins has been described which appear to have inflammatory properties, including chemoattractant effects on monocytes. We report here on the molecular cloning, characterization, and functional expression of mu RANTES, a new murine member of this family. mu RANTES expressed in a mammalian expression system is an approximately 8-kDa protein exhibiting immune cross-reactivity with a rabbit polyclonal antiserum generated against human RANTES. Boyden chamber chemotaxis experiments reveal some lack of species specificity in monocyte chemoattractant potential, as recombinant mu RANTES attracts human monocytes in a dose-dependent fashion in vitro. mu RANTES and its human homolog share approximately 85% amino acid identity, a higher level of conservation than that seen with any other species homologs in this cytokine family, and second only to transforming growth factor-beta among reported immune cytokines.

Identification of cysteine-rich domains of the type 1 tumor necrosis factor receptor involved in ligand binding.

The extracellular portion of the type 1 (p55) and type 2 (p75) tumor necrosis factor (TNF) receptors contains a repetitive amino acid sequence pattern of four cysteine-rich domains (CRDs). This pattern is found also in several other cell surface proteins, including the p75 nerve growth factor receptor and the CD40, 4-1BB, OX40, Fas, and CD27 antigens. To investigate whether CRDs play a role in TNF binding, we have constructed soluble variants of the extracellular portion of human type 1 TNF receptor (sTNFR1), in which CRD1 (N-terminal) or CRD4 (C-terminal) was deleted by mutagenesis. These variants or a wild type sTNFR1 were linked in their C terminus to the hinge and Fc portion of IgG1 heavy chain to create sTNFR1-IgG chimeras (immunoadhesins). Deletion of either CRD1 or -4 did not cause any major perturbations in the structure of the sTNFR1 variants, as evidenced by their efficient expression and secretion from transfected cells, and by their binding to conformation-dependent monoclonal antibodies that recognize diverse epitopes on sTNFR1. The wild type sTNFR1 immunoadhesin exhibited high affinity binding to TNF alpha (Kd = 65 pM) and TNF beta (Kd = 640 pM). Deletion of CRD4 resulted in about a 10-fold reduction in affinity for TNF alpha (Kd = 660 pM) and for TNF beta (Kd = 5.7 nM). In contrast, deletion of CRD1 resulted in a complete loss of binding to TNF alpha and to TNF beta. These results indicate that CRD4 is important but not necessary for TNF binding, while CRD1 is required. In addition, the results suggest some overlap between the TNFR1 binding sites for TNF alpha and TNF beta, despite low amino acid sequence homology between these cytokines.

Discontinuous polyacrylamide gradient agarose gels resolve a wide range of restriction fragments and optimize the efficiency of nucleic acid transfer.

A vertical electrophoresis procedure utilizing a discontinuous polyacrylamide gradient agarose gel was developed to resolve DNA fragments ranging in size from over 50 kb to less than 300 bp in length. The gel consisted of a polyacrylamide plug at the base of the gel followed by a gradient of agarose ranging from 0.3 to 0.9%. Restriction fragments migrated shorter distances than in a comparable polyacrylamide-0.3% agarose gel, and small fragments were retained. Southern transfer of DNA fragments from the gradient gel onto nitrocellulose was more efficient than transfer of fragments using a nongradient gel.