Structure and function of several anti-dansyl chimeric antibodies formed by domain interchanges between human IgM and mouse IgG2b

Artificial surface labelling of Escherichia coli with StrepTagII antigen to study how monoclonal antibodies drive complement-mediated killing

Antibodies play a key role in the immune defence against Gram-negative bacteria. After binding to bacterial surface antigens, IgG and IgM can activate the complement system and trigger formation of lytic membrane attack complex (MAC) pores. Molecular studies to compare functional activity of antibodies on bacteria are hampered by the limited availability of well-defined antibodies against bacterial surface antigens. Therefore, we genetically engineered E. coli by expressing the StrepTagII antigen into outer membrane protein X (OmpX) and validated that these engineered bacteria were recognised by anti-StrepTagII antibodies. We then combined this antigen-antibody system with a purified complement assay to avoid interference of serum components and directly compare MAC-mediated bacterial killing via IgG1 and pentameric IgM. While both IgG1 and IgM could induce MAC-mediated killing, we show that IgM has an increased capacity to induce complement-mediated killing of E. coli compared to IgG1. While Fc mutations that enhance IgG clustering after target binding could not improve MAC formation, mutations that cause formation of pre-assembled IgG hexamers enhanced the complement activating capacity of IgG1. Altogether, we here present a system to study antibody-dependent complement activation on E. coli and show IgM's enhanced capacity over IgG to induce complement-mediated lysis of E. coli.

IgM and its receptors: structural and functional aspects

This review combines the data obtained before the beginning of the 1990s with results published during the last two decades. The predominant form of the IgM molecule is a closed ring composed of five 7S subunits and a J chain. The new model of spatial structure of the pentamer postulates nonplanar mushroom-shaped form of the molecule with the plane formed by a radially-directed Fab regions and central protruding portion consisting of Cµ4 domains. Up to the year 2000 the only known Fc-receptor for IgM was pIgR. Interaction of IgM with pIgR results in secretory IgM formation, whose functions are poorly studied. The receptor designated as Fcα/µR is able to bind IgM and IgA. It is expressed on lymphocytes, follicular dendritic cells, and macrophages. A receptor binding IgM only named FcµR has also been described. It is expressed on T- and B-lymphocytes. The discovery of new Fc-receptors for IgM requires revision of notions that interactions between humoral reactions involving IgM and the cells of the immune system are mediated exclusively by complement receptors. In the whole organism, apart from IgM induced by immunization, natural antibodies (NA) are present and comprise in adults a considerable part of the circulating IgM. NA are polyreactive, germ-line-encoded, and emerge during embryogenesis without apparent antigenic stimuli. They demonstrate a broad spectrum of antibacterial activity and serve as first line of defense against microbial and viral infections. NA may be regarded as a transitional molecular form from invariable receptors of innate immunity to highly diverse receptors of adaptive immunity. By means of interaction with autoantigens, NA participate in maintenance of immunological tolerance and in clearance of dying cells. At the same time, NA may act as a pathogenic factor in atherosclerotic lesion formation and in development of tissue damage due to ischemia/reperfusion.

Function of FXYD proteins, regulators of Na, K-ATPase

In this short review, we summarize our work on the role of members of the FXYD protein family as tissue-specific modulators of Na, K-ATPase. FXYD1 or phospholemman, mainly expressed in heart and skeletal muscle increases the apparent affinity for intracellular Na(+) of Na, K-ATPase and may thus be important for appropriate muscle contractility. FXYD2 or gamma subunit and FXYD4 or CHIF modulate the apparent affinity for Na(+) of Na, K-ATPase in an opposite way, adapted to the physiological needs of Na(+) reabsorption in different segments of the renal tubule. FXYD3 expressed in stomach, colon, and numerous tumors also modulates the transport properties of Na, K-ATPase but it has a lower specificity of association than other FXYD proteins and an unusual membrane topology. Finally, FXYD7 is exclusively expressed in the brain and decreases the apparent affinity for extracellular K(+), which may be essential for proper neuronal excitability.

Role of FXYD proteins in ion transport

The FXYD proteins are a family of seven homologous single transmembrane segment proteins (FXYD1-7), expressed in a tissue-specific fashion. The FXYD proteins modulate the function of Na,K-ATPase, thus adapting kinetic properties of active Na+ and K+ transport to the specific needs of different cells. Six FXYD proteins are known to interact with Na,K-ATPase and affect its kinetic properties in specific ways. Although effects of FXYD proteins on parameters such as K(1/2)Na+, K(1/2)K+, K(m)ATP, and V(max) are modest, usually twofold, these effects may have important long-term consequences for homeostasis of cation balance. In this review we summarize basic features of FXYD proteins and present recent evidence for functional effects, structure-function relations and structural interactions with Na,K-ATPase. We then discuss possible physiological roles, based on in vitro observations and newly available knockout mice models. Finally, we also consider evidence that FXYD proteins affect functioning of other ion transport systems.

Identification of novel genes for secreted and membrane-anchored proteins in human keratinocytes

BACKGROUND

Both intercellular and intracellular signals are transduced primarily by interactions of secreted and/or membrane-anchored polypeptides, and they play a pivotal role in regulating proliferation, differentiation and apoptosis of keratinocytes within the epidermis. Despite recent identification of these polypeptides, it is likely that several important molecules remain undisclosed.

OBJECTIVES

To identify novel genes encoding secreted or membrane-anchored polypeptides expressed by human keratinocytes.

METHODS

We employed a signal sequence (SS) trap of a 5'-end-enriched cDNA library prepared from primary cultured human keratinocytes. Gene expression analysis was performed using Northern blotting. R Screening of 4018 cDNA clones yielded 82 positive clones (57 independent genes), most of which encoded SSs in their N-termini. Most of the positive clones were known genes registered in the GenBank database. Seven genes were identified in the EST database, four of which encoded novel membrane-anchored polypeptides with features of type I transmembrane proteins; the other three genes encoded novel non-type I transmembrane polypeptides. These EST genes were expressed differentially by keratinocytes subjected to low vs. high calcium concentrations and by basal vs. squamous cell carcinomas.

CONCLUSIONS

Using the SS trap, we isolated many genes known to be involved in constituting epidermal structures and others that had not previously been associated with keratinocytes. In addition, we identified novel genes (EST genes) that differ in kinetics of gene expression in keratinocyte differentiation. Our results validate the effective use of this SS trap method for identifying secreted and membrane-anchored polypeptides expressed by human keratinocytes. The identification will better illuminate the molecular mechanisms responsible for co-ordinated regulation of epidermal homeostasis.

The carboxyl-terminal domains of IgA and IgM direct isotype-specific polymerization and interaction with the polymeric immunoglobulin receptor

Mucosal surfaces are protected by polymeric immunoglobulins that are transported across the epithelium by the polymeric immunoglobulin receptor (pIgR). Only polymeric IgA and IgM containing a small polypeptide called the "joining" (J) chain can bind to the pIgR. J chain-positive IgA consists of dimers, and some larger polymers, whereas only IgM pentamers incorporate the J chain. We made domain swap chimeras between human IgA1 and IgM and found that the COOH-terminal domains of the heavy chains (Calpha3 and Cmu4, respectively) dictated the size of the polymers formed and also which polymers incorporated the J chain. We also showed that chimeric IgM molecules engineered to contain Calpha3 were able to bind the rabbit pIgR. Since the rabbit pIgR normally does not bind IgM, these results suggest that the COOH-terminal domain of the polymeric immunoglobulins is primarily responsible for interaction with the pIgR. Finally, we made a novel chimeric IgA immunoglobulin, containing the terminal domain from IgM. This recombinant molecule formed J chain-containing pentamers that could, like IgA, efficiently form covalent complexes with the human pIgR ectodomain, known as secretory component.

Targeting of Functional Antibody-Decay-accelerating Factor Fusion Proteins to a Cell Surface

Recombinant soluble complement inhibitors hold promise for the treatment of inflammatory disease and disease states associated with transplantation. Targeting complement inhibitors to the site of complement activation and disease may enhance their efficacy and safety. Data presented show that targeting of decay-accelerating factor (DAF, an inhibitor of complement activation) to a cell surface by means of antibody fragments is feasible and that cell-targeted DAF provides significantly enhanced protection from complement deposition and lysis compared with soluble untargeted DAF. An extracellular region of DAF was joined to an antibody combining site with specificity for the hapten dansyl, at the end of either C(H)1 or C(H)3 Ig regions. The recombinant IgG-DAF chimeric proteins retained antigen specificity and bound to dansylated Chinese hamster ovary cells. Both soluble C(H)1-DAF and C(H)3-DAF were effective at inhibiting complement-mediated lysis of untargeted Chinese hamster ovary cells at molar concentrations within the range reported by others for soluble DAF. However, when targeted to a dansyl-labeled cell membrane, C(H)1-DAF was significantly more potent at inhibiting complement deposition and complement-mediated lysis. Cell-bound C(H)1-DAF also provided cells with protection from complement lysis after removal of unbound C(H)1-DAF. Of further importance, the insertion of a nonfunctional protein domain of DAF (the N-terminal short consensus repeat) between C(H)1 and the functional DAF domain increased activity of the fusion protein. In contrast to C(H)1-DAF, C(H)3-DAF was not significantly better at protecting targeted versus untargeted cells from complement, indicating that a small targeting vehicle is preferable to a large one. We have previously shown that for effective functioning of soluble complement inhibitor CD59, binding of CD59 to the cell surface close to the site of complement activation is required. Significantly, such a constraint did not apply for effective DAF function.

Structural requirements for incorporation of J chain into human IgM and IgA

J chain is associated with pentameric IgM and dimeric IgA via disulfide bonds involving the penultimate cysteine residue in the secretory tailpiece of the mu or the alpha heavy chain. We have investigated the structural basis for incorporation of J chain by analyzing several IgM mutants, IgA mutants and IgG/IgM hybrid molecules. IgM mutants with the mu secretory tailpiece replaced by the alpha secretory tailpiece and/or Cys414 replaced by serine incorporated J chain, although in reduced amounts correlating with reduced pentamer/polymer formation. In addition to pentamers, tetramers of IgMC414S contained J chain, while no J chain was associated with smaller polymers or hexamers of IgM. An IgA/IgM hybrid tailpiece abolished J chain incorporation to pentameric IgM. Analysis of IgG molecules that have added a secretory tailpiece and/or have IgM domain replacements showed that J chain incorporation depends on regions of the C(mu)4 domain in addition to the tailpiece. Features of the C(mu)3 domain other than Cys414 also play a role in efficient formation of pentamers and J chain incorporation, while the C(mu)2 domain is not specifically required. By analysis of two IgA mutants that formed larger polymers than IgAwt, we found J chain equally incorporated into dimers, trimers, tetramers and pentamers. Thus, the results show that J chain incorporation into IgA does not depend on the polymeric structure, while J chain incorporation into IgM is restricted to certain polymeric conformations.

Structural requirements for polymeric immunoglobulin assembly and association with J chain

Both IgM and IgA exist as polymeric immunoglobulins. IgM is assembled into pentamers with J chain and hexamers lacking J chain. In contrast, polymeric IgA exists mostly as dimers with J chain. Both IgM and IgA possess an 18-amino acid extension of the C terminus (the tail-piece (tp)) that participates in polymerization through a penultimate cysteine residue. The IgM (mutp) and IgA (alphatp) tail-pieces differ at seven amino acid positions. However, the tail-pieces by themselves do not determine the extent of polymerization. We now show that the restriction of polymerization to dimers requires both C(alpha)3 and alphatp and that more efficient dimer assembly occurs when C(alpha)2 is also present; the dimers contain J chain. Formation of pentamers containing J chain requires C(mu)3, C(mu)4, and the mutp. IgM-alphatp is present mainly as hexamers lacking J chain, and mumugammamu-utp forms tetramers and hexamers lacking J chain, whereas IgA-mutp is present as high order polymers containing J chain. In addition, there is heterogeneous processing of the N-linked carbohydrate on IgA-mutp, with some remaining in the high mannose state. These data suggest that in addition to the tail-piece, structural motifs in the constant region domains are critical for polymer assembly and J chain incorporation.

Transcriptional control by corticosteroids of CHIF gene expression in the rat distal colon

1. Previous studies have shown that levels of CHIF mRNA are increased in the distal colon of the rat in response to corticosteroids. We have recently reported that this response occurs within 2 h of a single dose of either dexamethasone or aldosterone and that the response is mediated via both the mineralocorticoid and glucocorticoid receptors. 2. In the present study we sought to further define the nature of the various transcripts detected by a CHIF coding region cRNA probe in northern blot analysis of corticosteroid-stimulated colonic RNA. The identification of an intronic sequence was used to synthesize an intron-specific cDNA probe to characterize the transcripts. 3. The presence of an intronic sequence in the originally published sequence was confirmed using coupled reverse transcriptase-polymerase chain reactions with primers spanning and within the intronic sequence. The intronic cDNA probe hybridized to the higher molecular weight transcripts detected by the cRNA probe. These transcripts are induced in response to both corticosteroids. 4. Taken together with our observations that the increase in CHIF mRNA levels in the distal colon in response to corticosteroids is not blocked by prior cycloheximide treatment, the increase in the levels of the primary transcript and partially spliced forms argues that this is a primary transcriptional response. This is the first clear demonstration of an aldosterone-induced gene in vivo in a mammalian system.

Targeting of functional antibody-CD59 fusion proteins to a cell surface

Complement is involved in the pathogenesis of many diseases, and there is great interest in developing inhibitors of complement for therapeutic application. CD59 is a natural membrane-bound inhibitor of the cytolytic complement membrane attack complex (MAC). In this study, the preparation and characterization of antibody-CD59 (IgG-CD59) chimeric fusion proteins are described. Constructs were composed of soluble CD59 fused to an antibody-combining site at the end of CH1, after the hinge (H), and after CH3 Ig regions. The antigen specificity of each construct was for the hapten 5-dimethylamino-naphthalene-1-sulfonyl (dansyl). Correct folding of each IgG-CD59 fusion partner was indicated by recognition with anti-CD59 antibodies specific for conformational determinants and by IgG-CD59 binding to dansyl. The IgG-CD59 fusion proteins all bound specifically to dansyl-labeled Chinese hamster ovary cells and provided targeted cells, but not untargeted cells, with effective protection from complement-mediated lysis. Data indicate that CD59 must be positioned in close proximity to the site of MAC formation for effective function, and that modes of membrane attachment other than glycophosphatidylinositol linkage can affect CD59 functional activity.

Functional properties and physiological roles of organic solute channels

Membrane channels provide routes for the rapid, passive movement of solutes across plasma and intracellular membranes. It is generally assumed that the major physiological role of membrane channels is to transport inorganic ions for processes such as transepithelial salt absorption and secretion, cell volume regulation, signal transduction, and control of membrane electrical properties. Increasing evidence indicates, however, that channels play an important role in organic solute transport in a wide variety of cell types and organisms. Some of the major physiological roles of organic solute channels include uptake of nutrients, excretion of metabolic waste products, volume-regulatory organic osmolyte transport, and control of mitochondrial metabolism. This article reviews the functions and characteristics of channels that participate in the transport and regulation of both charged and electroneutral organic solutes.

Trials and tribulations: oncological antibody imaging comes to the fore

At the present time, there are three radiolabeled antibodies that have been approved by the US Food and Drug Administration (FDA) for imaging of cancer, a fourth commercially sponsored product recommended for approval (as of 10/29/96, cap romab pendetide (ProstaScint; Cytogen Corp., Princeton, NJ) was upgraded from recommended for approval to approved), and several additional agents in FDA-monitored trials. The majority of antibodies studied to date have been whole or fragmented murine monoclonals whereas the first of the human and humanized immunoglobulins are now entering clinical trials. While no antibody has behaved as a perfect imaging agent, they have consistently been shown to contribute to diagnosis, complementing and often exceeding the diagnostic ability of conventional modalities. Many promising new trends in antibody imaging, relating to the radiolabeled immunoglobulin, its route and manner of administration, and mode of detection, are under development. Because of the requisite several-year delay inherent in the (FDA) testing process, there is a lag before the most-promising of these innovations will achieve (FDA) approval and be incorporated into routine imaging studies. In spite of this effective performance, as "new kid on the block," radioimmunoscintigraphy may have often been expected to perform in an unrealistic manner, considering the great variation in biological behavior of primary and metastatic cancer and the consequent limitation of all diagnostic tests. Nonetheless, because radioimmunoscintigraphy identifies antigens on a cellular level, differing fundamentally from anatomic imaging modalities such as computed tomography and ultrasound which identify gross morphological changes, it has potential to impact significantly on patient care. With adequate resources focused on radioimmunoscintigraphy, this technology will continue to emerge as an important and unique diagnostic tool in the care of cancer patients, with demonstrable clinical efficacy and cost effectiveness.