Molecular and biological properties of an interleukin-1 receptor immunoadhesin

Recombinant anthrax toxin receptor-Fc fusion proteins produced in plants protect rabbits against inhalational anthrax

Inhalational anthrax, a zoonotic disease caused by the inhalation of Bacillus anthracis spores, has a ∼50% fatality rate even when treated with antibiotics. Pathogenesis is dependent on the activity of two toxic noncovalent complexes: edema toxin (EdTx) and lethal toxin (LeTx). Protective antigen (PA), an essential component of both complexes, binds with high affinity to the major receptor mediating the lethality of anthrax toxin in vivo, capillary morphogenesis protein 2 (CMG2). Certain antibodies against PA have been shown to protect against anthrax in vivo. As an alternative to anti-PA antibodies, we produced a fusion of the extracellular domain of human CMG2 and human IgG Fc, using both transient and stable tobacco plant expression systems. Optimized expression led to the CMG2-Fc fusion protein being produced at high levels: 730 mg/kg fresh leaf weight in Nicotiana benthamiana and 65 mg/kg in N. tabacum. CMG2-Fc, purified from tobacco plants, fully protected rabbits against a lethal challenge with B. anthracis spores at a dose of 2 mg/kg body weight administered at the time of challenge. Treatment with CMG2-Fc did not interfere with the development of the animals' own immunity to anthrax, as treated animals that survived an initial challenge also survived a rechallenge 30 days later. The glycosylation of the Fc (or lack thereof) had no significant effect on the protective potency of CMG2-Fc in rabbits or on its serum half-life, which was about 5 days. Significantly, CMG2-Fc effectively neutralized, in vitro, LeTx-containing mutant forms of PA that were not neutralized by anti-PA monoclonal antibodies.

Non-specific resistance mechanisms to listeriosis: implications for experimental and naturally occurring infection

Use of murine listeriosis as an experimental model has greatly increased our understanding of the complex interplay of cells and mediators in non-specific antibacterial resistance (innate immunity). Important contributions made with this experimental model include demonstrating the ability of inflammatory cytokines (i.e. IFN-gamma, IL-1 alpha, TNF-alpha) to protect against bacterial infection, and illustrating the rapidity of the host cytokine response (detectable within 1 h of challenge) during bacterial infection. Most experimental studies of host defense against Listeria monocytogenes (L. monocytogenes) have used a parenteral challenge (i.v. or i.p.). This ignores the pathogenesis of naturally occurring listeriosis, which usually results from ingestion of Listeria-contaminated food products. In this review, we will include consideration of the host-pathogen interactions that occur when L. monocytogenes invades through its natural portal of entry, the gastrointestinal tract. Although resistance to facultative intracellular pathogens, such as L. monocytogenes, was formerly thought to revolve exclusively around the T helper cell/macrophage axis, more recent evidence indicates that neutrophils are able to ingest and kill L. monocytogenes and prevent the unrestricted multiplication of listeriae in parenchymal cells. Exploring the mechanisms involved in this process will provide new insights into the communication between leukocytes and tissue cells in host defense.

CAR1, a TNFR-related protein, is a cellular receptor for cytopathic avian leukosis-sarcoma viruses and mediates apoptosis

Viral envelope (Env)-receptor interactions have been implicated in the cell death associated with infection by subgroups B and D avian leukosis-sarcoma viruses (ALVs). A chicken protein, CAR1, was identified that permitted infection of mammalian cells by these viral subgroups. CAR1 bound to a viral Env fusion protein, comprising an ALV-B surface Env protein and the Fc region of an immunoglobulin, indicating that it is a specific viral receptor. CAR1 contains two extracellular cysteine-rich domains characteristic of the TNFR family and a cytoplasmic region strikingly similar to the death domain of TNFR1 and Fas, implicating this receptor in cell killing. Chicken embryo fibroblasts susceptible to ALV-B infection and transfected quail QT6 cells expressing CAR1 underwent apoptosis in response to the Env-Ig fusion protein, demonstrating that this cytopathic ALV receptor can mediate cell death.

Residue Trp-48 of Tva is critical for viral entry but not for high-affinity binding to the SU glycoprotein of subgroup A avian leukosis and sarcoma viruses

Previously, mutant Tva receptors were classified as either partially or completely defective in mediating subgroup A avian leukosis and sarcoma virus (ALSV-A) entry (C. Bélanger, K. Zingler, and J. A. T. Young, J. Virol. 69:1019-1024, 1995; K. Zingler, C. Bélanger, R. Peters, D. Agard, and J. A. T. Young, J. Virol. 69:4261-4266, 1995). To specifically test the abilities of these mutant Tva proteins to bind ALSV-A surface (SU) protein, binding studies were performed with a subgroup A SU-immunoadhesin. This fusion protein is composed of the subgroup A Schmidt-Ruppin SU protein fused in frame to a rabbit immunoglobulin constant region. This reagent was conjugated to fluorescein isothiocyanate and used for flow cytometric analysis with transfected human 293 cells expressing different forms of Tva. The SU-immunoadhesin bound the wild-type Tva protein with a KD of approximately 1.5 nM. Amino acid substitutions that reduced viral entry at Asp-46 and at Cys-35 and Cys-50, which are predicted to form an intrachain disulfide bond in Tva, drastically reduced the binding affinity for the SU-immunoadhesin. Thus, the effects on viral entry of some mutations could be explained solely by changes in the binding affinity for ALSV-A SU. However, this was not true for other mutations tested, especially those with amino acid substitutions that replaced Trp-48. Compared with the wild-type receptor, these latter mutations led to approximately 43- to 200-fold reductions in viral infectivity but only to approximately 2.5- to 3.4-fold reductions in the binding affinity for the SU-immunoadhesin. These results support a role for Trp-48 of Tva in mediating steps of viral entry subsequent to binding ALSV-A SU.

Immunoadhesins: principles and applications

The prototypic immunoadhesin is an antibody-like molecule that fuses the Fc region of an immunoglobulin and the ligand-binding region of a receptor or adhesion molecule. In this article, we review some important structural and functional principles of immunoadhesins. In addition, we highlight some unique advantages of immunoadhesins as experimental tools in biology, as well as some of their exciting potential applications in medicine.

NF-kappa B activation by interleukin-1 (IL-1) requires an IL-1 receptor-associated protein kinase activity

An important function of interleukin-1 (IL-1) is activation of the transcription factor NF-kappa B, which is signaled via the type I IL-1 receptor (IL-1RI). By receptor mutagenesis studies, we have identified a region of the cytoplasmic domain of IL-1RI that is required for both IL-1-mediated NF-kappa B activation and IL-1-dependent activation of a receptor-associated protein kinase activity we term IRAK. No IL-1RI mutants were found that can activate NF-kappa B in the absence of IRAK activity. Therefore, we propose that IRAK activation is a necessary step in the activation of NF-kappa B by IL-1.

Generation of soluble interleukin-1 receptor from an immunoadhesin by specific cleavage

The extracellular portion of the interleukin-1 receptor (IL-1R) is sufficient for high-affinity binding to IL-1; however, the structural basis for binding of the receptor to IL-1 is not known. To produce individual domains of IL-1 receptor for structural studies, we constructed a molecular fusion of IL-1 receptor with immunoglobulin G heavy chain that contains a protease specific sequence joining the two portions of the molecule (IL-1R-G-IgG). We introduced the hexapeptide sequence AAHY:TL (where ":" denotes the scissile bond) at the junction of the IL-1R and IgG regions, for specific cleavage by an H64A variant of subtilisin BPN' (Genenase I), an endoprotease that cleaves selectively at this sequence (Carter et al., (1989) Proteins 6, 240-248). Plasmid DNA encoding the fusion protein was used to transfect human embryonic kidney 293 cells transiently, and secreted IL-1R-G-IgG was purified from cell supernatants by protein A chromatography. The IL-1 receptor's extracellular region was then generated by enzymatic cleavage with Genenase I which was immobilized on controlled-pore glass. Incubation of IL-1R-G-IgG with immobilized Genenase I resulted in specific cleavage at the target site, as confirmed by SDS-PAGE, immunoblotting and direct sequencing of the newly generated termini. The resulting soluble IL-1R was separated from the immunoglobulin Fc cleavage product by re-chromatography on protein A. The purified, soluble IL-1R retained quantitatively the ability to bind to its ligand, IL-1 beta. This approach offers a generic means by which the extracellular region of a given type I transmembrane receptor can be expressed as an immunoadhesin, released enzymatically and then easily purified for crystallographic or ligand binding studies.