We investigated whether protein stability controls antigen presentation using a four disulfide-containing snake toxin and three derivatives carrying one or two mutations (L1A, L1A/H4Y, and H4Y). These mutations were anticipated to increase (H4Y) or decrease (L1A) the antigen non-covalent stabilizing interactions, H4Y being naturally and frequently observed in neurotoxins. The chemically synthesized derivatives shared similar three-dimensional structure, biological activity, and T epitope pattern. However, they displayed differential thermal unfolding capacities, ranging from 65 to 98°C. Using these differentially stable derivatives, we demonstrated that antigen stability controls antigen proteolysis, antigen processing in antigen-presenting cells, T cell stimulation, and kinetics of expression of T cell determinants. Therefore, non-covalent interactions that control the unfolding capacity of an antigen are key parameters in the efficacy of antigen presentation. By affecting the stabilizing interaction network of proteins, some natural mutations may modulate the subsequent T-cell stimulation and might help microorganisms to escape the immune response.

Under CO 2 exposure at an intermediate temperature, typically 550°C, 9Cr-1Mo steel forms a duplex oxide scale made of an outer magnetite layer and an almost-as-thick inner Fe-Cr rich spinel oxide layer. It is proposed that the inner Fe-Cr spinel layer grows according to a mechanism involving void formation at the oxide/ metal interface. The driving force for pore formation is the outward magnetite growth: iron vacancies are injected at the oxide/metal interface then condense into voids. The fresh metallic surface made available is then oxidized by CO 2 , which diffuses fast through the scale. The physical aspects, the integrity and the nature of the scale are shown to be very dependent on the oxygen potential existing in the environment.

A calcium–sensing receptor (CaR) has functionally been described in the cortical thick ascending limb of Henle's loop (CTAL) of rat and mouse. This G protein–coupled receptor activates phospholipase C and increases the intracellular Ca²⁺ concentration. We observed that in the mouse CTAL cAMP formation, induced by 10^–8 mol/l AVP, was inhibited by more than 90% when the extracellular Ca²⁺ concentration ([Ca²⁺]_e) was increased from 0.5 to 3 mmol/l. Measurements of transepithelial potential difference (PD_te) in rat and mouse CTAL and medullary thick ascending limb (mTAL) segments and of transepithelial ion net fluxes in the mouse CTAL (isotonic perfusion conditions: 150 mmol/l NaCl in the lumen and bath) showed that an increase in the [Ca²⁺]_e had no effect on basal and arginine vasopressin (AVP, 10^–10 mol/l)–stimulated transepithelial PD_te, NaCl and Mg²⁺ transport. However, Ca²⁺ reabsorption was strongly inhibited by increased [Ca²⁺]_e. Addition of AVP reversed this inhibitory effect of increased [Ca²⁺]_e. Under hypotonic perfusion conditions (lumen 50 mmol/l NaCl; bath 150 mmol/l NaCl), a high [Ca²⁺]_e induced a 50% decrease in Mg²⁺ reabsorption which was restored by AVP. Under these conditions, the effects on Ca²⁺ transport described above were still observed. In conclusion, activation of the CaR in the mouse TAL has no effect on basal and AVP–stimulated transepithelial NaCl reabsorption despite its large inhibitory effect on cAMP synthesis. The CaR, however, could play a role in the regulation of transepithelial Ca²⁺ and Mg²⁺ reabsorption.

In parallel to the formation of a duplex oxide scale, 9Cr-1Mo steel carburizes strongly under CO 2 at 550°C and this carburization accelerates with time. It is observed that an increase of the total CO 2 pressure in the environment from 1 to 250 bars induces a higher carbon deposition in the inner Fe-Cr rich spinel oxide layer. In order to explain this phenomenon, modelling of the carburization process was carried out. A mechanism involving gas diffusion of CO 2 and CO through the oxide layer, the Boudouard reaction and carbon diffusion through the metallic substrate is proposed.

In the framework of a new generation of nuclear reactors, typically sodium fast reactors, supercritical carbon dioxide (CO 2 ) with a Brayton cycle (at 550°C and 250 bars) is identified as a promising energy conversion system to replace the traditional steam generators. Nevertheless, the long-term integrity of the heat exchanger structure in this environment has to be proven over at least 20 years. To this purpose, the corrosion behavior of different metallic materials under static CO 2 at 550°C and 250 bars is studied. The materials under study are one 9 wt% Cr ferrito-martensitic steel (T91) and several austenitic steels. The results about the nature of the corrosion product, morphology, and kinetics of formation were analyzed by glow discharge optical emission spectroscopy, scanning electron microscopy, wavelength-dispersive spectroscopy, and x-ray diffraction. A corrosion mechanism of the different steels is proposed and the most promising materials for heat exchanger applications are discussed.

Proteins are poor immunogens that require an adjuvant to raise an immune response. Here we show that the human immunodeficiency virus, type 1 Tat protein possesses an autoadjuvant property, and we have identified the determinants and the molecular events that are associated with this unusual property. Using a series of chemically synthesized Tat101 derivatives, we show that the core region controls the autoadjuvant phenomenon independently of the B-cell recognition and T-cell stimulation that are associated with epitopes respectively located on the N-terminal region and the cysteine-rich region. We also show that cysteine-mediated oligomerization is a key molecular event of the adjuvant-free antibody response. In particular, a Tat dimer formed by the oxidation of two cysteine residues, at position 34 only, raises an adjuvant-free antibody response that is comparable with that observed with the wildtype protein. Unlike the parent protein, the Tat dimer has no transactivating activity and remains homogeneous for several weeks in solution. This construct might be of value for the design of an adjuvant-free Tat-based vaccine. Furthermore, we suggest that the specific autoadjuvanticity determinant of Tat could be used to provide other proteins with adjuvant-free immunogenicity.Most free proteins injected in a soluble form in humans or animals are poor immunogens and can induce B-or T-cell tolerance or rapid proliferation followed by rapid death of Ag-specific T-cells. They usually become immunogenic when they are mixed with an adjuvant. It is not completely understood how adjuvants are able to convert a tolerogenic stimulus into an immunogenic one. The process seems to be related to an increase in immunogen half-life through the so-called "depot effect" (1) and to a series of immunological events that include induction of inflammation and of inflammatory cytokines (2, 3), improvement of Ag delivery to Ag-presenting cells (4), increase in Ag processing and presentation through the induction of major histocompatibility complex and/or costimulatory molecules (5), and induction of the production of immunomodulatory cytokines (6). Because most proteins cannot trigger such complex events, it is no surprise that, alone, they do not induce an immune response.The Tat (transcriptional transactivator) protein of HIV-1 2 is a regulatory protein that is produced early after infection and that is essential for viral replication (7,8). This molecule is released in the extracellular milieu (9, 10), where it exhibits numerous biological activities. In particular, Tat was suggested to induce angiogenesis (11-13), chemotaxis of monocytes (14), and secretion of proinflammatory cytokines such as IL-1, IL-6, and tumor necrosis factor ␣ (15). Furthermore, Tat was proposed to target monocyte-derived dendritic cells and enhance their maturation, function, and Ag-specific T-cell responses (16). Also, a recent report showed that Tat can reprogram immature dendritic cells to express chemoattractants for activated T-cells and macrophages, but in...

Material

Previous studies from our laboratory have shown that Ca2+ and Mg2+ absorption in the mouse cortical thick ascending limb of Henle's loop (cTAL) is a passive, paracellular process driven by the transepithelial voltage. The passive permeability of the epithelium is enhanced by peptide hormones. The present study investigated whether divalent cation absorption in the cTAL is influenced by cell maturation and/or gender. For this purpose, mouse cTAL segments were microdissected from kidneys of female and male animals aged 4 and 8 weeks. The microdissected tubules were perfused in vitro at a luminal flow rate of 1.5 to 2.5 nl/min. Transepithelial Na+, Cl-, Ca2+ and Mg2+ net fluxes (JX, pmol.min-1.mm-1) were measured using electron microprobe analysis, and the transepithelial potential difference (PDte) was measured continuously. No differences were found in the PDte, JNa and JCl of the various animal groups but the transepithelial Ca2+ and Mg2+ transport capacity of the cTAL was higher in adults (8 weeks) than in young animals (4 weeks). Furthermore, irrespective of age, transepithelial Ca2+ net absorption was greater in male than in female animals. In contrast, the NaCl transport was maximal at 4 weeks in both genders. We conclude therefore that transepithelial divalent cation absorption in the mouse cTAL is an inductive process influenced by cell maturation and gender. The molecular basis of these inductions remains to be elucidated.