On the nature of Bose-Einstein condensation enhanced by localization

In a previous paper we established that for the perfect Bose gas and the mean-field Bose gas with an external random or weak potential, whenever there is generalized Bose-Einstein condensation in the eigenstates of the single particle Hamiltonian, there is also generalized condensation in the kinetic energy states. In these cases Bose-Einstein condensation is produced or enhanced by the external potential. In the present paper we establish a criterion for the absence of condensation in single kinetic energy states and prove that this criterion is satisfied for a class of random potentials and weak potentials. This means that the condensate is spread over an infinite number of states with low kinetic energy without any of them being macroscopically occupied

Exactness of the Bogoliubov approximation in random external potentials

We investigate the validity of the Bogoliubov c-number approximation in the case of interacting Bose-gas in a \textit{homogeneous random} media. To take into account the possible occurence of type III generalized Bose-Einstein condensation (i.e. the occurrence of condensation in an infinitesimal band of low kinetic energy modes without macroscopic occupation of any of them) we generalize the c-number substitution procedure to this band of modes with low momentum. We show that, as in the case of the one-mode condensation for translation-invariant interacting systems, this procedure has no effect on the exact value of the pressure in the thermodynamic limit, assuming that the c-numbers are chosen according to a suitable variational principle. We then discuss the relation between these c-numbers and the (total) density of the condensate

On the nature of Bose-Einstein condensation in disordered systems

We study the perfect Bose gas in random external potentials and show that there is generalized Bose-Einstein condensation in the random eigenstates if and only if the same occurs in the one-particle kinetic-energy eigenstates, which corresponds to the generalized condensation of the free Bose gas. Moreover, we prove that the amounts of both condensate densities are equal. Our method is based on the derivation of an explicit formula for the occupation measure in the one-body kinetic-energy eigenstates which describes the repartition of particles among these non-random states. This technique can be adapted to re-examine the properties of the perfect Bose gas in the presence of weak (scaled) non-random potentials, for which we establish similar results

Scavenger receptor class B type I is a key host factor for hepatitis C virus infection required for an entry step closely linked to CD81.

International audienceHepatitis C virus (HCV) is a major cause of chronic hepatitis worldwide. Scavenger receptor class B type I (SR-BI) has been shown to bind HCV envelope glycoprotein E2, participate in entry of HCV pseudotype particles, and modulate HCV infection. However, the functional role of SR-BI for productive HCV infection remains unclear. In this study, we investigated the role of SR-BI as an entry factor for infection of human hepatoma cells using cell culture-derived HCV (HCVcc). Anti-SR-BI antibodies directed against epitopes of the human SR-BI extracellular loop specifically inhibited HCVcc infection in a dose-dependent manner. Down-regulation of SR-BI expression by SR-BI-specific short interfering RNAs (siRNAs) markedly reduced the susceptibility of human hepatoma cells to HCVcc infection. Kinetic studies demonstrated that SR-BI acts predominately after binding of HCV at an entry step occurring at a similar time point as CD81-HCV interaction. Although the addition of high-density lipoprotein (HDL) enhanced the efficiency of HCVcc infection, anti-SR-BI antibodies and SR-BI-specific siRNA efficiently inhibited HCV infection independent of lipoprotein. Conclusion: Our data suggest that SR-BI (i) represents a key host factor for HCV entry, (ii) is implicated in the same HCV entry pathway as CD81, and (iii) targets an entry step closely linked to HCV-CD81 interaction

La nature de la condensation de Bose-Einstein induite par la localisation

We investigate the phase transition exhibited by the Bose gas in systems which are not translation-invariant. Though it has been known since the sixties that Bose-Einstein condensation (BEC) cannot occur in translation invariant systems for dimension 1 or 2, one can nevertheless enhance this phase transition in low-dimensional Bose gases by the addition of suitable external potentials (thus losing translation invariance in the process). However, the resulting condensate is then found to be in localised states, while BEC is usually understood to be the macroscopic occupation of extended kinetic eigenstates. It is therefore not clear whether the phase transition obtained by means of localisation is of the same nature as the one related to the usual concept of BEC. In this thesis, we consider two classes of localised systems. The first one is a family of random systems, where the Bose gas is contained in a disordered medium, which is modelled by a random external potential. Our second model consists of weak (scaled) external potentials. We first recall necessary conditions on these external potentials to enhance condensation in the localised states. We then show under very general assumptions that in these models, BEC in the usual sense occurs also, in a generalised sense. This means that the particles condense on kinetic eigenstates with arbitrary small energy. For the non-interacting Bose gas, we can moreover show that the densities of both condensates are actually equal. Next, we investigate BEC on a finer scale, asking whether one can obtain condensation in a single kinetic eigenstate. We show that in spite of the existence of a phase transition, and the occurence of generalised BEC, no condensation exists in any single kinetic eigenstate. In particular, the so-called "ground-state condensation" does not occur in these localised systems. Finally, we establish a possible generalisation of the Bogoliubov c-number approximation to take into account the very specific properties of BEC in the presence of localisation, and discuss how to interpret the result of the corresponding variational problem.Nous étudions la transition de phase survenant dans le gaz de Bose pour des systèmes sans invariance par translation. Bien qu'il soit prouvé depuis les années 60 que la condensation de Bose Einstein (CBE) est absente des systèmes invariants par translation en dimension 1 ou 2, on peut néanmoins déclencher cette transition de phase dans des gaz de Bose en faible dimension en ajoutant un potentiel externe approprié (et par conséquent, en perdant l'invariance par translation). Cependant, le condensat ainsi obtenu se trouve dans des états localisés, alors que la CBE est généralement comprise comme l'occupation macroscopique d'états cinétiques étendus. Il n'est pas à priori évident que cette transition de phase obtenue grace à la localisation est de la même nature que celle reliée au concept habituel de CBE. Dans cette thèse, nous considérons deux classes de systèmes localisés. La première est une famille de modèles aléatoires, pour lesquels le gaz de Bose est contenu dans un milieu désordonné, ce que nous modélisons par un potentiel externe aléatoire. La deuxième est constituée de modèles incluant un potentiel externe faible (d'échelle). Nous commençons par un rappel des conditions nécessaires sur ces potentiels pour obtenir une condensation dans les états localisés. Nous montrons sous certaines hypothèses très générales que dans ces modèles, la CBE au sens habituel est aussi présente, dans un sens généralisé. Cela signifie que les particules sont condensées dans des états cinétiques ayant une énergie arbitrairement faible. Pour le gaz de Bose sans interactions, nous pouvons en plus prouver que les densités des deux condensats sont en fait égales. Nous approfondissons ensuite notre étude de la CBE, en demandant si il est possible d'obtenir une condensation sur un seul état cinétique. Nous montrons qu'en dépit de l'existence à la fois d'une transition de phase et de la CBE généralisée, aucune condensation ne survient sur un seul état cinétique. En particulier, la fameuse condensation sur l'état fondamental est absente pour ces modèles localisés. Finalement, nous établissons une généralisation possible de l'approximation de nombres complexes de Bogoliubov pour prendre en compte les propriétés très particulières de la CBE en présence de localisation, et nous discutons la faon d'interpréter le resultat du problème variationnel correspondant

Swiprosin-1/EFhd2 limits germinal center responses and humoral type 2 immunity

Activated Bcells are selected for in germinal centers by regulation of their apoptosis. The Ca2+-binding cytoskeletal adaptor protein Swiprosin-1/EFhd2 (EFhd2) can promote apoptosis in activated Bcells. We therefore hypothesized that EFhd2 might limit humoral immunity by repressing both the germinal center reaction and the expected enhancement of immune responses in the absence of EFhd2. Here, we established EFhd2(-/-) mice on a C57BL/6 background, which revealed normal B- and T-cell development, basal Ab levels, and T-cell independent type 1, and T-cell independent type 2 responses. However, Tcell-dependent immunization with sheep red blood cells and infection with the helminth Nippostrongylus brasiliensis (N.b) increased production of antibodies of multiple isotypes, as well as germinal center formation in EFhd2(-/-) mice. In addition, serum IgE levels and numbers of IgE(+) plasma cells were strongly increased in EFhd2(-/-) mice, both after primary as well as after secondary N.b infection. Finally, mixed bone marrow chimeras unraveled an EFhd2-dependent Bcell-intrinsic contribution to increased IgE plasma cell numbers in N.b-infected mice. Hence, we established a role for EFhd2 as a negative regulator of germinal center-dependent humoral type 2 immunity, with implications for the generation of IgE