Rapport quadriennal 1995-1998

Seismic approaches reveal the lithospheric structure and geodynamic contexts of Piton de la Fournaise and Etna. A set of tomographic experiments constrains their internal structures with a resolution providing insight into edifice evolution and eruptive behavior. The hotspot context of La Réunion island thus appears to differ from the Hawaiian one by a reduced plate flexure, large proportion of low velocity-density in the edifice, crustal underplating under the presently active part, which may be seen as the level of transposition from primary melts into the more tholeitic products erupted. Structural heterogeneities of the preexisting plate are documented which may control magmatism in addition to slow plate/hotspot motion. Two completely different tomographies establish a high-velocity central core under the active volcano summit area which contrasts with the surroundings interpreted as eruptives or transported material and rises from the top of the oceanic plate. Another distinct high-velocity body to the East, which top has been drilled, is imaged to be rooted in the plate. So-called rift-zones of the active volcano are not marked by velocity anomalies, hence do not appear to be magmatic. High resolution location of earthquakes, heterogeneity in Vp/Vs and magnitude-number evolution can be correlated for the preeruptive magma migration to the most recent eruption. Mt Etna is not in a simple geodynamic context typical for volcanism. Marine reflection seismics reveal extension of crustal scale, which has succeeded to the compression related to the subduction at the Calabro-Sicilian arc. It is interpreted as marking a change in slab coupling, with which the development of Etna appears coeval... (D'après résumé d'auteur

Monovalent ligation of the B cell receptor induces receptor activation but fails to promote antigen presentation

We explored the role of antigen valency in B cell receptor (BCR) activation and rearrangement of intracellular MHC class II compartments as factors that contribute to the efficacy of antigen presentation. Using primary B cells that express a hen egg lysozyme (HEL)-specific BCR, we found that oligomeric HEL more efficiently promoted both BCR activation and internalization than did monovalent HEL, although monovalent HEL, unlike monovalent Fab fragments of anti-Ig, readily triggered the BCR. Nonetheless, oligovalent ligation positions the BCR in a membrane microdomain that is distinct from one engaged in the course of monovalent ligation, as judged by detergent extraction of the BCR. Furthermore, oligovalent HEL induced more pronounced rearrangement of MHC class II-containing antigen-processing compartments. Using videomicroscopy we observed in real time the rearrangement of MHC class II compartments as well as delivery of antigen in primary B cells. The observed increase in rearrangement of MHC class II-positive compartments and the disposition of antigen-bound BCRs therein correlates with improved presentation of a HEL-derived epitope. Although monomeric HEL efficiently engages the BCR, presentation of HEL-derived epitopes is impaired compared to oligovalent antigens. This trait may help explain the known ability of soluble, disaggregated antigen to induce a state of B cell tolerance