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Somatic hypermutation specifically modifies rearranged immunoglobulin (Ig) genes in germinal center (GC) B cells. However, the bcl-6 gene can also acquire somatic mutations during the GC reaction, indicating that certain non-Ig genes can be targeted by the somatic hypermutation machinery. The CD95 gene, implicated in negative selection of B lymphocytes in GCs, is specifically expressed by GC B cells and was recently identified as a tumor suppressor gene being frequently mutated in (post) GC B cell lymphomas. In this study, the 5' region (5'R) and/or the last exon coding for the death domain (DD) of the CD95 gene were investigated in naive, GC, and memory B cells from seven healthy donors. About 15% of GC and memory, but not naive, B cells carried mutations within the 5'R (mutation frequency 2.5 x 10(-4) per basepair). Mutations within the DD were very rare but could be efficiently selected by inducing CD95-mediated apoptosis: in 22 apoptosis-resistant cells, 12 DD mutations were found. These results indicate that human B cells can acquire somatic mutations of the CD95 gene during the GC reaction, which potentially confers apoptosis resistance and may counteract negative selection through the CD95 pathway.

We report on the ferroelectric switching times [[Tau].sub.s] in polyvinylidene fluoride (PVDF), blended with polymethylmethacrylate, subjected to a mechanical stress [[Sigma].sub.2] in the transverse direction, and also with the use of different electrode materials. In contrast to theoretical predictions, [[Tau].sub.s] does not depend on [[Sigma].sub.2], which indicates that the importance of the pseudo-hexagonal crystallography of the crystalline [Beta]-modification of PVDF has been overestimated. The saturation polarization decreases severely with decreasing PVDF content whereas [[Tau].sub.s] stays constant. Finally, the injection of space charges is of minor importance for both the ferroelectric switching and the attainable polarization, since these do not depend on the electrode material. We conclude that the internal field is not influenced by blending but that this treatment reduces the amorphous phase contribution to the polarization. The dynamics and the strength of the polarization of PVDF can be understood only if the contributions of the orientation correlations and of the dynamics of the polymer chains in the crystal-amorphous interphase to the overall polarization are taken into account in this semi-crystalline material.

Polymeric actuator and sensor materials suffer from a low thermal and temporal stability. We report on an attempt to slow down the underlying structural relaxation in polyvinylidenefluoride (PVDF) films by irradiation with electrons of 1.5 MeV. This treatment yielded a dose-dependent crosslinking of the macromolecular chains. It caused a considerable rise of the ferroelectric remanent and saturation polarization, and the material became a harder ferroelectric. These changes were almost completely and irreversibly lost upon subsequent annealing. The electron spin resonance (ESR) measurements revealed that a large number of radicals and trapped electrons remained after irradiation (~1 per crystallite), and that the electric conductivity increased from 0.11 pS/m in the un-irradiated but poled sample to 1.1 pS/m at 450 kGy. The increase in conductivity with dose is caused completely by a corresponding increase of the product [Mu][Eta] of the charge carrier mobility [Mu] and the number density [Eta]. Based on a discussion of depolarization effects, we conclude that the change in ferroelectric behavior upon irradiation mainly is caused by the activity of those charges which are implanted or created by irradiation, and that the majority of them are annihilated by thermal treatment. However, the reduction of polarization with time, or at elevated temperature, is considerably slowed down due to the existence of crosslinks.

Presents information on Hodgkin's disease from citations and notes of clinicians. Loss of the B-lineage-specific gene expression programme in Hodgkin's and Reed-Sternberg cells of Hodgkin's lymphoma; Effectivity of the treatment of relapsed Castleman's disease (CD) 20 positive Hodgkin's lymphoma with the monoclonal antibody rituximab; Examination on the cases of Epstein-Barr-virus (EBV)-positive post-transplant lymphoproliferative disease for the pattern of EBV gene expression and the B cell phenotype by immunoglobulin heavy chain genotyping.