Observation of the radiative decay of the 229Th nuclear clock isomer.

The radionuclide thorium-229 features an isomer with an exceptionally low excitation energy that enables direct laser manipulation of nuclear states. It constitutes one of the leading candidates for use in next-generation optical clocks1-3. This nuclear clock will be a unique tool for precise tests of fundamental physics4-9. Whereas indirect experimental evidence for the existence of such an extraordinary nuclear state is substantially older10, the proof of existence has been delivered only recently by observing the isomer's electron conversion decay11. The isomer's excitation energy, nuclear spin and electromagnetic moments, the electron conversion lifetime and a refined energy of the isomer have been measured12-16. In spite of recent progress, the isomer's radiative decay, a key ingredient for the development of a nuclear clock, remained unobserved. Here, we report the detection of the radiative decay of this low-energy isomer in thorium-229 (229mTh). By performing vacuum-ultraviolet spectroscopy of 229mTh incorporated into large-bandgap CaF2 and MgF2 crystals at the ISOLDE facility at CERN, photons of 8.338(24) eV are measured, in agreement with recent measurements14-16 and the uncertainty is decreased by a factor of seven. The half-life of 229mTh embedded in MgF2 is determined to be 670(102) s. The observation of the radiative decay in a large-bandgap crystal has important consequences for the design of a future nuclear clock and the improved uncertainty of the energy eases the search for direct laser excitation of the atomic nucleus.

Magnesium-Vacancy Optical Centers in Diamond.

We provide the first systematic characterization of the structural and photoluminescence properties of optically active centers fabricated upon implantation of 30-100 keV Mg+ ions in synthetic diamond. The structural configurations of Mg-related defects were studied by the electron emission channeling technique for short-lived, radioactive 27Mg implantations at the CERN-ISOLDE facility, performed both at room temperature and 800 °C, which allowed the identification of a major fraction of Mg atoms (∼30 to 42%) in sites which are compatible with the split-vacancy structure of the MgV complex. A smaller fraction of Mg atoms (∼13 to 17%) was found on substitutional sites. The photoluminescence emission was investigated both at the ensemble and individual defect level in the 5-300 K temperature range, offering a detailed picture of the MgV-related emission properties and revealing the occurrence of previously unreported spectral features. The optical excitability of the MgV center was also studied as a function of the optical excitation wavelength to identify the optimal conditions for photostable and intense emission. The results are discussed in the context of the preliminary experimental data and the theoretical models available in the literature, with appealing perspectives for the utilization of the tunable properties of the MgV center for quantum information processing applications.

A polyvalent cellular vaccine induces T-cell responses against specific self-antigens overexpressed in chronic lymphocytic B-cell leukemia.

B cell-derived chronic lymphocytic leukemia (CLL) is an incurable disease that requires innovative therapeutic regimens. Experimental approaches of immunotherapy aiming at induction of systemic T-cell responses have been developed. Trioma cells provide a potent vaccine derived from malignant B cells that allows multiple antigens (Ags) from the parental tumor to be ingested by Ag-presenting cells. Like other strategies using modified whole tumor cells, this approach induces polyvalent responses. Using trioma cell-pulsed dendritic cells (DCs) for T-cell activation in vitro, we asked whether specific Ags overexpressed by CLL can be identified as target structures of such responses and what is the nature of these Ags. Expression levels of several genes in CLL samples were quantitated by reverse transcriptase-polymerase chain reaction. T lymphocytes were polyvalently stimulated by trioma-pulsed DCs and specificities were tested by determining cytokine secretion in the presence of target cells transfected with RNA coding for those Ags that were found to be overexpressed. We demonstrate that DCs pulsed with the modified tumor cells efficiently activate T lymphocytes against CLL and that overexpressed Ags related to leukemogenesis, such as BCL-2, MDM2, and ETV5, serve as targets for those T cells. Immune escape by Ag loss or mutation is less likely to occur if immunity is directed against altered self-proteins that are involved in malignant transformation. Therefore, vaccines based on modified tumor cells such as triomas hold promise for immunotherapy of CLL and other malignancies. Polyvalent vaccines originally designed as individualized therapeutics may be more broadly applicable, at least in patients showing similar Ag patterns.

Dendritic cell-based immunogens for B-cell chronic lymphocytic leukemia.

Hybrids generated from tumor cells and dendritic cells (DC) have been proposed as tools for treating malignant disease. Here, we study the underlying principles and the feasibility for the adjuvant therapy of human B cell chronic-lymphocytic leukemia (B-CLL). CLL cells and allogeneic DC were only mixed or additionally fused. Using a combination of FACS and fluorescence microscopic analyses, we show that DC-CLL hybrids can be successfully generated. However, fusion frequencies have to be critically evaluated because the number of fused cells is overestimated when based on FACS analyses alone. The capability of activating patients' PBMC was examined by measuring cytokine secretion in co-culture assays. We made a systematic comparison of the immunostimulatory capacities of different stimulator cell populations, including DC-CLL fusion samples, unfused mixtures of DC and CLL cells as well as DC or tumor cells alone. Surprisingly, even unfused mixtures had a pronounced tumor-directed immunostimulatory effect. This could be explained by the capture of antigens from surrounding leukemia cells by DC during co-cultivation. Although fusion frequencies were low, PBMC stimulation was significantly more effective when the mixtures were subjected to cell fusion. The most potent stimulus was provided by DC-CLL fusion samples derived from mature DC, probably due to their enhanced costimulatory capacity. In summary, DC-tumor cell hybrids might be feasible in the treatment of B-CLL. It should be considered that FACS analysis is not sufficient to assess fusion frequencies and that interactions between unfused DC and CLL cells also result in PBMC activation.

Vaccination against B-cell chronic lymphocytic leukemia with trioma cells: preclinical evaluation.

PURPOSE: Trioma cells are lymphoma cells that have been fused to a hybridoma and have thereby been modified to express an immunoglobulin directed against surface receptors of antigen-presenting cells. Trioma cells that potentially include all lymphoma-derived antigens will be targeted to professional antigen-presenting cells in vivo. This allows uptake, processing, and presentation of tumor-derived antigens to T lymphocytes. In a mouse model, vaccination with trioma cells conferred long-lasting, T cell-dependent tumor immunity and was even able to eradicate established lymphomas. Here, we investigated whether this potent approach is effective in the human system. EXPERIMENTAL DESIGN: Malignant cells from 11 patients with B cell chronic-lymphocytic leukemia (B-CLL) were fused to an anti-Fc receptor hybridoma. The resulting trioma cells were extensively characterized with respect to their clonal origin. The induction of autologous tumor-specific T lymphocytes in the presence of trioma and antigen-presenting cells was examined in vitro by determining cytokine secretion in coculture assays. RESULTS: In seven cases, trioma cells could successfully be generated from B-CLL cells. Stimulation of autologous lymphocytes with trioma cells induced a leukemia-specific T-cell response. Immunostimulatory trioma cells were also obtained from two patients with solid B-cell lymphoma. CONCLUSIONS: Trioma-mediated immunization may be a promising adjuvant treatment of human malignancies of the B-cell lineage, particularly of B-CLL, which has still a very poor prognosis. Our in vitro results pave the way for clinical application.

Impact of the lymphoma idiotype on in vivo tumor protection in a vaccination model based on targeting antigens to antigen-presenting cells.

Trioma cell vaccination is a potent new immunologic approach for the therapy of malignant B-cell lymphoma. It is based on targeting tumor antigens to internalizing receptors on antigen-presenting cells (APCs). Tumor cells are fused to an APC-specific hybridoma, where they are converted to trioma cells that include potentially all lymphoma-derived antigens and that express the APC-binding arm. In this study, the mechanisms of trioma-mediated tumor immunity in immunocompetent mice were dissected, and it was shown in this model system that humoral anti-idiotypic immunity is indeed detectable after idiotype-specific immunization but that it does not reflect the degree of tumor protection obtained in vivo. Immunization against the idiotype alone was not sufficient for efficient tumor rejection in vivo. Targeting tumor antigens to APCs is only successful in terms of inducing tumor protection when designed as a polyvalent vaccination protocol.