Activated T cells inhibit NK cell-mediated tumour rejection.

Previous studies have described the regulation of some T-cell subsets toward natural killer (NK) cells. Naturally occurring CD4(+)CD25(+) T regulatory cells can inhibit NK cell cytotoxicity, while activated interleukin-2 (IL-2) secreting T cells can stimulate NK cells. However, little is known about the impact of the integrity T-cell population on the final outcome of NK cell cytotoxicity. We thus examined the possible role of activated T cells in affecting NK cell cytotoxicity by mixed lymphocyte co-cultures in vitro and a B16 melanoma tumour model in vivo. In our study, activated T cells were found to be able to significantly inhibit NK cell cytotoxicity in vitro and blunt NK cell-mediated tumour rejection in vivo. The inhibition of NK cell function is a cell-cell contact dependent way. Results suggest that activated T cells may play an important role in limiting NK cell functions, which might be very significant for the design of biotherapy against tumour or infection in future.

Observation of Multi-TeV Gamma Rays from the Crab Nebula using the Tibet Air Shower Array.

The Tibet experiment, operating at Yangbajing (4300 m above sea level), is the lowest energy air shower array, and the new high-density array constructed in 1996 is sensitive to gamma-ray air showers at energies as low as 3 TeV. With this new array, the Crab Nebula was observed in multi-TeV gamma-rays and a signal was detected at the 5.5 sigma level. We also obtained the energy spectrum of gamma-rays in the energy region above 3 TeV which partially overlaps those observed with imaging atmospheric Cerenkov telescopes. The Crab spectrum observed in this energy region can be represented by the power-law fit dJ&parl0;E&parr0;&solm0;dE=&parl0;4.61+/-0.90&parr0;x10-12&parl0;E&solm0;3 TeV&parr0;-2.62+/-0.17 cm-2 s-1 TeV-1. This is the first observation of gamma-ray signals from point sources with a conventional air shower array using scintillation detectors.