Upon activation, naïve T helper cells can differentiate into two major distinct subsets, T helper 1 (Th1) and T helper 2 (Th2), as defined by their effector functions and cytokine secretion patterns. Cytokine milieu and costimulatory molecules have been shown to play an essential role in determining T helper differentiation. However, it is still unclear how the effects of signals of costimulatory molecules and cytokines are exerted during T helper differentiation. We show evidence suggesting that while cytokine signals initiate the differentiation program, the selective action of death effectors determines the end point balance of differentiating T helper subsets. We examined the expression of TNF-related apoptosis-inducing ligand (TRAIL) and CD95L in cloned and in vitro differentiated Th1 and Th2 cells. We found that activation-induced expression of TRAIL is exclusively observed in Th2 clones and primary T helper cells differentiated under the Th2 condition, while the expression of CD95L is mainly in Th1 cells. Furthermore, these two subsets exhibit distinct susceptibilities to TRAIL-and CD95L-mediated apoptosis. Th2 cells are more resistant to either TRAIL-or CD95L-induced apoptosis than Th1 cells. More importantly, both Th1 and Th2 cells could induce apoptosis in labeled Th1 but not Th2 cells. Blocking TRAIL and CD95L significantly enhance IFN-c production in vitro. Likewise, young MRL/MpJ-lpr/lpr mice also showed more Th1 response to ovalbumin immunization as compared to MRL/MpJ+/+. Therefore, apoptosis mediated by CD95L and TRAIL is critical in determining the fate of differentiating T helper cells.

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A thirty‐pair AlGaN/AlN distributed Bragg reflector (DBR) targeted at a center wavelength of 320 nm was grown on a 2‐inch sapphire substrate by metalorganic chemical vapor deposition (MOCVD). It is free of cracks in the main area of the wafer except for a ∼4 mm periphery observed under optical microscope. The measured reflectance spectrum shows a peak reflectivity of 93% at 313 nm and a bandwidth of 13 nm. Further investigation into the interfaces was performed using Auger electron spectroscopy (AES), scanning transmission electron microscope (STEM) and energy dispersive X‐ray fluorescence spectrometer (EDX). It was found that the constitutional change is abrupt when AlGaN is deposited on an AlN layer but gradual in reverse, which resulted in a stack of quasi‐three‐layer periods and high Al composition and therefore both the center reflectivity and the bandwidth were depressed. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)