Cell death is achieved by two fundamentally different mechanisms: apoptosis and necrosis. Apoptosis is dependent on caspase activation, whereas the caspase-independent necrotic signaling pathway remains largely uncharacterized. We show here that Fas kills activated primary T cells efficiently in the absence of active caspases, which results in necrotic morphological changes and late mitochondrial damage but no cytochrome c release. This Fas ligand-induced caspase-independent death is absent in T cells that are deficient in either Fas-associated death domain (FADD) or receptor-interacting protein (RIP). RIP is also required for necrotic death induced by tumor necrosis factor (TNF) and TNF-related apoptosis-inducing ligand (TRAIL). In contrast to its role in nuclear factor kappa B activation, RIP requires its own kinase activity for death signaling. Thus, Fas, TRAIL and TNF receptors can initiate cell death by two alternative pathways, one relying on caspase-8 and the other dependent on the kinase RIP.

TCRαβ thymocytes differentiate to either CD8αβ cytotoxic T lymphocytes or CD4+ T helper cells. This functional dichotomy is controlled by key transcription factors, including the T helper master regulator, ThPOK, which suppresses the cytolytic program in MHC class II-restricted CD4+ thymocytes. ThPOK continues to repress CD8-lineage genes in mature CD4+ T cells, even as they differentiate to T helper effector subsets. Here we show that the T helper-fate was not fixed and that mature antigen-stimulated CD4+ T cells could terminate Thpok expression and reactivate CD8-lineage genes. This unexpected plasticity resulted in the post-thymic termination of the T helper-program and the functional differentiation of distinct MHC class II-restricted CD4+ cytotoxic T lymphocytes.

The thymus leukemia antigen (TL) is a nonclassical class I molecule, expressed abundantly on intestinal epithelial cells. We show that, in contrast to other major histocompatibility complex (MHC) class I molecules that bind CD8alphabeta, TL preferentially binds the homotypic form of CD8alpha (CD8alphaalpha). Thus, TL tetramers react specifically to CD8alphaalpha-expressing cells, including most intestinal intraepithelial lymphocytes. Compared with CD8alphabeta, which recognizes the same MHC as the T cell receptor (TCR) and thus acts as a TCR coreceptor, high-affinity binding of CD8alphaalpha to TL modifies responses mediated by TCR recognition of antigen presented by distinct MHC molecules. These findings define a novel mechanism of lymphocyte regulation through CD8alphaalpha and MHC class I.

In humans suffering from dialysis-related amyloidosis, the protein ␤2-microglobulin (␤2M) is deposited as an amyloid; however, an amyloid of ␤2M is unknown in mice. ␤2M sequences from human and mouse are 70% identical, but there is a seven-residue peptide in which six residues differ. This peptide from human ␤2M forms amyloid in vitro, whereas the mouse peptide does not. Substitution of the human peptide for its counterpart in the mouse sequence results in the formation of amyloid in vitro. These results show that a seven-residue segment of human ␤2M is sufficient to convert ␤2M to the amyloid state, and that specific residue interactions are crucial to the conversion. These observations are consistent with a proposed Zipper-spine model for ␤2M amyloid, in which the spine of the fibril consists of an anhydrous ␤-sheet. More than 20 proteins have been found to aggregate into amyloids, elongated unbranched fibrils that bind the aromatic dyes Congo red and ThioflavinT (ThT) and have a common cross ␤ x-ray diffraction pattern (1, 2). The proteins that form amyloids differ in size, function, sequence, and native structure, but all form aggregates similar in structure and properties (3-5). It has long been recognized from the cross-␤ diffraction pattern that amyloids are formed from ␤-sheets Ϸ10-12 Å apart, each made up of extended strands stacked Ϸ4.7 Å apart (6, 7). There is evidence that in some amyloids, the ␤-strands run parallel to each other (8-10), and in others they may run antiparallel (11,12).Some models for amyloid structure depict the entire native protein as refolding into the amyloid (13-16); we term these Entire-refolding models. Other models depict the interactions of amyloid to be formed from only a small segment of the protein, with the rest retaining a native-like structure (17)(18)(19)(20). Entirerefolding models are based in part on the idea that amyloid formation is an inexorable tendency of all proteins, and that variations in rate of achieving the amyloid state are mainly a matter of amino acid composition (21). In contrast, models that depict amyloid formation as having its basis in a ''gain of interaction'' (18) focus on the formation of a new intermolecular bond contributed by a segment of the entire protein. The formation of these intermolecular bonds would in principle depend on the amino acid sequence, not just the composition. In this paper, we focus on a particular gain-of-interaction model, called the Zipper-spine model, in which the new interaction is a spine of ␤-sheet (17).One of the most intensively studied amyloid-forming proteins is ␤2-microglobulin (␤2M), a normally soluble protein that aggregates into pathogenic fibrils either at low pH (22) or under physiological conditions when divalent copper is present (23). The Entire-refolding view of amyloid depicts dialysis-related amyloidosis pathogenesis as destabilization of the native structure of ␤2M followed by formation of a nucleating ␤2M species that forms amyloid fibrils (24-26). However, there is accumulating evidence that s...