The leukocyte-specific adapter molecule SLP-76 (Src homology 2 domain-containing leukocyte protein of 76 kilodaltons) is rapidly phosphorylated on tyrosine residues after receptor ligation in several hematopoietically derived cell types. Mice made deficient for SLP-76 expression contained no peripheral T cells as a result of an early block in thymopoiesis. Macrophage and natural killer cell compartments were intact in SLP-76-deficient mice, despite SLP-76 expression in these lineages in wild-type mice. Thus, the SLP-76 adapter protein is required for normal thymocyte development and plays a crucial role in translating signals mediated by pre-T cell receptors into distal biochemical events.

Antigen receptor engagement on T lymphocytes activates transcription factors important for stimulating cytokine gene expression. This is critical for clonal expansion of antigen-specific T cells and propagation of immune responses. Additionally, under some conditions antigen receptor stimulation initiates apoptosis of T lymphocytes through the induced expression of CD95 ligand and its receptor. Here we demonstrate that the transcription factor, NFAT, which is critical for the inducible expression of many cytokine genes, also plays a critical role in the regulation of T cell receptor-mediated CD95 ligand expression. Two sites within the CD95 ligand promoter, identified through DNase I footprinting, bind NFAT proteins from nuclear extracts of activated T cells. Although both sites appear important for optimal expression of CD95 ligand in activated T cells, mutational analysis suggests that the distal NFAT site plays a more significant role. Furthermore, these sites do not appear to be required for constitutive CD95 ligand expression in Sertoli cells.Adaptive immune responses require the activation of T lymphocytes through antigen-specific T cell receptor (TCR) 1 stimulation. Signaling events initiated by TCR ligation lead to the activation of transcription factors that regulate expression of cytokine genes, such as IL-2. This is important for the clonal expansion of antigen-specific T cells and propagation of immune responses (1, 2). However, once the antigenic stimulus has been cleared, the expanded population of cells must be eliminated to prevent accumulation of excessive lymphocytes (2, 3). Recently, it has been proposed that one mechanism by which this occurs is through the induced expression of CD95 ligand (4 -7). Once expressed, CD95 ligand engages its receptor, CD95, also expressed on the population of activated lymphocytes (8). In the absence of costimulatory signals that can delay apoptosis (9 -12), CD95 ligation rapidly initiates the programmed cell death machinery thus efficiently eliminating excessive activated lymphocytes (8). Additionally, autoreactive T cells that are inappropriately activated in the periphery are believed to undergo apoptosis through a similar process (13).The significance of CD95 ligand expression in the process of activation-induced cell death has been highlighted by recent studies that demonstrate that blocking CD95/CD95 ligand interactions prevents apoptosis of TCR-stimulated lymphocytes (5, 7). Interestingly, in addition to its inducible expression on activated lymphocytes, CD95 ligand is constitutively expressed on epithelial cells within the eye and Sertoli cells within the testes (14 -16). Constitutive CD95 ligand expression participates in maintenance of the "immune privileged" status of these tissues by inducing apoptosis in infiltrating, CD95-bearing, activated lymphocytes (17). Despite improved understanding of the important physiological roles for CD95 ligand in immune privileged sites and in controlling T cell homeostasis, little is yet known about the regulation of ...

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The interaction between CD95 (Fas) and CD95L (Fas ligand) initiates apoptosis in a variety of cell types. Although the regulation of CD95L expression on activated T cells is an area of intense study, knowledge related to the induction of CD95L promoter activity in primary T cells is lacking. In this report we describe the generation of a novel transgenic mouse strain, CD95LP-Luc, in which murine CD95L promoter sequence controls the expression of a luciferase reporter gene. We use these mice to illustrate several important findings related to transcriptional regulation of CD95L in primary T cells. We demonstrate that maximal CD95L promoter activity occurs only after prolonged T cell stimulation and requires costimulation through CD28. We provide evidence that thymocytes express CD95L/luciferase after strong TCR ligation and that inducible CD95L promoter activation is present, but unequal, in both Th1 and Th2 effector cells. We also illustrate that while agonist peptide presentation by APCs generates robust proliferation during a primary T cell response, the same stimulus induces only modest CD95L promoter activity. These results suggest alternate explanations for the well-characterized delay in CD95-mediated activation-induced cell death following initial ligation of the TCR.