T cell-specific FADD-deficient mice: FADD is required for early T cell development

Abstract

FADD͞Mort1, initially identified as a Fas-associated death-domain containing protein, functions as an adapter molecule in apoptosis initiated by Fas, tumor necrosis factor receptor-I, DR3, and TRAILreceptors. However, FADD likely participates in additional signaling cascades. FADD-null mutations in mice are embryonic-lethal, and analysis of FADD ؊/؊ T cells from RAG-1 ؊/؊ reconstituted chimeras has suggested a role for FADD in proliferation of mature T cells. Here, we report the generation of T cell-specific FADDdeficient mice via a conditional genomic rescue approach. We find that FADD-deficiency leads to inhibition of T cell development at the CD4 ؊ CD8 ؊ stage and a reduction in the number of mature T cells. The FADD mutation does not affect apoptosis or the proximal signaling events of the pre-T cell receptor; introduction of a T cell receptor transgene fails to rescue the mutant phenotype. These data suggest that FADD, through either a death-domain containing receptor or a novel receptor-independent mechanism, is required for the proliferative phase of early T cell development. Fas-associated death domain (FADD)͞Mort1 was initially isolated as a protein that interacts with the cytoplasmic tail of the Fas receptor in a yeast two-hybrid system (1-3). FADD consists of a ''death effector domain'' at its amino terminus and a ''death domain'' at its carboxyl terminus. The death domain was shown to serve as a protein-protein interaction region with several death-domain containing members of the tumor necrosis factor receptor (TNFR) family. The FADD death effector domain recruits caspase-8 (4, 5). Upon interaction of Fas by FasL, for example, FADD, and subsequently caspase-8, are brought to the ''death-inducing signaling complex'' at the membrane (3, 6, 7). It was suggested that proximity among caspase-8 molecules within the death-inducing signaling complex leads to intermolecular processing and activation of caspase-8 (8, 9). This eventually leads to cleavage of the downstream targets and apoptosis. In addition to the Fas pathway, FADD also is required for signaling of cell death initiated by TNFR-I, DR3, and the TNF-related apoptosis-inducing ligand (TRAIL) receptors DR4 and DR5. FADD-deficient MEFs or FADD Ϫ/Ϫ Jurkat T cells are completely resistant to apoptosis mediated by anti-Fas agonist antibody, TNF␣,or TRAIL (10 -15).Apart from its role in apoptosis, analysis of FADD Ϫ/Ϫ mice showed that FADD is also involved in other biological processes (10, 11). FADD-deficient embryos die of heart defects at day 10 of gestation. This could be caused by either a requirement of a death-domain containing receptor in heart development or an involvement of FADD in other unknown processes during differentiation (10, 11). Additionally, FADD appears to play a role in T cell proliferation and͞or lymphocyte development. In The impaired T cell development obser ved in FADD Ϫ/Ϫ 3RAG-1 Ϫ/Ϫ chimeras could be caused by the early effect of the FADD mutation in lymphoid progenitor cells or later during T cell development. To explore thi...

Conditional Knockout Mice Reveal an Essential Role of Protein Phosphatase 4 in Thymocyte Development and Pre-T-Cell Receptor Signaling

“…3A and B). The increase in L DN3 cell numbers (approximately 5.2-fold) also indicates a developmental block which is likely due to a proliferation defect between the L DN3/DN4 and DP stages (14). Taking these findings together, we conclude that PP4 ablation in thymocytes leads to cell arrest at the DN3 stage ( Fig.…”

“…The DN3 cell population can be further subdivided into E and L cells based on cell size, determined by their forward versus side scatter parameters (8,10). Successful TCR gene rearrangement and functional pre-TCR signaling are required for E DN3 cell maturation into L DN3 cells (TCR ␤ selection), which will undergo proliferation and proceed to the DN4 stage (10,14). In Lck-Cre; F/F mice, where Cre-mediated DNA recombination was shown to end at the DN3 stage (27), we found that E DN3 cell numbers in the CD44 low CD25 ϩ DN3 cell population increased (about 3.3-fold), further suggesting a cell arrest at this stage which is likely due to an impaired differentiation of E DN3 cells and, potentially, dysregulated pre-TCR signaling ( Fig.…”

Caspase function in programmed cell death

“…Conditional fadd KO data indicate that like caspase-8, FADD is required for T-cell development and peripheral T-cell homeostasis. 88,89 The similarity in the phenotypes of casp8 and fadd mutant mice suggests that FADD and caspase-8 are essential for apoptosis mediated by many of the TNFRs, but also play context-dependent nonapoptotic functions (see article by Lamkanfi et al in this issue).…”

Apoptosis in the development of the immune system

“…In these mice, early thymocytes development was retarded at the double-negative stage and mature peripheral T-cells were severely reduced. 60,100,101 Blockade of FADD in recombination activating gene-1-deficient thymocytes, which cannot rearrange their TCR alleles, allowed some cells to progress to the DP stage illustrating a role for FADD in promoting the death of thymocytes lacking the pre-TCR. 102 Furthermore, transgenic expression of cFLIP has been shown to increase TCR-induced proliferation.…”