Caspase-10 is an initiator caspase in death receptor signaling

A role for caspase-10, previously implicated in the autoimmune lymphoproliferative syndrome, in death receptor signaling has not been directly shown. Here we show that caspase-10 can function independently of caspase-8 in initiating Fas- and tumor necrosis factor-related apoptosis-inducing ligand-receptor-mediated apoptosis. Moreover, Fas crosslinking in primary human T cells leads to the recruitment and activation of caspase-10. Fluorescent resonance energy transfer analysis indicates that the death-effector domains of caspase-8 and -10 both interact with the death-effector domain of FADD. Nonetheless, we find that caspase-8 and -10 may have different apoptosis substrates and therefore potentially distinct roles in death receptor signaling or other cellular processes.

Immunophenotypic profiles in families with autoimmune lymphoproliferative syndrome

Autoimmune lymphoproliferative syndrome (ALPS) type Ia is caused by inherited defects in apoptosis and is characterized by nonmalignant lymphoaccumulation, autoimmunity, and increased alpha/beta(+) double-negative T cells (alpha/beta(+)-DNT cells). This study reports immunophenotypic findings in 166 members of 31 families with ALPS type Ia, associated with genetic mutations in the TNFRSF6 gene encoding Fas. The ALPS type Ia probands (n = 31) and relatives having both a Fas mutation and clinically proven ALPS (n = 28) showed significant expansion of CD8(+) T cells, alpha/beta(+)-DNT cells, gamma/delta(+)-DNT cells, CD3(+)/ HLA-DR(+) T cells, CD8(+)/CD57(+) T cells, and CD5(+) B cells. Relatives with Fas mutations, but without all the required criteria for ALPS (n = 42), had expansions of CD8(+) T cells, alpha/beta(+)-DNT cells, and gamma/delta(+)-DNT cells. Interestingly, relatives without a Fas mutation and with no features of ALPS (n = 65) demonstrated a small but significant expansion of CD8(+) T cells, both DNT cell subsets, and CD5(+) B cells. As compared to unrelated healthy controls, lymphocyte subset alterations were greatest in the probands, followed by the relatives with mutations and ALPS. Probands and relatives with mutations and ALPS also showed a lower number of CD4(+)/CD25(+) T cells that, in combination with an independent increase in HLA-DR(+) T cells, provided a profile predictive of the presence of clinical ALPS. Because quantitative defects in apoptosis were similar in mutation-positive relatives regardless of the presence of clinical ALPS, factors, other than modifiers of the Fas apoptosis pathway, leading to these distinctive immunophenotypic profiles most likely contribute to disease penetrance in ALPS.

TcR-alpha/beta(+) CD4(-)CD8(-) T cells in humans with the autoimmune lymphoproliferative syndrome express a novel CD45 isoform that is analogous to murine B220 and represents a marker of altered O-glycan biosynthesis

Autoimmune lymphoproliferative syndrome (ALPS), caused by inherited defects in apoptosis secondary to mutations in genes encoding Fas/CD95/APO-1 and Fas ligand (Fasl)/CD95L, is characterized by nonmalignant lymphadenopathy and splenomegaly, increased T cell receptor alpha/beta(+) CD4(-)CD8(-) T cells (alpha/beta(+) double-negative T cells [alpha/beta(+)-DNT cells]), autoimmunity, hypergammaglobulinemia, and cytokine abnormalities. The alpha/beta(+)-DNT cells are immunophenotypically and functionally similar to alpha/beta(+)-DNT cells that accumulate in lpr and gld mice, which bear genetic mutations in Fas and FasL. In these mice, alpha/beta(+)-DNT cells express the B-cell-specific CD45R isoform B220. We show that alpha/beta(+)-DNT cells of ALPS patients, with either Fas or FasL mutations, also express B220. In addition, also similar to LPR/gLD mice, they have an unusual population of B220-positive CD4(+) T cells. B220 expression, together with our finding of characteristic lectin binding profiles, demonstrates that cell surface O-linked glycoproteins have undergone specific modifications, which may have consequences for lymphocyte trafficking, cell-cell interactions, and access to alternative apoptosis pathways.

The death effector domain-associated factor plays distinct regulatory roles in the nucleus and cytoplasm

Homophilic interactions of death effector domains (DEDs) are crucial for the signaling pathways of death receptor-mediated apoptosis. The machinery that regulates proper oligomerization and autoactivation of procaspase-8 and/or procaspase-10 during T lymphocyte activation determines whether the cells will undergo caspase-mediated apoptosis or proliferation. We screened a yeast two-hybrid library by using the DEDs contained in the prodomains of procaspase-8 and procaspase-10 and isolated a DED-associated factor (DEDAF) that interacts with several DED-containing proteins but does not itself contain a DED. DEDAF is highly conserved between human and mouse (98% amino acid identity) and is homologous to a nuclear regulatory protein YAF-2. DEDAF is expressed at the highest levels in lymphoid tissues and placenta. DEDAF interacts with FADD, procaspase-8, and procaspase-10 in the cytosol as well as with the DED-containing DNA-binding protein (DEDD) in the nucleus. At the cell membrane, DEDAF augmented the formation of CD95-FADD-caspase-8 complexes and enhanced death receptor- as well as DED-mediated apoptosis. In the nucleus, DEDAF caused the DEDD protein to relocalize from subnuclear structures to a diffuse distribution in the nucleoplasm. Our data therefore suggest that DEDAF may be involved in the regulation of both cytoplasmic and nuclear events of apoptosis.

Fluorescence resonance energy transfer analysis of cell surface receptor interactions and signaling using spectral variants of the green fluorescent protein

BACKGROUND

Fluorescence resonance energy transfer (FRET) is a powerful technique for measuring molecular interactions at Angstrom distances. We present a new method for FRET that utilizes the unique spectral properties of variants of the green fluorescent protein (GFP) for large-scale analysis by flow cytometry.

METHODS

The proteins of interest are fused in frame separately to the cyan fluorescent protein (CFP) or the yellow fluorescent protein (YFP). FRET between these differentially tagged fusion proteins is analyzed using a dual-laser FACSVantage cytometer.

RESULTS

We show that homotypic interactions between individual receptor chains of tumor necrosis factor receptor (TNFR) family members can be detected as FRET from CFP-tagged receptor chains to YFP-tagged receptor chains. Noncovalent molecular complexation can be detected as FRET between fusions of CFP and YFP to either the intracellular or extracellular regions of the receptor chains. The specificity of the assay is demonstrated by the absence of FRET between heterologous receptor pairs that do not biochemically associate with each other. Interaction between a TNFR-like receptor (Fas/CD95/Apo-1) and a downstream cytoplasmic signaling component (FADD) can also be demonstrated by flow cytometric FRET analysis.

CONCLUSIONS

The utility of spectral variants of GFP in flow cytometric FRET analysis of membrane receptors is demonstrated. This method of analyzing FRET allows probing of noncovalent molecular interactions that involve both the intracellular and extracellular regions of membrane proteins as well as proteins within the cells. Unlike biochemical methods, FRET allows the quantitative determination of noncovalent molecular associations at Angstrom level in living cells. Moreover, flow cytometry allows quantitative analyses to be carried out on a cell-by-cell basis on large number of cells. Published 2001 Wiley-Liss, Inc.

To B or not to B: TNF family signaling in lymphocytes

BLyS and family are known to affect B cells in a positive fashion. Knock-outs of BLyS receptors indicate some new functions, including negative regulation by one BLyS receptor, TACI.

The development of lymphomas in families with autoimmune lymphoproliferative syndrome with germline Fas mutations and defective lymphocyte apoptosis

Lymphomas were studied in kindreds with autoimmune lymphoproliferative syndrome (ALPS; Canale-Smith syndrome), a disorder of lymphocyte homeostasis usually associated with germline Fas mutations. Fas (CD95/APO-1) is a cell surface receptor that initiates programmed cell death, or apoptosis, of activated lymphocytes. Lymphoma phenotype was determined by immunohistochemistry, frequency of CD3(+)CD4(-)CD8(-) T-cell-receptor alpha/beta cells by flow cytometry, nucleotide sequences of the gene encoding Fas (APT1, TNFRSF6), and the percentage of lymphocytes undergoing apoptosis in vitro. Of 223 members of 39 families, 130 individuals possessed heterozygous germline Fas mutations. Eleven B-cell and T-cell lymphomas of diverse types developed in 10 individuals with mutations in 8 families, up to 48 years after lymphoproliferation was first documented. Their risk of non-Hodgkin and Hodgkin lymphomas, respectively, was 14 and 51 times greater than expected (each P <.001). Investigation of these 10 patients and their relatives with Fas mutations revealed that all had defective lymphocyte apoptosis and most had other features of ALPS. The tumor cells retained the heterozygous Fas mutations found in the peripheral blood and manifested defective Fas-mediated killing. These data implicate a role for Fas-mediated apoptosis in preventing B-cell and T-cell lymphomas. Inherited defects in receptor-mediated lymphocyte apoptosis represent a newly appreciated risk factor for lymphomas.

Effective antigen-specific immunotherapy in the marmoset model of multiple sclerosis

Mature T cells initially respond to Ag by activation and expansion, but high and repeated doses of Ag cause programmed cell death and can suppress T cell-mediated diseases in rodents. We evaluated repeated systemic Ag administration in a marmoset model of experimental allergic encephalomyelitis that closely resembles the human disease multiple sclerosis. We found that treatment with MP4, a chimeric, recombinant polypeptide containing human myelin basic protein and human proteolipid protein epitopes, prevented clinical symptoms and did not exacerbate disease. CNS lesions were also reduced as assessed in vivo by magnetic resonance imaging. Thus, specific Ag-directed therapy can be effective and nontoxic in primates.

The multifaceted role of Fas signaling in immune cell homeostasis and autoimmunity

Originally identified as a cell surface receptor that triggered the death of lymphocytes and tumor cells, it is now recognized that Fas (also known as CD95 or Apo-I) has distinct functions in the life and death of different cell types in the immune system. Fas signaling may also be involved in T cell costimulation and proliferation. Although Fas deficiency in humans and mice predisposes them towards systemic autoimmunity, Fas-FasL interactions can also facilitate organ-specific immunopathology. Proximal signaling by Fas and related receptors depends on subunit preassembly, which accounts for the dominant-negative effect of pathogenic receptor mutants and natural splice variants.

Generation of an apoptotic intracellular peptide by gamma-secretase cleavage of Alzheimer's amyloid beta protein precursor

The amyloid beta protein precursor (AbetaPP) is sequentially processed by beta- and gamma-secretases to generate the Abeta peptide. The biochemical path leading to Abeta formation has been extensively studied since extracellular aggregates of amyloidogenic forms of Abeta peptide (Abeta42) are considered the culprit of Alzheimer's disease. Aside from its pathological relevance, the biological role of AbetaPP proteolysis is unknown. Although never previously described, cleavage of AbetaPP by gamma-secretase should release, together with Abeta, a COOH-terminal AbetaPP Intracellular Domain, herein termed AID. We have now identified AID-like peptides in brain tissue of normal control and patients with sporadic Alzheimer's disease and demonstrate that AID acts as a positive regulator of apoptosis. Thus, overproduction of AID may add to the toxic effect of Abeta42 aggregates and further accelerate neurodegeneration.

A domain in TNF receptors that mediates ligand-independent receptor assembly and signaling

A conserved domain in the extracellular region of the 60- and 80-kilodalton tumor necrosis factor receptors (TNFRs) was identified that mediates specific ligand-independent assembly of receptor trimers. This pre-ligand-binding assembly domain (PLAD) is physically distinct from the domain that forms the major contacts with ligand, but is necessary and sufficient for the assembly of TNFR complexes that bind TNF-alpha and mediate signaling. Other members of the TNFR superfamily, including TRAIL receptor 1 and CD40, show similar homotypic association. Thus, TNFRs and related receptors appear to function as preformed complexes rather than as individual receptor subunits that oligomerize after ligand binding.

Fas preassociation required for apoptosis signaling and dominant inhibition by pathogenic mutations

Heterozygous mutations encoding abnormal forms of the death receptor Fas dominantly interfere with Fas-induced lymphocyte apoptosis in human autoimmune lymphoproliferative syndrome. This effect, rather than depending on ligand-induced receptor oligomerization, was found to stem from ligand- independent interaction of wild-type and mutant Fas receptors through a specific region in the extracellular domain. Preassociated Fas complexes were found in living cells by means of fluorescence resonance energy transfer between variants of green fluorescent protein. These results show that formation of preassociated receptor complexes is necessary for Fas signaling and dominant interference in human disease.

Measurement of molecular interactions in living cells by fluorescence resonance energy transfer between variants of the green fluorescent protein

Many signal transduction pathways operate through oligomerization of proteins into multi-subunit complexes. Although biochemical assays can identify potential protein-protein interactions, studying these interactions in living cells is more challenging. Fluorescence resonance energy transfer (FRET) has been used as a "spectroscopic ruler" to measure molecular proximity, but these methods have been limited by the need for chemical labeling of target proteins or labeled antibodies. We present methods for examining interactions between target proteins molecularly fused to cyan and yellow variants of the green fluorescent protein (GFP) by FRET in living cells. Flow cytometric and microscope-based methods are described that have been applied to a variety of interacting proteins.

TNF-alpha-induced secretion of C-C chemokines modulates C-C chemokine receptor 5 expression on peripheral blood lymphocytes

Peripheral blood lymphocytes express CCR5, a chemokine receptor for immune cell migration and calcium signaling that serves as an important coreceptor for the HIV. After in vitro stimulation, CCR5 expression is dramatically increased on mature T lymphocytes, especially on the CD45RO+ memory subset. In this study, we report that TNF-alpha delays the surface expression of CCR5 on PBLs after activation and diminishes CCR5 irrespective of its initial level. Functional loss of CCR5 is reflected in a decreased capability of the treated cells to migrate and signal calcium after MIP-1beta stimulation. The effect is mediated via the p80 type II TNF receptor (TNFR2), which induces NF-kappaB among other factors, leading to an enhanced secretion of the chemokines macrophage-inflammatory protein-1alpha, macrophage-inflammatory protein-1beta, and RANTES. Expression of these chemokines directly down-regulates CCR5. These findings reveal a new regulatory mechanism utilized by activated peripheral T cells to modulate their chemotaxis and potentially other functions mediated by CCR5, including the infection of T lymphocytes by macrophage-tropic HIV strains.

Roles of caspases in apoptosis, development, and cytokine maturation revealed by homozygous gene deficiencies

Caspases are a group of cysteine proteases critical for apoptosis of eukaryotic cells. Deletion of genes that encode murine caspases suggests that caspases are involved not only in apoptosis but also in cytokine maturation and cell growth and differentiation. Among them, caspase-1 and caspase-11 are primarily involved in the processing of pro-inflammatory cytokines. Caspase-3 and caspase-9 are essential for apoptosis during brain development. Caspase-8 is required for the development of heart muscle, cell proliferation in the hematopoietic lineage and death-receptor-mediated apoptosis. These studies suggest that caspases function in cell signaling events including apoptosis, cell growth and differentiation.

A crucial role for p80 TNF-R2 in amplifying p60 TNF-R1 apoptosis signals in T lymphocytes

Tumor necrosis factor-alpha (TNF-alpha) can elicit many cellular responses including programmed cell death or apotosis. TNF-alpha-induced apoptosis has been largely attributed to the p60 TNF-R1 receptor. The role of p80 in TNF-alpha-mediated apoptosis is largely unknown. We now present evidence that signaling through p80 switches on the previously dormant apoptotic machinery associated with p60. This effect on p60-associated apoptosis involves the proximal activation of caspases and proceeds in the presence of strong NF-kappaB induction. We evaluated the role of TRAF2 in p80-assisted apoptosis and found that a decrease in TRAF2 protein occurs with p80 but not p60 stimulation. However, the decrease in TRAF2 protein can be dissociated from apoptosis in the presence of a caspase inhibitor. Hence, one means by which p80 TNF-R2 regulates apoptosis is through its ability to amplify internally apoptotic signal transduction from p60 TNF-R1.

Inhibition of Fas-mediated apoptosis by the B cell antigen receptor through c-FLIP

Cross-linking of the B cell antigen receptor (BCR) induces resistance to Fas (APO-1 / CD95)-dependent apoptosis and thereby regulates one mechanism of B cell selection during antigen stimulation. To investigate the molecular mechanism by which BCR signaling regulates the Fas pathway, we examined the expression of constituents of the death-inducing signaling complex (DISC), including Fas, FADD, caspase-8 and cellular FLICE-inhibitory protein (c-FLIP). No significant changes in the cellular levels of Fas, FADD or caspase-8 were observed after BCR cross-linking. By contrast, the long isoform of c-FLIP (c-FLIP(L)) was significantly up-regulated by BCR cross-linking in primary B cells and in two B cell lines, A20 and WEHI-279. Moreover, transfection of c-FLIP(L) into A20 cells inhibited Fas-dependent apoptosis and suppressed recruitment of caspase-8 to the DISC. BCR cross-linking or FLIP overexpression also protects B cells from TRAIL-induced apoptosis. Thus, BCR signaling up-regulates c-FLIP(L) and suppresses the Fas- and TRAIL-receptor apoptosis pathways which could be important for tolerance and selection of antigen-specific B cells.

Inherited human Caspase 10 mutations underlie defective lymphocyte and dendritic cell apoptosis in autoimmune lymphoproliferative syndrome type II

Caspases are cysteine proteases that mediate programmed cell death in phylogenetically diverse multicellular organisms. We report here two kindreds with autoimmune lymphoproliferative syndrome (ALPS) type II, characterized by abnormal lymphocyte and dendritic cell homeostasis and immune regulatory defects, that harbor independent missense mutations in Caspase 10. These encode amino acid substitutions that decrease caspase activity and interfere with death receptor-induced apoptosis, particularly that stimulated by Fas ligand and TRAIL. These results provide evidence that inherited nonlethal caspase abnormalities cause pleiotropic apoptosis defects underlying autoimmunity in ALPS type II.

Caspases in T-cell receptor-induced thymocyte apoptosis

Apoptosis eliminates inappropriate or autoreactive T lymphocytes during thymic development. Intracellular mediators involved in T-cell receptor (TCR)-mediated apoptosis in developing thymocytes during negative selection are therefore of great interest. Caspases, cysteine proteases that mediate mature T-cell apoptosis, have been implicated in thymocyte cell death, but their regulation is not understood. We examined caspase activities in distinct thymocyte subpopulations that represent different stages of T-cell development. We found caspase activity in CD4+CD8+ double positive (DP) thymocytes, where selection involving apoptosis occurs. Earlier and later thymocyte stages exhibited no caspase activity. Only certain caspases, such as caspase-3 and caspase-8-like proteases, but not caspase-1, are active in DP thymocytes in vivo and can be activated when DP thymocytes are induced to undergo apoptosis in vitro by TCR-crosslinking. Thus, specific caspases appear to be developmentally regulated in thymocytes.

Requirement for Tec kinases Rlk and Itk in T cell receptor signaling and immunity

T cell receptor (TCR) signaling requires activation of Zap-70 and Src family tyrosine kinases, but requirements for other tyrosine kinases are less clear. Combined deletion in mice of two Tec kinases, Rlk and Itk, caused marked defects in TCR responses including proliferation, cytokine production, and apoptosis in vitro and adaptive immune responses to Toxoplasma gondii in vivo. Molecular events immediately downstream from the TCR were intact in rlk-/-itk-/- cells, but intermediate events including inositol trisphosphate production, calcium mobilization, and mitogen-activated protein kinase activation were impaired, establishing Tec kinases as critical regulators of TCR signaling required for phospholipase C-gamma activation.

Defective CD95/APO-1/Fas signal complex formation in the human autoimmune lymphoproliferative syndrome, type Ia

Heterozygous mutations in the CD95 (APO-1/Fas) receptor occur in most individuals with autoimmune lymphoproliferative syndrome (ALPS) and dominantly interfere with apoptosis by an unknown mechanism. We show that local or global alterations in the structure of the cytoplasmic death domain from nine independent ALPS CD95 death-domain mutations result in a failure to bind the FADD/MORT1 signaling protein. Despite heterozygosity for the abnormal allele, lymphocytes from ALPS patients showed markedly decreased FADD association and a loss of caspase recruitment and activation after CD95 crosslinking. These data suggest that intracytoplasmic CD95 mutations in ALPS impair apoptosis chiefly by disrupting death-domain interactions with the signaling protein FADD/MORT1.

An inherited disorder of lymphocyte apoptosis: the autoimmune lymphoproliferative syndrome

The autoimmune lymphoproliferative syndrome (ALPS) affords novel insights into the mechanisms that regulate lymphocyte homeostasis and underlie the development of autoimmunity. This syndrome arises early in childhood in persons who inherit mutations in genes that mediate apoptosis, or programmed cell death. The timely deletion of lymphocytes is a way to prevent their accumulation and the persistence of cells that can react against the body's own antigens. In ALPS, defective lymphocyte apoptosis permits chronic, nonmalignant adenopathy and splenomegaly; the survival of normally uncommon "double-negative" CD3+ CD4- CD8- T cells; and the development of autoimmune disease. Most cases of ALPS involve heterozygous mutations in the lymphocyte surface protein Fas that impair a major apoptotic pathway. Detailed immunologic investigations of the cellular and cytokine profiles in ALPS show a prominent skewing toward a T-helper 2 phenotype; this provides a rational explanation for the humoral autoimmunity typical of patients with ALPS. Prospective evaluations of 26 patients and their families show an ever-expanding spectrum of ALPS and its major complications: hypersplenism, autoimmune hemolytic anemia, thrombocytopenia, and neutropenia. Defective apoptosis may also contribute to a heightened risk for lymphoma.

Autoimmune lymphoproliferative syndrome with defective Fas: genotype influences penetrance

Autoimmune lymphoproliferative syndrome (ALPS) is a disorder of lymphocyte homeostasis and immunological tolerance. Most patients have a heterozygous mutation in the APT1 gene, which encodes Fas (CD95, APO-1), mediator of an apoptotic pathway crucial to lymphocyte homeostasis. Of 17 unique APT1 mutations in unrelated ALPS probands, 12 (71%) occurred in exons 7-9, which encode the intracellular portion of Fas. In vitro, activated lymphocytes from all 17 patients showed apoptotic defects when exposed to an anti-Fas agonist monoclonal antibody. Similar defects were found in a Fas-negative cell line transfected with cDNAs bearing each of the mutations. In cotransfection experiments, Fas constructs with either intra- or extracellular mutations caused dominant inhibition of apoptosis mediated by wild-type Fas. Two missense Fas variants, not restricted to patients with ALPS, were identified. Variant A(-1)T at the Fas signal-sequence cleavage site, which mediates apoptosis less well than wild-type Fas and is partially inhibitory, was present in 13% of African American alleles. Among the ALPS-associated Fas mutants, dominant inhibition of apoptosis was much more pronounced in mutants affecting the intracellular, versus extracellular, portion of the Fas receptor. Mutations causing disruption of the intracellular Fas death domain also showed a higher penetrance of ALPS phenotype features in mutation-bearing relatives. Significant ALPS-related morbidity occurred in 44% of relatives with intracellular mutations, versus 0% of relatives with extracellular mutations. Thus, the location of mutations within APT1 strongly influences the development and the severity of ALPS.

Determinant spreading associated with demyelination in a nonhuman primate model of multiple sclerosis

Definition of the immune process that causes demyelination in multiple sclerosis is essential to determine the feasibility of Ag-directed immunotherapy. Using the nonhuman primate, Callithrix jacchus jacchus (common marmoset), we show that immunization with myelin basic protein and proteolipid protein determinants results in clinical disease with significant demyelination. Demyelination was associated with spreading to myelin oligodendrocyte glycoprotein (MOG) determinants that generated anti-MOG serum Abs and Ig deposition in central nervous system white matter lesions. These data associate intermolecular "determinant spreading" with clinical autoimmune disease in primates and raise important issues for the pathogenesis and treatment of multiple sclerosis.

rlk/TXK encodes two forms of a novel cysteine string tyrosine kinase activated by Src family kinases

Rlk/Txk is a member of the BTK/Tec family of tyrosine kinases and is primarily expressed in T lymphocytes. Unlike other members of this kinase family, Rlk lacks a pleckstrin homology (PH) domain near the amino terminus and instead contains a distinctive cysteine string motif. We demonstrate here that Rlk protein consists of two isoforms that arise by alternative initiation of translation from the same cDNA. The shorter, internally initiated protein species lacks the cysteine string motif and is located in the nucleus when expressed in the absence of the larger form. In contrast, the larger form is cytoplasmic. We show that the larger form is palmitoylated and that mutation of its cysteine string motif both abolishes palmitoylation and allows the protein to migrate to the nucleus. The cysteine string, therefore, is a critical determinant of both fatty acid modification and protein localization for the larger isoform of Rlk, suggesting that Rlk regulation is distinct from the other Btk family kinases. We further show that Rlk is phosphorylated and changes localization in response to T-cell-receptor (TCR) activation and, like the other Btk family kinases, can be phosphorylated and activated by Src family kinases. However, unlike the other Btk family members, Rlk is activated independently of the activity of phosphatidylinositol 3-kinase, consistent with its lack of a PH domain. Thus, Rlk has two distinct isoforms, each of which may have unique properties in signaling downstream from the TCR.

The clinical spectrum in a large kindred with autoimmune lymphoproliferative syndrome caused by a Fas mutation that impairs lymphocyte apoptosis

Autoimmune lymphoproliferative syndrome (ALPS) is characterized by chronic, histologically benign splenomegaly and generalized lymphadenopathy, hypergammaglobulinemia, and autoantibody formation. ALPS has been attributed to defective programmed cell death of lymphocytes, most often arising as a result of mutations in the gene encoding the lymphocyte apoptosis receptor Fas/APO-l/CD95. We identified a novel mutation in the intracellular apoptosis signaling domain of Fas in 11 members of a family, individual members of which have been monitored for up to 25 years, with 1 or more features of ALPS. This study of a large number of family members carrying the same Fas defect demonstrates that ALPS is inherited in an autosomal dominant fashion but with a high degree of variability in clinical expression. Although 1 affected individual died of postsplenectomy sepsis and 1 has been treated for lymphoma, the Fas mutation in this family has been compatible with a healthy adulthood, as clinical features of ALPS have receded with increasing age.

Pathological findings in human autoimmune lymphoproliferative syndrome

The defects in lymphocyte apoptosis that underlie the autoimmune lymphoproliferative syndrome (ALPS) are usually attributable to inherited mutations of the CD95 (Fas) gene. In this report, we present the histopathological and immunophenotypic features seen in the lymph nodes (n = 16), peripheral blood (n = 10), bone marrow (n = 2), spleen (n = 3), and liver (n = 2) from 10 patients with ALPS. Lymph nodes showed marked paracortical hyperplasia. Interfollicular areas were expanded and populated by T cell receptor-alphabeta CD3+ CD4-CD8- (double-negative, DN) T cells that were negative for CD45RO. CD45RA+ T cells were increased in all cases studied. The paracortical infiltrate was a result of both reduced apoptosis and increased proliferation, as measured by in situ detection of DNA fragmentation and staining with MIB-1, respectively. The paracortical proliferation may be extensive enough to suggest a diagnosis of malignant lymphoma. Many of the paracortical lymphocytes expressed markers associated with cytotoxicity, such as perforin, TIA-1, and CD57. CD25 was negative. In addition, most lymph nodes exhibited florid follicular hyperplasia, often with focal progressive transformation of germinal centers; in some cases, follicular involution was seen. A polyclonal plasmacytosis also was present. The spleens were markedly enlarged, more than 10 times normal size. There was expansion of both white pulp and red pulp, with increased DN T cells. DN T cells also were observed in liver biopsies exhibiting portal triaditis. In the peripheral blood, the T cells showed increased expression of HLA-DR and CD57 but not CD25. CD45RA+ T cells were increased in the four cases studied. Polyclonal B cell lymphocytosis with expansion of CD5+ B cells was a characteristic finding. Taken together, the histopathological and immunophenotypic findings, particularly in lymph nodes and peripheral blood, are sufficiently distinctive to suggest a diagnosis of ALPS. Of note, two affected family members of one proband developed lymphoma (T-cell-rich B-cell lymphoma and nodular lymphocyte predominance Hodgkin's disease, respectively).

Molecular genetic studies in lymphocyte apoptosis and human autoimmunity

Using a genetic approach, we have studied the molecular basis of human autoimmunity with special emphasis on a disease that is due to defective lymphocyte apoptosis. Recently, we and our collaborators have found that the autoimmune/lymphoproliferative syndrome (ALPS), an inherited disease of children comprising marked lymphoid hyperplasia and autoimmune manifestations, is due to abnormalities in the CD95 gene that cause defective lymphocyte apoptosis. Our recent investigations have shown that the mutations in most families with ALPS cause either global or local changes in the structure of a cytoplasmic portion of the molecule called the 'death domain'. These death domain alterations impair binding of the adapter protein FADD/MORT1 and result in a failure to activate apoptotic caspases after CD95 (Fas/APO-1) cross-linking. Mutations in apoptotic caspases may also contribute to the pathogenesis of ALPS in individuals that have no CD95 gene mutations.

Differential TCR signaling regulates apoptosis and immunopathology during antigen responses in vivo

Clonal selection theories postulate that lymphocyte fate is regulated by antigen receptor specificity. However, lymphocyte apoptosis is induced through nonantigen-specific receptors such as Fas (CD95/APO-1) or TNFR. We define a selective TCR that controls apoptosis by Fas or TNFR stimulation. Variant ligands can deliver this "competence to die" signal without the full TCR signals necessary for cytokine synthesis. These partial agonists regulate T cell deletion in vivo even when Fas or TNF is provided by T cells of unrelated specificity, but they do not cause the liver necrosis that is associated with T cell elimination by the full agonist. Thus, selective signaling ligands regulate T cell deletion and immune damage in vivo and may be important for peripheral T cell tolerance.

Death-effector filaments: novel cytoplasmic structures that recruit caspases and trigger apoptosis

The death-effector domain (DED) is a critical protein interaction domain that recruits caspases into complexes with members of the TNF-receptor superfamily. Apoptosis can also be induced by expressing certain DED-containing proteins without surface receptor cross-linking. Using Green Fluorescent Protein to examine DED-containing proteins in living cells, we show that these proteins cause apoptosis by forming novel cytoplasmic filaments that recruit and activate pro-caspase zymogens. Formation of these filaments, which we term death-effector filaments, was blocked by coexpression of viral antiapoptotic DED-containing proteins, but not by bcl-2 family proteins. Thus, formation of death-effector filaments allows a regulated intracellular assembly of apoptosis-signaling complexes that can initiate or amplify apoptotic stimuli independently of receptors at the plasma membrane.

NMR structure and mutagenesis of the FADD (Mort1) death-effector domain

When activated, membrane-bound receptors for Fas and tumour-necrosis factor initiate programmed cell death by recruiting the death domain of the adaptor protein FADD to the membrane. FADD then activates caspase 8 (also known as FLICE or MACH) through an interaction between the death-effector domains of FADD and caspase 8. This ultimately leads to the apoptotic response. Death-effector domains and homologous protein modules known as caspase-recruitment domains have been found in several proteins and are important regulators of caspase (FLICE) activity and of apoptosis. Here we describe the solution structure of a soluble, biologically active mutant of the FADD death-effector domain. The structure consists of six antiparallel, amphipathic alpha-helices and resembles the overall fold of the death domains of Fas and p75. Despite this structural similarity, mutations that inhibit protein-protein interactions involving the Fas death domain have no effect when introduced into the FADD death-effector domain. Instead, a hydrophobic region of the FADD death-effector domain that is not present in the death domains is vital for binding to FLICE and for apoptotic activity.

HIV-1 directly kills CD4+ T cells by a Fas-independent mechanism

The mechanism by which HIV-1 induces CD4(+) T cell death is not known. A fundamental issue is whether HIV-1 primarily induces direct killing of infected cells or indirectly causes death of uninfected bystander cells. This question was studied using a reporter virus system in which infected cells are marked with the cell surface protein placental alkaline phosphatase (PLAP). Infection by HIV-PLAP of peripheral blood mononuclear cells (PBMCs) and T cell lines leads to rapid depletion of CD4(+) T cells and induction of apoptosis. The great majority of HIV-induced T cell death in vitro involves direct loss of infected cells rather than indirect effects on uninfected bystander cells. Because of its proposed role in HIV-induced cell death, we also examined the Fas (CD95/Apo1) pathway in killing of T cells by HIV-1. Infected PBMCs or CEM cells display no increase in surface Fas relative to uninfected cells. In addition, HIV-1 kills CEM and Jurkat T cells in the presence of a caspase inhibitor that completely blocks Fas-mediated apoptosis. HIV-1 also depletes CD4+ T cells in PBMCs from patients who have a genetically defective Fas pathway. These results suggest that HIV-1 induces direct apoptosis of infected cells and kills T cells by a Fas-independent mechanism.

Cell death attenuation by 'Usurpin', a mammalian DED-caspase homologue that precludes caspase-8 recruitment and activation by the CD-95 (Fas, APO-1) receptor complex

Apoptotic cell suicide initiated by ligation of CD95 (Fas/APO-1) occurs through recruitment, oligomerization and autocatalytic activation of the cysteine protease, caspase-8 (MACH, FLICE, Mch5). An endogenous mammalian regulator of this process, named Usurpin, has been identified (aliases for Usurpin include CASH, Casper, CLARP, FLAME-1, FLIP, I-FLICE and MRIT). This protein is ubiquitously expressed and exists as at least three isoforms arising by alternative mRNA splicing. The Usurpin gene is comprised of 13 exons and is clustered within approximately 200 Kb with the caspase-8 and -10 genes on human chromosome 2q33-34. The Usurpin polypeptide has features in common with pro-caspase-8 and -10, including tandem 'death effector domains' on the N-terminus of a large subunit/small subunit caspase-like domain, but it lacks key residues that are necessary for caspase proteolytic activity, including the His and Cys which form the catalytic substrates diad, and residues that stabilize the P1 aspartic acid in substrates. Retro-mutation of these residues to functional caspase counterparts failed to restore proteolytic activity, indicating that other determinants also ensure the absence of catalytic potential. Usurpin heterodimerized with pro-caspase-8 in vitro and precluded pro-caspase-8 recruitment by the FADD/MORT1 adapter protein. Cell death induced by CD95 (Fas/APO-1) ligation was attenuated in cells transfected with Usurpin. In vivo, a Usurpin deficit was found in cardiac infarcts where TUNEL-positive myocytes and active caspase-3 expression were prominent following ischemia/reperfusion injury. In contrast, abundant Usurpin expression (and a caspase-3 deficit) occurred in surrounding unaffected cardiac tissue, suggesting reciprocal regulation of these pro- and anti-apoptotic molecules in vivo. Usurpin thus appears to be an endogenous modulator of apoptosis sensitivity in mammalian cells, including the susceptibility of cardiac myocytes to apoptotic death following ischemia/ reperfusion injury.

Membrane oligomerization and cleavage activates the caspase-8 (FLICE/MACHalpha1) death signal

Many forms of apoptosis, including that caused by the death receptor CD95/Fas/APO-1, depend on the activation of caspases, which are proteases that cleave specific intracellular proteins to cause orderly cellular disintegration. The requirements for activating these crucial enzymatic mediators of death are not well understood. Using molecular chimeras with either CD8 or Tac, we find that oligomerization at the cell membrane powerfully induces caspase-8 autoactivation and apoptosis. Death induction was abrogated by the z-VAD-fmk, z-IETD-fmk, or p35 enzyme inhibitors or by a mutation in the active site cysteine but was surprisingly unaffected by death inhibitor Bcl-2. Amino acid substitutions that prevent the proteolytic separation of the caspase from its membrane-associated domain completely blocked apoptosis. Thus, oligomerization at the membrane is sufficient for caspase-8 autoactivation, but apoptosis could involve a death signal conveyed by the proteolytic release of the enzyme into the cytoplasm.

Selective induction of apoptosis in mature T lymphocytes by variant T cell receptor ligands

Activation, anergy, and apoptosis are all possible outcomes of T cell receptor (TCR) engagement. The first leads to proliferation and effector function, whereas the others can lead to partial or complete immunological tolerance. Structural variants of immunizing peptide-major histocompatibility complex molecule ligands that induce selective lymphokine secretion or anergy in mature T cells in association with altered intracellular signaling events have been described. Here we describe altered ligands for mature mouse CD4(+) T helper 1 cells that lead to T cell apoptosis by the selective expression of Fas ligand (FasL) and tumor necrosis factor (TNF) without concomitant IL-2, IL-3, or interferon gamma production. All ligands that stimulated cell death were found to induce FasL and TNF mRNA expression and TCR aggregation ("capping") at the cell surface, but did not elicit a common pattern of tyrosine phosphorylation of the TCR-associated signal transduction chains. Thus, TCR ligands that uniquely trigger T cell apoptosis without inducing cytokines that are normally associated with activation can be identified.

T cell growth cytokines cause the superinduction of molecules mediating antigen-induced T lymphocyte death

TCR stimulation of T lymphocytes that are activated and cycling in the presence of IL-2 leads to programmed cell death. We now show that this effect is at least partly attributable to the ability of IL-2 to dramatically increase the expression of mRNAs encoding ligands and receptors that mediate apoptosis. We also found that cyclosporin was not able to fully inhibit the TCR induction of death molecule mRNAs or TCR-induced apoptosis, although it could completely turn off IL-2 expression. The effect growth cytokines was further explored in T cells derived from mice bearing a homozygous deficiency of the IL-2R alpha-chain. We found that IL-2Ralpha-/- cells were resistant to death if IL-2 was used to induce apoptosis susceptibility, but that large amounts of other T cell growth cytokines, such as IL-4 and IL-7, could induce cell cycle progression and promote TCR-induced apoptosis. However, our findings suggest that autoimmunity and lymphoproliferation in IL-2Ralpha-/- mice can result from the loss of IL-2 stimulated feedback apoptosis and that other growth cytokines are not produced at levels sufficient to compensate for this deficit.

Molecules involved in cell death and peripheral tolerance

Apoptosis is important for maintaining peripheral lymphocyte homeostasis and for minimizing the accumulation of autoreactive lymphocytes. Disruption of apoptotic pathways has been linked to lymphadenopathy, breakdown of peripheral tolerance and the development of autoimmune diseases. Major progress has been made during the past year in understanding the critical roles of a variety of signaling molecules, especially a group of cysteine proteases, for the execution of apoptosis. These proteases appear to be the primary effector molecules responsible for carrying out lymphocyte apoptosis and may be critical for peripheral immunological tolerance.

Maintenance of clonotype specificity in CD95/Apo-1/Fas-mediated apoptosis of mature T lymphocytes

Ag-induced mature T cell apoptosis is the result of death-inducing cytokines, including the ligand for CD95 (Apo-1/Fas). This raises the possibility that expression of this death molecule could affect bystander T cells that were not directly antigenically stimulated but that express the CD95 receptor. We show here that bystander T cells, even if they express the CD95 receptor, are not killed when exposed to T cells undergoing Ag-induced apoptosis. Rather, cell death is restricted to T cells that bear the receptor clonotype that is specifically engaged by TCR ligands. At least one mechanism of clonotype restriction is a significant enhancement of CD95-induced apoptosis by TCR ligation. Our results demonstrate that, in addition to the well-known ability of TCR to stimulate apoptosis by inducing CD95 ligand expression, TCR signals at the time of CD95 engagement can effectively increase apoptosis. Therefore, we put forward the hypothesis that strict clonotype specificity is preserved when death cytokines such as CD95 ligand induce autoregulatory mature T cell apoptosis, at least in part through a sensitization signal provided by the TCR stimulation.

Maintenance of clonotype specificity in CD95/Apo-1/Fas-mediated apoptosis of mature T lymphocytes

Ag-induced mature T cell apoptosis is the result of death-inducing cytokines, including the ligand for CD95 (Apo-1/Fas). This raises the possibility that expression of this death molecule could affect bystander T cells that were not directly antigenically stimulated but that express the CD95 receptor. We show here that bystander T cells, even if they express the CD95 receptor, are not killed when exposed to T cells undergoing Ag-induced apoptosis. Rather, cell death is restricted to T cells that bear the receptor clonotype that is specifically engaged by TCR ligands. At least one mechanism of clonotype restriction is a significant enhancement of CD95-induced apoptosis by TCR ligation. Our results demonstrate that, in addition to the well-known ability of TCR to stimulate apoptosis by inducing CD95 ligand expression, TCR signals at the time of CD95 engagement can effectively increase apoptosis. Therefore, we put forward the hypothesis that strict clonotype specificity is preserved when death cytokines such as CD95 ligand induce autoregulatory mature T cell apoptosis, at least in part through a sensitization signal provided by the TCR stimulation.

Essential lymphocyte function associated 1 (LFA-1): intercellular adhesion molecule interactions for T cell-mediated B cell apoptosis by Fas/APO-1/CD95

B cells are susceptible to Fas ligand (FasL)+ CD4(+) Th1 cell-mediated apoptosis. We demonstrate that blocking the interactions between lymphocyte function associated (LFA)-1 and intercellular adhesion molecule(ICAM)-1 and ICAM-2 completely suppresses Fas-dependent B cell lysis. Antibodies to CD2 and CD48 partially suppress B cell apoptosis, whereas anti-B7.1 and anti-B7.2 antibodies have no effect. Also, B cells from ICAM-1-deficient mice are resistant to FasL+ T cell-mediated death. Our results suggest that LFA-1/ICAM interactions are crucial for Th1 cell-mediated B cell apoptosis and may contribute to the maintenance of B cell homeostasis in vivo.

Identification of a novel developmental stage marking lineage commitment of progenitor thymocytes

Bipotent progenitors for T and natural killer (NK) lymphocytes are thought to exist among early precursor thymocytes. The identification and functional properties of such a progenitor population remain undefined. We report the identification of a novel developmental stage during fetal thymic ontogeny that delineates a population of T/NK-committed progenitors (NK1. 1(+)/CD117(+)/CD44(+)/CD25(-)). Thymocytes at this stage in development are phenotypically and functionally distinguishable from the pool of multipotent lymphoid-restricted (B, T, and NK) precursor thymocytes. Exposure of multipotent precursor thymocytes or fetal liver- derived hematopoietic progenitors to thymic stroma induces differentiation to the bipotent developmental stage. Continued exposure to a thymic microenvironment results in predominant commitment to the T cell lineage, whereas coculture with a bone marrow-derived stromal cell line results in the generation of mature NK cells. Thus, the restriction point to T and NK lymphocyte destinies from a multipotent progenitor stage is marked by a thymus-induced differentiation step.

Death effector domain-containing herpesvirus and poxvirus proteins inhibit both Fas- and TNFR1-induced apoptosis

To identify novel antiapoptotic proteins encoded by DNA viruses, we searched viral genomes for proteins that might interfere with Fas and TNFR1 apoptotic signaling pathways. We report here that equine herpesvirus type 2 E8 protein and molluscum contagiosum virus MC159 protein both show sequence similarity to the death effector domains (DEDs) of the Fas/TNFR1 signaling components FADD and caspase-8. Yeast two-hybrid analysis revealed that E8 protein interacted with the caspase-8 prodomain whereas MC159 protein interacted with FADD. Furthermore, expression of either E8 protein or MC159 protein protected cells from Fas- and TNFR1-induced apoptosis indicating that certain herpesviruses and poxviruses use DED-mediated interactions to interfere with apoptotic signaling pathways. These findings identify a novel control point exploited by viruses to regulate Fas- and TNFR1-mediated apoptosis.

Characteristic T helper 2 T cell cytokine abnormalities in autoimmune lymphoproliferative syndrome, a syndrome marked by defective apoptosis and humoral autoimmunity

Autoimmune lymphoproliferative syndrome (ALPS) is marked by massive lymphadenopathy, hepatosplenomegaly, autoimmunity and the presence of increased numbers of circulating and tissue TCR-alpha beta, CD4- CD8- T cells. The underlying defect is that of decreased T cell and B cell apoptosis, due in most, but not all, cases to heterozygous mutations of the Fas gene and corresponding defective Fas signaling function. Here we measure in vivo and in vitro cytokine secretion in ALPS to shed light on the relation of apoptosis defects to the development of autoimmunity. In in vivo studies, ALPS patients manifested greatly increased circulating levels of IL-10 (> 100-fold), compared with both healthy individuals and various disease controls; in contrast, their levels of IL-1 beta, IL-4, and IFN-gamma were normal and their levels of IL-2 and TNF-alpha were marginally increased. In parallel in vitro studies, ALPS patients CD4+ DR+ T cells stimulated either with anti-CD3/CD28 or anti-CD2/CD28 produced increased amounts of IL-4 and IL-5 (10 to 20-fold) and decreased amounts of IFN-gamma (4-fold) as compared with those of control CD4+ DR+ T cells. In contrast, ALPS patients' CD4-/CD8- T cells produced very low amounts of cytokines. Finally, ALPS patients' peripheral monocytes/macrophages produced decreased amounts of IL-12 (30-fold) and increased amounts of IL-10 (5-fold). In conclusion, ALPS is marked by the presence of DR+ T cells that exhibit a skewed Th2 cytokine response upon various forms of stimulation. This cytokine response, in the presence of increased circulating IL-10 levels, is likely to define the cytokine milieu that accounts for the humoral autoimmune features of ALPS and, perhaps, of other humoral autoimmune states.

Clincal, immunologic, and genetic features of an autoimmune lymphoproliferative syndrome associated with abnormal lymphocyte apoptosis

Programmed cell death (apoptosis) of activated lymphocytes is critical to immune homeostasis. The cell surface protein Fas (CD95) and its ligand play a pivotal role in regulating lymphocyte apoptosis, and defective expression of either Fas or Fas ligand results in marked over accumulation of mature lymphocytes and autoimmune disease in mice. The results of recent studies suggest that defective lymphocyte apoptosis caused by mutations of the Fas gene can result in a severe autoimmune lymphoproliferative syndrome (ALPS) in humans. To define the clinical, genetic, and immunologic spectrum of ALPS, 9 patients and their families were extensively evaluated with routine clinical studies, lymphocyte phenotyping, genotyping, and in vitro assays for lymphocyte apoptosis. Individual patients were followed up for 3 months to 6 years. ALPS was identified in 9 unrelated children as manifested by moderate to massive splenomegaly and lymphadenopathy, hypergammaglobulinemia, autoimmunity, B-cell lymphocytosis, and the expansion of an unusual population of CD4- CD8- T cells that express the alpha/beta T-cell receptor (TCR). All patients showed defective lymphocyte apoptosis in vitro. Heterozygous mutations of the Fas gene were detected in 8 patients. One ALPS patient lacked a Fas gene mutation. Healthy relatives with Fas mutations were identified in 7 of 8 ALPS kindreds. These relatives also showed in vitro abnormalities of Fas-mediated lymphocyte apoptosis, but clinical features of ALPS were not present in the vast majority of these individuals. ALPS is a unique clinical syndrome in which in vitro abnormalities of lymphocyte apoptosis are associated with abnormal lymphoproliferation and autoimmunity. These findings provide evidence that apoptosis of activated lymphocytes is an important mechanism for maintaining immunologic homeostasis and self-tolerance in humans. Fas gene mutations account for impaired lymphocyte apoptosis in only a subset of patients with ALPS.

Characteristic T helper 2 T cell cytokine abnormalities in autoimmune lymphoproliferative syndrome, a syndrome marked by defective apoptosis and humoral autoimmunity

Autoimmune lymphoproliferative syndrome (ALPS) is marked by massive lymphadenopathy, hepatosplenomegaly, autoimmunity and the presence of increased numbers of circulating and tissue TCR-alpha beta, CD4- CD8- T cells. The underlying defect is that of decreased T cell and B cell apoptosis, due in most, but not all, cases to heterozygous mutations of the Fas gene and corresponding defective Fas signaling function. Here we measure in vivo and in vitro cytokine secretion in ALPS to shed light on the relation of apoptosis defects to the development of autoimmunity. In in vivo studies, ALPS patients manifested greatly increased circulating levels of IL-10 (&gt; 100-fold), compared with both healthy individuals and various disease controls; in contrast, their levels of IL-1 beta, IL-4, and IFN-gamma were normal and their levels of IL-2 and TNF-alpha were marginally increased. In parallel in vitro studies, ALPS patients CD4+ DR+ T cells stimulated either with anti-CD3/CD28 or anti-CD2/CD28 produced increased amounts of IL-4 and IL-5 (10 to 20-fold) and decreased amounts of IFN-gamma (4-fold) as compared with those of control CD4+ DR+ T cells. In contrast, ALPS patients' CD4-/CD8- T cells produced very low amounts of cytokines. Finally, ALPS patients' peripheral monocytes/macrophages produced decreased amounts of IL-12 (30-fold) and increased amounts of IL-10 (5-fold). In conclusion, ALPS is marked by the presence of DR+ T cells that exhibit a skewed Th2 cytokine response upon various forms of stimulation. This cytokine response, in the presence of increased circulating IL-10 levels, is likely to define the cytokine milieu that accounts for the humoral autoimmune features of ALPS and, perhaps, of other humoral autoimmune states.

TCR-mediated death of mature T lymphocytes occurs in the absence of p53

The p53 protein plays an important role in various forms of thymocyte apoptosis, however its role in mature T lymphocyte death caused by TCR stimulation has not been examined. We demonstrate here that T cell blasts derived from mice containing a germ-line deficiency of the p53 tumor suppressor gene are susceptible to TCR-induced apoptosis to the same degree as wild-type T cells. TCR stimulation of both resting and proliferatingT cells results in up-regulated expression of the p53-induced genes Bax and p21, and the induction of these genes appears reduced in T cells that are deficient in p53. Thus, while activation of p53-dependent genes following TCR stimulation is defective in p53-/- T cells, there is no impairment in their ability to undergo apoptosis. These results suggest that TCR-mediated apoptosis of mature T cells takes place via a pathway that is independent of p53.