Telomeres are specialized structures providing chromosome integrity during cellular division along with protection against premature senescence and apoptosis. Accelerated telomere attrition in patients with myelodysplastic syndrome (MDS) occurs by an undefined mechanism. Although the MDS clone originates within the myeloid compartment, T-lymphocytes display repertoire contraction and loss of naive T-cells. The replicative lifespan of T-cells is stringently regulated by telomerase activity. In MDS cases, we show that purified CD3+ T-cells have significantly shorter telomere length and reduced proliferative capacity upon stimulation compared with controls. To understand the mechanism, telomerase enzymatic activity and telomerase reverse transcriptase (hTERT), gene expression were compared in MDS cases (n=35) and healthy controls (n=42) within different T-cell compartments. Telomerase activity is greatest in naive T-cells illustrating the importance of telomere repair in homeostatic repertoire regulation. Compared with healthy controls, MDS cases had lower telomerase induction (P<0.0001) that correlated with significantly lower hTERT mRNA (P<0.0001), independent of age and disease stratification. hTERT mRNA deficiency affected naive but not memory T-cells, and telomere erosion in MDS occurred without evidence of an hTERT-promoter mutation, copy number variation or deletion. Telomerase insufficiency may undermine homeostatic control within the hematopoietic compartment and promote a change in the T-cell repertoire in MDS.

A hierarchical hematopoietic development with myeloid versus lymphoid bifurcation has been proposed downstream of the multipotent progenitor (MPP) stage, based on prospective isolation of progenitors capable of generating only myeloerythroid cells (common myeloid progenitor, CMP) or only lymphocytes (common lymphoid progenitor, CLP). By utilizing GATA-1 and PU.1 transcription factor reporters, here we identified progenitor populations that are precursors for either CMPs or CLPs. Two independent populations expressing either GATA-1 or PU.1 resided within the CD34+Sca-1+c-Kit+ MPP fraction. The GATA-1+ MPP displayed potent myeloerythroid potential without giving rise to lymphocytes, whereas the PU.1+ MPP showed granulocyte/monocyte/lymphoid-restricted progenitor activity without megakaryocyte/erythroid differentiation. Furthermore, GATA-1+ and PU.1+ MPPs possessed huge expansion potential and differentiated into the original CMPs and CLPs, respectively. Thus, the reciprocal activation of GATA-1 and PU.1 primarily organizes the hematopoietic lineage fate decision to form the earliest hematopoietic branchpoint that comprises isolatable myeloerythroid and myelolymphoid progenitor populations.

Response to immunosuppressive therapy (IST) in younger patients with myelodysplastic syndrome (MDS) has been linked to a T-cell-dominant autoimmune process that impairs hematopoiesis. Analysis of the age-adjusted CD4:CD8 ratio in 76 MDS patients compared with 54 healthy controls showed that inadequate CD4+, rather than expansion of CD8+ T cells, was associated with a lower ratio in a group that included both lower and higher risk MDS patients defined by the International Prognostic Scoring System. In younger MDS patients, naive and memory phenotypes defined by CD45RA and CD62L display showed depletion of naive CD4+ and CD8+ T cells, suggesting a possible relationship to IST responsiveness. To determine the correlation between T-cell subset distribution, T-cell turnover and autoimmunity, a cohort of 20 patients were studied before and after IST. The CD4:CD8 ratio correlated inversely with the proliferative T-cell index before treatment in IST-responsive patients, suggesting that proliferation may be linked to accelerated CD4+ T-cell turnover and hematopoietic failure. Our data show seminal findings that both CD4+ and CD8+ T-cell subsets are dysregulated in MDS. Association between these T-cell defects and response to IST suggests that aberrant T-cell homeostasis and chronic activation are critical determinants influencing autoimmune hematopoietic suppression in younger patients.

Selected patients with Myelodysplastic Syndromes (MDS) are responsive to immunosuppressive therapy, suggesting that hematopoietic suppressive T cells have a pathogenic role in ineffective hematopoiesis. We assessed T-cell receptor (TCR) clonality through combined flow cytometry and molecular analysis of the complementarity determining region (CDR)-3 of the T-cell receptor-V β gene. We identified clonal T cells in 50% of MDS patients ( n =52) compared to 5% of age-matched normal controls ( n =20). The presence of T-cell clones was not associated with features linked previously to immunosuppression response, including WHO diagnostic category, karyotype, marrow cellularity, IPSS category, sex or age ⩽60. Using flow cytometry to identify expanded V β -families, we found that T cells showed greater expansion in the bone marrow compared with peripheral blood, and were characterized as CD8 + /CD57 + /CD28 − effector T cells. Expanded effector T cell were CD62L negative and expressed the natural killer C-lectin-family receptor NKG2D and CD244 (2B4). We conclude that clonal T-cell expansion is common among all MDS prognostic subgroups.

Natural Killer (NK) function in patients with MDS as measured by non-MHC-restricted cytotoxicity and activation-dependent cell cytotoxicity (ADCC) are reduced in patients with MDS, however, the mechanisms of the functional impairment are not known. Tumor cytolysis occurs through orchestrated control by inhibitory NK receptors (NKRs) and activating NKRs, which control signaling events that lead to polarized movement of perforin-containing granules toward the NK-tumor contact area. We found that NK cells from 23 out of 35 patients with MDS (66%) displayed reduced lysis of K562 tumor cells compared to age-matched normal controls (p<0.01). To better characterize this defect, we evaluated patient NK function against differential tumor targets including the MDS1 cell line established from an MDS patients. We found that MDS1 incited non-MHC-restricted lysis. Unactivated PBMCs, unactivated NK cells, NK cell lines (NK92 and NKL) but not purified unactivated T cells from normal donors killed MDS1 in 4-hr 51Cr-release assays. Normal NK cells and NK cell lines were also found to rapidly redistrubute perforin granules after exposure to MDS1suggesting that a perforin-dependent lytic pathway was activated. We then performed simultaneous cytolytic assays with K562, MDS1, and the 721.221 B cell lymphoma cell line as target cells. We found that NK cells from MDS patients had greater lytic activity against MDS1 (average 24% vs. average 8% at 50:1 Effector:Target ratio, respectively, p<0.01) Antibody-blocking experiments demonstrated that the NKL cell line and PBMCs from 8 out of 10 MDS patients predominantly used the NKG2D activating receptor to kill MDS1. Consistent with this finding, we showed that MDS1 cells express the major human stress-inducible endogenous proteins MICA and MICB, which are NKG2D ligands. In contast, lysis by NK92 cells and normal PBMCs was not appreciably reduced by NKG2D blocking antibodies suggesting that other unidentified NKR(s) also mediate lysis. To identify the NKRs expressed in MDS patients, we performed immunophenotyping for both the activating NKRs and inhibitory NKRs compared to age-matched normal controls. We found that two activating receptors, NKp30 and CD244 (2B4), were significantly reduced on NK cells from all MDS patients regardless of their ability to lyse NK targets. Inhibitory NKR expression and function were normal. Interestingly, NKG2D expression correlated with reduced cytolytic function. Similar to studies on normal NK cells with low NKp30 and NKp46 (NCRdull) phenotypes, these results suggest that low NKp30 expression leads to predominant NKG2D utilization for tumor cell lysis, which is reduced in MDS patients with defective NK function. Our findings provide critical information about potential importance for immunosurviellance through NKG2D-NKG2D ligands.

Myelodysplastic Syndrome (MDS), a hematologic malignancy, is associated with cytogenetic and molecular abnormalities in maturing hematopoietic cells that occurs in 40–70% of cases. These abnormalities directly contribute to increased apoptosis and ineffective hematopoiesis of erythroid and myeloid progenitors while frequently sparing lymphoid subsets. In addition to these direct mechanisms of hematopoietic failure, failed hematopoiesis mediated indirectly by an autoimmune mechanism has been suggested to have importance in a subset of patients with the disease. Autoimmune-mediated bone marrow suppression is suggested by the positive outcome of several clinical trials using immunosuppressants. Depletion of autoreactive T cells with deleterious effects on bone marrow formation is considered the immunologic foundation for these therapeutic responses and bone marrow hypocellularity has been indicated as the best predictor of response. Antigen-driven expansion of immunodominant T cell clones can lead to overrepresentation of cells expressing individual T Cell Receptors (TCRs), which is known as TCR skewing. Treatment of hypocellular MDS patients with immunosuppressive therapies is associated with normalization of a skewed TCR- phenotype. The overall incidence of immunodominant T cell expansions has not been determined. The goal of our study was to assess the frequency of clonal T cell expansion in peripheral blood of MDS patients. Peripheral blood was analyzed from 52 patients for T cell CDR3-length skewing by genomic multi-plex PCR. All patients met the clinical criteria of MDS as defined by the WHO classification scheme. Patients with Refractory Anemia (RA) with and without Ringed Sideroblasts (RARS) represented 13% (n=7), Refractory Cytopenia with Multilineage Dysplasia (RCMD and RCMD-RS) represented 48% (n=25), and Refractory Anemia with Excess Blasts (RAEB1, RAEB2, and MDS that had progressed to AML) represented 38% (n=20). TCR-skewing occurred in 29 out of 52 patients with MDS (55%) compared to one out of 20 (5%) in age-matched normal controls. There was no difference in the frequency of clonal expansions based on the WHO classification. Based on the International Prognostic Scoring System (IPSS), we found that high risk patients (Int-2 and high) had a significantly higher incidence of clonal expansions than did patients with low risk disease (low and Int-1) (47% vs. 20%, respectively, p<0.05). Indeed, patients with the highest incidence of having clonal T cell expansion were in the Int-2 risk category (86% by TCR-Vβ analysis, n=7). Of 47 patients with known bone marrow cellularity classification, we enrolled only eight (17%) that were classified as hypocellular, ten (21%) normal cellular, and 29 (62%) hypercellular. No patient with a hypocellular bone marrow in our analysis showed evidence of clonal T cell expansion (p<0.05). These results suggest that clonal T cell expansion occurs prominately in high risk patients. We believe that cellular immunity in MDS could have both beneficial and deleterious effects. Antigen-specific cellular immune responses against pre-leukemic cells would be advantageous, while autoimmune destruction of normal bone marrow cells in the environment of an aggressive immune response would be deleterious. More information is needed about the role that clonal T cell expansion plays in high risk MDS.

Lenalidomide, which is a 4-amino-glutarimide analogue of thalidomide, has significant erythropoietic activity in patients with lower-risk MDS (List et al, NEJM , 351:26, 2004). Although its precise target of action in MDS is not known, lenalidomide modulates cellular response to varied stimuli including inhibition of angiogenic response and endotoxin induction of inflammatory cytokines and enhancement of antigen-induced immunologic response and erythropoietin receptor signaling. Clinical investigations in multiple myeloma indicate that the immunomodulatory effects of thalidomide and lenalidomide extends to the expansion of natural killer (NK) cells. We recently found that MDS patients have defective NK function, araising in part to reduced expression of activating NK receptors (NKRs) NKp30, CD244 (2B4), and NKG2D. To determine if lenalidomide may restore NK function in MDS, we investigated the effects of in vitro treatment with lenalidomide on NK function and phenotype. Lytic function was studied using peripheral blood mononuclear cells (PBMCs) as effector cells and the leukemia cell line, K562, as a target in standard 4-hr 51Cr-release assays at 12:1 and 25:1 effector:target (E:T) ratios. Among eight MDS patient's specimens evaluated, five patients had significant increase in tumor lysis after treatment with 1 μM lenalidomide for 72 hours (p ≤ 0.01, T-test). In similar experiments using PBMCs from normal donors, we found that NK lysis of K562 was 42% ± 15 (25:1 E:T ratio) pre-treatment which increased to 71% ± 17 (25:1 E:T ratio) after treatment, which was statistically significant (p ≤ 0.01, T-test). We also examined the in vitro effects of lenalidomide on lytic activity by the NK cell lines, NK92 and NKL, which was significantly increased after drug treatment. To discern the mechanisms of lenalidomide action in NK cell lines and normal NK cells, we evaluated NKR display by flow cytometry, and NKR function by antibody redirected cytotoxicity using the FcγR+ murine mastocytoma (P815) target cell line. Using NK92 and NKL cells, treatment with lenalidomide 1 μM for 72 hrs increased lysis by anti-NKG2D and anti-CD244 activating antibodies. We also found that NKG2D surface expression was increased on normal NK cells after lenalidomide treatment in vitro. These results suggest that some MDS patients may have improved NK function through the immunomodulatory effects of lenalidomide. The relationship between in vitro NK responsiveness to lenalidomide and in vivo hematological response warrants investigation in patients with MDS.

Chronic NK-LDGL is characterized by the expansion of CD3−, CD16+ and/or CD56+ mature NK cells associated with anemia and/or neutropenia. There are many reports of concurrent LDGL expansions in patients with other types of bone marrow failure syndromes such as myelodysplastic syndrome (MDS) and aplastic anemia (AA). Bone marrow suppression through autoimmune mechanisms has been suggested to have importance in these diseases along with failed hemaptopoiesis through ineffective stem cell differentiation. The critical mechanism for failed hematopoiesis in patients with NK-LDGL is thought to be through a mechanism of antigen-driven expansion of autoreactive NK cells that delete mature myeloid cells in the peripheral compartments. We found that patients with NK-LDGL express a skewed repertoire of NK receptors. (Epling-Burnette et al, Blood 103:3431, 2004) NK cells from these patients expressed a large number of activating NK receptors (NKRs) by genotype analysis and they also had potent cytolytic function in both direct and redirected cytotoxicity assays. Overexpression of activating receptors in the absence of the appropriate inhibitory receptors may cause these cells to recognize and lyse or produce inflammatory cytokines in response to autologous tissues. Therefore, our goal was to examine the colony forming capacity of CD34+- stem cells from patients with NK-LDGL and to determine whether the presence of autologous NK cells influence this maturation process. We isolated peripheral CD34+-stem cells from patients with NK-LDGL and normal controls by fluorescence-activated cell (FACS)-sorting using anti-CD34-PE-conjugated antibodies. Erythroid (BFU-E) and myeloid (CFU-GM) colony formation assays were performed by plating 500 CD34+ cells in methylcellulose in the absence and presence of CD3−/CD56+ NK cells that were simultaneously collected by FACS-sorting. On average, 274 ± 276 erythroid colonies and 253 ± 167 myeloid colonies were obtained from CD34+ cells from normal donors. Compared to each normal donor, CD34+cells from five out of seven NK-LDGL patients showed significantly reduced erythroid colony formation. Five out of six of these NK-LDGL patients also had reduced myeloid colony formation (p≤0.05). Myeloid colony formation was not determined in one patient. In addition to reduced colony formation, two patients demonstrated even further reduction in colony formation, erythroid or myeloid, in the presence of autologous CD3−/CD56+ or CD16+ NK cells. In contrast, there was no difference in colony formation when autologous NK cells were added to normal control CD34+-stem cells. Interestingly, the two patients with NK cell-mediated reduction in colony formation also had reduced functional capacity of their inhibitory NKRs against cross-reactive homozygous MHC-Class-I ligands. These results suggests that deficient levels of inhibitory NKRs were expressed to protect from autologous tissue recognition. These surprising results indicate that patients with NK-LDGL have defective CD34+ hematopoietic stem cells in addition to dysregulated autoimmunity that could contribute to development of failed hematopoiesis. Our findings have critical therapeutic implications for patients with NK-LDGL.

Low-dose methotrexate (MTX) is used as an immunosuppressive agent for the treatment of rheumatoid arthritis (RA), Large Granular Lymphocyte (LGL) leukemia, Cutaneous T Cell Lymphoma (CTCL), autoimmune diseases, and prevention of GvHD during bone marrow transplants. The mechanism for immunosuppression is not clearly understood but most data suggests that apoptosis of activated lymphocytes plays a critical role. In this study, we wanted to define the MTX-sensitive population and to determine the apoptotic pathway activated by MTX. Using a clinically relevant dosage range (8 nM- 1 μM), MTX-mediated apoptosis was first examined in a T lymphoblastic leukemia cell line (CEM). The apoptotic pathway induced by MTX included phosphotidylinositol externalization and caspase-3 activation along with a slight increase in mitochondrial membrane depolarization. We next examined a series of tumor cell lines and normal cells for evidence of MTX-induced apoptosis. Using the same clinically relevant dosage range, we found that MTX-induced apoptosis was primarily observed in the four T cell leukemia cell lines including CEM, Jurkat, MT-2, and HUT78 and in normal PBMCs activated with mitogens and IL-2. Less MTX-induced apoptosis was observed in two myeloid leukemia cell lines including HL-60 and K562 and in a B cell leukemia cell line Raji, and the multiple myeloma cell line 8226. Unactivated PBMCs were resistant to MTX-mediated apoptosis. T cells that are clonally expanded in patients with T-LGL leukemia have a CD8+ cytotoxic phenotype, whereas other diseases that are treated with low-dose MTX, such as CTCL and RA, are characterized by the expansion of CD4+ T cells. We found that both freshly sorted CD4+ and CD8+ cells were MTX resistant. In contrast, PHA plus IL-2 treatment induced MTX sensitivity in T cell with both immunophenotypes. We also examined clinical samples from patients with LGL leukemia. We found that freshly isolated PBMCs from T-LGL leukemia patients were resistant to MTX. Clonal cells from the peripheral blood of LGL leukemia patients are in G0/G1 phase of the cell cycle. Interestingly, we found that PHA plus IL-2 treatment induced the cells to enter S-phase and to become MTX sensitive. These results suggest that only fully activated, proliferating T cells from patients with LGL leukemia undergo apoptosis in response to low-dose MTX. Because there was only minor depolarization of mitochondria after MTX treatment in both CEM cells and normal activated PBMCs, we wanted to examine upstream apoptotic events after MTX treatment. We found that caspase-8 cleavage and enzymatic activity was induced by MTX in both CD95 Type I (HUT78) and Type II (CEM and Jurkat) cells but that there was a differential requirement for caspase-8 activity for apoptosis. We found that caspase-8 activation was independent of the Fas receptor as shown by immunoprecipitation experiments and MTX apoptotic assays in the JM3A5 Fas-receptor mutant Jurkat cell line. Using a Jurkat cell line with a homozygous deletion of the FADD gene, we found that caspase-8 activation, caspase-3 activation, and apoptosis in response to MTX were dependent on the adaptor protein FADD. These findings have important implications for understanding the mechanism of MTX for immunosuppressive therapy.

NK-LDGL is associated with the expansion of CD3−, CD16+ and/or CD56+ lymphocytes, which express a skewed repertoire of NK receptors. (Epling-Burnette et al, Blood103:3431, 2004) Single anti-KIR antibody reactivity was noted in seven patients (54%) and “defective” KIR reactivity in four patients (31%) (n=13). Defective KIR expression was characterized by the loss of reactivity to all available antibodies. These patients expressed a large number of activating KIRs by genotype analysis. Semi-quantitative RT-PCR demonstrated that lower than normal levels of RNA of the inhibitory KIR was present in some patients in contrast to normal NK cells. Consistent with a high level of activating receptors, we found the NK-LDGL cells have potent cytolytic function in both direct and redirected cytotoxicity assays. Overexpression of activating receptors in the absence of the appropriate inhibitory receptors may contribute to the bone marrow suppression commonly observed in these patients. We first performed Class I typing of the HLA-C locus of five patients with NK-LDGL using PCR-SSP (One Lambda, Inc, Canoga Park, CA). Phenotypes included Cw05/Cw16, Cw15/15, Cw07/12, Cw04/08, and Cw02/03. Each of the five patients had lower then normal levels of inhibitory KIR (KIR2DL1 and KIR2DL2/3) by flow-cytometry and by RT-PCR. In one patient, we examined the functional ability of these inhibitory receptors to block killing of tumor cells expressing cross-reactive Class-I ligands. Homozygous Cw15/15 was expressed by NK-LDGL5, which is a cross-reactive epitope to Cw3 and recognized by KIR2DL2/3. NK cells from NK-LDGL5 were capable of killing both the class I-negative tumor target 721.221 cells and HLA-Cw3-transfected 721.221 cells suggesting that deficient levels of inhibitory KIR were expressed on the patients NK cells to block activating NK receptor-mediated killing. These results suggest that NK cells from patients with NK-LDGL could mount dysregulated autoimmune-mediated killing, which may impact disease pathogenesis.

The natural killer (NK) type of lymphoproliferative disease of granular lymphocytes (LDGL) is associated with the expansion of CD3(-), CD16(+), and/or CD56(+) lymphocytes. We have examined the repertoire of NK receptors expressed on these cells and delineated the functional activity. We found skewed NK receptor expression on patient NK cells. Reactivity to a single anti-killer cell immunoglobulin-like receptor (anti-KIR) antibody was noted in 7 of 13 patients. LDGL patients variably expressed NKp30, NKp44, and NKp46 RNA. In contrast, CD94 and its inhibitory heterodimerization partner NKG2A were homogeneously expressed at high levels on these NK cells. Interestingly, these patients expressed a large number of activating KIR receptors by genotype analysis. Semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR) demonstrated that lower than normal levels of RNA of the inhibitory KIR was present in some patients in contrast to normal NK cells. Consistent with a high level of activating receptors, we found the NK-LDGL cells have potent cytolytic function in both direct and redirected cytotoxicity assays. These results demonstrate that patients with NK-LDGL have an increased activating-to-inhibitory KIR ratio. This altered ratio might induce inappropriate lysis or cytokine production and impact the disease pathogenesis.

We have previously shown that selection for resistance to the anthracenes, doxorubicin or mitoxantrone, results in coselection for resistance to CD95-mediated apoptosis (Landowski et al: Blood 89:1854, 1997). In the present study, we were interested in determining if the converse is also true; that is, does selection for CD95 resistance coselect for resistance to chemotherapeutic drugs. To address this question, we used two isogenic models of CD95-resistant versus CD95-sensitive cell lines: 8226/S myeloma cells selected for resistance to CD95-mediated apoptosis; and K562 cells expressing ectopic CD95. Repeated exposure of the CD95-sensitive human myeloma cell line, 8226/S, to agonistic anti-CD95 antibody resulted in a cell line devoid of CD95 receptor surface expression and completely resistant to CD95-mediated apoptosis. Multiple clonal populations derived from the CD95-resistant cell line showed no difference in sensitivity to doxorubicin, mitoxantrone, Ara-C, or etoposide, demonstrating that cross-resistance between Fas-mediated apoptosis and drug-induced apoptosis occurs only when cytotoxic drugs are used as the selecting agent. Using the inverse approach, we transfected the CD95-negative cell line, K562, with a CD95 expression vector. Clones expressing variable levels of cell-surface CD95 were isolated by limiting dilution, and analyzed for sensitivity to CD95-mediated apoptosis and response to chemotherapeutic drugs. We show that CD95 surface expression confers sensitivity to CD95-mediated apoptosis; however, it does not alter response to chemotherapeutic drugs. Similarly, doxorubicin-induced activation of caspases 3 and 8 was identical in the CD95-sensitive and CD95-resistant cell lines in both isogenic cell systems. In addition, prior treatment with the CD95 receptor-blocking antibody, ZB4, inhibited CD95-activated apoptosis in 8226/S cells, but had no effect on doxorubicin cytotoxicity. These results show that CD95 and chemotherapeutic drugs use common apoptotic effectors, but the point of convergence in these two pathways is downstream of CD95 receptor/ligand interaction.

Programmed cell death, or apoptosis, is well documented as a physiological means of eliminating activated lymphocytes and maintaining immune homeostasis. Apoptosis has also been implicated in the targeting of tumor cells by cytotoxic T lymphocytes and natural killer cells. One of the two primary mechanisms used in cell-mediated cytotoxicity is the Fas/FasLigand system. Activated or transformed cells expressing the Fas antigen on their surface are susceptible to killing by immune effector cells that express the Fas ligand. Many neoplastic cells, including those derived from patients with multiple myeloma, express Fas antigen on their surface, but do not undergo apoptosis in response to antigen crosslinking. One possibility for the lack of Fas-mediated apoptosis includes mutations in the Fas antigen. Loss of function mutations in the Fas antigen have been associated with congenital autoimmune disease in humans, and have been defined as the genetic defect the in lpr mice. Mutations in the Fas antigen have not been previously described in cancer patients. In this study, we show that mutations occur in the Fas antigen which may cause loss of function and contribute to the pathogenesis of the neoplastic disease, multiple myeloma. Using reverse transcriptase-polymerase chain reaction (RT-PCR), single-stranded conformation polymorphism (SSCP) analysis, and DNA sequencing, we examined the cDNA structure of the Fas antigen in 54 bone marrow (BM) specimens obtained from myeloma patients. Six patient specimens (11%) did not express detectable levels of Fas antigen mRNA. Of the 48 BM specimens which did express Fas antigen, 5 (10%) displayed point mutations. All of the mutations identified were located in the cytoplasmic region of the Fas antigen known to be involved in transduction of an apoptotic signal. Two separate individuals demonstrated an identical mutation at a site previously shown to be mutated in the congenital autoimmune syndrome, ALPS. One patient exhibited a point mutation at a site only two amino acids removed from the documented lesion of the lprcg mouse. Although the functional status of these point mutations remains to be determined, we propose that Fas antigen mutations may contribute to the pathogenesis and progression of myeloma in some patients.