Differential activation of the death receptor pathway in human target cells induced by cytotoxic T lymphocytes showing different kinetics of killing.

BACKGROUND AND OBJECTIVES: Cytotoxic T lymphocytes (CTL) may use two effector mechanisms to kill their target cells: perforin (PFN) and granzyme B (GrB)-dependent granule-mediated cell death and death receptor-mediated cell death. Controversy exists whether, in addition to PFN/GrB-mediated apoptosis, death receptor-induced apoptosis contributes to the elimination of human tumor cells by CTL. DESIGN AND METHODS: Since the two CTL-mediated effector mechanisms differ in time required to eliminate target cells, lysis of target cells was analyzed using CTL clones with slow and rapid kinetics of killing derived from a patient with chronic myeloid leukemia. To determine the involvement of the death receptor pathway, a retroviral construct encoding the antiapoptotic gene FLICE inhibitory protein (FLIP) was introduced into these target cells. RESULTS: A CTL clone capable of killing 50% of the target cells within 2 hours of incubation primarily acted by release of PFN and GrB. In contrast, two CTL clones showing slower target cell killing kinetics partially used the death receptor pathway (approximately 30% inhibition by FLIP). INTERPRETATION AND CONCLUSIONS: We demonstrated that the death receptor pathway contributes to T-cell-mediated cell death if not all target cells are destroyed by release of PFN and GrB.

Case-referent comparison of cognitive functions in patients receiving haematopoietic stem-cell transplantation for haematological malignancies: two-year follow-up results.

During bone marrow or haematopoietic stem-cell transplantation (HSCT), potentially neurotoxic treatments are used. Previous studies identified cognitive disturbances in patients treated with HSCT, but prospective studies with longitudinal assessment are sparse. We examined cognitive functions up to 20 months after a first baseline assessment in 101 patients undergoing HSCT and in 82 reference patients with a haematological malignancy treated with non-myeloablative cancer therapies. Baseline findings revealed no between-group differences and demonstrated mild cognitive impairments in both groups. Follow-up analyses showed no significant changes over time, though poorer performance in attention and executive function, and psychomotor function was found in HSCT patients. Our results suggest limited HSCT-related cognitive dysfunctions. Additional follow-up is necessary to assess long-term effects.

Assessment of disseminated adenovirus infections using quantitative plasma PCR in adult allogeneic stem cell transplant recipients receiving reduced intensity or myeloablative conditioning.

OBJECTIVES: Disseminated adenovirus (AdV) infections following allogeneic stem cell transplantation (allo-SCT) are increasingly recognised, particularly in children. This study evaluated the clinical relevance of disseminated AdV infections in adult allo-SCT recipients, after different conditioning regimens. METHODS: In a cohort of 107 adult allo-SCT recipients, receiving either reduced intensity conditioning (RIC, n = 48) or myeloablative conditioning (MAC, n = 59), AdV DNA levels in plasma were determined retrospectively at 1, 3 and 6 months following transplantation. Results of this screening regimen were compared with a cohort of 58 paediatric allo-SCT recipients, in whom AdV DNA load was monitored prospectively, as part of a pre-emptive treatment strategy. In positive cases, the course of AdV DNA load and clinical outcome were assessed. RESULTS: AdV DNA levels > or =1000 copies/mL were detected in five adults (4.7%) and eight children (13.8%). Screening for AdV viraemia at 1, 3 and 6 months would have detected seven of eight paediatric patients. One adult, receiving MAC, died with disseminated AdV disease and in four (three RIC and one MAC) AdV viraemia was transient without clinical symptoms specifically attributable to AdV. Seven paediatric patients with AdV viraemia were pre-emptively treated with ribavirin or cidofovir and in three of them disseminated AdV infection was related to a fatal outcome. CONCLUSIONS: Disseminated AdV infections following allo-SCT was a rare event in the adults and cause morbidity in a minority of these patients. In four of five adult patients, spontaneous clearance of AdV viraemia occurred. Results did not differ between the conditioning regimens that were applied in the adult cohort.

Outcomes after myeloablative unrelated donor stem cell transplantation using both in vitro and in vivo T-cell depletion with alemtuzumab.

HLA-matched unrelated donor (MUD) stem cell transplantation (MUD) is complicated by a high incidence of graft-versus-host-disease (GVHD) resulting in significant morbidity and mortality. To circumvent this problem we included alemtuzumab for in vivo and in vitro T-cell depletion in a myeloablative MUD-SCT regimen. After SCT, no severe acute GVHD was observed in the 30 transplanted patients. Donor lymphocyte infusion administered at a later time point resulted in sustained anti-tumor responses in most patients with chronic myeloid leukemia. After donor lymphocyte infusion three patients developed severe acute GVHD. Due to good responsiveness to immunosuppressive therapy only two patients developed persistent chronic GVHD. The main advantage of the transplantation regimen including alemtuzumab is that not only mortality due to GVHD is limited but also extensive chronic GVHD, which potentially leads to chronic morbidity and diminished quality of life, is hardly observed.

The mechanisms of Ara-C-induced apoptosis of resting B-chronic lymphocytic leukemia cells.

BACKGROUND AND OBJECTIVES: Cytarabine (Ara-C) is commonly used for the treatment of acute leukemia. Incorporation of Ara-C into DNA is a key event in the mechanism of killing of proliferating leukemic cells. Previously, we demonstrated that Ara-C was cytotoxic to proliferating but not to resting (G(O)) malignant cells from patients with acute leukemia. In contrast, here we show unexpected apoptosis of G(O) B-chronic lymphocytic leukemia (CLL) cells by Ara-C in a dose-dependent manner. In this study we analyzed which cellular processes were involved in Ara-C-mediated killing of G(O)-B-CLL cells. DESIGN AND METHODS: Using primary B-CLL cells (>98% in G(O)), we examined the mechanisms of Ara-C-mediated apoptosis in resting G(O) cells. CFSE-based cytotoxicity assays combined with cell cycle analysis were used to perform a long-term analysis of Ara-C-mediated killing of B-CLL cells. The effects of Ara-C on DNA and RNA synthesis were studied using various 3H-incorporation experiments. RESULTS: Ara-C-mediated cell death of B-CLL cells showed the characteristics of normal apoptosis, such as phosphatidyl serine exposure and caspase activation. The mechanism of killing of quiescent B-CLL cells by Ara-C was shown not to be dependent on DNA replication. In contrast, CD40L-activated B-CLL cells showed S-phase-specific depletion of proliferating CLL cells. We demonstrated that Ara-C was converted into its active triphosphate by G(O)-B-CLL cells, coinciding with a 30% inhibition of RNA synthesis. INTERPRETATION AND CONCLUSIONS: In conclusion, our data indicate that Ara-C can induce apoptosis in resting G(O)-B-CLL cells using a mechanism independent of cell proliferation and DNA replication but associated with inhibition of RNA synthesis and downregulation of Mcl-1.

Cytokine-dependent proliferation of human CD34+ progenitor cells in the absence of serum is suppressed by their progeny's production of serine proteinases.

In this study, we demonstrate that the synthesis and release of serine proteinases by hematopoietic cells affects the in vitro proliferation of hematopoietic progenitor cells (HPCs) in response to proteins, including hematopoietic growth factors (HGFs), transferrin, insulin, and albumin in serum-free cultures. In serum-free cultures, bone marrow mononuclear cells or the CD34- progeny of the CD34+ cells were shown to release the serine proteinases human neutrophil elastase (HNE), cathepsin G (Cath G), and proteinase 3 (Pr3). In the absence of serum, we showed that HNE, Cath G, and Pr3 rapidly and dose-dependently degraded HGF and other proteins present in the medium, resulting in decreased proliferation of HPCs. Addition of the serine proteinase inhibitors alpha1-proteinase inhibitor (alpha1-PI) or the secretory leukocyte proteinase inhibitor (SLPI), but not leupeptin, aprotinin, or AEBSF (4-[2-aminoethyl]-benzenesulfonylfluoride hydrochloride), could completely prevent the degradation of proteins relevant to the growth of hematopoietic cells. Thus, the addition of serine proteinase inhibitors like alpha1-PI or SLPI may be critical for the expansion of CD34+ cells or gene transfer into CD34+ cells or other hematopoietic cells in vitro using serum-free media under good manufacturing practice conditions.

Primary allogeneic T-cell responses against mantle cell lymphoma antigen-presenting cells for adoptive immunotherapy after stem cell transplantation.

PURPOSE: In patients treated with allogeneic stem cell transplantation for advanced mantle cell lymphoma (MCL), complete sustained remissions have been observed illustrating susceptibility of MCL cells to a graft-versus-lymphoma effect. To potentiate this graft-versus-lymphoma effect, adoptive transfer of in vitro selected MCL-specific CTL can be an attractive approach. The lack of expression of costimulatory molecules on MCL cells hampers the generation of MCL-reactive T-cell responses. The purpose of this study was to modify MCL cells into antigen-presenting cells (APC) and to use these MCL-APCs to induce allogeneic MCL-reactive T-cell responses. EXPERIMENTAL DESIGN: Interleukin (IL)-4, IL-10, CpG, and CD40 activation were tested for their capacity to up-regulate costimulatory molecules on MCL cells. Primary MCL cells or the modified MCL-APCs were then used to evaluate the induction of MCL-reactive T-cell responses in HLA-matched donors. RESULTS: Ligation of CD40 on MCL cells was essential to up-regulate costimulatory molecules and to induce production of high amounts of IL-12. In contrast to primary MCL cells, MCL-APC cells as stimulators were capable of inducing CD8+ CTL lines from HLA class I-matched donors. High numbers of CTL clones could be generated capable of efficiently killing the primary MCL cells and MCL-APC but not donor-specific targets. CONCLUSION: These results show the feasibility to generate primary allogeneic T-cell responses against MCL-APC, and may provide new immunotherapeutic tools to further exploit the graft-versus-lymphoma effect following allogeneic stem cell transplantation in patients with MCL.

Infectious complications of central venous catheters increase the risk of catheter-related thrombosis in hematology patients: a prospective study.

PURPOSE: We studied whether the risk of central venous catheter (CVC) -related thrombosis increased after an episode of CVC-related infection in patients undergoing intensive chemotherapy. Secondly, we determined whether thrombosis can be predicted or excluded by CVC lock fluid surveillance cultures. PATIENTS AND METHODS: In a prospective setting, 105 consecutive patients were carefully examined for CVC-related infection and thrombosis. In all patients, microbial surveillance cultures of CVC lock fluid were taken every other day. All patients with clinical suspicion of CVC-related thrombosis underwent Doppler ultrasound or additional venography. RESULTS: The cumulative incidence of CVC-related infection was 24% (25 of 105 patients). Clinically manifest thrombosis occurred in 13 (12%) of 105 patients. In patients with CVC-related infection, the risk of thrombosis increased markedly in comparison to those without infection (relative risk, 17.6; 95% CI, 4.1 to 74.1). In patients having two or more positive subsequent CVC lock fluid cultures with identical micro-organisms, 71.4% developed thrombosis, as compared with 3.3% in patients with negative or a single positive culture. CONCLUSION: The risk of clinically manifest thrombosis is increased after an episode of CVC-related infection in patients undergoing intensive chemotherapy. Surveillance culturing of CVC lock fluid may be clinically useful in estimating the risk for thrombosis and the instigation of focused early intervention.

Acute myeloid leukemia cells in G0 phase of the cell cycle that are unresponsive to conventional chemotherapy are sensitive to treatment with granulocyte-macrophage colony-stimulating factor/diphtheria toxin fusion proteins.

OBJECTIVE: Unresponsiveness to chemotherapy is a major problem in the treatment of leukemia, which can be caused by unresponsiveness of noncycling cells to cell cycle-dependent cytotoxic agents. Targeted toxins consisting of a targeting and activating cytokine (granulocyte-macrophage colony-stimulating factor [GM-CSF]) and diphtheria toxin (DT) can be used to overcome this kind of resistance of leukemic cells. In this study we manipulated the cell cycle and proliferative status of leukemic cells, explored the effect on sensitivity to DT, and determined the ability of DT388GMCSF fusion proteins to activate and subsequently kill leukemic cells. MATERIALS AND METHODS: We used the GM-CSF-dependent myeloid leukemic cell line AML-193 as a model. GM-CSF or granulocyte colony-stimulating factor (G-CSF) was used to manipulate the cell cycle and proliferative state of AML-193 cells. Cell death was quantified by 51Cr release assays. The results obtained in the AML-193 cell line model were confirmed using primary leukemic blasts. RESULTS: Similar to treatment with chemotherapy and immunotherapy, leukemic cells in resting G0 phase were relatively resistant to DT-induced cell death. Synchronized recruitment of leukemic cells into activated phases of the cell cycle by low concentrations of GM-CSF or G-CSF resulted in significant increased DT sensitivity. DT388GMCSF fusion proteins specifically targeted GM-CSF receptor-expressing cells, resulting in recruitment of leukemic cells from G0 phase of the cell cycle and subsequent kill of these cells. CONCLUSION: Leukemic cells in G0 phase, which are resistant to conventional chemotherapy, Fas-induced immunotherapy, and DT alone, can be synchronically activated and subsequently killed by DT388GMCSF fusion proteins.

Alpha-interferon with very-low-dose donor lymphocyte infusion for hematologic or cytogenetic relapse of chronic myeloid leukemia induces rapid and durable complete remissions and is associated with acceptable graft-versus-host disease.

Donor lymphocyte infusion (DLI) results in complete cytogenetic remission (CCR) of relapsed chronic-phase chronic myeloid leukemia (CML-CP) after allogeneic stem cell transplantation (SCT) in up to 80% of patients. The main complication of DLI is graft-versus-host disease (GVHD). Decreasing the dose of DLI is associated with less GVHD but also with a longer interval between treatment and CCR. We postulated that combining alpha-interferon (alpha-IFN) with DLI would enable us to decrease the dose of DLI, thereby limiting GVHD, and at the same time to decrease the interval between DLI and CCR for patients with either a hematologic or cytogenetic relapse. For molecular relapses, we hypothesized that because of a lower tumor load, very low doses of DLI without alpha-IFN could be an effective treatment. Two groups of CML-CP patients treated with DLI at a very low dose of 0.5 to 1.0 x 10(7) mononuclear cells per kilogram, containing 2 to 6 x 10(6) CD3+ T cells per kilogram, were analyzed: 13 patients with a cytogenetic or a hematologic relapse after allogeneic SCT (group A) were treated with additional alpha-IFN therapy at a dose of 3 x 10(6) U 5 d/wk, and 8 patients with a molecular relapse were treated without alpha-IFN (group B). Twelve patients from group A reached a CCR. The median interval between DLI and CCR was 7 weeks (range, 5-18 weeks) for group A. All patients with a CCR reached complete donor chimerism at a median of 10 weeks after DLI (range, 6-121 weeks). Eleven patients reached molecular remission at a median of 15 weeks after DLI (range, 8-34 weeks). In group B, all patients reached a molecular remission at a median of 14 weeks (range, 12-29 weeks). Five patients from group A developed acute GVHD grade II to IV and extensive chronic GVHD. In group B, 1 patient developed acute GVHD grade II to IV and subsequently developed extensive chronic GVHD. With a median follow-up of 62 months, 10 patients in group A are alive and in continuous CCR. One patient had a molecular relapse, for which she successfully received additional DLI; another patient reached molecular remission only after 5 doses of DLI. Two patients from group A died of a gram-negative sepsis, and 1 died of an acute myocardial infection. In group B, all patients are alive and in molecular remission with a median follow-up of 20 months. One patient's disease progressed but was successfully treated with DLI plus alpha-IFN. In conclusion, very-low-dose DLI in combination with alpha-IFN as treatment for cytogenetic or hematologic relapses of CML-CP after allogeneic SCT reduced the interval to obtain a CCR with acceptable GVHD when compared with the literature. Patients with a CCR also reached complete donor chimerism and complete molecular remissions. For patients with a molecular relapse, very-low-dose DLI alone is sufficient to induce molecular remissions in most patients and is associated with limited GVHD.

New CFSE-based assay to determine susceptibility to lysis by cytotoxic T cells of leukemic precursor cells within a heterogeneous target cell population.

For the clinical evaluation of the efficacy of cellular immunotherapy it is necessary to analyze the effector functions of T cells against primary leukemic target cell populations which are usually considerably heterogeneous caused by differential maturation stages of the leukemic cells. An appropriate assay should not only allow the quantitative analysis of rapid cell death induction as measured by the conventional 51Cr release assay but also of the more slowly executing pathways of T-cell-induced apoptosis occurring within days instead of hours which cannot be measured using this method. Furthermore, it should dissect the differential susceptibility to T-cell-induced cell death of various target cell subpopulations and characterize the malignant precursor cells capable of producing malignant progeny. To fulfill these requirements we developed a new assay based on carboxyfluorescein diacetate succinimidyl ester (CFSE) labeling of the target cell population combined with antibody staining of specific cell populations and addition of fluorescent microbeads to quantitatively monitor target cell death occurring within a longer time frame up to at least 5 days. This new assay facilitates the analysis of differential recognition of distinct cell types within a heterogeneous target cell population and allows simultaneously evaluation of the proliferative status of surviving target cells in response to relevant cytokines.

Central venous catheter related thrombosis in haematology patients and prediction of risk by screening with Doppler-ultrasound.

Patients with a central venous catheter (CVC) who receive intensive chemotherapy or a stem cell transplantation for haematological disease are at risk for developing CVC-related thrombosis. To study the incidence of thrombosis, 105 consecutive patients underwent serial Doppler-ultrasound and we evaluated whether clinically manifest thrombosis could be predicted by screening with Doppler-ultrasound. Patients with subclavian or jugular inserted CVCs were clinically assessed each day for signs and symptoms of thrombosis. Additional Doppler-ultrasound screens were performed weekly by an independent physician in all patients until CVC removal. Doppler-ultrasound recordings were assessed by two blinded observers. In cases of clinically suspected thrombosis, the attending physicians followed routine diagnostic and therapeutic procedures. The overall cumulative incidence of CVC-related thrombosis was 28.6% (30 of 105 patients). Of the 30 patients with thrombosis, 26 had subclinical thrombosis by Doppler-ultrasound, nine of whom developed clinically manifest thrombosis later. Four patients had clinically manifest thrombosis without prior abnormal Doppler-ultrasound. In cases of subclinical thrombosis the risk of developing symptomatic disease increased sevenfold (34.6% vs. 5.1%). Doppler-ultrasound screening may be useful to identify those patients that are at high and low risk for clinically manifest CVC-related thrombosis.

Minimal GVHD following in-vitro T cell-depleted allogeneic stem cell transplantation with reduced-intensity conditioning allowing subsequent infusions of donor lymphocytes in patients with hematological malignancies and solid tumors.

OBJECTIVE: Allogeneic stem cell transplantation (alloSCT) following reduced-intensity conditioning offers a relatively nontoxic regimen while preserving rapid and sustained engraftment. Acute and chronic graft-vs-host disease (GVHD) is, however, a significant cause of severe morbidity. To reduce the incidence of GVHD, we treated a group of high-risk patients with a reduced-intensity conditioning regimen followed by in vitro T-cell-depleted alloSCT using Campath 1-H incubation. PATIENTS AND METHODS: Eighteen patients were treated with fludarabine (6 x 30 mg/m(2)), busulphan (2 x 3.2 mg/kg), and ATG (4 x 10 mg/kg) followed by the infusion of high-dose T-cell-depleted peripheral stem cells from sibling donors. No posttransplant GVHD prophylaxis was administered. At 6 months after alloSCT, low-dose donor lymphocyte infusion (DLI) was administered. RESULTS: All patients had sustained engraftment of donor cells with a median of 95% donor cells at 3 months after alloSCT. Minimal acute and no chronic GVHD was observed after alloSCT. A high incidence of cytomegalovirus (CMV) reactivation but no CMV disease was observed. Eleven patients received DLI at a median of 6.5 months after alloSCT. Acute GVHD grade II-III developed in 6 patients. All patients showed improvement of donor chimerism after DLI. With a median follow-up of 211 days, 11 patients are alive. Particular in patients with chronic lymphocytic leukemia and acute myeloid leukemia, a significant graft-vs-tumor effect was observed. CONCLUSIONS: In vitro T-cell-depleted alloSCT following reduced-intensity conditioning leads to durable donor engraftment without GVHD. The high levels of donor chimerism allow the subsequent use of cellular immunotherapy to treat residual disease.

Partially T-cell-depleted allogeneic stem-cell transplantation for first-line treatment of multiple myeloma: a prospective evaluation of patients treated in the phase III study HOVON 24 MM.

PURPOSE: To determine in a prospective study the efficacy, toxicity, and long-term outcome of up-front allogeneic stem-cell transplantation (allo-SCT) in multiple myeloma (MM). PATIENTS AND METHODS: In the prospective phase III study by the Dutch-Belgian Hemato-Oncology Cooperative Group (HOVON), HOVON 24 MM, 53 patients with an HLA-identical sibling (median age at transplantation, 48 years; range, 31 to 56 years) were allocated to a partial T-cell-depleted allo-SCT after induction therapy. RESULTS: The overall response rate after allo-SCT was 89% (47 of 53 patients), including the 19% of patients (10 of 53 patients) with a complete remission (CR). Five patients achieved a CR only after allo-SCT. Five (71%) of seven primary refractory patients obtained a response to allo-SCT, all of whom had a partial remission. With a median follow-up of 38 months (range, 25 to 61 months), 20 patients are alive since allo-SCT and 33 patients have died (14 from progressive disease, 18 from treatment-related mortality [TRM], and one from another cause). Occurrence of acute graft-versus-host disease grades 2 to 4 predicted for higher TRM in a time-dependent analysis. The median progression-free survival time after allo-SCT was 17 months. Median overall survival time after allo-SCT was 25 months, or 29 months from the start of therapy. Only three patients are in continuing CR, indicating that the potential cure rate of this approach is, at best, 6%. CONCLUSION: This first prospective evaluation of up-front allo-SCT of MM in a multicenter setting does not support the use of T-cell-depleted myeloablative allo-SCT as part of first-line therapy.

Inactivation of deoxycytidine kinase and overexpression of P-glycoprotein in AraC and daunorubicin double resistant leukemic cell lines.

AraC resistance in vitro is explained by inactivation of dCK, while resistance to DNR is described by overexpression of multidrug efflux pumps like Pgp or MRP. Thus far, no correlation between resistance mechanisms in vitro and in patients with AML has been documented. We generated AraC and DNR double resistant cell lines to investigate resistance mechanisms of both agents. In these cell lines involvement of dCK was extensively investigated and Pgp expression and activity was determined. Our data implicate that similar resistance mechanisms like inactivation of dCK coincided by alternatively spliced dCK forms and overexpression of Pgp are induced in single-as well as in double resistant leukemic cell lines.

Deoxycytidine kinase expression and activity in patients with resistant versus sensitive acute myeloid leukemia.

Resistance to cytarabine (AraC) is a major problem in treatment of patients with acute myeloid leukemia (AML). In contrast to in vitro AraC resistance, deoxycytidine kinase (dCK) mutations are rarely found in patients with refractory or relapsed AML. Previously we have demonstrated alternatively spliced dCK mRNA predominantly expressed in leukemic blasts from patients with resistant AML. In this study we investigated wild-type (wt) dCK expression and activity to elucidate the possible role of decreased dCK expression or activity in unresponsiveness to AraC in patients with AML. No alterations in dCK mRNA and protein expression or in dCK activity were detected between patients with clinically resistant vs. sensitive AML. In addition, wt dCK expression and activity were not reduced in leukemic blasts expressing alternatively spliced dCK forms as compared to blasts with only wt dCK. Also, no major differences in wt dCK expression and activity were observed between samples obtained from patients with AML and bone marrow or peripheral blood samples from healthy donors. These data implicate that in our patient group of refractory or relapsed AML cases, alterations in dCK expression and/or activity cannot explain unresponsiveness to chemotherapy including AraC.

Functional role of alternatively spliced deoxycytidine kinase in sensitivity to cytarabine of acute myeloid leukemic cells.

Development of resistance to cytarabine (AraC) is a major problem in the treatment of patients with acute myeloid leukemia (AML). Inactivation of deoxycytidine kinase (dCK) plays an important role in AraC resistance in vitro. We have identified inactive, alternatively spliced dCK forms in leukemic blasts from patients with resistant AML. Because these dCK-spliced variants were only detectable in resistant AML, it was hypothesized that they might play a role in AraC resistance in vivo. In the current study, the biologic role of the alternatively spliced dCK forms in AraC resistance was further investigated by retroviral transductions in rat leukemic cells. Introduction of inactive, alternatively spliced dCK forms into AraC-resistant K7 cells, with no endogenous wild-type (wt) dCK activity, could not restore AraC sensitivity, whereas wt dCK fully restored the AraC-sensitive phenotype. Transfection of alternatively spliced dCK forms into AraC-sensitive KA cells, as well as in human leukemic U937 cells and in phytohemagglutinin-stimulated T cells, did not significantly change sensitivity toward AraC. In addition, cotransduction of wt dCK with alternatively spliced dCK in K7 cells did not result in altered sensitivity to AraC compared with K7 cells only transduced with wt dCK. These data indicate that the alternatively spliced dCK forms cannot act as a dominant-negative inhibitor on dCK wt activity when they are coexpressed in a single cell. However, a cell expressing alternatively spliced dCK forms that has lost wt dCK expression is resistant to the cytotoxic effects of AraC.