Capture-based targeted sequencing using a T-cell control in myeloid malignancies and idiopathic cytopenias.

We report on a study of next-generation sequencing in 257 patients undergoing investigations for cytopenias. We sequenced bone marrow aspirates using a target enrichment panel comprising 82 genes and used T cells from paired blood as a control. One hundred and sixty patients had idiopathic cytopenias, 81 had myeloid malignancies and 16 had lymphoid malignancies or other diagnoses. Forty-seven of the 160 patients with idiopathic cytopenias had evidence of somatic pathogenic variants consistent with clonal cytopenias. Only 39 genes of the 82 tested were mutated in the 241 patients with either idiopathic cytopenias or myeloid neoplasms. We confirm that T cells can be used as a control to distinguish between germline and somatic variants. The use of paired analysis with a T-cell control significantly reduced the time molecular scientists spent reporting compared to unpaired analysis. We identified somatic variants of uncertain significance (VUS) in a higher proportion (24%) of patients with myeloid malignancies or clonal cytopenias compared to less than 2% of patients with non-clonal cytopenias. This suggests that somatic VUS are indicators of a clonal process. Lastly, we show that blood depleted of lymphocytes can be used in place of bone marrow as a source of material for sequencing.

CREST-UK: Real-world effectiveness, safety and outpatient delivery of CPX-351 for first-line treatment of newly diagnosed therapy-related AML and AML with myelodysplasia-related changes in the UK.

Favourable outcomes with CPX-351 versus conventional 7 + 3 were demonstrated in the pivotal phase III trial in adults aged 60-75 years with newly diagnosed, highrisk/secondary acute myeloid leukaemia (AML). As a complement to the clinical trial and to address important data gaps, the CPX-351 Real-World Effectiveness and SafeTy (CREST-UK; NCT05169307) study evaluated the use of CPX-351 in routine clinical practice in the UK, in 147 patients with newly diagnosed therapy-related AML or AML with myelodysplasia-related changes. Best response of complete remission or complete remission with incomplete platelet or neutrophil recovery was achieved by 53% of evaluable patients. Kaplan-Meier median overall survival (OS) was 12.8 months (95% confidence interval 9.2-15.3). Fifty (34%) patients proceeded to haematopoietic cell transplantation (HCT); median OS landmarked from the HCT date was not reached. There were no new safety concerns with CPX-351 identified in CREST-UK. Patients treated with CPX-351 in the outpatient setting spent an average of 24.4, 16.7, 28.2, and 27.7 fewer days on the ward compared with inpatients during first induction, second induction, first consolidation, and second consolidation, respectively. The results from CREST-UK provide valuable insights into the effectiveness, safety, and outpatient delivery of CPX-351 in routine clinical practice in the UK.

Longitudinal kinetics of RBD+ antibodies in COVID-19 recovered patients over 14 months.

We describe the longitudinal kinetics of the serological response in COVID-19 recovered patients over a period of 14 months. The antibody kinetics in a cohort of 192 recovered patients, including 66 patients for whom follow-up serum samples were obtained at two to four clinic visits, revealed that RBD-specific antibodies decayed over the 14 months following the onset of symptoms. The decay rate was associated with the robustness of the response in that antibody levels that were initially highly elevated after the onset of symptoms subsequently decayed more rapidly. An exploration of the differences in the longitudinal kinetics between recovered patients and naïve vaccinees who had received two doses of the BNT162b2 vaccine showed a significantly faster decay in the naïve vaccinees, indicating that serological memory following natural infection is more robust than that following to vaccination. Our data highlighting the differences between serological memory induced by natural infection vs. vaccination contributed to the decision-making process in Israel regarding the necessity for a third vaccination dose.

Phosphonoacetate Modifications Enhance the Stability and Editing Yields of Guide RNAs for Cas9 Editors.

CRISPR gene editing and control systems continue to emerge and inspire novel research and clinical applications. Advances in CRISPR performance such as optimizing the duration of activity in cells, tissues, and organisms, as well as limiting off-target activities, have been extremely important for expanding the utility of CRISPR-based systems. By investigating the effects of various chemical modifications in guide RNAs (gRNAs) at defined positions and combinations, we find that 2'-O-methyl-3'-phosphonoacetate (MP) modifications can be substantially more effective than 2'-O-methyl-3'-phosphorothioate (MS) modifications at the 3' ends of single-guide RNAs (sgRNAs) to promote high editing yields, in some instances showing an order of magnitude higher editing yield in human cells. MP-modified 3' ends are especially effective at promoting the activity of guide RNAs cotransfected with Cas messenger RNA (mRNA), as the gRNA must persist in cells until the Cas protein is expressed. We demonstrate such an MP enhancement for sgRNAs cotransfected with a BE4 mRNA for cytidine base editing and also demonstrate that MP at the 3' ends of prime editing guide RNAs (pegRNAs) cotransfected with PE2 mRNA can promote maximal prime editing yields. In the presence of serum, sgRNAs with MP-modified 3' ends showed marked improvements in editing efficiency over sgRNAs with MS-modified 3' ends codelivered with Cas9 mRNA and showed more modest improvements at enhancing the activity of transfected ribonucleoprotein (RNP) complexes. Our results suggest that MP should be considered as a performance-enhancing modification for the 3' ends of synthetic gRNAs, especially in situations where the guide RNAs may be susceptible to exonuclease-mediated degradation.

Molecular MRD status and outcome after transplantation in NPM1-mutated AML.

Relapse remains the most common cause of treatment failure for patients with acute myeloid leukemia (AML) who undergo allogeneic stem cell transplantation (alloSCT), and carries a grave prognosis. Multiple studies have identified the presence of measurable residual disease (MRD) assessed by flow cytometry before alloSCT as a strong predictor of relapse, but it is not clear how these findings apply to patients who test positive in molecular MRD assays, which have far greater sensitivity. We analyzed pretransplant blood and bone marrow samples by reverse-transcription polymerase chain reaction in 107 patients with NPM1-mutant AML enrolled in the UK National Cancer Research Institute AML17 study. After a median follow-up of 4.9 years, patients with negative, low (<200 copies per 105ABL in the peripheral blood and <1000 copies in the bone marrow aspirate), and high levels of MRD had an estimated 2-year overall survival (2y-OS) of 83%, 63%, and 13%, respectively (P < .0001). Focusing on patients with low-level MRD before alloSCT, those with FLT3 internal tandem duplications(ITDs) had significantly poorer outcome (hazard ratio [HR], 6.14; P = .01). Combining these variables was highly prognostic, dividing patients into 2 groups with 2y-OS of 17% and 82% (HR, 13.2; P < .0001). T-depletion was associated with significantly reduced survival both in the entire cohort (2y-OS, 56% vs 96%; HR, 3.24; P = .0005) and in MRD-positive patients (2y-OS, 34% vs 100%; HR, 3.78; P = .003), but there was no significant effect of either conditioning regimen or donor source on outcome. Registered at ISRCTN (http://www.isrctn.com/ISRCTN55675535).

Repression of sphingosine kinase (SK)-interacting protein (SKIP) in acute myeloid leukemia diminishes SK activity and its re-expression restores SK function.

Previous studies have shown that sphingosine kinase interacting protein (SKIP) inhibits sphingosine kinase (SK) function in fibroblasts. SK phosphorylates sphingosine producing the potent signaling molecule sphingosine-1-phosphate (S1P). SKIP gene (SPHKAP) expression is silenced by hypermethylation of its promoter in acute myeloid leukemia (AML). However, why SKIP activity is silenced in primary AML cells is unclear. Here, we investigated the consequences of SKIP down-regulation in AML primary cells and the effects of SKIP re-expression in leukemic cell lines. Using targeted ultra-HPLC-tandem MS (UPLC-MS/MS), we measured sphingolipids (including S1P and ceramides) in AML and control cells. Primary AML cells had significantly lower SK activity and intracellular S1P concentrations than control cells, and SKIP-transfected leukemia cell lines exhibited increased SK activity. These findings show that SKIP re-expression enhances SK activity in leukemia cells. Furthermore, other bioactive sphingolipids such as ceramide were also down-regulated in primary AML cells. Of note, SKIP re-expression in leukemia cells increased ceramide levels 2-fold, inactivated the key signaling protein extracellular signal-regulated kinase, and increased apoptosis following serum deprivation or chemotherapy. These results indicate that SKIP down-regulation in AML reduces SK activity and ceramide levels, an effect that ultimately inhibits apoptosis in leukemia cells. The findings of our study contrast with previous results indicating that SKIP inhibits SK function in fibroblasts and therefore challenge the notion that SKIP always inhibits SK activity.

Venetoclax induces rapid elimination of NPM1 mutant measurable residual disease in combination with low-intensity chemotherapy in acute myeloid leukaemia.

Based on promising results in older adults with acute myeloid leukaemia (AML), we treated patients with NPM1mut measurable residual disease (MRD) using off-label venetoclax in combination with low-dose cytarabine or azacitidine. Twelve consecutive patients were retrospectively identified, including five with molecular persistence and seven with molecular relapse/progression. All patients with molecular persistence achieved durable molecular complete remission (CRMRD- ) without transplantation. Six of seven patients with molecular relapse/progression achieved CRMRD- after 1-2 cycles of venetoclax. This paper highlights the promising efficacy of venetoclax-based therapy to reduce the relapse risk in patients with persistent or rising NPM1mut MRD.

Improving CRISPR-Cas specificity with chemical modifications in single-guide RNAs.

CRISPR systems have emerged as transformative tools for altering genomes in living cells with unprecedented ease, inspiring keen interest in increasing their specificity for perfectly matched targets. We have developed a novel approach for improving specificity by incorporating chemical modifications in guide RNAs (gRNAs) at specific sites in their DNA recognition sequence ('guide sequence') and systematically evaluating their on-target and off-target activities in biochemical DNA cleavage assays and cell-based assays. Our results show that a chemical modification (2'-O-methyl-3'-phosphonoacetate, or 'MP') incorporated at select sites in the ribose-phosphate backbone of gRNAs can dramatically reduce off-target cleavage activities while maintaining high on-target performance, as demonstrated in clinically relevant genes. These findings reveal a unique method for enhancing specificity by chemically modifying the guide sequence in gRNAs. Our approach introduces a versatile tool for augmenting the performance of CRISPR systems for research, industrial and therapeutic applications.

Durable graft-versus-leukaemia effects without donor lymphocyte infusions - results of a phase II study of sequential T-replete allogeneic transplantation for high-risk acute myeloid leukaemia and myelodysplasia.

Allogeneic haematopoietic stem-cell transplantation remains the only curative treatment for relapsed/refractory acute myeloid leukaemia (AML) and high-risk myelodysplasia but has previously been limited to patients who achieve remission before transplant. New sequential approaches employing T-cell depleted transplantation directly after chemotherapy show promise but are burdened by viral infection and require donor lymphocyte infusions (DLI) to augment donor chimerism and graft-versus-leukaemia effects. T-replete transplantation in sequential approaches could reduce both viral infection and DLI usage. We therefore performed a single-arm prospective Phase II clinical trial of sequential chemotherapy and T-replete transplantation using reduced-intensity conditioning without planned DLI. The primary endpoint was overall survival. Forty-seven patients with relapsed/refractory AML or high-risk myelodysplasia were enrolled; 43 proceeded to transplantation. High levels of donor chimerism were achieved spontaneously with no DLI. Overall survival of transplanted patients was 45% and 33% at 1 and 3 years. Only one patient developed cytomegalovirus disease. Cumulative incidences of treatment-related mortality and relapse were 35% and 20% at 1 year. Patients with relapsed AML and myelodysplasia had the most favourable outcomes. Late-onset graft-versus-host disease protected against relapse. In conclusion, a T-replete sequential transplantation using reduced-intensity conditioning is feasible for relapsed/refractory AML and myelodysplasia and can deliver graft-versus-leukaemia effects without DLI.

Arginine deprivation using pegylated arginine deiminase has activity against primary acute myeloid leukemia cells in vivo.

The strategy of enzymatic degradation of amino acids to deprive malignant cells of important nutrients is an established component of induction therapy of acute lymphoblastic leukemia. Here we show that acute myeloid leukemia (AML) cells from most patients with AML are deficient in a critical enzyme required for arginine synthesis, argininosuccinate synthetase-1 (ASS1). Thus, these ASS1-deficient AML cells are dependent on importing extracellular arginine. We therefore investigated the effect of plasma arginine deprivation using pegylated arginine deiminase (ADI-PEG 20) against primary AMLs in a xenograft model and in vitro. ADI-PEG 20 alone induced responses in 19 of 38 AMLs in vitro and 3 of 6 AMLs in vivo, leading to caspase activation in sensitive AMLs. ADI-PEG 20-resistant AMLs showed higher relative expression of ASS1 than sensitive AMLs. This suggests that the resistant AMLs survive by producing arginine through this metabolic pathway and ASS1 expression could be used as a biomarker for response. Sensitive AMLs showed more avid uptake of arginine from the extracellular environment consistent with their auxotrophy for arginine. The combination of ADI-PEG 20 and cytarabine chemotherapy was more effective than either treatment alone resulting in responses in 6 of 6 AMLs tested in vivo. Our data show that arginine deprivation is a reasonable strategy in AML that paves the way for clinical trials.

Trs20 is required for TRAPP III complex assembly at the PAS and its function in autophagy.

The modular TRAPP complex acts as a guanine-nucleotide exchange factor (GEF) for Ypt/Rab GTPases. Whereas TRAPP I and TRAPP II regulate the exocytic pathway, TRAPP III functions in autophagy. The TRAPP subunit Trs20 is not required for assembly of core TRAPP or its Ypt1 GEF activity. Interestingly, mutations in the human functional ortholog of Trs20, Sedlin, cause spondyloepiphyseal dysplasia tarda (SEDT), a cartilage-specific disorder. We have shown that Trs20 is required for TRAPP II assembly and identified a SEDT-linked mutation, Trs20-D46Y, which causes a defect in this process. Here we show that Trs20 is also required for assembly of TRAPP III at the pre-autophagosomal structure (PAS). First, recombinant Trs85, a TRAPP III-specific subunit, associates with TRAPP only in the presence of Trs20, but not Trs20-D46Y mutant protein. Second, a TRAPP complex with Ypt1 GEF activity co-precipitates with Trs85 from wild type, but not trs20ts mutant, cell lysates. Third, live-cell colocalization analysis indicates that Trs85 recruits core TRAPP to the PAS via the linker protein Trs20. Finally, trs20ts mutant cells are defective in selective and non-selective autophagy. Together, our results show that Trs20 plays a role as an adaptor in the assembly of TRAPP II and TRAPP III complexes, and the SEDT-linked mutation causes a defect in both processes.

Loss of imprinting at the 14q32 domain is associated with microRNA overexpression in acute promyelocytic leukemia.

Distinct patterns of DNA methylation characterize the epigenetic landscape of promyelocytic leukemia/retinoic acid receptor-α (PML-RARα)-associated acute promyelocytic leukemia (APL). We previously reported that the microRNAs (miRNAs) clustered on chromosome 14q32 are overexpressed only in APL. Here, using high-throughput bisulfite sequencing, we identified an APL-associated hypermethylation at the upstream differentially methylated region (DMR), which also included the site motifs for the enhancer blocking protein CCCTC-binding factor (CTCF). Comparing the profiles of diagnostic/remission paired patient samples, we show that hypermethylation was acquired in APL in a monoallelic manner. The cytosine guanine dinucleotide status of the DMR correlated with expression of the miRNAs following a characteristic position-dependent pattern. Moreover, a signature of hypermethylation was also detected in leukemic cells from an established transgenic PML-RARA APL mouse model at the orthologous region on chromosome 12, including the CTCF binding site located upstream from the mouse miRNA cluster. These results, together with the demonstration that the region does not show DNA methylation changes during myeloid differentiation, provide evidence that 14q32 hypermethylation is implicated in the pathogenesis of APL. We propose a model in which loss of imprinting at the 14q32 domain leads to overexpression of the miRNAs in APL.

Acute myeloid leukemia does not deplete normal hematopoietic stem cells but induces cytopenias by impeding their differentiation.

Acute myeloid leukemia (AML) induces bone marrow (BM) failure in patients, predisposing them to life-threatening infections and bleeding. The mechanism by which AML mediates this complication is unknown but one widely accepted explanation is that AML depletes the BM of hematopoietic stem cells (HSCs) through displacement. We sought to investigate how AML affects hematopoiesis by quantifying residual normal hematopoietic subpopulations in the BM of immunodeficient mice transplanted with human AML cells with a range of genetic lesions. The numbers of normal mouse HSCs were preserved whereas normal progenitors and other downstream hematopoietic cells were reduced following transplantation of primary AMLs, findings consistent with a differentiation block at the HSC-progenitor transition, rather than displacement. Once removed from the leukemic environment, residual normal hematopoietic cells differentiated normally and outcompeted steady-state hematopoietic cells, indicating that this effect is reversible. We confirmed the clinical significance of this by ex vivo analysis of normal hematopoietic subpopulations from BM of 16 patients with AML. This analysis demonstrated that the numbers of normal CD34(+)CD38(-) stem-progenitor cells were similar in the BM of AML patients and controls, whereas normal CD34(+)CD38(+) progenitors were reduced. Residual normal CD34(+) cells from patients with AML were enriched in long-term culture, initiating cells and repopulating cells compared with controls. In conclusion the data do not support the idea that BM failure in AML is due to HSC depletion. Rather, AML inhibits production of downstream hematopoietic cells by impeding differentiation at the HSC-progenitor transition.

Trs20 is required for TRAPP II assembly.

The modular TRAPP complexes act as nucleotide exchangers to activate the Golgi Ypt/Rab GTPases, Ypt1 and Ypt31/Ypt32. In yeast, TRAPP I acts at the cis-Golgi and its assembly and structure are well characterized. In contrast, TRAPP II acts at the trans-Golgi and is poorly understood. Especially puzzling is the role of Trs20, an essential TRAPP I/II subunit required neither for the assembly of TRAPP I nor for its Ypt1-exchange activity. Mutations in Sedlin, the human functional ortholog of Trs20, cause the cartilage-specific disorder SEDT. Here we show that Trs20 interacts with the TRAPP II-specific subunit Trs120. Furthermore, the Trs20-Trs120 interaction is required for assembly of TRAPP II and for its Ypt32-exchange activity. Finally, Trs20-D46Y, with a single-residue substitution equivalent to a SEDT-causing mutation in Sedlin, interacts with TRAPP I, but the resulting TRAPP complex cannot interact with Trs120 and TRAPP II cannot be assembled. These results indicate that Trs20 is crucial for assembly of TRAPP II, and the defective assembly caused by a SEDT-linked mutation suggests that this role is conserved.

Regulation of selective autophagy onset by a Ypt/Rab GTPase module.

The key regulators of intracellular trafficking, Ypt/Rab GTPases, are stimulated by specific upstream activators and, when activated, recruit specific downstream effectors to mediate membrane-transport events. The yeast Ypt1 and its human functional homolog hRab1 regulate both endoplasmic reticulum (ER)-to-Golgi transport and autophagy. However, it is not clear whether the mechanism by which these GTPases regulate autophagy depends on their well-documented function in ER-to-Golgi transport. Here, we identify Atg11, the preautophagosomal structure (PAS) organizer, as a downstream effector of Ypt1 and show that the Ypt1-Atg11 interaction is required for PAS assembly under normal growth conditions. Moreover, we show that Ypt1 and Atg11 colocalize with Trs85, a Ypt1 activator subunit, and together they regulate selective autophagy. Finally, we show that Ypt1 and Trs85 interact on Atg9-containing membranes, which serve as a source for the membrane component of the PAS. Together our results define a Ypt/Rab module--comprising an activator, GTPase, and effector--that orchestrates the onset of selective autophagy, a process vital for cell homeostasis. Furthermore, because Atg11 does not play a role in ER-to-Golgi transport, we demonstrate here that Ypt/Rabs can regulate two independent membrane-transport processes by recruiting process-specific effectors.

A dual-channel electrochemical biosensor enables concurrent detection of pathogens and antibiotic resistance.

Diarrheagenic E. coli infections, commonly treated with ß-lactam antibiotics, contribute to antibiotic resistance - a pressing public health concern. Rapid monitoring of pathogen antibiotic resistance is vital to combat antimicrobial spread. Current bacterial diagnosis methods identify pathogens or determine antibiotic resistance separately, necessitating multiple assays. There is an urgent need for tools that simultaneously identify infectious agents and their antibiotic resistance at the point of care (POC). We developed an integrated electrochemical chip-based biosensor for detecting enteropathogenic E. coli (EPEC), a major neonatal diarrheal pathogen, using an antibody against a virulence marker, termed EspB, and the ß-lactam resistance marker, ß-lactamase. A dual-channel microfabricated chip, bio-functionalized with a specific EspB monoclonal antibody, and nitrocefin, a ß -lactamase substrate was utilized. The chip facilitated electrochemical impedance spectroscopy (EIS)-based detection of EspB antigen and EspB-expressing bacteria. For ß-lactam resistance profiling, a second channel enabled differential-pulse voltammetric (DPV) measurement of hydrolyzed nitrocefin. EIS-based detection of EspB antigen was calibrated (LOD: 4.3 ng/mL ±1 and LOQ: 13.0 ng/mL ±3) as well as DPV-based detection of the antibiotic resistance marker, ß-lactamase (LOD: 3.6 ng/mL ±1.65 and LOQ: 10 ng/mL ±4). The integrated EIS and DPV biosensor was employed for the simultaneous detection of EspB-expressing and ß-lactamase-producing bacteria. The combined readout from both channels allowed the distinction between antibiotic-resistant and -sensitive pathogenic bacteria. The integrated electrochemical biosensor successfully achieved simultaneous, rapid detection of double positive EspB- and ß-lactamase-expressing bacteria. Such distinction enabled by a portable device within a short assay time and a simplified sample preparation, may be highly valuable in mitigating the spread of AMR. This new diagnostic tool holds promise for the development of POC devices in clinical diagnosis.

Venetoclax-based low intensity therapy in molecular failure of NPM1-mutated AML.

ABSTRACT: Molecular failure in NPM1-mutated acute myeloid leukemia (AML) inevitably progresses to frank relapse if untreated. Recently published small case series show that venetoclax combined with low-dose cytarabine or azacitidine can reduce or eliminate measurable residual disease (MRD). Here, we report on an international multicenter cohort of 79 patients treated for molecular failure with venetoclax combinations and report an overall molecular response (≥1-log reduction in MRD) in 66 patients (84%) and MRD negativity in 56 (71%). Eighteen of 79 patients (23%) required hospitalization, and no deaths were reported during treatment. Forty-one patients were bridged to allogeneic transplant with no further therapy, and 25 of 41 were MRD negative assessed by reverse transcription quantitative polymerase chain reaction before transplant. Overall survival (OS) for the whole cohort at 2 years was 67%, event-free survival (EFS) was 45%, and in responding patients, there was no difference in survival in those who received a transplant using time-dependent analysis. Presence of FLT3-ITD mutation was associated with a lower response rate (64 vs 91%; P < .01), worse OS (hazard ratio [HR], 2.50; 95% confidence interval [CI], 1.06-5.86; P = .036), and EFS (HR, 1.87; 95% CI, 1.06-3.28; P = .03). Eighteen of 35 patients who did not undergo transplant became MRD negative and stopped treatment after a median of 10 months, with 2-year molecular relapse free survival of 62% from the end of treatment. Venetoclax-based low intensive chemotherapy is a potentially effective treatment for molecular relapse in NPM1-mutated AML, either as a bridge to transplant or as definitive therapy.

Long-term survival with low toxicity after allogeneic transplantation for acute myeloid leukaemia and myelodysplasia using non-myeloablative conditioning without T cell depletion.

The toxicity burden and long-term anti-leukaemic effect of non-myeloablative (NMA) allogeneic haematopoietic stem-cell transplantation (AHSCT) for acute myeloid leukaemia (AML) and myelodysplasia (MDS) remains undefined. We report the outcome of 56 patients with AML/MDS transplanted from human leucocyte antigen-matched donors using NMA conditioning without T-cell depletion. With a median follow-up of 5 years, treatment-related mortality was 9% and current disease-free survival (CDFS) was 45% (overall) and 55% (patients transplanted in remission). Development of graft-versus-host disease upon withdrawal of post-transplant immunosuppression was associated with less relapse and better CDFS. These data confirm that NMA AHSCT without T-cell depletion is safe and can result in sustained remissions of AML/MDS.

Differential and tumor-specific expression of CD160 in B-cell malignancies.

CD160 is a human natural killer (NK)-cell-activating receptor that is also expressed on T-cell subsets. In the present study, we examined 811 consecutive cases of B-cell lymphoproliferative disorders (B-LPDs), and demonstrated CD160 expression in 98% (590 of 600) of chronic lymphocytic leukemia (CLL) cases, 100% (32 of 32) of hairy cell leukemia (HCL) cases, 15% (5 of 34) of mantle cell lymphoma (MCL) in the leukemic phase, and 16% (23 of 145) of other B-LPD cases. CD160 transcript and protein were absent in the normal B-cell hierarchy, from stem cells, B-cell precursors, maturing B cells in the germinal center, and circulating B cells, including CD5(+)CD19(+) B1 cells in umbilical cord. CD160 positivity was significantly higher in CLL and HCL in terms of percentage (65.9% and 67.8%, respectively, P < .0001) and median fluorescence intensity (552 and 857, respectively, P < .0001) compared with all other B-LPD cases. Lymph node CLL samples were also CD160(+). Using the disease-specific expression of CD5, CD23, and CD160, a score of 3 characterized CLL (diagnostic odds ratio, 1430); a score of 0 excluded CLL, MCL, and HCL; and the CD23/CD5 ratio differentiated CLL from leukemic CD23(+) MCL. In the B-cell lineage, CD160 is a tumor-specific antigen known to mediate cellular activation signals in CLL, and is a novel target for therapeutic manipulation and monitoring of minimal residual disease.