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Adenosine-to-inosine RNA editing and the catalyzing enzyme adenosine deaminase are both essential for hematopoietic development and differentiation. However, the RNA editome during hematopoiesis and the underlying mechanisms are poorly defined. Here, we sorted 12 murine adult hematopoietic cell populations at different stages and identified 30 796 editing sites through RNA sequencing. The dynamic landscape of the RNA editome comprises stage- and group-specific and stable editing patterns, but undergoes significant changes during lineage commitment. Notably, we found that antizyme inhibitor 1 (Azin1) was highly edited in hematopoietic stem and progenitor cells (HSPCs). Azin1 editing results in an amino acid change to induce Azin1 protein (AZI) translocation to the nucleus, enhanced AZI binding affinity for DEAD box polypeptide 1 to alter the chromatin distribution of the latter, and altered expression of multiple hematopoietic regulators that ultimately promote HSPC differentiation. Our findings have delineated an essential role for Azin1 RNA editing in hematopoietic cells, and our data set is a valuable resource for studying RNA editing on a more general basis., (© 2021 by The American Society of Hematology.)

Erythropoiesis is a complex multistage process that involves differentiation of early erythroid progenitors to enucleated mature red blood cells, in which lineage-specific transcription factors play essential roles. Erythroid Krüppel-like factor (EKLF/KLF1) is a pleiotropic erythroid transcription factor that is required for the proper maturation of the erythroid cells, whose expression and activation are tightly controlled in a temporal and differentiation stage-specific manner. Here, we uncover a novel role of G-protein pathway suppressor 2 (GPS2), a subunit of the nuclear receptor corepressor/silencing mediator of retinoic acid and thyroid hormone receptor corepressor complex, in erythrocyte differentiation. Our study demonstrates that knockdown of GPS2 significantly suppresses erythroid differentiation of human CD34+ cells cultured in vitro and xenotransplanted in nonobese diabetic/severe combined immunodeficiency/interleukin-2 receptor γ-chain null mice. Moreover, global deletion of GPS2 in mice causes impaired erythropoiesis in the fetal liver and leads to severe anemia. Flow cytometric analysis and Wright-Giemsa staining show a defective differentiation at late stages of erythropoiesis in Gps2-/- embryos. Mechanistically, GPS2 interacts with EKLF and prevents proteasome-mediated degradation of EKLF, thereby increasing EKLF stability and transcriptional activity. Moreover, we identify the amino acids 191-230 region in EKLF protein, responsible for GPS2 binding, that is highly conserved in mammals and essential for EKLF protein stability. Collectively, our study uncovers a previously unknown role of GPS2 as a posttranslational regulator that enhances the stability of EKLF protein and thereby promotes erythroid differentiation., (© 2020 by The American Society of Hematology.)

Adult patients with disseminated nontuberculous mycobacterial (dNTM) infections usually have severe immune system defects. Recently, several studies have shown that anti-IFN-γ autoantibodies may play an important role in the pathogenicity of dNTM infections. A considerable proportion of reported cases of anti-IFN-γ autoantibodies show either clinical or laboratory evidence of autoimmune disease. In the present study, we identified 19 formerly healthy adults who later developed dNTM infections, of whom 17 were further investigated immunologically. High-titer anti-IFN-γ autoantibodies capable of inhibiting IL-12 production in vitro were found in the plasma of all of these patients. In addition to dNTM infection, 35% and 71% of our patients also suffered from salmonellosis and herpes zoster, respectively. This observation suggests that IFN-γ may be crucial in controlling salmonella infection and reactivating latent varicella-zoster virus infection in humans. 2 HLA alleles, DRB1*16:02 DQB1*05:02 (odds ratio 8.68; 95% confidence interval, 3.47-21.90; P = 1.1 × 10(-6); Pc = 3.08 × 10(-5) and odds ratio 7.16; 95% confidence interval, 3.02-17.05; P = 1 × 10(-7); Pc = 1.4 × 10(-6), respectively), were found in 82% (14 of 17) of our patients. In conclusion, our data suggest that anti-IFN-γ autoantibodies may play a critical role in the pathogenesis of dNTM infections and reactivation of latent varicella-zoster virus infection and are associated with HLA-DRB1*16:02 and HLA-DQB1*05:02.

A multicenter Eastern Cooperative Group (ECOG) phase 2 trial assessed whether adding prednisone to lenalidomide would improve previously reported responses in persons with myelofibrosis (MF). Forty-eight subjects with anemia (42 evaluable) received lenalidomide, 10 mg/d, with a 3-month low-dose prednisone taper. Ten subjects received 3 months, and 25 received 6 months of therapy. Myelosuppression was the main toxicity with 88% with ≥ grade 3 hematologic toxicity and 45% ≥ grade 3 nonhematologic toxicity. There were responses in 10 subjects (23%) using the International Working Group for Myelofibrosis Research and Treatment (IWG-MRT)-defined clinical improvement of anemia in 8 (19%) and/or decreased spleen size in 4 (10%). Serial bone marrow analysis showed no resolution of disease-related fibrosis or angiogenesis. With a median follow-up of 2.3 years, 23 subjects are alive. Lenali-domide and prednisone for myelofibro-sis evaluated through a multicentered-cooperative group mechanism is only modestly active and myelosuppre-sive. This study was registered at http://clinicaltrials.gov as NCT00227591.

The prognostic heterogeneity of the World Health Organization category of "systemic mastocytosis with associated clonal hematologic nonmast cell lineage disease" (SM-AHNMD) has not been systematically validated by primary data. Among 138 consecutive cases with SM-AHNMD, 123 (89%) had associated myeloid neoplasm: 55 (45%) myeloproliferative neoplasm (SM-MPN), 36 (29%) chronic myelomonocytic leukemia, 28 (23%) myelodysplastic syndrome (SM-MDS), and 4 (3%) acute leukemia. Of the myeloid subgroups, SM-MPN displayed a 2- to 3-fold better life expectancy (P = .003), whereas leukemic transformation was more frequent in SM-MDS (29%; P = .02). The presence of eosinophilia, although prevalent (34%), was prognostically neutral, and the overall results were not affected by exclusion of FIP1L1-PDGFRA-positive cases. We conclude that it is clinically more useful to consider specific entities, such as SM-MPN, systemic mastocytosis with chronic myelomonocytic leukemia, SM-MDS, and systemic mastocytosis with-acute leukemia, rather than their broad reference as SM-AHNMD.

Clinical phenotype in systemic mastocytosis (SM) is markedly variable, which complicates prognostication and decision making regarding the choice and timing of therapy. In a retrospective study of 342 consecutive adult patients with SM seen at the Mayo Clinic between 1976 and 2007, disease subdesignation according to the World Health Organization (WHO) proposal was indolent (ISM) in 159 (46%), with associated clonal hematologic non-mast cell lineage disease (SM-AHNMD) in 138 (40%), aggressive (ASM) in 41 (12%), and mast cell leukemia in 4 (1%). KITD816V was detected in bone marrow-derived DNA by allele-specific polymerase chain reaction (PCR) in 68% of 165 patients evaluated (ISM, 78%; ASM, 82%; SM-AHNMD, 60%; P = .03); JAK2V617F was detected in 4%, all in SM-AHNMD. Compared with those with nonindolent SM, life expectancy in ISM was superior and not significantly different from that of the age- and sex-matched US population. In addition, multivariable analysis identified advanced age, weight loss, anemia, thrombocytopenia, hypoalbuminemia, and excess bone marrow blasts as independent adverse prognostic factors for survival. The current study validates the prognostic relevance of the WHO subclassification of SM and provides additional information of value in terms of both risk stratification and interpretation of clinical presentation and laboratory results.

Among 2333 consecutive patients with myelofibrosis with myeloid metaplasia (MMM) seen at our institution, 91 fulfilled the World Health Organization (WHO) criteria for leukemic transformation (LT). All episodes of LT were myeloid in origin (acute myeloid leukemia [AML]) with all French-American-British (FAB) subtypes represented except M3; the most frequent subtypes were M7 (25.4%), M0 (22.4%), and M2 (17.9%). Cytogenetic studies during LT were available in 56 patients and revealed a clonal abnormality in 51 (91%): 30 patients had complex karyotype, 2 had core-binding factor gene lesions, and 18 had abnormalities of chromosome 5 or 7. Karyotypic evolution was documented in the majority of the patients in whom serial analysis was possible. In general, LT was fatal in 98% of the cases after a median of 2.6 months (range, 0-24.2 months). Twenty-four patients received AML-like induction chemotherapy that resulted in no complete remission: 41% reverted into chronic-phase disease and the incidence of treatment-related mortality was 33%. The remaining 67 patients received either supportive care alone (48 patients) or low-intensity chemotherapy (19 patients). Overall, survival was similarly poor in all 3 treatment categories. The outcome of LT in MMM with current therapies is dismal and either supportive care alone or appropriate clinical trials should be considered.

A novel oncogenic mutation (FIP1L1-PDGFRA), which results in a constitutively activated platelet-derived growth factor receptor-alpha (PDGFRA), has been invariably associated with a primary eosinophilic disorder. The current study examines both the prevalence and the associated clinicopathologic features of this mutation in a cohort of 89 adult patients presenting with an absolute eosinophil count (AEC) of higher than 1.5 x 10(9)/L. A fluorescence in situ hybridization (FISH)-based strategy was used to detect FIP1L1-PDGFRA in bone marrow cells. None of 8 patients with reactive eosinophilia displayed the abnormality, whereas the incidence of FIP1L1-PDGFRA in the remaining 81 patients with primary eosinophilia was 14% (11 patients). None (0%) of 57 patients with the hypereosinophilic syndrome (HES) but 10 (56%) of 19 patients with systemic mast cell disease associated with eosinophilia (SMCD-eos) carried the specific mutation. The bone marrow mast cell infiltration pattern in FIP1L1-PDGFRA(+) SMCD-eos was distinctly diffuse with loose tumoral aggregates. Treatment with low-dose imatinib (100 mg/d) produced complete and durable responses in all 8 FIP1L1-PDGFRA(+) cases treated. In contrast, only 40% partial response rate was seen in 10 HES cases. FIP1L1-PDGFRA is a relatively infrequent but treatment-relevant mutation in primary eosinophilia that is indicative of an underlying systemic mastocytosis.

Imatinib mesylate is effective in the treatment of hematologic malignancies that are characterized by either abl- or PDGFR beta- activating mutations. The drug is also active in a subset of patients with eosinophilic disorders and systemic mast cell disease (SMCD). Recently, a novel tyrosine kinase that is generated from fusion of the Fip1-like 1 (FIP1L1) and PDGFR alpha (PDGFRA) genes has been identified as a therapeutic target for imatinib mesylate in hypereosinophilic syndrome (HES). We used fluorescence in situ hybridization (FISH) to detect deletion of the CHIC2 locus at 4q12 as a surrogate for the FIP1L1-PDGFRA fusion. CHIC2 deletion was observed in bone marrow cells for 3 of 5 patients with SMCD associated with eosinophilia. Deletion of this locus and expression of the FIP1L1-platelet-derived growth factor receptor alpha (PDGFRA) fusion was also documented in enriched eosinophils, neutrophils, or mononuclear cells by both FISH and reverse transcriptase-polymerase chain reaction (RT-PCR) for one patient. While all 3 patients with the FIP1L1-PDGFRA rearrangement achieved a sustained complete response with imatinib mesylate therapy, the other two, both carrying the c-kit Asp816 to Val (Asp816Val) mutation, did not. These observations suggest that the FIP1L1-PDGFRA rearrangement occurs in an early hematopoietic progenitor and suggests that the molecular pathogenesis for a subset of SMCD patients is similar to that of HES. Screening for the FIP1L1-PDGFRA rearrangement and Asp816Val mutation will advance rational therapy decisions in SMCD.

Imatinib mesylate (Gleevec), a small molecule inhibitor of abl, kit, and platelet-derived growth factor receptor (PDGFR) tyrosine kinases, has been reported to be effective in the treatment of hypereosinophilic syndrome (HES) and a rare eosinophilia-associated chronic myeloid disorder (eos-CMD) characterized by the t(5;12)(q33;p13) cytogenetic abnormality. In the current study, we sought to confirm the preliminary observations in HES as well as evaluate the therapeutic value of imatinib in eos-CMD that is not associated with t(5;12)(q33;p13). Five patients with HES (all men, median age = 46 years) and 2 with eos-CMD (both men, aged 45 and 58 years) were treated with imatinib at a starting dose of 100 to 400 mg/day. Cytogenetic studies showed no evidence of either the bcr-abl translocation or t(5;12)(q33;p13) in any patient. Screening of exons encoding the intracellular catalytic domains and extracellular ligand binding domains of PDGFR beta (exons 2-23) and c-kit (exons 1-21) in 6 patients demonstrated mostly previously known polymorphisms. At a median follow-up of 17 weeks (range, 10-33 weeks), 2 patients with HES and 1 with eos-CMD have achieved complete clinical remission and 1 additional patient with HES has achieved a partial remission. In contrast to previous observations, all 4 responding patients had elevated serum interleukin-5 levels. Although the drug was well tolerated in most patients, a previously unrecognized treatment toxicity of acute left ventricular dysfunction occurred in a responding patient with HES within the first week of treatment. Myocardial biopsy revealed eosinophilic infiltration and degranulation, and the cardiogenic shock was reversed with the prompt institution of corticosteroid therapy.

Single-agent thalidomide (THAL) at "conventional" doses (> 100 mg/d) has been evaluated in myelofibrosis with myeloid metaplasia (MMM) based on its antiangiogenic properties and the prominent neoangiogenesis that occurs in MMM. THAL monotherapy at such doses produces approximately a 20% response rate in anemia but is poorly tolerated (an adverse dropout rate of > 50% in 3 months). To improve efficacy and tolerability, we prospectively treated 21 symptomatic patients (hemoglobin level < 10 g/dL or symptomatic splenomegaly) with MMM with low-dose THAL (50 mg/d) along with a 3-month oral prednisone (PRED) taper (beginning at 0.5 mg/kg/d). THAL-PRED was well tolerated in all enrolled patients, with 20 patients (95%) able to complete 3 months of treatment. An objective clinical response was demonstrated in 13 (62%) patients, all improvements in anemia. Among 10 patients who were dependent on erythrocyte transfusions, 7 (70%) improved and 4 (40%) became transfusion independent. Among 8 patients with thrombocytopenia (platelet count < 100 x 10(9)/L), 6 (75%) experienced a 50% or higher increase in their platelet count. In 4 of 21 patients (19%), spleen size decreased by more than 50%. Responses observed were mostly durable after discontinuation of the PRED. The dose of THAL in this study (50 mg/d) was better tolerated than the higher doses used in previous studies. Adverse events associated with corticosteroid therapy were mild and transient. Clinical responses did not correlate with improvements in either intramedullary fibrosis or angiogenesis. THAL-PRED is well tolerated and preliminarily appears to be a promising drug regimen for treating cytopenias in patients with MMM.

Although numerous reports indicate that patients receiving autotransplants for lymphoma are at increased risk for myelodysplastic syndrome (MDS)/acute myeloid leukemia (AML), the separate contributions of pretransplantation- and transplantation-related therapy are not well characterized. We conducted a case-control study of 56 patients with MDS/AML and 168 matched controls within a cohort of 2 739 patients receiving autotransplants for Hodgkin disease or non-Hodgkin lymphoma at 12 institutions (1989-1995). Detailed abstraction of medical records was undertaken to determine all pre- and posttransplantation therapy, and transplantation-related procedures. In multivariate analyses, risks of MDS/AML significantly increased with the intensity of pretransplantation chemotherapy with mechlorethamine (relative risks [RRs] = 2.0 and 4.3 for cumulative doses < 50 mg/m2 and > or = 50 mg/m,2 respectively; trend over dose categories, P =.04) or chlorambucil (RRs = 3.8 and 8.4 for duration < 10 months or > or = 10 months, respectively; trend, P =.009), compared with cyclophosphamide-based therapy. Transplantation-conditioning regimens including total-body irradiation (TBI) at doses 12 Gy or less did not appear to elevate leukemia risk (RR = 1.3; P =.48) compared with non-TBI regimens; however, a statistically significant increased risk was found for TBI doses of 13.2 Gy (RR = 4.6; P =.03). Peripheral blood stem cells were associated with a nonsignificant increased risk of MDS/AML (RR = 1.8; P =.12) compared with bone marrow grafts. Our data show that type and intensity of pretransplantation chemotherapy with alkylating agents are important risk factors of MDS/AML following autotransplantation. Transplantation-related factors may also modulate this risk; however, the apparent contribution of high-dose TBI requires confirmation.

Despite its wide use as a marker for hematopoietic stem cells (HSCs), the function of stem cell antigen-1 (Sca-1) (also known as lymphocyte activation protein-6A [Ly-6A]) in hematopoiesis remains poorly defined. We have previously established that Sca-1(-/-) T cells develop normally, although they are hyperresponsive to antigen. Here, we report detailed analysis of hematopoiesis in Sca-1-deficient animals. The differentiation potential of Sca-1-null bone marrow was determined from examination of the most mature precursors (culture colony-forming units [CFU-Cs]) to less committed progenitors (spleen CFUs [CFU-Ss]) to long-term repopulating HSCs. Sca-1-null mice are mildly thrombocytopenic with a concomitant decrease in megakaryocytes and their precursors. Bone marrow cells derived from Sca-1(-/-) mice also have decreased multipotential granulocyte, erythroid, macrophage, and megakaryocyte CFU (GEMM-CFU) and CFU-S progenitor activity. Competitive repopulation assays demonstrated that Sca-1(-/-) HSCs are at a competitive disadvantage compared with wild-type HSCs. To further analyze the potential of Sca-1(-/-) HSCs, serial transplantations were performed. While secondary repopulations using wild-type bone marrow completely repopulated Sca-1(-/-) mice, Sca-1(-/-) bone marrow failed to rescue one third of lethally irradiated wild-type mice receiving secondary bone marrow transplants from irradiation-induced anemia and contributed poorly to the surviving transplant recipients. These data strongly suggest that Sca-1 is required for regulating HSC self-renewal and the development of committed progenitor cells, megakaryocytes, and platelets. Thus, our studies conclusively demonstrate that Sca-1, in addition to being a marker of HSCs, regulates the developmental program of HSCs and specific progenitor populations.

The lack of diagnostic certainty in some patients makes it difficult to distinguish between primary and secondary forms of thrombocytosis. To augment current diagnostic studies for thrombocytosis, we retrospectively evaluated clinical records and bone marrow trephine specimens of 183 patients with thrombocytosis-164 with essential thrombocythemia (ET), 19 with reactive thrombocytosis (RT)-for bone marrow angiogenesis, bone marrow megakaryocyte c-Mpl staining, and morphologic evidence of megakaryocyte proliferation. Angiogenesis was increased in patients with ET compared with healthy controls (P <.0001) and patients with RT (P =.006). In addition, an increase in angiogenesis was associated with certain disease features such as splenomegaly (P =.004) and reticulin fibrosis (P =.005). Decreased megakaryocyte c-Mpl staining was observed in a heterogeneous pattern in ET compared with healthy controls (P <.0001) and RT (P <.0001). Histologic stratifying criteria incorporating increased angiogenesis, decreased megakaryocyte c-Mpl expression, and marked megakaryocyte proliferation in the bone marrow was highly sensitive (97%) and specific (95%) for distinguishing ET from RT (P <.0001). However, with the current duration of follow-up available on the patients, none of the histologic features evaluated have yet demonstrated prognostic value for subsequent clinical course, vascular events, or survival.

Patients with myelofibrosis with myeloid metaplasia (MMM) often experience debilitating constitutional symptoms such as drenching night sweats, profound fatigue, unexplained fevers, and unintentional weight loss. Tumor necrosis factor (TNF) contributes to organ fibrosis and hypercatabolic symptoms in a variety of disease states. We conducted an open-label pilot study of etanercept, a soluble TNF receptor, administered at a dose of 25 mg subcutaneously twice weekly for up to 24 weeks in 22 patients with MMM. Of 20 evaluable patients, 12 (60%) experienced an improvement in constitutional symptoms, and 4 (20%) had an objective response (improvement in peripheral cytopenias or spleen size). The degree of marrow fibrosis was unchanged, and only minor changes in overall marrow cellularity were observed. Toxicity was mild, with injection site reactions (20%) and minor infections (10%) as the most common side effects. One patient developed reversible pancytopenia. Etanercept may be useful for palliation of constitutional symptoms in MMM.

Acquired pure red cell aplasia (PRCA) can be associated with lymphoproliferative disease of granular T lymphocytes (T-LDGL), also known as T-cell large granular lymphocyte leukemia. Fifteen adult patients with PRCA associated with T-LDGL comprise this study. Neutropenia and rheumatoid arthritis were uncommon. All patients responded to immunosuppressive therapy. The 2 most commonly used treatments were prednisone and cyclophosphamide +/- corticosteroids, producing overall response rates of 50% and 60%, respectively. Treatment with cyclophosphamide was associated with a more durable remission (median, 60 versus 7.5 months). After a median follow-up of 67 months, 2 patients died of treatment-related complications, one from myelodysplasia and another from cyclosporine-induced renal failure. The clinical course and treatment responses of PRCA associated with T-LDGL in this series were similar to the general group of PRCA. Because T-LDGL is frequently underdiagnosed, it is likely that a significant proportion of idiopathic or primary PRCA is in fact secondary to T-LDGL.

Splenic extramedullary hematopoiesis is an integral component of myelofibrosis with myeloid metaplasia (MMM) and may be classified into 3 distinct histologic patterns of infiltration by myeloid precursors: diffuse, nodular, and a predominance of immature granulocytes. These 3 histologic patterns occurred in 121 (56.8%), 75 (35.2%), and 17 (8%), respectively, of 213 patients with MMM who underwent splenectomy at a single institution. In general, karyotypic findings in splenic tissue (n = 92) were similar to those seen in the bone marrow. The histologic pattern of immature granulocyte predominance, the presence of microscopic splenic infarcts (26 patients), or the detection of an abnormal splenic karyotype (52 patients) was significantly associated with decreased postsplenectomy survival. These adverse features were also associated with characteristics of advanced disease. These observations support the bone marrow origin of the myeloid progenitor pool in the spleen of patients with MMM and suggest a prognostic value for splenic histopathology and karyotype. (Blood. 2001;97:3665-3667)

Lymphoproliferative disease of granular T lymphocyte (T-LDGL), also known as T-cell large granular lymphocyte leukemia, is a clonal disorder of cytotoxic T lymphocytes that is clinically manifested as chronic neutropenia and anemia. Association with autoimmune disorders is common. In 9 patients, T-LDGL is reported as presenting as aplastic anemia. The clinical characteristics were similar to acquired aplastic anemia. Morphologic evidence of increased granular lymphocytes in the peripheral blood and an excess of CD3(+)/CD8(+)/CD57(+) cells in the bone marrow were found in most cases. Cyclophosphamide was ineffective, but noncytotoxic immunosuppressive agents generally produced a good response. After a median follow-up of 49 months, 5 patients had died from the disease or related complications. Median survival was 40 months. Aplastic anemia can be a presenting manifestation of T-LDGL, and T-LDGL should be considered in the differential diagnosis of acquired aplastic anemia.

Recent studies have shown decreased megakaryocyte expression of the thrombopoietin receptor (c-mpl) in patients with polycythemia vera (PV) but not in those with reactive erythrocytosis. We examined the diagnostic utility of this observation in 22 patients with PV, 7 patients with secondary erythrocytosis (SE), and 10 normal controls. Commercial antibodies against c-mpl were used with standard immunoperoxidase methods. Megakaryocyte c-mpl staining intensity was uniformly moderate-to-strong in the healthy controls and in all the patients with SE. In contrast, staining intensity in 9 patients with PV (41%) was uniformly weak. Furthermore, in 12 of the remaining 13 patients with PV, the c-mpl staining pattern in each case was heterogeneous and was associated with weak staining intensity in more than 20% of the megakaryocyte population. These preliminary data suggest that c-mpl immunostains may complement bone marrow histopathology in distinguishing PV from nonclonal causes of erythrocytosis. (Blood. 2000;96:771-772)

This study evaluated the contributing roles of flow cytometric immunophenotyping of blood and bone marrow and immunohistochemical paraffin section staining of bone marrow biopsies in the staging of B-cell malignant lymphoma. Flow immunophenotyping was performed on a marrow specimen in 175 cases; a corresponding blood specimen was also immunophenotyped in 135 of these cases. Morphologic marrow involvement by lymphoma was found in 59 cases; flow immunophenotyping identified 54 cases with a monoclonal B-cell process: morphology-positive/flow-positive (n = 49), morphology-positive/flow-negative (n = 10), morphology-negative/flow-positive (n = 5), and morphology-negative/flow-negative (n = 111). The 10 morphology-positive/flow-negative cases included 5 follicular and 5 large-cell lymphomas with minimal marrow involvement. All 5 morphology-negative/flow-positive cases were from patients with large-cell lymphomas and bulky clinical disease. Because the blood contained the same B-cell clone in 2 of 2 morphology-negative/flow-positive cases studied, blood contamination of marrow may account for these findings. Blood flow cytometric immunophenotyping studies were positive in 32 cases; 30 had marrow involvement by morphology and were from patients with follicular, mantle cell, lymphoplasmacytic, small lymphocytic, or marginal zone lymphomas. From our results, we conclude that (1) bone marrow flow cytometric immunophenotyping is not a cost-effective replacement for good morphologic evaluation in lymphoma staging and that (2) a positive peripheral blood flow cytometric immunophenotyping study when performed in low-grade lymphomas correlates with marrow involvement.