Multidimensional flow cytometric blood cell differentiation without erythrocyte lysis.

Forward light scattering, orthogonal light scattering, and the fluorescence intensities of unlysed peripheral blood cells, labeled with CD45-phycoerythrin and the nucleic acid dyes LDS-751 and thiazole orange, were measured simultaneously, utilizing a flow cytometer. Erythrocytes, reticulocytes, platelets, neutrophils, eosinophils, basophils, monocytes, lymphocytes, nucleated erythrocytes, and immature nucleated cells occupied unique positions in the five-dimensional space created by the listmode storage of the five independent parameters. A software program was developed which identified and enumerated each of these cell populations. Platelets in this study were identified by LDS-751 staining, in addition to their forward and orthogonal light-scattering characteristics. Validation of this approach was obtained by demonstrating that all CD41- or CD42-expressing platelets also stained with LDS-751. Furthermore, the staining by LDS-751 did not change following platelet activation with ADP. The quantification of erythrocytes, platelets, neutrophils, eosinophils, monocytes, and lymphocytes correlated well with data obtained with a commercial hematology whole blood analyzer (H-1). Reproducibility of the identification of these populations was shown by repeated measurement of the same sample and by staining and analysis of multiple aliquots of identical blood samples. Stability studies demonstrated that 8 hours after blood collection, the number of damaged cells increased. This could be measured by a greater thiazole orange uptake by the damaged cells. This investigation demonstrates the feasibility of multidimensional flow cytometric blood cell differentiation for an automated whole blood cell analysis without the necessity of erythrocyte lysis. The ability to simultaneously identify reticulocytes, nucleated erythrocytes, and immature nucleated cells in one measurement is unique and promises to be a powerful tool for the assessment of abnormal blood samples.     

Five-dimensional flow cytometry as a new approach for blood and bone marrow differentials

We have used five independent variables on a flow cytometer to discriminate and to quantify the cellular components within both blood and bone marrow aspirates. The signals were stored in list mode by which a five-dimensional space was created. The cells--differentiated into: 1) erythrocytes, 2) reticulocytes, 3) nucleated erythroid cells, 4) platelets, 5) lymphocytes, 6) monocytes, 7) neutrophils, 8) eosinophils, and 9) immature leukocytes--had to meet unique criteria with regard to their characteristics in the created five-dimensional space in order to be classified in a specific cell category. Forward and orthogonal light-scattering signals were matched with three fluorescence variables to obtain discrimination without necessitating erythrocyte lysis. Thiazole orange (binding predominantly to RNA) and LDS-751 (principally detecting DNA) were used to differentiate erythrocytes, platelets, reticulocytes, and nucleated cells. A monoclonal antibody, CD45, conjugated with phycoerythrin, was used to aid in discriminating between lineages of nucleated cells.     

CD2 is expressed by a subpopulation of normal B cells and is frequently present in mature B-cell neoplasms

BACKGROUND: CD2 is expressed by T and natural killer (NK) cells and has been reported in T/NK cell lineage neoplasms as well as in immature B-lymphoblastic and myeloid leukemias. Although CD2+ B-cells have been identified in normal fetal and postnatal thymus, they have not been reported in adults. METHODS: We retrospectively reviewed flow cytometric immunophenotypic data on consecutive low-grade B-cell leukemias and lymphomas to investigate the frequency of CD2 expression. We also reviewed samples from normal healthy donors to determine whether there is a normal CD2+ B-cell population. RESULTS: CD2 expression (partial or complete) was observed in 13 of 83 (16%) chronic lymphocytic leukemias (CLL), 16 of 29 (55%) follicle center lymphomas (FCL), 3 of 12 (25%) hairy cell leukemias (HCL), 0 of 6 mantle cell lymphomas (MCL), 8 of 28 (29%) large cell lymphomas (LCL), and in 0 of 5 marginal zone/mucosa-associated lymphoid tissue lymphomas (MZL/MALT). We determined that 5.74 +/- 2.46% (mean +/- SD) of normal peripheral blood B cells and 6.48 +/- 1.62 % (mean +/- SD) of normal bone marrow B cells coexpress CD2. CONCLUSIONS: CD2 expression in B-cell neoplasia is a more prevalent phenomenon than previously appreciated. Normal CD2+ B-cell populations are observed in adults and may represent the nonmalignant counterpart of CD2+ B-cell neoplasms.     

Kinetics of megakaryocyte progenitor cells in idiopathic thrombocytopenic purpura

The number and proliferative state of megakaryocyte progenitor cells (CFU-Meg) were compared between 13 patients with idiopathic thrombocytopenic purpura (ITP) and hematologically normal controls. The mean frequency of CFU-Meg assayed by the plasma clot method was 27.8 +/- 12.2 (+/- SD)/2 x 10(5) bone marrow light-density cells for the ITP patients, which did not differ significantly from the control value of 31.9 +/- 16.1. The percentage of CFU-Meg in DNA synthesis estimated by the 3H-thymidine suicide technique was 41.3% +/- 9.2% in ITP, which was significantly greater than the control value of 27.1% +/- 7.4% (P less than 0.01). The megakaryocyte counts for histological sections prepared from bone marrow aspirates from the ITP patients and controls were 34.5 +/- 8.5/mm2 and 11.2 +/- 5.8/mm2, respectively, with the difference being highly significant (P less than 0.001). These results suggest that increased cycling activity in a quantitatively unchanged CFU-Meg pool may lead to increased megakaryocytes in the bone marrow of ITP patients.     

Peripheral blood reticulocytes and their reference range values for percentage, absolute count, and immature fraction in children, measured with flow cytometry

Reticulocyte count by manual method has been the assay traditionally used to evaluate the status of erythropoiesis in hematological disorders with disturbances in erythropoietic activity. However, due to its variability, it is rather a semiquantitative method. Automated reticulocyte counting based on flow cytometry has provided more objective and exact measure of reticulocytes. Besides traditional parameters, such as percentage and absolute number of reticulocytes, automatic reticulocyte counters can detect differences in the amounts of cellular RNA present in immature erythrocytes that reflect their maturational stages and evaluate indices of reticulocyte maturation such as RMI, HFR, IRF. These parameters can be used as the earliest signs of marrow engraftment after autologous or allogeneic bone marrow transplantation. Currently there is no strict agreement between various automated methods of reticulocyte evaluation. Additionally, lack of standardization and quality control materials for this assay compels determination of own, interlaboratory ranges of reference values for new proposed parameters. A group of 102 children aged from 3 months to 18 years with normal hematological parameters was examined. Samples of blood stained supravitally with thiazole orange were analyzed in a flow cytometer. Results for percentage, absolute number and immature reticulocyte fraction expressed as a mean +/- 2SD were: 2.00 +/- 1.56%, 88.8 +/- 68.94 x 10(3)/microliter, and 0.22 +/- 0.16, respectively. A poor correlation was found between IRF and other parameters, suggesting its independent role as a marker of erythropoietic activity. Automated reticulocyte counting will probably improve the diagnosis and monitoring of many hematological diseases.     

Discriminating between damaged and intact cells in fixed flow cytometric samples

A vital, nucleic acid stain (LDS-751) was used to discriminate intact from damaged cells in a flow cytometer even after the samples had been fixed with paraformaldehyde. Three major cell populations with different fluorescence properties with LDS-751 were found in the fixed samples. Cells not staining or only dimly staining with LDS-751 were identified as erythrocytes and platelets, respectively. Cells staining with intermediate amounts of LDS-751 were found to be intact cells, while cells intensively stained were identified as damaged cells. Confirmation of the identity of the populations was obtained by light microscopic examination of the sorted populations and by correlating the fluorescent signals of FDA and LDS-751 in nonfixed cell preparations. Agglutinated cells could also be identified by the increased fluorescent signal in the LDS-751 channel as compared with single cells. The spectral properties of this dye permit excitation at 488 nm with emission in the far red portion of the spectrum. This allowed two-color immunofluorescence to be combined with the intact/damaged cell discrimination on fixed samples. Therefore, intact single cells could be distinguished during flow cytometric analysis, increasing the accuracy of the immunofluorescence measurements. The visualization of the multidimensional data was facilitated using color to discriminate cell populations depicted in multiple perspectives.     

Detection of granulocyte reactive oxygen species formation in whole blood using flow cytometry.

We have developed a technique for analysis of granulocyte reactive oxygen species formation in whole blood using flow cytometry and two color immunofluorescence. This technique relies upon the use of specific fluorescent dye (LDS-751) to stain nucleated cells, eliminating erythrocytes from analysis. Using LDS-751, forward angle light scatter, and 90 degrees side scatter, a granulocyte gate, monocyte gate, and lymphocyte gate were identified. Analysis with multiple FITC conjugated monoclonal antibodies demonstrated greater than 95% purity of a flow cytometrically identified granulocyte population in whole blood without physical manipulation of the blood. Utilizing 2'7' dichlorofluorescein diacetate (DCFH-DA), we were able to measure granulocyte intracellular reactive oxygen species production. Dose response curves were obtained for the effect of granulocyte agonists phorbol myristate acetate, FMLP, and heat fixed Staphylococcus aureus on reactive oxygen species production. The techniques described in this paper should be useful for measuring granulocyte activation in vivo with flow cytometry.     

Simplified mouse peripheral reticulocyte micronucleus test with dimethylnitrosamine.

The induction of micronuclei by treatment with dimethylnitrosamine was evaluated and compared in peripheral blood and bone marrow cells of male CD-1 mice. Peripheral blood preparations were made on acridine orange (AO)-coated slides and scanned by fluorescence microscopy. A significant increase in micronuclei was observed 24 h after treatment in bone marrow polychromatic erythrocytes, and 24-48 h after treatment in peripheral reticulocytes. The peak frequency of micronuclei in peripheral reticulocytes was delayed by about 24 h relative to bone marrow polychromatic erythrocytes. This micronucleus test using peripheral blood was shown to be easy to do and as sensitive as the test using bone marrow cells. From this result, it is concluded that the method with AO-coated slides and peripheral blood is as suitable as bone marrow cells for the micronucleus assay.     

Synthesis and assembly of membrane skeletal proteins in mammalian red cell precursors

The synthesis of membrane skeletal proteins in avian nucleated red cells has been the subject of extensive investigation, whereas little is known about skeletal protein synthesis in bone marrow erythroblasts and peripheral blood reticulocytes in mammals. To address this question, we have isolated nucleated red cell precursors and reticulocytes from spleens and from the peripheral blood, respectively, of rats with phenylhydrazine-induced hemolytic anemia and pulse-labeled them with [35S]methionine. Pulse-labeling of nucleated red cell precursors shows that the newly synthesized alpha- and beta-spectrins are present in the cytosol, with a severalfold excess of alpha-spectrin over beta-spectrin. However, in the membrane-skeletal fraction, newly synthesized alpha- and beta-spectrins are assembled in stoichiometric amounts, suggesting that the association of alpha-spectrin with the membrane skeleton may be rate-limited by the amount of beta-spectrin synthesized, as has been shown recently in avian erythroid cells (Blikstad, I., W. J. Nelson, R. T. Moon, and E. Lazarides, 1983. Cell, 32:1081-1091). Pulse-chase experiments in the rat nucleated red cell precursors show that the newly synthesized alpha- and beta-spectrin of the cytosol turn over coordinately and extremely rapidly. In contrast, in the membrane-skeletal fraction, the newly synthesized polypeptides of spectrin are stable. In contrast to nucleated erythroid cells, in reticulocytes the synthesis of alpha- and beta-spectrins is markedly diminished compared with the synthesis and assembly of proteins comigrating with bands 2.1 and 4.1 on SDS gels. Thus, in nucleated red cell precursors, the newly synthesized spectrin may be attached to the plasma membrane before proteins 2.1 and 4.1 are completely synthesized and incorporated in the membrane.     

Differential recovery of polymorphonuclear neutrophils,B and T cell subpopulations in the thymus,bone marrow,spleen and blood of mice following split-dose polychemotherapy

In these studies, we examined the effect of a maximum-tolerated, split-dose chemotherapy protocol of cyclophosphamide, cisplatin, and 1,3-bis(2-chloroethyl)-1-nitrosourea carmustine on neutrophil and lymphocyte subpopulations in the peripheral blood (PBL), thymus, bone marrow and spleen. It was found that this protocol of polychemotherapy, modeled after the induction protocol used with autologous bone marrow transplantation for breast cancer, suppressed both B and T cell populations and T cell function at times when the absolute neutrophil count had returned to normal or supernormal numbers. In the peripheral blood, 7 days following initiation of chemotherapy, there was a twofold increase in the percentage of granulocytes as compared to the level in control animals on the basis of a differential count. The polymorphonuclear neutrophil (PMN) frequency in the bone marrow was increased on day 14 and statistically identical to that in control mice on all other days analyzed. In contrast to the bone marrow cells and PBL on day 7, the frequency of PMN in the spleen and thymus was depressed. B cells (B220+) were depressed in the PBL, spleen and bone marrow and took 18–32 days to return to their normal frequency, while the frequency of B cells in the thymus was increased owing to a loss of immature T cells. The percentage of CD3+ cells in the thymus, spleen and bone marrow was significantly increased and required 10–18 days to return to normal levels, while the absolute number of CD3+ cells in the blood varied around the normal value. The ratio of CD4+ to CD8+ cells in all the organs studied varied only slightly owing to a similar reconstitution of CD4+ and CD8+ cells. In contrast to the phenotypic recovery of the CD3+, CD4+ and CD8+ cells, the ability of the splenic lymphocytes to respond to concanavalin-A was depressed and remained depressed, despite the phenotypic reconstitution of the T cell subsets, on the basis of both percentage and absolute cell number. These results show a selective T and B cell depression following multi-drug, split-dose chemotherapy in tissue and blood leukocyte populations and a chronic depression in T cell function.     

Effect of blood transfusion on erythropoiesis of homozygotic beta-thalassemia patients

The influence of blood transfusion on erythropoiesis (bone marrow erythroblasts, peripheral blood erythroblasts and reticulocytes) has been studied in 20 non splenectomized homozygous beta thalassaemia patients aged 3 to 16 years and in 10 splenectomized patients aged 8 to 24 years affected with the same disease. The number of reticulocytes was the same in the two groups but the number of erythroblasts in the splenectomized group was higher than in the other group. There was no correlation between the erythroblasts and the reticulocytes of the peripheral blood on one hand and the haemoglobin level proceeding from the same sample on the other hand. In the non splenectomized group of patients, an inverse relationship was found between the percentage of bone marrow erythroblasts and the mean annual haemoglobin level (r = -0.71; p less than 0.01). These results demonstrate the effect of blood transfusion on the erythroid cell line in homozygous beta thalassaemia and the delay between the transfusions and the medullary erythroblastic response.     

The micronucleus assay using peripheral blood reticulocytes from mitomycin C- and cyclophosphamide-treated rats.

It used to be believed that the use of rat peripheral blood for the micronucleus assay would be difficult because micronucleated erythrocytes are captured and destroyed by the spleen quickly. We have applied an acridine orange (AO) supravital staining method to rat peripheral blood using AO-coated glass slides. Normal and splenectomized SD rats were treated once with mitomycin C (i.p.) or cyclophosphamide (p.o.), and 5 microliters of blood was collected at intervals from the tail vein between 0 and 72 h after treatment. For comparison, bone marrow cells were smeared conventionally 30 h after treatment. Although the frequencies of spontaneous and chemically induced micronucleated reticulocytes (MNRETs) from normal rats were lower on average in the highest dose group than those of splenectomized rats, the incidence of micronuclei among type I and II reticulocytes in normal rats at 48 h was almost identical to the incidence of RNA-containing erythrocytes with micronucleus in bone marrow. Thus, we suggest that rat peripheral reticulocytes can be used as target cells for the micronucleus assay.     

Differential Sensitivity of Mouse Hematopoietic Stem Cells to Merocyanine 540

In vivo and in vitro clonal assays of immature mouse blood cells showed that diffferent populations of hematopoietic progenitor cells differ considerably with respect to their sensitivity to photodynamic damages caused by the fluorescent dye Merocyanine 540. Late erythroid progenitors were the most sensitive cells followed in order of decreasing sensitivity by pluripotent stem cells, early erythroid progenitors, and granulocyte/macrophage progenitors. Only about 2%–4% of all nucleated marrow cells were stained with Merocyanine 540 which correlated well with current frequency estimates of progenitor cells in mouse bone marrow. Our findings indicate that the expression of Merocyanine binding sites is developmentally regulated and might, therefore, provide a useful molecular marker for blood cell differentiation and a basis for an effective purification of hematopoietic progenitor cells.     

CFU-F from dog marrow: a colony assay and its significance

A colony assay method is described for studying dog fibroblast colony development in marrow cells derived from resected ribs. The assay showed an increased number of fibroblast colony forming units (CFU-F) in cell suspensions prepared from resected ribs compared to cell suspensions prepared from bone marrow aspirates or from peripheral blood. A linear relationship between the number of cells plated and the number of fibroblastoid colonies was demonstrated in each case. The proportion of phagocytic cells was lower in cultures prepared from resected ribs than in those prepared from bone marrow aspirates. Staining for acid phosphatase and with sudan black showed differences between phagocytic cells and non-phagocytic fibroblasts. When left in plastic dishes for 2 hrs, 81% +/- 10% of the CFU-F adhered to the plastic dishes. Velocity sedimentation separation showed a modal sedimentation rate of 6.49 mm/h.     

Hematopoietic Stem/Progenitor Cell Sources to Generate Reticulocytes for Plasmodium vivax Culture

The predilection of Plasmodium vivax (P. vivax) for reticulocytes is a major obstacle for its establishment in a long-term culture system, as this requires a continuous supply of large quantities of reticulocytes, representing only 1–2% of circulating red blood cells. We here compared the production of reticulocytes using an established in vitro culture system from three different sources of hematopoietic stem/progenitor cells (HSPC), i.e. umbilical cord blood (UCB), bone marrow (BM) and adult peripheral blood (PB). Compared to CD34⁺-enriched populations of PB and BM, CD34⁺-enriched populations of UCB produced the highest amount of reticulocytes that could be invaded by P. vivax. In addition, when CD34⁺-enriched cells were first expanded, a further extensive increase in reticulocytes was seen for UCB, to a lesser degree BM but not PB. As invasion by P. vivax was significantly better in reticulocytes generated in vitro, we also suggest that P. vivax may have a preference for invading immature reticulocytes, which should be confirmed in future studies.     

Maturation-dependent adhesion of human B cell precursors to the bone marrow microenvironment

Murine B cell precursors can be induced to proliferate in culture if allowed to bind to bone marrow derived adherent cells prepared under specific conditions. We studied the binding of human B cell precursor subpopulations to various in vitro microenvironments to determine which conditions may potentially be suitable models for human B precursor differentiation. Using the markers CD10, CD34, and CD20, B lineage populations of increasing maturation were quantitated: CD10+/CD34+, CD10+/CD20-, CD10+/CD20+, and CD10-/CD20+ cells in marrow, and CD10-/CD20+ mature B cells in peripheral blood. The adhesion of subpopulations of blood and marrow-derived light density cells to adherent cell layers or matrix was studied following a 2-h incubation in 24-well plates. The absolute number of bound B lineage cells was determined by cell counts and flow cytometry analysis. The adherence of B lineage cells to passaged human marrow fibroblasts (BM-FB) was highest in the most immature CD10+/CD34+ cells (34.3 +/- 4.2%), decreasing steadily with each stage of maturation to the peripheral blood B cells (11.2 +/- 2.4%). Increased adhesion of CD10+ B cell precursors relative to CD10-/CD20+ marrow B cells was confirmed by adhesion studies using sorted cells. The two most immature B lineage cells (CD10+CD34+ and CD10+/CD20-) showed more adherence to BM-FB than any other cell type tested, except for monocytes. Only B lineage precursor cells, erythroid precursors and CD10-/CD34+ cells showed significantly greater binding to BM-FB than to plastic. B lineage precursors bound equally well to primary and passaged human marrow fibroblasts, but bound significantly less well to passaged human foreskin fibroblasts, primary human marrow stroma, extracellular matrix of marrow fibroblasts, or fibronectin. These results suggest that specific binding to marrow fibroblasts is part of the differentiation program of early B lineage precursors. This binding activity gradually and predictably decreases during B lineage differentiation, in contrast to expression of other binding receptors, such as LFA-1 and CD44, which increase during B lineage maturation.     

Concentrations of ciclosporin in allogeneic bone marrow recipients. Comparison of assay methods

Thirteen patients in complete remission from acute nonlymphoblastic leukaemia or in chronic phase of chronic myelocytic leukaemia were treated with total body irradiation, cyclophosphamide and allogeneic bone marrow transplantation (BMT). Ciclosporin (CS) was administered for the prevention and the treatment of Graft versus Host Disease. Blood concentrations of CS were determined by Radioimmunoassay (RIA) and by High Performance Liquid Chromatography (HPLC). Trough levels of CS in peripheral blood as measured by RIA exceeded HPLC derived levels in nearly all (56/58) samples with a ratio of RIA:HPLC ranging from 2.43 +/- 1.42 at day 12 to 3.65 +/- 1.86 at day 26 after BMT (means +/- SD). A comparable ratio was found as regards the peak concentrations of CS in peripheral blood. Neither the dose of CS (0.5-3.0 mg/kg/day intravenously; 3.0-5.0 mg/kg/day per os) nor the duration of treatment (12, 19, 26 or 33 days after start of CS) were a significant factor as regards the ratio between HPLC and RIA. Concentrations of CS were also determined in bone marrow nucleated cells at 1 hour after the drug infusion had started. Here the ratio of RIA versus HPLC varied upon the duration of CS treatment with a highest ratio of 8.75 +/- 8.74 at day 12 after BMT. Bone marrow levels corresponded well with blood trough concentrations (p less than 0.01). It is concluded that the concentrations of CS in blood and bone marrow as determined by RIA and HPLC differ significantly, though consistently. At present, no advantage can be attributed to either method of analysis for routine clinical monitoring, as long as detailed information on the immunosuppressive and the toxic characteristics of CS metabolites in humans is lacking.     

Neutrophil maturation and activation determine anatomic site of clearance from circulation

The long-term disposition of circulating neutrophils and the site of disappearance from circulation remain unclear. We investigated neutrophil localization in mice using (111)In-labeled murine peripheral blood neutrophils, mature bone marrow neutrophils, and peritoneal exudate neutrophils to track in vivo localization of these different cell populations. Infused peripheral neutrophils were found to localize equally between liver and marrow sites by 4 h (31.2 +/- 1.9 vs. 31.9 +/- 1.8%), whereas exudate neutrophils predominantly localized to liver (42.0 +/- 1.1%) and marrow-derived neutrophils to the marrow (65.9 +/- 6.6%) where they were found to localize predominantly in the hematopoietic cords. Stimulation of marrow neutrophils before infusion caused a shift in localization from marrow to liver, and subsequent induction of an inflammatory site after infusion and marrow sequestration led to remobilization of infused marrow neutrophils but not of peripheral neutrophils. These results indicate that the marrow participates in removing neutrophils from circulation, with evidence supporting both storage and perhaps disposal functions. Furthermore, models for circulating neutrophil homeostasis should consider that the site of retention is governed by the maturation and activation states of the cell.     

A novel cell surface antigen (T305) found in increased frequency on acute leukemia cells and in autoimmune disease states

Monoclonal antibody T305, prepared by immunizing mice with the T-ALL derived cell line RPMI-8402, immunoprecipitates a single chain glycoprotein with m.w. 160,000 daltons (under reducing conditions) or 180,000 daltons (under nonreducing conditions). In immunofluorescence assays, antibody T305 reacted with a subpopulation of T cells in normal blood (22 +/- 6%), thymus (28 +/- 11%), and lymph node (24 +/- 6%). Increased frequency of T cells reactive with antibody T305 was found in peripheral blood of patients with infectious mononucleosis (greater than 80%), graft-vs-host disease after bone marrow transplantation (65 +/- 11%), acquired immunodeficiency syndrome (53 +/- 12%). The T cells in synovial fluid of patients with rheumatoid arthritis had increased frequency of antibody T305 reactive cells (59 +/- 8%) as compared to their peripheral blood (18 +/- 7%). Two color immunofluorescent studies demonstrated that the T305+ T cells predominantly co-stained with antibody Leu 2a (suppressor/cytotoxic subset) in both normals and disease state blood. After cell sorting to obtain T305+ and T305- subpopulations, we demonstrated that a) natural killer and antibody-dependent cellular cytotoxicity activity in normal blood was in the T305+ but not T305- T cells; b) cytotoxic T cells induced by mixed lymphocyte reaction were predominantly T305+; c) T305- T cells could be induced in vitro to express T305 antigen by mitogens or allogeneic B cells; d) the DNA content of T305+ and T305- T cells in normal blood was similar (greater 95% of cells with G0/G1 level); e) after mitogen stimulation, T305 antigen induction on previously T305- cells occurs before S-phase; and f) significantly more [3H]-thymidine after mitogen stimulation was incorporated by originally T305- cells than by originally T305+ cells (p less than 0.001). The T305 antigen was not restricted to T cells because it was also found on myeloid precursors in bone marrow but was not present on polymorphonuclear leukocytes, red blood cells, platelets, muscle, liver, skin, kidney, lung, or brain. Antibody T305 was found on 24/25 cases of acute leukemia (6 T-ALL, 10/11 cALL, 7 AML, and 1 AMOL) but not on 18 cases of chronic leukemia (B-CLL, T-CLL, null CLL, CML). The importance of the T305 antigen is that it is present on a high number of T cells in certain autoimmune diseases and on virtually all acute leukemia cells. Its distribution on immature and in vitro activated cells suggests that it may represent a receptor for signals related to cellular replication or differentiation.