Phosphatidylinositol 3-kinases are involved in the all-trans retinoic acid-induced upregulation of CD38 antigen on human haematopoietic cells

All-trans retinoic acid (ATRA) is a specific inducer of CD38 antigen on marrow CD34+ cells as well as on blast cells in acute promyelocytic and myeloblastic leukaemia. The CD38 antigen contributes to the control of blast cell proliferation, and the upregulation of CD38 might constitute an element in the pathogenesis of retinoic acid syndrome. The aim of this study was to determine whether phosphoinositide 3-kinase (PI3-K) is involved in the modification of CD38 antigen expression on myeloid cells, as PI3-K plays a major role in the ATRA-induced granulocytic differentiation of HL-60 cells. We evaluated the effects of PI3-K inhibitors (wortmannin and LY294002) on the levels of CD38 antigen and mRNA in HL-60 and normal marrow CD34+ cells exposed to ATRA (1 micromol/l). The inhibitors prevented increase in CD38 mRNA expression and the overexpression of membrane CD38 antigen, without modification of the cytoplasmic level of this antigen. Interestingly, PI3-K activity was also necessary for CD38 expression on normal marrow CD34+ cells and for the ATRA-induced upregulation of CD157, a CD38-related antigen. In conclusion, PI3-K activity plays an essential role in the regulation of CD38 expression on human haematopoietic cells, and might constitute an interesting therapeutic target in haematological disorders involving CD38 overexpression.     

Cell surface expression of c-kit receptors by childhood acute myeloid leukemia blasts is not of prognostic value: a report from the Childrens Cancer Group

The prognostic significance of c-kit receptor expression on leukemic blast cells was determined in 122 children with acute myeloid leukemia (AML) entered onto Childrens Cancer Group protocol 213. Clinical and laboratory characteristics as well as outcome were analyzed according to the percentage of blast cells expressing c-kit receptors and the relative number of c-kit receptors per cell as determined by indirect immunofluorescence. c-kit receptor expression was strongly associated with the expression of the CD34 antigen. However, contrary to findings in adult patients with AML, c-kit receptor expression by childhood AML blast cells was not predictive of a poor response to therapy.     

All-trans retinoic acid promotes a differential regulation of adhesion molecules on acute myeloid leukaemia blast cells

Summary. In the present study we investigated the membrance expression of selectin ligands (CD15/Le^x, CDw65/VIM2, CD15s/sLe^x), β2 integrins (CD11a/LFA-1, CD11b/Mac-1) and CD45 phosphatase isoforms (CD45RA, CD45RO on leukaemic cells from 28 patients with acute myeloid malignancies cultured with and without all-trans retinioc acid (ATRA). Within each adhesion system, ATRA was bale to differentially regulate distinct molecules. Furthermore, it was able to exert effects specific for acute promyelocytic leukaemia (APL) blast cells, as well as to induce a series of non-cytotype-restricted phenotypic changes. An impressive feature of ATRA induction was a simultaneous increase in the expression of CD15, CDw65 and CD11b on leukaemic promyelocytes. The sialylated antigen CD15s, however, Showed results independent from the other two carbohydrates (CD15 and CDw65), suggesting a differential enzymatic regulation within the selectin ligands system.
In spite of the well-recognized expression of CD11a throughout all stages of normal myeloid differentiation, APL blast cells were found to virtually lack LFA-1 expression. Moreover, ATRA was unable to promote and up-regulation of this antigen in APL, while inducing a frequent down-modulation in non-APL cases constitutively expressing this antigen. In APL cases ATRA generated an asynchronous phenotype (CD15⁺, CDw65⁺, CD11b⁺, CD11a^-), undetectable on normally maturing myeloid cells, but consistent with the concept that incomplete differentation, in terms of surface molecule expression, can be sufficient to obtain therapeutic results.     

Differentiation Linked Expression of p28,33 (Ia-like) Structures on Human Leukaemic Cells

S ummary . A differentiation linked expression of 'Ia-like' (B cell associated) structures was detected by heterologous antisera against p28,33 antigen on human leukaemic cells in immunofluorescence test. A double fluorochrome immunofluorescence test was used to distinguish Ia⁺ ( p28,33 , ⁺) leukaemic cells from B lymphocytes. Leukaemic cells with varying intensities of anti-p28,33 staining were separated using the Fluorescence Activated Cell Sorter. The results indicate that p28,33 is very strongly expressed on undifferentiated blast cells from Philadelphia chromosome (Ph¹) positive 'lymphoid' blast crisis and non-T,non-B acute lymphoid leukaemia. In Ph¹ positive myeloid blast crisis and in acute myeloid leukaemia the p28,33 antigen was only moderately strongly expressed on myeloblasts and virtually absent on promyelocytes. The p28,33 antigen was also absent on the relatively more mature cells during the chronic phase of chronic myeloid leukaemia. Non-leukaemic samples such as bone marrow samples from fetus, and from patients with severe megaloblastic anaemia or myeloid hyperplasia (due to hypersplenism in Felty syndrome) were also found to contain p28,33 positive (and membrane immunoglobulin negative) immature blast cells.
On the basis of these observations the hypothesis is proposed that 'Ia-like' antigens on leukaemic cells reflect the origin of these leukaemias from haemopoietic stem cells which are themselves Ia positive. The corollary of this view is that 'Ia-like' membrane structures may play an important role not only in immunological responses of B cell, but also in the regulation of early haemopoietic cell differentiation.     

Differentiation patterns in the blastic phase of chronic myeloid leukemia

The surface antigen phenotype of 30 patients with the blast phase of chronic myeloid leukemia (CML) was determined using a panel of monoclonal antibodies recognizing differentiation antigens of normal myeloid, erythroid, megakaryocyte, and lymphoid cells. Ten patients' cells expressed a phenotype corresponding to an immature myeloid cell and were felt to have "myeloid" blast crisis. None of these myeloid leukemias were TdT+ or responded to vincristine (V) and prednisone (P). Eleven patients expressed a phenotype similar to acute lymphoblastic leukemia cells and probably reflect maturation to an early B lymphocyte. All of these "lymphoid" leukemias were TdT+, and 67% of evaluable patients had a complete response to V and P. One leukemia had the phenotype of an erythroleukemia, one patient's cells expressed the phenotype of megakaryoblastic leukemia, and one leukemia had populations of both myeloid and lymphoid blasts. Six leukemias did not express surface markers characteristic of any lineage and were termed "undifferentiated." This group was heterogeneous with respect to TdT expression, but no patient had a complete response to V and P. Determination of surface antigen phenotype in CML blast crisis thus provides clinically useful information for the structuring of treatment protocols.     

The expression of myeloid-specific antigens on myeloid leukemia cells: correlations with leukemia subclasses and implications for normal myeloid differentiation

The expression of three distinct myeloid-specific cell surface antigens detected by monoclonal antibodies (PMN 6, PMN 29, and AML-2-23) on acute and chronic myeloid leukemia cells is correlated with blast cell morphology and normal myeloid cell antigen display. In studies on normal peripheral blood cells, monoclonal antibodies PMN 6 and PMN 29 have previously been shown to react exclusively with neutrophils while AML-2-23 reacts with both neutrophils and monocytes. The present report demonstrates that these antigens are absent from blast cells of patients with acute myelocytic leukemia (AML) classified as M1 and M2 in the French-American-British system and chronic myelocytic leukemia in myeloid blast crisis. However, leukemia cells with myelomonocytic morphology (M4) expressed all three antigens, while cells with pure monocytic features (M5) were generally only positive for AML-2-23. Based on the absence of these antigens on both leukemic and normal myeloblasts and granulocyte-monocyte progenitors and their characteristic patterns of display on more differentiated leukemic and normal cells, we propose a modified concept of normal myelopoiesis. In this hypothesis, the myeloblast is an uncommitted cell that gives rise to a series of intermediate precursors that acquire committment to either the granulocytic or monocytic lineage marked by the acquisition of specific cell surface markers.     

Differential expression of CD54/intercellular adhesion molecule-1 in myeloid leukemias and in lymphoproliferative disorders

Indirect immunofluorescence staining with monoclonal antibody (MoAb) CL203.4 of malignant cells from 269 patients with hematologic malignancies showed a heterogeneous expression of CD54/intercellular adhesion molecule-1 (ICAM-1). This marker was expressed by malignant cells of 57 out of 118 patients with myeloid malignancies and 69 out of 135 with B-lymphoid malignancies. On the other hand, CD54 was not detected on malignant cells of 16 patients with T-lymphoid malignancies. In myeloid malignancies, CD54 is preferentially expressed by "stem cell-derived" malignancies, being detectable on blast cells from almost all patients affected by chronic myelogenous leukemia in blast phase or myelodysplastic syndromes and by only 34% of patients with de novo acute myeloid leukemia (AML). The expression of CD54 did not correlate with any specific myeloid FAB subtype, although three cases of highly undifferentiated AML (FAB MO) displayed maximal levels of the antigen. The expression of CD54 in AML was significantly associated with that of CD34 and HLA-DR antigens. In B-lymphoid malignancies, CD54 expression appears to correlate with the differentiation stage of malignant cells, since B-origin acute lymphoblastic leukemias and conventional B-chronic lymphocytic leukemias (B-CLL; ie, "dim SIg" CLL) expressed lower levels of CD54 than more mature lymphoproliferative disorders ("bright SIg" CLL, prolymphocytic leukemias, and lymphoplasmacytic tumors). "High-grade" B-cell non-Hodgkin's lymphomas (B-NHL) express in general a higher level of CD54 than "low-grade" ones. This finding in conjunction with the expression of CD54 in all 17 patients with "bright SIg" CLL investigated (characterized by marked organomegaly and poor prognosis) suggest that the differential expression of CD54 in lymphoproliferative disorders may also relate to their degree of malignancy.     

Absence of common ALL antigen on normal bipotent myeloid, erythroid, and granulocyte progenitors

The presence of the common acute lymphoblastic leukemia antigen (CALLA) on leukemic cells from the great majority of patients with non-T cell acute lymphoblastic leukemia and chronic myelogenous leukemia in blast crisis suggests that CALLA could be differentiation antigen expressed by normal lymphoid and myeloid stem cells. Treatment with a murine monoclonal anti-CALLA antibody and complement lysed CALLA-positive leukemic cells quantitatively, whereas similar treatment of nucleated cells from peripheral blood and bone marrow failed to affect the expression, in semisolid culture, of CFU-G/E, BFU-E, CFU-E, or CFU-C. These data suggest that CALLA is not a normal differentiation antigen of the myeloid bipotent cell or its committed progenitors.     

High CD33 expression levels in acute myeloid leukemia cells carrying the nucleophosmin (NPM1) mutation

The CD33 antigen is expressed on the blast cells of most cases of acute myeloid leukemia and represents a suitable tumor-associated target antigen for antibody-based therapies. The aim of this study was to investigate the relationship between the CD33 levels quantified by mean fluorescence intensity and antibody binding capacity, and the presence/absence of NPM1 and FLT3 gene mutations in 99 newly diagnosed acute myeloid leukemia cases. The CD33 intensity evaluated as mean fluorescence intensity and antibody binding capacity was significantly higher in the NPM1-mutated acute myeloid leukemia cases compared to the NPM1-unmutated cases (P=0.0001 and P=0.0088, respectively). On the contrary, FLT3 gene mutations did not influence the levels of CD33 expression on the leukemic cells. These results establish a rational basis for the therapeutic use of anti-CD33 antibodies in NPM1-mutated acute myeloid leukemia patients.     

Induction of differentiation in blast cells and leukemia colony-forming cells from patients with acute myeloid leukemia

The characteristic lesion in acute myeloid leukemia (AML) is the failure of myeloid cells to differentiate normally, leading to the accumulation of immature blast cells (BC) in the bone marrow. We determined whether BC and leukemia colony-forming cells (L-CFC) from AML patients could differentiate in vitro after short-term culture with interferon-gamma (IFN gamma), 1,25 dihydroxyvitamin D3 (D3), retinoic acid (RA), tumor necrosis factor-alpha (TNF alpha), and granulocyte-monocyte colony-stimulating factor (GM-CSF). Expression of myeloid differentiation antigens CD15, CD14, CD33, and p124 was determined on the BC by immunofluorescence and on the L-CFC by monoclonal antibody (MoAb) and complement (C')-mediated cytotoxicity followed by cloning in methylcellulose. We found that 26 of 39 (67%) cases demonstrated changes in the expression of myeloid differentiation antigens on the BC, and 6 of 7 (86%) cases showed an altered L-CFC myeloid antigen phenotype after short-term culture with differentiating agents. Alterations in myeloid antigen expression in the L-CFC population correlated with a reduction in L-CFC cloning potential. In the BC, alterations of myeloid differentiation antigens occurred in a manner consistent with those observed during normal myelopoiesis. For example, CD14 antigen expression (a late-stage monocyte antigen) increased on BC from 12 of 39 (31%) cases, and p124 (an antigen expressed both by myeloid progenitor cells and by a subset of monocytes) increased on 15 of 39 (38%) cases. Changes in the expression of CD33 antigens (expressed normally by myeloid progenitor cells and by mature monocytes) on the BC were variable, with 7 of 29 cases (24%) showing a decrease and 7 of 29 cases (24%) showing an increase. When comparisons were made between pairs of differentiation agents that caused the altered expression of an antigen on either the BC or L-CFC of a patient, the majority of changes were in the same direction (either both "increased" or both "decreased"). This suggests that the direction of antigen change is characteristic of the leukemia cell subpopulation for each patient and not of the stimulatory agent. This study demonstrates that cells from more than two thirds of AML cases examined responded to various differentiation agents in vitro as measured by changes in the expression of myeloid cell-associated surface antigens and by alterations in cloning potential of the L-CFC, a finding of potential clinical significance.     

Phenotypic conversion of TdT+ adult AML to CALLA+ ALL

A case of acute leukaemia is described in which the blast cells showed Sudan Black and terminal deoxynucleotidyl transferase (TdT) positivity at presentation and did not react with the monoclonal antibodies (McAb) My9 (anti-myeloid) and J5 (anti-CALL antigen). The myeloid lineage of these cells was confirmed by the myeloperoxidase (MPO) reaction at electron microscopic level. The patient entered complete remission but relapsed one year later with blasts showing negative Sudan Black and MPO reactions. These cells were still TdT+ but J5 was now positive. This case points to the existence of a clonogenic leukaemic cell with potential for evolution in both myeloid and lymphoid lineages. This is also suggested by the reactivity of both types of blast cells with the McAb 3C5 which recognises an antigen present both on early myeloid and lymphoid precursors. It is possible that patients with such leukaemias may benefit from combined therapy directed against cells of both myeloid and lymphoid lineages.     

Expression of Ia antigens on myeloid progenitor cells in chronic myeloid leukemia: direct analysis using partially purified colony- forming cells

The regulation of Ia (HLA-DR) antigen expression on myeloid progenitor cells may be closely related to the control of myelopoiesis in both normal individuals and chronic myeloid leukemia (CML) patients. In an effort to study directly the expression and behavior of Ia surface molecules on myeloid progenitor cells, we used an immunologic purification technique to enrich these cells approximately 100-fold from the peripheral blood of CML patients. The majority of cells in this blast population expressed HLA-DR antigens. Thirty percent to 40% of cells could form a granulocyte or monocyte colony in agar, and these cells tended to express the highest levels of HLA-DR. The number of HLA- DR molecules per cell increased about twofold as the cells tranversed the cell cycle from G0/G1 to G2/M. This was true for unstimulated cells or cells exposed to colony-stimulating factors. Some of this increase was related to a corresponding increase in cell size and is also seen with other cell surface antigens such as beta-2-microglobulin. Ia antigen expression was not modified by culture with colony-stimulating factors, fetal calf serum, or serum-free, prostaglandin-free medium for periods of up to 24 hours. These results demonstrate that Ia antigens are expressed on the myeloid progenitor cells of CML, are increased in the S and G2/M phases of the cell cycle, and are stable under most in vitro culture conditions for at least 24 hours of culture.     

急性粒细胞白血病CD33、CD45及MDR表达和靶抗原的筛选

目的 :观察急性粒细胞白血病 (AML)CD33、CD4 5及MDR表达和抗原强度的变化 ,筛选特异的靶抗原 ,探讨抗体靶向治疗的应用价值。方法 :应用间接免疫荧光法检测AML细胞CD33、CD4 5及MDR的表达和抗原强度的变化。结果 :CD33、CD4 5表达阳性率分别为 81.5 %、90 .6 % ,CD33抗原表达比CD4 5强 ,差异有显著性意义(P <0 .0 5 )。CD33与MDR在白血病细胞上表达差异无显著性意义。结论 :CD33特异性强 ,抗原性强 ,适合作靶抗原 ;MDR也是耐药白血病治疗的较好靶抗原。抗体靶向治疗将是AML治疗的一条新途径。     

Increased DR antigen expression by committed hemopoietic progenitor cells but not primitive progenitor cells in chronic myeloid leukemia

We used a complement-dependent cytotoxic assay to study the expression of DR antigens by hemopoietic progenitor cells derived from normal marrow and from the blood of patients with chronic phase chronic myeloid leukemia (CML). Almost all normal and CML day-12 granulocyte-macrophage colony-forming cells (d12 GM-CFC) and erythroid burst-forming units (BFU-E) expressed DR antigens. A primitive progenitor cell, the "blast colony-forming cell" (Bl-CFC), also expressed DR antigens whether derived from normal marrow or CML blood. The expression of DR antigen by normal and CML Bl-CFC was similar but the density of DR antigens on CML d12 GM-CFC and BFU-E was much greater than that of their counterparts from normal marrow. The similarity in DR antigen density on normal and CML Bl-CFC means that purging of CML marrow using a DR-specific antibody would not select in favor of normal cells and thus would not be useful in an autograft setting.     

Simultaneous expression of lymphoid and myeloid phenotypes in acute leukemia arising from myelodysplastic syndrome

A 57-year-old female developed myelodysplastic syndrome (MDS) that terminated as a biphenotypic leukemia after exposure to chemoradiotherapy. Double staining of blast cells, using monoclonal antibodies specific for myeloid and lymphoid lineage, demonstrated that one-third of the leukemic cells simultaneously expressed the E rosette-associated antigen (OKT11) and myeloid-associated antigen (MY7). This finding suggests the possibility that some cases of MDS are clonal disorders that arise in a pluripotent stem cell that can also differentiate to T cell lineage.     

Myeloid-associated antigen expression lacks prognostic value in childhood acute lymphoblastic leukemia treated with intensive multiagent chemotherapy

Frequency and clinical significance of myeloid-associated antigen expression in blast cells were assessed in 372 consecutive children with acute lymphoblastic leukemia (ALL). A comprehensive panel of myeloid monoclonal antibodies representing seven cluster groups showed myeloid-associated antigen expression in 61 cases (16.4%), 18 of which expressed two or more antigens. The antigens expressed comprised CD11b (8.9% of the total series), CD13 (6.5%), CD33 (3.2%), CD36 (1.9%), CD15 (1.6%), CD14 (1.3%), and CDw12 (1.1%). No significant associations were found between myeloid-associated antigen expression and the presence of known adverse prognostic features (eg, higher leukocyte count, nonwhite race, older age). Myeloid-associated antigen expression had no effect on remission induction or event-free survival for the 267 children who had been treated with the same combination of chemotherapeutic agents (P = .34). Thus, blast cell expression of myeloid-associated antigens in childhood ALL appears to lack prognostic value in the context of contemporary intensive chemotherapy.     

Expression of lymphoid-associated cell surface antigens by childhood acute myeloid leukemia cells lacks prognostic significance.

The prognostic significance of cell surface antigens associated with lymphoid and myeloid lineage differentiation on the blasts of children with acute myeloid leukemia (AML) was evaluated. Leukemic blasts from 176 patients enrolled on Childrens Cancer Study Group Protocol 213 determined to have AML by standard morphologic and cytochemical criteria were immunophenotyped. Cell surface antigens associated with myeloid differentiation were found on blasts from 88.1% of patients (CD15, 44%; CD33, 65%; CD36, 53%; glycoprotein Ib, 9.3%). However, blasts from 30.7% of patients expressed surface antigens thought to be specific for lymphoid lineage differentiation (CD2, 9.4%; CD5, 2.7%; CD19, 34.5%; CD20, 0.8%). In addition, CD34, a glycoprotein found on immature cells of both myeloid and lymphoid lineages, was expressed on the blast cells of 48.2% of patients. With the exception of the lymphoid lineage nonspecific antigen CD4, no correlation was found between white blood cell count at diagnosis, age, and French-American-British morphology, and the expression of any of the lymphoid- or myeloid-associated cell surface antigens studied. Nor was the expression of these antigens prognostically significant with respect to response to induction therapy, death during induction, survival, event-free survival, or survival/event-free survival following remission induction. Multivariate analysis showed that CD4 expression was not an independent predictor of outcome. Thus, this prospective study suggests that expression of lymphoid-associated cell surface antigens as well as myeloid-associated antigens by childhood AML blasts lacks prognostic significance.     

Surface antigens on normal and leukaemic human cells detected by monoclonal antibodies

Surface antigens were analyzed on normal human marrow and chronic myeloid leukaemic cells using 4 monoclonal mouse anti-human antibodies. The fluorescence-activated cell sorter was used to quantify the binding of each antibody to different subpopulations of cells, and sorted fractions were cultured in agar-medium to assay for granulocyte-macrophage and eosinophil precursors. All cells in the granulocyte series including colony-forming cells bound a similar quantity of an antibody to the human leucocyte common antigen. This antibody did not bind to cells in the erythroid series. A monoclonal antibody to antigen present on brain, lymphocytes and granulocytes (and almost certainly homologous to the W3/13 antigen of the rat) bound to the cells in the order: blast greater than promyelocytes and myelocytes greater than granulocytes. The third monoclonal antibody was directed against a determinant of the leucocyte common antigen present predominantly on B lymphocytes and absent from the myeloid series. The fourth antibody, directed against the human homologue of Thy-1, reacted with less than 1% of marrow cells, none of which appeared to be granulocyte or eosinophil progenitors. The leucocoyte common antigen and the brain-lymphocyte-granulocyte-antigen were also present on colony- and cluster-forming cells from a patient with chronic myeloid leukemia. Using the low angle and wide angle light scatter properties of CML blood cells, 7-fold enrichment was obtained for progenitor cells from chronic myeloid leukaemia. With the monoclonal antibodies up to 4-fold enrichment was obtained.     

Possible mechanism of selective killing of myeloid leukemic blast cells by lymphokine-activated killer cells.

Major histocompatibility complex-unrestricted lymphokine-activated killer (LAK) cells have been proposed as therapy for a variety of hematologic malignancies. Because these cells recognize and kill their targets independently of their antigen specific CD3 receptor, it is unclear how they might discriminate between normal and malignant cells. We now propose one such mechanism for the selective killing of myeloid leukemia blasts. While both CD2+ and CD2- activated killer cells may inhibit the clonogenic growth of myeloid leukemia cells, only the CD2+ subset effectively inhibits the growth of normal myeloid (granulocyte-macrophage and granulocyte) progenitors. This difference appears to reflect differential requirements for cell adhesion molecule recognition between normal and malignant progenitor cells. Inhibition of the growth of normal granulocyte-macrophage colonies by CD2+ LAK cells is blocked by antibodies to the CD2-lymphocyte function-associated antigen 3 (LFA-3) (CD58) cell adhesion system. In contrast, these antibodies have no effect on CD2+ LAK-mediated inhibition of malignant cell clonogenic growth. Instead, antibodies to the LFA-1 (CD11a/CD18)-intercellular adhesion molecule 1 (ICAM-1) (CD54) adhesion system reduce inhibition. These differences correspond to differential expression of the CD54 cell adhesion molecule by normal and malignant myeloid progenitor cells because less than 15% of normal CD34 positive cells are CD54+ while greater than 85% of CD34+ acute myeloid leukemia blasts express the CD54 antigen. LFA-3, the ligand for CD2, is strongly expressed by erythrocytes, and these cells competitively inhibit killing of normal but not malignant clonogenic cells in an analogous way to the effects of monoclonal antibody to the CD2-LFA-3 adhesion system. The operation of this effect in vivo may be a basis for selective cytotoxicity by CD2+ LAK against clonogenic myeloid blast cells, and could be exploited further with infusion of appropriate monoclonal antibodies.     

Nonhistone protein antigen profiles of five leukemic cell lines reflect the extent of myeloid differentiation

The human leukemic cell lines, K562, KG-1, and HL-60, and the blast subclones, KG-1a and HL-60 blast, were utilized to relate differences in nonhistone protein antigens to stages of myeloid cell differentiation. Chromatin proteins were separated on SDS- polyacrylamide gels, transferred electrophoretically to nitrocellulose sheets, and visualized by the peroxidase-antiperoxidase method of Sternberger. Screening with antisera raised against total and dehistonized chromatin and a nuclear extract from these cells revealed quantitative as well as qualitative differences between the cell lines. A decrease in antigen content seemed to parallel progressive stages of myeloid cell development. The results indicate that a number of chromosomal protein antigens are lost or modified during differentiation. An antigen(s) of approximately 55,000 molecular weight was found in HL-60 chromatin, but was not present in its less differentiated subclone or in the other lines representative of earlier stage cells. Upon the induction of HL-60 cells to mature to end stages with 4 microM retinoic acid, a significant increase in the mol wt 55,000 activity was seen. This antigen was detected only with antisera against HL-60 total chromatin and granulocyte nuclei, and it was found only in normal mature granulocytes and in the later stage cells of the HL-60 culture. Thus, the antigen appears to be associated with a differentiated myeloid function.