Murine pluripotential stem cells lack Ia antigen

Mapping cell surface antigen expression of haemopoietic progenitor cells using monoclonal antibodies

A library of monoclonal antibodies which show selective reactivity with particular cells or gene products (e.g. HLA-DR, glycophorin) of different cell lineages in the haemopoietic system has been compiled. Using these probes in conjunction with the fluorescence-activated cell sorter (FACS) the pattern and sequence of cell surface antigenic expression on haemopoietic progenitor cells have been mapped. A cell is identified in bone marrow which has a unique membrane phenotype and the nuclear enzyme terminal deoxynucleotidyl transferase. Its composite phenotype is identical to that seen in the common variant of acute lymphoblastic leukaemia (ALL). It is suggested that this cell is a putative B lineage progenitor which provides the major target for ALL. Detailed analysis of erythroid differentiation with monoclonal antibodies on the FACS reveals an intriguing pattern of antigenic expression in which HLA-DR, glycophorin and band III appear in sequence. HLA-DR (Ia-like antigen) may be present on all or most committed haemopoietic progenitor cells and could play an important role in cell interactions regulating early haemopoiesis.

Absence of major histocompatibility class II expression does not impair hematopoiesis in mice

OBJECTIVE

Major histocompatibility class II (MHC II) molecules are among the earliest antigens to be expressed in hematopoietic progenitor cells; however, the functional role of these molecules in hematopoiesis remains controversial. We examined the role of MHC II antigens during hematopoiesis using a mouse model of MHC II deficiency related to the absence of the critical transcriptional activator, CIITA.

METHODS

Sca-1(-), Sca-1(+)lin(+), and Sca-1(+)lin(-) populations of marrow cells from CIITA(-)(/-) and wild-type mice were analyzed by immunofluorescence for MHC II expression. Hematopoietic capacity was assessed in CIITA(-/-) and wild-type mice by CFU-S, CFU-GM, and radiation sensitivity assays.

RESULTS

Flow cytometric characteristics of hematopoietic progenitors from CIITA(-/-) and wild-type mice were identical except for the absence of MHC II expression in CIITA null mice. There were no significant differences in capacity for hematopoietic reconstitution and clonogenicity as measured by radiation sensitivity, CFU-S, and CFU-GM assays among CIITA(-/-) and wild-type mice.

CONCLUSIONS

These experiments show that downregulation of MHC II gene transcription does not effectively alter normal hematopoiesis, and provide strong evidence that MHC II expression on hematopoietic progenitors is not required for normal hematopoietic development.

Self-renewal, maturation, and differentiation of the rat myelomonocytic hematopoietic stem cell

Hematopoiesis is viewed as a differentiating system emanating from a pluripotent hematopoietic stem cell capable of both self-renewal and differentiation. By identifying and characterizing a novel and highly specific in vitro mitogenic response to the N-acetyl glucosamyl/sialic acid specific, stem cell-binding lectin wheat germ agglutinin (WGA), we demonstrate the existance of a rare (0.1%), plastic adherent precursor in rat bone marrow capable of proliferation (two to seven divisions) in response to WGA. Stimulated cells possess a lineage (lin)low/- immunophenotype and immature blastoid morphology (WGA blasts). A subsequent proliferative response to stem cell factor (SCF), the ligand for the proto-oncogene receptor tyrosine kinase c-kit, is characterized by an initial maturation in immunophenotype and subsequent self-renewal of cells (SCF blasts) without differentiation for at least 50 generations. Although granulocyte colony-stimulating factor (G-CSF), interleukin (IL) -6, IL-7, and IL-11 synergize with SCF to increase blast colony formation, cytokines such as granulocyte-macrophage CSF or IL-3 are without significant effect. At all time points in culture, however, cells rapidly differentiate to mature neutrophils with dexamethasone or to mainly monocytes/macrophages in the presence of 1alpha,25-dihydroxyvitamin D3, characterized by cell morphology and cytochemistry. Removal of SCF during blast maturation, self-renewal, or induction of differentiation phases results in apoptotic cell death. Data indicate a pivotal role for SCF/c-kit interaction during antigenic maturation, self-renewal, and apoptotic protection of these lineage-restricted progenitors during non-CSF-mediated induction of differentiation. This approach provides a source of many normal, proliferating myelomonocytic precursor cells, and introduces possible clinical applications of ex vivo expanded myeloid stem cells.

Cytofluorometric and cytomorphologic analysis of human bone marrow cells derived from stromal cultures stimulated by granulocyte-macrophage colony-stimulating factor, interferon-gamma and splenopentin pentapeptide

We studied the influence of human recombinant granulocyte-macrophage colony-stimulating factor (hrGM-CSF), human recombinant interferon-gamma (hrIFN-gamma) and splenopentin pentapeptide (Sp-5), either alone or in combination, on the proliferation and differentiation of human bone marrow cells in modified Dexter's cultures. After 10, 14 and 21 days cells were analyzed by classical staining according to Pappenheim and by cytofluorometry with a set of different monoclonal antibodies. IFN-gamma inhibited the proliferation of progenitor cells and provided signals promoting monocytic differentiation, whereas GM-CSF induced the proliferation of blastoid elements which expressed HLA-DR and M2 (VIM-2 monoclonal antibody), but progressively lost surface CD34. Furthermore, an increase of CD15+ cells was also observed. When GM-CSF was tested in combination with IFN-gamma, it abolished the inhibitory effect of IFN-gamma and both cytokines synergized to promote the expression of CD11c, CD14 and M2 surface antigens. Sp-5 alone had only a marginal activity, but it potentiated the effects of GM-CSF. These findings suggest that GM-CSF may induce the transition from stem cells to committed myeloid progenitors. In contrast to IFN-gamma, Sp-5 can serve as an additional proliferative signal with negligible effects on cell maturation.

Membrane antigen expression during hemopoietic differentiation

The pattern of certain groups of antigens expressed on the surface of hemopoietic cells changes either during the course of differentiation from pluripotent stem cells to mature functional cells or as a function of the proliferative state of the cells. A map of these changes is emerging and is providing valuable information for selecting and purifying rare stem cells and for classifying the acute leukemias. This knowledge is also beginning to provide insights into physiological and pathological cellular interactions affecting the early stages of hemopoiesis, and is being exploited to remove T lymphocytes from allogeneic bone marrow grafts in order to prevent graft-vs.-host disease as well as leukemic cells from bone marrow before autologous reinfusion. In this article I will briefly review the cellular basis of hemopoiesis and then discuss the methods used to determine the presence of antigens on normal hemopoietic cells. I will then summarize the pattern of membrane antigens expressed during differentiation and conclude by discussing the biological and therapeutic implications.

Immune regulation of in vitro murine megakaryocyte development. Role of T lymphocytes and Ia antigen expression

Mitogen-activated murine T lymphocytes or T cell hybridomas produce an activity (megakaryocyte [Mk] potentiator activity) that enhances the in vitro growth and development of Mk colonies. This activity was found in optimal concentrations (2.5%) in T cell hybridoma-conditioned medium, and was also produced by feeder layers of concanavalin A-activated T cells. A subpopulation of murine Mk progenitor cells (colony-forming units; CFU-Mk) bears the Ia antigen. Separate experiments indicated that T cell products stimulate CFU-Mk by increasing their basal levels of Ia expression as well as the frequency of cells actively synthesizing DNA. The hypothesis that the expression of this antigen was related to the cell cycle status of these progenitor cells was confirmed in studies that indicated that ablation of actively cycling cells in vivo abrogated the cytotoxic effects of anti-Ia monoclonal antibodies. The interdependence of T cell lymphokine regulation of both Ia expression and cell cycle status was also seen in in vitro experiments in which Ia+ progenitor cells were eliminated by complement-dependent cytotoxicity. The removal of Ia+ cells prevented 5-hydroxyurea-mediated inhibition of cells in S phase. We hypothesize that immune modulation of megakaryocytopoiesis occurs via soluble T cell products that augment Mk differentiation. Further, the mechanism of immune recognition/modulation may occur via Ia antigens present on the surface of these progenitor cells.

Heterogeneity of Ia antigen expression by proliferating nonlymphocytic leukemia blast cells

In vitro systems were used to detect Ia-like antigens on proliferating normal myeloid and acute nonlymphocytic leukemia (ANLL) blast cells. Incubation of normal bone marrow cells with a monoclonal anti-Ia antibody and complement resulted in toxicity for both granulocyte/macrophage progenitors (CFU-GM) (toxicity 79%-100%) and cells proliferating in liquid culture in response to placenta-conditioned medium colony-stimulating factor (CSF) or medium conditioned by normal, phytohemagglutinin (PHA)-stimulated mononuclear cells. In contrast, effects of anti-Ia antibody and complement on blast colony-forming cells and 3H-TdR incorporation in liquid culture from eight patients with ANLL were variable. Colony growth with CSF after treatment was 0% to 91% of control growth and did not correlate with display of Ia-like antigens. Survival of ANLL cells growing in liquid cultures was even more variable after anti-Ia+ complement treatment (28%-227% of control). The presence of Ia-like antigens did not distinguish ANLL cells responding to PHA-conditioned medium from those responding to CSF in either colony or liquid culture. Dose-response curves for ANLL cells in liquid culture were similar before and after treatment with anti-Ia+ complement. In contrast to normal myeloid precursor cells, which show uniform display of Ia-like antigens, display of Ia antigen by proliferating leukemia cells is highly variable from patient to patient. Anti-Ia reagents such as this one would not be effective in treating ANLL marrow for autologous transplantation.

Human hematopoietic progenitor cells in long-term cultures express HLA-DR antigens and lack HLA-DQ antigens

The expression of HLA-DR antigenic determinants on human hematopoietic progenitor cells (HPC) capable of differentiating into mature blood cells, as determined in semisolid cultures, has been demonstrated previously (3-7). Here, we investigated the expression of class II determinants on HPC responsible for the sustained proliferation of colony-forming units of granulocyte/macrophage (CFU-GM), of multilineage HPC (CFU-GEMM, granulocyte/erythrocyte/macrophage/megakaryocyte), and burst-forming units of erythroid cells (BFU-E) in liquid long-term cultures. Using both fluorescence-activated cell sorting and complement-dependent cytotoxicity assays, HLA-DR determinants could be identified on virtually all these HPC capable of proliferating in long-term cultures. Experiments in which the stromal layer had been irradiated provided evidence that the HPC themselves were truly HLA-DR+, and that the sustained proliferation of HPC was not due to activation of HLA-DR- residual HPC in the stromal layer by reinoculated HLA-DR+ accessory cells. Furthermore, it was shown that all HPC recognized in semisolid and liquid long-term cultures were HLA-DQ-. These results suggest that the human true pluripotential stem cell is HLA-DR+. These results open the possibility of studying class II-dependent regulation of hematopoiesis in liquid long-term cultures.

Colony formation by primitive haemopoietic progenitors in cocultures of bone marrow cells and stromal cells

Human bone marrow contains a class of human haemopoietic progenitor cells that adhere to cultured marrow stromal cells and form colonies of blast cells. These progenitor cells are found in the non-adherent mononuclear fraction of normal human bone marrow. They are not in active cell cycle and do not express Ia-like (HLA-DR) antigens but appear to be capable of self-renewal in vitro. These properties indicate that they should be classified as members of the primitive haemopoietic progenitor cell compartment.

Expression of tumor-associated surface membrane antigens on marrow CFU-s, CFC-gm, BFU-e, and brain CFU-s

Antiserum raised against a mouse mast cell line (FMP1) reacts with 90% to 100% of spleen colony-forming units (CFU-s), granulocyte/macrophage colony-forming cells (CFC-gm), erythroid burst-forming units (BFU-e), and 15% of nucleated marrow cells, using a complement-dependent cytotoxicity assay. We demonstrated that bone marrow, spleen, or thymus cells are able to absorb this activity from the antiserum. Although mouse brain cells have low reactivity with anti-FMP1 serum, the cytolysis level was reduced to background when antiserum was absorbed with brain cells. In addition, colony formation by marrow CFU-s, CFC-gm, and BFU-e was no longer prevented when the cells were incubated with brain-absorbed anti-FMP1 serum and complement. These findings suggest the presence of brain-associated antigens on CFU-s, CFC-gm, and BFU-e. To test whether a CFU-s accessory cell population in marrow is affected by treatment with anti-FMP1 serum and complement, antibody-treated marrow cells were mixed with large numbers of thymocytes and injected into recipient mice. Colony formation was not altered, indicating that the antiserum reacted directly with antigens on CFU-s and not on CFU-s accessory cells.

Expression of cell surface determinant(s) on murine myeloma stem cells and hematopoietic stem cells

Murine B cell lymphomas and myelomas were examined for the expression of a determinant previously found exclusively on normal pluripotent stem cells colony-forming unit-spleen (CFU-s). This determinant(s), which is defined by a rabbit antimouse brain antiserum (R alpha MB), is present on the tumor stem cell population of some but not all B cell neoplasms examined. The determinant is not detected on tumor cells of the macrophage or T cell lineage. Absorption of the activity in R alpha MB with myeloma cells, concomitantly removed reactivity with the normal stem cell, CFU-s, and the myeloma stem cell, plasmacytoma CFU-s. Sorting analysis further showed that the antigen was diminished within a positive tumor population as cells acquired the capacity to secrete immunoglobulin. These studies suggest that this normal stem cell-associated antigen may also be an early differentiation antigen for the B cell lineage, and is expressed on some stem cells of B cell tumors.

Colony assays of hematopoietic progenitor cells and correlations to clinical situations

The production of blood cells is a dynamic process that is noticeably aberrant during disease. The availability of colony assays in vitro that allow detection of hematopoietic stem and progenitor cells for the neutrophil, monocyte-macrophage, erythroid and/or megakaryocyte lineages has been of importance for the present understanding of the mechanisms controlling the proliferation, self-renewal capacity, and differentiation of morphologically nonrecognizable immature cells which give rise to the mature progeny circulating in the blood. It is through the use of these assays that the existence of potentially relevant stimulatory and inhibitory feedback interactions has been demonstrated. Abnormalities in these interactions, which may be of significance during leukemia and related disorders, have been uncovered. This communication will discuss regulatory interactions detected via the colony assays, their potential relevance physiologically and pathologically, and the use of these assays for diagnosis, prognosis, and for monitoring the clinical status of patients.

Studies on the in vitro microenvironment in man

In a preliminary manner, the data presented here characterize some features of MSC and their progenitors. The progenitors, at least in chronic myelogenous leukemia, are derived from the neoplastic pluripotent stem cell that also differentiates along lymphoid and myeloid pathways. In addition, we have demonstrated that the precursor for MSC is lacking both the Ia and CALLA determinants. Several antigenic and functional characteristics of the mature stromal cell population have also been identified. Stromal cells express CALLA, synthesize types I, III, and IV collagen, and may express factor VIII associated antigen. It is of interest that fibroblasts do not express factor VIII associated antigen, do not synthesize type IV collagen in measurable quantities, but do express CALLA [9]. Endothelial cells express factor VIII associated antigen, synthesize type IV collagen, but are not CALLA positive. Thus, MSC have some features in common with fibroblasts and others with endothelial cells. The unique characteristics of MSC are that they are transplantable and are derived from a common progenitor with other hematopoietic cells. These features clearly distinguish this cell population from fibroblasts, which are neither transplantable nor derived from the neoplastic clone in CML.

HLA antigens expressed on human erythroid burst forming cells

The human major histocompatibility antigens HLA, play an important role in transplantation. To ascertain the expression of the antigens of A and B loci on the surface of human hemopoietic progenitors, the cytotoxic effect of specific anti-HLA sera was examined. Anti-HLA A2 and B5 sera were used in the present experiments. Circulating blood mononuclear cells and nucleated marrow cells, pretreated with appropriate specific anti-HLA sera and complement, formed fewer colonies from BFU-e and CFU-c in a culture medium than the controls which were treated with autologous serum. In our experiments, the HLA antisera also killed macrophages, monocytes, and T cells which carry the surface antigen. Then, the influence of macrophages, monocytes, and T cells on colony formation from BFU-e and CFU-c was examined. These results indicated that BFU-e as well as CFU-c expressed HLA antigens on their surface.

Expression of cell-surface HLA-DR, HLA-ABC and glycophorin during erythroid differentiation

The unexpected discovery that Ia-like (HLA-DR) antigens in humans were present on blast cells from acute myeloblastic leukaemia led to the finding that normal granulocytic progenitors, in contrast to their mature descendents, also expressed HLA-DR antigens. Thus, anti-Ia sera stain a proportion of myeloblasts in normal bone marrow, inhibit myeloid progenitor (CFU-GM) colony formation in the presence of complement and can be used to label and separate CFU-GM on a fluorescence-activated cell sorter (FACS). Winchester et al. subsequently reported that erythroid progenitors (BFU-E and CFU-E) were also inhibited or killed by anti-Ia (p28,37) and complement. These observations raised the possibility that HLA-DR (or presumptive I-region equivalent) products might have a regulatory role in early haematopoiesis. We have now analysed HLA-DR and HLA-ABC antigen expression on normal erythroid progenitors using monoclonal antibodies to non-polymorphic determinants and fluorescence-activated cell sorting. In parallel experiments, we tested a monoclonal antibody to glycophorin, a well defined erythroid-specific cell-surface membrane glycoprotein. We report that HLA-DR, HLA-ABC and glycophorin are all expressed at various stages during erythroid differentiation.

Fc and C3 receptors as membrane differentiation markers of acute myelogenous leukaemia cells

Acute myelogenous leukaemia cells (AML) and cells of chronic myelogenous leukaemia blast crisis (CGL-CB) were examined for the presence of receptors for Fc IgG fragment (FcR), receptors for the complement components (CR1 and CR2), and the surface immunoglobulins including the light kappa and lambda type chains. The leukaemia blasts were found to be the cells poor in receptors and poorly differentiated. As a rule, they contained very small amount of detectable FcR, CR2, and CR1. Analysis of AML cell populations separated on the discontinuous density gradient revealed that the appearance of FcR was followed by CR2 and CR1. The CGL-CB cells were more differentiated immunologically since, in comparison with the AML cells, in greater percentage they expressed the FcR, and the receptors for complement. Assays for surface immunoglobulins indicated that they were not an active product of the leukaemic blasts, but rather exogenous in origin.

The use of in vitro technics to study drug-induced pancytopenia

The confirmation of drug-induced marrow aplasia is difficult since rechallenging the patient can lead to serious morbidity. We used marrow-culture technics to challenge in vitro the bone marrow of a patient with marrow aplasia after ingestion of quinidine. There was no clinical or laboratory evidence of quinidine-mediated destruction of erythrocytes, leukocytes or platelets. By contrast, use of quinidine in combination with the patient's serum substantially inhibited in vitro growth of allogeneic marrow granulocytic and erythroid series. Furthermore, use of quinidine in combination with acute-phase serum (but not acute-phase serum alone or quinidine in combination with recovery-phase serum) inhibited growth of the patient's marrow. This observation suggests that both a transient serum factor and quinidine were responsible for the marrow aplasia. These technics could be applied with minimal risk to similar patients and would permit in vitro rechallenge with the suspected drug.

The expression of histocompatibility-2 antigens on hemopoietic stem cells

Antisera directed against histocompatibility-2 antigens of the mouse suppress the formation of spleen colonies by pluripotent hemopoietic stem cells. Of antisera which are specific for subregions of the H-2 complex, only those which contain anti-H-2K or anti-H-2D activity are effective. Specific anti-Ia sera do not react with the stem cell. If the titer of the CFU-s suppressive effect of the antisera is compared to their toxicity to spleen lymphocytes, it can be concluded that CFU-s express antigens in the same density as spleen lymphocytes. The amount of H-2 antigens expressed on the surface of CFU-s is independent of its rate of proliferation.

Inhibition of human erythropoietic colony formation in culture by treatment with Ia antisera

Incubation with Ia antiserum, followed by complement, markedly inhibited erythroid colonies arising from hematopoietic cells present in the nonadherent low density fractions of normal bone marrow. Both erythropoietin-dependent colonies and bursts were eliminated at dilutions of antiserum equivalent to, or greater than the dilutions required to abolish the granulocyte-macrophage colony formation. The inhibitory effect of the Ia antiserum was abolished by absorption with B but not T cells from lymphoid lines. Available evidence suggested that Ia determinants are expressed on the erythropoietin-sensitive progenitors of the erythroid series in precise analogy to their sequence of expression on the granulocyte lineage. In both lineages, as shown previously, the Ia determinants become undetectable during subsequent stages of differentiation.