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

Skewed fate and hematopoiesis of CD34+ HSPCs in umbilical cord blood amid the COVID-19 pandemic

Umbilical cord blood (UCB) is an irreplaceable source for hematopoietic stem progenitor cells (HSPCs). However, the effects of SARS-CoV-2 infection and COVID-19 vaccination on UCB phenotype, specifically the HSPCs therein, are currently unknown. We thus evaluated any effects of SARS-CoV-2 infection and/or COVID-19 vaccination from the mother on the fate and functionalities of HSPCs in the UCB. The numbers and frequencies of HSPCs in the UCB decreased significantly in donors with previous SARS-CoV-2 infection and more so with COVID-19 vaccination via the induction of apoptosis, likely mediated by IFN-γ-dependent pathways. Two independent hematopoiesis assays, a colony forming unit assay and a mouse humanization assay, revealed skewed hematopoiesis of HSPCs obtained from donors delivered from mothers with SARS-CoV-2 infection history. These results indicate that SARS-CoV-2 infection and COVID-19 vaccination impair the functionalities and survivability of HSPCs in the UCB, which would make unprecedented concerns on the future of HSPC-based therapies.

Frequent HLA-DR loss on hematopoietic stem progenitor cells in patients with cyclosporine-dependent aplastic anemia carrying HLA-DR15

To determine whether antigen presentation by HLA-DR on hematopoietic stem progenitor cells (HSPCs) is involved in the development of acquired aplastic anemia (AA), we studied the HLA-DR expression on CD45dimCD34+CD38+ cells in the peripheral blood of 61 AA patients including 23 patients possessing HLA-class I allele-lacking (HLA-class I[-]) leukocytes. HLA-DR-lacking (DR[-]) cells accounted for 13.0-57.1% of the total HSPCs in seven (11.5%) patients with HLA-DR15 who did not possess HLA-class I(-) leukocytes. The incubation of sorted DR(-) HSPCs in the presence of IFN-γ for 72 h resulted in the full restoration of the DR expression. A comparison of the transcriptome profile between DR(-) and DR(+) HSPCs revealed the lower expression of immune response-related genes including co-stimulatory molecules (e.g., CD48, CD74, and CD86) in DR(-) cells, which was not evident in HLA-class I(-) HSPCs. DR(-) cells were exclusively detected in GPI(+) HSPCs in four patients whose HSPCs could be analyzed separately for GPI(+) and GPI(-) HSPCs. These findings suggest that CD4+ T cells specific to antigens presented by HLA-DR15 on HSPCs may contribute to the development of AA as well as the immune escape of GPI(-) HSPCs in a distinct way from CD8+ T cells recognizing HLA-class I-restricted antigens.

DNA methylation dysregulates and silences the HLA-DQ locus by altering chromatin architecture

The MHC-II locus encodes a cluster of highly polymorphic genes HLA-DR, -DQ, and -DP that are co-expressed in mature B lymphocytes. Two cell lines were established over 30 years ago from a patient diagnosed with acute lymphocytic leukemia. Laz221 represented the leukemic cells of the patient; whereas Laz388 represented the normal B cells of the patient. Whereas Laz388 expressed both HLA-DR and HLA-DQ surface and gene products, Laz221 expressed only HLA-DR genes. The discordant expression of HLA-DR and HLA-DQ genes was due to epigenetic silencing of the HLA-DQ region CTCF-binding insulators that separate the MHC-II subregions by DNA methylation. These epigenetic modifications resulted in the loss of binding of the insulator protein CTCF to the HLA-DQ flanking insulator regions and the MHC-II specific transcription factors to the HLA-DQ promoter regions. These events led to the inability of the HLA-DQ promoter regions to interact with flanking insulators that control HLA-DQ expression. Inhibition of DNA methylation by treatment with 5’deoxyazacytidine reversed each of these changes and restored expression of the HLA-DQ locus. These results highlight the consequence of disrupting an insulator within the MHC-II region and may be a normal developmental mechanism or one used by tumor cells to escape immune surveillance.

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.

Coreceptor restriction within the HLA-DQ locus for Epstein-Barr virus infection

Epstein-Barr virus (EBV) is a ubiquitous human herpesvirus that causes infectious mononucleosis and is etiologically associated with malignancies of multiple origins. EBV enters cells through a cascade of interactions between its envelope glycoprotein gp350 and the gp42-gH-gL complex with cellular receptors. Membrane fusion is catalyzed by the binding of gp42, a member of the C type lectin family, to HLA class II molecule HLA-DR, -DP, or -DQ. Here we demonstrate that only a subset of HLA-DQ alleles mediates EBV entry, indicating that individuals expressing these alleles may offer unique sites for EBV infection and subsequent sequelae. Additionally, the specific site within HLA-DQ determined to be essential for EBV entry is homologous to a site within MHC class I shown by structural studies to bind to the C type-lectin-like natural killer receptor, providing insight into the biochemical nature of the gp42-HLA class II interaction.

Expression of MHC class II antigens on rat bone marrow cells and macrophages, and their modulation during culture with murine GM-CSF or M-CSF

Flow cytometric analysis employing MRC OX 6 and MRC OX17 monoclonal antibodies recognizing determinants on RT1.B or RT1.D molecules, equivalent to murine I-A and I-E, respectively, was used to detect rat MHC class II antigen (Ag) expression. Approximately 5% of freshly isolated rat bone marrow cells (BMC) expressed RT1.B and over 30% displayed RT1.D molecules. The RT1.D+ cells were W3/13+, OX 7+, OX 19- and OX 22-. After one week culture of BMC with murine recombinant granulocyte/macrophage colony-stimulating factor (GM-CSF), regardless of concentrations, 90 to 95% of the cells were scored as bone marrow-derived macrophages (BMDM phi), and over 30% expressed both RT1.B and RT1.D Ag. GM-CSF increases the percentage of BMDM phi bearing MHC class II Ag in a concentration-dependent manner. This effect seems to be specific because antibodies to interferon-gamma, tumor necrosis factor-alpha or interleukin-4 did not reduce the number of cells expressing RT1.B and RT1.D Ag. Furthermore, GM-CSF was able to trigger expression of class II molecules on rat peritoneal macrophages (M phi) and BMDM phi resulted from cultures of BMC with mouse M phi-CSF (M-CSF), and the RT1.B and RT1.D inducing effect of GM-CSF was opposed by M-CSF, and by anti-GM-CSF antibodies. The induction of MHC class II Ag synthesis by GM-CSF on rat BMDM phi was confirmed at the mRNA level by Northern blot analysis employing cDNA probes encoding the RT1.B alpha.

Polymorphism in the upstream regulatory region of DQA1 genes and DRB1, QAP, DQA1, and DQB1 haplotypes in the German population

Polymorphism in the URR of the MHC class II DQA1 gene defines ten different alleles named QAP. Oligotyping for the alleles of DRB1, QAP, DQA1, and DQB1 have been performed in 210 unrelated healthy controls from Germany. Moreover, 83 HTCs from the Tenth IHWS have been tested. Four point loci haplotypes (DRB1, QAP, DQA1, and DQB1) have been analyzed in the unrelated healthy population sample. Computer analysis of the linkage disequilibria leads to the conclusion that QAP alleles are in strong linkage disequilibrium with alleles either the DQA1 or the DRB1 locus. One typical ("common") haplotype was found to be associated with each DRB1 allele in the majority (86%) of the tested persons. Apart from that, 25 other less frequent ("unusual") haplotypes, with an overall frequency of 14% have been defined. Some of these "unusual" MHC class II haplotypes were found to differ only in the regulatory alleles of DQA1 (QAP alleles) while they are identical for the alleles coding for structural elements (DRB1, DQA1, and DQB1). Most of the "unusual" haplotypes were found to carry HLA-DQ6. Assuming that "unusual" (= rare) haplotypes have arisen from "common" (= frequent) haplotypes by point mutation and recombination, we propose the existence of three recombination sites in the MHC DR-DQ region: one between DRB1 and QAP, the second between QAP and DQA1, and the third between DQA1 and DQB1.

Characterization of CD34+HLA-DR-CD38+ and CD34+HLA-DR-CD38- progenitor cells from human umbilical cord blood

In this study we show that depletion of cells expressing mature cell markers, including HLA-DR, followed by positive cell sorting for cells expressing CD34 and CD38, can be used to define functionally distinct hematopoietic cells from human umbilical cord blood (HUCB). The CD34+HLA-DR-CD38+ population contained the majority of directly clonogenic cells, while the optimal ability to maintain long term co-culture with bone marrow stromal cells was present within the CD34+HLA-DR-CD38- population. 1.2 +/- 0.4% of the CD34+HLA-DR-CD38- cells plated at 1 cell/well and grown in the presence of hematopoietic growth factors (HGF) formed hemopoietic colonies. Mesenchymal elements were observed in 20% of these cultures. No cell growth, however, was observed when the CD34+HLA-DR-CD38- cells were cultured in the absence of HGF. This is in contrast with the findings in fetal bone marrow which demonstrated the presence of stem cells that were independent of HGF. Thus, while it is possible to isolate very immature hemopoietic progenitor cells from HUCB defined by the phenotype Lin-CD34+HLA-DR-CD38-, these cells do not appear to exhibit the pluripotentiality of the analogous population reported in fetal bone marrow. We conclude that these cells are absent or at a very small frequency in HUCB.

Locus- and allele-specific DNA-protein interactions in the HLA-DQB1 X box

Expression of MHC class II genes is regulated by a complex series of protein-DNA interactions which lead to the initiation of transcription. Although the different MHC class II loci are generally coordinately expressed, important differences in expression can be seen among loci and among individual alleles. The major sites of transcriptional control in the human MHC consist of several highly conserved nucleotide sequence elements located upstream of each MHC class II gene. We have analyzed the interlocus and interallelic variation in one of these key regulatory regions of the HLA-DQB1 promoter, the X box, and identified several sites of protein-DNA interaction. Two protein-DNA complexes were found which differ between the DQ and DR loci as well as two distinct complexes which differed between DQ alleles. These nuclear protein-X box interactions are likely to influence the differential expression of the MHC class II loci and alleles in tissue-specific or developmentally regulated pathways.

Phenotypic characterization of normal and CML CD34-positive cells: only the most primitive CML progenitors include Ph-neg cells

We studied the sequence of acquisition of CD33, CD38 and HLA-DR antigens on CD34+ cells from marrow and blood of Ph-chromosome positive CML patients and normal marrow. We examined the Ph status of the various CML cell populations. The mean proportions of normal and CML CD34+ cells expressing CD33 and CD38 were not significantly different. However, a significantly greater proportion of CML CD34+ cells expressed HLA-DR antigens compared with normal CD34+ cells and the level of HLA-DR expression per CML cell was abnormally high. When the sequence of acquisition of these antigens on normal and CML CD34+ cells was evaluated using 3-colour fluorescence analysis, the results suggested that HLA-DR was expressed earlier than CD38 or CD33 and these findings were confirmed by following the acquisition of CD38 and CD34+/DR+/CD38-subpopulation during liquid culture. We performed cytogenetic studies on CD34+ subpopulations in 6 cases. In 4 cases there were some Ph-negative metaphases detectable in the CD34+/DR-subpopulation (range 12.5 to 60%). In the CD34+/DR+ fractions, however, all 6 patients had only Ph-positive metaphases and only 1/5 patients had detectable Ph-negative metaphases in the CD34+/CD38-subpopulation. We conclude that expression of HLA-DR antigens may precede the expression of CD38 on CD34+ cells during normal stem cell differentiation. In CML DR may be expressed aberrantly and Ph-negative cells are found predominantly in the DR negative subpopulation.

Ex VivoExpansion and Differentiation of Hematopoietic Stem Cells

Hematopoietic stem cells are phenotypically very heterogeneous, probably reflecting the degree of activation and/or differentiation. This cell population is capable of high-level proliferative activity and multilineage differentiation. Despite its potential for self-renewal, the hematopoietic stem cell exists in a quiescent state for prolonged periods of time. The mechanism(s) involved in triggering these cells to enter the cell cycle is/are not totally clear; however, cytokines (both positive and negative regulators) are implicated. Most, if not all known cytokines that interact at the stem cell level do so not only by inducing proliferation but also differentiation. The ability to maintain a population of truly primitive stem cells for extended periods of time in vitro is currently under investigation by many research groups.

Two DNA-binding proteins discriminate between the promoters of different members of the major histocompatibility complex class II multigene family

The regulation of major histocompatibility complex (MHC) class II gene expression is a key feature of the control of normal and abnormal immune responses. In humans, class II alpha - and beta-chain genes are organized in a multigene family with three distinct subregions, HLA-DR, -DQ, and -DP. The regulation of these genes is generally coordinated, and their promoters contain highly conserved motifs, in particular the X and Y boxes. We have identified five distinct proteins that bind to specific DNA sequences within the first 145 base pairs of the HLA-DR promoter, a segment known to be functionally essential for class II gene regulation. Among these, RF-X is of special interest, since mutants affected in the regulation of MHC class II gene expression have a specific defect in RF-X binding. Unexpectedly, RF-X displays a characteristic gradient of binding affinities for the X boxes of three alpha-chain genes (DRA greater than DPA much greater than DQA). The same observation was made with recombinant RF-X. We also describe a novel factor, NF-S, which bound to the spacer region between the X and Y boxes of class II promoters. NF-S exhibited a reverse gradient of affinity compared with RF-X (DQA greater than DPA much greater than DRA). As expected, RF-X bound well to the mouse IE alpha promoter, while NF-S bound well to IA alpha. The drastic differences in the binding of RF-X and NF-S to different MHC class II promoters contrasts with the coordinate regulation of HLA-DR, -DQ, and -DP genes.

Two DNA-binding proteins discriminate between the promoters of different members of the major histocompatibility complex class II multigene family

The regulation of major histocompatibility complex (MHC) class II gene expression is a key feature of the control of normal and abnormal immune responses. In humans, class II alpha - and beta-chain genes are organized in a multigene family with three distinct subregions, HLA-DR, -DQ, and -DP. The regulation of these genes is generally coordinated, and their promoters contain highly conserved motifs, in particular the X and Y boxes. We have identified five distinct proteins that bind to specific DNA sequences within the first 145 base pairs of the HLA-DR promoter, a segment known to be functionally essential for class II gene regulation. Among these, RF-X is of special interest, since mutants affected in the regulation of MHC class II gene expression have a specific defect in RF-X binding. Unexpectedly, RF-X displays a characteristic gradient of binding affinities for the X boxes of three alpha-chain genes (DRA greater than DPA much greater than DQA). The same observation was made with recombinant RF-X. We also describe a novel factor, NF-S, which bound to the spacer region between the X and Y boxes of class II promoters. NF-S exhibited a reverse gradient of affinity compared with RF-X (DQA greater than DPA much greater than DRA). As expected, RF-X bound well to the mouse IE alpha promoter, while NF-S bound well to IA alpha. The drastic differences in the binding of RF-X and NF-S to different MHC class II promoters contrasts with the coordinate regulation of HLA-DR, -DQ, and -DP genes.

Cellularly defined minor histocompatibility antigens are differentially expressed on human hematopoietic progenitor cells

Previously, five CTL lines directed against minor histocompatibility (mH) antigens designated HA-1-5 have been established from peripheral blood of patients after allogeneic bone marrow transplantation (BMT), and have been characterized using population and family studies. All cell lines showed specific HLA class I-restricted lysis of PHA-stimulated peripheral blood target cells from donors positive for the particular mH antigens. After 4 h of incubation of the mH antigen HA-3-specific CTL line with bone marrow cells from HA-3+ donors, complete class I-restricted inhibition of colony growth of the hematopoietic progenitor cells was observed even at low E/T ratios, indicating that the HA-3 antigen is strongly expressed on hematopoietic stem cells. Therefore, this antigen may be a target structure in the immune-mediated rejection of the hematopoietic graft in case of incompatibility for this determinant between donor and recipient in allogeneic BMT. In contrast, incubation of bone marrow cells with the antigen-specific anti-HA-1, -2, -4, and -5 CTL lines did not result in growth inhibition of the hematopoietic progenitor cells tested. After a prolonged incubation time and using a very high E/T ratio, progenitor cells from HA-2+ or HA-5+ donors were killed to some extent by the anti-mH-specific CTL lines, although the growth inhibition observed was minor and variable. Our results show that mH antigens are differentially expressed on human hematopoietic progenitor cells. Therefore, only some of these antigens may be targets in immune-mediated rejection of the bone marrow graft.

Characterization of a human hematopoietic progenitor cell capable of forming blast cell containing colonies in vitro

A hematopoietic cell (CFU-B1) capable of producing blast cell containing colonies in vitro was detected using a semisolid culture system. The CFU-B1 has the capacity for self-renewal and commitment to a number of hematopoietic lineages. Monoclonal antibody to the human progenitor cell antigen-1 (HPCA-1) and a monoclonal antibody against the major histocompatibility class II antigen (HLA-DR) were used with fluorescence activated cell sorting to phenotype the CFU-B1. The CFU-B1 was found to express My10 but not HLA-DR antigen; experiments using complement-dependent cytotoxicity to eliminate DR positive cells confirmed this finding. Pretreatment of marrow cells with two chemotherapeutic agents, 5-fluorouracil and 4-hydroperoxycyclophosphamide facilitated detection of CFU-B1 derived colonies, while diminishing or totally inhibiting colony formation by other hematopoietic progenitor cells. CFU-B1-derived colony formation was dependent upon the addition of exogenous hematopoietic growth factors. Media conditioned either by the human bladder carcinoma cell line 5637 or lectin stimulated leukocytes, as well as recombinant granulocyte-macrophage colony stimulating factor, interleukin 3 or interleukin 1 alpha promoted blast cell colony formation. By contrast, neither recombinant erythropoietin, recombinant interleukin 4, purified macrophage colony stimulating factor or recombinant granulocyte colony-stimulating factor alone promoted blast cell colony formation.

Minor histocompatibility antigen H-Y is expressed on human hematopoietic progenitor cells

Polymorphic minor transplantation antigens probably play an important role in immune mediated graft rejections of bone marrow transplants. Mapping of these antigens on hematopoietic progenitor cells (HPC) is important since these antigenic determinants may serve as target structures in the rejection process, and it ultimately opens the possibility to match for these antigens. Using a cell-mediated cytotoxicity assay with H-Y-specific cytotoxic T lymphocytes as effector cells, a dose-dependent growth inhibition up to 100% of myeloid (CFU-GM), erythroid (BFU-E) and multipotential (CFU-GEMM) HPC of male donors was obtained, indicating expression of the H-Y antigen on these progenitor cells. In contrast, inhibition of relatively mature erythroid and myeloid progenitor cells was only 40-50%, indicating that the recognition of the H-Y antigen diminished during maturation of erythroid and myeloid HPC. Our results show that the H-Y antigen can be recognized on HPC as a target for cytotoxic T cell responses. This may be important in graft rejection of male donor bone marrow grafts by female recipients.

Regulation of genes for HLA class II antigens in cell lines from patients with severe combined immunodeficiency

HLA Class II-negative severe combined immunodeficiency (SCID) results from a congenital defect characterized by an absence of HLA Class II antigens. Patients with the disorder have no HLA-DR, DQ, or DP antigens or mRNAs in their peripheral-blood lymphocytes. The affected gene is a recessive, transacting regulatory gene that controls the expression of Class II genes. We studied the regulation of HLA Class II gene expression with the use of established Epstein-Barr virus-transformed B-cell lines and skin fibroblast lines from a group of patients with SCID. Lymphoblastoid B-cell lines from the patients contained no mRNA for HLA-DR, DQ, and DP alpha and beta polypeptides, but did express mRNA for the HLA-associated invariant chain, which is normally coregulated with HLA Class II antigens. In the B-cell line from one patient, a very low amount of DR mRNA could be detected, indicating some heterogeneity in SCID. The lymphokine gamma-interferon, a strong inducer of Class II genes in a variety of normal cells, did not restore Class II gene expression in any of the SCID B-cell lines. More important, gamma-interferon was unable to induce any Class II mRNA in fibroblast lines from patients with SCID, in contrast to the efficient induction observed in normal fibroblasts. The invariant-chain gene, however, was induced in the SCID fibroblasts, confirming a unique uncoupling in the regulation of invariant and Class II genes. Thus, the genetic defect in patients with SCID affects not only the B-cell lineage but also the inducible expression of HLA Class II genes that is normally observed in Class II-negative cells, such as fibroblasts. This unresponsiveness to gamma-interferon in vitro indicates that patients with SCID will not respond to treatment with this lymphokine. Our data also increase understanding of the normal mechanisms regulating the genes for the HLA Class II cell-surface glycoproteins.

The production of myeloid blood cells and their regulation during health and disease

The regulation of myelopoiesis in vivo most likely entails a complex set of interactions between cell-derived biomolecules and their target cells: hematopoietic stem and progenitor cells and accessory cells. Stimulating and suppressing factors have been characterized through in vitro studies, and their mechanisms of action in vitro and in vivo have begun to be elucidated. Among those factors being studied are the hematopoietic colony-stimulating factors (CSF): interleukin-3 (multi-CSF), granulocyte-macrophage-CSF, granulocyte-CSF, and macrophage-CSF; other molecules include erythropoietin, B-cell-stimulating factor-1, interleukin-1, interleukin-2, prostaglandin E, leukotrienes, acidic ferritins, lactoferrin, transferrin, the interferons-gamma, -alpha, and -beta, and the tumor necrosis factors-alpha and -beta (lymphotoxin). These factors interact to modulate blood cell production in vitro and in vivo. The proposed review characterizes these biomolecules biochemically and functionally, including receptor-ligand interactions and the secondary messengers within the cell which mediate their functional activity. The production and action of the molecules are described under conditions of hematopoietic disorders, as well as under normal conditions. Studies in vitro are correlated with studies in vivo using animal models to give an overall view of what is known about these molecules and their relevance physiologically and pathologically.

Expression of HPCA-1 and HLA-DR antigens on growth factor- and stroma-dependent colony forming cells

The expression of HLA-DR and HPCA-1 antigens (recognized by the L243 and BI.3C5 antibodies respectively) on adult human bone marrow cells was examined by fluorescence activated cell sorting and colony assays. Nearly all the (day 14) lineage restricted and multipotential colony forming cells analysed in methylcellulose cultures in the presence of added growth factors express HLA-DR and HPCA-1 determinants. Two colour cell sorting reveals that the lineage restricted HLA-DR positive progenitors express variable levels of BI.3C5 positivity whereas most of the multipotential progenitors, the multi-CFC or CFU-GEMM, are highly BI.3C5 positive. The isolated HLA-DR and BI.3C5 positive populations also contain haemopoietic precursors which adhere to and form colonies on pre-formed stromal layers. Thus, haemopoietic progenitors assayed in both types of culture system can be analysed and enriched by simultaneous two-colour sorting using anti-HLA-DR and BI.3C5 monoclonal antibodies. Similarities in the antigenic phenotype of such cells, however, precludes the use of these reagents for segregating growth factor-dependent from stroma-dependent progenitors.

HLA-class II antigens on human hematopoietic progenitors

A panel of alloindifferent monoclonal antibodies (MAB's) was used in complement-dependent lysis to characterize human myeloid, erythroid and multipotential progenitors (CFU-GM, BFU-E, CFU-GEMM) for their expression of MHC class II HLA-DR, -DP, and -DQ products. 7-16 donors were tested in each system. MAB Tü 34, detecting DR products, caused reduction of CFU-GM by a mean of 89%, whereas BFU-E and CFU-GEMM were reduced by 67% and 66% respectively. 35% of CFU-GM, 27% of BFU-E and 32% of CFU-GEMM were lysed by MAB B7/21, recognizing HLA-DP determinants, while Tü 22, binding HLA-DQ antigens, lysed 32% only of CFU-GM and did not lyse the other progenitors. Employing the "broad" MAB Tü 39, which binds at least DR and DP, inhibition of colony formation by CFU-GM was generally greater than that caused by Tü 34 alone or even by combinations of Tü 34, Tü 22, and B7/21. This suggests that there may be a subset of DR-, DP-, DQ- hematopoietic progenitors, which nonetheless bind MAB Tü 39, previously proposed as a candidate for the recognition of novel class II antigens.

Co-ordinate expression of BI.3C5 and HLA-DR antigens on haemopoietic progenitors from chronic myeloid leukaemia

Haemopoietic cells isolated from the peripheral blood of patients with chronic myeloid leukaemia (CML), have been extensively purified and enriched using either Percoll density gradients or Percoll density gradients combined with elutriation. The quantitative expression of the BI.3C5 associated antigen and the co-expression of BI.3C5 and HLA-DR antigens on these two populations has been studied using either single or simultaneous two colour FACS sorting, following by in-vitro culture for single and multilineage haemopoietic progenitors thus obtained. The data show that the CFU-GEMM are always found in the most strongly BI.3C5 positive fraction, irrespective of the separation procedure and that the bulk of the CFU-GEMM co-express BI.3C5 and HLA-DR. The cell types initiating these CFU-GEMM are morphologically immature blasts. The more mature cells of the myelomonocytic and erythroid lineages forming single lineage colony types show variable BI.3C5 expression, although most are HLA-DR positive. Such enriched populations of malignant progenitors could provide a useful source of material to study both gene expression and the molecular mechanisms underlying malignant transformation.