Expression of CD7 on normal human myeloid progenitors

Existence of biphenotypic leukemias co-expressing CD7 and CD34 has prompted the question of whether a similar population of cells is present in normal human bone marrow. As CD7 is considered to be a T cell-restricted Ag, the co-expression of CD7 with the "human stem cell Ag" CD34 may identify a bipotent stage within hemopoietic differentiation. Cells with this phenotype have previously been isolated from human thymus. In this report we provide evidence that human marrow mononuclear cells also contain a minor subpopulation of cells co-expressing CD7 and CD34. The CD7+/CD34+ cells were found to contain committed myeloid progenitors assayed both as CFU in semi-solid media and by their ability to produce granulocytes in long term marrow cultures. Expression of CD7 on myeloid committed progenitors was further confirmed in a C-mediated cytotoxic assay. We conclude that CD7 expression is not restricted to T cells but is also expressed during early stages of myeloid differentiation.

Vascular cell adhesion molecule-1 expressed by bone marrow stromal cells mediates the binding of hematopoietic progenitor cells

Human bone marrow-derived CD34+ cells were analyzed for the expression of the beta 1-family of integrin adhesion molecules. Integrin alpha 4 beta 1 was consistently expressed by greater than 90% of CD34+ cells, including essentially all assayable granulocyte-macrophage colony-forming cells (CFU-GM) and erythroid bursts (BFU-E) as shown by fluorescence-activated cell sorting studies. Adhesion of highly enriched CD34+ cells to cultured allogeneic marrow stromal cells was largely inhibited both by monoclonal antibody to alpha 4 beta 1 and to vascular cell adhesion molecule-1 (VCAM-1), a ligand for alpha 4 beta 1. VCAM-1 was found to be expressed by bone marrow stromal elements in vitro both constitutively at low level and at high levels after treatment with cytokines. Induction of VCAM-1 was cytokine- and time-dependent with maximum levels being obtained after 4 hours of exposure to a combination of interleukin-4 and tumor necrosis factor-alpha. Cytokine-induced stromal cells bound threefold higher numbers of CFU-GM and BFU-E, this increase being abrogated by anti-alpha 4 beta 1 and anti-VCAM-1 antibodies. In addition, the adhesion to stroma of more immature progenitors, the long-term culture initiating cells, also occurred through an alpha 4 beta 1/VCAM-1-dependent mechanism. These studies identify an adhesion mechanism of potential importance in the localization of primitive progenitors within the hematopoietic microenvironment.

Production of granulocyte colony-stimulating factor and granulocyte/macrophage-colony-stimulating factor after interleukin-1 stimulation of marrow stromal cell cultures from normal or aplastic anemia donors

We have studied stromal cell function in naive or interleukin-1 (IL-1)-stimulated (100 pg/ml) long-term marrow cultures (LTC) from 12 normal donors and 21 patients with severe aplastic anemia (AA). Conditioned media (CM) from normal LTC contained levels of erythroid burst-promoting activity (BPA) and granulocyte/macrophage (GM) colony-stimulating activity (CSA) comparable to those previously described (Migliaccio et al., [1990] Blood, 75:305-312). The addition of IL-1 to these cultures increased the level of CSA and, specifically, of granulocyte colony-stimulating factor (G-CSF) released. Anti-GM-CSF antibody neutralized BPA and CSA in normal naive LTC CM but only the CSA in the CM from IL-1-stimulated LTC. Since the concentrations of GM-CSF, as detected with a specific immunoassay, did not increase after IL-1 treatment, these data suggest that IL-1-stimulated cultures contain an unidentified growth factor having BPA. CM from AA stromal cells contained levels of CSA comparable to those observed in normal stromal cell CM but had significantly lower levels of BPA. Neither anti-GM-CSF nor anti-IL-3 antibodies neutralized the BPA in AA stromal cell CM. This activity may be related to that found in the CM of IL-1-treated normal stromal cells. In nearly 50% of stromal cell cultures of AA patients, addition of IL-1 failed to increase the BPA, CSA, or G-CSF. The presence of an inhibitor in naive or IL-1-treated AA stromal cell CM was excluded by adding the CM to IL-3-stimulated cultures. These findings suggest that G-CSF and GM-CSF genes are differentially regulated in the marrow microenvironment. Furthermore, a marrow microenvironment, deficient in BPA production and, in some cases, unresponsive to IL-1 could contribute to marrow failure in some patients with AA.

Cytomegalovirus and marrow function

Infection with cytomegalovirus (CMV) continues to be one of the most common complications following allogeneic bone marrow transplantation. A proportion of patients with CMV infection also experience neutropenia. To investigate the possible role of CMV in the suppression of hematopoiesis, we have examined the effect of CMV on the growth of isolated myeloid progenitors and on the production of myeloid cells in the long-term bone marrow culture (LTMC) system. In these studies, various isolates of CMV were added either directly to cultures of progenitors or to LTMC established from normal CMV-seronegative donors. In the first system, myelosuppression is manifested by a reduction in the number of colonies that grow. In the second system, myelosuppression is manifested by a reduction in the number of myeloid cells produced and released into the culture supernatant. Analysis of the data observed indicated that myelosuppression could in some cases be attributed to direct infection of myeloid progenitors. In other cases stromal cells were infected. In the latter cases, myelosuppression was then caused by an alteration in cytokines produced by the stromal cells. These observations made in vitro raise the possibility that comparable mechanisms may be responsible for the myelosuppression observed with CMV infection in vivo. To pursue this possibility we proposed to detect the CMV genome in defined subpopulations of marrow cells isolated from infected patients. Given the technical restrictions imposed by the small sample size available from patient marrow aspirations, our initial attempts to develop on appropriate technique involved isolation of cells from CMV-seropositive normal bone marrow donors. Using the polymerase chain reaction we were able to amplify CMV DNA contained within marrow cells of some healthy CMV-seropositive marrow donors.

CD34 expression by stromal precursors in normal human adult bone marrow

Normal bone marrow cells were isolated by fluorescence-activated cell sorting (FACS) on the basis of CD34 antigen expression and then assayed in vitro for colonies of fibroblastic cells (fibroblast colony-forming units [CFU-F]). Greater than 95% of detectable CFU-F were recovered in the CD34+ population, while their numbers were markedly depleted in the CD34- population. Additional experiments showed that the majority of CFU-F exhibited high forward and perpendicular light scatter and low-density CD34 antigen. Growth of sorted cells in medium optimized for long-term marrow culture (LTMC) produced a complex mixture of adherent stromal elements including fibroblasts, adipocytes, smooth muscle cells, and macrophages. Monoclonal antibody STRO-1, which identifies bone marrow stromal cells, reacted with approximately 5% of CD34+ cells, which included all CFU-F and stromal precursors in LTMC. Experiments using soybean agglutinin (SBA) further showed that these stromal elements were restricted to a population of bone marrow cells with the phenotype CD34+/SBA+. These properties of stromal precursors are quite distinct from those of primitive hematopoietic progenitors, showing that although the precursors of the hematopoietic and stromal systems share expression of CD34, they are otherwise phenotypically distinct cell types.

Identification of stromal cell precursors in human bone marrow by a novel monoclonal antibody, STRO-1

Murine IgM monoclonal antibody STRO-1 identifies a cell surface antigen expressed by stromal elements in human bone marrow (BM). STRO-1 binds to approximately 10% of BM mononuclear cells, greater than 95% of which are nucleated erythroid precursors, but does not react with committed progenitor cells (colony-forming unit granulocyte-macrophage [CFU-GM], erythroid bursts [BFU-E], and mixed colonies [CFU-Mix]). Fibroblast colony-forming cells (CFU-F) are present exclusively in the STRO-1+ population. Dual-color cell sorting using STRO-1 in combination with antibody to glycophorin A yields a population approximately 100-fold enriched in CFU-F in the STRO-1+/glycophorin A- population. When plated under long-term BM culture (LTBMC) conditions, STRO-1+ cells generate adherent cell layers containing multiple stromal cell types, including adipocytes, smooth muscle cells, and fibroblastic elements. STRO-1+ cells isolated from LTBMC at later times retain the capacity to generate adherent layers with a cellular composition identical to that of the parent cultures. The STRO-1-selected adherent layers are able to support the generation of clonogenic cells and mature hematopoietic cells from a population of CD34+ cells highly enriched in so-called long-term culture-initiating cells. We conclude that antibody STRO-1 binds to BM stromal elements with the capacity to transfer the hematopoietic microenvironment in vitro.

Selective elimination of breast cancer cells from human bone marrow using an antibody-Pseudomonas exotoxin A conjugate

A pancarcinoma monoclonal antibody (NR-LU-10), homogeneously reactive with human breast cancer cells, was conjugated to Pseudomonas exotoxin A. The immunotoxin was evaluated for its potential for purging breast cancer cells from human bone marrow. The immunotoxin NR-LU-10 antibody did not react with normal bone marrow preparations yet readily detected 1% contamination of bone marrow by MCF-7 breast cancer cells added to normal bone marrow without significantly inhibiting the colony-forming ability of bone marrow progenitor cells. NR-LU-10-Pseudomonas exotoxin A has potential for purging bone marrow of breast cancer cells without impairing the growth of bone marrow progenitor cells.

Mechanisms of cytomegalovirus-mediated myelosuppression: perturbation of stromal cell function versus direct infection of myeloid cells

Infection with cytomegalovirus (CMV) continues to be one of the most common complications following allogeneic bone marrow transplantation. To study the role of CMV in the suppression of hemopoiesis that frequently accompanies infection, we investigated the effect of CMV on the growth of isolated committed myeloid progenitors and on hemopoiesis in long-term bone marrow cultures. Laboratory strain AD169 had no effect on the growth and development of progenitor cells. In contrast, 40% of clinical isolates of CMV inhibited colony formation by up to 100%. In long-term bone marrow cultures all CMV isolates resulted in myelosuppression, which in the majority of cases was associated with the infection of stromal elements. Analysis of RNA from stromal cells infected with AD169 and one clinical isolate demonstrated a specific deficiency of granulocyte colony-stimulating factor transcripts. For a small proportion of the clinical isolates tested in long-term bone marrow cultures, suppression of hemopoiesis was correlated with infection of developing granulocytes. These studies suggest that CMV can impair hemopoiesis either through infection of stromal cells and consequent perturbation of growth factor production or by direct infection of myeloid cells.

Comparative analysis of hematopoietic growth factors released by stromal cells from normal donors or transplanted patients

We compared the erythroid burst-promoting activity (BPA) and colony-stimulating activity (CSA) released under serum-deprived conditions by stromal cells derived from nine normal subjects and from nine patients after bone marrow transplantation. BPA and CSA were defined according to the capacity of the conditioned media (CM) to stimulate formation of erythroid bursts and granulocyte/macrophage (GM) colonies in serum-deprived cultures of nonadherent marrow cells. Six patients (group A) failed to establish or maintain successful allografts during the study. The remaining three (group B) did not experience problems with engraftment. CM from all stromal cell cultures contained detectable levels of BPA. Preincubation of the CM with an anti-GM colony-stimulating factor (GM-CSF) monoclonal antibody (MoAb), but not with a rabbit anti-interleukin-3 (IL-3) serum, reduced BPA by an average of 94%. CM from normal and group B stromal cell cultures contained detectable CSA, and the levels correlated with the amounts of granulocyte-CSF (G-CSF) detected by a specific bioassay. G-CSF was not detectable in medium conditioned by stromal cells from transplanted patients with poor marrow function. These results indicate that CM from stromal cells from normal subjects and transplanted patients with good marrow function contain both GM-CSF and G-CSF, while CM from stromal cells from transplanted patients with poor marrow function contain detectable levels of GM-CSF only. The reduced capacity of these stromal cells to produce G-CSF is associated with a reduced capacity of the CM to sustain GM colony formation and may be associated with the inability of these patients to sustain their neutrophil counts in vivo.

Etiological mechanisms in immune-mediated aplastic anemia

An immune-mediated etiology for aplastic anemia was first supported by the observation that some patients recovered their marrow function after immunosuppressive or cytoreductive therapy. Subsequently, investigators tried to define this immune process by studying the effects of patient lymphocytes on the growth of hematopoietic colonies in vitro. Many investigators were able to demonstrate inhibition of in vitro hematopoiesis by mononuclear cells from aplastic anemia patients. In many cases, however, it appeared that allosensitization via blood transfusions was responsible for the inhibitory activity. In general, it has been difficult to determine if any abnormal lymphocyte activity observed in these patients is causal as opposed to consequential. Although these initial studies provided very little conclusive information regarding the pathogenesis of aplastic anemia, they did lead to a more complete understanding of the potential immune modulation of hematopoietic regulation. Taken together with the major technological advances achieved over the past decade, this new knowledge has made it possible to functionally dissect the hematopoietic system. By identifying the interactive cell populations in this system and determining what factors they produce and what activities these factors have, it should be possible to precisely define defects than can result in marrow failure.

CD8+/DR+/CD25--T-lymphocytes associated with marrow graft failure

Phenotypic and functional characteristics of peripheral blood mononuclear cells (PBMC) were studied in eight patients with poor graft function following HLA-identical T cell-depleted marrow transplantation. Similar patients with good graft function and normal individuals were used as controls. Freshly isolated PBMC from patients with failing grafts contained more CD3+ and CD8+ cells than PBMC from well engrafted patients. The CD8+ cells appeared activated insofar as they expressed DR antigens, but they did not express the low affinity IL-2 receptor recognized by Tac antibody (CD25) and they did not have increased cytolytic activities. After culture with phytohemagglutinin (PHA) and IL-2, PBMC from patients with poor graft function contained fewer CD2+ and CD4+ cells than cultured PBMC from patients with good graft function. Cultured cells from patients with poor graft function acquired lymphokine activated killer (LAK) activity against NK-sensitive and NK-insensitive targets, but still did not express CD25. Host-mediated anti-donor cytotoxic activity could be demonstrated in one patient only after presensitization with donor cells and culture with IL-2 and PHA. The abnormalities in T cell activation observed in patients with poor graft function did not correlate with the donor or host origin of lymphoid cells. These data indicate that some cases of graft failure may be associated with defective T cell maturation. These abnormalities may simply represent a consequence of marrow failure or they may actually contribute to failure by not providing critical hematopoietic accessory functions.

Antibody-mediated marrow failure after allogeneic bone marrow transplantation

Marrow graft failure observed in association with histocompatibility differences between donor and recipient is often attributed to rejection mediated by host-derived cytolytic T lymphocytes. The data presented in this report indicate that persistent host antibodies specific for donor antigen may also mediate graft failure, either by antibody-dependent cell-mediated cytotoxicity (ADCC), or complement-mediated cytotoxicity. In the case of HLA Class I disparity, where all donor cells express the target antigen, the presence of alpha-donor antibody was associated with complete graft failure and death. In the case of ABO blood group antigen disparity, the presence of alpha-donor antibody resulted in erythroid hypoplasia. The latter cases proved informative insofar as they established that host antibodies could persist for more than 18 months after chemoradiotherapy and impair marrow function.

Inhibition of erythroid progenitor growth is mediated by cytotoxic lymphocytes and not by natural killer cells or IFN-gamma

Alloantigen primed T cells (PTC) were recovered from MLR at day 6 and 12, then added to cultures of erythroid progenitors, erythroid burst-forming units, BFU-E. The PBMC source of BFU-E was prepared either to retain or deplete APC by treatment with appropriate mAb and C. BFU-E grown in cocultures were counted at day 14 and replicate cultures assayed for IFN-gamma production on days 1 to 7. Analysis of MLR cells indicated that large, rapidly cycling cells recovered from MLR at day 6 have significant NK activity, whereas CTL activity is minimal, and production of IFN-gamma requires reexposure to APC. The smaller, noncycling cells recovered from MLR at day 12 have comparable NK activity, also require reexposure to APC for IFN-gamma production, but in addition have significant CTL activity. The addition of day 12 MLR cells to BFU-E cultures results in MHC restricted inhibition of BFU-E growth, suggesting that the CTL activity and not the NK activity contained within this population of cells is responsible for BFU-E inhibition. Studies using enriched population of BFU-E indicated that appropriate APC are needed to trigger both IFN-gamma production and BFU-E inhibition by the PTC. By using various APC-BFU-E combinations it was determined that after reexposure of PTC to appropriate APC, the inhibition of BFU-E was still target-specific indicating a direct effect between the PTC and BFU-E.

Inhibition of erythroid progenitor growth is mediated by cytotoxic lymphocytes and not by natural killer cells or IFN-gamma

Alloantigen primed T cells (PTC) were recovered from MLR at day 6 and 12, then added to cultures of erythroid progenitors, erythroid burst-forming units, BFU-E. The PBMC source of BFU-E was prepared either to retain or deplete APC by treatment with appropriate mAb and C. BFU-E grown in cocultures were counted at day 14 and replicate cultures assayed for IFN-gamma production on days 1 to 7. Analysis of MLR cells indicated that large, rapidly cycling cells recovered from MLR at day 6 have significant NK activity, whereas CTL activity is minimal, and production of IFN-gamma requires reexposure to APC. The smaller, noncycling cells recovered from MLR at day 12 have comparable NK activity, also require reexposure to APC for IFN-gamma production, but in addition have significant CTL activity. The addition of day 12 MLR cells to BFU-E cultures results in MHC restricted inhibition of BFU-E growth, suggesting that the CTL activity and not the NK activity contained within this population of cells is responsible for BFU-E inhibition. Studies using enriched population of BFU-E indicated that appropriate APC are needed to trigger both IFN-gamma production and BFU-E inhibition by the PTC. By using various APC-BFU-E combinations it was determined that after reexposure of PTC to appropriate APC, the inhibition of BFU-E was still target-specific indicating a direct effect between the PTC and BFU-E.

Effects of treating marrow with a CD3-specific immunotoxin for prevention of acute graft-versus-host disease

Data from human clinical trials and animal experiments have suggested that T lymphocytes in donor marrow help to facilitate engraftment after allogeneic bone marrow transplantation, possibly through a suppressive effect on the immunity of the recipient. In previous studies marrows from HLA-identical donors were treated ex vivo with a mixture of eight monoclonal antibodies together with rabbit complement to achieve a 3-log depletion of T cells and CD3-negative lymphoid cells. Transplantation of this marrow was associated with a 27% actuarial risk of graft failure in leukemic recipients conditioned with cyclophosphamide (120 mg/kg) and 15.75 Gy fractionated total body irradiation. In the present study, we employed an anti-CD3 ricin A-chain-containing immunotoxin (64.1-A) together with 20 mM NH4Cl to achieve a selective 3-log depletion of CD3-positive cells. The patient entry criteria and pretransplant conditioning regimen were identical to those used in previous studies. Despite the differences in marrow treatment, the clinical outcome of the present study was similar to that obtained previously. Graft-versus-host disease (GVHD) was largely prevented without the need for post-transplant immunosuppression, but two of the eight patients developed graft failure. These results indicate that CD3-negative cells have little or no ability to initiate GVHD. To the extent that graft failure in this study was not caused by stem cell damage or loss of CD3-negative cells during ex vivo processing of the marrow, it appears that the lymphoid cells required for facilitating allogeneic engraftment under these conditions are CD3-positive.

Graft failure in patients receiving T cell-depleted HLA-identical allogeneic marrow transplants

Results of a previous study suggested that the risk of graft failure after transplantation of HLA-identical T cell-depleted marrow may be influenced by the preparative regimen. Subsequent clinical trials were carried out to clarify this relationship and to determine whether post-transplant immunosuppression could have an effect on graft durability. Two factors were found to be associated with graft failure. Patients with hematologic malignancy given a preparative regimen of cyclophosphamide (120 mg/kg) and 15.75 Gy fractionated total body irradiation (TBI) had a 27% cumulative incidence of graft failure, which was less than the 69% incidence seen previously in patients given cyclophosphamide and 12.0 Gy fractionated TBI (p less than 0.05, log-rank test). Patients with acute leukemia had a higher risk of graft failure than patients with chronic myelogenous leukemia (p less than 0.005). The incidence of graft failure was not influenced by post-transplant immunosuppression with cyclosporine, methotrexate or a combination of cyclosporine plus methotrexate or by the omission of all post-transplant immunosuppression. Similarly, graft failure was not associated with the complement lot used for marrow treatment, the recovery of BFU-E or CFU-GM, or with the number of nucleated cells or T cells in the graft. The effect of primary diagnosis and the inverse relationship between the amount of pretransplant TBI and the graft failure rate suggest that a host factor may have been involved in a presumably immune-mediated rejection. This observation further leads to the inference that certain T cells present in donor marrow can suppress host immunity or help to maintain function of the graft.

Monitoring of cyclosporine therapy with in vitro biological assays

We examined the correlation between cyclosporine (CsA) levels and in vitro assays of immune function and hematopoiesis. Mixed lymphocyte reaction (MLR), mitogen responses, suppressor cell (SC), cell-mediated lympholysis (CML), and erythroid colony (EC) assays were studied in dogs, and in vitro megakaryocytopoiesis was studied in mice. Serum CsA concentrations were measured by radioimmunoassay. After oral or intramuscular CsA dosing, lymphocyte proliferation, as measured by MLR, inversely correlated with in vivo serum CsA concentration. MLR responses decreased rapidly, and nearly complete inhibition coincided with peak CsA levels. While CsA concentration-related suppression of lymphocyte stimulation was also observed in mitogen-stimulated cultures, results were less predictable and similar to results with in vitro CsA addition, and higher serum CsA levels were required to achieve comparable suppression. In vivo serum or in vitro CsA levels greater than 300 ng/ml completely inhibited the development of cytotoxic effector cells but had no measureable effect on the expression of suppressor cells in the same cultures. Furthermore, CsA serum also caused concentration-related inhibition of EC growth. The addition of human embryonic kidney-conditioned medium, however, abrogated CsA-related inhibition of EC growth, which suggested that CsA indirectly inhibited EC growth, presumably by interfering with CsA-sensitive accessory cells. This was supported by studies in an in vitro model of murine megakaryocytopoiesis. In normal conditioned medium, megakaryocyte colonies were unaffected by the presence of CsA. However, when cells were cultured in conditioned medium prepared in the presence of CsA, profound inhibition of megakaryocyte growth was observed. These studies show that biologic assays can be used reliably to measure concentration-related changes in immunosuppressive activity of CsA. Further clinical studies are needed to evaluate the usefulness of pharmacodynamic monitoring of CsA therapy.

Host origin of marrow stromal cells following allogeneic bone marrow transplantation

Although it is generally agreed that stromal cells are important in the regulation of haematopoietic cell development, the origin of these phenotypically diverse cells has been a subject for debate for more than 50 years. Data which support the concept of a separate origin for the haematopoietic stem cell and the marrow stroma are derived from cytogenetic or enzyme marker studies of explanted and expanded stromal cells grown under conditions that do not allow haematopoiesis in vitro. Recent evidence in man and in mouse suggesting that the stromal cells capable of transferring the haematopoietic microenvironment in vitro are transplantable seemingly questions this dichotomy, one interpretation being the existence of a common haematopoietic/stromal 'stem cell'. We used in situ hybridization to discriminate donor cells from host in blood and bone marrow samples obtained from patients with functioning sex-mismatched but HLA-identical allografts. Without exception, marrow-derived stromal cells that proliferate in long-term cultures were found to be of host genotype, whereas the macrophage component of the adherent layer in these cultures originated from the donor.

Gamma-interferon in aplastic anemia: inability to detect significant levels in sera or demonstrate hematopoietic suppressing activity

A radioimmunoassay (RIA) was used to quantitate biologically active gamma interferon (INF-gamma) in sera and in supernatants of cultured mononuclear cells obtained from 50 patients with aplastic anemia. Only five of the 50 serum samples had INF-gamma levels above background (greater than 0 less than 0.5 units per mL). Detectable levels of spontaneous INF-gamma (0.3 to 868 U/mL) were found in 18 of the 50 mononuclear cell supernatants tested. The addition of patient sera or INF-gamma positive supernatants to cultures of normal hematopoietic colonies did not result in reduced colony growth. Flow cytofluorimetric analysis of mononuclear cells failed to establish a correlation between the presence of INF-gamma in supernatants and the number of activated T cells or natural killer (NK) cells in the mononuclear cell population. However, a significant correlation did exist between the presence of monocytes and the production of INF-gamma. Contrary to previous reports, our data suggest that patients with aplastic anemia do not have high circulating levels of INF-gamma. Unstimulated mononuclear cells from some patients will produce significant levels of INF-gamma, but this does not result in decreased in vitro hematopoiesis.