Human T, B, natural killer, and dendritic cells arise from a common bone marrow progenitor cell subset

Dendritic cells in allogeneic hematopoietic stem cell transplantation

In allogeneic hematopoietic stem cell transplantation (SCT), dendritic cells (DCs) as the most potent antigen-presenting cells play a central role in the development of acute and chronic graft-vs-host disease (GVHD), in graft-vs-leukemia or -malignancy reactions and in fighting infectious complications. Functional maturity and distribution of DC sub-types (DC1 and DC2) differ between the different stem cell sources used (bone marrow, granulocyte colony-stimulating factor-mobilised peripheral blood and cord blood) resulting in various rates of graft-vs-host disease and graft-vs-leukemia activity. Although DC recovery following stem cell transplantation is prompt, graft-vs-host disease and the use of immunosuppressive drugs result in qualitative and quantitative disturbances in DC homeostasis and have been observed for up to 1 year after transplantation. Complete donor DC chimerism seems to be a pre-requisite for the development of chronic GVHD and for graft-vs-leukemia activity, at least following reduced-intensity transplants, although in the early phase of acute graft-vs-host disease the presence of host antigen-presenting cells is essential. Preliminary data show promising results with DC-based immunotherapy for treatment of viral and fungal infections and of leukemic relapse following allogeneic stem cell transplantation. More information on the mechanisms and interactions between dendritic cells and regulatory T cells is needed for DC vaccination concepts for modulation of graft-vs-host disease.

A clonogenic bone marrow progenitor specific for macrophages and dendritic cells

Macrophages and dendritic cells (DCs) are crucial for immune and inflammatory responses and belong to a network of cells that has been termed the mononuclear phagocyte system (MPS). However, the origin and lineage of these cells remain poorly understood. Here, we describe the isolation and clonal analysis of a mouse bone marrow progenitor that is specific for monocytes, several macrophage subsets, and resident spleen DCs in vivo. It was also possible to recapitulate this differentiation in vitro by using treatment with the cytokines macrophage colony-stimulating factor and granulocyte-macrophage colony-stimulating factor. Thus, macrophages and DCs appear to renew from a common progenitor, providing a cellular and molecular basis for the concept of the MPS.

Human skin cells support thymus-independent T cell development

Thymic tissue has previously been considered a requirement for the generation of a functional and diverse population of human T cells. We report that fibroblasts and keratinocytes from human skin arrayed on a synthetic 3-dimensional matrix support the development of functional human T cells from hematopoietic precursor cells in the absence of thymic tissue. Newly generated T cells contained T cell receptor excision circles, possessed a diverse T cell repertoire, and were functionally mature and tolerant to self MHC, indicating successful completion of positive and negative selection. Skin cell cultures expressed the AIRE, Foxn1, and Hoxa3 transcription factors and a panel of autoantigens. Skin and bone marrow biopsies can thus be used to generate de novo functional and diverse T cell populations for potential therapeutic use in immunosuppressed patients.

Identification of bovine dendritic cell phenotype from bovine peripheral blood

Dendritic cells (DCs) are professional antigen presenting cells, which initiate primary immune responses and also play an important role in the generation of peripheral tolerance. There is no reliable method established for the isolation of bovine peripheral blood DCs, and furthermore, the phenotypes and the functions of bovine DCs are still not fully clear. In the present study, we have attempted to identify bovine peripheral blood DCs by negative-selection. In bovine peripheral blood mononuclear cells (PBMC), we have newly characterized the phenotype of DCs, which is CD11c+/CD172a+. These cells display features of myeloid type DCs. In the thymic medulla, CD11c+/CD172a+ cells were also present and CD1+/CD172a+ cells were additionally detected as a population of DCs. The data suggest that one of the bovine DCs phenotypes from PBMC is derived from myeloid lineages lacking a CD1 molecule, which then drift to several tissues, and that they then may express a CD1 molecule upon their functional differentiation.

Expression of CD24 on CD19- CD79a+ early B-cell progenitors in human bone marrow

CD24 is a surface marker expressed in immature and mature B cells and involved in cellular adhesion and apoptosis. There are no data, which delineate the stage in early development of human B cells, which marks the expression of CD24. We studied lymphopoiesis in normal pediatric bone marrow (BM) and found that 1.5+/-0.2% of WBC were CD24(+) lymphocytes which did not express CD19. A significant fraction of these cells expressed low levels of CD45 (CD19- CD24+ CD45low cells). Small numbers of CD19- CD24+ CD45low cells were found in the regenerating BM of children with acute lymphoblastic leukemia after the completion of chemotherapy and in normal adult BM. Flow cytometric analyses have shown that CD19- CD24+ CD45low lymphocytes express CD10, CD34, CD79a, CD179a (VpreB), and TdT markers, i.e., displayed antigenic properties of early B-cell progenitors. Our data indicate that CD19- early B-cell progenitors in human BM express CD24, and that the expression of CD24 in human B-cell development precedes the expression of CD19.

Unilineage monocytopoiesis in hematopoietic progenitor culture: switching cytokine treatment at all Mo developmental stages induces differentiation into dendritic cells

We have developed a new culture system whereby human hematopoietic progenitors purified from adult peripheral blood extensively proliferate and gradually differentiate into >95% pure monocytic (Mo) cells. At all developmental stages treatment with interleukin (IL)-4+granulocyte-macrophage colony-stimulating factor or IL-4+c-Kit-ligand+FLT-3 ligand switched the Mo precursors into dendritic cells (DCs). The switching capacity decreased only at the end of the culture, when most Mo cells matured to macrophages. Moreover, the Mo precursors were highly susceptible to transduction with lentiviral vectors: once switched to DCs, they maintained the transgene expression, as well as the phenotype and function of the DC lineage. Our results provide new insight into the potential role of the Mo lineage as a reservoir of DCs in vivo. Furthermore, the methodology for transduction of Mo precursors provides a tool to generate genetically modified, normally functioning DCs potentially useful for immunotherapy.

Mobilization of Human Lymphoid Progenitors after Treatment with Granulocyte Colony-Stimulating Factor

Hemopoietic stem and progenitor cells ordinarily residing within bone marrow are released into the circulation following G-CSF administration. Such mobilization has a great clinical impact on hemopoietic stem cell transplantation. Underlying mechanisms are incompletely understood, but may involve G-CSF-induced modulation of chemokines, adhesion molecules, and proteolytic enzymes. We studied G-CSF-induced mobilization of CD34+ CD10+ CD19- Lin- and CD34+ CD10+ CD19+ Lin- cells (early B and pro-B cells, respectively). These mobilized lymphoid populations could differentiate only into B/NK cells or B cells equivalent to their marrow counterparts. Mobilized lymphoid progenitors expressed lymphoid- but not myeloid-related genes including the G-CSF receptor gene, and displayed the same pattern of Ig rearrangement status as their bone marrow counterparts. Decreased expression of VLA-4 and CXCR-4 on mobilized lymphoid progenitors as well as multipotent and myeloid progenitors indicated lineage-independent involvement of these molecules in G-CSF-induced mobilization. The results suggest that by acting through multiple trans-acting signals, G-CSF can mobilize not only myeloid-committed populations but a variety of resident marrow cell populations including lymphoid progenitors.

Hematopoietic stem and progenitor cells: clinical and preclinical regeneration of the hematolymphoid system

A vast literature exists on the biology of blood formation and regeneration under experimental and clinical conditions. The field of hematopoiesis was recently advanced by the capacity to purify to homogeneity primitive hematopoietic stem and progenitor cells. Isolation of cells at defined maturational stages has enhanced the understanding of the fundamental nature of stem cells, including how cell fate decisions are made, and this understanding is relevant to the development of other normal as well as malignant tissues. This review updates the basic biology of hematopoietic stem cells (HSC) and progenitors, the evolving use of purified HSC as grafts for clinical hematopoietic cell transplantation (HCT) including immune tolerance induction, and the application of HSC biology to other stem cell fields.

Engraftment of NOD/SCID/gammacnull mice with multilineage neoplastic cells from patients with juvenile myelomonocytic leukaemia

Several lines of evidence indicate the clonal nature of juvenile myelomonocytic leukaemia (JMML), involving myeloid, erythroid, megakaryocyte and B-lymphoid lineages. However, it is unclear whether the T-lymphocyte lineage is involved. We demonstrated that cells from six patients with JMML repopulated in non-obese diabetic/severe combined immunodeficient/gammac(null) mice and differentiated into granulocytes, monocytes, erythrocytes, B lymphocytes, T lymphocytes and natural killer cells. The percentage of human CD45 antigen-positive cells ranged from 41% to 73% in the murine bone marrow 12 weeks after transplantation. To examine the involvement of lymphocyte subpopulations, we purified human CD3(+), CD19(+) and CD56(+) cells from murine bone marrow cells transplanted from a patient with monosomy 7. Fluorescence in situ hybridization (FISH) showed the clonal marker in 96-100% of purified CD3(+), CD19(+) and CD56(+) subpopulations. These findings support the concept that JMML originates in transplantable multilineage haematopoietic stem cells. This novel murine xenotransplant model should be useful for investigating the nature of stem cells and testing new therapies for patients with JMML.

Growth hormone-induced stimulation of multilineage human hematopoiesis

Growth hormone (GH) has been shown to have significant positive effects on hemato-lymphopoiesis in rodent models and, more recently, to increase thymic mass and circulating naïve CD4+ T cells in humans infected with the human immunodeficiency virus, type 1. To determine whether the latter effects on human T lymphopoiesis might be due, at least in part, to effects on the bone marrow (BM), we examined the specific effects of GH and its proximal mediator, insulin-like growth factor I (IGF-I), on human multilineage hematopoiesis in fetal BM (FBM). Using in vitro analysis, we found that GH and IGF-I each stimulated the expansion of primitive multilineage CD34+CD38- hematopoietic progenitor cells and increased yields of several hematopoietic subpopulations, including CD34+CD38+CD10+ lymphoid progenitor cells. Additionally, GH and IGF-I had direct effects on FBM stromal elements, inducing the expansion of myeloid-like CD45+CD14+ FBM stromal cells and enhancing production of the hematopoietic cytokine interleukin-3 by fibroblast-like CD45-CD10+ FBM stromal cells. Surface expression of GH and type-I IGF receptors correlated with the observed biologic responses to these hormones. Whereas GH enhanced the proliferation of FBM progenitors and stroma, IGF-I exerted a predominantly antiapoptotic effect. Finally, both GH and IGF-I stimulated the generation of hematopoietic colony forming cells. These findings identify specific targets of GH and IGF-I within human FBM, and demonstrate direct and indirect effects that may contribute to GH-mediated enhancement of human hemato-lymphopoiesis.

Development of functional human blood and immune systems in NOD/SCID/IL2 receptor {gamma} chain(null) mice

Here we report that a new nonobese diabetic/severe combined immunodeficient (NOD/SCID) mouse line harboring a complete null mutation of the common cytokine receptor gamma chain (NOD/SCID/interleukin 2 receptor [IL2r] gamma(null)) efficiently supports development of functional human hemato-lymphopoiesis. Purified human (h) CD34(+) or hCD34(+)hCD38(-) cord blood (CB) cells were transplanted into NOD/SCID/IL2rgamma(null) newborns via a facial vein. In all recipients injected with 10(5) hCD34(+) or 2 x 10(4) hCD34(+)hCD38(-) CB cells, human hematopoietic cells were reconstituted at approximately 70% of chimerisms. A high percentage of the human hematopoietic cell chimerism persisted for more than 24 weeks after transplantation, and hCD34(+) bone marrow grafts of primary recipients could reconstitute hematopoiesis in secondary NOD/SCID/IL2rgamma(null) recipients, suggesting that this system can support self-renewal of human hematopoietic stem cells. hCD34(+)hCD38(-) CB cells differentiated into mature blood cells, including myelomonocytes, dendritic cells, erythrocytes, platelets, and lymphocytes. Differentiation into each lineage occurred via developmental intermediates such as common lymphoid progenitors and common myeloid progenitors, recapitulating the steady-state human hematopoiesis. B cells underwent normal class switching, and produced antigen-specific immunoglobulins (Igs). T cells displayed the human leukocyte antigen (HLA)-dependent cytotoxic function. Furthermore, human IgA-secreting B cells were found in the intestinal mucosa, suggesting reconstitution of human mucosal immunity. Thus, the NOD/SCID/IL2rgamma(null) newborn system might be an important experimental model to study the human hemato-lymphoid system.

IL-3 or IL-7 increases ex vivo gene transfer efficiency in ADA-SCID BM CD34+ cells while maintaining in vivo lymphoid potential

To improve maintenance and gene transfer of human lymphoid progenitors for clinical use in gene therapy of adenosine deaminase (ADA)-deficient SCID we investigated several gene transfer protocols using various stem cell-enriched sources. The lymphoid differentiation potential was measured by an in vitro clonal assay for B/NK cells and in the in vivo SCID-hu mouse model. Ex vivo culture with the cytokines TPO, FLT3-ligand, and SCF (T/F/S) plus IL-3 or IL-7 substantially increased the yield of transduced bone marrow (BM) CD34(+) cells purified from ADA-SCID patients or healthy donors, compared to T/F/S alone. Moreover, the use of IL-3 or IL-7 significantly improved the maintenance of in vitro B cell progenitors from ADA-SCID BM cells and allowed the efficient transduction of B and NK cell progenitors. Under these optimized conditions transduced CD34(+) cells were efficiently engrafted into SCID-hu mice and gave rise to B and T cell progeny, demonstrating the maintenance of in vivo lymphoid reconstitution capacity. The protocol based on the T/F/S + IL-3 combination was included in a gene therapy clinical trial for ADA-SCID, resulting in long-term engraftment of stem/progenitor cells. Remarkably, gene-corrected BM CD34(+) cells obtained from one patient 4 and 11 months after gene therapy were capable of repopulating the lymphoid compartment of SCID-hu hosts.

Roles of lymphoid cells in the differentiation of Langerhans dendritic cells in mice

Langerhans dendritic cells are antigen presenting cells (APC) that reside within the epidermis and are capable of stimulating naive T cells. Reciprocally, lymphocytes may play a role in Langerhans cells (LC) differentiation. Our results show that the differentiation of skin LC is unaffected in the absence of lymphocytes and/or signaling through the common cytokine receptor gamma chain (gammac) required for IL-2, IL-4, IL-7, IL-9, IL-15 and IL-21 signaling. Migration of LC and other dendritic cells (DC) from the skin to the draining lymph nodes (LNs) after FITC skin sensitization, is unaffected in the absence of lymphocytes or CD40. FITC+ LC/DC sorted from the LNs of lymphoid deficient or control mice stimulated naive T cells with similar efficiency. However, while the absence of lymphocytes did not appear to affect the phenotype or number of emigrating LN DC/LC, their persistence in the LN appears to depend on alphabeta T cells. Thus, DC are strikingly reduced in numbers in the peripheral LNs of T-cell deficient mice. Finally, CD8alpha expression on skin emigrants was low and dependent on the presence of CD8+ lymphocytes, while spleen CD8+ DC were present in the absence of lymphocytes. We conclude that the presence of T cells is not required for the differentiation and migration of resident skin DC but is critical for the maintenance of DC and LC migrating into the LNs.

A human CD34(+) subset resides in lymph nodes and differentiates into CD56bright natural killer cells

In humans, T cells differentiate in thymus and B cells develop in bone marrow (BM), but the natural killer (NK) precursor cell(s) and site(s) of NK development are unclear. The CD56bright NK subset predominates in lymph nodes (LN) and produces abundant cytokines compared to the cytolytic CD56dim NK cell that predominates in blood. Here, we identify a novel CD34dimCD45RA(+) hematopoietic precursor cell (HPC) that is integrin alpha4beta7bright. CD34dimCD45RA(+)beta7bright HPCs constitute <1% of BM CD34(+) HPCs and approximately 6% of blood CD34(+) HPCs, but >95% of LN CD34(+) HPCs. They reside in the parafollicular T cell regions of LN with CD56bright NK cells, and when stimulated by IL-15, IL-2, or activated LN T cells, they become CD56bright NK cells. The data identify a new NK precursor and support a model of human NK development in which BM-derived CD34dimCD45RA(+)beta7bright HPCs reside in LN where endogenous cytokines drive their differentiation to CD56bright NK cells in vivo.

CD34+ cells cultured in stem cell factor and interleukin-2 generate CD56+ cells with antiproliferative effects on tumor cell lines

In vitro stimulation of CD34+ cells with IL-2 induces NK cell differentiation. In order to define the stages of NK cell development, which influence their generation from CD34 cells, we cultured G-CSF mobilized peripheral blood CD34+ cells in the presence of stem cell factor and IL-2. After three weeks culture we found a diversity of CD56+ subsets which possessed granzyme A, but lacked the cytotoxic apparatus required for classical NK-like cytotoxicity. However, these CD56+ cells had the unusual property of inhibiting proliferation of K562 and P815 cell lines in a cell-contact dependent fashion.

Phosphatidic acid and tumor necrosis factor-alpha induce the expression of CD83 through mitogen activated protein kinase pathway in a CD34+ hematopoietic progenitor cell line, KG1

To elucidate the signaling pathways involved in the expression of CD83, which is linked to the differentiation and maturation states of dendritic cells, we examined the effect of phosphatidic acid (PA) on the expression of CD83 in KG1, a CD34(+) hematopoietic progenitor cell. In the presence of tumor necrosis factor (TNF)-alpha, PA but not lyso-PA up-regulated CD83 on KG1 cells. Moreover, PA and TNF-alpha-induced expression of CD83 was slightly increased by propranolol, an inhibitor of PA phosphohydrolase but was unaffected by phospholipase A2 inhibitor. PA and TNF-alpha increased the phosphorylation of extracellular signal-regulated kinase (ERK)-1/2, p38-kinase, and c-Jun N-terminal kinase (JNK) by Western blotting. However, the up-regulation of CD83 by PA/TNF-alpha on KG1 was significantly abrogated by PD98059, a specific inhibitor of ERK kinase, but was enhanced by SP600125, a JNK inhibitor. Bis-indolylmaleimide, an inhibitor of protein kinase C, partially blocked the up-regulation of CD83 and ERK phosphorylation induced by PA and TNF-alpha. Moreover, the incubation of KG1 cells with phorbol ester and TNF-alpha for 5 days increased the protein level of phospholipase D. These results suggest that PA and TNF-alpha induce the up-regulation of CD83 and that their action is regulated by ERK and JNK.

Clonal type I interferon-producing and dendritic cell precursors are contained in both human lymphoid and myeloid progenitor populations

Because of different cytokine responsiveness, surface receptor, and transcription factor expression, human CD11c⁻ natural type I interferon–producing cells (IPCs) and CD11c⁺ dendritic cells were thought to derive through lymphoid and myeloid hematopoietic developmental pathways, respectively. To directly test this hypothesis, we used an in vitro assay allowing simultaneous IPC, dendritic cell, and B cell development and we tested lymphoid and myeloid committed hematopoietic progenitor cells for their developmental capacity. Lymphoid and common myeloid and granulocyte/macrophage progenitors were capable of developing into both functional IPCs, expressing gene transcripts thought to be associated with lymphoid lineage development, and into dendritic cells. However, clonal progenitors for both populations were about fivefold more frequent within myeloid committed progenitor cells. Thus, in humans as in mice, natural IPC and dendritic cell development robustly segregates with myeloid differentiation. This would fit with natural interferon type I–producing cell and dendritic cell activity in innate immunity, the evolutionary older arm of the cellular immune system.

Identification of hematopoietic stem/progenitor cells: strength and drawbacks of functional assays

A major challenge in hematopoiesis is to conceive assays that could bring useful insights into experimental and clinical hematology. This means identifying separately the various classes of hematopoietic progenitors that are produced sequentially during the progression from stem cells to differentiated functional cells. Standardized short-term colony assays easily quantify lineage-committed myeloid precursors, but identification of primitive cells, which have both the ability to repopulate durably myeloid and lymphoid lineages and perhaps to self-renew, still depends on in vivo assays. Whatever the assay, two important requisites have to be solved: one is the definition of appropriate read-outs that will depend solely on the function of these cells, and the second is to evaluate precisely their numbers and proliferative potential in quantitative assays. When evaluating hematopoiesis, three parameters have to be taken into account: (1) the lack of reliable correlation between the phenotype of a given cell and its function. This is especially problematic in post-transplantation situations where cells from transplanted animals are analysed; (2) functionally heterogeneous cells are identified in a single assay; and (3) ontogeny-related changes in hematopoietic cell proliferation and self-renewal that, in human beings, hampers the exploration of adult stem cells. Nevertheless, years of progress in the manipulation of hematopoietic stem cells have recently resulted in the purification of a cell subset that repopulates irradiated recipients with absolute efficiency.

Acute lymphoblastic leukemia with the phenotype of a putative B-cell/T-cell bipotential precursor

Biphenotypic acute leukemias (BALs) are uncommon. Most are of myeloid-B-cell or myeloid-T-cell lineage. We report herein a 70-year-old man with an unusual acute leukemia where the blasts expressed both B- and T-lymphoid markers. He presented to us with an enlarging cutaneous tumor. The presenting peripheral blood and bone marrow aspirate showed 40% and 90% blasts, respectively, which were negative for the usual cytochemical stains. The flow cytometric analysis revealed that the blasts were positive for CD19, CD20, CD22, cytoplasmic (Cyt) CD79a, CD10, Cyt CD3, CD5, CD7, CD4, HLA-DR, TdT, and were negative for myeloid markers. According to the scoring system from the European Group for the Immunological Characterization of Acute Leukaemias (EGIL), this case was an unequivocal B-cell/T-cell BAL. Conventional cytogenetic analysis revealed 46XY [t(4;11)(q31;q13), add(8)(q24), der(9)del(9)(p21)del(9)(q32q34), -13, +mar] in all 25 metaphases analyzed. Fluorescence in situ hybridization (FISH) and polymerase chain reaction (PCR) for 11q23 rearrangements as well as t(9;22) were negative. PCR for both TCR-gamma and IgH gene analyses revealed polyclonal rearrangements. We postulate that this case of BAL might have arisen from the putative common lymphoid progenitor cell.

Blood dendritic cells in patients with acute lymphoblastic leukaemia

Myeloid and plasmacytoid dendritic cells (MDCs, PDCs) play a key role in the initiation of immune responses. In this study, we show a severe reduction of MDCs and PDCs in patients with B lineage acute lymphoblastic leukaemia (B-ALL; P = 0.01 vs. controls). DCs from patients with T lineage ALL (T-ALL) were quantitatively and functionally comparable to healthy donors, as demonstrated by secretion of interleukin (IL)-12p70 and interferon-alpha. In vitro, the circulating CD34(+) fraction of B-ALL cases did not generate either CD1a(+) MDCs or PDCs, suggesting that DC development is probably affected in B-ALL, but not in T-ALL.

G-CSF-mobilized CD34+ cells cultured in interleukin-2 and stem cell factor generate a phenotypically novel monocyte

To study the early stages of development from stem cells of the CD56+ cell population [which includes natural killer (NK) cells], granulocyte-colony stimulating factor-mobilized peripheral blood CD34+ cells from healthy donors were sorted to >99% purity and cultured in the presence of stem cell factor and interleukin (IL)-2. After 3 weeks in culture, the majority of cells acquired CD33, with or without human leukocyte antigen-DR and CD14. In 20 stem cell donors tested, 8.7 +/- 8.8% of cells were CD56+. Two major CD56+ subsets were identified: CD56(bright), mainly CD33- cells (7+/-10%, n=11) with large, granular lymphocyte morphology, and CD56dim, mainly CD33+ (2.5+/-2, n=11) cells with macrophage morphology. The CD56bright population had cytoplasmic granzyme A but lacked killer inhibitory receptor, suggesting they were immature NK cells. The CD56dim, CD33+, population lacked NK markers. They may represent a minor subset of normal monocytes at a developmental stage comparable with the rare CD56+ CD33+ hybrid myeloid/NK cell leukemia. Consistent with a monocyte nature, CD56dimCD33+ proliferated and produced a variety of cytokines upon lipopolysaccharide stimulation, including IL-8, IL-6, monocyte chemoattractant protein-1, and macrophage-derived chemokine but not interferon-gamma. In a short-term cytotoxicity assay, they failed to kill but powerfully inhibited the proliferation of the NK-resistant cell line P815. The generation of CD56+ cells was negatively regulated by hyaluronic acid and IL-4, indicating that extracellular matrix may play an important role in the commitment of CD34+ cells into CD56 myeloid and lymphoid lineages.

Selective Generation of Different Dendritic Cell Precursors from CD34+ Cells by Interleukin-6 and Interleukin-3

There is a growing interest in generating dendritic cells (DCs) for using as vaccines. Several cytokines, especially stem cell factor (SCF) and FLT3-ligand (FL), have been identified as essential to produce large numbers of myeloid precursors and even to increase DC yield obtained by the action of granulocyte-macrophage colony-stimulating factor (GM-CSF) and tumor necrosis factor alpha (TNF-alpha). However, there are few studies on the effect of the early-acting cytokines, commonly used to expand CD34+ progenitor cells, on DC generation. We report here that in the absence of serum, SCF, FL, and thrombopoietin (TPO) plus interleukin-6 (IL-6) and SCF, FL, and TPO plus IL-3 were able to generate CD14+CD1a- and CD14- CD1a+ myeloid DC precursors from CD34+ cells, but IL-6 had an inhibitory effect on the generation of CD14- CD1a+ cells. Both DC precursors differentiated into mature DCs by GM-CSF, IL-4, and TNF-alpha, and DCs obtained from both types of culture exhibited equal allostimulatory capacity. CD1a+ DCs generated could be identified on the basis of DC-specific intracellular adhesion molecule-grabbing nonintegrin (DC-SIGN) expression, a novel C-type lectin receptor expressed on dermal DCs but not on Langerhans cells. In addition, the inclusion of IL-3 to the culture medium induced the appearance of CD13- cells that differentiated into plasmacytoid DC (DC2) on the addition of TNF-alpha, allowing the identification of developmental stages of DC2. Like true plasmacytoid DCs, these cells secreted interferon-alpha after TLR9-specific stimulation with a specific CpG nucleotide.

The dynamic life of natural killer cells

Natural killer (NK) cells play important roles in immunological processes, including early defense against viral infections. This review provides an overview of the dynamic in vivo life of NK cells from their development in the bone marrow to their mature peripheral responses and their ultimate demise, with particular emphasis on mouse NK cells and viral infections.

Engraftment of quiescent progenitors and conversion to full chimerism after nonmyelosuppressive conditioning and hematopoietic cell transplantation in miniature swine

Our laboratory has previously reported a nonmyelosuppressive preparative regimen for hematopoietic cell transplantation that leads to mixed chimerism and allograft tolerance in miniature swine across minor and major histocompatibility disparities. Stable chimerism persisted in most of these animals but was restricted to T cells and confined to peripheral blood. Because of the importance of myeloid and erythroid progenitors for the treatment of hematologic disorders, the objective of this study was to assess whether such cells existed in the bone marrow of these lymphoid chimeras as an indication of functional engraftment. Colony-formation assays were performed on donor inocula before infusion and on bone marrow cells harvested from the transplant recipients. Donor-origin myeloid/erythroid progenitor colonies were detected in bone marrow from 6 of 7 lymphoid chimeric recipients. A delayed donor leukocyte infusion successfully converted a stable lymphoid chimera to full multilineage chimerism within 2 weeks. Donor-origin myeloid/erythroid progenitors could be detected in the bone marrow of a host-matched recipient after myeloablation and adoptive transfer of mobilized cells from one of the engrafted lymphoid chimeras. These data suggest that even when only lymphoid chimerism is readily detected by flow cytometry, dormant myeloid/erythroid progenitors can exist and subsequent conversion to full donor chimerism can be achieved. The ability to establish multilineage engraftment and chimerism without significant toxicity may have important clinical implications for the management of nonmalignant hematopoietic disorders and hematologic malignancies.

Hemopoietic precursors and development of dendritic cell populations

The antigen presenting dendritic cells (DCs) are bone marrow (BM) derived cells. Despite their common functions of antigen-processing and T-lymphocyte activation, DCs are diverse in surface markers, migratory patterns and cytokine output. These differences can determine the fate of the T cells they activate. Several subsets of mature DCs have been described in both mouse and human, but tracing the origin of these specialised DC subsets has not been a trivial task. The original concept that all DCs were of myeloid origin was questioned by several recent studies, which demonstrated that in addition to the DCs derived from conventional myeloid precursors, some DCs could also be efficiently generated from lymphoid-restricted precursors. Moreover, it has been shown that both myeloid-restricted and lymphoid-restricted precursors were able to generate DC subsets with similar surface phenotype. These observations demonstrate the existence of both myeloid- and lymphoid-derived DC lineages and suggest an early developmental flexibility of DC precursors. The downstream points where the DC sub-lineages branch off from the conventional myeloid and lymphoid precursors, and the cytokines and environmental factors required for inducing their specialised functions are yet to be determined.

Absolute values of dendritic cell subsets in bone marrow, cord blood, and peripheral blood enumerated by a novel method

Dendritic cells (DCs) are pivotal in inducing immunity or alternatively downregulating immune reactivity. In humans, the opposing phenotypic subsets of CD11c(+)/CD123(-) "myeloid" DCs and CD123(+)/CD11c(-) "lymphoid" DCs have been proposed to orchestrate these immune responses. In this study we determined the absolute numbers of both subsets in three resting hematopoietic tissues by employing a novel flow cytometry method, eliminating processing steps and calculations based on mononuclear cell percentages. Internal bead standards along with the cells of interest were simultaneously acquired directly from unmanipulated whole blood specimens. We found significant differences (p < 0.001) between the mean absolute numbers of CD123(+)/CD11c(-) lymphoid DCs among the three sources, with the fewest present in peripheral blood (8.2/ micro l), about 50% more in cord blood (12.2/ micro l), and fivefold more in bone marrow (40.2/ micro l). Cord blood and bone marrow CD11c(+)/CD123(-) myeloid DC counts did not differ from each other (23.5/ micro l and 28.9/ micro l, respectively) but peripheral blood contained significantly fewer (15.5/ micro l, p = 0.006). CD11c(+)/CD123(-) DCs had significantly higher surface expression of HLA-DR (p < 0.001) in all three sources. To test for association with the DC subsets, T, B, and natural killer (NK) lymphocytes were also enumerated. In bone marrow only, significant correlations were found between the size of the CD123(+)/CD11c(-) lymphoid DC pool and NK cells (p = 0.0029) and B cells (p = 0.0033). These data support the hypothesis that a common CD123(+)/CD11c(-) DC, NK cell, and B cell progenitor is resident in marrow, and this cell may be identical to the common lymphoid progenitor previously described in mice and/or the human CD34(+)/Lin(-)/CD10(+) progenitor.

Dendritic cells: friend or foe in autoimmunity?

The large amount of information that has been acquired from human and animal models substantiates that the DC lineage system represents a double-edged sword in the immune system. Presumably, in normal physiology, tolerizing DCs guard against autoimmunity and control established immune reactions, whereas immunogenic DCs provide active host defenses. In autoimmune diseases, there is strong evidence to support the idea that tolerance is overridden by the development of immunogenic DCs that favor cross-priming. Based on the wide range of possible clinical applications, it is not surprising that manipulation of DCs for clinical benefit is rampant. Indeed, multiple clinical strategies are currently underway, including the development of DC immunotherapy for cancer vaccines and graft survival. In cancer, DC-based vaccines for solid tumors, such as melanoma, were well-tolerated and produced beneficial antitumor responses, even in patients who had advanced disease. Although initial trials such as these are highly promising, the ultimate goal is to develop DC-based strategies that will lead to highly specific, long-lasting immunity against the cancer cells. In autoimmune diseases and transplant settings, the goal is to devise strategies that will block the initiation and maintenance of autoreactive and antigraft responses, respectively. Specific strategies for autoimmune diseases might include interference with cross-priming events that activate autoreactive T cells and genetic engineering to introduce molecules that have immunosuppressive functions, such as IL-10, TGF3, Fas ligand, ILT3, and ILT4. Successful application to these diseases will necessitate high specificity. In this regard, recent preliminary studies that described antigen-specific suppression of a primed immune response by tolerogenic DCs are especially informative.

NK cell recovery, chimerism, function, and recognition in recipients of haploidentical hematopoietic cell transplantation following nonmyeloablative conditioning using a humanized anti-CD2 mAb, Medi-507

OBJECTIVE

Natural killer (NK) cells kill allogeneic cells that lack a class I MHC ligand for clonally distributed killer inhibitory receptors (KIR). Following HLA-mismatched hematopoietic cell transplantation (HCT), donor NK cells might mediate graft-vs-host (GVH) reactions that promote donor chimerism and mediate anti-tumor effects. Additionally, recipient NK cells might mediate donor marrow rejection. We have developed a nonmyeloablative approach to haploidentical HCT involving recipient treatment with a T cell-depleting mAb, Medi-507, that can achieve donor engraftment and mixed hematopoietic chimerism without graft-vs-host disease (GVHD). Donor lymphocyte infusions (DLI) are later administered in an effort to achieve graft-vs-leukemia/lymphoma (GVL) effects without GVHD. It is unknown whether NK cell "tolerance" develops in human mixed chimeras.

METHODS

We have addressed these issues in 12 patients receiving Medi-507-based nonmyeloablative haploidentical HCT.

RESULTS

NK cells recovered relatively early, despite the presence of circulating anti-CD2 mAb, but the majority of initially recovering cells lacked CD2 expression. These NK cells showed a reduced capacity, compared to those from normal donors, to kill class I-deficient targets. No association was detected between KIR mismatches in the host-vs-graft (HVG) or GVH direction and graft or tumor outcomes in this small series. NK cell chimerism did not correlate with chimerism in other lineages in mixed chimeras. NK cell tolerance to the host was not observed in a patient with full donor chimerism. One patient developed NK cell reactivity against donor-derived lymphoblast targets after loss of chimerism, despite the absence of an HVG KIR mismatch.

CONCLUSION

Our results do not show an impact of NK cells on the outcome of nonmyeloablative, even T cell-depleted, HCT across haplotype barriers using an anti-CD2 mAb. Our data also raise questions about the applicability of observations made with NK cell clones to the bulk NK cell repertoire in humans.

Enforced expression of EBF in hematopoietic stem cells restricts lymphopoiesis to the B cell lineage

Mice deficient in early B cell factor (EBF) are blocked at the progenitor B cell stage prior to immunoglobulin gene rearrangement. The EBF-dependent block in B cell development occurs near the onset of B-lineage commitment, which raises the possibility that EBF may act instructively to specify the B cell fate from uncommitted, multipotential progenitor cells. To test this hypothesis, we transduced enriched hematopoietic progenitor cells with a retroviral vector that coexpressed EBF and the green fluorescent protein (GFP). Mice reconstituted with EBF-expressing cells showed a near complete absence of T lymphocytes. Spleen and peripheral blood samples were >95 and 90% GFP+EBF+ mature B cells, respectively. Both NK and lymphoid-derived dendritic cells were also significantly reduced compared with control-transplanted mice. These data suggest that EBF can restrict lymphopoiesis to the B cell lineage by blocking development of other lymphoid-derived cell pathways.

Unique Properties of Fetal Lymphoid Progenitors Identified According to RAG1 Gene Expression

RAG1/GFP knockin mice were exploited to isolate and characterize fetal lymphoid progenitors. CD11b and IL-7Ralpha are expressed in a developmental stage-dependent fashion, revealing how substantial numbers of early lymphoid progenitors were discarded or neglected in previous studies. The myeloerythroid potential of fetal progenitors in clonal assays declined in synchrony with activation of the RAG1 locus but was not completely extinguished. Lymphoid differentiation corresponded to patterns of gene expression previously found for adult marrow, but no fraction of fetal liver was enriched with respect to B + T progenitors. Also, unlike adults, fetal lymphoid progenitors transiently expressed endothelial cell markers. These findings help to reconcile discrepancies in previous reports and suggest that the fetal immune system arises via unique mechanisms.

CD84 expression on human hematopoietic progenitor cells

OBJECTIVE

CD84 is a member of the CD2 subgroup of the immunoglobulin receptor superfamily. Members of this family have been implicated in the activation of T cells and NK cells. Expression of CD84 was originally described on most mononuclear blood cells as well as platelets. To elucidate its presence on other blood cell types, we analyzed the expression pattern of CD84 on human immature CD34+ and mature hematopoietic cells.

METHODS

Expression analysis was carried out by flow cytometry. The differentiation potential of CD84+ progenitor cells was assessed by colony-forming assays and long-term cultures. RT-PCR was used to analyze CD84 mRNA isoforms.

RESULTS

In addition to monocytes, macrophages, B cells, and some T cells, CD84 is expressed on the cell surface of the majority of granulocytes. In addition, 64%+/-5% of CD34+ progenitor cells isolated from peripheral blood and 30.5%+/-5% from bone marrow of healthy volunteers also express CD84. The majority of CD34+ cells coexpressing lineage antigens were CD84+. In methylcellulose CD34+CD84+ cells formed primarily erythroid colonies, whereas myeloid or mixed colonies were scarce. The frequency of long-term culture-initiating cells in peripheral blood was approximately fivefold higher in CD34+CD84- vs CD34+CD84+ cells. In short-term cultures, 95% of the initially CD34+CD84- cells became CD84+ after 72 hours.

CONCLUSIONS

CD84 is expressed on cells from almost all hematopoietic lineages and on CD34+ hematopoietic progenitor cells. The proliferative potential of CD34+ cells decreases with increasing CD84 expression, suggesting that CD84 serves as a marker for committed hematopoietic progenitor cells.

Biology of hematopoietic stem cells and progenitors: implications for clinical application

Stem cell biology is scientifically, clinically, and politically a current topic. The hematopoietic stem cell, the common ancestor of all types of blood cells, is one of the best-characterized stem cells in the body and the only stem cell that is clinically applied in the treatment of diseases such as breast cancer, leukemias, and congenital immunodeficiencies. Multicolor cell sorting enables the purification not only of hematopoietic stem cells, but also of their downstream progenitors such as common lymphoid progenitors and common myeloid progenitors. Recent genetic approaches including gene chip technology have been used to elucidate the gene expression profile of hematopoietic stem cells and other progenitors. Although the mechanisms that control self-renewal and lineage commitment of hematopoietic stem cells are still ambiguous, recent rapid advances in understanding the biological nature of hematopoietic stem and progenitor cells have broadened the potential application of these cells in the treatment of diseases.

Common lymphoid progenitors rapidly engraft and protect against lethal murine cytomegalovirus infection after hematopoietic stem cell transplantation

Lymphoid deficiency after allogeneic hematopoietic cell transplantation (HCT) results in increased susceptibility to infection; however, transplantation of mature lymphocytes frequently results in a serious complication known as graft-versus-host disease (GVHD). Here we demonstrate in mice that both congenic as well as allogeneic transplantation of low numbers of highly purified common lymphoid progenitors (CLPs)-a rare population of lymphoid-lineage-committed bone marrow cells-accelerates immune reconstitution after lethal irradiation and rescue with hematopoietic stem cells (HSCs). After congenic transplantation, 3 x 10(3) CLPs protected against murine cytomegalovirus (MCMV) infection at a level roughly equivalent to 107 unfractionated lymph node cells. In the allogeneic model of matched unrelated donor HSC transplantation, cotransplantation of 3 x 10(3) CLPs protected thymus-bearing as well as thymectomized hosts from MCMV infection and attenuated disease severity. Immunohistochemistry in combination with antibody depletion of T and natural killer (NK) cells confirmed that CLP-derived as well as residual host lymphocytes contribute to antiviral protection. Importantly, transplantation of allogeneic CLPs provided a durable antiviral immunity without inducing GVHD. These data support the potential for composing grafts with committed progenitors to reduce susceptibility to viral infection following HCT.

In vitro identification of human pro-B cells that give rise to macrophages, natural killer cells, and T cells

In this study we report the molecular and functional characterization of very early interleukin 7 receptor alpha (IL-7Ralpha)+-CD79a+CD19- B-cell progenitors, produced by human CD34+CD19-CD10- cord blood cells grown in the presence of stromal cells and cytokines. Purified IL-7Ralpha+CD79a+CD19- cells transcribed the B-lymphoid specific genes E2A, EBF, TdT, Rag-1, had initiated DJH rearrangements, but almost lacked Pax-5 mRNA. When exposed to appropriate environmental conditions, these cells repressed B-cell genes and completely differentiated into CD14+ macrophages, CD56+ natural killer cells, and CD4high T cells. Retention of the DJH rearranged genes in both CD14+ and CD56+ cells unambiguously demonstrates that early B-cell genes, expressed prior to Pax-5, can be activated in a multipotent human progenitor cell whose final fate, including in non-B lineages, is determined by external signals.

Functional characterization of human CD34+ cells that express low or high levels of the membrane antigen CD111 (nectin 1)

Nectins are recently described adhesion molecules that are widely expressed on many tissues, including the hematopoietic tissue. Nectin 1 (CD111) is expressed on a higher proportion of mobilized peripheral blood (mPB) than cord blood (CB) CD34+ cells, and of CD34+/CD38+ cells when compared with CD34+/CD38- cells. We studied functional properties of human CB and mPB CD34+ cells that express low or high levels of CD111. CD34+/CD111(dim) cells contain a higher proportion of cells in G0/G1 phase than CD34+/CD111(bright) cells. CD34+/CD111(bright) cells contain more erythroid progenitors: CFU-E, than their counterparts, which on the opposite contain more HPP-CFC. Limiting dilution analyses demonstrate a higher frequency of immature progenitors: cobblestone-area colony-forming cells, CD34+/CD111(dim) than in CD34+/CD111(bright) cells. In vitro differentiation assays demonstrate a higher frequency of B-, T- and dendritic-cell precursors, but less NK-cell precursors in CD34+/CD111(dim) cells. Evaluation of engraftment in NOD-SCID mice shows that SCID repopulating cells are more frequent among mPB CD34+/CD111(dim) cells. Liquid culture of CD34+/CD111(dim) cells with erythropoietin shows that CD111 expression increases simultaneously with CD36, following CD71 and before glycophorin A expression. In conclusion, immature human hematopoietic progenitors express low levels of CD111 on their surface. During erythroid differentiation CD34+ cells acquire higher levels of the CD111 antigen.

The lymphoid past of mouse plasmacytoid cells and thymic dendritic cells

There has been controversy over the possible lymphoid origin of certain dendritic cell (DC) subtypes. To resolve this issue, DC and plasmacytoid pre-DC isolated from normal mouse tissues were analyzed for transient (mRNA) and permanent (DNA rearrangement) markers of early stages of lymphoid development. About 27% of the DNA of CD8(+) DC from thymus, and 22-35% of the DNA of plasmacytoid pre-DC from spleen and thymus, was found to contain IgH gene D-J rearrangements, compared with 40% for T cells. However, the DC DNA did not contain IgH gene V-D-J rearrangements nor T cell Ag receptor beta gene D-J rearrangements. The same DC lineage populations containing IgH D-J rearrangements expressed mRNA for CD3 chains, and for pre-T alpha. In contrast, little of the DNA of the conventional DC derived from spleen, lymph nodes, or skin, whether CD8(+) or CD8(-), contained IgH D-J rearrangements and splenic conventional DC expressed very little CD3 epsilon or pre-T alpha mRNA. Therefore, many plasmacytoid pre-DC and thymic CD8(+) DC have shared early steps of development with the lymphoid lineages, and differ in origin from conventional peripheral DC.

Human cord blood CD34+Pax-5+ B-cell progenitors: single-cell analyses of their gene expression profiles

Circulating CD34(+) cells are used in reparative medicine as a stem cell source, but they contain cells already committed to different lineages. Many think that B-cell progenitors (BCPs) are confined to bone marrow (BM) niches until they differentiate into B cells and that they do not circulate in blood. The prevailing convention is that BCP transit a CD34(+)CD19(-)10(+) early-B-->CD34(+)CD19(+)CD10(+) B-cell progenitor (pro-B)-->CD34(-)CD19(+)CD10(+) B-cell precursor (pre-B) differentiation pathway within BM. However, populations of CD34(+)CD10(+) and CD34(+)CD19(+) cells circulate in adult peripheral blood and neonatal umbilical cord blood (CB) that are operationally taken as BCPs on the basis of their phenotypes, although they have not been submitted to a systematic characterization of their gene expression profiles. Here, conventional CD34(+)CD19(+)CD10(+) and novel CD34(+)CD19(+)CD10(-) BCP populations are characterized in CB by single-cell sorting and multiplex analyses of gene expression patterns. Circulating BCP are Pax-5(+) cells that span the early-B, pro-B, and pre-B developmental stages, defined by the profiles of rearranged V-D-J(H), CD79, VpreB, recombination activating gene (RAG), and terminal deoxynucleotidyl transferase (TdT) expression. Contrary to the expectation, circulating CD34(+)CD19(-)CD10(+) cells are essentially devoid of Pax-5(+) BCP. Interestingly, the novel CD34(+)CD19(+)CD10(-) BCP appears to be the normal counterpart of circulating preleukemic BCPs that undergo chromosomal translocations in utero months or years before their promotion into infant acute lymphoblastic B-cell leukemia after secondary postnatal mutations. The results underscore the power of single-cell analyses to characterize the gene expression profiles in a minor population of rare cells, which has broad implications in biomedicine.

The origin and development of the immune system with a view to stem cell therapy

Careful study of the phylogeny and ontogeny of the three components of the immune system reveals that the macrophage, lymphatic, and hematopoietic systems originate independently of each other. Chronologically, the most ancient is the macrophage system, which arises in the coelomic cavity as mesenchymal ameboid cells having the properties to recognize self from non-self and to ingest foreign particles. The lymphatic system later develops from the endoderm of pharyngeal pouches, where the thymic anlage differentiates. The lymphocytes that originate here seed all lymphatic organs and retain the ability to divide and thereby form multiple colonies (lymphatic nodules) in the respiratory and digestive tract; further diversification of lymphocytes follows after confrontation with antigens. The last component of the immune system to appear is the hematopoietic system, which originates from the splanchnic mesoderm of the yolk sac as hematogenic tissue, containing hemangioblasts. The hematogenic tissue remains attached to the outer wall of the vitelline vessels, which provides an efficient mechanism for introducing the hematogenic tissue into the embryo. In an appropriate microenvironment, the hemangioblasts give rise to sinusoidal endothelium and to hemocytoblasts - the bone marrow stem cells for erythrocytes, myeloid cells, and megakaryocytes. The facts and opinions presented in this article are not in agreement with the currently accepted dogma that a common "hematolymphatic stem cell" localized in the marrow generates all of the cellular components of blood and the immune system.

Transcriptional profiling identifies Id2 function in dendritic cell development

Dendritic cells (DCs) are potent antigen-presenting cells with a pivotal role in antigen-specific immune responses. Here, we found that the helix-loop-helix transcription factor Id2 is up-regulated during DC development in vitro and crucial for the development of distinct DC subsets in vivo. Id2-/- mice lack Langerhans cells (LCs), the cutaneous contingent of DCs, and the splenic CD8alpha+ DC subset is markedly reduced. Mice deficient for transforming growth factor (TGF)-beta also lack LCs, and we demonstrate here that, in DCs, TGF-beta induces Id2 expression. We also show that Id2 represses B cell genes in DCs. These findings reveal a TGF-beta-Id2 signaling pathway in DCs and suggest a mechanism by which Id2 affects the lineage choice of B cell and DC progenitors.

Analysis of TCR, pT alpha, and RAG-1 in T-acute lymphoblastic leukemias improves understanding of early human T-lymphoid lineage commitment

T-acute lymphoblastic leukemias (T-ALLs) derive from human T-lymphoid precursors arrested at various early stages of development. Correlation of phenotype and T-cell receptor (TCR) status with RAG-1 and pT alpha transcription in 114 T-ALLs demonstrated that they largely reflect physiologic T-lymphoid development. Half the TCR alpha beta lineage T-ALLs expressed a pre-TCR, as evidenced by RAG-1, pT alpha, and cTCR beta expression, absence of TCR delta deletion, and a sCD3(-), CD1a(+), CD4/8 double-positive (DP) phenotype, in keeping with a population undergoing beta selection. Most TCR gamma delta T-ALLs were pT alpha, terminal deoxynucleotidyl transferase (TdT), and RAG-1(lo/neg), double-negative/single-positive (DN/SP), and demonstrated only TCR beta DJ rearrangement, whereas 40% were pT alpha, TdT, and RAG-1 positive, DP, and demonstrated TCR beta V(D)J rearrangement, with cTCR beta expression in proportion. As such they may correspond to TCR alpha beta lineage precursors selected by TCR gamma delta expression, to early gamma delta cells recently derived from a pT alpha(+) common alpha beta/gamma delta precursor, or to a lineage-deregulated alpha beta/gamma delta intermediate. Approximately 30% of T-ALLs were sCD3/cTCR beta(-) and corresponded to nonrestricted thymic precursors because they expressed non-T-restricted markers such as CD34, CD13, CD33, and CD56 and were predominantly DN, CD1a, pT alpha, and RAG-1 low/negative, despite immature TCR delta and TCR gamma rearrangements. TCR gene configuration identified progressive T-lymphoid restriction. T-ALLs, therefore, provide homogeneous expansions of minor human lymphoid precursor populations that can aid in the understanding of healthy human T-cell development.

Two developmentally distinct populations of dendritic cells inhabit the adult mouse thymus: demonstration by differential importation of hematogenous precursors under steady state conditions

Although a variety of lymphoid and myeloid precursors can generate thymic dendritic cells (DCs) under defined experimental conditions, the developmental origin(s) of DCs in the steady state thymus is unknown. Having previously used selective combinations of normal, parabiotic, and radioablated mice to demonstrate that blood-borne prothymocytes are imported in a gated and competitive manner, we used a similar approach in this study to investigate the importation of the hematogenous precursors of thymic DCs. The results indicate that two developmentally distinct populations of DC precursors normally enter the adult mouse thymus. The first population is indistinguishable from prothymocytes according to the following criteria: 1) inefficient (<20%) exchange between parabiotic partners; 2) gated importation by the thymus; 3) competitive antagonism for intrathymic niches; 4) temporally linked generation of thymocytes and CD8alpha(high) DCs; and 5) absence from prothymocyte-poor blood samples. The second population differs diametrically from prothymocytes in each of these properties, and appears to enter the thymus in at least a partially differentiated state. The resulting population of DCs has a CD8alpha(-/low) phenotype, and constitutes approximately 50% of total thymic DCs. The presence of two discrete populations of DCs in the steady state thymus implies functional heterogeneity consistent with evidence implicating lymphoid DCs in the negative selection of effector thymocytes and myeloid DCs in the positive selection of regulatory thymocytes.

Lineage-negative side-population (SP) cells with restricted hematopoietic capacity circulate in normal human adult blood: immunophenotypic and functional characterization

Side-population (SP) cells are a recently described rare cell population detected in selected tissues of various mammalian species, but not yet described in the peripheral circulation. In the present study, we have identified for the first time SP cells in lineage-negative adult human blood and have provided an extensive functional and immunophenotypic characterization of these cells. Adult peripheral blood was depleted of mature leukocyte cell types by density gradient and immunomagnetic separation. The SP cell population was identified by its characteristic Hoechst 33342 profile. Immunophenotypic analysis revealed that blood SP cells expressed high levels of CD45, CD59, CD43, CD49d, CD31, and integrin markers and lacked CD34. Highly purified SP cells were put into cobblestone area-forming cell (CAFC), long-term culture-initiating cell (LTC-IC), and liquid cell culture assays; repopulating assays were performed utilizing nonobese diabetic/severe combined immunodeficient mice. Circulating SP cells were shown to exhibit verapamil sensitivity and a low growth rate. LTC-IC, CAFC, and engraftment assays indicated that circulating SP cells had lost the multipotentiality described in murine bone marrow SP cells. However, outgrowth of mature cell types from liquid cell culture suggests the presence of common lymphoid (T and natural killer) and dendritic cell precursor(s) within circulating SP cell populations. The absence of SP cell growth in the LTC-IC, CAFC, and repopulating assays might be intrinsic to the tissue source (marrow versus blood) or species (mouse versus human) tested. Thus, human blood SP cells, although rare, may serve as a source of selected leukocyte progenitor cells. The immunophenotype of circulating SP cells may provide clues to their seeding and homing capacity.

Highly active antiretroviral therapy corrects hematopoiesis in HIV-1 infected patients: interest for peripheral blood stem cell-based gene therapy

OBJECTIVES

To study, in asymptomatic HIV-1-infected (HIV+) patients, whether peripheral blood hematopoietic progenitor/stem cells (PBPC) mobilized by granulocyte colony stimulating factor (G-CSF), can be used as a source of cells for retroviral gene therapy.

DESIGN

PBPC from two groups of HIV+ patients (treated or untreated by highly active antiretroviral therapy) and from seronegative donors were mobilized with G-CSF.

METHODS

PBPC collected by leukapheresis were enriched for CD34 cells, immunophenotypically and functionally characterized, cultured and infected with retroviral vectors. HIV proviral integration was studied on fresh and cultured cells.

RESULTS

G-CSF moderately and transiently increased the viral load in untreated patients only, and induced in both groups of HIV+ patients mobilization of percentages and numbers of CD34 cells comparable to those of seronegative volunteers. The most immature CD34 cell subset, the clonogenic progenitor and long-term culture initiating cells were significantly decreased in leukapheresis products and CD34-enriched fractions from untreated HIV+ patients but not in those from treated HIV+ patients. Cell cycle activation and growth factor responses of CD34 cells from both groups of HIV+ patients were not different from those of the control group. Culture and retroviral infection of CD34 cells from HIV+ patients did not enhance HIV replication, and yielded transduction levels similar to those obtained using CD34 cells from seronegative donors.

CONCLUSIONS

G-CSF-mobilized PBPC can be safely used for HIV retroviral gene therapy in asymptomatic treated patients while highly active antiretroviral therapy would control the G-CSF-induced increase in viral load and correct the defective hematopoiesis observed in untreated patients, without inhibiting the retroviral transduction of PBPC.

Impaired NK cell differentiation of blood-derived CD34+ progenitors from patients with myeloid metaplasia with myelofibrosis

Cultured blood CD34(+) progenitors from patients with myeloid metaplasia with myelofibrosis (MMM) failed to differentiate into natural killer (NK) cells with recombinant interleukin (IL)-15. No NK cells either could be induced in coculture with IL-15-expressing fibroblasts from MMM patients' spleens. The impaired NK differentiation could be circumvented by using normal blood CD34(+) cells in the coculture. In this case, cell-to-cell contact and IL-15 interaction were crucial for NK cell differentiation. Pretreatment of normal CD34(+) progenitors with anti-IL-15 monoclonal antibody markedly reduced NK cell production while MMM fibroblast pretreatment did not. Both normal and MMM progenitors constitutively expressed IL-15. Analysis of endogenous IL-15 signaling pathway revealed a constitutive gammac/Jak3 association and STAT3 activation in the two types of progenitors. Anti-IL-15 monoclonal antibody treatment caused a downregulation of IL-15 signaling in normal but not MMM blood cells. The impaired NK differentiation in MMM may thus arise from a deregulated control of an endogenous IL-15 involved in hematopoietic progenitors' lymphoid differentiation.

Fc epsilon RI and CD22 mRNA are expressed in early B-lineage and myeloid leukemia cell lines

CD22, one of the important markers for diagnosing B-lineage acute leukemia, was expressed in mature basophil granulocytes. We then investigated the expression of CD22 and other B cell- and basophil-related molecules in 25 human acute leukemia cell lines to find the phenotype of the virtual common progenitor of B and myeloid lineage. Surface and cytoplasmic expressions of antigens were analyzed using a flow cytometer and an essential antibody panel used for diagnosing acute leukemia as well as cytokine receptors and basophil-related enzymes. Messenger RNA expression of Fc epsilon R1 and CD22 was also analyzed. Peroxidase-positive and -negative myeloid leukemias showed eosinophil- and basophil-type expression of enzymes, respectively. Early myeloid and B-lineage cells expressed basically similar combinations of cytokine receptors and various combinations of mRNA listed above, while T-lineage cells did not. The virtual common progenitor of B and myeloid lineage cells may be defined as immature cells simultaneously expressing B and basophil phenotypes.

B lymphopoiesis is active throughout human life, but there are developmental age-related changes

This study addressed several questions concerning age-related changes in human B lymphopoiesis. The relative abundance of pro-B, pre-B, immature, naive, and mature B cells among the CD19(+) lymphocyte fraction of human bone marrow was found not to change appreciably over the interval between 24 and 88 years of age. Moreover, proliferation of pro-B and large pre-B cells in adult marrow equaled that observed with fetal marrow specimens. Exceptionally low numbers of lymphocyte precursors were found in some marrow samples, and the values obtained were used to determine parameters that best reflect B lymphopoiesis. Cord blood always contained higher incidences of functional precursors than adult cells. However, sorted CD34(+) Lin(-) CD10(+) progenitors from cord blood and adult marrow had equivalent potential for differentiation in culture, and notable age-related changes were found in more primitive subsets. A recently described subset of CD34(+)CD38(-)CD7(+) cord blood cells had no exact counterpart in adult marrow. That is, all adult CD34(+)Lin(-)CD7(+)CD10(-) cells expressed CD38, displayed less CD45RA, and had little B-lineage differentiation potential. The CD7(+) fractions in either site contained progenitors for erythroid and natural killer (NK) lineages, and ones sorted from marrow expressed high levels of transcripts for the CD122 interleukin 2 (IL-2)/IL-15 receptor required by NK-lineage precursors. Dramatic changes in human B lymphopoiesis occur early in life, and more information is required to construct a probable sequence of differentiation events prior to the acquisition of CD10.

Characterization of human blood dendritic cell subsets

Dendritic cells (DCs) are key antigen-presenting cells for stimulating immune responses and they are now being investigated in clinical settings. Although defined as lineage-negative (Lin(-)) HLA-DR(+) cells, significant heterogeneity in these preparations is apparent, particularly in regard to the inclusion or exclusion of CD14(+), CD16(+), and CD2(+) cells. This study used flow cytometry and a panel of monoclonal antibodies (mAbs), including reagents from the 7th Leukocyte Differentiation Antigen Workshop, to define the cellular composition of 2 standardized peripheral blood mononuclear cell (PBMCs)-derived Lin(-) HLA-DR(+) preparations. Lin(-) cells were prepared from PBMCs by depletion with CD3, CD14, CD19, CD11b, and either CD16 or CD56 mAbs. Analysis of the CD16-replete preparations divided the Lin(-) HLA-DR(+) population into 5 nonoverlapping subsets (mean +/- 1 SD): CD123 (mean = 18.3% +/- 9.7%), CD1b/c (18.6% +/- 7.6%), CD16 (49.6% +/- 8.5%), BDCA-3 (2.7% +/- 1.4%), and CD34 (5.0% +/- 2.4%). The 5 subsets had distinct phenotypes when compared with each other, monocytes, and monocyte-derived DCs (MoDCs). The CD85 family, C-type lectins, costimulatory molecules, and differentiation/activation molecules were also expressed differentially on the 5 Lin(-) HLA-DR(+) subsets, monocytes, and MoDCs. The poor viability of CD123(+) DCs in vitro was confirmed, but the CD16(+) CD11c(+) DC subset also survived poorly. Finally, the individual subsets used as stimulators in allogeneic mixed leukocyte reactions were ranked by their allostimulatory capacity as CD1b/c > CD16 > BDCA-3 > CD123 > CD34. These data provide an opportunity to standardize the DC populations used for future molecular, functional and possibly even therapeutic studies.

T cell-specific expression of the murine CD3delta promoter

T cell-specific expression of human and mouse CD3delta is known to be governed by an enhancer element immediately downstream from the gene. Here we demonstrate by transgenic and in vitro studies that the murine CD3delta (mCD3delta) promoter prefers to be expressed in cells of the T lineage. Deletion analyses of a promoter segment (-401/+48 bp) followed by transient transfections indicate that upstream elements between -149 and -112 bp contribute to full expression of the gene. Furthermore, a core promoter region -37/+29 appears to contribute to a T cell specificity. Using substitution mutant scanning, four positive and one negative regulatory elements were found within the mCD3delta core promoter. The first two positive elements comprise two classical initiator-like sites, which recruit TFII-I, whereas a third contains a functional Ets binding site. Immediately adjacent to the observed transcription start site is a negative element that utilizes the transcription regulator YY1. The last positive regulatory element contains a potentially functional CREB binding site and the minor transcriptional start site. Because NERF-2, Elf-1, and Ets-1 are expressed preferentially in lymphocytes and because, in addition, YY1 represses the promoter activity strongly in non-T cells, we conclude that the combination of these transcription factors contributes to the T cell-specific expression pattern of mouse CD3delta.

Origin and filiation of human plasmacytoid dendritic cells

Human plasmacytoid dendritic cells represent a rare population of leukocytes which produce high amounts of type I interferon in response to certain viruses. Although those cells were first described in 1958, there are still unsolved issues related to their origin and function. Recently, a leukemic counterpart of plasmacytoid dendritic cells was identified. Molecular approaches using either normal or leukemic plasmacytoid dendritic cells provide some new insights into the controversial lymphoid origin of those cells. The need for specific markers is still a critical aspect for the identification of plasmacytoid dendritic cells, whatever stage of differentiation, in normal as well as in pathological conditions. Hopefully, novel markers will allow delineation of the relationships between dendritic cells at different stages of differentiation/maturation along the myeloid and lymphoid lineages.