Rat T lymphocyte progenitors in bone marrow can be induced to express more Thy 1

We used a rat thymic epithelial cell line, IT-45R1, as a differentiation inducer to identify rat bone marrow (BM) pre-T cells. The following observations strongly suggest that BM pre-T cells are relatively small to medium-sized and weakly Thy 1 positive (the amount of Thy 1 is much lower than that of majority of adult thymocytes) lymphoid cells: (1) Fraction (Fr.) 5 cells, separated by bovine serum albumin (BSA) density gradient centrifugation, were induced to express more Thy 1. (2) In a time course study and a study with a beta-stimulator, Fr. 5 cells were almost identical to mouse pre-T cells. (3) Thy 1 expression of Fr. 5 cells appeared to be related to the syntheses of DNA, RNA and protein, being in good agreement with the results of human and mouse pre-T cells. (4) The increase of Thy 1 content by STEL was confirmed in the cell sorter (ABCASS) analysis. (5) ABCASS revealed that Thy 1 content of fetal thymocytes of 18 days' gestation was just between that of BM cells and adult thymocytes, indicating that the increase in Thy 1 content may occur also in vivo.     

Bipotential B-macrophage progenitors are present in adult bone marrow

According to the current model of adult hematopoiesis, differentiation of pluripotential hematopoietic stem cells into common myeloid- and lymphoid-committed progenitors establishes an early separation between the myeloid and lymphoid lineages. This report describes a rare and previously unidentified CD45R-CD19+ B cell progenitor population in postnatal bone marrow that can also generate macrophages. In addition to the definition of this B-lineage intermediate, the data indicate that a developmental relationship between the B and macrophage lineages is retained during postnatal hematopoiesis.     

The bone marrow constitutes a reservoir of pericyte progenitors

Adult bone marrow is a rich reservoir of hematopoietic and mesenchymal stem and progenitor cells. Mobilization and recruitment of bone marrow-derived cells to injured or ischemic tissue or tumors endorse the initiation and maintenance of angiogenic processes in the adult by incorporating endothelial progenitor cells (EPC) into the developing vasculature and by recruiting accessory hematopoietic cells. Recent data have now revealed that the origin of bone marrow-derived vascular cells is not restricted to endothelial cells but also includes pericytes--the perivascular support cells. Several laboratories have now reported the existence of pericyte progenitor cells, and these cells, like EPC, can be mobilized and recruited to the remodeling vasculature under ischemic conditions and in tumors. This review focuses on pericytes in vessel formation and on recent discoveries about their bone marrow origin in the adult.     

Identification of a third evolutionarily conserved gene within the RAG locus and its RAG1-dependent and -independent regulation

Recombination-activating gene (RAG)1 and RAG2 encode T and B lymphocyte-specific endonucleases indispensable for rearrangements of antigen-receptor gene segments but also capable of causing deleterious chromosome rearrangements. The mechanisms regulating RAG expression and repression are not clear. Here we identify NWC, a third evolutionarily conserved gene within the RAG locus, and show that it is ubiquitously expressed, with the notable exception of RAG-nonexpressing immature and mature T and B lymphocytes because in lymphocytes it is regulated by the RAG1 promoter and transcribed as RAG1-NWC hybrid mRNA molecules. We also show that in all other cells NWC is controlled by the RAG2 intragenic promoter, which in immature and mature T and B lymphocytes is silent. The possible implications of these findings for understanding the activation and inactivation of RAG genes in lymphocytes and their repression in other cells are discussed.     

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.     

Interleukin-3 does not affect the differentiation of mast cells derived from human bone marrow progenitors

Although IL-3 is commonly used for culture of human progenitor-derived mast cells together with Stem cell factor (SCF) and IL-6, the effect of IL-3 on human mast cell differentiation has not been well elucidated. Human bone marrow CD34+ progenitors were cultured for up to 12 weeks in the presence of rhSCF and rhIL-6 either with rhIL-3 (IL-3 (+)) or without rhIL-3 (IL-3 (-)) for the initial 1-week of culture. Total cell number increased at 2 weeks in IL-3 (+), as compared to IL-3 (-), but changes in the appearance of mast cells were delayed. When IL-3 was present for the initial 1-week culture, granules looked more mature with IL-3 than without IL-3. However, tryptase and chymase contents, and surface antigen expression (CD18, CD51, CD54, and CD117) were not altered by IL-3. Surface expression and mRNA level of FcepsilonRIalpha and histamine release by crosslinking of FcepsilonRIalpha did not differ from one preparation to the next. GeneChip analysis revealed that no significant differences were observed between IL-3 (+) and IL-3 (-) cells either when inactivated or activated by aggregation of FcepsilonRIalpha. These findings indicate that initial incubation of human bone marrow CD34+ progenitors with IL-3 does not affect the differentiation of mast cells.     

Morphologic and radiological observations on the earliest bone marrow formation in human embryos and fetuses

Morphologic and radiologic studies were undertaken on 26 human embryos and fetuses to determine the stage and site of the earliest bone marrow formation. Up to the 10th week of gestation, primary bone marrow is not present anywhere although the intramembranous ossification occurs in the maxilla and mandible and also in the middle portion of the clavicle. At the 11th week of gestation, X-ray examination showed in two fetuses the bone formation in the clavicle, scapula, maxilla, mandible, and the diaphysis of the long bones. Serial sections of these fetuses revealed that the primary bone marrow occurs first in the middle portion of the clavicle. From a series of our embryological studies, the concept of the mononuclear phagocyte system which involves the bone-marrow-derived monocytic origin of tissue macrophages, is not accepted, at least, on the origin of Kupffer cells in human fetal livers.     

Unraveling the origin of lymphocyte progenitors

Lymphocytes are ultimately derived from hematopoietic stem cells, however the intervening steps that give rise to lymphocyte progenitors are still under intense investigation. In particular, whether a common lymphocyte progenitor serves exclusively as a lymphoid precursor, or whether this cell retains limited myeloid progenitor potential are addressed by Rolink and colleagues. This issue is highlighted by their identification within the bone marrow of early progenitors with lymphoid and myeloid developmental potential or EPLMs. How these cells fit into our current understanding of lymphocyte development, and how this new subset helps to further refine our view of the common lymphocyte progenitor are discussed.     

T lymphocyte reconstitution following autologous bone marrow transplantation.

Peripheral blood T lymphocyte subpopulations were analysed using OKT3, OKT4 and OKT8 monoclonal antibodies in eleven patients undergoing autologous bone marrow transplantation following high dose chemotherapy +/- total body irradiation for bronchial carcinoma or as intensification therapy for acute myeloblastic leukaemia. Marked inversion of the OKT4:OKT8 ratio was noted in all patients following the procedure. This is due to both a fall in the number of OKT4 positive cells and a rise in the number of OKT8 positive cells in all but one patient. Sequential analysis of six patients shows a return to the normal ratio in two with time and a trend towards normalization in the others. We suggest that the subpopulation inversion noted following bone marrow transplantation is an effect of marrow regeneration and is not causally related to graft versus host disease.