Evidence that the protein tyrosine phosphatase (PC12,Br7,Sl) gamma (-) isoform modulates chondrogenic patterning and growth.

One of the earliest events in bone morphogenesis is the condensation of embryonic mesenchymal cells into chondroblasts and their subsequent proliferation and differentiation into chondrocytes. During this time, certain signaling cascades operate to establish proper patterning and differentiation of the cartilaginous skeleton. Characterization of the signaling pathways involved in these processes remains to be accomplished. We have identified a novel murine cytosolic tyrosine phosphatase termed PTPPBS gamma (+/-) which is a member of the PTP PC12,Br7,Sl (PTPPBS) family. Spatio-temporal expression analysis of the members of this tyrosine phosphatase family demonstrates significant expression of the gamma (-) splice variant in the cartilaginous skeleton. Using an embryonic mandibular explant culture system to serve as a model for cartilage formation, we examined the potential roles of the PTPPBS gamma phosphatase by loss-of-function studies achieved with antisense oligodeoxynucleotides. These studies demonstrated that loss of expression of the PTPPBS gamma (-) isoform resulted in abnormal patterning of Meckel's cartilage and an increase in the size of the chondrogenic regions. In gamma antisense-treated explants, bromodeoxyuridine-pulse labeling studies revealed increased proliferation of chondroblasts bordering along precartilaginous condensations and bordering populations of maturing chondrocytes. These studies provide evidence that in early skeletal development, PTPPBS gamma may regulate the rate of chondroblast proliferation in the cartilaginous skeleton.

Protein tyrosine phosphatase (PC12, Br7,S1) family: expression characterization in the adult human and mouse.

Protein tyrosine phosphatases (PTPs) play important roles in modulating signals transduced by tyrosine kinases. Certain phosphatases have been implicated as having important roles in embryonic development as well as in adult physiology. Although both kinases and phosphatases are equally important in regulating signal transduction, phosphatases as a group have not been well characterized. Thus, characterization of sequence, expression, and biological function for additional phosphatases is informative. PTPBr7/PC12 and PTPSl are mouse receptor PTPs sharing similar amino acid sequences. Northern blot analysis demonstrated expression of these genes in adult rodent brain and revealed previously uncharacterized transcripts in the brain and other tissues. Our results demonstrate that PTPBr7/PC12 and PTPSl are members of a larger family of PTPs. We have identified two novel family members as well as several novel transcriptional splice variants from both human and mouse colon cDNA libraries. Expression analysis demonstrated that the various mRNA transcripts are differentially expressed, with the highest levels found in the brain, intestinal tract, uterus, and placenta. In situ hybridization analysis of mouse brain and intestinal tissues established that each isoform has a unique expression pattern in specific cell populations as well as in tissue regions. Furthermore, these restricted patterns suggest that the encoded family of phosphatases may play roles in modulating signal transduction pathways important for specific cell types and biological processes.

Anemia and perinatal death result from loss of the murine ecotropic retrovirus receptor mCAT-1.

The mCAT-1 gene encodes a basic amino acid transporter that also acts as the receptor for murine ecotropic leukemia viruses. Targeted mutagenesis in embryonic stem cells has been used to introduce a germ-line null mutation into this gene. This mutation removes a domain critical for virus binding and inactivates amino acid transport activity. Homozygous mutant pups generated from these cells were approximately 25% smaller than normal littermates, very anemic, and died on the day of birth. Peripheral blood from homozygotes contained 50% fewer red blood cells, reduced hemoglobin levels, and showed a pronounced normoblastosis. Histological analyses of bone marrow, spleen, and liver showed a decrease in both erythroid progenitors and mature red blood cells. Mutant fetal liver cells behaved normally in in vitro hematopoietic colony-forming assays but generated an anemia when transplanted into irradiated C.B.-17 SCID mice. Furthermore, reconstitution of the white cell compartment of SCID mice by mutant fetal liver cells was less complete than that observed with a mixed population of wild-type and heterozygous fetal liver cells. Primary embryo fibroblasts from mutant mice were completely resistant to ecotropic retrovirus infection. Thus, mCAT-1 not only appears to be the sole receptor for a group of murine ecotropic retroviruses associated with hematological disease but also plays a critical role in both hematopoiesis and growth control during mouse development.

A selective culture system for generating terminal deoxynucleotidyl transferase-positive lymphoid precursor cells in vitro. IV. Properties and developmental relationships of the lymphoid cells in the adherent and nonadherent compartments of the culture.

In a selective culture system, the in vitro generation of terminal deoxynucleotidyl transferase positive (TdT+) B cell progenitors from rat, mouse, and human bone marrow (BM) is characterized by initial lymphoid colonization of a mouse BM feeder layer. In the present study, four sequentially appearing subsets of B cell progenitors in the culture system are characterized. The first lymphoid subsets to appear, and presumably the least mature, are the adherent TdT- and TdT+ cells that reach plateau numbers on days 3 and 7, respectively. The appearance of TdT+ cells in the adherent phase of the culture is closely followed by a parallel increase in the number of TdT+ cells in the nonadherent phase, suggesting the release of a constant proportion of the adherent lymphoid cells from the feeder layer. Lastly, a wave of TdT- lymphoid cells is observed in the nonadherent fraction. All of the lymphoid cells generated in the cultures expressed the rat equivalent of the B220 antigen, whereas only one-half of the lymphoid cells (and one-half of the TdT+ cells) expressed the heat stable antigen (HSA), while none expressed surface or cytoplasmic immunoglobulin (Ig). This phenotypic profile identifies one-half of the generated lymphoid cells as pre-pro-B cells and the remainder as early to late pro-B cells. It also demonstrates that TdT is initially expressed prior to HSA expression in rat B cell differentiation. In vitro transfer experiments demonstrate that virtually all the precursors of TdT+ lymphoid cells (pre-TdT cells) in rat BM adhere to the mouse BM feeder layer during the first 24 hours of culture, and by day 7 of culture, the total pre-TdT cell activity in the adherent cell compartment increases more than 20-fold on a per cell basis and more than 70-fold on a per well basis. Finally, it was observed that a decrease in the concentration of fetal bovine serum (FBS) in the culture medium resulted in the selective release of the adherent TdT+, but not TdT-, lymphoid cells into the nonadherent compartment. These studies indicated that a stepwise progression of the earliest detectable stages in lymphoid development was associated with changes in stromal cell/lymphoid cell interactions that were partly regulated by serum-dependent adhesion mechanisms.

Long-term impaired neutrophil migration in mice overexpressing human interleukin-8.

The proinflammatory chemokine interleukin-8 (IL-8/NAP-1) has been implicated in recruiting neutrophils to sites of acute and chronic tissue inflammation. In transgenic mice, elevated serum IL-8 levels ranging from 1 to 118 ng/ml were correlated with proportional increases in circulating neutrophils and proportional decreases in L-selectin expression on the surface of blood neutrophils. No change in the expression of the beta 2-integrins Mac-1 and LFA-1 was apparent on peripheral blood neutrophils of the IL-8 transgenic mice. Additionally, L-selectin expression on bone marrow neutrophils and neutrophil precursors was normal in all transgenic lines. IL-8 transgenic mice demonstrated an accumulation of neutrophils in the microcirculation of the lung, liver and spleen. Moreover, there was no evidence of neutrophil extravasation, plasma exudation or tissue damage in any IL-8 transgenic mice. Neutrophil migration into the inflamed peritoneal cavity was severely inhibited in IL-8 transgenic mice, but not in nontransgenic littermates. The IL-8 transgenic mice should serve as useful models for studying the putative role of neutrophils in mediating tissue damage in models of inflammation, such as hepatic ischemia and reperfusion injury, cecal puncture and ligation, and glomerulonephritis.

A selective culture system for generating terminal deoxynucleotidyl transferase-positive lymphoid cells in vitro. III. Structure of the bone marrow microenvironment for early lymphopoiesis.

BACKGROUND: We have previously demonstrated the feasibility of generating terminal deoxynucleotidyl transferase-positive (TdT+) lymphoid precursor cells in vitro in the nonadherent compartment of a long-term xenogeneic culture system in which rat bone marrow (BM) cells are seeded onto established mouse BM adherent cell layers. We have also noted that the appearance of TdT+ cells in these cultures is preceded by the formation of clusters of lymphoblasts in close association with the mouse BM adherent cell layer. Inasmuch as the selective generation of such primitive lymphoid cells is not ordinarily observed in homogeneic (i.e., mouse: mouse, rat:rat) BM cultures, the nature of the microenvironment for the generation of committed lymphoid stem/progenitor cells has not yet been detailed. Consequently, the aim of this study was to define the cellular components in the adherent compartment of our xenogeneic culture system that are associated with the earliest stages of lymphopoiesis in vitro. EXPERIMENTAL DESIGN: The nature of the interactions between rat BM lymphoid precursor cells and mouse BM adherent microenvironmental cells was investigated by a combination of immunophenotyping and scanning and transmission electron microscopy of primary cultures. The kinetics of formation and composition of lymphoid clusters were also determined morphologically and phenotypically. Results were compared with those of other investigators who have studied lymphopoiesis in intact BM or in homogeneic cultures of pre-B cells. RESULTS: Two distinct microenvironmental regions are represented within the mouse BM adherent cell layer: (a) paucilayer (PL) regions, composed of two or three horizontally oriented layers of alkaline phosphatase-positive mouse stromal cells; and (b) multilayer (ML) regions, containing 4 to 8 layers of such stromal cells. In both regions, proliferating rat lymphoid cells, expressing the HIS24 (B220) and/or HIS50 (heat stable antigen) early B-lineage antigens, are "sandwiched" between adjacent layers of stromal cells and enveloped by cytoplasmic processes from interdigitating mouse macrophages (pseudoemperipolesis). More than 95% of the lymphoid cells are of rat origin, whereas more than 95% of the nonlymphoid cells are of mouse origin. Large clusters, containing up to 1,000 lymphoid cells, preferentially develop in the ML regions and are comprised primarily of TdT+ cells. Small clusters containing 5 to 50 lymphoid cells, preferentially develop in the PL regions and are comprised primarily of TdT- cells, that can generate TdT+ cells upon transfer onto fresh adherent cells layers. Formation of individual small clusters, which outnumber large clusters by approximately 10-fold, is initiated by as few as 25 unfractionated rat BM cells. This process is not preceded by a lag period, and is linear with respect to time and cell dose. Formation of large clusters requires approximately 30 times more input cells, and is linear with respect to time after a lag of 5 days. CONCLUSIONS: The number of small lymphoid clusters formed in vitro closely approximates the frequency of lymphoid stem/progenitor cells in the BM inoculum (3 to 5%). This suggests that, under ideal conditions, individual clusters are clonally derived and the seeding efficiency of the culture system approaches 100%. The results further suggest that large clusters are formed by the coalescence of numerous small clusters within ML regions of the adherent cell layer; and that the formation of ML regions may be supported by an underlying monolayer of macrophages. A novel aspect of this system appears to be the frequency of pseudoemperipolesis, rather than phagocytosis, of primitive lymphoid cells by macrophages, that has also been noted in vivo. Moreover, the ML regions themselves bear a close resemblance to the recently described pro-B cell-enriched, multicellular aggregate fraction of freshly harvested mouse BM. Hence, this system appears to structurally recreate in vitro the

Granulocyte colony-stimulating factor crosses the placenta and stimulates fetal rat granulopoiesis.

We studied the effect of recombinant human granulocyte colony-stimulating factor (rhG-CSF) administration to pregnant rats upon fetal and neonatal myelopoiesis. Pregnant rats were treated with rhG-CSF twice daily for 2, 4, and 6 days before parturition. rhG-CSF crossed the placenta and reached peak fetal serum concentrations 4 hours after administration. Peak fetal serum levels were 1,000-fold lower than levels detected in the dam. Hematopoietic effects of rhG-CSF were assessed by cytologic analysis of the newborn blood, spleen, bone marrow, thymus, and liver. White blood cell counts were increased twofold to fourfold in newborns. This increase was due to circulating numbers of polymorphonuclear cells (PMN). rhG-CSF induced a myeloid hyperplasia in the newborn marrow consisting of immature and mature myeloid cells in the day-2 and day-4 treated pups. Bone marrow of pups treated for 6 days contained mostly hyper-segmented PMN with little or no increase in myeloid precursors. An increase in the number of postmitotic (PMN, bands, and metamyelocytes) and mitotic (promyeloblasts, myeloblasts, and metamyeloblasts) myeloid cells in the spleen of neonates was observed. No change was detected in splenic lymphocytes or monocytes. No effect of rhG-CSF was noted in the newborn liver or thymus. These results demonstrate that maternally administered rhG-CSF crosses the placenta and specifically induces bone marrow and spleen myelopoiesis in the fetus and neonate. The significant myelopoietic effects of rhG-CSF at low concentrations in the fetus suggest an exquisite degree of developmental sensitivity to this cytokine and may provide enhanced defense mechanisms to the neonate.

Maternal administration of granulocyte colony-stimulating factor improves neonatal rat survival after a lethal group B streptococcal infection.

Maternally administered recombinant human granulocyte colony-stimulating factor (rhG-CSF) has been shown to cross the placenta and induce a peripheral neutrophilia and increases in the marrow and spleen neutrophil storage pools in fetal and newborn rats. In the present study, we have used this model system to investigate the efficacy of prenatally administered rhG-CSF on neonatal defense to a lethal challenge with Group B-beta hemolytic Streptococcus (GBS). Pregnant rats were injected with rhG-CSF twice daily beginning 6 days before parturition. At birth, all pups were infected with a dose of GBS that is lethal for 90% of infected pups (LD90). Survival was monitored daily for 5 days. Survival of infected pups from saline-treated mothers beyond 60 hours after infection was 10%. No difference in survival was observed among pups from mothers treated 2 and 4 days before parturition. In contrast, we determined that survival was 82.5% among infected pups from mothers treated for 6 days before parturition with rhG-CSF. Our results demonstrate that maternal administration of rhG-CSF augments neonatal defenses against a lethal bacterial challenge.

Isolation of rat bone marrow mast lineage cells using Thy 1.1 and rat stem cell factor.

Recent reports have shown that various marrow-derived cell populations respond vigorously to recombinant rat stem cell factor (rrSCF164), one form of the kit-ligand. In the present study, we isolated cell populations from rat bone marrow using the Thy 1.1 antigen (an antigen that in the rat is differentially expressed on primitive hemopoietic progenitor cells) and fluorescently conjugated rrSCF164 (rrSCF164-PE). We show that rrSCF164 only stimulates cells that are enriched in the brightest Thy 1.1 populations (Thy 1.1bright). Numerous cell lines were generated by serial passage in rrSCF164 containing medium, and the prototypic cell lines have been designated SRT002 and SRT003. Each cell line retains the Thy 1.1bright phenotype and does not respond to interleukins (IL) 1-8, IL-10, granulocyte (G) colony-stimulating factor (CSF), granulocyte macrophage (GM) CSF, M-CSF, or crude preparations of mitogen-stimulated T-cell supernatants. The Thy 1.1bright population of rat marrow was subdivided into a subset that binds rrSCF164-PE (Thy 1.1bright, rrSCF164+). The majority of these cells possess certain characteristics in common with marrow-derived mast cells and the Thy 1.1bright, rrSCF164 responsive cell lines, having similar granule morphology, being metachromatic, and reacting positively with alcian blue. Moreover, rats treated with rrSCF164 displayed significant increases in Thy 1.1bright, rrSCF164+ cells in the bone marrow. These studies show that the combination of Thy 1.1 and rrSCF164 makes possible the isolation of a unique subset of rat bone marrow cells that differentially express the Thy 1.1 antigen and the cell surface receptor c-kit, the majority of which are morphologically similar to marrow-derived mast cells.

Rat stem-cell factor induces splenocytes capable of regenerating the thymus.

Cytokine regulation of prethymic T-lymphoid progenitor-cell proliferation and/or differentiation has not been well-defined, although much is known of cytokine regulation of hemopoietic stem- and progenitor-cell development. Here we use a recently identified hemopoietic growth factor, stem-cell factor (SCF) (a form of the c-kit ligand), and a transplant model of thymocyte regeneration to assess the effect of SCF on the in vivo generation of prethymic, thymocyte progenitor-cell activity. We show that recombinant rat SCF (rrSCF164) administered to weanling rats selectively induces an increase in thymocyte progenitor activity in the spleens of treated rats as compared to rats treated with vehicle, polyethylene glycol (PEG)-conjugated rat albumin, or recombinant human granulocyte colony-stimulating factor (rhG-CSF). These data demonstrate that administration of SCF in vivo affects extrathymic-origin thymocyte regenerating cells and may influence, directly or indirectly, early prethymic stages of T-cell lymphopoiesis in addition to its known effect on early stages of myelopoiesis and erythropoiesis.

Characterization of RT6 bearing rat lymphocytes. I. Ontogeny of the RT6+ subset.

The RT6 alloantigen is present on approximately 70% of peripheral T cells in the rat, but is absent from thymocytes and bone marrow lymphocytes. The results of further phenotypic analysis in the present study demonstrated that the RT6 alloantigen is expressed on approximately 45% of the helper/inducer (CD4; W3/25+) and 80% of the cytotoxic/suppressor (CD8; OX8+) peripheral T-cell subsets. Ontogenetic and thymus ablation studies indicated that the RT6+ T-cell subset is thymus-dependent and normally develops after the appearance of RT6-T cells in neonatal rats, and that the expression of RT6 is a post-thymic maturational event. Furthermore, intrathymic adoptive transfer of bone marrow cells demonstrated that RT6+ T cells are thymus-derived cells. These results show that most if not all RT6+ T cells are the progeny of RT6- T cells. However, they do not exclude the possibility that a separate lineage of RT6- T cells exists, which also has OX8+ and W3/25+ subsets. The possible developmental and functional relationships of RT6- and RT6+ T cells in the rat are discussed.

Rat immunoglobulin genes have comparable patterns of JH rearrangement in normal peripheral B cells and in pre-B and cultured TdT+ cells from bone marrow.

Pre-B cells from normal rat bone marrow were isolated by first rosette-depleting granulocytes, T cells and stem cells with W3/13 mAb and mature B cells with anti-Ig mAb, and then in some samples enriching by FACS-sorting HIS24+ cells. Presumptive precursors of these pre-B cells possessing terminal deoxy-nucleotidyl transferase were prepared by culture of normal marrow. The extent of rearrangement of kappa and heavy chain genes was estimated on both these types of cells and on mature peripheral B cells by measuring the disappearance of the germ-line restriction fragments encoding J segments (other than JH1) relative to a non-rearranging CK fragment. Pre-B cells and B cells showed similar degrees of rearrangement of their kappa genes, leaving roughly half in germ-line form; the TdT+-enriched cells showed approximately similar heavy chain rearrangement as the peripheral B cells. Some non-germ-line restriction fragments hybridising with JH in sufficient amount to register as discrete bands on Southern blots were found in common between these different cell populations taken from different individual donors. These may represent preferred D-JH or perhaps V-D-JH rearrangement arising repetitively in independent clones, or may be due to dominance by a few preferred clones.

Colony-stimulating factors regulate the development of multinucleated osteoclasts from recently replicated cells in vitro.

Osteoclasts mediate the process of bone resorption. However, little is known about the mechanisms that regulate the formation of either osteoclasts or osteoclast precursors. In contrast, colony-stimulating factors (CSFs) are well-known to regulate the formation of myeloid cells and their precursors. Because osteoclasts and myeloid cells may originate from a common stem cell, we examined the effects of two CSFs, granulocyte-macrophage CSF (GM-CSF) and interleukin 3 (IL-3), on bone resorption, osteoclast formation, and the incorporation of recently replicated nuclei into the osteoclasts of mouse bone cultures. CSFs had little effect on the formation rate of osteoclasts or their resorptive activity but significantly decreased the percentage of recently replicated osteoclast progenitor cell nuclei present in the osteoclasts of bones treated with parathyroid hormone. GM-CSF also increased the number of myeloid cells in the marrow space of the cultures and the percentage of these cells derived from recently replicated progenitors. These results demonstrate that GM-CSF and IL-3 can regulate the development of osteoclasts from recently replicated precursor cells in cultured fetal mouse long bones. However, the mechanisms by which CSFs influence osteoclast formation are difficult to determine from these studies because markers for the osteoclast progenitor and precursor do not exist. These data also provide evidence that the differentiation of osteoclast progenitors is regulated by different factors at different points in their ontogeny.

Defective lymphopoiesis in the bone marrow of motheaten (me/me) and viable motheaten (mev/mev) mutant mice. II. Description of a microenvironmental defect for the generation of terminal deoxynucleotidyltransferase-positive bone marrow cells in vitro.

We have presented evidence in a previous paper that the development of prothymocytes, pre-B cells, and TdT+ lymphoid precursor cells in the bone marrow of motheaten (me/me) and viable motheaten (mev/mev) mice is defective. In the present study, we have used a selective culture system that supports the generation of rat- and mouse-origin TdT+ bone marrow lymphoid cells in vitro to further investigate the early stages of lymphopoiesis in me/me and mev/mev mice. The results demonstrate that bone marrow stromal cell feeder layers derived from me/me and mev/mev mice do not support the growth of rat TdT+ cells in vitro, whereas stromal cell feeder layers from heterozygous (+/-) littermates and wild type (+/+) control mice do. Moreover, composite feeder layers formed by mixing as few as one part me/me and mev/mev bone marrow cells with 7 to 9 parts +/- littermate bone marrow cells also fail to effectively support the generation of TdT+ cells in vitro. In contrast to me/me and mev/mev mice, other mutant mouse models of autoimmune (NZB, NZB/W), immunodeficient (nu/nu), and hemopoietic (W/Wv, Sl/Sld) disorders form feeder layers that support normal to elevated levels of TdT+ cell growth in vitro. Thus, to date, only the me/me and mev/mev mutant mice have been found to lack the appropriate microenvironment for the generation of TdT+ bone marrow cells. Histologic analysis of the stromal cell feeder layers that are formed in our culture system shows that multilayered cellular patches, which normally are the most active sites of TdT+ cell development in vitro, are absent in feeder layers of me/me and mev/mev cells. Moreover, feeder layers from mev/mev mice contain a population of MAC 1+, basophilic, nonvacuolated, macrophage-like cells; whereas feeder layers from control mice contain MAC 1+, eosinophilic, vacuolated macrophage-like cells. Stromal cell feeder layers formed by mixtures of me/me or mev/mev and control mouse bone marrow cells contain numerous multilayered cellular patches and vacuolated mononuclear cells, but also contain large numbers of basophilic mononuclear cells. These composite feeder layers have a disproportionately reduced capacity to support the generation of TdT+ cells in vitro. Although the stromal microenvironment of me/me and mev/mev bone marrow does not support the growth of TdT+ cells in vivo or in vitro, the bone marrow from these mutant mice contains detectable numbers of pre-TdT+ cells. Thus, when cultured on normal mouse feeder layers, mutant mouse bone marrow rapidly generates TdT+ cells in vitro, albeit at significantly reduced levels as compared to +/- littermate controls.(ABSTRACT TRUNCATED AT 400 WORDS)

Defective lymphopoiesis in bone marrow of motheaten (me/me) and viable motheaten (mev/mev) mutant mice. I. Analysis of development of prothymocytes, early B lineage cells, and terminal deoxynucleotidyl transferase-positive cells.

This study identifies defects in the early stages of lymphopoiesis that may contribute to the abnormalities in the development and/or function of peripheral T and B lymphocytes in mice homozygous for the motheaten (me/me) and viable motheaten (mev/mev) mutations. The results indicate that in me/me and mev/mev mice prothymocytes in bone marrow are present in essentially normal numbers, as determined by intrathymic injection, but apparently lack the ability to home effectively to the thymus, as determined by intravenous transfer; early B lineage cells in bone marrow, identified by the B220 antigen, are markedly depleted, including immature B cells (sIg+), pre-B cells (cIg+, sIg-), and pro-B cells (B220+, cIg-, sIg-); TdT+ bone marrow cells, especially a subset that expresses the B220 B lineage antigen, are markedly depleted by two weeks of age; normal numbers of TdT+ thymocytes are present during the first 3 wk of postnatal life, but rapidly decrease thereafter. The results further indicate that neither the defective thymus homing capacity of prothymocytes nor the deficiency of TdT+ bone marrow cells is due to autoantibodies. The possible relationship of the defective development of lymphoid precursor cells to the premature onset of thymic involution and to the abnormalities of peripheral T and B lymphocytes in me/me and mev/mev mice is discussed; as are the results of in vitro studies (presented in a companion paper), which suggest that a primary defect in the stromal microenvironment of the bone marrow is responsible for the abnormal development of the lymphoid precursor cells.

Phenotypic heterogeneity of Gross virus-induced thymic lymphomas in the rat: cellular origins and migratory properties.

Neoplastic thymocytes from rat thymic lymphoma-leukemias induced by the rat-adapted Gross leukemia virus (RAGV) were analyzed for a variety of differentiation markers. The neoplasms from individual rats all expressed the antigenic phenotype MP+, W3/13+, Thy-1+, RT-1+, RT-7+, W3/25-. However, approximately two-thirds of the neoplasms were positive for the OX 8 antigen, and one-third were negative. The OX 8- neoplasms only involved the thymus, whereas approximately 40% of the OX 8+ neoplasms involved the spleen as well as the thymus. Virtually all OX 8+ and OX 8- neoplastic cells contained terminal deoxynucleotidyl transferase (TdT), and both OX 8+ and OX 8- lymphomas expressed the lactate dehydrogenase (LDH)-5' isozyme and the primary, but not the secondary, ADA isozyme. This enzymatic phenotype is characteristic of thymocyte precursors, but not thymocytes. Our results therefore indicate that RAGV-induced lymphomas arise from transformed prethymic TdT+ cells which contain the LDH-5' and the primary ADA isozymes. These preleukemic cells presumably migrate to the thymus where they express the RT-7 pan-T-cell antigen and, in some instances, the OX 8 antigen during the development of overt leukemia. The OX 8+ neoplasms, being more differentiated than their OX 8- counterparts, then migrate to peripheral lymphoid tissues.

A selective culture system for generating terminal deoxynucleotidyl transferase-positive (TdT+) lymphoid precursor cells in vitro. I. Description of the culture system.

A primary xenogeneic culture system has been devised that selectively generates undifferentiated TdT+ lymphoblasts from rat bone marrow under conditions that do not support the growth or maintenance of rat colony-forming unit-spleen (CFU-S) or granulocyte/macrophage colony-forming cells (GM-CFC). The culture system requires a mouse bone marrow feeder layer, and a serum supplement that has markedly reduced levels of cortisol. The growth of TdT+ cells can be significantly enhanced by the addition of mesodermalizing factors (e.g., fibroblast growth factor, guinea pig bone marrow extract) to the culture medium, and the serum supplement can be decreased by the addition of selenium, transferrin, and T3. The cultured TdT+ cells are antigenically "null" cells that further resemble their normal counterparts in bone marrow with respect to morphology, size, cortisone sensitivity, and pattern of TdT fluorescence. The TdT+ cells are generated with equal facility from bone marrow of normal and congenitally athymic rats, can be maintained in logarithmic growth for at least 10 mos by serial passage in vitro, and do not cause leukemia when infused into irradiated recipients. Although the lineage relationships of these immature lymphoid cells have not yet been established, our working hypothesis, based on preliminary evidence, is that the cultured TdT+ cells are primitive members of the T cell series.

Putative B lymphocyte lineage precursor cells in early murine embryos.

The emergence of putative B lymphocyte lineage cells in embryonic and neonatal tissues of mice was studied using monoclonal antibodies which recognize virtually all B cells, pre-B cells and an additional population of lymphoid cells. An extensive survey of extraembryonic and intraembryonic hemopoietic tissues indicated that cells displaying this marker first appear within liver of 45-50 somite stage embryos. Prior to this developmental stage, none were found in yolk sac or peripheral blood. Although some 14.8 antigen bearing cells were found in suspensions of ectoplacenta, these were small surface IgM+ lymphocytes which were presumably of maternal origin. Culture of embryonic liver fragments which initially contained small numbers of antigen positive cells resulted in their expansion. However, this did not occur when very early liver rudiments containing virtually no lymphoid cells were cultured. These findings document the earliest appearance of cells during ontogeny which may be committed to differentiate into functional B lymphocytes.

The liver hemopoietic environment: I. Developing hepatocytes and their role in fetal hemopoiesis.

Mouse fetal liver was studied ultrastructurally to identify and characterize the developing hepatic parenchyma or prehepatocyte which may be responsible for producing the liver hemopoietic environment. It was observed that as the liver develops, there is close association of endodermal and mesenchymal cells in the region of the septum transversum. Numerous intercellular adhesions were observed between endodermal cells and mesenchymal cells. Twenty-four after endodermal and mesenchymal cells first intermingle, the liver extravascular space consisted of spherical hemopoietic cells dispersed among a heterogenous population of dark and light cells. The reticulum of prehepatocytes formed a three-dimensional cellular network which structurally supported the hemopoietic cells residing in the liver. By 12 days of gestation, prehepatocytes were a homogenous population of dark, stellate cells joined together by numerous intercellular adhesions. Broad areas of intercellular association were noted between processes and prehepatocytes and hemopoietic cells; however, no intercellular junctions between these two disparate cell populations were observed at this or any stage in development. Characteristics reflecting a cell population capable of synthesis and secretion of proteinaceous substances, namely, dilated Golgi apparati, increased numbers of polyribosomes and profiles of rough endoplasmic reticulum (RER), two types of vacuoles and/or vesicles, and intercellular microvillus-lined spaces, were observed in the prehepatocytes between 12 and 17 days gestation. By day 17 of gestation, glycogen accumulation, biliary channel development, appearance of a subendothelial microvillus surface, nuclear shape and chromatin pattern, and arrangement of cytoplasmic organelles reflected the maturation of prehepatocytes into hepatocytes, the adult liver parenchyma.