Linkage analysis of HSP70 genes and historecognition locus in botryllus schlosseri

The protochordate allorecognition system has long invited comparison with the vertebrate major histocompatibility complex (MHC). In the colonial species Botryllus schlosseri, a rapid fusion or rejection response resembling graft acceptance or rejection in vertebrates is controlled by a single highly polymorphic genetic region. Because linkage between heat shock protein 70 (HSP70) genes and the MHC appears to be conserved within the vertebrate lineage, linkage relationships between two HSP70 genes (HSP70.1 and HSP70.2) and the historecognition locus (FuHC) have been analyzed in B. schlosseri. Segregation patterns of restriction fragment length polymorphisms located in the 3' flanking regions of HSP70.1 and HSP70.2 were determined for progeny of defined crosses. These progeny were also analyzed for fusibility type by an in vivo cut colony assay. No close linkage was detected between any of the three loci. These results do not support the hypothesis that the allorecognition response in B. schlosseri is determined by an MHC homologue. However, it remains a possibility that orthologues of other MHC-linked genes will be linked to the B. schlosseri FuHC.

Systemic overexpression of BCL-2 in the hematopoietic system protects transgenic mice from the consequences of lethal irradiation

A new transgenic mouse has been generated in which the proto-oncogene BCL-2 is ubiquitously overexpressed. H2K-BCL-2 transgenic mice overexpress BCL-2 in all cells of the hematolymphoid system and have been used to assess the role of BCL-2 in protecting cells of the hematolymphoid system from the consequences of ionizing radiation. We have expanded on previous studies that have demonstrated protection for specific (lymphoid) cell populations and show that systemic overexpression of BCL-2 can protect the hematopoietic system as a whole, including hematopoietic stem cells (HSC), thus increasing the radioresistance of the animal. The increase in radioresistance in H2K-BCL-2 transgenic mice has two components: an increase in the radioresistance of individual cells and, to a lesser extent, an increase in the size of certain critically important cell populations, such as HSC. Bone marrow transplantation experiments show that the increased radioresistance of the transgenic animals is provided by cells of the hematopoietic system. Protection against the consequences of irradiation is not limited to the increased expression levels of BCL-2 in transgenic mice; levels of endogenous BCL-2 are higher in lymphocyte populations that survive irradiation in wild-type mice. We show that ubiquitous overexpression of BCL-2 in the hematopoietic system can be used to increase the resistance of animals to lethal challenges such as irradiation.

Helios, a T cell-restricted Ikaros family member that quantitatively associates with Ikaros at centromeric heterochromatin

The Ikaros gene encodes multiple protein isoforms that contribute critical functions during the development of lymphocytes and other hematopoietic cell types. The intracellular functions of Ikaros are not known, although recent studies have shown that Ikaros proteins colocalize with inactive genes and centromeric heterochromatin. In this study, Ikaros proteins were found to be components of highly stable complexes. The complexes from an immature T cell line were purified, revealing associated proteins of 70 and 30 kD. The p70 gene, named Helios, encodes two protein isoforms with zinc finger domains exhibiting considerable homology to those within Ikaros proteins. Helios and Ikaros recognize similar DNA sequences and, when overexpressed, Helios associates indiscriminately with the various Ikaros isoforms. Although Ikaros is present in most hematopoietic cells, Helios was found primarily in T cells. The relevance of the Ikaros-Helios interaction in T cells is supported by the quantitative association of Helios with a fraction of the Ikaros. Interestingly, the Ikaros-Helios complexes localize to the centromeric regions of T cell nuclei, similar to the Ikaros localization previously observed in B cells. Unlike the B cell results, however, only a fraction of the Ikaros, presumably the fraction associated with Helios, exhibited centromeric localization in T cells. These results establish immunoaffinity chromatography as a useful method for identifying Ikaros partners and suggest that Helios is a limiting regulatory subunit for Ikaros within centromeric heterochromatin.

Two distinct pathways of positive selection for thymocytes

Most mouse thymocytes undergoing positive selection are found on one of two pathways; the c-Kit+ and the c-Kit- pathways. Here, we show that c-Kit and interleukin-7 receptor (IL-7R)-mediated signals support positive selection during the transition from the subpopulation that first expresses cell surface T cell receptor (TCR)-the TCRalpha/betaloCD4(int)/CD8(int) (DPint) c-Kit+ cells to TCRalpha/betamedc-Kit+ transitional intermediate cells (the c-Kit+ pathway). Cells that fail positive selection on the c-Kit+ pathway become TCRalpha/betaloc-Kit- (DPhi) blasts that appear to undergo alternative TCRalpha rearrangements. The rare DPhic-Kit- blast cells that thus are salvaged for positive selection by expressing a self-major histocompatibility complex selectable TCRalpha/beta up-regulate IL-7R, but not c-Kit, and are the principal progenitors on the c-Kit- pathway; this c-Kit-IL-7R+ pathway is mainly CD4 lineage committed. Cell division is a feature of the TCRlo-medc-Kit+ transition, but is not essential for CD4 lineage maturation from DPhic-Kit- blasts. In this view, positive selection on the c-Kit- path results from a salvage of cells that failed positive selection on the c-Kit+ path.

High doses of purified stem cells cause early hematopoietic recovery in syngeneic and allogeneic hosts

In humans, autologous transplants derived from bone marrow (BM) usually engraft more slowly than transplants derived from mobilized peripheral blood. Allogeneic BM transplants show a further delay in engraftment and have an apparent requirement for donor T cells to facilitate engraftment. In mice, Thy-1.1(lo)Lin-/loSca-1+ hematopoietic stem cells (HSCs) are the principal population in BM which is responsible for engraftment in syngeneic hosts at radioprotective doses, and higher doses of HSCs can radioprotect an allogeneic host in the absence of donor T cells. Using the mouse as a preclinical model, we wished to test to what extent engraftment kinetics was a function of HSC content, and whether at high doses of c-Kit+Thy-1.1(lo)Lin-/loSca-1+ (KTLS) cells rapid allogeneic engraftment could also be achieved. Here we demonstrate that engraftment kinetics varied greatly over the range of KTLS doses tested (100-10,000 cells), with the most rapid engraftment being obtained with a dose of 5,000 or more syngeneic cells. Mobilized splenic KTLS cells and the rhodamine 123(lo) subset of KTLS cells were also able to engraft rapidly. Higher doses of allogeneic cells were needed to produce equivalent engraftment kinetics. This suggests that in mice even fully allogeneic barriers can be traversed with high doses of HSCs, and that in humans it may be possible to obtain rapid engraftment in an allogeneic context with clinically achievable doses of purified HSCs.

Characterization of a population of cells in the bone marrow that phenotypically mimics hematopoietic stem cells: resting stem cells or mystery population?

We have identified a population of cells in murine bone marrow that has many of the phenotypic characteristics attributed to resting hematopoietic stem cells but does not reconstitute irradiated mice. These cells express high levels of Sca-1, H-2K and CD38 and low levels of Thy-1.1, but do not express CD34 nor any of the lineage markers including CD3, CD4, CD5, CD8 NK1.1, I-A, B220, Ig(MGA), CD40, kappa, Mac-1, Gr-1 or Ter119. In addition, this population can be found at normal frequency in nu/nu as well as rag-1-/- mice. These cells incorporate only low levels of Rh123, are resistant to the cytotoxic effects of 5-fluorouracil and, consistent with their resting phenotype, less than 2% of these cells are in the S/G2/M phases of the cell cycle. The only phenotypic characteristic that distinguishes these cells from the lineage- Sca-1+, Thy-1.1low long-term reconstituting hematopoietic stem cell population is their lack of c-kit expression. Here we have explored the possibility that these cells represent a truly resting population of hematopoietic stem cells. We found that the lineage-, Sca-1+, c-kit- cells do not respond to hematopoietic growth factors in vitro, either alone or in combination with stromal layers. Furthermore, these cells do not form in vivo spleen colonies nor do they have the ability to reconstitute irradiated mice. Thus, this population may represent either a population of resting stem cells for which we lack the appropriate activating stimulus, or simply represent a "mystery population" that phenotypically mimics most of the physical properties of resting stem cells. Given the close phenotypic similarity of the c-kit- mystery population cells to the c-kit+ long-term reconstituting stem cells, investigators must be rigorous to exclude their effects from other stem cell assays.

Tolerance of allogeneic heart grafts in mice simultaneously reconstituted with purified allogeneic hematopoietic stem cells

BACKGROUND

Animals reconstituted with allogeneic whole bone marrow (WBM) are often tolerant of donor-specific solid organ grafts. Clinical application of bone marrow transplantation in solid organ transplantation has been limited, however, principally by graft-versus-host disease. We previously demonstrated that hematopoietic stem cells (HSCs) reconstitute lethally irradiated allogeneic mice without producing graft-versus-host disease. The purpose of this study was to determine whether tolerance to solid organ grafts could be induced in mice reconstituted with HSCs.

METHODS

BALB/c mice were lethally irradiated and reconstituted with allogeneic C57BL/Ka, Thy-1.1 WBM or HSCs. An isolated group was given a limited number of HSCs (250 cells) and a subpopulation of allogeneic cells known to facilitate HSC engraftment (facilitators). C57BL/Ka, Thy-1.1 neonatal heart grafts were placed in reconstituted animals either at the time of hematopoietic transplant or 35 days later. Third-party C3H grafts were placed over 2 months after hematopoietic reconstitution. Tolerance was defined as the persistence of cardiac contraction for the duration of evaluation (125-270 days).

RESULTS

All surviving mice that were reconstituted with C57BL/Ka, Thy-1.1 HSCs, WBM, or HSCs and facilitators were tolerant of C57BL/Ka grafts long-term. Third-party C3H grafts placed in reconstituted animals were rejected by day 12, whereas those placed in unmanipulated mice were rejected by day 9.

CONCLUSION

These data indicate that tolerance to concurrently or subsequently placed solid organ grafts can be reliably achieved with limited numbers of purified HSCs in a model where immunocompetence to third-party major histocompatibility complex antigens is delayed but intact.

Hematopoietic stem cells and lymphoid progenitors express different Ikaros isoforms, and Ikaros is localized to heterochromatin in immature lymphocytes

The generation of lymphoid cells in mice depends on the function of the Ikaros protein. Ikaros has been characterized as a lymphoid-restricted, zinc-finger transcription factor that is derived from an alternatively spliced message. Ikaros knockout mice have defects in multiple cell lineages, raising the question of whether the protein regulates multiple committed progenitors and/or multipotent stem cells. To address this issue, we examined Ikaros expression in purified populations of multipotent cells and more committed progenitors. We found that the DNA-binding isoforms of Ikaros were localized in the nucleus of the most primitive hematopoietic stem cell subset. Changes in the RNA splicing pattern of Ikaros occurred at two stages: (i) as long-term self-renewing stem cells differentiated into short-term self-renewing stem cells and (ii) as non-self-renewing multipotent progenitors differentiated into lymphoid-committed progenitors. Unexpectedly, we found Ikaros localized to heterochromatin in Abelson-transformed pre-B lymphocytes by using immunogold electron microscopy. These observations suggest a complex role for Ikaros in lymphoid development.

HSP70 genes and historecognition in Botryllus schlosseri: implications for MHC evolution

The colonial protochordate Botryllus schlosseri possesses a historecognition system which has long invited comparison to the vertebrate MHC. Upon contact, colonies either fuse or reject one another in a manner resembling graft acceptance or rejection in vertebrates. This response is controlled by a single highly polymorphic genetic region, the FuHC locus. Colonial protochordates such as B. schlosseri are among the closest relatives of the vertebrate lineage, and therefore may possess a recognizable MHC homologue. Since linkage between heat shock protein 70 (HSP70) genes and MHC appears to be conserved within the vertebrate lineage, we have analyzed HSP70 genes from B. schlosseri as a first step toward isolating the historecognition locus. Two HSP70 genes (HSP70.1 and HSP70.2) have been cloned and sequenced, and exhibit 93.6% sequence identity within the predicted coding regions. The B. schlosseri genes share a number of characteristics with vertebrate MHC-linked HSP70 genes: Northern blotting and sequence analysis suggest that the protochordate genes are cytoplasmically-expressed heat-inducible members of the HSP70 gene family (FAGAN and WEISSMAN 1996). However, unlike vertebrate MHC-linked HSP70 genes, HSP70.1 and HSP70.2 are not closely linked (FAGAN and WEISSMAN 1997). Furthermore, neither is closely linked to the locus determining historecognition (FAGAN and WEISSMAN 1997). These results do not support the hypothesis that the B. schlosseri FuHC locus is an MHC homolog. A discussion of the implications of these results for evolution of the vertebrate MHC is included.

Hydroxyurea can be used to increase mouse c-kit+Thy-1. 1(lo)Lin-/loSca-1(+) hematopoietic cell number and frequency in cell cycle in vivo

The DNA synthesis inhibitor hydroxyurea (HU) was administered to determine whether it induces changes in the cell-cycle status of primitive hematopoietic stem cells (HSCs)/progenitors. Administration of HU to mice leads to bone marrow accumulation of c-kit+Thy-1.1(lo)Lin-/loSca-1(+) (KTLS) cells in S/G2/M phases of the cell cycle. HU is a relatively nontoxic, reversible cell-cycle agent that can lead to approximately a threefold expansion of KTLS cells in vivo and approximately an eightfold increase in the number of KTLS cells in S/G2/M. HSCs in HU-treated mice have undiminished multilineage long-term and short-term clonal reconstitution activity.

Identification of clonogenic common lymphoid progenitors in mouse bone marrow

The existence of a common lymphoid progenitor that can only give rise to T cells, B cells, and natural killer (NK) cells remains controversial and constitutes an important gap in the hematopoietic lineage maps. Here, we report that the Lin(-)IL-7R(+)Thy-1(-)Sca-1loc-Kit(lo) population from adult mouse bone marrow possessed a rapid lymphoid-restricted (T, B, and NK) reconstitution capacity in vivo but completely lacked myeloid differentiation potential either in vivo or in vitro. A single Lin(-)IL-7R(+)Thy-1(-)Sca-1loc-Kit(lo) cell could generate at least both T and B cells. These data provide direct evidence for the existence of common lymphoid progenitors in sites of early hematopoiesis.

Developing lymph nodes collect CD4+CD3- LTbeta+ cells that can differentiate to APC, NK cells, and follicular cells but not T or B cells

For a brief period during fetal lymph node organogenesis in mice, lymph node postcapillary high endothelial venules surprisingly express the Peyer's patch addressin MAdCAM-1. This expression allows initial seeding of this incipient structure by two unusual lymphocyte populations selectively expressing the Peyer's patch homing receptor integrin alpha4beta7: CD4+CD3- oligolineage progenitors and TCR gammadelta+ T cells. We show here that CD4+CD3- cells are lineage-restricted progenitors that express surface lymphotoxin-beta (LTbeta) and the chemokine receptor BLR1 and that can become natural killer cells, dendritic antigen-presenting cells, and follicular cells of unknown outcome, but these cells do not become T or B lymphocytes. Since the necessity of lymphotoxin in lymphoid organ development has been shown, we propose that the novel subset of CD4+CD3-LTbeta+ fetal cells is instrumental in the development of lymphoid tissue architecture.

Highly polymorphic microsatellite loci in the colonial ascidian Botryllus schlosseri

Five mircosatellite loci are characterized for the colonial ascidian Botryllus schlosseri. Within one population from Monterey, California, these loci have 3 to 17 alleles, observed heterozygosities from 0.40 to 0.63, expected heterozygosities from 0.52 to 0.84, and an overall paternity exclusion rate (QT) of 0.78. Three of the five loci demonstrated Mendelian patterns of inheritance in laboratory crosses. The size distribution of alleles suggests that most allelic diversity within these loci is generated by single-step and less frequently multistep mutations. However, several alleles may also have been generated by single based insertions or deletions. Mutation rates for the five microsatellite loci are less than 1 x 10(-2) per generation. Because or their highly polymorphic nature, these loci should prove useful for exploring issues of identity, kinship, population structure, and phylogenetics.

Bcl-2 rescues T lymphopoiesis, but not B or NK cell development, in common gamma chain-deficient mice

The common cytokine receptor gamma chain (gamma(c)) is an indispensable subunit for the formation of lymphoid-related cytokine receptors, including IL-7 and IL-15 receptors, that mediate nonredundant or critical signals for the differentiation of T and B cells and natural killer (NK) cells, respectively. We introduced the bcl-2 transgene driven by E mu or H-2K promoters into gamma(c)-deficient mice that lack all three lymphoid subclasses. The forced expression of Bcl-2 restored all stages of T lymphopoiesis, but not B or NK cell development, indicating that a primary function of gamma(c)-mediated signals in the T lineage might be to maintain cell survival. Therefore, the development of T, B, and NK cells may be influenced by distinct intracytoplasmic signaling cascades that are activated by coupling of gamma(c)-related receptors.

Bcl-2 rescues T lymphopoiesis in interleukin-7 receptor-deficient mice

Mice lacking functional IL-7 or IL-7R alpha genes are severely deficient in developing thymocytes, T cells, and B cells. IL-7 and IL-7 receptor functions are believed to result in lymphoid cell proliferation and cell maturation, implying signal transduction pathways directly involved in mitogenesis and elaboration of developmentally specific new gene programs. Here, we show that enforced expression of the bcl-2 gene in T-lymphoid cells (by crossing in the Emu-bcl-2 transgene) in IL-7R alpha-deficient mice results in a significant restoration of thymic positive selection and T cell numbers and function. We propose cell survival signals to be the principal function of IL-7R engagement in thymic and T cell development.

Enforced expression of Bcl-2 in monocytes rescues macrophages and partially reverses osteopetrosis in op/op mice

Osteopetrotic (op/op) mice lack functional M-CSF and have depressed levels of macrophages and osteoclasts. We prepared transgenic mice (hMRP8bcl-2) that express human Bcl-2 in monocytes. In vitro hMRP8bcl-2 monocytes do not undergo apoptosis in the absence of serum and M-CSF, while op/op and wild-type monocytes die. These Bcl-2-expressing monocytes spontaneously undergo macrophage differentiation. In vivo, the op/op hMRP8bcl-2 mice show significant replenishment of tissue macrophages. Their long bone osteopetrosis is largely reversed, and extensive medullary hematopoiesis appears in the bone marrow. We propose that M-CSF augments monocyte survival, permitting them to respond to internal and external cues for their differentiation.

From stem cells to lymphocytes: biology and transplantation

We review the development of the hematopoietic system, focusing on the transition from hematopoietic stem cells (HSCs) to T cells. This includes the isolation of HSCs, and recent progress in understanding their ontogeny, homing properties, and differentiation. HSC transplantation is reviewed, including the kinetics of reconstitution, engraftment across histocompatibility barriers, the facilitation of allogeneic engraftment, and the mechanisms of graft rejection. We describe progress in understanding T-cell development in the bone marrow and thymus as well as the establishment of lymph nodes. Finally, the role of bcl-2 in regulating homeostasis in the hematopoietic system is discussed.

Phenotypic and functional changes induced at the clonal level in hematopoietic stem cells after 5-fluorouracil treatment

A significant fraction of hematopoietic stem cells (HSCs) have been shown to be resistant to the effects of cytotoxic agents such as 5-fluorouracil (5-FU), which is thought to eliminate many of the rapidly dividing, more committed progenitors in the bone marrow and to provide a relatively enriched population of the most primitive hematopoietic progenitor cells. Although differences between 5-FU-enriched progenitor populations and those from normal bone marrow have been described, it remained unclear if these differences reflected characteristics of the most primitive stem cells that were revealed by 5-FU, or if there were changes in the stem-cell population itself. Here, we have examined some of the properties of the stem cells in the bone marrow before and after 5-FU treatment and have defined several activation-related changes in the stem-cell population. We found that long-term reconstituting stem cells decrease their expression of the growth factor receptor c-kit by 10-fold and increase their expression of the integrin Mac-1 (CD11b). These changes begin as early as 24 hours after 5-FU treatment and are most pronounced within 2 to 3 days. This activated phenotype of HSCs isolated from 5-FU-treated mice is similar to the phenotype of stem cells found in the fetal liver and to the phenotype of transiently repopulating progenitors in normal bone marrow. We found that cell cycle is induced concomitantly with these physical changes, and within 2 days as many as 29% of the stem-cell population is in the S/G2/M phases of the cell cycle. Furthermore, when examined at a clonal level, we found that 5-FU did not appear to eliminate many of the transient, multipotent progenitors from the bone marrow that were found to be copurified with long-term repopulating, activated stem cells. These results demonstrate the sensitivity of the hematopoietic system to changes in its homeostasis and correlate the expression of several important surface molecules with the activation state of HSCs.

Identification of a lineage of multipotent hematopoietic progenitors

All multipotent hematopoietic progenitors in C57BL-Thy-1.1 bone marrow are divided among three subpopulations of Thy-1.1(lo) Sca-1+ Lin(-/lo) c-kit+ cells: long-term reconstituting Mac-1- CD4- c-kit+ cells and transiently reconstituting Mac-1(lo) CD4- or Mac-1(lo) CD4(lo) cells. This study shows that the same populations, with similar functional activities, exist in mice whose hematopoietic systems were reconstituted by hematopoietic stem cells after lethal irradiation. We demonstrate that these populations form a lineage of multipotent progenitors from long-term self-renewing stem cells to the most mature multipotent progenitor population. In reconstituted mice, Mac-1- CD4- c-kit+ cells gave rise to Mac-1(lo) CD4- cells, which gave rise to Mac-1(lo) CD4(lo) cells. Mac-1- CD4- c-kit+ cells had long-term self-renewal potential, with each cell being capable of giving rise to more than 10(4) functionally similar Mac-1- CD4- c-kit+ cells. At least half of Mac-1(lo) CD4- cells had transient self-renewal potential, detected in the spleen 7 days after reconstitution. Mac-1(lo) CD4(lo) cells did not have detectable self-renewal potential. The identification of a lineage of multipotent progenitors provides an important tool for identifying genes that regulate self-renewal and lineage commitment.

Hematopoietic stem cells: challenges to expectations

The past year provided a number of challenges to our expectations regarding hematopoietic stem cell (HSC) biology. Evidence has emerged that HSCs arise intraembryonically before they can be detected in the yolk sac. A number of genes that may regulate the formation, self-renewal, or differentiation of HSC have been identified. New markers for purifying HSCs have also been described. Although different groups have attributed different properties to HSCs, it now appears that the differences may be explained by variations in assay conditions rather than by differences in the HSCs themselves. Finally, insights have emerged into the complexity of the regulation of HSC proliferation and adhesion properties.

A PMLRARalpha transgene initiates murine acute promyelocytic leukemia

The malignant cells of acute promyelocytic leukemia (APL) contain a reciprocal chromosomal translocation that fuses the promyelocytic leukemia gene (PML) with the retinoic acid receptor alpha gene (RAR alpha). To test the hypothesis that the chimera PMLRAR alpha plays a role in leukemogenesis, we expressed a PMLRAR alpha cDNA in myeloid cells of transgenic mice. PMLRAR alpha transgenic mice exhibited impaired neutrophil maturation early in life, which progressed at a low frequency over the course of several months to overt APL. Both the preleukemic state and the leukemia could be transplanted to nontransgenic mice, and the transplanted preleukemia could progress to APL. The APL recapitulated features of the human disease, including a response to retinoic acid. Retinoic acid caused the leukemic cells to differentiate in vitro and in vivo, eliciting remissions of both the preleukemic state and APL in mice. Our results demonstrate that PMLRAR alpha impairs neutrophil differentiation and initiates the development of APL. The transgenic mice described here provide an apparently accurate model for human APL that includes clear evidence of tumor progression. The model should be useful for exploring the molecular pathogenesis of APL and the mechanisms of the therapeutic response to retinoic acid, as well as for preclinical studies of therapeutic regimens.

Cyclophosphamide/granulocyte colony-stimulating factor induces hematopoietic stem cells to proliferate prior to mobilization

We isolated hematopoietic stem cells (HSC) from mice treated with cyclophosphamide (CY) and granulocyte colony-stimulating factor (G-CSF). All mobilized multipotent progenitor activity was contained in two populations: Thy-1(lo) Sca-1+ Lin- Mac-1- CD4- c-kit+ long-term reconstituting progenitors and Thy-1(lo) Sca-1+ Lin- Mac-1(lo) CD4- transiently reconstituting progenitors. CY/G-CSF treatment drove both long-term and transient multipotent progenitors into cycle, leading to a more than 12-fold expansion in the number of long-term self-renewing HSC prior to mobilization. After CY and 2 days of G-CSF treatment the number of bone marrow HSC began to decline and the number of blood and splenic HSC increased. HSC continued to proliferate in the bone marrow and spleen through 8 days of G-CSF treatment, but HSC released into the blood tended to be in G0/G1 phase. Mobilized multipotent progenitors isolated from the spleen were less efficient than normal bone marrow multipotent progenitors in engrafting irradiated mice but did not differ in colony forming unit-spleen (CFU-S) activity or single cell in vitro assays of primitive progenitor activity. The data suggest that mobilized HSC isolated from the spleen are less efficient at homing to and engrafting the bone marrow of irradiated recipient mice.

Somatic and germ cell parasitism in a colonial ascidian: possible role for a highly polymorphic allorecognition system

A colonial protochordate, Botryllus schlosseri, undergoes a natural transplantation reaction in the wild that results alternatively in colony fusion (chimera formation) or inflammatory rejection. A single, highly polymorphic histocompatibility locus (called Fu/HC) is responsible for rejection versus fusion. Gonads are seeded and gametogenesis can occur in colonies well after fusion, and involves circulating germ-line progenitors. Buss proposed that colonial organisms might develop self/non-self histocompatibility systems to limit the possibility of interindividual germ cell "parasitism" (GCP) to histocompatible kin [Buss, L. W. (1982) Proc. Natl. Acad. Sci. USA 79, 5337-5341 and Buss, L. W. (1987) The Evolution of Individuality (Princeton Univ. Press, Princeton]. Here we demonstrate in laboratory and field experiments that both somatic cell and (more importantly) germ-line parasitism are a common occurrence in fused chimeras. These experiments support the tenet in Buss's hypothesis that germ cell and somatic cell parasitism can occur in fused chimeras and that a somatic appearance may mask the winner of a gametic war. They also provide an interesting challenge to develop formulas that describe the inheritance of competing germ lines rather than competing individuals. The fact that fused B. schlosseri have higher rates of GCP than unfused colonies additionally provides a rational explanation for the generation and maintenance of a high degree of Fu/HC polymorphism, largely limiting GCP to sibling offspring.

Flow cytometric identification of murine neutrophils and monocytes

Flow cytometry and cell sorting have been employed in order to identify and purify murine neutrophils and monocytes. Using a combination of antibodies, we were able to distinguish between these two subsets of myeloid cells. The method described in this paper is simple to perform and can be applied to characterize myeloid cells from blood, spleen, bone marrow and after an induced inflammation.