The thrombopoietin receptor, c-Mpl, is a selective surface marker for human hematopoietic stem cells.

BACKGROUND: Thrombopoietin (TPO), the primary cytokine regulating megakaryocyte proliferation and differentiation, exerts significant influence on other hematopoietic lineages as well, including erythroid, granulocytic and lymphoid lineages. We previously demonstrated that the receptor for TPO, c-mpl, is expressed by a subset of human adult bone marrow hematopoietic stem/progenitor cells (HSC/PC) that are enriched for long-term multilineage repopulating ability in the SCID-hu Bone in vivo model of human hematopoiesis. METHODS: Here, we employ flow cytometry and an anti-c-mpl monoclonal antibody to comprehensively define the surface expression pattern of c-mpl in four differentiation stages of human CD34+ HSC/PC (I: CD34+38--, II: CD34+38dim, III: CD34+38+, IV: CD34dim38+) for the major sources of human HSC: fetal liver (FL), umbilical cord blood (UCB), adult bone marrow (ABM), and cytokine-mobilized peripheral blood stem cells (mPBSC). We use a surrogate in vivo model of human thymopoiesis, SCID-hu Thy/Liv, to compare the capacity of c-mpl+ vs. c-mpl-- CD34+38--/dim HSC/PC for thymocyte reconstitution. RESULTS: For all tissue sources, the percentage of c-mpl+ cells was significantly highest in stage I HSC/PC (FL 72 +/- 10%, UCB 67 +/- 19%, ABM 82 +/- 16%, mPBSC 71 +/- 15%), and decreased significantly through stages II, III, and IV ((FL 3 +/- 3%, UCB 8 +/- 13%, ABM 0.6 +/- 0.6%, mPBSC 0.2 +/- 0.1%) [ANOVA: P < 0.0001]. The relative median fluorescence intensity of c-mpl expression was similarly highest in stage I, decreasing through stage IV [ANOVA: P < 0.0001]. No significant differences between tissue sources were observed for either % c-mpl+ cells [P = 0.89] or intensity of c-mpl expression [P = 0.21]. Primary Thy/Liv grafts injected with CD34+38--/dimc-mpl+ cells showed slightly higher levels of donor HLA+ thymocyte reconstitution vs. CD34+38--/dimc-mpl---injected grafts and non-injected controls (c-mpl+ vs. c-mpl--: CD2+ 6.8 +/- 4.5% vs. 2.8 +/- 3.3%, CD4+8-- 54 +/- 35% vs. 31 +/- 29%, CD4--8+ 29 +/- 19% vs. 18 +/- 14%). CONCLUSION: These findings support the hypothesis that the TPO receptor, c-mpl, participates in the regulation of primitive human HSC from mid-fetal through adult life. This study extends our previous work documenting human B-lineage, myeloid and CD34+ cell repopulation by c-mpl+ progenitors to show that c-mpl+ HSC/PC are also capable of significant T-lineage reconstitution in vivo. These results suggest that c-mpl merits consideration as a selective surface marker for the identification and isolation of human HSC in both basic research and clinical settings.

Bryozoan larvae as mosaics of multifunctional ciliary fields: Ultrastructure of the sensory organs of Bugula solonifera (Cheilostomata: Cellularioidea).

The ultrastructure of ciliated sensory and effector organs has been examined by light microscopy and by scanning and transmission electron microscopy in larvae of the cellularioid cheilostome, Bugula stolonifera. The larval surface is formed by a mosaic of ciliary fields, each distinctive in organization and function. The most extensive ciliary field consists of the motile cilia of the corona, which collectively constitute the primary larval locomotory organ and form most of the larval surface. The corona is innervated basally by a nerve ring that circumscribes the larval equator. A circular field of immotile shorter cilia at the aboral pole arise from bands of radial cells in the apical disc. The radial ciliated cells form chemical synapses with underlying nerves of the paired aboral nerve cords, which extend to an equatorial nerve plexus on the median anterior side of the larva, beneath the pyriform organ. The pyriform organ is a complex of ciliated and glandular regions aligned along the anterior midline of the larva. It consists of, in aboral to oral sequence: the superior glandular field, the vibratile plume, the inferior glandular field, and the ciliated cleft. The vibratile plume is distinctive from the surrounding ciliation by its length and pattern of beating. It consists of four bundles of cilia approximately 64 µm long that beat rhythmically in synchrony into the ciliated cleft. Each bundle of cilia arises from a single cell which is further distinguished by apical pits of stout microvilli. The glandular fields and the vibratile plume are bordered by a variety of previously undescribed ciliated cells, longitudinal myoepithelial cells, and a pair of antero-medial eyespots. Another type of ciliated sensory cell is located between cornal cells. These intercoronal cells form synapses directly with the adjacent coronal cells. On the basis of these ultrastructural observations, the radial ciliated cells of the apical disc and the intercoronal cells are classified as primary sensory cells with different motor pathways. The radial ciliated cells are associated with the various effector organs of the larva via the nervous system. The intercoronal cells may combine sensory and motor functions by synapsing directly with the larval locomotory organ.

Expansion of myeloid suppressor cells in SHIP-deficient mice represses allogeneic T cell responses.

Previously we demonstrated that SHIP(-/-) mice accept allogeneic bone marrow transplants (BMT) without significant acute graft-vs-host disease (GvHD). In this study we show that SHIP(-/-) splenocytes and lymph node cells are poor stimulators of allogeneic T cell responses that cause GvHD. Intriguingly, SHIP(-/-) splenocytes prime naive T cell responses to peptide epitopes, but, conversely, are partially impaired for priming T cell responses to whole Ag. However, dendritic cells (DC) purified from SHIP(-/-) splenocytes prime T cell responses to allogeneic targets, peptide epitopes, and whole Ag as effectively as SHIP(+/+) DC. These findings point to an extrinsic effect on SHIP(-/-) DC that impairs priming of allogeneic T cell responses. Consistent with this extrinsic effect, we found that a dramatic expansion of myeloid suppressor cells in SHIP(-/-) mice impairs priming of allogeneic T cells. These findings suggest that SHIP expression or its activity could be targeted to selectively compromise T cell responses that mediate GvHD and graft rejection.

Influence of SHIP on the NK repertoire and allogeneic bone marrow transplantation.

Natural killer cell (NK) receptors for major histocompatibility complex (MHC) class I influence engraftment and graft-versus-tumor effects after allogeneic bone marrow transplantation. We find that SH2-containing inositol phosphatase (SHIP) influences the repertoire of NK receptors. In adult SHIP-/- mice, the NK compartment is dominated by cells that express two inhibitory receptors capable of binding either self or allogeneic MHC ligands. This promiscuous repertoire has significant functional consequences, because SHIP-/- mice fail to reject fully mismatched allogeneic marrow grafts and show enhanced survival after such transplants. Thus, SHIP plays an important role in two processes that limit the success of allogeneic marrow transplantation: graft rejection and graft-versus-host disease.