Cartilage oligomeric matrix protein is a natural inhibitor of thrombin.

Thrombin is an effector enzyme for hemostasis and thrombosis; however, endogenous regulators of thrombin remain elusive. Cartilage oligomeric matrix protein (COMP), a matricellular protein also known as thrombospondin-5, is essential for maintaining vascular homeostasis. Here, we asked whether COMP is involved in the process of blood coagulation. COMP deficiency shortened tail-bleeding and clotting time and accelerated ferric-chloride-induced thrombosis in mice. The absence of COMP had no effect on platelet count. In contrast, COMP specifically inhibited thrombin-induced platelet aggregation, activation, and retraction and the thrombin-mediated cleavage of fibrinogen. Furthermore, surface plasmon resonance analysis revealed direct thrombin-COMP binding (KD = 1.38 ± 0.24 µM). In particular, blockage of thrombin exosites with compounds specific for exosite I (hirudin and HD1 aptamer) or exosite II (heparin and HD22 aptamer) impaired the COMP-thrombin interaction, indicating a 2-site binding mechanism. Additionally, epidermal growth factor-like repeats (amino acids 84-261) were identified as a COMP binding site for thrombin. Moreover, COMP was expressed in and secreted by platelets. Using bone marrow transplantation and platelet transfusion to create chimeric mice, platelet-derived but not vessel-wall-derived COMP was demonstrated to inhibit coagulation. Taken together, COMP is an endogenous thrombin inhibitor and negative regulator of hemostasis and thrombosis.

Tumor necrosis factor-alpha-converting enzyme (ADAM17) mediates GPIbalpha shedding from platelets in vitro and in vivo.

Interaction of the platelet receptor glycoprotein (GP) Ib-V-IX with von Willebrand factor exposed at a site of vascular injury is an essential step in the initiation of a hemostatic plug. Proteolytic cleavage (shedding) of the GPIbalpha subunit was first described >25 years ago, the protease mediating this event as well as its physiological function, however, have not been elucidated. We reported recently that shedding of GPIbalpha induced by platelet storage or mitochondrial injury involves a platelet-derived metalloproteinase(s). Here we show that GPIbalpha shedding in response to mitochondrial injury or physiological activation is inhibited in platelets obtained from chimeric mice, which express inactive tumor necrosis factor-alpha converting enzyme (TACE(DeltaZn/DeltaZn)) in blood cells only. Shedding was also inhibited in mouse and human platelets in the presence of 2 potent TACE inhibitors: TAP1 and TMI-1. Our data further suggest that TACE is important in the regulation of GPIbalpha expression in vivo because we observed an approximately 90% reduction in soluble GPIbalpha (glycocalicin) in plasma of TACE(DeltaZn/DeltaZn) chimeras as well as significantly increased levels of GPIbalpha on circulating platelets. In contrast, shedding of P-selectin from activated platelets was not affected by the mutation in TACE. Damaged TACE(DeltaZn/DeltaZn) platelets were further characterized by a markedly improved post-transfusion recovery and hemostatic function in mice. In conclusion, our data demonstrate that TACE is expressed in platelets and that it is the key enzyme mediating shedding of GPIbalpha.

A murine model of antimetabolite-based, submyeloablative conditioning for bone marrow transplantation: biologic insights and potential applications.

OBJECTIVE: Nonmyeloablative conditioning regimens for marrow transplantation are desirable in many settings. Because repeated doses of the antimetabolite 5-fluorouracil (5-FU) decreases marrow long-term repopulating ability (LTRA) upon transplantation into lethally irradiated hosts, we hypothesized that mice given sequential doses of 5-FU (termed paired dose 5-FU) may permit substantial syngeneic marrow engraftment. METHODS: C57Bl/6 or X-linked chronic granulomatous disease (X-CGD) mice were administered 5-FU (150 mg/kg) on days -5 and -1. Assessment of host marrow phenotype and repopulating ability occurred on day 0. Transplantation of syngeneic donor marrow occurred on day 0 or day +15. RESULTS: We confirmed that the number of Sca-1+lin- cells and the LTRA of marrow from paired dose 5-FU-treated animals were diminished. C57Bl/6 hosts conditioned with paired doses of 5-FU followed by transplantation of 20 x 10(6) fresh B6.SJL marrow cells on day 0 displayed 44.9% +/- 7.1% donor chimerism 2 months posttransplant, and 34.4% +/- 8.6% donor chimerism 6 months posttransplant. In contrast, paired dose 5-FU-conditioned hosts transplanted with similar numbers of donor cells on day +15 exhibited only 3.4% +/- 1.2% donor chimerism at 2 months. Paired dose 5-FU-conditioned X-CGD hosts transplanted with MSCV-m91Neo-transduced X-CGD marrow averaged 6.6% +/- 2.3% (range, 4%-10%) NADPH oxidase-reconstituted neutrophils 12-16 months after transplant. CONCLUSION: These findings support the concept that impairment of host stem cell competitiveness may be an important mechanism for permitting engraftment of donor cells, and suggest that only a brief period of modest host stem cell impairment may be necessary to achieve substantial donor cell engraftment.

Reconstitution of functional human B lymphocytes in NOD/SCID mice engrafted with ex vivo expanded CD34(+) cord blood cells.

OBJECTIVE: Functional capacity of B cells developed from ex vivo expanded hematopoietic stem cells has not been fully evaluated. Therefore, we investigated the antigen-specific antibody production in human B cells maturated from ex vivo expanded cord blood (CB) CD34(+) cells in NOD/Shi-scid (NOD/SCID) mice. MATERIALS AND METHODS: CB CD34(+) cells were cultured for 5 days in the presence of human cytokines and the murine stromal cell line HESS-5, and transplanted into irradiated NOD/SCID mice. These mice, reconstituted with human hematopoietic cells, were challenged with T-cell-independent (TI) or T-cell-dependent (TD) antigens after CD19(+) cells appeared at 6 weeks. RESULTS: Three months later, anti-dinitrophenol (DNP)-specific antibody was detected in both mice immunized with DNP-Ficoll (TI) and those immunized with DNP-keyhole limpet hemocyanin or DNP-ovalbumin (TD). The anti-DNP antibody was mainly immunoglobulin M, but a small amount of immunoglobulin G also was detected. In the spleen, the majority of CD19(+) cells expressed mature B-cell markers such as CD40, immunoglobulin M, immunoglobulin D, cytoplasmic Cmu, and light chains kappa, and lambda. CONCLUSIONS: These results indicate that human B cells develop from CD34(+) cells in NOD/SCID mice to produce antigen-specific antibody with in vivo primary stimulation. This system provides a powerful and versatile tool for studying the entire process of human B-lymphocyte development and producing specific human monoclonal antibodies.

Regeneration capability of Lin-/c-Kit+/Sca-1+ cells with or without radiation exposure for repopulation of peripheral blood in lethally irradiated mice monitored using Ly5.1 isotype on days 35, 90, and 270 after transplantation.

OBJECTIVE: Hematopoietic stem cells are supposed to repopulate and maintain long-term regeneration of the recipient's bone marrow and peripheral blood. In this study, we evaluated the regeneration capability of Lin(-)/c-Kit(+)/Sca-1(+) (LKS) cells, the putative hematopoietic stem cells, after radiation exposure at graded doses, for long-term regeneration of peripheral blood in lethally irradiated recipients. MATERIALS AND METHODS: LKS primitive progenitor cells, collected from the bone marrow of Ly5.1 mice that had been irradiated at graded increased doses (0.5, 1, 1.5, and 2 Gy) were transfused into lethally irradiated (9.5 Gy) Ly5.2 mice. Then, the Ly5.1 chimeric ratio in repopulated peripheral blood cells in the recipients was monitored. A reactive oxygen species (ROS)-reacting CM-H(2)DCFDA dye was used to evaluate the amount of ROS in LKS primitive progenitor cells with/without irradiation. Moreover, the amount of intracytoplasmic ROS generated after irradiation was estimated in terms of percent attenuation of cellular increase in number by the treatment with 100 microM N-acetyl-L-cysteine before irradiation. RESULTS: Differential regeneration capability of LKS cells irradiated at graded increased doses showed a dose-dependent suppression of regeneration of peripheral blood in the recipient mice as compared with LKS cells without radiation exposure. The amount of intracytoplasmic ROS in LKS cells was much smaller than that in mature bone marrow cells, and that of ROS in LKS increased slightly after radiation exposure, as evaluated by CM-H(2)DCFDA dye fluorescence analysis. The estimated amount of ROS generated in LKS cells after radiation exposure was different between progenitor cells for early regeneration and those for late regeneration; namely, the amount of ROS in progenitors on day 270 were estimated to be smaller than that in progenitors for day 35 or day 90. CONCLUSIONS: Because of the small amount of generated radiation-induced ROS calculated in terms of attenuation rate after N-acetyl-L-cysteine treatment, progenitor cells regenerating peripheral blood cells 270 days after transfusion were assumed to be anaerobic and more immature and radioresistant than those on day 35 or day 90. However, limited long-term regeneration capability (up to 270 days) of steady-state LKS cells than that of unfractionated rescue bone marrow cells suggests that LKS cells do not seem to be true hematopoietic stem cells.

Reconstitution of lethally irradiated mice by cells isolated from adipose tissue.

It is suggested that hematopoietic stem cells (HSC) could be found in several tissues of mesodermic origin. Among these, adipose tissue can expand throughout adult life and its expansion is not only due to mature adipocyte hypertrophy but also to the presence of precursor cells in stroma-vascular fraction (SVF). Here we report that transplantation of cells isolated from mice adipose tissue can efficiently rescue lethally irradiated mice and results in a reconstitution of major hematopoietic lineages. Donor cells can be detected in blood and in hematopoietic tissues of recipient mice. Adipose tissue contains a significant percentage of CD34, CD45 positive cells, and SVF cells were able to give rise to hematopoietic colonies in methylcellulose. We demonstrate the presence of hematopoietic progenitors in adipose tissue by phenotypic and functional characteristics. Thus adipose tissue could be considered as an important and convenient source of cells able to support hematopoiesis.

Dissociation of hemopoietic chimerism and allograft tolerance after allogeneic bone marrow transplantation.

Creation of stable hemopoietic chimerism has been considered to be a prerequisite for allograft tolerance after bone marrow transplantation (BMT). In this study, we demonstrated that allogeneic BMT with bone marrow cells (BMC) prepared from either knockout mice deficient in both CD4 and CD8 T cells or CD3E-transgenic mice lacking both T cells and NK cells maintained a high degree of chimerism, but failed to induce tolerance to donor-specific wild-type skin grafts. Lymphocytes from mice reconstituted with T cell-deficient BMC proliferated when they were injected into irradiated donor strain mice, whereas lymphocytes from mice reconstituted with wild-type BMC were unresponsive to donor alloantigens. Donor-specific allograft tolerance was restored when donor-type T cells were adoptively transferred to recipient mice given T cell-deficient BMC. These results show that donor T cell engraftment is required for induction of allograft tolerance, but not for creation of continuous hemopoietic chimerism after allogeneic BMT, and that a high degree of chimerism is not necessarily associated with specific allograft tolerance.