Effects of mixed chimeric bone marrow repopulation on platelet storage pool-associated bleeding defects in mouse mutants.

We have previously shown mouse platelet storage pool deficiency (SPD) to be associated with lesions at eight different genetic loci, each of which is sufficient to produce murine SPD. We have also shown that normal bleeding times and normal platelet functions are restored when mice with SPD are transplanted with marrow from normal mice. Conversely, when normal mice are transplanted with mutant marrow, they present symptoms of SPD. In order to determine the amount of normal platelets needed to prevent the prolonged bleeding times associated with SPD, we established stable mixed chimeric mice by transplanting various ratios of normal and mutant marrow into lethally irradiated host animals. The proportion of normal input marrow correlated well with the proportion of normal peripheral red blood cells and platelets determined in chimerae 100 days after transplantation using direct morphology and electrophoretic variants of glucose phosphate isomerase to identify normal and mutant cell populations. The proportions of normal input marrow were also reflected in the proportions of platelets with normal and mutant platelet morphology in the chimerae. This confirms that the platelet abnormality in SPD is intrinsic to the stem cell population from which the platelets are derived. When bleeding times were determined in the mixed chimeric mice, a surprisingly high percentage of normal platelets (greater than 50% and sometimes greater than 75%) were needed to stop bleeding. These results suggest that the mutant platelets in the mixed chimeric mice may interfere with normal platelet aggregation patterns. They also raise some important considerations in devising treatment for SPD. Bleeding episodes in human SPD are normally treated by platelet transfusion. The results suggest that, at least in some cases, transfusions may not be effective. Also, in future gene therapy of this disease, it is like that a functional gene will have to be present in greater than 50% of stem cells for therapy to be effective.

Restoration of neutrophil and platelet function in feline Chediak-Higashi syndrome by bone marrow transplantation.

Allogeneic bone marrow transplantation (BMT) was successfully performed in four Chediak-Higashi (CHS) syndrome affected cats. Preparatory regimens included selective intestinal flora decontamination, fractionated total body irradiation for myeloablation, and prophylactic treatment for graft-versus-host disease with cyclosporin A. Neutrophil chemotaxis under-agarose and whole-blood platelet aggregation/secretion were characterized prior to BMT and after engraftment of donor-origin marrow cells. Liver and kidney biopsies were obtained and evaluated by light and electron microscopy before, and at 6 months post-BMT to determine what effect BMT might have on abnormal lysosome fusion in hepatocytes and renal tubule cells. The platelet storage pool defect was resolved by day 40 post-BMT. In vitro neutrophil migration in all cats appeared to improve with time after BMT and complete restoration was evident by day 175 post-BMT. No apparent differences were evident in either the liver or the kidney at 6 months post-BMT. One cat developed seizures and one developed posterior paresis 5 months post-BMT; neurologic impairment ultimately resulted in death of two cats at 6 and 8 months post-BMT, respectively. Neurologic lesions in both cats were characterized by non-suppurative encephalitis. Allogeneic BMT successfully corrected the neutrophil migration defect and platelet storage pool deficiency but had no effect on lysosome distribution in liver and kidney cells of CHS cats.

The use of recombinant factor VIIa in children with inherited platelet function disorders.

Inherited deficiencies of platelet surface glycoproteins such as Glanzmann's thrombasthenia (GT) or Bernard-Soulier syndrome (BSS) can lead to a severe bleeding diathesis. In the past, bleeding episodes in these patients have often required platelet transfusion to secure haemostasis but recently a number of patient reports have suggested that recombinant factor VIIa (rVIIa) may also be effective. We have used rVIIa on 33 occasions in seven children with inherited platelet function disorders over a 2-year period: five had GT, one had BSS and one had storage pool disease with a severe phenotype. Bleeding ceased with rVIIa alone in 10 of 28 acute bleeding episodes, but recurred in two of these. The two features that predicted response to rVIIa were the severity of the bleeding and the delay from the onset of bleeding to treatment. Five episodes of planned surgical intervention were treated successfully with rVIIa. Eighteen out of the 28 acute episodes and none of the planned surgical episodes required blood product support. We have found variable efficacy of rVIIa for acute bleeding episodes in this small series of children with inherited platelet function defects but larger studies are warranted, particularly as rVIIa is a relatively low-risk treatment approach for these disorders.