Circulating CD14+ monocytes are precursors of phagocytes, such as macrophages and dendritic cells. Here we report primitive cells with a fibroblast-like morphology derived from human peripheral blood CD14+ monocytes that can differentiate into several distinct mesenchymal cell lineages. We named this cell population monocyte-derived mesenchymal progenitor (MOMP). MOMPs were obtained in vitro from human peripheral blood mononuclear cells cultured on fibronectin in the presence of fetal bovine serum alone as a source of growth factors. MOMPs had a unique molecular phenotype-CD14+CD45+CD34+type I collagen+-and showed mixed morphologic and molecular features of monocytes and endothelial and mesenchymal cells. MOMPs were found to be derived from a subset of circulating CD14+ monocytes, and their differentiation required that they bind fibronectin and be exposed to one or more soluble factors derived from peripheral blood CD14- cells. MOMPs could be expanded in culture without losing their original phenotype for up to five passages. The induction of MOMPs to differentiate along multiple limb-bud mesodermal lineages resulted in the expression of genes and proteins specific for osteoblasts, skeletal myoblasts, chondrocytes, and adipocytes. Our findings represent the first evidence that human circulating CD14+ monocytes are a source of progenitors that exhibit mesenchymal cell differentiation.

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We have recently reported that in patients with chronic immune thrombocytopenic purpura (IMTP), circulating T and B cells that are responsive to gpIIb-IIIa can induce anti-platelet autoantibody production. In this study, the frequencies and activation status of gpIIb-IIIa-reactive T and B cells were evaluated in the peripheral blood and spleen obtained from nine IMTP patients undergoing splenectomy. There was no difference in gpIIb-IIIa-reactive T cell frequencies between peripheral blood and spleen (6.4 ± 2.6 vs 5.2 ± 2.4 per 105 T cells), as determined by limiting dilution analysis, but activated T cells responsive to gpIIb-IIIa showing accelerated proliferation kinetics and those expressing CD154 were more frequent in spleen than in peripheral blood. The frequencies of anti-gpIIb-IIIa Ab-producing B cells, as determined by ELISPOT assay, were also similar in peripheral blood and spleen (61.2 ± 24.0 vs 77.7 ± 45.3 per 105 B cells); however, an anti-gpIIb-IIIa Ab was spontaneously produced by splenocytes in vitro, but scarcely secreted by PBMCs. CD19−/surface Ig−/CD38+/CD138+ plasma cells secreting anti-gpIIb-IIIa Ab were exclusively detected in the spleen. In serial analysis, the frequencies of circulating gpIIb-IIIa-reactive T and B cells were markedly decreased after splenectomy in patients with a complete response, but were unchanged in nonresponders. These findings indicate that an interaction between gpIIb-IIIa-reactive T and B cells inducing anti-platelet Ab production in IMTP patients occurs primarily in the spleen and that the significant number of gpIIb-IIIa-reactive T and B cells activated in the spleen are released into the circulation as memory cells.

ABSTRACT

To evaluate the mechanisms underlying prolonged thrombocytopenia after allogeneic hematopoietic stem cell transplantation (SCT), an index for plasma glycocalicin normalized for the individual platelet count (GCI), plasma thrombopoietin (TPO), and circulating B cells producing anti-GPIIb-IIIa antibodies were measured in 50 SCT recipients with or without prolonged thrombocytopenia, 42 patients with idiopathic thrombocytopenic purpura, nine patients with aplastic anemia, and 22 healthy individuals. All three indices were significantly higher in the SCT recipients with thrombocytopenia than in those without (Po0.01 for all comparisons), and were significantly correlated with the platelet count in SCT recipients.Stepwise multiple regression analysis of the samples from the SCT recipients revealed that GCI and TPO independently pointed to specific mechanisms of thrombocytopenia. The GCI and TPO status in SCT recipients with thrombocytopenia had a pattern similar to that seen in aplastic anemia, suggesting a major role for impaired thrombopoiesis. An antiplatelet antibody response was frequently detected in SCT recipients, but the development of thrombocytopenia is likely to depend on additional factors, such as reticuloendothelial function. In summary, post transplant prolonged thrombocytopenia is associated with complex mechanisms, including impaired thrombopoiesis and increased platelet turnover.