Colony-forming precursors of circulating blood fibroblasts

Circulating connective tissue precursors: extreme rarity in humans and chondrogenic potential in guinea pigs

Using a variety of cell separation techniques and cultivation conditions, circulating, adherent, connective tissue, clonogenic cells were found in just 3 donors out of 66, demonstrating that these precursors are extremely rare in postnatal human blood. Contrary to humans, guinea pig blood shows much more reproducible connective tissue colony formation; it was therefore chosen to study the differentiation potential of adherent blood-derived clonogenic cells. Out of 22 single colony-derived strains of various morphologies, only 5 spindle-shaped strains showed extensive proliferative capacity in vitro. None of these strains formed bone upon in vivo transplantation, whereas two strains formed cartilage in high-density pellet cultures in vitro. Both chondrogenic strains included cells expressing aggrecan, whereas nonchondrogenic strains did not. Out of four polyclonal strains studied, one formed both cartilage and abundant bone accompanied by hematopoiesis-supporting stroma. Evidently, there are cells in adult guinea pig blood capable of both extensive proliferation and differentiation toward cartilage: circulating chondrogenic precursors. Although some of these cells lack osteogenic potential and therefore represent committed chondrogenic precursors, others may be multipotential and consequently belong to the family of skeletal stem cells. This is the first demonstration of postnatal circulating chondrogenic precursors, as well as of precursor cells with chondrogenic but not osteogenic potential. Disclosure of potential conflicts of interest is found at the end of this article.

Regulated Production of Type I Collagen and Inflammatory Cytokines by Peripheral Blood Fibrocytes

We recently described a novel population of blood-borne cells, termed fibrocytes, that display a distinct cell surface phenotype (collagen+/CD13+/CD34+/CD45+), rapidly enter sites of tissue injury, and contribute to scar formation. To further characterize the role of these cells in vivo, we examined the expression of type I collagen and cytokine mRNAs by cells isolated from wound chambers implanted into mice. Five days after chamber implantation, CD34+ fibrocytes but not CD14+ monocytes or CD90+ T cells expressed mRNA for type I collagen. Fibrocytes purified from wound chambers also were found to express mRNA for IL-1β, IL-10, TNF-α, JE/MCP, MIP-1α, MIP-1β, MIP-2, PDGF-A, TGF-β1, and M-CSF. The addition of IL-1β (1–100 ng/ml), a critical mediator in wound healing, to fibrocytes isolated from human peripheral blood induced the secretion of chemokines (MIP-1α, MIP-1β, MCP-1, IL-8, and GROα), hemopoietic growth factors (IL-6, IL-10, and macrophage-CSF), and the fibrogenic cytokine TNF-α. By contrast, IL-1β decreased the constitutive secretion of type I collagen as measured by ELISA. Additional evidence for a role for fibrocytes in collagen production in vivo was obtained in studies of livers obtained from Schistosoma japonicum-infected mice. Mouse fibrocytes localized to areas of granuloma formation and connective matrix deposition. We conclude that fibrocytes are an important source of cytokines and type I collagen during both the inflammatory and the repair phase of the wound healing response. Furthermore, IL-1β may act on fibrocytes to effect a phenotypic transition between a repair/remodeling and a proinflammatory mode.

Possible monocytic origin of chondrosarcoma: in vitro transdifferentiation of HLA-DR blood monocyte-like cells from a patient with chondrosarcoma, into neo-fibroblasts and chondrocyte-like cells

Nodules and multilayered areas composed of fibroblasts and chondrocyte-like cells embedded in an abundant extracellular matrix appeared spontaneously in in vitro culture of mononucleated blood cells taken from a patient with chondrosarcoma. Using specific antibodies it was demonstrated that the neo-fibroblasts which developed in the culture resulted from a direct transdifferentiation of monocytes expressing HLA-DR specificity. The experiment was carried out twice, once before surgery and then two years later. In both cases the spontaneous transdifferentiation of HLA-DR monocytes into neo-fibroblasts was observed. Previously it was shown that normal monocytes were also able to give rise in vitro to neo-fibroblasts. However, the latter are normally rapidly destroyed by cell-cell contact with T-cells. Normal T-cells adhere to normal neo-fibroblasts by which they are finally engulfed. As a result, the neo-fibroblasts lose their fibroblastic shape, no longer adhere to their support and die. Therefore the abnormal proliferation and persistence of neo-fibroblasts in pathological situations such as the present case may result either from an intrinsic defect in monocytes, T-cells or both. The question is whether or not this transdifferentiation process observed in vitro accounts for the development of chondrosarcoma in vivo. The present results suggest that in vivo chondrosarcoma may start in a necrotic zone (resulting for instance from trauma) and attract HLA-DR monocytes, where they accumulate and transdifferentiate into neo-fibroblasts and chondrocyte-like cells. The uncontrolled transdifferentiation of these HLA-DR monocytes resulting from a dysregulation of the immune system is probably linked to the malignant process which may have a retroviral origin. The question is raised regarding the embryologic origin of this special sub-population of blood monocytes in which pluripotential capabilities are retained; its origin may differ from that of the other circulating monocytes.