Haemopoietic Origin of Myofibroblasts Formed in the Peritoneal Cavity in Response to a Foreign Body

Abstract

This study utilized both in vivo and in vitro techniques to investigate whether cells of bone marrow origin can differentiate into smooth muscle-like cells (myofibroblasts) with contractile filaments and proteins. Female C57BL/6 mice expressing the Ly5.2 antigen on the surface of their haemopoietic cells had four pieces of silastic tubing (3 × 0.5 mm outer diameter) or boiled blood clot (2–3 mm diameter) placed in their peritoneal cavity. After 3, 5, 7 and 14 days (n = 4/group) the implants were removed and those that had remained free-floating were processed for light microscopy, immunohistochemistry and electron microscopy. In the first 3–5 days, rounded cells adhered to the entire surface of the tubing then flattened. These cells stained with fluoresceinated antibodies to Ly5.2 showing that they were derived from haemopoietic cells. By 14 days the cells had become elongated and multilayered in a collagen matrix, forming a thick tissue capsule around the tubing or boiled clot. They contained contractile filaments and stained with antibodies to α-smooth muscle actin but no longer stained for Ly5.2. A separate set of female C57BL/6 Ly5.2 mice were X-irradiated to destroy bone marrow then immediately transfused with 10⁶ nucleated bone marrow cells taken from the femur and tibia of a congenic strain of male mice expressing the Ly5.1 allele. Eight of the female mice with successful engraftment (80–99%) had silastic tubing implanted in the peritoneal cavity. After 14 days, in situ hybridization with Y chromosome probe confirmed the male donor, and thus bone marrow, origin of the elongated cells that formed the capsule. In vitro studies showed that cells of the murine macrophage cell lines RAW 264.7 and J774 express α-smooth muscle actin after exposure to the cytokine γ-interferon in vitro. These data show that bone marrow-derived cells can differentiate into smooth muscle-like cells and raises the possibility that blood-derived cells may contribute to the development of fibro-proliferative vascular diseases such as atherosclerosis.

Cellular Plasticity of Inflammatory Myeloid Cells in the Peritoneal Foreign Body Response

“…These results confirm earlier electron microscopic studies, which reported the presence of cells intermediate in morphology between macrophage and fibroblast/myofibroblast within the tissue capsule 30 and provide evidence that the hematopoietic progenitor of the myofibroblast is the macrophage. 4 Macrophages have many trophic roles in development and are essential for effective tissue regeneration. 37 Although the role of macrophages was previously attributed to the paracrine production of cytokine/growth factors and extracellular matrix proteins to recruit and support other cell types, 2,38 the present study provides evidence for a more direct role in this process by way of their capacity to transdifferentiate into myofibroblasts.…”

“…Given the prevalence of macrophages within the tissue capsule through to day 28, along with previous research showing the hematopoietic origin of capsular myofibroblasts, 4 we sought to determine whether cells of myeloid origin (ie, monocyte/macrophages expressing EGFP) within the tissue capsule can transdifferentiate into ␣-SM actin-expressing myofibroblasts. We reasoned that such cells would retain the stable EGFP protein even if transcription of macrophage-specific genes was extinguished.…”

“…Within the first 3 to 5 days after implantation, the object is covered by rounded cells, many of which have a macrophage-like morphology and express the common leukocyte antigen Ly-5 (CD45). 4 After 2 to 3 weeks, a tissue capsule comprising multiple layers of myofibroblasts and extracellular matrix and covered by a continuous layer of mesothelial cells surrounds the object. 5 In contrast with the tissue surrounding foreign material at other anatomical sites, the tissue encapsulating free-floating foreign objects in the peritoneal cavity is avascular.…”

“…We have previously demonstrated that myofibroblasts within the tissue capsule are of bone marrow (hematopoietic) origin using sex-mismatched bone marrow transplant experiments. 4 Others have demonstrated that labeled peripheral blood mononuclear cells, injected into the peritoneal cavity at the same time as foreign object implantation, contribute to tissue capsule formation. 18 However, it is not clear whether monocyte/macrophages are a cellular source of tissue capsule myofibroblasts.…”

Transdifferentiation, a Potential Mechanism for Covering Vascular Grafts Grown Within Recipient’s Peritoneal Cavity With Endothelial-Like Cells

“…The other marker, CD18, suggests that these intimal cells may indeed be derived from leukocytes, either as a direct result of blood perfusion or as part of the foreign body reaction, as suggested by both the Campbell and Cebotari groups. 3 In any case, this mixed phenotype is indicative of a transdifferentiation process, which may be further stimulated 4,5 and exploited.…”