Stable expression of angiopoietin-1 and other markers by cultured pericytes: phenotypic similarities to a subpopulation of cells in maturing vessels during later stages of angiogenesis in vivo.

Pericytes have been difficult cells to study because they do not maintain their characteristic phenotype in vitro, and they begin to express fibroblast markers after only a few days in culture. We now report methods for the isolation, purification, culture, and repurification of human dermal pericytes from mixed cell populations using an immunoaffinity-magnetic bead approach coupled with the 3G5 IgM monoclonal antibody that is specific for a pericyte surface ganglioside. These purified cells could be expanded in culture, and they maintained their pericyte phenotype for up to 8 days. In addition, they strongly expressed angiopoietin-1 (Ang-1) but not angiopoietin-2, Tie-1, or Tie-2; in contrast, dermal microvascular endothelial cells exhibited a reciprocal expression pattern. These findings are important because the close proximity of endothelial cells and pericytes has often made it difficult to determine with certainty the specific cell type(s) that expressed each of these proteins in situ. Extending our in vitro findings to two models of angiogenesis in vivo, we demonstrated a subpopulation of Ang-1-expressing cells that appeared in maturing microvessels during later stages of cutaneous wound healing and vascular permeability factor/vascular endothelial growth factor-induced angiogenesis. Our results provide strong evidence that Ang-1 is expressed by pericytes in vitro and in vivo and that the role proposed for Ang-1 in vessel maturation in development can be extended to vessel maturation after angiogenesis in adult tissues.