[A human pluripotent stem cell in the blood of adults: towards a new cellular therapy for tissue repair].

The presence in normal adult man of stem cells sharing the properties of embryonic stem cells opens new avenues for basic and therapeutic research. We describe a stem cell present in normal adult human blood, probably able to give rise to the "reserve" stem cells in charge of repair, present in different organs. These monocytoid circulating cells are able to transdifferentiate into several cell types. In normal man, they are almost quiescent and are strictly controlled by a special subpopulation of T lymphocytes. In diseases such as fibrosis and chondrosarcoma, these cells proliferate and the differentiated cells escape T lymphocyte control. As a consequence, these cells accumulate, giving rise in vitro to a tissue which evoke the lesions characterizing the disorder of the patient, showing spontaneously their pluripotentiality. Neural cell markers are present in this migrating cell, suggesting that pluripotent stem cells present in adult man may derive from the neural crest. These circulating cells could offer a source of stem cells for cellular and gene therapy provided the normal cells could be expanded, their transdifferentiation directed and the control by T lymphocytes maintained.