Ontogeny of murine T cells: thymus-regulated development of T cell receptor-bearing cells derived from embryonic yolk sac

Abstract

Pre-thymic stem cells which arise extrinsically to the thymus differentiate into mature T cells after colonizing the fetal thymus, but the origin of the stem cells during mammalian embryogenesis remains unknown. More than one potential site has been proposed, including the liver, the omentum and the embryonic yolk sac (YS). Since the murine YS appears earliest in gestation and is the first site of hematopoiesis, our studies focus on the mouse YS as the early major source of pre-thymic stem cells. Previous studies showed that freshly obtained YS cells expressing no known major T cell marker can develop into Thy-1+ cells in vitro either transiently or stably. However, the critical steps leading to functional maturation of T cells involve the rearrangement and expression of the T cell receptor (TcR) genes. We report here that YS cells grown in short-term cell culture show no detectable rearrangement or transcription at the TcR gamma gene locus, although the TcR gamma gene of in vitro transformed YS cells rearranges in the absence of the thymus. However, after seeding lymphocyte-depleted thymuses in vitro, YS cells can differentiate into various subsets of cells appearing in the thymus, including CD3/TcR alpha/beta- and gamma/delta-bearing cells. TcR V gamma 3 which is expressed mainly on early fetal thymocytes and on Thy-1+ dendritic cells can be detected on YS-derived cells. TcR V beta 8-expressing cells which appear later in thymic ontogeny are also detected. The kinetics for reconstituting the thymus varies among YS cells of different ages (day 10 to 13 gestation). These results indicate that a single wave of early mouse YS cells can undergo the molecular and cellular changes associated with T cell differentiation and acquire mature T cell characteristics in the absence of continuous inflow of other stem cells. The thymus acts in an inductive/regulatory role for that differentiation.