DEVELOPMENTALLY REGULATED FETAL THYMIC AND EXTRATHYMIC T-CELL RECEPTOR-GAMMA-DELTA GENE-EXPRESSION

The γδ T-cell receptor (TCR) is the first TCR to be expressed in ontogeny in all vertebrates in which it has been examined thoroughly. Murine γδ cell-surface protein is detected by the fourteenth day of gestation. In this work, the activation of γδ RNA has been studied. Data indicate that the first TCR protein to appear in the thymus is encoded by γ genes that are activated after cells colonize the thymus. However, the sequential appearance of different γδ TCR proteins during thymic ontogeny cannot be readily explained by differential temporal activation of Vγ genes in the thymus. There are distinct patterns of γ and δ genes expression during fetal liver development and in the fetal gut (or tissue associated with it). Cells apparent in the liver of mice at birth express γδ cell-surface protein, but they disappear from the liver very soon afterward. One Vγ gene is rearranged and expressed prethymically. In addition, γ gene expression is detectable in the livers of newborn athymic mice. Together, these observations indicate a thymic-independent pathway of activation of TCR genes.