The avian transcription factor c-Rel is expressed in lymphocyte precursor cells and antigen-presenting cells during thymus development.

Transcription factors of the Rel/NF-kappaB family are widely involved in the immune system. In this study, we investigate the in vivo expression of the avian protein c-Rel in the T-cell lineage during thymus development. The majority of thymocytes do not express the c-Rel protein. However, lymphocyte precursor cells that colonize the thymus express the c-Rel protein shortly after their homing in the organ and before they begin to differentiate. c-Rel is also detected in different subsets of antigen-presenting cells such as epithelial cells, dendritic cells, and macrophages. In vitro studies have shown that Rel/NF-kappaB proteins are sequestered in an inactive form in the cytoplasm by interaction with the IkappaBalpha inhibitory protein. By immunocytochemistry, we show that in vivo c-Rel is localized in the cytoplasm of antigen-presenting cells but in both the cytoplasm and nucleus of lymphocyte precursor cells. The cytoplasmic localization of c-Rel in antigen-presenting cells correlates with a high expression of IkappaBalpha, whereas the nuclear localization of c-Rel in lymphocyte precursor cells correlates with a much lower expression of IkappaBalpha. These results suggest that c-Rel might be constitutively activated in lymphocyte precursor cells.