Co-ordinate expression of the pre-T-cell receptor complex and a novel immature thymocyte-specific antigen, IMT-1, during thymocyte development

Previously we described a monoclonal antibody (mAb) that reacted with a cell-surface antigen, immature thymocyte antigen-1 (IMT-1), which is expressed on thymocytes of late CD4^- CD8^- (double negative) to early CD4⁺ CD8⁺ (double positive) differentiation stages. In this study, we investigated the expression of IMT-1 on various cell lineages in thymus as well as in peripheral lymphoid organs. We found that IMT-1 is expressed on T- cell receptor (TCR)-β(lo) and TCR-δ(lo) thymocytes, but not on TCR-β(hi), TCR-δ(hi) or natural killer (NK) 1.1⁺ thymocytes, or on peripheral αβ or γδ T cells. We also investigated the kinetics of expression of IMT-1 during fetal thymocyte development and compared it with the expression of the pre- TCR complex, comprising CD3, pre-TCR-α (pTα) and TCR-β. We found that expression of both was similar, starting at day 14.5 of gestation, peaking on day 16.5 and gradually decreasing thereafter. Furthermore, the expression of both IMT-1 and pTα was drastically reduced when DN thymocytes in recombination activating gene (RAG)-2(-/-) mice were challenged in vivo with anti-CD3 mAb. These results indicate that IMT-1 is expressed on not only immature thymocytes of αβ T-cell lineage but also on those of γδ T-cell lineage, and that the expression oF IMT-1 and the pre-TCR complex is co- ordinately regulated during the αβ lineage thymocyte development.