Signals transduced by CD3epsilon, but not by surface pre-TCR complexes, are able to induce maturation of an early...

Development of immature CD4-CD8- (double-negative) thymocytes to the CD4+CD8+ (double-positive) stage is linked to productive rearrangement of the TCRbeta locus by signals transduced through the pre-TCR. However, the mechanism whereby pre-TCR signaling is initiated remains unclear, in part due to the lack of an in vitro model system amenable to both biochemical and genetic analysis. In this study, we establish the thymic lymphoma Scid.adh as such a model system. Scid.adh responds to Ab engagement of surface IL-2Ra (TAC):CD3epsilon molecules (a signaling chimera that mimics pre-TCR signaling in vivo) by undergoing changes in gene expression observed following pre-TCR activation in normal thymocytes. These changes include down-regulation of CD25, recombinase-activating gene (RAG)-1, RAG-2, and pTalpha; and the up-regulation of TCRalpha germline transcripts. We term this complete set of changes in gene expression, in vitro maturation. Interestingly, Scid.adh undergoes only a subset of these changes in gene expression following Ab engagement of the pre-TCR. Our findings make two important points. First, because TAC:CD3epsilon stimulation of Scid.adh induces physiologically relevant changes in gene expression, Scid.adh is an excellent cellular system for investigating the molecular requirements for pre-TCR signaling. Second, Ab engagement of CD3epsilon signaling domains in isolation (TAC:CD3epsilon) promotes in vitro maturation of Scid.adh, whereas engagement of CD3epsilon molecules contained within the complete pre-TCR fails to do so. Our current working hypothesis is that CD3epsilon fails to promote in vitro maturation when in the context of an Ab-engaged pre-TCR because another pre-TCR subunit(s), possibly TCRzeta, qualitatively alters the CD3epsilon signal.