Signals Transduced by CD3ε, But Not by Surface Pre-TCR Complexes, Are Able to Induce Maturation of an Early Thymic Lymphoma In Vitro

Development of immature CD4−CD8− (double-negative) thymocytes to the CD4+CD8+ (double-positive) stage is linked to productive rearrangement of the TCRβ 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):CD3ε 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 pTα; and the up-regulation of TCRα 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:CD3ε 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 CD3ε signaling domains in isolation (TAC:CD3ε) promotes in vitro maturation of Scid.adh, whereas engagement of CD3ε molecules contained within the complete pre-TCR fails to do so. Our current working hypothesis is that CD3ε fails to promote in vitro maturation when in the context of an Ab-engaged pre-TCR because another pre-TCR subunit(s), possibly TCRζ, qualitatively alters the CD3ε signal.

The Mechanism of Chromosome 7 Inversion in Human Lymphocytes Expressing Chimeric γβ TCR

Functional chimeric TCR chains, encoded by VγJγCβ or VγJβCβ hybrid gene TCR, are expressed at the surface of a small fraction of αβ T lymphocytes in healthy individuals. Their frequency is dramatically increased in patients with ataxia-telangiectasia, a syndrome associated with inherited genomic instability. As the TCR γ and β loci are in an inverted orientation on chromosome 7, the generation of such hybrid genes requires at least an inversion event. Until now, neither the sequences involved in this genetic mechanism nor the number of recombinations leading to the formation of functional transcriptional units have been characterized. In this manuscript, we demonstrate that at least two rearrangements, involving classical recombination signal sequence and the V(D)J recombinase complex, lead to the formation of productive hybrid genes. A primary inversion 7 event between Dβ and Jγ genic segments generates CγVβ and CβVγ hybrid loci. Within the CγVβ locus, secondary rearrangements between Vγ and Jγ or Vγ and Jβ elements generate functional genes. Besides, our results suggest that secondary rearrangements were blocked in the CβVγ locus of normal but not ataxia-telangiectasia T lymphocytes. We also provide formal evidence that the same Dβ-3′ recombination signal sequence can be used in successive rearrangements with Jγ and Jβ genic segments, thus showing that a signal joint has been involved in a secondary recombination event.

Development of Human Peripheral TCRBJ Gene Repertoire

Previous studies of TCRBJ gene repertoires of human peripheral T lymphocytes showed that all TCRBV family transcripts had some common features in BJ gene usage, and nevertheless, transcripts of each BV family gene had a distinct pattern. To discern how the development of the peripheral BJ repertoire is controlled, the effects of preferential BJ gene rearrangement, thymic selection, and peripheral stimulation on the repertoire formation were investigated. A PCR-ELISA technique was used to examine the immature CD3−CD4+CD8−, and mature CD3+CD4+CD8− and CD3+CD4−CD8+ thymocytes, and peripheral CD4+ and CD8+ T lymphocytes for their BJ gene repertoires. Analogous to the peripheral repertoire, the BJ gene repertoires of the immature thymocytes displayed common features, and each BV transcript had a distinct pattern. All features were conserved well by those of mature thymocytes and peripheral T lymphocytes. In addition, the BJ gene repertoires of mature CD4 and CD8 thymocytes and peripheral lymphocytes with the same coreceptors were apparently different in a few BV-BJ combinations. The results showed that the overall BJ gene repertoire pattern was developed before antigenic selection. Thus, the preferential BJ gene expression, primarily based on preferential use of certain BJ gene rearrangements, dictates the peripheral BJ gene repertoire, which is then further modified by thymic selection and peripheral stimulation.