A delta T-cell receptor deleting element transgenic reporter construct is rearranged in alpha beta but not gamma delta T-cell lineages

Chromosomal excision of TCRδ chain genes is dispensable for αβ T cell lineage commitment

TCRbeta, delta and gamma chain genes are assembled and expressed in double-negative thymocytes prior to alphabeta or gammadelta T cell lineage commitment. Thus, cells committed to the alphabeta T cell lineage can possess completely assembled TCRdelta and/or TCRgamma chain genes. However, these genes are not expressed. TCRgamma chain gene expression may be silenced through the activity of a cis-acting silencer element. In the TCRalpha/delta locus, the TCRdelta genes lie between the Valpha and Jalpha gene segments, which rearrange by deletion. Moreover, Valpha to Jalpha rearrangements occur on both alleles in essentially all developing alphabeta T cells. Consequently, both TCRdelta chain genes are excised from the chromosome and placed on extrachromosomal circles in mature alphabeta T cells. It has been proposed that this excision process is important for silencing TCRdelta gene expression and permitting alphabeta T cell lineage commitment. A gene-targeting Cre-loxP strategy was used to invert a 75-kb region of the TCRalpha/delta locus encompassing all the Jalpha gene segments, generating the TCRalpha/delta(I) allele. Initial Valpha to Jalpha rearrangements on the TCRalpha/delta(I) allele occur by inversion, resulting in chromosomal retention of TCRdelta chain genes. These TCRdelta chain genes can be productively rearranged and are expressed at levels similar to TCRdelta chain genes in gammadelta T cells. However, alphabeta T cell development appears unperturbed in TCRalpha/delta(I/I) mice. Thus, excision of TCRdelta genes from the chromosome per se is not required for commitment of developing lymphocytes to the alphabeta T cell lineage.

Interleukin-7: An Interleukin for Rejuvenating the Immune System

Infection of an individual (aged 20-30 years) by a virus will cause a response from the T (thymus derived) lymphocytes of which there are approximately 3 x 10(11). If the individual has not met the virus before, the response will come from the naive T cell subset (50 +/- 10% of the total T cell pool at this age) containing recent thymic emigrants produced from the thymus at approximately 10(8) per day. Their antigen-specific receptor has a defined specificity governed by the conformation of its two chains (alpha and beta), and the repertoire of specificities is somewhere in the region of 2 x 10(7) to 10(8). A successful response leads to clonal expansion and the generation of memory T cells to the infecting agent.

Reversal of thymic atrophy

Age-associated thymic atrophy is a key event preceding the inefficient functioning of the immune system, resulting in a diminished capacity to generate new T-cells. This thymic involution has been proposed to be due to changes in the thymic microenvironment resulting in its failure to support thymopoiesis. A key cytokine in the early stages of thymocyte development is IL-7 and expression levels are greatly reduced with age. The ability of IL-7 to restore the immune system by enhancing thymic output remains controversial. In this review, we highlight the advances in molecular approaches used to evaluate recent thymic emigrants and assess the success of these strategies in determining whether IL-7 can lead to immune reconstitution.

Quantification of newly developed T cells in mice by real-time quantitative PCR of T-cell receptor rearrangement excision circles

OBJECTIVE

Thymic output of newly developed ab T cells in humans can be measured via signal joint T-cell receptor rearrangement excision circles (sjTRECs). Deletion of the TCRD locus via dRec to psiJa recombination during TCRA rearrangement results in the production of such sjTRECs. The deleting elements dRec and psiJa are highly conserved between humans and mice and used in a comparable manner. We developed and evaluated a real-time quantitative PCR (RQ-PCR) to detect and quantify dRec-psiJa sjTRECs in murine peripheral blood leukocytes for estimation of thymic output of newly developed ab T cells in mice.

METHODS

The threshold cycle (Ct) of the sjTREC RQ-PCR was related to the Ct value of an endogenous reference gene. The difference in Ct value (DCt) was correlated to the absolute numbers of CD45+ and CD3+ cells per mL of blood, as obtained by a single platform flow cytometric assay, resulting in the frequency of sjTRECs in CD45+ and CD3+ cells.

RESULTS

The RQ-PCR proved to be sensitive with a detection level of approximately one sjTREC copy in 100 ng of DNA. SjTRECs could not be detected in peripheral blood leukocytes of RAG-1(-/-) mice, demonstrating the specificity of the assay. As in humans and primates, sjTREC levels declined in aging and thymectomized mice. Remarkably, significant mouse strain-dependent differences in sjTREC levels were observed. 129Sv and C57BL/6 mice had significantly lower sjTREC levels in blood than Balb/c and DBA2 mice.

CONCLUSION

Quantification of murine sjTRECs by RQ-PCR may allow for accurate assessment of thymic output in mice.

Effects of exogenous interleukin-7 on human thymus function

Immune reconstitution is a critical component of recovery after treatment of human immunodeficiency virus (HIV) infection, cancer chemotherapy, and hematopoietic stem cell transplantation. The ability to enhance T-cell production would benefit such treatment. We examined the effects of exogenous interleukin-7 (IL-7) on apoptosis, proliferation, and the generation of T-cell receptor rearrangement excision circles (TRECs) in human thymus. Quantitative polymerase chain reaction demonstrated that the highest level of TRECs (14 692 copies/10 000 cells) was present in the CD1a(+)CD3(-)CD4(+)CD8(+) stage in native thymus, suggesting that TREC generation occurred following the cellular division in this subpopulation. In a thymic organ culture system, exogenous IL-7 increased the TREC frequency in fetal as well as infant thymus, indicating increased T-cell receptor (TCR) rearrangement. Although this increase could be due to the effect of IL-7 to increase thymocyte proliferation and decrease apoptosis of immature CD3(-) cells, the in vivo experiments using NOD/LtSz-scid mice given transplants of human fetal thymus and liver suggested that IL-7 can also directly enhance TREC generation. Our results provide compelling evidence that IL-7 has a direct effect on increasing TCR-alphabeta rearrangement and indicate the potential use of IL-7 for enhancing de novo naïve T-cell generation in immunocompromised patients.

Kinetics of T cell receptor beta, gamma, and delta rearrangements during adult thymic development: T cell receptor rearrangements are present in CD44(+)CD25(+) Pro-T thymocytes

We performed a comprehensive analysis of T cell receptor (TCR) gamma rearrangements in T cell precursors of the mouse adult thymus. Using a sensitive quantitative PCR method, we show that TCRgamma rearrangements are present in CD44(+)CD25(+) Pro-T thymocytes much earlier than expected. TCRgamma rearrangements increase significantly from the Pro-T to the CD44(-)CD25(+) Pre-T cell transition, and follow different patterns depending on each Vgamma gene segment, suggesting that ordered waves of TCRgamma rearrangement exist in the adult mouse thymus as has been described in the fetal mouse thymus. Recombinations of TCRgamma genes occur concurrently with TCRdelta and D-Jbeta rearrangements, but before Vbeta gene assembly. Productive TCRgamma rearrangements do not increase significantly before the Pre-T cell stage and are depleted in CD4(+)CD8(+) double-positive cells from normal mice. In contrast, double-positive thymocytes from TCRdelta-/- mice display random proportions of TCRgamma rearranged alleles, supporting a role for functional TCRgamma/delta rearrangements in the gammadelta divergence process.

Developmental regulation of V(D)J recombination at the TCR alpha/delta locus

The T-cell receptor (TCR) alpha/delta locus includes a large number of V, D, J and C gene segments that are used to produce functional TCR delta and TCR alpha chains expressed by distinct subsets of T lymphocytes. V(D)J recombination events within the locus are regulated as a function of developmental stage and cell lineage during T-lymphocyte differentiation in the thymus. The process of V(D)J recombination is regulated by cis-acting elements that modulate the accessibility of chromosomal substrates to the recombinase. Here we evaluate how the assembly of transcription factor complexes onto enhancers, promoters and other regulatory elements within the TCR alpha/delta locus imparts developmental control to VDJ delta and VJ alpha rearrangement events. Furthermore, we develop the notion that within a complex locus such as the TCR alpha/delta locus, highly localized and region-specific control is likely to require an interplay between positive regulatory elements and blocking or boundary elements that restrict the influence of the positive elements to defined regions of the locus.

Identification of a DNA segment exhibiting rearrangement modifying effects upon transgenic delta-deleting elements

Control of the rearrangement and expression of the T cell receptor alpha and delta chains is critical for determining T cell type. The process of delta deletion is a candidate mechanism for maintaining separation of the alpha and delta loci. Mice harboring a transgenic reporter delta deletion construct show alpha/beta T cell lineage-specific use of the transgenic elements. A 48-basepair segment of DNA, termed HPS1A, when deleted from this reporter construct, loses tight lineage-specific rearrangement control of transgenic elements, with abundant rearrangements of transgenic delta-deleting elements now in gamma/delta T cells. Furthermore, HPS1A augments recombination frequency of extrachromosomal substrates in an in vitro recombination assay. DNA binding proteins recognizing HPS1A have been identified and are restricted to early B and T cells, during the time of active rearrangement of endogenous TCR and immunoglobulin loci. These data are consistent with delta deletion playing an important role in maintaining separate TCR alpha and delta loci.

Intrathymic delta selection events in gammadelta cell development

The major pathway of gammadelta cell development is shown to be regulated by in-frame rearrangements at the T cell receptor (TCR) delta locus. Such "delta selection" occurs at or around the same point in thymocyte development as selection for in-frame rearrangements at the TCRbeta locus. However, there are at least two major differences with beta selection: first, delta selection commonly involves selection on the cognate TCR chain, gamma, suggesting that there is no "preTgamma" chain of major biological significance; second, most gammadelta-selected thymocytes differentiate rather than proliferate. Nonetheless, some delta selection events seemingly facilitate thymocyte expansion, similar to alphabeta T cell development. In these cases, TCRgamma selection is less obvious. Furthermore, the capacity of individual gamma chains to facilitate gammadelta selection is shown to vary with developmental age. The results further clarify early T cell development at the beta selection/delta selection stage and place clear constraints on models of cell fate determination.