Delayed and restricted expression limits putative instructional opportunities of Vgamma1.1/Vgamma2 gammadelta TCR in alphabeta/gammadelta lineage choice in the thymus

Effective differentiation of double negative thymocytes requires high fidelity replication of mitochondrial DNA in an age dependent manner

One of the most proliferative periods for T cells occurs during their development in the thymus. Increased DNA replication can result in increased DNA mutations in the nuclear genome, but also in mitochondrial genomes. A high frequency of mitochondrial DNA mutations can lead to abnormal mitochondrial function and have negative implications on human health. Furthermore, aging is accompanied by an increase in such mutations through oxidative damage and replication errors. Increased mitochondrial DNA mutations cause loss of mitochondrial protein function, and decrease energy production, substrates, and metabolites. Here we have evaluated the effect of increased mitochondrial DNA mutations on T cell development in the thymus. Using mice carrying a mutant mitochondrial DNA polymerase γ (PolG) that causes increased mitochondrial DNA mutations, we show that high fidelity replication of mitochondrial DNA is pivotal for proper T cell development. Reducing the fidelity of mitochondrial DNA replication results in a premature age-dependent reduction in the total number of CD4/CD8 double negative and double positive thymocytes. Analysis of mitochondrial density in thymocyte subpopulations suggests that this may be due to reduced proliferation in specific double negative stages. Taken together, this work suggests that T cell development is regulated by the ability of mitochondria to faithfully replicate their DNA.

Enhancement of cyclophosphamide-induced antitumor effect by a novel polysaccharide from Ganoderma atrum in sarcoma 180-bearing mice

The aim of this study was to investigate the enhancement of Ganoderma atrum polysaccharide (PSG-1) on cyclophosphamide (CTX)-induced antitumor effect in sarcoma 180 (S-180)-bearing mice. Results showed that both CTX and PSG-1 delayed tumor growth and resulted in tumor apoptosis. The combined regimen was superior to either modality alone. Moreover, the combined treatment-induced apoptosis was mediated via mitochondrial pathway, as evidenced by alterations of Bcl-2 family proteins, loss of mitochondrial membrane potential (Δψ(m)), cytochrome c release, and caspases activation. Our results also showed that thymus and spleen indexes, lymphocytes proliferation, and concentrations of cytokine in the CTX group were decreased, which were alleviated by PSG-1. Additionally, the combined treatment ameliorated oxidative stress as compared with CTX alone. Taken together, we conclude that PSG-1 improved the antitumor effect of CTX, possibly in part mediated by enhancing the induction of apoptosis via mitochondrial pathways, activating host immune function, and modifying the redox system in S-180-bearing mice.

Disorderly conduct in gammadelta versus alphabeta T cell lineage commitment

The mechanism of T cell precursor commitment to the gammadelta or alphabeta T cell lineage remains unclear. While TCR signal strength has emerged as a key factor in lineage commitment based on TCR transgenic models, the entire TCR repertoire may not possess the same discriminatory power. A counterbalance to the TCR as the lineage determinant is the pre-existing heterogeneity in gene expression among precursors, which suggests that single precursors are unlikely to respond homogeneously to a given instructive signal.

The thymus chapter in the life of gut-specific intra epithelial lymphocytes

The intestinal intraepithelial lymphocytes (IEL) represent multi-lineage T cell populations. In addition to a major gammadeltaTCR(+) T cell subset, many IEL express alphabetaTCRs and they can be separated into alphabeta sublineages. Some TCRalphabeta(+)IEL have characteristics in common with conventional TCRalphabeta(+)T cells whereas others share an unconventional phenotype with their TCRgammadelta(+) counterparts. Because the latter are enriched for autoreactive TCRs and can be generated in the absence of a thymus, it has long been postulated that some IEL subsets develop locally in the intestine. Several new data however, indicate that under physiological conditions, IEL require a thymic education that directs lineage commitment and functional differentiation. This review will discuss the contributions of the thymus in shaping the various intestinal IEL sublineages.

T cell receptor-instructed alphabeta versus gammadelta lineage commitment revealed by single-cell analysis

alphabeta and gammadelta T cell lineages develop in the thymus from a common precursor. It is unclear at which stage of development commitment to these lineages takes place and in which way T cell receptor signaling contributes to the process. Recently, it was demonstrated that strong TCR signals favor gammadelta lineage development, whereas weaker TCR signals promote alphabeta lineage fate. Two models have been proposed to explain these results. The first model suggests that commitment occurs after TCR expression and TCR signaling directly instructs lymphocytes to adopt one or the other lineage fate. The second model suggests that commitment occurs before TCR expression and that TCR signaling merely confirms the lineage choice. By tracing the fate of single T cell precursors, this study shows that there is no commitment to either the alphabeta or gammadelta lineage before TCR expression and that modulation of TCR signaling in progeny of a single TCR-expressing cell changes lineage commitment.

Key factors in the organized chaos of early T cell development

A fundamental issue in T cell development is what controls whether a thymocyte differentiates into a gammadelta T cell or an alphabeta T cell, each defined by their distinct T cell receptor. Most likely, lessons learned in studying that issue will also provide insight into how the thymus produces T cell subsets with distinct functional and regulatory potentials. Here we review recent experiments, focusing on three factors that regulate thymocyte differentiation up to and including the expression of the first products of antigen receptor gene rearrangements. Those factors are the archetypal developmental regulator Notch, intrinsic signals emanating from antigen-receptor complexes, and trans conditioning, which reflects communication between different subsets of thymocytes. We also review new findings on the positive selection of gammadelta T cells and on extrathymic T cell development.

Metalloproteinase-dependent control of thymocyte differentiation and proliferation

The development of T cells in the thymus is dependent on interactions between thymocytes and thymic stromal cells, on stimulation by growth factors, and on the binding to and migration along extracellular matrix (ECM) components. As metalloproteinases (MP) are involved in processes such as growth factor release and ECM modelling, we assessed the effect of MP inhibitors on T-cell development using fetal thymic organ culture systems. MP inhibitors significantly reduced the numbers of CD4/CD8 double-positive (DP) and mature single-positive thymocytes generated, correlated with a reduced number of cell cycles between the double-negative (DN)3 and DP stages. The progression of early thymocyte progenitors through the DN1-4 stages of development was also severely affected, including incomplete upregulation of CD25, decreased DN3 cell numbers, reduced rearrangement of the T-cell receptor (TCR)-beta locus and expression of intracellular TCR-beta by fewer DN3 cells. When purified DN1 cells were utilized as donor cells in reaggregate thymic organ cultures, essentially no DP thymocytes were produced in the presence of MP inhibitors. The results suggest that MP inhibitors affect the differentiation of developing thymocytes before, and reduce proliferation after, pre-TCR-mediated selection.

Recent insights into the signals that control alphabeta/gammadelta-lineage fate

During thymopoiesis, two major types of mature T cells are generated that can be distinguished by the clonotypic subunits contained within their T-cell receptor (TCR) complexes: alphabeta T cells and gammadelta T cells. Although there is no consensus as to the exact developmental stage where alphabeta and gammadelta T-cell lineages diverge, gammadelta T cells and precursors to the alphabeta T-cell lineage (bearing the pre-TCR) are thought to be derived from a common CD4- CD8- double-negative precursor. The role of the TCR in alphabeta/gammadelta lineage commitment has been controversial, in particular whether different TCR isotypes intrinsically favor adoption of the corresponding lineage. Recent evidence supports a signal strength model of lineage commitment, whereby stronger signals promote gammadelta development and weaker signals promote adoption of the alphabeta fate, irrespective of the TCR isotype from which the signals originate. Moreover, differences in the amplitude of activation of the extracellular signal-regulated kinase- mitogen-activated protein kinase-early growth response pathway appear to play a critical role. These findings will be placed in context of previous analyses in an effort to more precisely define the signals that control T-lineage fate during thymocyte development.

Visualization of the earliest steps of γδ T cell development in the adult thymus

The checkpoint in gammadelta cell development that controls successful T cell receptor (TCR) gene rearrangements remains poorly characterized. Using mice expressing a reporter gene 'knocked into' the Tcrd constant region gene, we have characterized many of the events that mark the life of early gammadelta cells in the adult thymus. We identify the developmental stage during which the Tcrd locus 'opens' in early T cell progenitors and show that a single checkpoint controls gammadelta cell development during the penultimate CD4- CD8- stage. Passage through this checkpoint required the assembly of gammadelta TCR heterodimers on the cell surface and signaling via the Lat adaptor protein. In addition, we show that gammadelta selection triggered a phase of sustained proliferation similar to that induced by the pre-TCR.

The TCRbeta enhancer is dispensable for the expression of rearranged TCRbeta genes in thymic DN2/DN3 populations but not at later stages

The Ebeta enhancer has been shown to be dispensable for germline transcription of nonrearranged TCRbeta segments but appears to be required for TCRbeta V to DJ rearrangement. Ebeta dependency of the subsequent expression of VDJ-rearranged TCRbeta genes in thymic subpopulations has so far not been analyzed. We generated transgenic mice, using a Vbeta8.2Dbeta1Jbeta1.3-rearranged TCRbeta bacterial artificial chromosome, which lacked Ebeta, and monitored transgene expression by flow cytometry using Vbeta-specific mAbs and an IRES-eGFP reporter. Transgene expression was found in double negative (DN)2 and DN3 but not at later stages of thymopoesis. There was no toxicity associated with the transgene given that apoptosis in DN3, DN4 was not increased, and the number of DN4 cells generated from DN3 cells in reaggregate thymic organ cultures was not diminished. The transgenic TCRbeta gave rise to a pre-TCR, as suggested by its ability to suppress endogenous TCRbeta rearrangement, to facilitate beta-selection on a TCRbeta-deficient background and to inhibit gammadelta T cell lineage development. The results suggest that the Vbeta8.2 promoter is sufficient to drive expression of rearranged TCRbeta VDJ genes Ebeta independently in DN2/DN3 but not at later stages.

Gammadelta T cell development--having the strength to get there

Gammadelta T cells play critical roles in immune regulation, tumour surveillance and specific primary immune responses. Mature gammadelta cells derive from thymic precursors that also generate alphabeta T cells. Recent reports have highlighted the impact of the strength of signal received via the T cell receptor on T cell lineage commitment, and the importance of cross-talk between committed alphabeta thymocytes and bipotential progenitors for normal gammadelta T cell differentiation. Studies on T cell receptor-mediated selection of gammadelta cells have supported the view that these unconventional T cells are positively rather than negatively selected on cognate self antigen.