T-cell lineage fate: instructed by receptor signals?

Description of an elasmobranch TCR coreceptor: CD8α from Rhinobatos productus

Cell-mediated immunity plays an essential role for the control and eradication of intracellular pathogens. To learn more about the evolutionary origins of the first signal (Signal 1) for T-cell activation, we cloned CD8α from an elasmobranch, Rhinobatos productus. Similar to full-length CD8α cDNAs from other vertebrates, Rhpr-CD8α (1800bp) encodes a 219 amino acid open reading frame composed of a signal peptide, an extracellular IgSF V domain and a stalk/hinge region followed by a well-conserved transmembrane domain and cytoplasmic tail. Overall, the mature Rhpr-CD8α protein (201 aa) displays ∼ 30% amino acid identity with mammalian CD8α including absolute conservation of cysteine residues involved in the IgSf V domain fold and dimerization of CD8αα and CD8αβ. One prominent feature is the absence of the LCK association motif (CXC) that is needed for achieving signal 1 in tetrapods. Both elasmobranch and teleost CD8α protein sequences possess a similar but distinctly different motif (CXH) in the cytoplasmic tail. The overall genomic structure of CD8α has been conserved during the course of vertebrate evolution both for the number of exons and phase of splicing. Finally, quantitative RTPCR demonstrated that elasmobranch CD8α is expressed in lymphoid-rich tissues similar to CD8 in other vertebrates. The results from this study indicate the existence of CD8 prior to the emergence of the gnathostomes (>450 MYA) while providing evidence that the canonical LCK association motif in mammals is likely a derived characteristic of tetrapod CD8α, suggesting potential differences for T-cell education and activation in the various gnathostomes.

Target-organ specificity of autoimmunity is modified by thymic stroma and bone marrow-derived cells

Physical contact between thymocytes and the thymic stroma is essential for the establishment of self-tolerance, and Aire in thymic epithelial cells plays an important role in this action. As expected, the autoimmune phenotypes of Aire-deficient mice are thymic stroma-dependent. Interestingly, the spectrum of the organs involved differs depending on the genetic background of non-autoimmune-prone mouse strains. Furthermore, deficiency of Aire in an autoimmune-prone strain of NOD also modifies target-cell specificity in the pancreas. In order to clarify the factors that regulate target-organ specificity in Aire-dependent autoimmunity, I have generated both thymic and bone-marrow chimeras, making it possible to evaluate the contribution of thymic stroma and bone-marrow-derived cells to this pathogenic process. The findings suggested that the genetic background of bone-marrow-derived cells contributes to the strain-dependent target-organ specificity of non-autoimmune-prone strains. Furthermore, in a study using NOD mice with a fixed genetic background, thymic stromal cells but not bone-marrow-derived cells were found to be relevant to the Aire-dependent alteration of target-cell specificity in the pancreas. These results clearly underscore the significance of immunological and/or genetic complexity that underlies Aire-deficiency monogenic disease together with critical dialogue between thymic stroma and bone-marrow-derived cells in the organized thymic microenvironment.

ShcA mediates the dominant pathway to extracellular signal-regulated kinase activation during early thymic development

During thymic development, the beta selection checkpoint is regulated by pre-T-cell receptor-initiated signals. Progression through this checkpoint is influenced by phosphorylation and activation of the serine/threonine kinases extracellular signal-regulated kinase 1 (ERK1) and ERK2, but the in vivo relevance of specific upstream players leading to ERK activation is not known. Here, using mice with a conditional loss of the shc1 gene or expressing mutants of ShcA, we demonstrate that the adapter protein ShcA is responsible for up to 70% of ERK activation in double-negative (DN) thymocytes in vivo and ex vivo. We also identify two specific tyrosines on ShcA that promote ERK phosphorylation in vivo, and mice expressing ShcA with mutations of these tyrosines show impaired DN thymocyte development. This work provides the first in vivo demonstration of the relative requirement of upstream adapters in controlling ERK activation during beta selection and suggests a dominant role for ShcA.

The RhoA- and CDC42-specific exchange factor Dbs promotes expansion of immature thymocytes and deletion of double-positive and single-positive thymocytes

Specific members of the Rho family of GTPases exert unique influences on thymocyte proliferation, differentiation and deletion. Dbs is a guanine nucleotide exchange factor which is expressed throughout thymocyte development and is able to activate the Rho family GTPases CDC42, RhoA and RhoG. Transgenic mice expressing an activated form of Dbs had increased numbers of double-negative thymocytes. The Dbs transgene promoted expansion of double-negative thymocytes in the absence of pre-TCR, but had no effect on pre-TCR-dependent differentiation of double-negative thymocytes into double-positive thymocytes. Transgenic double-positive thymocytes were proliferative in vivo, but were also susceptible to apoptosis in vivo and in vitro. The transgenic single-positive thymocytes had attenuated proliferative responses following TCR ligation, and were depleted rather than expanded during culture in the presence of anti-CD3. When expressing a positively selectable TCR, transgenic double-positive thymocytes were increased in number and activated, but the output of single-positive thymocytes was reduced. Transgenic double-positive thymocytes were acutely sensitive to deletion by TCR ligation in vivo. These results indicate that activation of Dbs has the potential to promote proliferation throughout thymocyte development, but also sensitizes double-positive and single-positive thymocytes to deletion.

Somatostatin-14 alters the thymus size and relation among the thymocyte subpopulations in peripubertal rats

It is well known that somatostatin exerts a wide range of effects in the body, and acts as an autocrine or paracrine factor in the thymus. However, it has not been investigated yet whether somatostatin alters the thymus size and relation among the thymocyte subpopulations in the peripubertal rats. For this purpose, the peripubertal AO male rats were cannulated intracerebroventriculary and treated with repeated, low doses of somatostatin-14 (experimental group) or saline (control group). Twenty-four hours after the last treatment, we removed and prepared the thymuses for determination of thymocyte subpopulations by flow cytometry. After five days, animals were sacrificed and their thymuses taken for morphometrical analysis by stereological methods. We noticed that somatostatin-14 decreased volumes of thymus cortex and medulla, total number of thymocytes, number of thymocytes in the cortex and medulla and numerical density of thymocytes in deeper cortex. As a consequence of these changes, thymus size was also diminished. The phenotypic analysis of thymocyte subpopulations showed that somatostatin-14 decreased the percentage of CD4(+)CD8(+) cells with low level of TCR alphabeta expression, positively selected CD4(+)CD8(+)TCRalphabeta (high) cells and the most mature CD4(-)CD8(+)TCRalphabeta (high) cells, while the percentage of CD4(+)CD8(-)TCRalphabeta (high) thymocytes was slightly increased. Somatostatin-14 increased the relative proportion of the least mature CD4(-)CD8(-)TCRalphabeta (-/low), CD4(+)CD8(+)TCRalphabeta (-) cells and both of TCRalphabeta (-/low) single positive subpopulations. These results show that centrally applied somatostatin-14, induces hypotrophy of the thymus in peripubertal rats by changing the volumes and cellularities of the thymic compartments. Additionally, increased number of the least mature thymocytes and a deficiency of double positive cells indicate the involvement of somatostatin in the modulation of T cells maturation.

Sequential roles of Brg, the ATPase subunit of BAF chromatin remodeling complexes, in thymocyte development

T cells develop through distinct stages directed by a series of signals. We explored the roles of SWI/SNF-like BAF chromatin remodeling complexes in this process by progressive deletion of the ATPase subunit, Brg, through successive stages of early T cell development. Brg-deficient cells were blocked at each of the developmental transitions examined. Bcl-xL overexpression suppressed cell death without relieving the developmental blockades, leading to the accumulation of Brg-deleted cells that were unexpectedly cell cycle arrested. These defects resulted partly from the disruptions of pre-TCR and potentially Wnt signaling pathways controlling the expression of genes such as c-Kit and c-Myc critical for continued development. Our studies indicate that BAF complexes dynamically remodel chromatin to propel sequential developmental transitions in response to external signals.

Combined deletion of CD8 locus cis-regulatory elements affects initiation but not maintenance of CD8 expression

Developmental stage-, subset-, and lineage-specific CD8 enhancers have been identified recently by transgenic reporter analyses. Enhancer E8(II) (CIV-4,5) is active in both immature double-positive thymocytes (DP) and mature CD8 single-positive (SP) thymocytes and T cells, whereas E8(I) (CIII-1,2) directs expression only in mature cells. In mice lacking either E8(I) (CIII-1,2) or E8(II) (CIV-4,5), there was no effect on CD8 expression in DP thymocytes. However, deletion of both enhancers resulted in variegated expression of CD8, with appearance of CD4(+)CD8(-) SP thymocytes expressing surface markers characteristic of DP thymocytes. Consequently, fewer mature CD8(+) T cells developed from the reduced pool of DP cells. These results suggest that the initiation of CD8 expression is mediated by cis-regulatory elements that are distinct from any that may be involved in maintenance of expression.

Duration of calcineurin and Erk signals regulates CD4/CD8 lineage commitment of thymocytes

CD4/CD8 lineage commitment of thymocytes is controlled by the T cell receptor-mediated signals and is mimicked in vitro by a long-pulse stimulation of isolated CD4(+)CD8(+) thymocytes with proper combinations of phorbol myristate acetate and the calcium ionophore ionomycin. CD4 lineage commitment required higher intracellular Ca(2+) levels than CD8 lineage commitment in this culture system. The calcineurin inhibitor FK506 at 1nM inhibited the development of thymocytes to either lineage, but 0.3nM FK506 significantly switched the development from the CD4 cell fate to the CD8 cell fate. The switch in lineage commitment was also observed when 1nM FK506 was added 8h after the start of the culture. Delayed addition of 20microM U0126, an Mek (Erk kinase) inhibitor, also induced the switch. These results suggest that the intensity of calcineurin activity and the duration of both calcineurin and Erk pathway activation are crucial for thymocyte lineage commitment.

Lineage fate alteration of thymocytes developing in an MHC environment containing MHC/peptide ligands with antagonist properties

A18 TCR transgenic thymocytes which are H-2E(k) restricted and normally selected into the CD4 lineage, are exclusively selected into the CD8 lineage in an H-2(q) MHC background. CD8 T cell selection in the H-2(q) background is far more efficient than default selection of A18 CD8 cells on a CD4(-/-) H-2E(k +) background. This suggests the involvement of special selecting ligands. Analogues of the cognate peptide for A18 with antagonist properties for the A18 TCR have previously been shown to effect a lineage diversion from CD4 to CD8 in fetal thymic organ cultures and intriguingly the MHC(q) background contains unidentified natural MHC class II ligands which similarly show antagonist properties for the A18 TCR. Despite the presence of these unidentified MHC class II ligands in the H-2(q) background and their potential influence on developing A18 thymocytes, however, MHC class I molecules were essential for thymic selection of A18 CD8 T cells.

Epigenetic silencing of CD4 in T cells committed to the cytotoxic lineage

The process of thymocyte development culminates in the maturation of helper (CD4+) and cytotoxic (CD8+) T cells from their common precursors, the CD4+CD8+ double-positive cells. A crucial step during lineage specification is the termination of expression of either the CD4 or the CD8 coreceptor. A silencer element within the first intron of the CD4 gene is sufficient for CD4 transcriptional repression in cells of the cytotoxic lineage, as well as in thymocytes at earlier stages of differentiation. Here we show that the function of the CD4 silencer is required only at distinct stages of development. Its deletion before the initiation of lineage specification resulted in CD4 derepression throughout thymocyte differentiation. By contrast, once cells committed to the cytotoxic CD8+ lineage, the CD4 locus remained silent through subsequent mitoses, even when the silencer element was excised. The epigenetic inheritance of the silenced CD4 locus was not affected by the inhibition of DNA methylation or histone deacetylation, and may thus involve other mechanisms that ensure a stable state of gene expression.