Thymic differentiation of TCRαβ+ CD8αα+ IELs

Summary:  Intraepithelial lymphocytes (IELs) contain several subsets, but the origin of the T-cell receptor (TCR)αβ⁺ CD8αα⁺ IELs has been particularly controversial. Here we provide a synthesis, based on recent work, that attempts to unify the divergent views. The intestine has a primordial function in lymphopoiesis, and precursors with the potential to differentiate into T cells are found both in the epithelium and underlying lamina propria. Moreover, the thymus has been reported to export cells to the intestine that are not fully differentiated. TCRαβ⁺ CD8αα⁺ IELs can differentiate in the intestine from each of these sources, but in normal euthymic mice, the thymus appears to be the major source for TCRαβ⁺ CD8αα⁺ IELs. This unique IEL subset is a self-reactive population that requires exposure to self-agonists for selection in the thymus, similar to other regulatory T-cell populations. IELs transition through a double-positive (DP) intermediate in the thymus, but they originate from a subset of the DP cells that can be identified by its expression of CD8αα homodimers. The agonist-selected cells in the thymus are TCRβ⁺ but CD4 and CD8 double negative. The evidence suggests that reacquired expression of CD8αα and downregulation of CD5 occur after thymus export, perhaps in the intestine under the influence of interleukin-15. As a result of agonist exposure, a new gene expression program is activated. Therefore, the increased understanding of the developmental origin of TCRαβ⁺ CD8αα⁺ IELs may help us to understand how they participate in immune regulation and protection in the intestine.     

Crucial function of the pre-T-cell receptor (TCR) in TCRP selection, TCRβ allelic exclusion and αβ versus γδ lineage commitment

Summary: The analysis of T-cell receptor (TCR) βselection, TCRβ allelic exclusion and TCRβ rearrangement in γδ T cells from normal and pre-TCR-deficient mice has shown that the pre-TCR has a crucial role in T-lyinpbocyte development:

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  The pre-TCR is by far the most effective receptor that generates large numbers of CD4⁺8⁺ T cells with productive TCRβ rearrangements.
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  In the absence of the pre-TCR, TCRβ rearrangement proceeds in developing cells irrespective of whether they already contain a productive TCRβ gene.
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  The pre-TCR directs developing T cells to the αβ lineage because y5 T cells from pTα^-/- mice proceed much further in TCRβ rearrangement than γδ T cells from wild-type mice. It is argued that the pre-TCR commits developing T cells to the αβ lineage by an instructive mechanism, which has largely replaced an evolutionarily more ancient mechanism that involves stochastic αβ lineage commitment.

     

CD8alpha+beta(low) effector T cells in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a complex autoimmune disorder characterized by the loss of self-tolerance to nuclear antigens. Aberrant T-cell function plays a central role in lupus pathogenesis. We and others previously demonstrated that peripheral TCRαβ⁺CD3⁺ T cells express CD8β either at a high (CD8β^high) or low density (CD8β^low), thereby defining two functionally distinct subsets. CD8β^low T cells express predominantly CD8αα and less CD8αβ as a coreceptor, display a differentiated phenotype and exert effector function. CD8β^high T cells appear to be the precursors expressing predominantly the heterodimeric efficient CD8αβ coreceptor, exhibiting a naïve phenotype and high proliferative capacity. In the present study, the distribution and functional properties of CD8β^high and CD8β^low T cells of SLE patients were compared ( n  = 20) with those of healthy subjects ( n  = 16). It was found that expansion of CD8β^low T-cell subset correlated with disease activity indicating chronic antigenic stimulation leading to a major lack of naïve CD8β^high precursor T cells in SLE. Functional characteristics of CD8β^low T cells including production of cytokines and cytotoxic granules were not significantly different between patients with SLE and healthy individuals. We speculate that unbalanced CD8β^high/CD8β^low T-cell relation reflects a skewed homeostasis within the CD8⁺ T-cell compartment towards fully differentiated effector T cells possibly due to persistent antigen stimulation in SLE.     

Oestrogen Treatment Depletes Extrathymic T Cells from Intestinal Lymphoid Tissues

Different lineages of thymic and extrathymic T cells are found in the epithelial layer and in the lamina propria of the small and large intestine of euthymic and athymic mice. A single subcutaneous injection of oestradiolvalorat (Progynon^®-Depot-10, Schering, Berlin, Germany) into athymic mice led to a dose-dependent depletion of extrathymic T cells from the intraepithelial and lamina propria compartments of the small and large intestine. TCRαβ and TCRγδ, CD4⁺ and CD8α⁺ T cell subsets were affected. The depletion of intraepithelial, extrathymic T cells by oestradiol treatment was striking. Oestrogen, therefore, has an effect not only on genital mucous membranes, but also on the large, diffuse lymphoid tissues of the gut, in that it selectively depletes the intestinal, extrathymic T-cell subsets.     

CD3−4−8− Thymocyte Precursors with Interleukin-2 Receptors Differentiate Phenotypically in Coculture with Thymic Stromal Cells

CD3⁻4⁻8⁻ interleukin-2 receptor positive (IL-2R⁺) thymocyte precursors from adult mice were cocultured with thymic stromal cells from syngeneic adult mice. The IL-2R⁺CD3⁻4⁻8⁻ thymocytes were obtained by positive panning of IL-2R⁺ cells followed by either sorting or negative panning of triple negative cells, and they were cocultured with primary or secondary cultures of heterogeneous thymic stromal cells. Phenotypic maturation of these precursor cells was extremely rapid. Within 2½ days significant numbers of CD4⁺8⁺ and CD3⁺4⁺8⁻ cell populations developed, the latter expressing the αβ T-cell receptor (αβ-TCR). Thus heterogeneous stromal cell cultures support the development of IL-2R⁺ precursors and with these methods it will now be possible to isolate the particular stromal cells involved at each stromal-dependent step.     

Regulatory T cells in spontaneous autoimmune encephalomyelitis

Summary: Spontaneous experimental autoimmune encephalomyelitis (EAE) develops in 100% of mice harboring a monoclonal myelin basic protein (MBP)-specific CD4⁺αβ T-cell repertoire. Monoclonality of the αβ T-cell repertoire can be achieved by crossing MBP-specific T-cell receptor (TCR) transgenic mice with either RAG^−/− mice or TCR α^−/−/TCR β^−/− double knockout mice. Spontaneous EAE can be prevented by a single administration of purified CD4⁺ splenocytes or thymocytes obtained from wild-type syngeneic mice. The regulatory T cells (T-reg) that protect from spontaneous EAE need not express the CD25 marker, as effective protection can be attained with populations depleted of CD25⁺ T cells. Although the specificity of the regulatory T cells is important for their generation or regulatory function, T cells that protect from spontaneous EAE can have a diverse TCR α and β chain composition. T-reg cells expand poorly in vivo , and appear to be long lived. Finally, precursors for T-reg are present in fetal liver as well as in the bone marrow of aging mice. We propose that protection of healthy individuals from autoimmune diseases involves several layers of regulation, which consist of CD4⁺CD25⁺ regulatory T cells, CD4⁺CD25⁻ T-reg cells, and anti-TCR T cells, with each layer potentially operating at different stages of T-helper cell-mediated immune responses.     

γδ T cells: firefighters or fire boosters in the front lines of inflammatory responses

Summary:  Intradermal inoculation of cloned self-reactive αβ T cells into the footpads of mice induced cutaneous graft-versus-host disease (GVHD), and after recovery from GVHD, the epidermis became resistant to subsequent attempts to induce GVHD. Resistance to GVHD was not induced in the epidermis of T-cell receptor δ-deficient (TCRδ^−/−) mice that lacked γδ T cells bearing canonical Vγ5/Vδ1⁺γδTCRs, known as dendritic epidermal T cells (DETCs), and resistance was restored by reconstitution of these mutant mice with precursors of Vγ5⁺ DETCs. Pulmonary infection by Cryptococcus neoformans induced an increase of γδ T cells in the lung, and in comparison with wildtype mice, TCRδ^−/− mice eliminated C. neoformans more rapidly and synthesized more interferon-γ in the lung. In the mouse small intestine, the absence of γδ T cells is associated with a reduction in epithelial cell turnover and downregulation of the expression of major histocompatibility complex class II molecules. The protective role of γδ T cells was verified in a dextran sodium sulfate-induced inflammatory bowel disease (IBD) model, whereas in a spontaneous model of IBD, γδ T cells were involved in the exacerbation of colitis in TCRα^−/− mice. Taken together, in addition to the homeostatic regulation of epithelial tissues, γδ T cells appear to play a pivotal role in the modification of inflammatory responses induced in many organs containing epithelia.     

The Tec kinases Itk and Rlk regulate conventional versus innate T-cell development

Summary:  Tec family kinases are important components of antigen receptor signaling pathways in B cells, T cells, and mast cells. In T cells, three members of this family, inducible T-cell kinase (Itk), resting lymphocyte kinase (Rlk), and Tec, are expressed. In the absence of Itk and Rlk, T-cell receptor signaling is impaired, with defects in mitogen-activated protein kinase activation, Ca²⁺ mobilization, and actin polymerization. During T-cell development in the thymus, no role has been found for these kinases in the CD4⁺ versus CD8⁺ T-cell lineage decision; however, several studies indicate that Itk and Rlk contribute to the signaling leading to positive and negative selection. In addition, we and others have recently described an important role for Itk and Rlk in the development of conventional as opposed to innate CD4⁺ and CD8⁺ T cells. Natural killer T and γδ T-cell populations are also altered in Itk- and Rlk/Itk-deficient mice. These findings strongly suggest that the strength of T-cell receptor signaling during development determines whether T cells mature into conventional versus innate lymphocyte lineages. This lineage decision is also influenced by signaling via signaling lymphocytic activation molecule (SLAM) family receptors. Here we discuss these two signaling pathways that each contribute to conventional versus innate T-cell lineage commitment.     

γδ T-Cell Precursor-Derived CD4− CD8−αβ T Cells Retain γδ Cell Function

We have previously shown that some of the DNαβ⁺ T cells arising in TcRα-chain transgenic mice are of γδ T cell origin, based on phenotypic data and on their status of TcR gene rearrangements. In the present report we investigated the impact of αβ TcR expression on the functional programme of the mature γδ precursor-derived DNαβ⁺ T cells. Our results demonstrate that both their proliferative capacity and their cytokine production profile are similar to that of γδ T cells. Furthermore, both transgenic DNαβ⁺ T cells and DNγδ⁺ T cells up-regulate CD8α expression after activation, but, in contrast to CD4⁺αβ T cells, are unable to induce proliferation of naive B cells. Thus, our results suggest that the effector functions of mature T cells are determined independently of the TcR isotype, probably at an early stage of differentiation, and thereby have important implications for current models of T-cell lineage commitment.     

Roles of Cav channels and AHNAK1 in T cells: the beauty and the beast

Summary:  T lymphocytes require Ca²⁺ entry though the plasma membrane for their activation and function. Recently, several routes for Ca²⁺ entry through the T-cell plasma membrane after activation have been described. These include calcium release-activated channels (CRAC), transient receptor potential (TRP) channels, and inositol-1,4,5-trisphosphate receptors (IP₃Rs). Herein we review the emergence of a fourth new route for Ca²⁺ entry, composed of Ca_v channels (also known as L-type voltage-gated calcium channels) and the scaffold protein AHNAK1 (AHNAK/desmoyokin). Both helper (CD4⁺) and killer (CD8⁺) T cells express high levels of Ca_v1 α1 subunits (α1S, α1C, α1D, and α1F) and AHNAK1 after their differentiation and require these molecules for Ca²⁺ entry during an immune response. In this article, we describe the observations and open questions that ultimately suggest the involvement of multiple consecutive routes for Ca²⁺ entry into lymphocytes, one of which may be mediated by Ca_v channels and AHNAK1.     

The extrathymic T-cell differentiation in the murine gut

Summary:  The gut epithelial border is in continuous contact with exogenous antigens and harbors a distinctive and very abundant CD8αα intraepithelial T-lymphocyte effector population. We describe here the characteristics of these cells that distinguish them from all other T-cell types in the body as well as their functions in local protection. We also describe how these cells differentiate from local precursors present in the gut cryptopatches (CPs) following a pathway of T-cell differentiation unique to the gut wall. Finally, we describe the origin of the precursors of CD8αα T cells, which come from the bone marrow in athymic mice but are first imprinted in the thymus in euthymic mice. Indeed, CD3⁻CD4⁻CD8⁻ T-cell-committed precursors can leave the thymus before T-cell receptor rearrangements and then colonize the gut CPs, proceeding with their differentiation within the gut wall.     

γδ intraepithelial lymphocytes drive tumor necrosis factor-α responsiveness to intestinal iron challenge: relevance to hemochromatosis

Summary: The dependence of intestinal epithelial cell (IEC) growth and differentiation on intraepithelial lymphocytes (IELs) expressing the gamma/delta (γδ) T-cell receptor (TCR), suggested a potential role for γδ⁺ IELs in the regulation of iron absorption. We therefore examined the levels of hepatic iron and the IEL cytokine responses in C57BL/6J control and class I and TCR knockout lines (placed on a C57BL/6J genetic background) following the administration of supplemental dietary iron. The highest level of liver iron was found in the β2-microglobulin knockout (β2m^-/-) mice followed by the TCR-δ knockout (TCRδ^-/-) animals. TCR-α knockout (TCRα^-/-) and control animals did not differ in their iron levels. Liver iron loading correlated inversely with rhe ability of the mice to generate an IEL tumor necrosis factor (TNE)-α response. These observations suggest a model in which IEC iron loading is communicated to IELs via the HFE class I protein. The result of this communication is the initiation of TNE-α release by γδ⁺ IELs (sustained by macrophages and dendritic cells) contributing to the upregulation of ferritin expression and possibly to the normal maintenance of the IEC apoptotic pathway.     

Human NK Cells Expressing α4β1/β7 Adhere to VCAM-1 Without Preactivation

The authors demonstrate that resting CD56⁺/CD3⁻ NK cell adhesion to the endothelial VCAM-1 is over three-fold higher than CD56⁻/CD3 ⁺ T-cell adhesion. T-cell, but not NK-cell adhesion, to VCAM-1 is enhanced significantly by stimulation. The expression of VCAM-I receptor subunits α4 and β1 on both effector cells remains unchanged upon stimulation. A subpopulation of NK cells, as well as of T cells, was found to express β7, whose expression was not altered upon stimulation. The authors conclude that the adhesive properties of the same receptor structures on these distinct cell populations are regulated in a different manner, according to the specific functions of the effector cells of the immune system.     

Lineage commitment and differentiation of T and natural killer lymphocytes in the fetal mouse

Summary: T cells and natural killer (NK) cells are presumed to share a common intrathymic precursor. The development of conventional a|3 T lymphocytes begins within the early fetal thymus, after the colonization of multipotent CDl1 71 precursors. Irrevocable commitment to the T lineage is marked by thymus-induced expression of CD25. However, the contribution of the fetal thymus to NK lineage commitment and differentiation remains largely unappreciated. Recently, we demonstrated that the development of functional mouse NK cells occurs first in the fetal thymus. Moreover, the appearance of mature fetal thymic NK cells (NK1.1⁺/CD 117^-) is preceded by a thymus-induced developmental stage (NKl.1⁺/CD1 17⁺) that marks lineage commitment of multipotent hematopoietic precursors to the T and NK-cell fates. Commitment to the T/NK bipotent stage is induced by fetal thymic stroma, but is not thymus dependent. Recent data indicate that CD90⁺/CD117^lo fetal blood prothymocytes exhibit NK lineage potential and are phenotypically and functionally identical to fetal thymic NK1,1⁺/CD1 17⁺ progenitors. This finding also indicates that full commitment of circulating precursors to the T-cell lineage occurs after thymus colonization. In this review, we discuss recent insights into the cellular and molecular events involved in fetal mouse T and NK lineage commitment and differentiation to unipotent progenitors.     

Leukaemic T Cells from Patients with Chronic Lymphocytic Leukaemia of T-Cell Origin Respond to Staphylococcus aureus Enterotoxin Superantigens

We investigated the in vitro responsiveness of peripheral blood lymphocytes from two patients with T-cell chronic lymphocytic leukaemia (T-CLL) to Staphylococcus aureus enterotoxin (SE) superantigens. T-cell receptor (TcR) αβ(Vβ 7.1)-expressing CD4⁺ leukaemic T cells from patient HE (white blood cell count 480,000/μl) proliferated in response to SEA and, only at 1000-fold higher concentrations, to SEB, SED, and SEE. CD4⁺ CD8⁺ TcRαβ (Vβ 12.1)-expressing leukaemic T cells from patient KO (white blood cell count 120,000/μl) were activated by SEB but not by the other tested SEs. In both instances, the activation of leukaemic T cells by SE was dependent on the presence of HLA-DR⁺ cells. Southern blot analysis of TcRβ gene rearrangement confirmed that the proliferating cells were derived from the leukaemic T-cell clone and not from contaminating normal T cells. These data indicate that leukaemic T cells from patients with T-CLL exert a clonally variable responsiveness to SE superantigens. We conclude that recognition of specific antigen and subsequent signal transduction can be initiated via the TcR of leukaemic T-CLL cells.     

Abnormal T-Cell Expansion and V-Gene Usage in Myasthenia Gravis Patients

To determine the extent of V-gene heterogeneity of blood T lymphocytes in patients suffering from Myasthenia Gravis (MG), we used eight recently available monoclonal antibodies (MoAb), directed against different Vα and Vβ gene products of the variable part of the T-cell receptor (TCR), covering approximately 25% of the α/β T cells in normal peripheral blood (PBL) of healthy individuals. Using a two-colour immunofluorescence method, we could calculate the expression of α/β V segments within the two major T-cell subsets, CD4^-/CD8⁺ and CD4⁺/CD8^- lymphocytes. Twenty-seven per cent (4/15) of the MG patients had T cells showing signs of abnormal expansion. Furthermore, among these expanded T cells, a restricted Vβ12 gene expansion could be seen, in three out of four patients. No correlation between TCR V-gene usage and HLA haplotypes (HLA-A, -B, -DR and -DQ) could be seen. Our data suggest that the majority of MG patients have abnormally expanded T-cell clones. The relevance of these findings is discussed.