Influence of site of expression on the fetal gamma delta T-cell receptor repertoire

Butyrophilin-like proteins display combinatorial diversity in selecting and maintaining signature intraepithelial γδ T cell compartments

Butyrophilin-like (Btnl) genes are emerging as major epithelial determinants of tissue-associated γδ T cell compartments. Thus, the development of signature, murine TCRγδ⁺ intraepithelial lymphocytes (IEL) in gut and skin depends on Btnl family members, Btnl1 and Skint1, respectively. In seeking mechanisms underlying these profound effects, we now show that normal gut and skin γδ IEL development additionally requires Btnl6 and Skint2, respectively, and furthermore that different Btnl heteromers can seemingly shape different intestinal γδ⁺ IEL repertoires. This formal genetic evidence for the importance of Btnl heteromers also applied to the steady-state, since sustained Btnl expression is required to maintain the signature TCR.Vγ7⁺ IEL phenotype, including specific responsiveness to Btnl proteins. In sum, Btnl proteins are required to select and to maintain the phenotypes of tissue-protective γδ IEL compartments, with combinatorially diverse heteromers having differential impacts on different IEL subsets.

Characterization of a Putative Receptor Binding Surface on Skint-1, a Critical Determinant of Dendritic Epidermal T Cell Selection

Dendritic epidermal T cells (DETC) form a skin-resident γδ T cell population that makes key contributions to cutaneous immune stress surveillance, including non-redundant contributions to protection from cutaneous carcinogens. How DETC become uniquely associated with the epidermis was in large part solved by the identification of Skint-1, the prototypic member of a novel B7-related multigene family. Expressed only by thymic epithelial cells and epidermal keratinocytes, Skint-1 drives specifically the development of DETC progenitors, making it the first clear candidate for a selecting ligand for non-MHC/CD1-restricted T cells. However, the molecular mechanisms underpinning Skint-1 activity are unresolved. Here, we provide evidence that DETC selection requires Skint-1 expression on the surface of thymic epithelial cells, and depends upon specific residues on the CDR3-like loop within the membrane-distal variable domain of Skint-1 (Skint-1 DV). Nuclear magnetic resonance of Skint-1 DV revealed a core tertiary structure conserved across the Skint family, but a highly distinct surface charge distribution, possibly explaining its unique function. Crucially, the CDR3-like loop formed an electrostatically distinct surface, featuring key charged and hydrophobic solvent-exposed residues, at the membrane-distal tip of DV. These results provide the first structural insights into the Skint family, identifying a putative receptor binding surface that directly implicates Skint-1 in receptor-ligand interactions crucial for DETC selection.

Six-of-the-best: unique contributions of γδ T cells to immunology

γδ T cells are a unique and conserved population of lymphocytes that have been the subject of a recent explosion of interest owing to their essential contributions to many types of immune response and immunopathology. But what does the integration of recent and long-established studies really tell us about these cells and their place in immunology? The time is ripe to consider the evidence for their unique and crucial functions. We conclude that whereas B cells and αβ T cells are commonly thought to contribute primarily to the antigen-specific effector and memory phases of immunity, γδ T cells are distinct in that they combine conventional adaptive features (inherent in their T cell receptors and pleiotropic effector functions) with rapid, innate-like responses that can place them in the initiation phase of immune reactions. This underpins a revised perspective on lymphocyte biology and the regulation of immunogenicity.

Comparison of the peripheral blood leukocyte population between Japanese Black and Holstein calves

Japanese black (JB) calves have greater susceptibility to infectious diseases compared to Holstein (Hol) calves. In order to clarify the differences in cellular immune status between JB and Hol calves, the leukocyte population and lymphocyte proliferative ability were analyzed. In total 200 healthy calves, 1 day to 14 weeks of age, were examined: 105 JB and 95 Hol calves. Lower numbers in peripheral blood and percentage in peripheral blood mononuclear cells of CD3(+)TcR1-N12(+) T cells and major histocompatibility complex class-II(+)CD14(-) B cells were observed in the JB compared to the Hol. The percentage of TcR1-N12(+)CD25(+) T cell in the JB was significantly lower than that of the Hol at 4-6, and 8-10 weeks. Interleukin (IL)-2 sensitivity in the JB was lower than that in the Hol, and significant differences were observed in age groups of 6-8 weeks and 10-14 weeks. These findings indicated that the lower numbers of γδ T cells and B cells in the JB compared to the Hol might be associated with the specificity of the immune systems in JB calves.

Distribution and development of the postnatal murine Vδ1 T-cell receptor repertoire

Murine γ/δ T cells express canonical Vγ5Vδ1 chains in the epidermis and Vγ6Vδ1 chains at reproductive sites. Both subsets carry an identical Vδ1-Dδ2-Jδ2 chain which completely lacks junctional diversity. These cells are thought to monitor tissue integrity via recognition of stress-induced self antigens. In this study, we showed by reverse transcription-polymerase chain reaction (RT-PCR), complementarity determining region 3 (CDR3) spectratyping and sequencing of the junctional regions of Vδ1 chains from C57BL/6 mice (aged 1 day to 14 months) that the canonical Vδ1-Dδ2-Jδ2 chain is also consistently present at other sites such as the thymus, gut, lung, liver, spleen and peripheral blood. In addition, we found multiple Vδ1 chains with fetal type rearrangements which were also shared among organs and among animals. These Vδ1 chains were typically characterized by a conserved amino acid motif, 'GGIRA'. Furthermore, by analysing the early postnatal period at days 10 and 16, we demonstrated that the diversification of the thymic Vδ1 repertoire is not paralleled by a diversification of extrathymic Vδ1+γ/δ T cells. This indicates that only fetal type rearrangements survive at extrathymic sites. In conclusion, γ/δ T cells expressing the canonical Vδ1-Dδ2-Jδ2 chain are not unique to the skin and reproductive sites. Furthermore, we found other γ/δ T cells expressing fetal type Vδ1 chains which were shared among different organs and animals. Thus, γ/δ T cells expressing conserved Vδ1 chains are likely to have important functions. We suggest a model in which this subset continuously recirculates throughout the organism and rapidly responds to stress-induced self antigens.

Selection of the cutaneous intraepithelial gammadelta+ T cell repertoire by a thymic stromal determinant

Intraepithelial lymphocytes constitute a group of T cells that express mainly monospecific or oligoclonal T cell receptors (TCRs). Like adaptive TCR alphabeta+ T cells, intraepithelial lymphocytes, a subset enriched in TCR gammadelta+ T cells, are proposed to be positively selected by thymically expressed self agonists, yet no direct evidence for this exists at present. Mouse dendritic epidermal T cells are prototypic intraepithelial lymphocytes, displaying an almost monoclonal TCR gammadelta+ repertoire. Here we describe an FVB substrain of mice in which this repertoire was uniquely depleted, resulting in cutaneous pathology. This phenotype was due to failure of dendritic epidermal T cell progenitors to mature because of a heritable defect in a dominant gene used by the thymic stroma to 'educate' the natural, skin-associated intraepithelial lymphocyte repertoire to be of physiological use.

Homogeneous epithelial gamma delta T cell repertoire of the skin is shaped through peripheral selection

In contrast to the T cell receptor (TCR) diversity of major alpha beta T cells in lymphoid tissues, epithelial T cells of the murine skin, called dendritic epidermal T cells (DETC), express exclusively an invariant gamma delta TCR. Fetal thymic precursors of DETC immigrate to the skin before birth, and in adult mice T cells expressing the canonical gamma delta TCR identical to that of DETC are not found in other lymphoid or epithelial tissues. Here, we show that DETC precursors migrate to the gut as well as to the skin during fetal periods, but preferentially survive and expand in the skin after birth. We propose that similar to the thymic selection of the diverse alpha beta T cell repertoire, 'peripheral selection' of the homogeneous epithelial gamma delta T cell repertoire may be mediated by TCR signaling upon the recognition of the self-ligand, because the ligand for the DETC TCR was expressed only in the skin.

Disruption of epithelial gamma delta T cell repertoires by mutation of the Syk tyrosine kinase

Chimeric mice in which lymphocytes are deficient in the Syk tyrosine kinase have been created. Compared with Syk-positive controls, mice with Syk -/- lymphocytes display substantial depletion of intraepithelial gamma delta T cells in the skin and gut, with developmental arrest occurring after antigen receptor gene rearrangement. In this dependence on Syk, subsets of intraepithelial gamma delta T cells are similar to B cells, but distinct from splenic gamma delta T cells that develop and expand in Syk-deficient mice. The characteristic associations of certain T-cell receptor V gamma/V delta gene rearrangements with specific epithelia are also disrupted by Syk deficiency.

Repertoires of antigen receptors in Tdt congenitally deficient mice

Tdt deficient mice show lack of N region in V(D)J junctions of immunoglobulin and T cell receptor genes and revealed that "immature recombinase" in fetal stage would boil down to no more than a lack of Tdt. Although particular junctions which are thought to be created by homology-mediated joining are frequently observed, one fourth of junctions lacked even one bp of overlap, indicating the existence of a V(D)J joining pathway that is homology independent. Lymphocyte repertoire which express VH81X gene without N region is negatively selected, which shows that the repertoire of Tdt deficient mice is not a truly fetal repertoire. Positive selection of thymocytes is more efficient in Tdt deficient mice. Furthermore Tdt-/- mice produce significant amounts of anti-dsDNA antibodies as Tdt+/+ mice, indicating that increased diversity of the third complementarity-determining region (CDR3) by Tdt is not essential for the expansion of precursor B cells programmed to produce anti-DNA antibodies.

T cell receptor gamma cDNA in human fetal liver and thymus: variable regions of gamma chains are restricted to V gamma I or V9, due to the absence of splicing of the V10 and V11 leader intron

Although complete in-frame transcripts of the human T cell receptor gamma V10 and V11 genes have been described, the corresponding gamma chains have never been found in gamma delta T cell receptors. In this study, we show that the leader intron of all V10 and V11 cDNA isolated from fetal thymus, fetal liver and adult peripheral blood lymphocytes are unspliced. We demonstrate that, due to the absence of splicing, V10 and V11 are pseudogenes and cannot be expressed in gamma chains. They are the first pseudogenes of this type described in a rearranging T cell receptor/immunoglobulin locus. Therefore the gamma repertoire at the protein level is limited to subgroup V gamma I and to V9. By analysis of the gamma polymerase chain reaction products from total cDNA, we find that the gamma locus is active in early ontogeny (8 weeks), as shown by the presence of rearranged V9 and V10 gene transcripts in the liver. At 13 weeks, the V gamma I genes as well as V9 and V10 have undergone productive rearrangements in the liver, and in the thymus. Most rearrangements, if not all, involve the T cell receptor gamma C1 region (JP1, JP, J1 segments) in both tissues, confirming the accessibility of the C1 region in early stages of development.

Lack of N regions in antigen receptor variable region genes of TdT-deficient lymphocytes

During the assembly of immunoglobulin and T cell receptor variable region genes from variable (V), diversity (D), and joining (J) segments, the germline-encoded repertoire is further diversified by processes that include the template-independent addition of nucleotides (N regions) at gene segment junctions. Terminal deoxynucleotidyl transferase (TdT)-deficient lymphocytes had no N regions in their variable region genes, which shows that TdT is responsible for N region addition. In addition, certain variable region genes appeared at increased frequency in TdT-deficient thymocytes, which indicates that N region addition also influences repertoire development by alleviating sequence-specific constraints imposed on the joining of particular V, D, and J segments.

The level of N-region diversity in T cell receptors is not pre-ordained in the stem cell

The alpha beta T cell repertoires of adults and neonates are distinctly different. For example, T cell receptors (TcR) from adult animals have substantial N-nucleotide addition at their V-D-J junctions while those from neonatal animals do not. This dichotomy reflects a rather abrupt change in expression of the terminal deoxynucleotidyl transferase (TdT) gene in thymocytes on day 4 after birth. We have asked whether this change is due to the differentiation of successive waves of stem cells harboring different potentials for TdT expression, a scenario like the one proposed to explain developmental regulation of gamma delta T cell repertoires. Reconstitution of adult severe combined immunodeficiency mice with either fetal liver or adult bone marrow precursors gave rise to T cells with substantial N-region diversity in their TcR, even at the earliest points of reconstitution. It is most likely, then, that the abrupt change in TdT gene expression in day 4 thymocytes is due to an environmentally induced switch-on.

Thymic origin of embryonic intestinal gamma/delta T cells

Current evidence suggests both thymic and extrathymic origins for T cells. Studies in mice favor an in situ origin for a prominent population of intestinal intraepithelial lymphocytes that express gamma/delta T cell receptor (TCR). This developmental issue is explored in an avian model in which the gamma/delta lymphocytes constitute a major T cell subpopulation that is accessible for study during the earliest stages of lymphocyte development. In the chick embryo, cells bearing the gamma/delta TCR appear first in the thymus where they reach peak levels on days 14-15 of embryogenesis, just 2 d before gamma/delta T cells appear in the intestine. Using two congenic chick strains, one of which expresses the ov antigen, we studied the origin and kinetics of intestinal colonization by gamma/delta T cells. The embryonic gamma/delta+ thymocytes homed to the intestine where they survived for months, whereas an embryonic gamma/delta- thymocyte population enriched in thymocyte precursors failed to give rise to intestinal gamma/delta+ T cells. Embryonic hemopoietic tissues, bone marrow, and spleen, were also ineffective sources for intestinal gamma/delta+ T cells. Intestinal colonization by gamma/delta+ thymocytes occurred in two discrete waves in embryos and newly hatched birds. The data indicate that intestinal gamma/delta T cells in the chicken are primarily thymic migrants that are relatively long-lived.

Regulation of N-region diversity in antigen receptors through thymocyte differentiation and thymus ontogeny

The random addition of "N nucleotides" by terminal deoxynucleotidyltransferase (TdT) is an important component of the diversity of T-cell receptor genes. We have investigated the expression of TdT during thymocyte differentiation and thymus ontogeny. TdT gene transcripts are confined to immature thymocytes of the cortex, being down-regulated concomitantly with recombination-activating gene transcripts after positive selection of mature medullary T cells. According to in situ hybridization, TdT RNA is absent from the neonatal thymus, but it appears 3 to 5 days after birth, just before the appearance of significant N-region diversity in T-cell receptor junctional sequences but clearly after the thymus attains competence at clonal deletion.

Haematopoietic cell lines capable of colonizing the thymus following in vivo transfer expressed T-cell receptor gamma-gene immature mRNA

To clarify the mechanism by which progenitor T (pro-T) cells recognize and enter the thymus, an attempt was made to produce haematopoietic cell lines by the fusion of BALB/c nude mouse bone marrow or foetal liver cells (gestation 14 and 15 days) with AKR thymoma BW5147, thereby immortalizing cells with potency to colonize the thymus, a characteristic of pro-T cells rarely found in adult bone marrow or foetal liver. The hybridomas thus produced were classified according to the phenotype of surface markers, T-cell receptor (TcR) gene configuration and expression. All hybridomas were negative in the surface expression of T-cell markers such as TcR alpha beta, TcR gamma delta, CD3, CD4 and CD8. They had TcR beta-, gamma- and delta-genes, each with a different status with respect to configuration and transcription. Some possessed partially rearranged TcR genes and others expressed immature TcR mRNA. The cell lines were examined for their capacity to colonize the thymus following intravenous injection into recipient mice. It was found that the cells with capacity of colonizing the thymus expressed immature TcR delta mRNA, while the cell lines lacking TcR delta-genes did not home to the thymus. These findings imply that the potency for migrating to thymus is closely associated with the particular stage of prethymic cell differentiation which could be estimated by the analysis of TcR genes, and that some cell lines with the expression of TcR delta-gene mRNA and the ability to colonize the thymus are derived from pro-T cells.

The development of functionally responsive T cells

The work reviewed in this article separates T cell development into four phases. First is an expansion phase prior to TCR rearrangement, which appears to be correlated with programming of at least some response genes for inducibility. This phase can occur to some extent outside of the thymus. However, the profound T cell deficit of nude mice indicates that the thymus is by far the most potent site for inducing the expansion per se, even if other sites can induce some response acquisition. Second is a controlled phase of TCR gene rearrangement. The details of the regulatory mechanism that selects particular loci for rearrangement are still not known. It seems that the rearrangement of the TCR gamma loci in the gamma delta lineage may not always take place at a developmental stage strictly equivalent to the rearrangement of TCR beta in the alpha beta lineage, and it is not clear just how early the two lineages diverge. In the TCR alpha beta lineage, however, the final gene rearrangement events are accompanied by rapid proliferation and an interruption in cellular response gene inducibility. The loss of conventional responsiveness is probably caused by alterations at the level of signaling, and may be a manifestation of the physiological state that is a precondition for selection. Third is the complex process of selection. Whereas peripheral T cells can undergo forms of positive selection (by antigen-driven clonal expansion) and negative selection (by abortive stimulation leading to anergy or death), neither is exactly the same phenomenon that occurs in the thymic cortex. Negative selection in the cortex appears to be a suicidal inversion of antigen responsiveness: instead of turning on IL-2 expression, the activated cell destroys its own chromatin. The genes that need to be induced for this response are not yet identified, but it is unquestionably a form of activation. It is interesting that in humans and rats, cortical thymocytes undergoing negative selection can still induce IL-2R alpha expression and even be rescued in vitro, if exogenous IL-2 is provided. Perhaps murine thymocytes are denied this form of rescue because they shut off IL-2R beta chain expression at an earlier stage or because they may be uncommonly Bcl-2 deficient (cf. Sentman et al., 1991; Strasser et al., 1991). Even so, medullary thymocytes remain at least partially susceptible to negative selection even as they continue to mature.(ABSTRACT TRUNCATED AT 400 WORDS)