T cell function and expression are dramatically altered in T cell receptor V gamma 1.1J gamma 4C gamma 4 transgenic mice

DKK1 mediated inhibition of Wnt signaling in postnatal mice leads to loss of TEC progenitors and thymic degeneration

Background

Thymic epithelial cell (TEC) microenvironments are essential for the recruitment of T cell precursors from the bone marrow, as well as the subsequent expansion and selection of thymocytes resulting in a mature self-tolerant T cell repertoire. The molecular mechanisms, which control both the initial development and subsequent maintenance of these critical microenvironments, are poorly defined. Wnt signaling has been shown to be important to the development of several epithelial tissues and organs. Regulation of Wnt signaling has also been shown to impact both early thymocyte and thymic epithelial development. However, early blocks in thymic organogenesis or death of the mice have prevented analysis of a role of canonical Wnt signaling in the maintenance of TECs in the postnatal thymus.

Methodology/Principal Findings

Here we demonstrate that tetracycline-regulated expression of the canonical Wnt inhibitor DKK1 in TECs localized in both the cortex and medulla of adult mice, results in rapid thymic degeneration characterized by a loss of ΔNP63⁺ Foxn1⁺ and Aire⁺ TECs, loss of K5K8DP TECs thought to represent or contain an immature TEC progenitor, decreased TEC proliferation and the development of cystic structures, similar to an aged thymus. Removal of DKK1 from DKK1-involuted mice results in full recovery, suggesting that canonical Wnt signaling is required for the differentiation or proliferation of TEC populations needed for maintenance of properly organized adult thymic epithelial microenvironments.

Conclusions/Significance

Taken together, the results of this study demonstrate that canonical Wnt signaling within TECs is required for the maintenance of epithelial microenvironments in the postnatal thymus, possibly through effects on TEC progenitor/stem cell populations. Downstream targets of Wnt signaling, which are responsible for maintenance of these TEC progenitors may provide useful targets for therapies aimed at counteracting age associated thymic involution or the premature thymic degeneration associated with cancer therapy and bone marrow transplants.

Development and selection of gammadelta T cells

Two main lineages of T cells develop in the thymus: those that express the alphabeta T-cell receptor (TCR) and those that express the gammadelta TCR. Whereas the development, selection, and peripheral localization of newly differentiated alphabeta T cells are understood in some detail, these processes are less well characterized in gammadelta T cells. This review describes research carried out in this laboratory and others, which addresses several key aspects of gammadelta T-cell development, including the decision of precursor cells to differentiate into the gammadelta versus alphabeta lineage, the ordered differentiation over the course of ontogeny of functional gammadelta T-cell subsets expressing distinct TCR structures, programming of ordered Vgamma gene rearrangement in the thymus, including a molecular switch that ensures appropriate Vgamma rearrangements at the appropriate stage of development, positive selection in the thymus of gammadelta T cells destined for the epidermis, and the acquisition by developing gammadelta T cells of cues that determine their correct localization in the periphery. This research suggests a coordination of molecularly programmed events and cellular selection, which enables specialization of the thymus for production of distinct T-cell subsets at different stages of development.

Positive selection of dendritic epidermal gammadelta T cell precursors in the fetal thymus determines expression of skin-homing receptors

The role of cellular selection in the development of gammadelta T cells remains unclear. Knockout mice lacking a subset of Vgamma genes, including Vgamma3, contain abundant gammadelta T cells but are devoid of dendritic epidermal gammadeltaT cells (DETCs), which normally express an invariant Vgamma3/Vdelta1 gammadelta TCR. A rearranged Vgamma2 transgene restored DETC development, but the restored DETCs selectively expressed a unique Vdelta gene other than Vdelta1, indicating that DETC development involves TCR-based selection. In both normal and transgenic/knockout mice, specific DETC precursors in the fetal thymus were activated and expressed the IL-15 receptor beta chain, skin-homing receptors, and thymic exiting receptors. In vitro activation of irrelevant precursors also led to upregulation of the skin-homing receptor, providing an explanation for how thymic selection is coordinated with development of epidermal gammadelta T cells.

Resident skin-specific gammadelta T cells provide local, nonredundant regulation of cutaneous inflammation

The function of the intraepithelial lymphocyte (IEL) network of T cell receptor (TCR) gammadelta(+) (Vgamma5(+)) dendritic epidermal T cells (DETC) was evaluated by examining several mouse strains genetically deficient in gammadelta T cells (delta(-/-) mice), and in delta(-/-) mice reconstituted with DETC or with different gammadelta cell subpopulations. NOD.delta(-/-) and FVB.delta(-/-) mice spontaneously developed localized, chronic dermatitis, whereas interestingly, the commonly used C57BL/6.delta(-/-) strain did not. Genetic analyses indicated a single autosomal recessive gene controlled the dermatitis susceptibility of NOD.delta(-/-) mice. Furthermore, allergic and irritant contact dermatitis reactions were exaggerated in FVB.delta(-/-), but not in C57BL/6.delta(-/-) mice. Neither spontaneous nor augmented irritant dermatitis was observed in FVB.beta(-/-) delta(-/-) mice lacking all T cells, indicating that alphabeta T cell-mediated inflammation is the target for gammadelta-mediated down-regulation. Reconstitution studies demonstrated that both spontaneous and augmented irritant dermatitis in FVB.delta(-/-) mice were down-regulated by Vgamma5(+) DETC, but not by epidermal T cells expressing other gammadelta TCRs. This study demonstrates that functional impairment at an epithelial interface can be specifically attributed to absence of the local TCR-gammadelta(+) IEL subset and suggests that systemic inflammatory reactions may more generally be subject to substantial regulation by local IELs.

Lymphostromal interactions in thymic development and function

The generation of a peripheral T-cell pool is essential for normal immune system function. CD4+ and CD8+ T cells are produced most efficiently in the thymus, which provides a complexity of discrete cellular microenvironments. Specialized stromal cells, that make up such microenvironments, influence each stage in the maturation programme of immature T-cell precursors. Progress has recently been made in elucidating events that regulate the development of intrathymic microenvironments, as well as mechanisms of thymocyte differentiation. It is becoming increasingly clear that the generation and maintenance of thymic environments that are capable of supporting efficient T-cell development, requires complex interplay between lymphoid and stromal compartments of the thymus.

Forced expression of terminal deoxynucleotidyl transferase in fetal thymus resulted in a decrease in gammadelta T cells and random dissemination of Vgamma3Vdelta1 T cells in skin of newborn but not adult mice

The repertoire of lymphocyte receptor genes encoded in a germline is further diversified by a number of processes, including the template-independent addition of nucleotides (N regions) by means of terminal deoxynucleotidyl transferase (TdT). Normally, mouse gammadelta T cells in the early fetal thymus, whose T-cell receptor (TCR) genes lack N regions and are encoded by Vgamma3-Jgamma1 and Vdelta1-Ddelta2-Jdelta2 with canonical junctions (invariant Vgamma3Vdelta1), are thought to be the precursors of dendritic epidermal T cells (DETC). We generated mutant mice whose endogenous TdT promoter was replaced with the lck promoter through homologous recombination. These mutant mice expressed TdT in fetal thymus, had abundant N regions and infrequent canonical junctions in gamma and delta rearrangements, and showed a decreased number of gammadelta T cells. Various Vgamma3Vdelta1 T cells, most of which had N regions in their TCR genes, were found to disseminate in the skin of newborn mutant mice, whereas normal numbers of DETCs with the invariant Vgamma3Vdelta1 rearrangement were observed in adult mutants. These data demonstrate that the regulation of TdT expression during fetal development is important for the generation of gammadelta T cells, and that Vgamma3Vdelta1 T cells, which have various junctional sequences in their TCR genes, randomly disseminate in skin, but invariant Vgamma3Vdelta1 T cells have a great advantage for proliferation in skin.

Activation and control of self-reactive gammadelta T cells

Mammalian and avian CD3+ T cells can be separated into two lymphocyte subsets bearing heterodimeric T-cell receptors (TCR) composed of either alphabeta or gammadelta chains. Although it is now widely accepted that gammadelta and alphabeta T cells fulfill mandatory and nonredundant roles, the generality of this assumption and the exact functions played by gammadelta T cells remain uncertain. While an early protective role of gammadelta T cells has long been suspected, recent observations drawn in particular from transgenic models suggest their implication in the homeostatic control of immune and nonimmune processes. This hypothesis is also supported by the existence of several self-reactive gammadelta T-cell subsets in rodents and humans, whose specificity and effector properties will be detailed and discussed here. The present review will also describe several mechanisms that could allow efficient control of these self-reactive subsets while permitting expression of their regulatory and/or protective properties.

Cytokine transcription in lymph nodes of cattle in different stages of bovine leukemia virus infection

Bovine leukemia virus (BLV) is a transforming oncovirus that contains no oncogenes or preferred site of proviral integration. The role of cytokines in the disease process of BLV is potentially important due to the similarity of BLV with other retroviruses in which cytokines play a role, such as HTLV-I and -II. Mesenteric and supra-mammary lymph nodes were obtained from a panel of nine cattle. Three were non-infected controls, three were BLV-positive aleukemic (AL), and three were BLV-positive persistent lymphocytotic (PL). Mononuclear cells were perfused from the organs and total RNA extracted from either 1 x 10(8) unseparated cells or 1 x 10(7) purified CD4/CD8 T-cells. cDNA was generated and subjected to RT-PCR to analyze cytokine transcription during disease progression. cDNA levels were normalized using beta-actin PCR at sub-plateau cycle number, enabling a semi-quantitative assessment of cytokine gene transcripts. Using this approach, IL-2, IL-10 and IFN-gamma message was detected in the T-cell fractions of all of the BLV-infected animals, but not in the non-infected controls.

Bovine cytokine expression during different phases of bovine leukemia virus infection

The potential role of aberrant cytokine production in the pathogenesis of bovine leukemia virus (BLV) was studied by analyzing cytokine mRNA expression in pokeweed-stimulated PBMLs of cows in different phases of disease progression. To analyze the mRNA, a semi-quantitative RT-PCR assay was developed. The RT-PCR assay was developed for detection of IL-2, -4, -6, -10, -12, IFN-gamma and actin using cDNA derived from phorbol-stimulated peripheral blood mononuclear leukocytes. Using a PCR specific for BLV tax, agar gel immunodiffusion and white blood cell counts, BLV-negative, BLV-positive aleukemic (AL), and BLV-positive persistently lymphocytotic (PL) cattle were identified. Peripheral blood lymphocytes cultured in vitro for 24 h in pokeweed mitogen were analyzed for cytokine production using the RT-PCR assay. Consistently elevated levels of IL-2 and IL-12 in AL and PL cattle in pokeweed mitogen-stimulated cells was detected, while IFN-gamma was elevated in the AL but not the PL cattle.

Defects of the Mismatch Repair Gene MSH2 Are Implicated in the Development of Murine and Human Lymphoblastic Lymphomas and Are Associated With the Aberrant Expression of Rhombotin-2 (Lmo-2) and Tal-1 (SCL)

Mutations in the DNA mismatch repair (MMR) gene hMSH2 underlie a novel pathway of tumorigenesis for some cancers of epithelial origin. Mice deficient in MSH2 are susceptible to lymphomas but defects in this gene have not been identified in human lymphoid tumors. To determine if the lymphomas these mice develop are related to a particular subtype of human lymphoma we evaluated 20 clinically ill homozygous MSH2−/− mice ranging in age from 2 to 13 months. The murine tumors comprised a single histopathologic entity representing the malignant counterpart of precursor thymic T cells and closely resembled human precursor T-cell lymphoblastic lymphoma (LBL). Evaluation of the expression of three T-cell malignancy associated genes showed that Rhombotin-2 (RBTN-2 also known as Lmo-2), TAL-1 (also known as SCL), and HOX-11 were expressed in 100%, 40%, and 0% of the murine tumors, respectively. The MSH2−/− murine model of precursor T-cell LBL was substantiated by the finding of a nearly identical expression profile of RBTN-2, TAL-1, and HOX-11 in 10 well-characterized cases of human LBL. Direct evidence for MSH2 abnormalities in human LBL was established by sequence analysis of exon 13 of hMSH2, which revealed coding region mutations in 2 of 10 cases. Our findings implicate defects in the MMR system with the aberrant expression of T-cell specific proto-oncogenes and define a new pathway of human lymphomagenesis.

Spontaneous resistance to acute T-cell leukaemias in TCRV gamma 1.1J gamma 4C gamma 4 transgenic mice

The concept of tumour surveillance implies that specific and non-specific components of the immune system eliminate tumours in the early phase of malignancy. The immunological mechanisms that control growth of preneoplastic cells are, however, not known. T cells expressing gamma delta T-cell receptors (TCR) were first described as lymphocytes with reactivity against various tumour cells, which suggests that gamma delta T cells could mediate tumour surveillance. Here we show that TCRV gamma 1.1J gamma 4C gamma 4 transgenic mice are spontaneously resistant to acute T-cell leukaemias but cannot reject non-haematopoietic tumours. TCRV gamma 1.1J gamma 4C gamma 4+ hybridomas isolated from these mice react in vitro against almost all haematopoietic tumour cell lines tested. Recognition of tumour cells depends on the gamma delta TCR but is independent of major histocompatibility complex (MHC) class I, MHC class II, or TAP-2 peptide transporter expression. Ligand recognition is influenced by the murine Nromp gene, which confers resistance or susceptibility to tuberculosis, lepra and leishmaniasis. These data indicate that TCRV gamma 1.1+ T cells confer spontaneous immunity against haematopoietic tumours in vivo and link innate resistance to bacterial infections with tissue-specific tumour surveillance by gamma delta+ T cells.

Differential effects of zeta and eta transgenes on early alpha/beta T cell development

The zeta-family dimers (zeta, eta, and gamma) are a group of structurally and functionally related proteins that are expressed in developing thymocytes and function as signal transducing subunits of the T cell antigen receptor (TCR) and certain Ig Fc receptors. Zeta, eta, and gamma each contain one or more copies of a conserved tyrosine-based activation motif (TAM) that is known to be required for signal transduction. To examine the developmental importance of multiple or individual TAM elements we generated transgenic mice that express: (a) full-length (FL) zeta-chain (3 TAMs); (b) eta-chain, a naturally occurring variant of zeta that is derived from alternative splicing (2 TAMs); or (c) truncated zeta-chain (CT108; 1 TAM), under the control of the human CD2 promoter and regulatory elements. Unexpectedly, we found that overexpression of the FL zeta chain caused premature termination of RAG-1 and RAG-2 expression, prevented productive rearrangement of the TCR-alpha and TCR-beta genes and blocked entry of thymocytes into the CD4/CD8 developmental pathway. In contrast, we found that overexpression of eta or CT108 had no effect on normal thymocyte maturation. These results suggest that an early signaling pathway exists in precursor TCR- thymocytes that can regulate RAG-1 and RAG-2 expression and is differentially responsive to individual members of the zeta-family dimers.

Development of murine pre-T cells into gamma delta T-cell receptor bearing cells

Murine T cells bearing the gamma delta T-cell receptor (gamma delta TCR) are the major lymphocyte subset in the thymus early in fetal development, and postnatally they are the major population of T cells in the epithelia of nonlymphoid tissues including the intestine, skin, tongue, lung, and reproductive organs. The site of origin of gamma delta T-cell precursors (pre-T cells) changes during fetal development, reflecting the sites of active hematopoiesis. In addition, the pattern of expression of specific gamma delta TCR variable (V) region genes changes during fetal and neonatal development, and is unique in different epithelial tissues postnatally. We herein review the literature describing these developmental changes and provide a model for the developmental pathways of murine gamma delta T cells.

Tolerance and self-reactivity in V gamma 1.1C gamma 4 transgenic mice

Immunological tolerance is the process of inhibiting or eliminating lymphocytes that recognize self-derived antigens. By removing potentially harmful self-reactive clones, this mechanism allows for the random generation of a diverse repertoire of T-cells capable of responding to foreign pathogens. Although all self-reactive T-cells should be removed from the repertoire, it is quite clear from many recent studies that a significant fraction of T-cells bearing gamma delta T-cell receptors (TCR) recognize self-derived antigens in normal healthy mice. The presence of self-reactive T-cells in healthy animals presents a paradox which may be explained by understanding the transient expression of the antigens (e.g., MHC class Ib, Heat Shock Proteins) that have been identified for gamma delta T-cells thus far. Data from experiments with V gamma 1.1C gamma 4 transgenic mice demonstrating the presence of self-reactive gamma delta T-cells and their influence on lymphoid development and immune surveillance will be examined in this review.

T cell receptor delta gene mutant mice: independent generation of alpha beta T cells and programmed rearrangements of gamma delta TCR genes

T cells bearing T cell receptor (TCR) gamma and delta chain heterodimers are first generated early in ontogeny. They form distinct subsets that differ in their TCR repertoires and tissue distribution. Disruption of the mouse TCR C delta gene segment by a gene targeting method caused the complete loss of T cells bearing TCR gamma delta chains, but had little or no effect on the development of T cells bearing TCR alpha beta chains. The analyses of TCR gamma and delta genes in the mutant mice suggest that intracellular mechanisms acting at the level of DNA rearrangement play key roles in the differential gamma and delta gene rearrangements and in the generation of the highly restricted junctional sequences during fetal thymic development.

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)

Expansion of myelopoietic precursors and inhibition of B-cell precursors in mice that express a T-cell receptor gamma (V gamma 1.1J gamma 4C gamma 4) transgene

Knowledge of the genetic determinants that can affect renewal of multipotential stem cells and their commitment to specific cell lineages is essential to our understanding of multicellular development. However, despite the vast amount of accumulated knowledge in this area, genetic determinants that affect renewal and commitment of precursor cells are unknown. In this study, we demonstrate that three independently derived founder mouse strains, transgenic for the TcR V gamma 1.1J gamma 4C gamma 4 (TcR gamma 4) chain gene, differed significantly from normal mice in their development of T and B cells as well as myelopoietic precursor cells. Ontogenic programs consistent with an acceleration of T-cell development and a delayed appearance and suppressed levels of pre-B- and B-cell precursors were evident in these transgenic mice. In addition, TcR gamma 4 transgenic mice possessed a significantly elevated level of myelopoietic pluripotential precursors. 3H-thymidine cell suicide studies suggest that higher percentages of pluripotent precursors from the bone marrow of the TcR gamma 4 transgenic mice were in the S phase of the cell cycle. These modulations of the lymphoid and myelopoietic compartments, however, were not found in other T-cell receptor transgenic mice (e.g., TcR V gamma 1.2J gamma 2C gamma 2, TcR gamma 2; or V beta 8.1D beta J beta 2.4C beta 2, TcR beta) constructed with the same or similar cDNA expression vector. The results suggest that the expression of a specific T-cell receptor gamma chain gene, and/or an elevated level of particular subset of TcR gamma delta cells, may affect the proliferation and relative proportions of haemopoietic and lymphoid precursors.

T cell receptor expression by T cells that mature extrathymically in nude mice

The expression of TCR by T cells that mature extrathymically in nude mice was determined by staining Ig- cells from B10 nude mice that were 5 months of age or older with mAbs specific for CD3, alpha/beta or gamma/delta TCR. Although the majority of Ig- cells in older nude mice express TCR, the distribution of alpha/beta and gamma/delta TCR in relation to CD4 and CD8 expression is markedly different compared to T cells from euthymic mice. Approximately half of the CD3+ T cells found in the spleen and lymph nodes of nude mice express gamma/delta TCR that is equally distributed between CD4-8- double-negative and CD8+ single-positive T cells. These data provide the first quantitative measure of the expression of TCR by T cells that mature in the absence of a thymus and suggest that the extrathymic environment, although not efficient, is permissive for the maturation of T cells that express alpha/beta and gamma/delta TCR.

Repertoire development and ligand specificity of murine TCR gamma delta cells

During the past several years, we have been studying the circulating TCR gamma delta cells expressed in peripheral lymphoid tissues. Biochemical and molecular characterization of the TCR gamma delta heterodimers present on these TCR gamma delta cells identified 3 TCR gamma proteins, V gamma 2-C gamma 1, V gamma 1.2-C gamma 2, and V gamma 1.1-C gamma 4. In addition, at least 6 different V delta gene products (V delta 2,4,5,6,V alpha 10, V alpha 11) are expressed in peripheral lymphoid tissue. Nucleotide sequence analysis has revealed a great deal of junctional diversity present among the different V gamma and V delta proteins. Thus, compared to other nonlymphoid tissues (e.g., skin), this population of TCR gamma delta cells appears quite extensive. The development and specificity of TCR gamma delta cells has been pursued by two approaches. First, different TCR gamma delta cells clones were generated which recognize MHC-encoded gene products. One clone recognizes an unconventional TL-encoded antigen, whereas others have been shown to recognize either classical MHC class I or class II antigens. The TCR gamma delta receptor genes have been cloned from the TL-specific TCR gamma delta cell and used to construct transgenic mice to examine the development of TCR gamma delta cells. Although the Tg+ TCR gamma delta cells are tolerized by thymic clonal tolerance similar to TCR alpha beta cells, the epithelial Tg+ TCR gamma delta cells are subjected to non-deletional tolerance (anergy). A second approach towards examining the development of TCR gamma delta cells has been to compare the repertoire of TCR gamma delta splenocytes in a variety of inbred and MHC-congenic strains of mice using subset-specific anti-murine TCR gamma delta mAb. The percentage of individual subsets of splenic TCR gamma delta cells differ widely between different inbred strains of mice due to both MHC- and TCR-encoded genetic differences. In summary, these studies provides a basis for understanding and determining the ligand(s) of the TCR gamma delta heterodimer and the factors which shape the peripheral TCR gamma delta repertoire.

Generation and function of gamma delta T cells after allogeneic bone marrow transplantation in humans: comparison in absence or presence of HLA-matched or mismatched thymus

We have observed two patients who exhibited an exclusive increase of delta TCS1+ subset of gamma delta T cells in the peripheral blood after bone marrow transplantation (BMT). In one case with severe combined immunodeficiency (SCID) who received haploidentical BMT from his father, gamma delta T cells appeared only after thymus transplantation. However, his T cell-mediated immunity remained severely defective despite the generation of T cells of donor origin. In the other case with aplastic anemia, delta TCS1- gamma delta T cells began to increase in the peripheral blood later. This indicates that the thymus is necessary for the generation of gamma delta T cells and that the delta TCS1+ subset is dominant in the early stages of their ontogeny. delta TCS1+ T cell lines were established from both patients, and allo-reactivity was investigated. The cell line from the latter case reacted to recipient cells in a mixed lymphocyte reaction, but did not show cytotoxity to the allogeneic cells including recipient cells. The other cell line, from the former case, did not react to either donor or recipient cells. This indicates that an intact thymus is needed for gamma delta T cells to acquire allo-reactivity. Both cell lines showed MHC non-restricted cytotoxity against NK-sensitive target cells.

Expression of chicken liver cell adhesion molecule fusion genes in transgenic mice

The tissue-specific expression of the chicken liver cell adhesion molecule (L-CAM) was studied by generating transgenic mice. The rat insulin II promoter was fused to a chicken L-CAM cDNA or to chicken genomic L-CAM sequences. Mice carrying the cDNA showed no expression of L-CAM. Mice carrying L-CAM genomic sequences showed expression in the beta cells of the pancreas, suggesting that sequences in introns or in flanking regions are required for expression. Murine L-CAM was undetectable in the beta cells of the pancreas of those transgenic mice expressing chicken L-CAM and thus appeared to be down-regulated, but expression of the mouse protein was not altered at other sites. Chicken L-CAM was also found in extrapancreatic tissues such as skin, kidney, liver, lung, intestine, blood vessels, and the choroid plexus and leptomeninges of the central nervous system. These findings raised the possibility that the chicken L-CAM gene contains cis regulatory elements that interfere with the specificity of a tissue-specific promoter such as the rat insulin promoter. To test this hypothesis, transgenic mice were produced with a construct containing the murine neurofilament promoter fused to genomic chicken L-CAM sequences. Chicken L-CAM was expressed in the brain and spinal cord, where L-CAM is not normally found, but it was also found in some nonneural tissues (kidney, liver, intestine, lung) in which L-CAM is normally expressed. The combined results suggest that tissue-specific cis-acting elements in the chicken L-CAM gene, when combined with heterologous promoters/enhancers, can generate novel patterns of gene expression.

Porcine gamma/delta T lymphocyte subsets differing in their propensity to home to lymphoid tissue

Three types of T cell receptors (TcR) gamma/delta were identified in swine, all of which are disulfide-bonded cell surface heterodimers. The first, a 38/40-kDa molecule, is related to the TcR gamma/delta of sheep and cattle, and defines in swine a subset of CD4-CD8- T lymphocytes that is enriched in the circulating T lymphocyte pool. The other two receptors are private to swine. The second, a 37/40-kDa molecule, is expressed by the major subset of CD2-CD4-CD8- T lymphocytes that is neither enriched in nor excluded from lymphoid tissue. By contrast, the third type of TcR gamma/delta, a 40/46-kDa molecule, is found on CD2+CD4-CD8- T lymphocytes accumulating in lymphoid tissue. Thus, TcR gamma/delta T lymphocyte subsets differing in homing propensity are typified by the expression of distinct molecular forms of the TcR gamma/delta.

Recognition of heat shock proteins and gamma delta cell function

Recently evidence has accumulated suggesting that gamma delta cells may participate in the immune response to mycobacteria and other infectious organisms. Many mouse gamma delta cells are stimulated by the 65 kDa heat shock protein of M. bovis and human gamma delta cell lines reactive with this mycobacterial protein have also been isolated. Indirect evidence further suggests that gamma delta cells can recognize autologous heat shock proteins. In this article, Willi Born and colleagues focus on these and other recent findings and speculate on their importance to gamma delta cell function in vivo.