Impaired development of V gamma 3 dendritic epidermal T cells in p56lck protein tyrosine kinase-deficient and CD45 protein tyrosine phosphatase-deficient mice

Skin γδ T Cells and Their Function in Wound Healing

For the skin immune system, γδ T cells are important components, which help in defensing against damage and infection of skin. Compared to the conventional αβ T cells, γδ T cells have their own differentiation, development and activation characteristics. In adult mice, dendritic epidermal T cells (DETCs), Vγ4 and Vγ6 γδ T cells are the main subsets of skin, the coordination and interaction among them play a crucial role in wound repair. To get a clear overview of γδ T cells, this review synopsizes their derivation, development, colonization and activation, and focuses their function in acute and chronic wound healing, as well as the underlining mechanism. The aim of this paper is to provide cues for the study of human epidermal γδ T cells and the potential treatment for skin rehabilitation.

Epidermal resident γδ T cell development and function in skin

Epidermal resident γδ T cells, or dendritic epidermal T cells (DETCs) in mice, are a unique and conserved population of γδ T cells enriched in the epidermis, where they serve as the regulators of immune responses and sense skin injury. Despite the great advances in the understanding of the development, homeostasis, and function of DETCs in the past decades, the origin and the underlying molecular mechanisms remain elusive. Here, we reviewed the recent research progress on DETCs, including their origin and homeostasis in the skin, especially at transcriptional and epigenetic levels, and discuss the involvement of DETCs in skin diseases.

Get in Touch With Dendritic Epithelial T Cells!

Innate and adaptive immune systems continuously interchange information and orchestrate their immune responses to protect the host. γδT cells play crucial roles, as they incorporate both innate and adaptive immune characteristics. Dendritic epidermal T cells (DETC) are specialized γδT cells, which are uniquely positioned to rapidly respond to skin wounds and infections. Their elongated dendrite morphology allows them to be in continuous contact with multiple neighboring keratinocytes and Langerhans cells. Cellular interactions are fundamental to the formation, activation and maintenance of immune cell functions during steady state and pathology. Recent technological advances, especially in the field of cellular imaging, have contributed greatly to the characterization of complex cellular interactions in a spatiotemporally resolved manner. In this review, we will highlight the often-underappreciated function of DETC and other γδT cells during steady state and an ongoing immune response. More specifically, we discuss how DETC-precursors are shaped in the fetal thymus during embryogenesis as well as how direct cell-cell interactions of DETC with neighboring epidermal cells shape skin homeostasis and effector functions. Furthermore, we will discuss seminal work and recent discoveries made in the γδT cell field, which have highlighted the importance of γδT cells in the skin, both in humans and mice.

Tissue Adaptations of Memory and Tissue-Resident Gamma Delta T Cells

Epithelial and mucosal barriers are critical interfaces physically separating the body from the outside environment and are the tissues most exposed to microorganisms and potential inflammatory agents. The integrity of these tissues requires fine tuning of the local immune system to enable the efficient elimination of invasive pathogens while simultaneously preserving a beneficial relationship with commensal organisms and preventing autoimmunity. Although they only represent a small fraction of circulating and lymphoid T cells, γδ T cells form a substantial population at barrier sites and even outnumber conventional αβ T cells in some tissues. After their egress from the thymus, several γδ T cell subsets naturally establish residency in predetermined mucosal and epithelial locations, as exemplified by the restricted location of murine Vγ5⁺ and Vγ3Vδ1⁺ T cell subsets to the intestinal epithelium and epidermis, respectively. Because of their preferential location in barrier sites, γδ T cells are often directly or indirectly influenced by the microbiota or the pathogens that invade these sites. More recently, a growing body of studies have shown that γδ T cells form long-lived memory populations upon local inflammation or bacterial infection, some of which permanently populate the affected tissues after pathogen clearance or resolution of inflammation. Natural and induced resident γδ T cells have been implicated in many beneficial processes such as tissue homeostasis and pathogen control, but their presence may also exacerbate local inflammation under certain circumstances. Further understanding of the biology and role of these unconventional resident T cells in homeostasis and disease may shed light on potentially novel vaccines and therapies.

IL-7 produced by thymic epithelial cells plays a major role in the development of thymocytes and TCRγδ+ intraepithelial lymphocytes

IL-7 is a cytokine essential for T cell development and survival. However, the local function of IL-7 produced by thymic epithelial cells (TECs) is poorly understood. To address this question, we generated IL-7-floxed mice and crossed them with FoxN1 promoter-driven Cre (FoxN1-Cre) mice to establish knockout mice conditionally deficient for the expression of IL-7 by TECs. We found that αβ and γδ T cells were significantly reduced in the thymus of IL-7(f/f) FoxN1-Cre mice. Proportion of mature single-positive thymocytes was increased. In lymph nodes and the spleen, the numbers of T cells were partially restored in IL-7(f/f) FoxN1-Cre mice. In addition, γδ T cells were absent from the fetal thymus and epidermis of IL-7(f/f) FoxN1-Cre mice. Furthermore, TCRγδ(+) intraepithelial lymphocytes (IELs) were significantly decreased in the small intestines of IL-7(f/f) FoxN1-Cre mice. To evaluate the function of IL-7 produced in the intestine, we crossed the IL-7(f/f) mice with villin promoter-driven Cre (Vil-Cre) mice to obtain the mice deficient in IL-7 production from intestinal epithelial cells. We observed that αβ and γδ IELs of IL-7(f/f) Vil-Cre mice were comparable to control mice. Collectively, our results suggest that TEC-derived IL-7 plays a major role in proliferation, survival, and maturation of thymocytes and is indispensable for γδ T cell development. This study also demonstrates that IL-7 produced in the thymus is essential for the development of γδ IELs and indicates the thymic origin of γδ IELs.

Molecular aspects of epithelial γδ T cell regulation

γδ T cells lie at the interface between innate and adaptive immunity, sharing features with both arms of the immune system. The vast majority of γδ T cells reside in epithelial layers of tissues such as skin, gut, lung, tongue and reproductive tract where they provide a first line of defense against environmental attack. The existence of epithelium-resident γδ T cells has been known for over 20 years but our understanding of the molecular events regulating development and function of these cells is incomplete. We review recent advances in the field, with particular emphasis on the γδ T cell population resident in mouse epidermis. These studies have enhanced our knowledge and understanding of the life cycle of this enigmatic population of cells.

CD45 regulates migration, proliferation, and progression of double negative 1 thymocytes

CD45 is a protein tyrosine phosphatase that is expressed on all nucleated hematopoietic cells, from stem cells to memory cells. Although its function in regulating the threshold of Ag receptor signaling is well established, its role in other leukocytes, particularly progenitor cells, is not well defined. In this study, we find CD45 affects early thymocyte development. Examination of the CD4(-)CD8(-) double negative (DN) populations revealed a significant reduction in the DN1 population, in both the numbers of CD117(+) DN1 cells (the early thymocyte progenitors) and the CD117(-) DN1 cells in the thymus of CD45(-/-) mice. There was also a reduced frequency of CCR9(+) Lin(-)Sca-1(+)c-Kit(+) cells and common lymphoid progenitors in the CD45(-/-) bone marrow. Competitive bone marrow reconstitution showed a reduced contribution of DN1 cells from CD45(-/-) cells, consistent with an intrinsic defect in these cells. CD45(-/-) DN1 cells exhibited reduced proliferation in vivo and reduced CXCL12-mediated migration in vitro. The loss of CD45 led to the accumulation of an intermediate DN1.5 thymocyte population in vivo that was dependent on Notch for progression. In vivo, CD117(-) DN1 cells gave rise to gammadelta T cells. In vitro, CD117(-) DN1 cells progressed to DN4 on OP9-DL1 cells but CD117(-) DN1 cells lacking CD45 did not. CD45(-/-) CD117(-) DN1 cells were also deficient in TCRbeta expression. Thus, CD45 deficiency affects the development and progression of DN1 thymocytes.

Differential roles of IL-2-inducible T cell kinase-mediated TCR signals in tissue-specific localization and maintenance of skin intraepithelial T cells

Tissue-specific innate-like gammadelta T cells are important components of the immune system critical for the first line of defense, but mechanisms underlying their tissue-specific development are poorly understood. Our study with prototypical skin-specific intraepithelial gammadeltaT lymphocytes (sIELs) found that among different thymic gammadelta T cell subsets fetal thymic precursors of sIELs specifically acquire a unique skin-homing property after positive selection, suggesting an important role of the TCR selection signaling in "programming" them for tissue-specific development. In this study, we identified IL-2-inducible T cell kinase (ITK) as a critical signal molecule regulating the acquirement of the skin-homing property by the fetal thymic sIEL precursors. In ITK knockout mice, the sIEL precursors could not undergo positive selection-associated upregulation of thymus-exiting and skin-homing molecules sphingosine-1-phosphate receptor 1 and CCR10 and accumulated in the thymus. However, the survival and expansion of sIELs in the skin did not require ITK-transduced TCR signaling, whereas its persistent activation impaired sIEL development by inducing apoptosis. These findings provide insights into molecular mechanisms underlying differential requirements of TCR signaling in peripheral localization and maintenance of the tissue-specific T cells.

Roles of the Src tyrosine kinases Lck and Fyn in regulating gammadeltaTCR signal strength

Lck and Fyn, members of the Src family of tyrosine kinases, are key components of the αβTCR-coupled signaling pathway. While it is generally accepted that both Lck and Fyn positively regulate signal transduction by the αβTCR, recent studies have shown that Lck and Fyn have distinct functions in this signaling pathway, with Lck being a positive regulator and Fyn being a negative regulator of αβTCR signal transduction. To determine whether Lck and Fyn also differentially regulate γδTCR signal transduction, we analyzed γδ T cell development and function in mice with reduced Lck or Fyn expression levels. We found that reducing Lck or Fyn levels altered the strength of the γδTCR signaling response, with low levels of Lck weakening γδTCR signal strength and low levels of Fyn augmenting γδTCR signal strength. These alterations in γδTCR signal strength had profound effects not only on αβ/γδ lineage choice, but also on γδ thymocyte maturation and γδ T cell effector function. These results indicate that the cellular levels of Lck and Fyn play a role in regulating the strength of the γδTCR signaling response at different stages in the life of the γδ T cell.

Development of promyelocytic zinc finger and ThPOK-expressing innate gamma delta T cells is controlled by strength of TCR signaling and Id3

The broad-complex tramtrack and bric a brac-zinc finger transcriptional regulator (BTB-ZF), promyelocytic leukemia zinc finger (PLZF), was recently shown to control the development of the characteristic innate T cell phenotype and effector functions of NK T cells. Interestingly, the ectopic expression of PLZF was shown to push conventional T cells into an activated state that seems to be proinflammatory. The factors that control the normal expression of PLZF in lymphocytes are unknown. In this study, we show that PLZF expression is not restricted to NK T cells but is also expressed by a subset of gammadelta T cells, functionally defining distinct subsets of this innate T cell population. A second BTB-ZF gene, ThPOK, is important for the phenotype of the PLZF-expressing gammadelta T cells. Most importantly, TCR signal strength and expression of inhibitor of differentiation gene 3 control the frequency of PLZF-expressing gammadelta T cells. This study defines the factors that control the propensity of the immune system to produce potentially disease-causing T cell subsets.

A retrospective on the requirements for gammadelta T-cell development

Since the discovery of gammadelta T cells two decades ago, considerable effort has been made to understand their developmental program, their antigen specificity, and their contribution to the immune response. In this review, we focus on what is known about gammadelta T-cell development and on the advances that have been made in determining which genes are required. In addition, we compare the genetic requirements for alphabeta and gammadelta T-cell development with the hope of gaining a better picture of the signaling pathways that govern the development of gammadelta lineage cells.

Attenuation of gammadeltaTCR signaling efficiently diverts thymocytes to the alphabeta lineage

The role of the T cell antigen receptor complex (TCR) in alphabeta/gammadelta lineage commitment remains controversial, in particular whether different TCR isoforms intrinsically favor adoption of a certain lineage. Here, we demonstrate that impairing the signaling capacity of a gammadeltaTCR complex enables it to efficiently direct thymocytes to the alphabeta lineage. In the presence of a ligand, a transgenic gammadeltaTCR mediates almost exclusive adoption of the gammadelta lineage, while in the absence of ligand, the same gammadeltaTCR promotes alphabeta lineage development with efficiency comparable to the pre-TCR. Importantly, attenuating gammadeltaTCR signaling through Lck deficiency causes reduced ERK1/2 activation and Egr expression and diverts thymocytes to the alphabeta lineage even in the presence of ligand. Conversely, ectopic Egr overexpression favors gammadelta T cell development. Our data support a model whereby gammadelta versus alphabeta lineage commitment is controlled by TCR signal strength, which depends critically on the ERK MAPK-Egr pathway.

Seeding of dendritic epidermal T cells in the neonatal skin is reduced in 129 strain of mice

Precursors for Thy-1(+) dendritic epidermal T cells (DETC) develop as Vgamma3(+) T cells in the fetal thymus and become distributed in the adult skin. DETC are variably distributed from site to site and from strain to strain. To elucidate the basis of strain variation, we first compared the density of DETC in the ear epidermis among different mouse strains. In the ear epidermis, we detected the highest level of DETC in C57BL/6 mice, intermediate levels in C3H and CBA/J mice, and the lowest levels in other strains including BALB/c and 129 mice. Although BALB/c and 129+Ter/Sv mice showed higher levels of DETC in the abdomen than in the ear, the levels were significantly lower than C57BL/6 mice. Furthermore, in neonatal abdominal epidermis we detected considerably lower numbers of DETC in BALB/c and 129+Ter/Sv mice than in C57BL/6 mice. In contrast, Vgamma3(+) DETC precursors in the fetal thymus are rather increased in 129+Ter/Sv mice. These results suggest that fewer DETC precursors are seeded in the neonatal skin of BALB/c and 129+Ter/Sv mice and that their expansion in the skin during neonatal to adult stages does not reach the levels in C57BL/6 mice.

Differential roles of cytokine receptors in the development of epidermal gamma delta T cells

IL-7 and IL-15 play important roles in gammadelta T cell development. These receptors transmit proliferation and/or survival signals in gammadelta T cells. In addition, the IL-7R promotes recombination and transcription in the TCR gamma locus. To clarify the role of the cytokine receptors in the development of epidermal gammadelta T cells, we introduced a Vgamma3/Vdelta1 TCR transgene, derived from Thy-1+ dendritic epidermal T cells (DETC), into IL-7Ralpha-deficient mice, and we found that they partly rescued gammadelta T cells in the adult thymus but not in the spleen. Introduction of an additional Bcl-2 transgene had a minimal effect on gammadelta T cells in the adult thymus of these mice. In contrast to the adult thymus, the introduction of the Vgamma3/Vdelta1 TCR transgene into IL-7Ralpha-/- mice completely restored Vgamma3+ T cells in the fetal thymus and DETC in the adult skin. On the contrary, the same Vgamma3/Vdelta1 TCR transgene failed to rescue DETC in the skin of IL-2Rbeta-deficient mice, even with the additional Bcl-2 transgene. These results suggest that the IL-2/IL-15R, rather than the IL-7R, plays an essential role in proliferation and survival of DETC in the fetal thymus and the skin. In contrast, the IL-7R is probably essential in the induction of V-J recombination of TCRgamma genes. Thus, this study proves that IL-7R and IL-2/IL-15R serve differential functions in epidermal gammadelta T cell development.

Immune competence involving the natural killer cell lineage promotes placental growth

Very small placentae and absence of uterine natural killer (uNK) cells are amongst the reproductive deficits found in the natural killer (NK) cell and thymus-derived (T) cell immunodeficient mouse tgepsilon26. These defects can be reversed by grafting of adult tgepsilon26 females with bone marrow from T and B cell immunodeficient scid/scid donors. We report here that a second protocol, grafting of neonatal tgepsilon26 females with immunocompetent bone marrow pretreated with antibody to Thy-1, successfully established the uNK cell lineage and ameliorated the phenotype. Further, comparisons of mid-gestation (days 10-16) placental area measurements from tgepsilon26 and seven other immunodeficient strains to time-matched tissues from four strains of immunocompetent mice indicate that lymphocytes of the NK but not the T or B cell lineages are able to influence placental size during normal gestation and that this action is independent of interleukin 2. Area measurements of placentae produced in manipulated tgepsilon26 pregnancies (maternal bone marrow engraftment, outcrossing to immunocompetent males and reciprocal embryo transfers with an immunocompetent strain) suggest that NK cell competence is required in each of the maternal and fetal compartments to optimize placental growth.

Differential Requirements of CD45 Protein Tyrosine Phosphatase for Cytolytic Activities and Intrathymic and Extrathymic Development of Intestinal Intraepithelial Lymphocytes

CD45 is a transmembrane protein tyrosine phosphatase essential for Ag receptor-mediated signaling in both T and B cells. In this study we investigated roles of CD45 in development and cytolytic activities of murine intestinal intraepithelial lymphocytes (i-IEL) using CD45 exon 6 knockout (CD45−/−) mice. Interestingly, the total cell number of i-IEL was significantly reduced in CD45−/− mice during aging (10–20 wk of age), whereas the i-IEL number was normally increased in the wild-type littermates. Especially, the number of γδTCR+ i-IEL decreased markedly in CD45−/− mice during aging. The i-IEL in CD45−/− mice were more susceptible to in vitro spontaneous apoptosis than the normal i-IEL, implying that CD45 is required for maintenance of the cellularity of i-IEL. Results from in vivo analyses of the extrathymic and intrathymic development of i-IEL suggested that CD45-mediated signaling is required for the intrathymic, but not the extrathymic, development of i-IEL. Moreover, the whole i-IEL from CD45−/− mice showed a significantly reduced cytolytic activity, and the residual cytolytic activity was completely diminished by depleting CD45+ i-IEL, suggesting that CD45 is indispensable for the TCR-mediated cytolytic activity of i-IEL. Furthermore, we found differential contributions of CD45 and p56lck to development and induction of cytolytic activities of i-IEL.

Defective expression of p56lck in an infant with severe combined immunodeficiency

Severe combined immune deficiency (SCID) is a heterogeneous disorder characterized by profound defects in cellular and humoral immunity. We report here an infant with clinical and laboratory features of SCID and selective CD4 lymphopenia and lack of CD28 expression on CD8(+) T cells. T cells from this patient showed poor blastogenic responses to various mitogens and IL-2. Other T cell antigen receptor- induced responses, including upregulation of CD69, were similarly inhibited. However, more proximal T cell antigen receptor signaling events, such as anti-CD3 induced protein tyrosine phosphorylation, phosphorylation of mitogen-associated protein kinase, and calcium mobilization were intact. Although p59fyn and ZAP-70 protein tyrosine kinases were expressed at normal levels, a marked decrease in the level of p56lck was noted. Furthermore, this decrease was associated with the presence of an alternatively spliced lck transcript lacking the exon 7 kinase encoding domain. These data suggest that a deficiency in p56lck expression can produce a SCID phenotype in humans.

Requirement of the IL-2 receptor beta chain for the development of Vgamma3 dendritic epidermal T cells

Vgamma3 TCR cells develop in the fetal thymus and migrate to the skin as dendritic epidermal T cells (DETC). Fetal Vgamma3 thymocytes differentiate from immature heat stable antigen (HSA)high cells to mature HSAlow cells and the latter subset predominantly expresses IL-2 receptor beta chain (IL-2Rbeta). In this study, the role of IL-2Rbeta in the development of Vgamma3 cells was determined in IL-2Rbeta-deficient mice. There was a moderate reduction of mature HSAlow Vgamma3 thymocytes in IL-2Rbeta-deficient mice. Small numbers of Vgamma3 DETC were detected in the fetal skin of IL-2Rbeta-deficient mice, but they were absent in newborn and adult mice. These results suggest that IL-2Rbeta may transduce the crucial signal for survival and/or expansion of Vgama3 cells in the fetal thymus and in the fetal skin. In normal mice, IL-15 but not IL-2 mRNA was expressed in the fetal epidermis and exogenous addition of low concentration of IL-15 to fetal skin organ culture induced proliferation of Vgamma3 DETC. The dependence of fetal Vgamma3 DETC on the expression of IL-2Rbeta and the presence of IL-15 mRNA in the fetal epidermis imply an essential role of IL-15 signaling through IL-2Rbeta in the selective localization of this gammadelta T cell subpopulation in the skin.

Cellular functions regulated by Src family kinases

Src family protein tyrosine kinases are activated following engagement of many different classes of cellular receptors and participate in signaling pathways that control a diverse spectrum of receptor-induced biological activities. While several of these kinases have evolved to play distinct roles in specific receptor pathways, there is considerable redundancy in the functions of these kinases, both with respect to the receptor pathways that activate these kinases and the downstream effectors that mediate their biological activities. This chapter reviews the evidence implicating Src family kinases in specific receptor pathways and describes the mechanisms leading to their activation, the targets that interact with these kinases, and the biological events that they regulate.

Murine mast cells and monocytes express distinctive sets of CD45 isoforms

CD45 isoform expression patterns were examined in various mast and monocyte cell populations. The reverse transcription-polymerase chain reaction (RT/PCR) and Southern analysis showed these myeloid cells express characteristic sets of CD45 isoforms. Mast cells produce mRNA for two splice variants, one containing exons 5, 7 and 8 of the alternatively expressed exons (therefore lacking exons 4 and 6) and another containing variable exons 7 and 8. Monocytes express three prominent CD45 mRNA species, one which includes exons 5, 7 and 8, another with exons 7 and 8 and the third containing only exon 8 of the variable exons. These results show that there are clear differences within the myeloid lineage sub-populations with respect to CD45 exon usage which appear to delineate mast cell and monocyte specific patterns.

Normal macrophage functions, but impaired induction of gamma delta T cells, at the site of bacterial infection in CD45 exon 6-deficient mice

We investigated the protective functions of macrophages and gamma delta T cells in adult CD45 exon 6-deficient (CD45 -/-) mice against an intraperitoneal (i.p.) infection with Listeria monocytogenes. gamma delta T cells are preferentially localized in the spleen, liver, and intraperitoneal cavity of the adult CD45-/- mice. Increased numbers of gamma delta T cells were observed after i.p. infection with L. monocytogenes in the peritoneal cavity of C57BL/6 (CD45 +/+) mice but not in CD45 -/- mice. The gamma delta T cells showed predominant usage of V delta 5 and V delta 6 rearranged to J delta 1 in the infected CD45 -/- mice which are the same as those used by resident gamma delta T cells of noninfected CD45 +/+ and CD45 -/- mice. Furthermore, we analyzed the protective abilities of the CD45 -/-, CD45 +/+, and gamma delta T cell-depleted mice at the early stage of the listerial infection. The numbers of bacteria in the spleens and livers of the CD45 -/- mice 5 days after the listerial infection were almost ten times larger than those in the CD45 -/- and gamma delta T cell-depleted CD45 +/+ mice. Macrophages showed normal antigen presentation, nitric oxide production and bactericidal activity for L. monocytogenes despite their lacking CD45 surface expression, suggesting that CD45-negative macrophages have a minimal influence on the increased bacterial multiplication in the CD45-/- mice. These results suggest that the gamma delta T cells are induced by the bacterial infection in a CD45-dependent manner, and that unresponsiveness of the gamma delta T cells results in only weak protection against L. monocytogenes in CD45 -/- mice.

The role of CD45 and CD45-associated molecules in T cell activation

CD45 (lymphocyte common antigen) is a receptor-linked protein tyrosine phosphatase that is expressed on all leucocytes, and which plays a crucial role in the function of these cells. On T cells the extracellular domain of CD45 is expressed in several different isoforms, and the particular isoform(s) expressed depends on the particular subpopulation of cell, their state of maturation, and whether or not they have previously been exposed to antigen. It has been established that the expression of CD45 is essential for the activation of T cells via the TCR, and that different CD45 isoforms display a different ability to support T cell activation. Although the tyrosine phosphatase activity of the intracellular region of CD45 has been shown to be crucial for supporting signal transduction from the TCR, the nature of the ligands for the different isoforms of CD45 have been elusive. Moreover, the precise mechanism by which potential ligands may regulate CD45 function is unclear. Interestingly, in T cells CD45 has been shown to associate with numerous molecules, both membrane associated and intracellular; these include components of the TCR-CD3 complex and CD4/CD8. In addition, CD45 is reported to associate with several intracellular protein tyrosine kinases including p56lck and p59fyn of the src family, and ZAP-70 of the Syk family, and with numerous proteins of 29-34 kDa. These CD45-associated molecules may play an important role in regulating CD45 tyrosine phosphatase activity and function. However, although the role of some of the CD45-associated molecules (e.g. CD45-AP and LPAP) has become better understood in recent years, the role of others still remains obscure. This review aims to summarize recent findings on the role of CD45 and CD45-associated molecules in T cell activation, and to highlight issues that seem relevant to ongoing research in this area.

Role of phosphatases in lymphocyte activation

Many lymphocyte signaling pathways are regulated by protein tyrosyl phosphorylation, which is controlled by protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs). Substantial progress has been made in defining the functions of lymphocyte PTPs. Individual PTPs can enhance or diminish cell signaling levels. The transmembrane PTP CD45 is a key positive element in multiple lymphocyte signaling pathways in vivo. New insights into the function of individual CD45 isoforms have emerged. Anti-CD45 antibodies with potent immunosuppressant activity have been identified, suggesting that CD45 may be a propitious target for drug design. Progress has also been made in elucidating the function and targets of specific nontransmembrane PTPs, particularly those with Src homology 2 domains.

CD45 molecule in gammadelta T-cell generation: disruption of CD45 exon 6 does not affect Vgamma3 dendritic epidermal T-cell development

There are two distinct lineages of T cells: T-cell receptor (TCR) alphabeta-bearing cells (alphabeta T cells) and TCR gammadelta-bearing cells (gammadelta T cells). All of the alphabeta T cells and most subsets of gammadelta T cells develop in the thymus. It has been demonstrated that the protein tyrosine phosphatase CD45 plays a pivotal role in the intrathymic development of alphabeta T cells. Thymocyte maturation is arrested at the transitional stage from immature CD4+ CD8+ double-positive to mature CD4+ or CD8+ single-positive cells after CD45 exon 6 gene disruption. In this study, we examined whether Vgamma3 dendritic epidermal T cells (DETC), a subset of thymus-dependent gammadelta T cells uniquely residing in the murine epidermis, are altered in the CD45 exon 6-deficient mice. In situ immunolabeling on epidermal sheets demonstrated that the CD45-deficient mice had a normal density and immunophenotype of Vgamma3 DETC compared with the wild-type control mice. Reverse transcriptase polymerase chain reaction revealed that similar levels of Vgamma3 TCR mRNA were present in the epidermis of CD45-deficient mice and wild-type controls. Flow cytometry demonstrated no significant difference in the proportion of Vgamma3 T cells in the epidermis between the genotypes. In addition, Vgamma2 T cells, another subset of gammadelta T cells, were also examined by flow cytometry. The frequency of Vgamma2 T cells in lymph nodes was normal in the CD45-deficient mice. Our results indicate that although CD45 is crucial for the development of alphabeta T cells, this molecule is not necessary for the thymic maturation of gammadelta T cells, including Vgamma3 DETC and Vgamma2 T cells.

alpha beta T cell development is abolished in mice lacking both Lck and Fyn protein tyrosine kinases

Two families of protein tyrosine kinases (PTKs), the Src and Syk/ZAP-70 families, are required for T cell development. Lck is the major Src family member required for thymopoiesis, since there is a severe deficit of CD4+CD8+ thymocytes and mature T cells in its absence. However, some peripheral T cells are evident in these mice, suggesting that additional PTKs may contribute to T cell development. Here we show that the combined disruption of Lck and Fyn (lck(-/-)fyn(-/-)) completely arrests alpha beta T cell development at the CD4-CD8- stage. The development of V gamma 3+ dendritic epidermal T cells is also severely impaired, but natural killer cell development and cytolytic activity is unaffected in lck(-/-)fyn(-/-) mice. These findings reveal the potential for redundant functions mediated by Src family PTKs while emphasizing crucial roles for Lck and Fyn in T cell development.

Fyn can partially substitute for Lck in T lymphocyte development

Lck, a Src family tyrosine kinase, transduces signals important for the development of alphabeta and gammadelta T cells. However, T cell development is only partially compromised in Lck-deficient mice, suggesting that other kinases may also transduce pre-TCR or TCR signals. One candidate is Fyn, a Src kinase coexpressed with Lck in immature and mature T cells. Here we show that T cell development is completely compromised in lck(-/-)fyn(-/-) mice. In addition, we demonstrate that expression of a gain-of-function mutant fyn(T) transgene completely restores production of immature CD4/CD8 double positive thymocytes and gammadelta T cells and improves the representation of CD4 or CD8 single positive thymocytes. These observations reveal that Fyn can subserve some Lck-like functions in T cell development.

Protein tyrosine phosphatases in T-cell development, apoptosis and signalling

The study of phosphatases was viewed as a rather esoteric subject for immunologists until eight years ago, when the discovery that CD45 is a protein tyrosine phosphatase (PTPase) began to make the topic respectable. Now, as reviewed by Julie Frearson and Denis Alexander, PTPases are increasingly being shown to play key roles in the molecular physiology of haematopoietic cells and some have been shown to regulate critical events in T-cell development and signalling.

Interleukin 7 receptor-deficient mice lack gammadelta T cells

The interleukin 7 receptor (IL-7R) plays a crucial role in early B- and T-cell development. It consists of a unique a chain and a common gamma chain [IL-2 receptor gamma chain (IL-2Rgamma)]. Gene inactivation of IL-7, IL-7R, and IL-2Rgamma resulted in severe impairment of B and T lymphopoiesis in mice. In addition, IL-2Rgamma-deficient mice lack gammadelta T cells in the skin and have the impaired development of natural killer (NK) cells and intraepithelial lymphocytes. To explore the role of IL-7/IL-7R system in gammadelta T- and NK-cell development, we have generated and analyzed IL-7R-deficient mice. gammadelta T cells were absent from skin, gut, liver, and spleen in the deficient mice. In contrast, alphabeta T and B cells were detected in reduced, but certain, numbers, and NK cells developed normally. The gammadelta T-cell development in fetal and adult thymus was also completely blocked. These results clearly demonstrate that the signal from IL-7R is indispensable for gammadelta T-cell development in both thymic and extrathymic pathways. On the contrary, it is suggested that NK-cell development requires cytokine(s) other than IL-7.

Positive and negative selection invoke distinct signaling pathways

During T cell development, interaction of the T cell receptor (TCR) with cognate ligands in the thymus may result in either maturation (positive selection) or death (negative selection). The intracellular pathways that control these opposed outcomes are not well characterized. We have generated mice expressing dominant-negative Ras (dnRas) and Mek-1 (dMek) transgenes simultaneously, either in otherwise normal animals, or in animals expressing a transgenic TCR, thereby permitting a comprehensive analysis of peptide-specific selection. In this system, thymocyte maturation beyond the CD4+8+ stage is blocked almost completely, whereas negative selection, assessed using an in vitro deletion protocol, is quantitatively intact. This suggests that activation of the mitogen-activated protein kinase (MAPK) cascade is necessary for positive selection, but irrelevant for negative selection. Generation of gamma/delta and of CD4-8- alpha/beta T cells proceeds normally despite blockade of the MAPK cascade. Hence, only cells that mature via conventional, TCR-mediated repertoire selection require activation of the MAPK pathway to complete their maturation.

Functions of TCR and pre-TCR subunits: lessons from gene ablation

Recent gene-targeting experiments have highlighted the existence of checkpoints that ensure that alpha beta T cells do not complete intrathymic differentiation if they have not attained certain landmark events. These 'proofreading' mechanisms operate by way of the pre-TCR and TCR complexes, which are sequentially expressed during T-cell development. These complexes are likely to signal via their associated CD3 subunits. By activating intracellular effectors, the CD3 subunits probably modulate gene expression profiles and drive the maturing alpha beta T cells through a precise developmental sequence.

T lymphocyte development in p56lck deficient mice: allelic exclusion of the TcR beta locus is incomplete but thymocyte development is not restored by TcR beta or TcR alpha beta transgenes

The protein tyrosine kinase, p56lck, is involved in signal transduction in mature T cells and in the molecular events controlling early thymocyte differentiation. Thymuses of mice deficient for p56lck expression (p56lck-/-) consist of immature CD4-CD8- double-negative (DN) and CD4+CD8+ double-positive (DP) thymocytes and are severely reduced in total cell number. In this report we have studied DN thymocytes from p56lck-/- mice and found an increase in the proportion of the CD44-CD25+ subset, suggesting that transit through this stage, which is known to require T cell receptor (TcR) beta expression, may be delayed in the absence of p56lck expression. In addition, the expression of a transgenic TcR beta chain or TcR alpha beta pair did not restore thymic development in p56lck-/- mice. However, in contrast to mice expressing a dominant negative isoform of p56lck in which DP thymocytes do not develop, DP thymocytes still develop in nontransgenic and TcR transgenic p56lck-/- mice. These results demonstrate that expansion of the DP subset is impaired in p56lck-/- mice. In contrast, allelic exclusion is not severely compromised. Although there was an increase in the number of peripheral T cells expressing more than one V beta chain in TcR transgenic p56lck-/- mice, we found that inhibition of endogenous TcR beta gene rearrangement was almost complete in thymocytes of V beta transgenic p56lck-/- mice and we could not detect any peripheral T cells that expressed more than one V beta chain in non-transgenic p56lck-/- mice.