Phenotypic properties, interleukin 2 production, and developmental origin of a "mature" subpopulation of Lyt-2- L3T4- mouse thymocytes

Natural Killer T (NKT) Cells in Autoimmune Hepatitis: Current Evidence from Basic and Clinical Research

Natural killer T (NKT) cells are unconventional T cells that are activated by glycolipid antigens. They can produce a variety of inflammatory and regulatory cytokines and, therefore, modulate multiple aspects of the immune response in different pathological settings, including autoimmunity. NKT cells have also been implicated in the immunopathogenesis of autoimmune hepatitis, and in this review we summarize and analyze the main studies investigating the involvement and/or homeostasis of NKT cells in this disease. In detail, the evidence from both basic and clinical research has been specifically analyzed. Even though the experimental murine models supported a relevant role of NKT cells in immune-mediated hepatic injury, very few studies specifically investigated NKT cell homeostasis in patients with autoimmune hepatitis; however, these initial studies reported some alterations of NKT cells in these patients, which may also correlate with the disease activity to some extent. Further clinical studies are needed to investigate the potential role and use of NKT cell analysis as a disease marker of clinical relevance, and to better understand the precise cellular and molecular mechanisms by which NKT cells contribute to the pathogenesis of autoimmune hepatitis.

Novel lipid antigens for NKT cells in cancer

Cancer immunotherapy aims to unleash the power of the immune system against tumors without the side effects of traditional chemotherapy. Immunotherapeutic methods vary widely, but all follow the same basic principle: overcome the barriers utilized by cancers to avoid immune destruction. These approaches often revolve around classical T cells, such as with CAR T cells and neoantigen vaccines; however, the utility of the innate-like iNKT cell in cancer immunotherapy has gained significant recognition. iNKT cells parallel classic T cell recognition of peptide antigens presented on MHC through their recognition of lipid antigens presented on the MHC I-like molecule CD1d. Altered metabolism and a lipogenic phenotype are essential properties of tumor cells, representing a unique feature that may be exploited by iNKT cells. In this review, we will cover properties of iNKT cells, CD1d, and lipid antigen presentation. Next, we will discuss the cancer lipidome and how it may be exploited by iNKT cells through a window of opportunity. Finally, we will review, in detail, novel lipid antigens for iNKT cells in cancer.

Modulation of Immune Responses to Influenza A Virus Vaccines by Natural Killer T Cells

Influenza A viruses (IAVs) circulate widely among different mammalian and avian hosts and sometimes give rise to zoonotic infections. Vaccination is a mainstay of IAV prevention and control. However, the efficacy of IAV vaccines is often suboptimal because of insufficient cross-protection among different IAV genotypes and subtypes as well as the inability to keep up with the rapid molecular evolution of IAV strains. Much attention is focused on improving IAV vaccine efficiency using adjuvants, which are substances that can modulate and enhance immune responses to co-administered antigens. The current review is focused on a non-traditional approach of adjuvanting IAV vaccines by therapeutically targeting the immunomodulatory functions of a rare population of innate-like T lymphocytes called invariant natural killer T (iNKT) cells. These cells bridge the innate and adaptive immune systems and are capable of stimulating a wide array of immune cells that enhance vaccine-mediated immune responses. Here we discuss the factors that influence the adjuvant effects of iNKT cells for influenza vaccines as well as the obstacles that must be overcome before this novel adjuvant approach can be considered for human or veterinary use.

Transcription factor YY1 is essential for iNKT cell development

Invariant natural killer T (iNKT) cells develop from CD4⁺CD8⁺ double-positive (DP) thymocytes and express an invariant V_α14-J_α18 T-cell receptor (TCR) α-chain. Generation of these cells requires the prolonged survival of DP thymocytes to allow for V_α14-J_α18 gene rearrangements and strong TCR signaling to induce the expression of the iNKT lineage-specific transcription factor PLZF. Here, we report that the transcription factor Yin Yang 1 (YY1) is essential for iNKT cell formation. Thymocytes lacking YY1 displayed a block in iNKT cell development at the earliest progenitor stage. YY1-deficient thymocytes underwent normal V_α14-J_α18 gene rearrangements, but exhibited impaired cell survival. Deletion of the apoptotic protein BIM failed to rescue the defect in iNKT cell generation. Chromatin immunoprecipitation and deep-sequencing experiments demonstrated that YY1 directly binds and activates the promoter of the Plzf gene. Thus, YY1 plays essential roles in iNKT cell development by coordinately regulating cell survival and PLZF expression.

Possible Immune Regulation of Natural Killer T Cells in a Murine Model of Metal Ion-Induced Allergic Contact Dermatitis

Metal often causes delayed-type hypersensitivity reactions, which are possibly mediated by accumulating T cells in the inflamed skin, called irritant or allergic contact dermatitis. However, accumulating T cells during development of a metal allergy are poorly characterized because a suitable animal model is unavailable. We have previously established novel murine models of metal allergy and found accumulation of both metal-specific T cells and natural killer (NK) T cells in the inflamed skin. In our novel models of metal allergy, skin hypersensitivity responses were induced through repeated sensitizations by administration of metal chloride and lipopolysaccharide into the mouse groin followed by metal chloride challenge in the footpad. These models enabled us to investigate the precise mechanisms of the immune responses of metal allergy in the inflamed skin. In this review, we summarize the immune responses in several murine models of metal allergy and describe which antigen-specific responses occur in the inflamed skin during allergic contact dermatitis in terms of the T cell receptor. In addition, we consider the immune regulation of accumulated NK T cells in metal ion-induced allergic contact dermatitis.

CD8(+)NKT-like cells regulate the immune response by killing antigen-bearing DCs

CD1d-dependent NKT cells have been extensively studied; however, the function of CD8(+)NKT-like cells, which are CD1d-independent T cells with NK markers, remains unknown. Here, we report that CD1d-independent CD8(+)NKT-like cells, which express both T cell markers (TCRβ and CD3) and NK cell receptors (NK1.1, CD49b and NKG2D), are activated and significantly expanded in mice immunized with GFP-expressing dendritic cells. Distinct from CD1d-dependent NKT cells, CD8(+)NKT-like cells possess a diverse repertoire of TCRs and secrete high levels of IFN-gamma but not IL-4. CD8(+)NKT-like cell development is normal in CD1d(-/-) mice, which suggests that CD8(+)NKT-like cells undergo a unique development pathway that differs from iNKT cells. Further functional analyses show that CD8(+)NKT-like cells suppress T-cell responses through elimination of dendritic cells in an antigen-specific manner. Adoptive transfer of antigen-specific CD8(+)NKT-like cells into RIP-OVA mice prevented subsequent development of diabetes in the animals induced by activated OT-I CD8 T cells. Our study suggests that CD8(+)NKT-like cells can function as antigen-specific suppressive cells to regulate the immune response through killing antigen-bearing DCs. Antigen-specific down regulation may provide an active and precise method for constraining an excessive immune response and avoiding bypass suppression of necessary immune responses to other antigens.

CD1d Expression and Invariant NKT Cell Responses in Herpesvirus Infections

Invariant natural killer T (iNKT) cells are a highly conserved subset of unconventional T lymphocytes that express a canonical, semi-invariant T cell receptor and surface markers shared with the natural killer cell lineage. iNKT cells recognize exogenous and endogenous glycolipid antigens restricted by non-polymorphic CD1d molecules, and are highly responsive to the prototypical agonist, α-galactosylceramide. Upon activation, iNKT cells rapidly coordinate signaling between innate and adaptive immune cells through the secretion of proinflammatory cytokines, leading to the maturation of antigen-presenting cells, and expansion of antigen-specific CD4+ and CD8+ T cells. Because of their potent immunoregulatory properties, iNKT cells have been extensively studied and are known to play a pivotal role in mediating immune responses against microbial pathogens including viruses. Here, we review evidence that herpesviruses manipulate CD1d expression to escape iNKT cell surveillance and establish lifelong latency in humans. Collectively, published findings suggest that iNKT cells play critical roles in anti-herpesvirus immune responses and could be harnessed therapeutically to limit viral infection and viral-associated disease.

Immunology in the Clinic Review Series; focus on host responses: invariant natural killer T cell activation following transplantation

Invariant natural killer T (iNKT) cells have been shown to play a key role in the regulation of immunity in health and disease. However, iNKT cell responses have also been found to influence both rejection and the induction of tolerance following transplantation of allogeneic cells or organs. Although a number of mechanisms have been identified that lead to iNKT cell activation, how iNKT cells are activated following transplantation remains unknown. This review will attempt to identify potential mechanisms of iNKT cell activation in the context of transplantation by applying knowledge garnered from other disease situations. Furthermore, we put forward a novel mechanism of iNKT cell activation which we believe may be the dominant mechanism responsible for iNKT activation in this setting, i.e. bystander activation by interleukin-2 secreted by recently activated conventional T cells.

The role of iNKT cells in the immunopathology of systemic lupus erythematosus

An increasing body of evidence suggests that CD1d-restricted invariant natural killer T (iNKT) cells play an important immunoregulatory role in a variety of autoimmune diseases in both humans and mouse models. Their role in systemic lupus erythematosus (SLE), however, is not fully determined, as SLE mouse models have yielded conflicting results demonstrating both a protective function and a pathogenic role. The reduced frequency of iNKT cells in peripheral blood of lupus patients supports the idea of a protective role for these cells in the immunopathology of SLE. Therapeutic approaches using glycolipids provide a promising tool to correct numerical iNKT cell deficiencies and to modulate their function. This review highlights the potential role of iNKT cells in lupus immunopathology and summarizes recent studies concerning iNKT cells in SLE patients, lupus-prone murine models and glycolipid therapy.

NKT cells: In the beginning

Invariant natural killer T (iNKT) cells as we know them today are a unique subset of mature T cells co-expressing a semi-invariant Valpha14/Vbeta8 TCR and surface markers characteristic of NK cells. The semi-invariant TCR on iNKT cells recognizes glycolipids bound to monomorphic CD1d molecules, leading to rapid cytokine production. The purpose of this historical perspective is to describe how a series of seemingly unrelated findings in the late 1980s and early 1990s crystallized in the discovery of iNKT cells. The story is told from a personal viewpoint, with a particular effort to place both breakthroughs and misinterpretations in the context of their era.

Natural killer T cells: a bridge to tolerance or a pathway to rejection?

Over the past 20 years, natural killer T (NKT) cells have been shown to play an important role in both innate and adaptive immune responses. In this review, the potential role of NKT cells in transplantation will be discussed, particularly their role in rejection and the induction of a state of tolerance.

Advances in the understanding of acute graft-versus-host disease

Allogeneic stem cell transplantation (SCT) remains the definitive immunotherapy for malignancy. However, morbidity and mortality due to graft-vs.-host disease (GVHD) remains the major barrier to its advancement. Emerging experimental data highlights the immuno-modulatory roles of diverse cell populations in GVHD, including regulatory T cells, natural killer (NK) cells, NK T cells, gammadelta T cells, and antigen presenting cells (APC). Knowledge of the pathophysiology of GVHD has driven the investigation of new rational strategies to both prevent severe GVHD and treat steroid-refractory GVHD. Novel cytokine inhibitors, immune-suppressant agents known to preserve or even promote regulatory T-cell function and the depletion of specific alloreactive T-cell sub-populations all promise significant advances in the near future. As our knowledge and therapeutic options expand, the ability to limit GVHD whilst preserving anti-microbial and tumour responses becomes a realistic prospect.

Participatory role of natural killer and natural killer T cells in atherosclerosis: lessons learned from in vivo mouse studies

Atherosclerosis is a multifactor, highly complex disease with numerous aetiologies that work synergistically to promote lesion development. One of the emerging components that drive the development of both early- and late-stage atherosclerotic lesions is the participation of both the innate and acquired immune systems. In both humans and animal models of atherosclerosis, the most prominent cells that infiltrate evolving lesions are macrophages and T lymphocytes. The functional loss of either of these cell types reduces the extent of atherosclerosis in mice that were rendered susceptible to the disease by deficiency of either apolipoprotein E or the LDL (low density lipoprotein) receptor. In addition to these major immune cell participants, a number of less prominent leukocyte populations that can modulate the atherogenic process are also involved. This review will focus on the participatory role of two "less prominent" immune components, namely natural killer (NK) cells and natural killer T (NKT) cells. Although this review will highlight the fact that both NK and NKT cells are not sufficient for causing the disease, the roles played by both these cells types are becoming increasingly important in understanding the complexity of this disease process.

A positive look at double-negative thymocytes

In some respects, our understanding of the cellular and molecular aspects of early T-cell differentiation is lagging behind that of B cells. Papers describing gene-knockout and reporter-transgenic mice in which thymocyte development is affected are often difficult to interpret. Progress in this field will be hampered unless a more detailed phenotypic and molecular analysis of progenitor thymocytes at the single-cell level is carried out.

A natural killer T (NKT) cell developmental pathway iInvolving a thymus-dependent NK1.1(-)CD4(+) CD1d-dependent precursor stage

The development of CD1d-dependent natural killer T (NKT) cells is poorly understood. We have used both CD1d/alpha-galactosylceramide (CD1d/alphaGC) tetramers and anti-NK1.1 to investigate NKT cell development in vitro and in vivo. Confirming the thymus-dependence of these cells, we show that CD1d/alphaGC tetramer-binding NKT cells, including NK1.1(+) and NK1.1(-) subsets, develop in fetal thymus organ culture (FTOC) and are completely absent in nude mice. Ontogenically, CD1d/alphaGC tetramer-binding NKT cells first appear in the thymus, at day 5 after birth, as CD4(+)CD8(-)NK1.1(-)cells. NK1.1(+) NKT cells, including CD4(+) and CD4(-)CD8(-) subsets, appeared at days 7-8 but remained a minor subset until at least 3 wk of age. Using intrathymic transfer experiments, CD4(+)NK1.1(-) NKT cells gave rise to NK1.1(+) NKT cells (including CD4(+) and CD4(-) subsets), but not vice-versa. This maturation step was not required for NKT cells to migrate to other tissues, as NK1.1(-) NKT cells were detected in liver and spleen as early as day 8 after birth, and the majority of NKT cells among recent thymic emigrants (RTE) were NK1.1(-). Further elucidation of this NKT cell developmental pathway should prove to be invaluable for studying the mechanisms that regulate the development of these cells.

Characterization of CD4- CD8- CD3+ T-cell receptor-alphabeta+ T cells in murine cytomegalovirus infection

In this study, we have investigated that after the intraperitoneal infection with murine cytomegalovirus (MCMV), the CD3+ CD4- CD8-(double negative; DN) T-cell receptor (TCR)alphabeta+ T cells increased in peritoneal cavity, liver and spleen in both resistant C57BL/6 and susceptible BALB/c mice. The total cellular population of these cells showed peak levels around day 5 after infection in all the three investigated organs and the following phenotypical and functional characteristics emerged. The peritoneal DN TCRalphabeta+ T cells expressed highly skewed TCRVbeta8 on day 5 after infection compared with the uninfected mice, but those in spleen and liver showed moderate and low skewed TCRVbeta8, respectively. The percentages of NK1.1+ DN TCRalphabeta+ T cells gradually decreased as did modulation of some of their activation markers consistent with an activated cell phenotype. The peritoneal DN TCRalphabeta+ T cells on day 5 after infection expressed the genes of interferon-gamma (IFN-gamma), tumour necrosis factor-alpha, Eta-1 (early T-cell activation-1) and MCP-1 (monocyte chemoattractant protein 1) but lacked expression of interleukin-4 (IL-4). After in vitro stimulation with phorbol 12-myristate 13-acetate and calcium ionophore in the presence of Brefeldin A, higher frequencies of intracellular IFN-gamma+ DN TCRalphabeta+ T cells were detected in all three investigated organs of infected mice compared with those of uninfected mice. Stimulation of peritoneal DN TCRalphabeta+ T cells with plate-bound anti-TCRbeta monoclonal antibodies showed proliferation and also produced IFN-gamma but not IL-4. These results suggest that DN TCRalphabeta+ T cells were activated and may have an antiviral effect through producing IFN-gamma and some macrophage-activating factors during an early phase of MCMV infection.

Distribution of alpha 4 beta 7 and alpha E beta 7 integrins on thymocytes, intestinal epithelial lymphocytes and peripheral lymphocytes

Beta 7 is expressed on subsets of thymocytes, while T and B lymphocytes show heterogeneous expression of beta 7. Here, we examine the phenotype of the thymocyte and lymphocyte subsets which express alpha 4 beta 7 and alpha E beta 7 using mAb against alpha E, beta 7 and mAb DATK32 which recognizes a combinatiorial epitope on alpha 4 beta 7+ thymocytes have a mature phenotype: TcR+, CD11a(hi)CD44(hi)HSA(dull). Small subsets of double-negative CD4-CD8-, single-positive CD4+ and CD8+ thymocytes express beta 7, while double-positive CD4+CD8+ thymocytes are beta 7-. However, two integrins alpha E beta 7 and alpha 4 beta 7 recognized by anti-beta 7 are not expressed on an identical subpopulation of thymocytes, as alpha E beta 7+ alpha 4 beta 7-, alpha E beta 7 + alpha 4 beta 7+ and alpha E beta 7- alpha 4 beta 7+ thymocyte subsets are evident. Similarly, intraepithelial lymphocytes express high levels of alpha E beta 7 but little alpha 4 beta 7. In the spleen, Peyer's patches and lymph nodes, alpha 4 beta 7 is expressed at higher levels on most B lymphocytes than on the majority of T lymphocytes, while a small subset of T lymphocytes, which includes both CD4+ and CD8+ lymphocytes, express high levels of beta 7 in the form of alpha 4 beta 7 and alpha E beta 7, although, as observed with lymphocytes, not all alpha 4 beta 7 hi CD4+ lymphocytes expressed alpha 4 beta 7. The population of alpha 4 beta 7 hi CD4 lymphocytes are enriched in Peyer's patches and form subsets of the memory CD4+ lymphocyte population, which can be further subdivided on the basis of alpha E beta 7, L-selectin and alpha 4 expression. Therefore, memory CD4+ lymphocytes are highly heterogeneous in their expression of adhesion receptors, and presumably these subpopulations will exhibit very different trafficking properties.

Early appearance of T cell receptor alpha beta + CD4- CD8- T cells with a skewed variable region repertoire after infection with Listeria monocytogenes

We found that the number of T cell receptor (TCR) alpha beta + CD4- CD8- T cells increased in the peritoneal cavity on day 5 after an intraperitoneal infection with Listeria monocytogenes strain EGD together with TCR gamma delta + CD4- CD8- T cells. Thereafter, the TCR alpha beta + CD4- CD8- T cells decreased to a normal level by day 14. The TCR alpha beta + CD4- CD8- T cells showed an activated T cell phenotype (L-selectin CD44 +) and expressed CD45/B220 and interleukin-2 receptor beta, but did not express heat stable antigen, which is expressed by the immature CD4- CD8- thymocytes. Furthermore, 20-30% of the TCR alpha beta + CD4- CD8- T cells expressed the NK1.1 natural killer cell marker. Analysis of the TCR V region repertoire of the TCR alpha beta + CD4- CD8- T cells induced by L. monocytogenes infection showed that more than 80% of the TCR alpha beta + CD4- CD8- T cells expressed TCR V beta 8 detected by anti-TCR V beta 8.1 and 8.2 mAb, and a reverse transcription-polymerase chain reaction analysis of V alpha 14 relative to V alpha 11 expression revealed that the TCR alpha beta + CD4- CD8- T cells expressed a higher level of V alpha 14, which was reported to be preferentially expressed by TCR alpha beta + CD4- CD8- thymocytes rather than conventional CD4+ T cells. The TCR alpha beta + CD4- CD8-T cells showed a proliferative response to anti-TCR alpha beta mAb stimulation. In contrast, they showed no response to stimulation with either Listeria antigen or 65-kDa heat shock protein of Mycobacterium bovis, which do stimulate the Listeria-specific TCR alpha beta + CD4- CD8- T cells and the Listeria-induced TCR gamma delta + T cells, respectively. These results suggest that the TCR alpha beta + CD4- CD8- T cells may recognize a restricted set of self antigens induced by L. monocytogenes infection, and that they contribute to host protection at an early stage of infection.

Class II major histocompatibility complex-restricted T cell function in CD4-deficient mice

Previously, we and others have demonstrated that CD4-deficient mice have a normal number of T cells and B cells with a significant population of CD4-8-TcR alpha beta+ T cells. Surprisingly, however, these mice lacking CD4 show in vivo immunoglobulin isotype class switching from IgM to IgG in response to sheep erythrocytes and vesicular stomatitis virus. In this study we have depleted various subpopulations of T cells in vivo and shown that the population of CD4-8-TcR alpha beta+ T cells is responsible for providing "help" in the antibody response of CD4-deficient mice to vesicular stomatitis virus infection. We have used antigen-specific proliferation assays and blocking studies with class I and II major histocompatibility complex (MHC)-specific purified antibodies to show that these cells are class II MHC-restricted in responses against the T cell-dependent antigen keyhole limpet hemocyanin (KLH). Finally, phenotypic analysis of the CD4-CD8- thymocytes in CD4-deficient mice shows that these cells have a more mature phenotype than the CD4-8- thymocytes in wild type mice. These results indicate that CD4 is not absolutely necessary for positive selection or effector function of class II MHC-restricted helper T cells.

Cytotoxicity of fresh NK1.1+ T cell receptor alpha/beta+ thymocytes against a CD4+8+ thymocyte population associated with intact Fas antigen expression on the target

Recent studies have revealed that 10-20% of CD4+8- or CD4-8- thymocyte populations contain NK1.1+ T cell receptor (TCR)-alpha/beta+ cells. This subpopulation shows characteristics that are different from NK1.1- CD4+ or NK1.1- CD8+ T cells and seems to have developed in a manner different from NK1.1- T cells. Although extensive studies have been performed on the NK1.1+ TCR-alpha/beta+ thymocytes, the physiological role of the NK1.1+ TCR-alpha/beta+ thymocytes has been totally unclear. In the present study, we found that freshly isolated NK1.1+ TCR-alpha/beta+ thymocytes, but neither whole thymocytes nor lymph node T cells, directly killed CD4+8+ thymocytes from normal syngeneic or allogeneic mice by using a long-term cytotoxic assay in which flow cytometry was used to detect the cytotoxicity. However, only weak cytotoxicity was detected against thymocytes from lpr mice on which the Fas antigen that transduces signals for apoptosis into the cells is not expressed. Furthermore, the NK1.1+ TCR-alpha/beta+ thymocytes exhibited high cytotoxicity against T lymphoma targets transfected with fas genes as compared with the parental T lymphoma targets or target cells transfected with mutated fas genes, which lack the function of transducing signals. On the other hand, NK1.1+ effector thymocytes from gld mice that carry a point mutation in Fas ligand did not kill thymocyte targets from normal mice. The present findings, thus, consistently suggest that the NK1.1+ TCR-alpha/beta+ thymocytes kill a subpopulation among CD4+8+ thymocytes via Fas antigen and in this way regulate generation of T lineage cells in the thymus.

Class I dependence of the development of CD4+ CD8- NK1.1+ thymocytes

A small subset of functionally active CD4+ CD8- thymocytes express the NK1.1 marker, as do most CD4-CD8- NK1.1+ thymocytes. Previous studies have failed to implicate a role for major histocompatibility complex (MHC) or related molecules in the selection of the CD4+ CD8- NK1.1+ subset. We report here that the development of most of these cells is sharply reduced in class I-deficient mice, but not in class II-deficient mice. Hence, some CD4+ T cells are class I dependent and not class II dependent. Unlike conventional T cells, however, the development of NK1.1+ thymocytes in both the CD4+ CD8- and CD4- CD8- subsets is dependent on class I MHC expression by hematopoietic cells and not thymic epithelial cells. We propose that these populations are selected by nonpolymorphic class Ib or CD1 molecules.

Adult T-cell leukemia derived from S100 beta positive double-negative (CD4- CD8-) T cells

Adult T-cell leukemia (ATL) is a mature T-cell malignancy which is caused by human T lymphotropic virus type-I (HTLV-I). Most of the ATL cells are CD3+, CD4+, CD8-, and T-cell receptor (TCR) alpha beta+ and also express activated antigens such as HLA-DR and interleukin-2 receptor (IL2R) alpha chain (CD25). Diminished surface expression of the TCR alpha beta/CD3 complex is a specific feature of ATL cells. Since the gene transcript for each chain of this complex has been detected and surface expression of this complex is further decreased, accompanied by the induction of IL2R alpha chain, after stimulation with anti-CD3 monoclonal antibody (mAb), the TCR alpha beta/CD3 complex is considered to be down-modulated in vivo. We recently reported four ATL patients whose leukemic cells were CD4-, CD8- (double-negative; DN), TCR alpha beta+. These DN-ATL cells expressed S100 beta protein which was not detected in CD4+ ATL cells. Similar to CD4+ ATL cells, surface expression of the TCR alpha beta/CD3 complex on DN-ATL cells was decreased in vivo despite the lack of CD4 or CD8 as coreceptor. Therefore, the TCR alpha beta+ T-cell with or without CD4 is the sole target of HTLV-I induced leukemia and the down-modulation of the TCR alpha beta/CD3 complex is considered to play a key role in the development of ATL.

Defective development of thymocytes overexpressing the costimulatory molecule, heat-stable antigen

Heat-stable antigen (HSA) is a small, glycosyl phosphatidylinositol-anchored protein that can act as a costimulatory molecule for antigen-dependent activation of helper T cells. In addition to being expressed on antigen-presenting B cells, HSA is also expressed during the initial stages of T cell development in the thymus. HSA levels are very high on immature CD4-, CD8- double negative thymocytes, but are reduced on CD4+, CD8+ double positive cells undergoing selection in the thymus, and are entirely eliminated when these cells differentiate into immunologically competent CD4+ or CD8+ single positive T cells. To examine the potential roles of this molecule in T cell development and selection, we generated transgenic mice in which HSA was highly expressed on all classes of thymocytes. The consequence of deregulated HSA expression was a pronounced reduction in the numbers of double positive and single positive thymocytes, whereas the numbers of their double negative precursors were largely unaffected. These results demonstrate that downregulation of HSA expression at the double positive stage is a critical event in thymocyte development. The depletion of thymocytes resulting from HSA overexpression begins at the same time as the onset of negative selection, suggesting that HSA may provide signals that contribute to determining the efficiency of this process.

Positive selection of V beta 8+ CD4-8- thymocytes by class I molecules expressed by hematopoietic cells

A small subset of T cells of mature phenotype express the alpha/beta T cell receptor, but not CD4 and CD8 coreceptors (alpha/beta double-negative [DN] cells). The repertoire of V beta usage of alpha/beta DN cells is strongly biased towards V beta 8 expression, suggesting that the formation of the population is subject to selection. We now report that deficiency of class I expression leads to a strongly depressed frequency of V beta 8+ DN cells, but has little effect on V beta 8- DN cells. Studies of hematopoietic chimeras between class I+ and class I- mice demonstrated that expression of class I molecules by hematopoietic cells is necessary and sufficient for selection of most V beta 8 DN cells. The lack of a role for class I expression by thymic epithelial cells suggests that the mechanism of selection of these cells by class I differs significantly from the mechanism of selection of conventional T cells. Models to explain the selection of these cells as well as their possible function in vivo are discussed.

Analysis of T cell antigen receptor (TCR) expression by human peripheral blood CD4-8- alpha/beta T cells demonstrates preferential use of several V beta genes and an invariant TCR alpha chain

CD4-CD8- (double negative [DN]) alpha/beta T cells are a largely uncharacterized subpopulation of unknown function. To investigate whether these cells are selected to recognize particular antigens or antigen-presenting molecules, DN alpha/beta T cells were purified from the peripheral blood of five normal donors and their T cell receptor (TCR) alpha and beta chains were examined. Random cloning of TCR alpha chains by single-sided polymerase chain reaction (PCR) amplification identified an invariant rearrangement between V alpha 24 and J alpha Q, with no N region diversity, which was expressed preferentially by DN alpha/beta T cells from all donors. Random cloning also identified a precise V alpha 7.2-J alpha (IGRJa14) rearrangement, with two variable amino acids encoded in the V-J junction, which was enriched in the DN alpha/beta T cell preparations from some, but not all, donors. Analysis of TCR beta chains by quantitative PCR amplification demonstrated that the expression of four V beta gene families, V beta 2, 8, 11, and 13, was markedly increased in these DN alpha/beta T cell preparations. The expression of particular TCRs by DN alpha/beta T cells from multiple donors indicates that these cells, or at least a subpopulation of cells with this phenotype, recognize a limited spectrum of antigens and suggests that they may use nonpolymorphic antigen-presenting molecules.

Irreversible suppression of CD8 expression in CD4-CD8+ thymocytes upon in vitro stimulation

CD8 (Ly-2) expression was suppressed in purified murine CD4-CD8+ thymocytes at the mRNA level upon continuous stimulation with PMA and ionomycin in the presence of rIL-2. The level of CD8 expression on CD4-CD8+ thymocytes was reduced gradually during the culture and a majority of them turned into CD4-CD8- cells after 48 hr. This suppression was not transient, since CD8 expression was not recovered on these cells in additional 48 hr of culture without PMA and ionomycin. The suppression was dependent on the concentrations of PMA and ionomycin, and inhibited by adding an immunosuppressant, CsA to the culture. Treatment with either PMA or ionomycin alone did not induce suppression of CD8. Crosslinking of CD3-epsilon chains also induced suppression of CD8 for a part of CD4-CD8+ thymocytes. Interestingly, CD8 expression was hardly suppressed in CD4-CD8+ peripheral T lymphocytes, suggesting that the mechanisms of suppression of CD8 is developmentally regulated. We propose that the suppression of CD8 expression at CD4-CD8+ stage involves an additional mechanism of negative selection of thymic T cells.

MHC class I-deficient mice

A great deal has already been learned from the analysis of beta 2m-mutant mice, but it is clear that a great deal remains to be learned. A significant (though unanticipated) problem with this model system is that it is functionally leaky: residual functional class I expression can be detected in beta 2m- mice, and small numbers of functional CD8+ lymphocytes are present in the animals. In many cases, this has frustrated the initial attempts at obtaining immediate definitive resolution of important questions regarding the function of class I molecules. This has occurred primarily in instances in which the class I-deficient mice fail to express an expected phenotype--for example, in studies showing that beta 2m- mice make adequate protective immune responses against certain intracellular pathogens, and are able to reject some allogeneic tissues with a relatively normal pace. On the other hand, it appears that combining the use of beta 2m- mice with other methods (for example, antibody-mediated depletion of CD8+ T cells) is usually adequate to circumvent these difficulties. It remains to be seen whether other better class I deficiencies can be engineered--for example, large deletions of class I genes or mutations in transcription factors essential for class I gene expression. The extent of immunocompetence of beta 2m- mice was somewhat surprising. It was widely expected that class I-deficient mice would be exquisitely sensitive to many viral infections, though the results indicate that sensitivity varies dramatically with the virus and conditions of infection. However, it appears that in lieu of one major arm of the immune system, compensatory immune mechanisms are in many cases able to deal with infection. Similar conclusions are developing from the analysis of several other recently generated mutant mice. Nevertheless, the results indicate a very important role for class I-directed responses in clearing infections mediated by various viral and parasitic agents, particularly in the case of more severe conditions of infection. Although the class I-deficient mice were initially considered primarily a vehicle for analysis of the role of CD8+ T cells, evidence is accumulating that they manifest deficiencies in several other types of lymphocytes, including NK cells, TCR alpha beta+CD4-CD8- cells, and a subset of TCR gamma delta+ cells. This has been a boon for analysis of the development of these cells, but at the same time it has created difficulties in assigning a biological effect of the mutation to a specific lymphocyte deficiency.(ABSTRACT TRUNCATED AT 400 WORDS)

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)

CD4-CD8- T cell receptor alpha beta T cells: generation of an in vitro major histocompatibility complex class I specific cytotoxic T lymphocyte response and allogeneic tumor rejection

The generation of an in vitro major histocompatibility complex class I specific response of CD4-CD8- T cell receptor (TCR) alpha beta cytotoxic T lymphocytes (CTL) and their allogeneic tumor rejection were investigated. Inocula of BALBRL male 1 were rejected in C57BL/6 (B6) mice treated with minimum essential medium (MEM) (control), anti-L3T4 (CD4) monoclonal antibody (mAb) or anti-Lyt-2.2 (CD8) mAb and CTL against the tumor were generated in vitro. No rejection and no induction of CTL were observed in B6 mice treated with anti-L3T4 (CD4) plus anti-Lyt-2.2 (CD8) mAb. CTL with the classical Thy-1+ CD3+CD4-CD8+ TCR alpha beta phenotype were generated in mixed lymphocyte tumor cell culture (MLTC) spleen cells from B6 mice treated with MEM (control) or anti-L3T4 (CD4) mAb, whereas CTL with an unusual Thy-1+CD3+CD4-CD8- TCR alpha beta phenotype were generated in MLTC spleen cells from anti-Lyt-2.2 (CD8) mAb-treated B6 mice. Both types of CTL were reactive with both H-2Kd and Dd (Ld) class I antigen. These findings suggest that when CD4+ cells were blocked by anti-L3T4 (CD4) mAb, CD8+ CTL mediated rejection, and when CD8+ cells were blocked by anti-Lyt-2.2 (CD8) mAb, CD4+ cells were capable of mediating rejection, although less efficiently than CD8+ cells, by inducing CD4-CD8- TCR alpha beta CTL. The finding that adoptive transfer of CD4 and CD8-depleted MLTC spleen cells, obtained from anti-Lyt-2.2 (CD8) mAb-treated B6 mice that had rejected BALBRL male 1, resulted in rejection of BALBRL male 1 inoculated into B6 nu/nu mice confirmed the above notion. CTL clones with the CD4-CD8- TCR alpha beta phenotype specific for Ld were established.

Effects of cyclosporin A on T-cell development in organ-cultured foetal thymus

The effects of cyclosporin A (CsA) on T-cell development were assessed in an organ culture of murine foetal thymus. Applying three-colour flow cytometric analysis, we showed that the agent inhibits the development of mature CD3/T-cell receptor alpha beta (TcR alpha beta)+ cells both in CD4+8- and CD4-8+ populations. CD4-8- cells appeared to be accumulated by CsA. We examined the heterogeneity of CD4-8- cells generated in the organ culture, and defined five subpopulations by the expression of the cell-surface molecules CD3/TcR, J11d and CD25. It has been demonstrated that only the CD3/TcR alpha beta+ J11d- CD25- subpopulation is susceptible to the suppressive effects of CsA among CD4-8- cells, whereas all the other four subpopulations, including CD3/TcR gamma delta+ cells, are resistant. Thus, all of the TcR alpha beta-bearing cells, including CD4-8- cells but none of the TcR alpha beta- cells, are CsA sensitive. Because it is known that CsA inhibits the TcR-mediated signalling events in mature T cells and that signallings mediated via the interaction of TcR with major histocompatibility complex (MHC) molecules on thymic stroma cells are crucial for thymic selection of T cells, these results indicate that TcR alpha beta-bearing CD4-8- cells but not TcR gamma delta-bearing CD4-8- cells undergo thymic positive selection.

A synthetic thymic hormone, THF-gamma 2, repairs immunodeficiency of mice cured of plasmacytoma by melphalan

BALB/c mice cured of large MOPC-315 plasmacytomas by melphalan remain deficient in their spleen T-cell functions. This was manifested by impairment of the allogeneic and the antibody responses in vitro to SRBC and in decreased numbers of T-cells including their subsets CD4 and CD8. IL-2 production and specific cytotoxicity against MOPC-315 tumor cells were, on the other hand, maintained. Treatment of these cured mice by in-vivo administration of THF-gamma 2, an octapeptide from calf thymus, repaired these deficits. This was evidenced by in vitro tests with spleen cells which manifested an increased allogeneic response and elevated generation of primary antibody response, restoration of T-cell subpopulations to normal and an enhanced IL-2 production above normal levels. The potential use of THF-gamma 2 as supportive therapy in cancer treatment is suggested.

Tolerance-related V beta clonal deletions in normal CD4-8-, TCR-alpha/beta + and abnormal lpr and gld cell populations

We have analyzed tolerance-related clonal deletion of Mls-and I-E-reactive thymocytes at the RNA level using a multi-V beta probe RNAse protection assay, and used this phenomenon to identify the maturation stage of the abnormally expanded CD4-8-, TCR-alpha/beta + subset in lpr and gld homozygous mice, and of the phenotypically similar minor thymocyte subset found in normal mice. Essentially complete V beta clonal deletions were detected in lpr and gld cells of all appropriate background strains. Substantial, but not complete, V beta clonal deletions were also detected in the CD4-8- TCR-alpha/beta + subset of normal mice. Since expression of CD4/CD8 is required for V beta clonal deletions to occur, we conclude that lpr and gld cells, and at least a portion of CD4-8- TCR-alpha/beta + thymocytes in normal mice, are derived by secondary loss of CD4/CD8 accessory molecules from more mature CD4+8+ precursors. One possible interpretation of these findings is that such CD4/CD8 loss may affect a class of self-reactive thymocytes that have escaped direct clonal deletion. Exportation and expansion of such cells in the periphery may be an important contributory factor in the induction of systemic autoimmunity.

Expression and function of CD2 during murine thymocyte ontogeny

CD2, originally recognized as the sheep erythrocyte receptor of human T cells, has been implicated in early T cell development in the thymus. In this report, we examined the expression and functional role of CD2 during murine thymocyte ontogeny by using monoclonal antibodies to murine CD2. Surface expression of CD2 was first detected in Thy-1+ fetal thymocytes at day 14 of gestation and it progressively increased during CD4-CD8- phenotype. Surface IL 2 receptor (CD25) expression was readily detected in surface CD2- cells at day 13 of gestation and the majority of CD2+ cells appeared to be generated from CD25+ cells thereafter. In adult CD4-CD8- thymocytes, the expression of CD2 and CD25 was mutually exclusive. These results indicate that surface CD2 expression is not a prerequisite for CD25 induction during murine thymocyte ontogeny. This was further confirmed by fetal thymus organ culture in which anti-murine CD2 mAb was included. The antibody treatment led to a suppressed CD2 expression on thymocytes; however, there was no effect on the appearance of CD25. Moreover, no influence on the development of mature CD3+ thymocytes was observed after fetal thymus organ culture in the presence of anti-murine CD2 mAb, and a substantial number of CD3+CD2- cells was demonstrated in fetal and adult CD4-CD8- thymocytes. These findings argue against the functional relevance of CD2 expression during early T cell development as proposed in humans.

The methylation state of the T cell antigen receptor beta chain gene in subpopulations of mouse thymocytes

Previous analyses of T cell receptor beta chain (TcR beta) genomic DNA from subsets of human peripheral blood leukocytes suggested that the TcR beta methylation pattern might reflect distinct differentiation pathways. The studies presented here, using murine thymocyte subsets, have specifically addressed the question of whether methylation of TcR beta DNA is related to the cellular maturity and type of TcR beta mRNA expressed in the different subsets. We have observed that the DNA isolated from either CD4+ or CD8+ thymocytes, the more mature thymic subsets, is less methylated in the TcR beta region than DNA isolated from the CD4-CD8-, double-negative population containing the more immature thymocytes. In addition, this pattern of DNA methylation is directly related to the ratio of 1.3-kb to 1.0-kb TcR beta mRNA seen in these different cell types. Although a quantitative difference in the level of TcR beta mRNA was noted for the two mature subsets, no qualitative difference in the ratio of 1.3-kb to 1.0-kb mRNA was detected. Furthermore, these DNA methylation patterns appear to be lineage related, because the TcR beta region of genomic DNA isolated from mouse macrophages is heavily methylated.

Mouse strain variation in Ly-24 (Pgp-1) expression by peripheral T cells and thymocytes: implications for T cell differentiation

The cell surface glycoprotein Ly-24 has been proposed as a useful marker for the identification of in vivo-primed T cells. Analysis of Ly-24 surface expression by T cells from different mouse strains has shown variation in Ly-24 expression that is not H-2 linked; however, mice of the Ly-24.1 allele (e.g. BALB/c) express relatively high amounts, whereas Ly-24.2 strains (e.g. C57BL/6) are low expressors. In BALB/c (Ly-24 high) and C57BL/6 (Ly-24 low) mice, Ly-24 was expressed by both CD4- CD8+ and CD4+ CD8- subpopulations of single-positive T cells and thymocytes Among CD4- CD8- thymocytes, the overall expression of Ly-24 was similar in both mouse strains. Analysis of CD4+ and CD8+ single-positive thymocytes from newborn and adult BALB/c mice showed that the neonatal population contained fewer Ly-24+ cells. However, using the cell surface markers J11d and CD3, neonatal single-positive thymocytes were found to contain larger numbers of cells with the Ly-24-J11d+CD3 low to negative phenotype. Taken together, these results show that in BALB/c (Ly-24 high) mice, as soon as functional mature phenotype (CD3+) CD4+ and CD8+ single-positive thymocytes are generated, they already express Ly-24. These data cast doubt on the usefulness of Ly-24 expression as a universal marker of in vivo-primed T cells and suggest that in BALB/c mice thymus migrants may well be Ly-24+. Expression of Ly-24 by thymocytes is discussed in the context of current models of intrathymic T cell differentiation.

Functional responsiveness in vitro and in vivo of alpha/beta T cell receptors expressed by the B2A2 (J11d)- subset of CD4-8- thymocytes

B2A2-CD4-8- cells represent a rare subpopulation of thymocytes normally comprising 0.5% of the total adult thymus. These cells are known to express CD3-associated T cell receptor (TcR) alpha/beta molecules. In the present study we have examined the functional capacity of alpha/beta heterodimers on B2A2-CD4-8- cells. In the presence of monoclonal antibody (mAb) specific for either murine CD3 or TcR expressing the V beta 8-encoded beta chain (F23.1), B2A2-CD4-8- cells proliferated. Such proliferation was blocked by mAb to interleukin 2 receptor (IL 2R), suggesting an autocrine mechanism involving IL 2 production and subsequent utilization. IL 2 and also IL 3 production by mAb-stimulated B2A2-CD4-8- cells was directly confirmed. Furthermore, a panel of B2A2-CD4-8- clones were derived to assess the role of the TcR in cytolysis. Many clones were isolated which killed Fc receptor-bearing P815 target cells only in the presence of F23.1 mAb. Finally, in vivo treatment of neonatal mice with F23.1 mAb resulted in a marked reduction of V beta 8+ B2A2-CD4-8- thymocytes. Collectively, these results indicate that the TcR alpha/beta complex on CD4-CD8-B2A2- cells is fully capable of transducing signals that lead to proliferation, lymphokine production and cytolysis in vitro, as well as to disappearance of this subset from the thymus in vivo.

Tissue-specific and cell surface expression of human major histocompatibility complex class I heavy (HLA-B7) and light (beta 2-microglobulin) chain genes in transgenic mice

We introduced the human genes HLA-B7 and B2M encoding the heavy (HLA-B7) and light [beta 2-microglobulin (beta 2m)] chains of a human major histocompatibility complex class I antigen into separate lines of transgenic mice. The tissue-specific pattern of HLA-B7 RNA expression was similar to that of endogenous class I H-2 genes, although the HLA-B7 gene was about 10-fold underexpressed in liver. Identical patterns of RNA expression were detected whether the HLA-B7 gene contained 12 or 0.66 kilobase(s) (kb) of 5' flanking sequence. The level of expression was copy number dependent and as efficient as that of H-2 genes; gamma interferon enhanced HLA-B7 RNA expression in parallel to that of H-2. In addition to the mechanism(s) responsible for gamma interferon-enhanced expression, there must be at least one other tissue-specific mechanism controlling the constitutive levels of class I RNA. Tissue-specific human beta 2m RNA expression was similar to that of mouse beta 2m, including high-level expression in liver. Cell surface HLA-B7 increased 10- to 17-fold on T cells and on a subset of thymocytes from HLA-B7/B2M doubly transgenic mice compared to HLA-B7 singly transgenic mice. The pattern of expression of HLA-B7 on thymocytes resembled that of H-2K as opposed to H-2D. These results confirm that coexpression of both human chains is required for efficient surface expression and that HLA-B7 may share a regulatory mechanism with H-2K, which distinguishes it from H-2D.

In vivo dynamics of CD4-8- thymocytes. Proliferation, renewal and differentiation of different cell subsets studied by DNA biosynthetic labeling and surface antigen detection

The proliferative status of CD4-8- thymocyte subsets was determined by in vivo or in vitro labeling with bromodeoxyuridine (BrdUrd), a nonreutilized thymidine analogue detectable with a monoclonal antibody, simultaneously with relevant surface proteins. An actively cycling subset [J11d+, interleukin 2 receptor-positive (IL 2R+)] was defined, besides a relatively resting one (J11d-, Pgp1+, T cell receptor-positive). Continuous per os administration of BrdUrd showed that 85% only of CD4-8- thymocytes were labeled in 6 days confirming the existence of a relatively long-term resting subset. By contrast, CD4+8+ thymocytes were all labeled in 3-4 days. Observation of labeled CD4-8- cells after pulse labeling showed an immediate decrease of their absolute number per thymus, confirming their low autorenewal capacity. However, a small number of labeled cells which were hydroxyurea or colchicine resistant remained CD4-8- for several days and progressively acquired surface expression of IL 2R. IL 2R expression by cycling CD4-8- cells during thymus regeneration after antimitogenic drug treatment was rapid, but very transient. According to these results most CD4-8- thymocytes appear as largely engaged in a proliferation-dependent differentiative process, and do not behave as true stem cells. Consequently, this subset is principally renewed by thymic immigration of exogenously produced resting cells. However, a tenfold expansion of CD4-8- cells was found in the fetal and regenerating thymus, suggesting two proliferative phases during intrathymic CD4-8- cell maturation, the first one yielding to cell expansion and the second to cell differentiation. A tentative evaluation of daily cell immigration is proposed starting with the determination of the number of cells beginning DNA synthesis each day. A global model is finally discussed by confronting our kinetic results with the known reconstitution capacities of CD4-8- thymocyte subsets.