Interactions between double positive thymocytes and high affinity ligands presented by cortical epithelial cells generate double negative thymocytes with T cell regulatory activity

Exosomes in malignant pleural effusion from lung cancer patients impaired the cytotoxicity of double-negative T cells

CD3+CD4-CD8- double-negative T (DNT) cells are new weapons in cancer immunotherapy. Here, we explored DNT cells in malignant pleural effusions (MPEs) from lung cancer patients. DNT cells, especially TCRαβ+CD56- DNT cells, were increased in MPE from lung cancer patients. DNT cells highly expressed PD-1, TRAIL, NKG2D and DNAM-1. In contrast, FasL was barely detected in DNT cells. Compared with non-MPE cells, MPE-derived DNT cells expressed much higher levels of PD-1 and TRAIL. DNT cells from healthy peripheral blood donors potentially killed lung cancers, which was decreased by MPE supernatant. Exosomes from MPE supernatant expressed PD-1 and CEACAM1 and impaired the cytotoxicity of DNT cells. Blocking PD-1 and TIM3 rescued the cytotoxicity of DNT cells treated with MPE-derived exosomes. Overall, we demonstrated that the frequency of DNT cells in MPE from lung cancer patients was increased and that MPE-derived exosomes impaired the cytotoxicity of DNT cells via the PD-1/PD-L1 and CEACAM1/TIM3 pathways.

CD3+CD4-CD8- (Double-Negative) T Cells in Inflammation, Immune Disorders and Cancer

The crucial role of CD4⁺ and CD8⁺ T cells in shaping and controlling immune responses during immune disease and cancer development has been well established and used to achieve marked clinical benefits. CD3⁺CD4^-CD8^- double-negative (DN) T cells, although constituting a rare subset of peripheral T cells, are gaining interest for their roles in inflammation, immune disease and cancer. Herein, we comprehensively review the origin, distribution and functions of this unique T cell subgroup. First, we focused on characterizing multifunctional DN T cells in various immune responses. DN regulatory T cells have the capacity to prevent graft-versus-host disease and have therapeutic value for autoimmune disease. T helper-like DN T cells protect against or promote inflammation and virus infection depending on the specific settings and promote certain autoimmune disease. Notably, we clarified the role of DN tumor-infiltrating lymphocytes and outlined the potential for malignant proliferation of DN T cells. Finally, we reviewed the recent advances in the applications of DN T cell-based therapy for cancer. In conclusion, a better understanding of the heterogeneity and functions of DN T cells may help to develop DN T cells as a potential therapeutic tool for inflammation, immune disorders and cancer.

Double-negative T cells in autoimmune diseases

PURPOSE OF REVIEW

TCRαβ+CD4-CD8- double-negative T (DNT) cells, a principal subset of mature T lymphocytes, have been closely linked with autoimmune/inflammatory conditions. However, controversy persists regarding their ontogeny and function. Here, we present an overview on DNT cells in different autoimmune diseases to advance a deeper understanding of the contribution of this population to disease pathogenesis.

RECENT FINDINGS

DNT cells have been characterized in various chronic inflammatory diseases and they have been proposed to display pathogenic or regulatory function. The tissue location of DNT cells and the effector cytokines they produce bespeak to their active involvement in chronic inflammatory diseases.

SUMMARY

By producing various cytokines, expanded DNT cells in inflamed tissues contribute to the pathogenesis of a variety of autoimmune inflammatory diseases. However, it is unclear whether this population represents a stable lineage consisting of different subsets similar to CD4+ T helper cell subset. Better understanding of the possible heterogeneity and plasticity of DNT cells is needed to reveal interventional therapeutic opportunities.

HIV Infection and Persistence in Pulmonary Mucosal Double Negative T Cells In Vivo

The lungs are relatively unexplored anatomical human immunodeficiency virus (HIV) reservoirs in the antiretroviral therapy (ART) era. Double negative (DN) T cells are a subset of T cells that lack expression of CD4 and CD8 (CD4^- CD8^-) and may have both regulatory and effector functions during HIV infection. Notably, circulating DN T cells were previously described as cellular HIV reservoirs. Here, we undertook a thorough analysis of pulmonary versus blood DN T cells of people living with HIV (PLWH) under ART. Bronchoalveolar lavage (BAL) fluid and matched peripheral blood were collected from 35 PLWH on ART and 16 uninfected volunteers without respiratory symptoms. Both PLWH and HIV-negative (HIV^-) adults displayed higher frequencies of DN T cells in BAL versus blood, and these cells mostly exhibited an effector memory phenotype. In PLWH, pulmonary mucosal DN T cells expressed higher levels of HLA-DR and several cellular markers associated with HIV persistence (CCR6, CXCR3, and PD-1) than blood. We also observed that DN T cells were less senescent (CD28^- CD57⁺) and expressed less immunosuppressive ectonucleotidase (CD73/CD39), granzyme B, and perforin in the BAL fluid than in the blood of PLWH. Importantly, fluorescence-activated cell sorter (FACS)-sorted DN T cells from the BAL fluid of PLWH under suppressive ART harbored HIV DNA. Using the humanized bone marrow-liver-thymus (hu-BLT) mouse model of HIV infection, we observed higher infection frequencies of lung DN T cells than those of the blood and spleen in both early and late HIV infection. Overall, our findings show that HIV is seeded in pulmonary mucosal DN T cells early following infection and persists in these potential cellular HIV reservoirs even during long-term ART.IMPORTANCE Reservoirs of HIV during ART are the primary reasons why HIV/AIDS remains an incurable disease. Indeed, HIV remains latent and unreachable by antiretrovirals in cellular and anatomical sanctuaries, preventing its eradication. The lungs have received very little attention compared to other anatomical reservoirs despite being immunological effector sites exhibiting characteristics ideal for HIV persistence. Furthermore, PLWH suffer from a high burden of pulmonary non-opportunistic infections, suggesting impaired pulmonary immunity despite ART. Meanwhile, various immune cell populations have been proposed to be cellular reservoirs in blood, including CD4^- CD8^- DN T cells, a subset that may originate from CD4 downregulation by HIV proteins. The present study aims to describe DN T cells in human and humanized mice lungs in relation to intrapulmonary HIV burden. The characterization of DN T cells as cellular HIV reservoirs and the lungs as an anatomical HIV reservoir will contribute to the development of targeted HIV eradication strategies.

DNT Cell-based Immunotherapy: Progress and Applications

Cancer immunotherapy has firmly established a dominant status in recent years. Adoptive cellular immunotherapy (ACI) is the main branch of immunotherapy. Recently, the immune effector cells of ACI, such as T cells, NK cells, and genetically engineered cells, have been used to achieve significant clinical benefits in the treatment of malignant tumors. However, the clinical applications have limitations, including toxicity, unexpectedly low efficiency, high costs and strict technical requirements. More exploration is needed to optimize ACI for cancer patients. CD3+CD4-CD8- double negative T cells (DNTs) have emerged as functional antitumor effector cells, according to the definition of adoptive immunotherapy. They constitute a kind of T cell subset that mediates nontumor antigen-restricted immunity and has important immune regulatory functions. Preclinical experiments showed that DNTs had a dual effect by killing tumor cells and inhibiting graft-versus-host disease. Notably, DNTs can be acquired from healthy donors and expanded in vitro; thus, allogeneic DNTs may be provided as “off-the-shelf” cellular products that can be readily available for direct clinical application. We review the progress and application of DNTs in immunotherapy. DNTs may provide some novel perspectives on cancer immunotherapy.

TCRαβ+CD3+CD4-CD8- (double negative) T cells in autoimmunity

TCRαβ⁺CD3⁺CD4^-CD8^- "double negative" (DN) T cells comprise a small subset of mature peripheral T cells. The origin and function of DN T cells are somewhat unclear and discussed controversially. While DN T cells resemble a rare and heterogeneous T cell subpopulation in healthy individuals, numbers of TCRαβ⁺ DN T cells are expanded in several inflammatory conditions, where they also exhibit distinct effector phenotypes and infiltrate inflamed tissues. Thus, DN T cells may be involved in systemic inflammation and tissue damage in autoimmune/inflammatory conditions, including SLE, Sjögren's syndrome, and psoriasis. Here, the current understanding of the origin and phenotype of DN T cells, and their role in the instruction of immune responses, autoimmunity and inflammation will be discussed in health and disease.

Temporal Expression of Bim Limits the Development of Agonist-Selected Thymocytes and Skews Their TCRβ Repertoire

CD8αα TCRαβ⁺ intestinal intraepithelial lymphocytes play a critical role in promoting intestinal homeostasis, although mechanisms controlling their development and peripheral homeostasis remain unclear. In this study, we examined the spatiotemporal role of Bim in the thymic selection of CD8αα precursors and the fate of these cells in the periphery. We found that T cell-specific expression of Bim during early/cortical, but not late/medullary, thymic development controls the agonist selection of CD8αα precursors and limits their private TCRβ repertoire. During this process, agonist-selected double-positive cells lose CD4/8 coreceptor expression and masquerade as double-negative (DN) TCRαβ^hi thymocytes. Although these DN thymocytes fail to re-express coreceptors after OP9-DL1 culture, they eventually mature and accumulate in the spleen where TCR and IL-15/STAT5 signaling promotes their conversion to CD8αα cells and their expression of gut-homing receptors. Adoptive transfer of splenic DN cells gives rise to CD8αα cells in the gut, establishing their precursor relationship in vivo. Interestingly, Bim does not restrict the IL-15-driven maturation of CD8αα cells that is critical for intestinal homeostasis. Thus, we found a temporal and tissue-specific role for Bim in limiting thymic agonist selection of CD8αα precursors and their TCRβ repertoire, but not in the maintenance of CD8αα intraepithelial lymphocytes in the intestine.

MHC class II restricted innate-like double negative T cells contribute to optimal primary and secondary immunity to Leishmania major

Although it is generally believed that CD4⁺ T cells play important roles in anti-Leishmania immunity, some studies suggest that they may be dispensable, and that MHC II-restricted CD3⁺CD4⁻CD8⁻ (double negative, DN) T cells may be more important in regulating primary anti-Leishmania immunity. In addition, while there are reports of increased numbers of DN T cells in Leishmania-infected patients, dogs and mice, concrete evidence implicating these cells in secondary anti-Leishmania immunity has not yet been documented. Here, we report that DN T cells extensively proliferate and produce effector cytokines (IFN-γ, TNF and IL-17) and granzyme B (GrzB) in the draining lymph nodes and spleens of mice following primary and secondary L. major infections. DN T cells from healed mice display functional characteristics of protective anti-Leishmania memory-like cells: rapid and extensive proliferation and effector cytokines production following L. major challenge in vitro and in vivo. DN T cells express predominantly (> 95%) alpha-beta T cell receptor (αβ TCR), are Leishmania-specific, restricted mostly by MHC class II molecules and display transcriptional profile of innate-like genes. Using in vivo depletion and adoptive transfer studies, we show that DN T cells contribute to optimal primary and secondary anti-Leishmania immunity in mice. These results directly identify DN T cells as important players in effective and protective primary and secondary anti-L. major immunity in experimental cutaneous leishmaniasis.

Although it is generally believed that CD4⁺ T cells mediate anti-Leishmania immunity, some studies suggest that CD3⁺CD4⁻CD8⁻ (double negative, DN) T cells may play a more important role in regulating primary anti-Leishmania immunity. Here, we report that DN T cells extensively proliferate and produce effector cytokines in mice following primary and secondary L. major infections. Leishmania-reactive DN T cells utilize αβ T cell receptor (TCR) and are restricted by MHC class II molecules. Strikingly, DN T cells from healed mice display functional characteristics of protective anti-Leishmania memory-like cells: rapid and extensive proliferation, effector cytokine production in vitro and in vivo, and accelerated parasite control following secondary L. major challenge. These results directly identify DN T cells as important players in protective primary and secondary anti-L. major immunity in experimental cutaneous leishmaniasis.

The unique surface molecules on intestinal intraepithelial lymphocytes: from tethering to recognizing

Interspersed among epithelial cells (ECs), intraepithelial lymphocytes (IELs) might be important constituents of the physiological and immunological barriers of the intestinal epithelial layer. IELs are composed of memory-effector T cell subtypes bearing the T cell receptor-γδ (TCRγδ) and TCRαβ. The intimate cell adhesion molecules- and tight junction proteins-mediated biological interactions between IELs and ECs ensure that IELs can reside within the intraepithelial compartment and survey large areas of the villus epithelium. As sentinels in this critical interface, IELs express TCRs that recognize antigenic peptides presented by conventional major histocompatibility complex (MHC) molecules or by non-classical MHC molecules. Moreover, IELs monitor for stressed or damaged ECs to mediate pathological responses and maintain intestinal homeostasis. In this review, we address how IELs reside within the epithelium and exert their sentinel functions.

Clonal deletion and the fate of autoreactive thymocytes that survive negative selection

Clonal deletion of autoreactive thymocytes is important for self-tolerance, but the intra-thymic signals that induce clonal deletion have not been clearly identified. We now report that clonal deletion during negative selection requires CD28 costimulation of autoreactive thymocytes at the CD4⁺CD8^lo intermediate stage of differentiation. Autoreactive thymocytes were prevented from undergoing clonal deletion by either absent CD28 costimulation or transgenic over-expression of the anti-apoptotic factors Bcl-2 or Mcl-1, with surviving thymocytes differentiating into anergic T cell receptor αβ⁺ double negative thymocytes that preferentially migrated to the intestine where they re-expressed CD8α and were sequestered as CD8αα intraepithelial lymphocytes (IELs). This study identifies CD28 costimulation as the intrathymic signal required for clonal deletion and identifies CD8αα IELs as the developmental fate of autoreactive thymocytes that survive negative selection.

Double-negative T cells during HIV/SIV infections: potential pinch hitters in the T-cell lineup

PURPOSE OF REVIEW

This review summarizes the role of CD3+CD4-CD8- double-negative T cells, which have both regulatory and helper T-cell functions and may have the potential to compensate for the reduced levels of CD4 T cells during SIV/HIV infection.

RECENT FINDINGS

Double-negative T cells have been characterized in several human diseases and in murine models of autoimmunity and transplantation, where they exhibit both immunoregulatory and helper T-cell-like function. During the natural nonpathogenic SIV infection of African nonhuman primates, the lack of clinical disease progression is associated with the presence of double-negative T cells that maintain helper T-cell functions while remaining refractory to viral infection. Moreover, DN T cells may compensate for very low levels of CD4+ T cells observed in a cohort of SIV-infected sooty mangabeys that have remained free of clinical AIDS for over 10 years. These studies identify a potential for double-negative T cells to provide critical helper function during HIV infection.

SUMMARY

Double-negative T cells with some CD4+ T-cell functions are associated with a nonpathogenic outcome during SIV infection and represent a potential immune therapeutic target in HIV-infected patients.

Inflamed gut mucosa: downstream of interleukin-10

BACKGROUND

Interleukin-10 is a pleiotropic cytokine, whose main function is limitation and ultimately termination of immune responses. This is especially true for environmental interfaces such as the gastrointestinal tract. IL-10 acts as a key mediator for maintaining gut homeostasis. IL-10 knockout mice are well established as a genetic model for inflammatory bowel disease (IBD), and sequence variants in the IL-10 locus contribute to ulcerative colitis (UC).

DESIGN

This review covers the significance of IL-10 signalling in the intestinal immune response both in health and disease. It explains the biological role of IL-10, its deregulation in IBD and its contribution to intestinal inflammation via endoplasmic reticulum stress response.

RESULTS

Many IBD susceptibility genes have been discovered in the past years, linking fundamental biological systems, like innate and adaptive immunity, stress responses, autophagy and mucosal barrier to the pathogenesis of Crohn's disease (CD) and UC. IL-10 has long been known for its substantial role in regulating gut immunity, but its contribution to IBD was somewhat elusive. A recent study identified mutations in either IL-10 receptor subunits that are associated with early-onset enterocolitis, a severe phenotype of IBD. Other than genetic variants of IL-10 receptors, IL-10 and STAT3 genes are also associated with IBD, emphasizing the involvement of the IL-10 signalling cascade in the pathogenesis of CD and UC.

CONCLUSIONS

  The discovery of inherited deregulations in the IL-10 signalling cascade is not only considered the missing link between IL-10 and intestinal homeostasis, but also demonstrates how findings made in animal models help explaining human disease.

The light and dark sides of intestinal intraepithelial lymphocytes

The intraepithelial lymphocytes (IELs) that reside within the epithelium of the intestine form one of the main branches of the immune system. As IELs are located at this critical interface between the core of the body and the outside environment, they must balance protective immunity with an ability to safeguard the integrity of the epithelial barrier: failure to do so would compromise homeostasis of the organism. In this Review, we address how the unique development and functions of intestinal IELs allow them to achieve this balance.

CD3+CD4-CD8- (double negative) T cells: saviours or villains of the immune response?

Recent studies have shown that T cells are not just the latecomers in inflammation but might also play a key role in the early phase of this response. In this context, a number of T cell subsets including NKT cells, mucosal-associated invariant T cells and γ/δ T cells have been shown, together with classical innate immune cells, to contribute significantly to the development and establishment of acute and chronic inflammatory diseases. In this commentary we will focus our attention on a somewhat neglected class of T cells called CD3(+)CD4(-)CD8(-) double negative T cells and on their role in inflammation and autoimmunity. We will summarize the most recent views on their origin at the thymic and peripheral levels as well as their tissue localization in immune and non-lymphoid organs. We will then outline their potential pathogenic role in autoimmunity as well as their homeostatic role in suppressing excessive immune responses deleterious to the host. Finally, we will discuss the potential therapeutic benefits or disadvantages of targeting CD3(+)CD4(-)CD8(-) double negative T cells for the treatment of autoimmune disease. We hope that this overview will shed some light on the function of these immune cells and attract the interest of investigators aiming at the design of novel therapeutic approaches for the treatment of autoimmune and inflammatory conditions.

Double negative (CD3+ 4- 8-) TCR alphabeta splenic cells from young NOD mice provide long-lasting protection against type 1 diabetes

BACKGROUND

Double negative CD3(+)4(-)8(-) TCR alphabeta splenic cells (DNCD3) can suppress the immune responses to allo and xenografts, infectious agents, tumors, and some autoimmune disorders. However, little is known about their role in autoimmune diabetes, a disease characterized by the reduction of insulin production subsequent to destruction of pancreatic beta-cells by a polyclonal population of self-reactive T-cells. Herein, we analyzed the function and phenotype of DNCD3 splenic cells in young NOD mice predisposed to several autoimmune disorders among which, the human-like autoimmune diabetes.

METHODOLOGY/PRINCIPAL FINDINGS

DNCD3 splenic cells from young NOD mice (1) provided long-lasting protection against diabetes transfer in NOD/Scid immunodeficient mice, (2) proliferated and differentiated in the spleen and pancreas of NOD/Scid mice and pre-diabetic NOD mice into IL-10-secreting T(R)-1 like cells in a Th2-like environment, and (3) their anti-diabetogenic phenotype is CD3(+)(CD4(-)CD8(-))CD28(+)CD69(+)CD25(low) Foxp3(-) iCTLA-4(-)TCR alphabeta(+) with a predominant Vbeta13 gene usage.

CONCLUSIONS/SIGNIFICANCE

These findings delineate a new T regulatory component in autoimmune diabetes apart from that of NKT and CD4(+)CD25(high) Foxp3(+)T-regulatory cells. DNCD3 splenic cells could be potentially manipulated towards the development of autologous cell therapies in autoimmune diabetes.

ALTERATIONS IN THE THYMOPOIESIS IN EXPERIMENTAL AUTOIMMUNE MYASTHENIA GRAVIS

Experimental autoimmune myasthenia gravis (EAGM) was induced in female AO rats, by a single immunization with Torpedo acetylcholine receptor (AChR). Animals injected with saline served as controls. The study showed substantial changes in EAMG rats in the thymopoiesis, causing an increase in the relative proportion of mature CD8+ and, particularly, CD4+ (possibly autoreactive) single positive (SP) cells expressing TCRalphabeta at high level (TCRalphabeta(high)), as well as in that of mature double negative (DN) TCRalphabeta(high) cells, which are believed to be the immunoregulatory cells that augment autoantibody (autoAb) production. These results indicate that an augmented production of autoreactive CD4+ cells, on one side, and an increase of the immunoregulatory T cells that augment autoAb production, on the other side (reflecting, most likely, an increased entry of activated autoreactive CD4+ T cells from the periphery into the thymus), may have a significant role in the sustention of immune response in EAMG, and may suggest a putative mechanism underlying the sustention of autoimmune response in acquired MG.

SIT and TRIM determine T cell fate in the thymus

Thymic selection is a tightly regulated developmental process essential for establishing central tolerance. The intensity of TCR-mediated signaling is a key factor for determining cell fate in the thymus. It is widely accepted that low-intensity signals result in positive selection, whereas high-intensity signals induce negative selection. Transmembrane adaptor proteins have been demonstrated to be important regulators of T cell activation. However, little is known about their role during T cell development. Herein, we show that SIT (SHP2 Src homology domain containing tyrosine phosphatase 2-interacting transmembrane adaptor protein) and TRIM (TCR-interacting molecule), two structurally related transmembrane adaptors, cooperatively regulate TCR signaling potential, thereby influencing the outcome of thymic selection. Indeed, loss of both SIT and TRIM resulted in the up-regulation of CD5, CD69, and TCRbeta, strong MAPK activation, and, consequently, enhanced positive selection. Moreover, by crossing SIT/TRIM double-deficient mice onto transgenic mice bearing TCRs with different avidity/affinity, we found profound alterations in T cell development. Indeed, in female HY TCR transgenic mice, positive selection was completely converted into negative selection resulting in small thymi devoided of double-positive thymocytes. More strikingly, in a nonselecting background, SIT/TRIM double-deficient single-positive T cells developed, were functional, and populated the periphery. In summary, we demonstrated that SIT and TRIM regulate cell fate of developing thymocytes, thus identifying them as essential regulators of central tolerance.

Doubting the TCR coreceptor function of CD8alphaalpha

"The beginning of wisdom is found in doubting; by doubting we come to question, and by seeking we may come upon the truth." -Pierre Abélard. CD8 is a glycoprotein expressed on hematopoietic cells. Two isoforms of CD8, CD8alphabeta and CD8alphaalpha, have been identified that are distinct in their expression and function. Whereas CD8alphabeta serves as a T cell receptor (TCR) coreceptor to enhance the functional avidity and is constitutively expressed on MHC class I-restricted T cells, CD8alphaalpha marks T cells that are distinct from the conventional thymus-selected and MHC-restricted CD4(+) or CD8alphabeta(+) T cells. Inconsistent with a coreceptor function, CD8alphaalpha decreases antigen sensitivity of the TCR, and it can be transiently or permanently expressed on T cells, regardless of the MHC restriction of the TCR or the presence of conventional coreceptors. Together, these observations indicate that CD8alphaalpha on T cells marks a differentiation stage and that it likely functions as a TCR corepressor to negatively regulate T cell activation.

Changes in thymus size, cellularity and relation between thymocyte subpopulations in young adult rats induced by somatostatin-14

The role of somatostatin on inhibition of both normal and tumor cell cycle, secretion of endocrine and exocrine cells, as well as induction apoptosis is well documented. However, its effect on T cell development and thymic structure is not fully clarified. In order to investigate the influence of somatostatin in vivo on the thymus structure and T cell development, the young adult Albino Oxford male rats were intracerebroventriculary treated with somatostatin-14. We examined the thymus compartments and its cellularity, through assessment of morphometric parameters by stereological method, and the relation between thymocytes subpopulations, over expression of CD4, CD8 and T-cell receptor (TCR) alpha beta by flow cytometry. Additionally, we also determined the body and thymus weight of the rats, during the first three months of life, to define the time of SRIH-14 application. A decrease of relative thymus weight from the fourth weeks of postnatal life, and an unchanged relative thymus weight obtained in treated group indicates that SRIH-14 in young adult rats inhibits growth of whole organism, not only thymus. The changes in the absolute number and numerical density of cortical thymocytes indicate that SRIH-14 alters the true lymphoid tissue. SRIH-14 changes relation between thymocyte subsets, increase number of CD4(-)CD8(-)TCR alpha beta(-) and CD4(-)CD8(+)TCR alpha beta(hi) thymocyte subsets as well as the CD4(-)CD8(-)TCR alpha beta(low/hi) thymocytes, while decrease number of CD4(+)CD8(+) TCR alpha beta(-/low/hi) thymocyte subsets. These results indicate that somatostatin-14 is not involved in the control of the physiologic involution of the thymus, although induces thymic weight loss through the reduction of true lymphoid tissue. In addition, changes in frequency of thymocyte subpopulations, especially immature cells, indicate that SRIH-14 modulates thymocytes development and maturation.

Somatostatin modulates T cells development in adult rat thymus

It is well known that somatostatin modulates thymic functions, such as binding to receptors. In order to elucidate the influence of somatostatin on the thymus architecture and the T cells maturation, young adult male rats were treated with somatostatin-28. The results showed that somatostatin-28 decreased thymus weight and cellularity, probably due to alterations in the thymic morphometric parameters. Our results also demonstrated that SRIH treatment reduces number of cells with undetectable alphabetaTCR and cells with low expression of alphabetaTCR, while the number of TCRalphabeta(hi) cells remains approximately the same as the values obtained from the control rats. Besides, in the least mature thymocytes (DNTCR TCRalphabeta(-)) and among the most mature the SPCD4 TCRalphabeta(hi) subset remained unaltered, while SPCD8 TCRalphabeta(hi) decreased. At last, it should be noted that SRIH treatment increases DN thymocytes subsets expressing TCRalphabeta(low/hi) (TCRalphabeta(+)). These results suggest that somatostatin-28 induces reshaping of T cells maturation and, at least partly, contributes to thymic weight loss, through the modulation of the complex neuroendocrine-immune network.

Trypanosoma cruzi: populations bearing opposite virulence induce differential expansion of circulating CD3+CD4-CD8- T cells and cytokine serum levels in young and adult rats

The JG strain is the least virulent while the CL-Brener clone is one of the most virulent Trypanosoma cruzi populations in young rats. In this study, we determined that the parasitemia peak values in CL-Brener clone-infected adult rats were 50-fold lower than in young rats and that mortality was null as compared to 45% death in young rats. Low parasitemia, milder and sustained myocarditis and myositis characterized JG infections. CL-Brener clone caused a significantly higher production of pro-inflammatory cytokines and higher expansion of CD3(+)CD4(-)CD8(-), double-negative (DN) T cells, during the acute phase in both adult and young rats. DN T cell frequencies correlated with IFN-gamma levels. These findings may explain the higher inflammation and fast acute phase resolution in CL-Brener infection. In young rats, IL-10 levels were similar in both infections. The IL-10/IFN-gamma ratio was higher in JG acute infection in accordance with the milder inflammation and parasite persistence leading to a chronic phase. In conclusion, virulence and pathogenicity depend on T. cruzi ability to induce expansion of DN T cells and production of specific cytokines.

Double-negative regulatory T cells: non-conventional regulators

The crucial role of regulatory T (Treg) cells in self-tolerance and downregulating immune responses has been clearly established. Numerous different Treg subsets have been identified that possess distinct phenotypes and functions in various disease models. Among these subsets, alphabeta-TCR+CD3+CD4-CD8- double-negative (DN) Treg cells have been shown to be able to inhibit a variety of immune responses in part via direct killing of effector T cells in an antigenspecific manner in both mice and humans. This was shown to occur at least partially by acquisition of MHC-peptide complexes from antigen-presenting cells (APCs) and subsequent Fas/Fas-ligand interactions. In addition, DN Treg cells have been shown to express several molecules uncommon to other Treg cell subsets, such as IFN-gamma, TNF-alpha, Ly6A, FcRgamma, and CXCR5, which may contribute to their unique regulatory ability. Understanding the development and regulatory functions of DN Treg cells may elucidate the etiology for loss of self-tolerance and serve as a therapeutic modality for various diseases. This review will summarize the characteristics, developmental pathways, and mechanisms of action of DN Treg cells, as well as their role in transplant tolerance, autoimmunity, and anticancer immunity.

Regulatory T cell-mediated transplantation tolerance

The existence of naturally occurring regulatory T cells in normal hosts and their pivotal role in maintaining both auto- and allo-tolerance have direct implications on the therapy of autoimmune disorders and for achieving immunosuppression-free allotransplantation. Among the various forms of regulatory T cells described, CD4(+)CD25(+) T cells have emerged as one of the most potent tolerogenic subsets. In this review, we discuss the molecular basis of development and function of these regulatory T cells and their potential role in the context of chronic lung allograft rejection.

T cell tolerance induced by cross-reactive TCR ligands can be broken by superagonist resulting in anti-inflammatory T cell cytokine production

Cross-reactive activation of potentially autoreactive T cells by high-affinity nonself ligands may be important in breaking self-tolerance in autoimmunity. In a mouse transgenic for a cross-reactive TCR, we have previously shown that a hyper-stimulating altered peptide ligand, L144, induced unresponsiveness to the self peptide, proteolipid protein 139-151. In this study, we demonstrate that a superagonist ligand can break T cell tolerance induced by the lower affinity cognate Ag. T cells tolerant to the cognate ligand, Q144, responded to superagonist, L144, by proliferation and the production of mainly IL-4 and IL-10 in vitro. In contrast, T cells that were tolerized to the superagonist were unable to respond to any peptide that cross-reacted with the transgenic TCR. Low-dose immunization with the superagonist L144 was able to break tolerance to the cognate ligand in vivo and resulted in a blunted proliferative response with production of Th2 cytokines.

The timing of TCR alpha expression critically influences T cell development and selection

Sequential rearrangement of the T cell receptor for antigen (TCR) beta and alpha chains is a hallmark of thymocyte development. This temporal control is lost in TCR transgenics because the alpha chain is expressed prematurely at the CD4- CD8- double negative (DN) stage. To test the importance of this, we expressed the HY alpha chain at the physiological CD4+ CD8+ double positive (DP) stage. The reduced DP and increased DN cellularity typically seen in TCR transgenics was not observed when the alpha chain was expressed at the appropriate stage. Surprisingly, antigen-driven selection events were also altered. In male mice, thymocyte deletion now occurred at the single positive or medullary stage. In addition, no expansion of CD8 alpha alpha intestinal intraepithelial lymphocytes (IELs) was observed, despite the fact that HY transgenics have been used to model IEL development. Collectively, these data establish the importance of proper timing of TCR expression in thymic development and selection and emphasize the need to use models that most accurately reflect the physiologic process.

Gonadotropin-releasing hormone agonist administration affects the thymopoiesis in adult female rats independently on gonadal hormone production

PROBLEM

In addition to having an indirect effect on the T-cell development by controlling the production of ovarian steroids, an accumulating body of evidence suggest that GnRH analogue (GnRH-A) administration may exert a thymopoietic regulatory effect that is not mediated by ovarian hormones.

METHOD OF STUDY

In non-ovariectomized (non-OVX) and OVX adult female AO rats treated s.c. with GnRH-A or saline (controls), over 14 days, were estimated the thymic cellularity and thymocyte expression of CD4/CD8/TCRalphabeta by stereological analysis and three-color flow cytometry, respectively.

RESULTS

GnRH-A in both groups of rats diminished the thymic cellularity. In non-OVX rats GnRH-A increased the relative numbers of immature cells (CD4-8-TCRalphabeta(-), CD4-8-TCRalphabeta(low) and CD4+8-TCRalphabeta(low)), and reduced those of positively selected CD4+8+TCRalphabeta(high) and mature (CD4-8+TCRalphabeta(high), CD4(+8)-TCRalphabeta(high)) cells, suggesting decelerated expression of TCRalphabeta followed by less efficient positive selection and further maturation of the selected cells. Differently, in OVX rats GnRH-A decreased the percentage of immature (CD4-8-TCRalphabeta(-), CD4+8+ TCRalphabeta(-)) cells and increased those of all TCRalphabeta(high) subsets, suggesting an increased rate of early thymocyte differentiation, more efficient positive selection and further maturation of the selected cells.

CONCLUSIONS

The effect of GnRH-A administration is affected by the presence of ovarian steroids.

Changes in thymopoiesis in myasthenia gravis

This study was undertaken to investigate T-cell maturation in hyperplastic thymi of patients suffering from myasthenia gravis (MG). For this purpose, the expression of the major differentiational molecules (CD4, CD8, and CD3/TCRalphabeta) and that of the regulatory and activation molecules on thymocytes from MG patients and control subjects were estimated by flow cytometric analysis. In the MG patients the increase in relative proportion of immature (CD4-8- TCRalphabeta-) and the most mature (CD4+8- TCRalphabetahigh and CD4-8- TCRhigh encompassing immunoregulatory NKT) thymocytes followed by a decrease in that of CD4+8+CD3-/TCRalphabeta- cells was found. Furthermore, in these patients the relative proportion of CD4+HLA-DR+ and CD4+71+ cells was increased, whereas that of CD4+25+ cells was slightly, but significantly, decreased (reflecting, most likely, decreased contribution of T reg cells bearing this phenotype). Moreover, in MG thymi the percentage of CD45RA+ cells was reduced indicating changes in the selection processes. In keeping with this finding the reduced thymocyte apoptotic index and percentage of cells bearing apoptosing (CD4-8- TCRalphabetalow) phenotype were detected. In conclusion, the study demonstrates substantial changes in intrathymic differentiation of T cells in hyperplastic MG thymi and suggests alterations in selection events providing an increased escape of potentially autoreactive T-cell clones, on one side, and an altered maturation and/or selection of immunoregulatory cells (NKT and CD4+8-25+ T reg cells) keeping these cell clones under control, on the other side.

The fourth way? Harnessing aggressive tendencies in the thymus

During late stages of thymic development, T cells must chose between different fates, dictated by their TCR specificity. Typically, this is thought of as a choice between three alternatives (being positive selection for useful T cells vs negative selection or neglect for harmful or useless T cells). However, there is growing evidence for a fourth alternative, in which T cells are positively selected by agonist ligands, which would normally be expected to induce T cell deletion. In this review, we will discuss where and when agonist selection is induced and whether this should be considered as a novel form of thymic selection or as an alternative differentiation state for Ag-exposed T cells.

Exposure to forced swim stress alters morphofunctional characteristics of the rat thymus

The aim of this study was to investigate whether chronic stress, induced by repeated daily swimming during 21 days, alters the morphofunctional parameters in the thymus of adult rats. Our results showed that chronic stress reduced thymus mass, total number of thymocytes, volume of the thymus compartments and numerical density of thymocytes within thymus inner cortex and medulla. However, the percentage of apoptotic cells and the level of corticosterone were significantly increased. The percentages of CD4-CD8-TCRalphabeta(low/high) and CD4-CD8+TCRalphabeta(-)thymocytes were significantly increased, while the percentage of the least mature CD4+CD8-SP TCRalphabeta(-) thymocytes was significantly decreased. These results show that recurred swimming procedure induces thymus hypotrophy and elevated percentage of DN TCRalphabeta(+) cells.

Neonatal androgenization affects the intrathymic T-cell maturation in rats

The thymus structure, expression of CD4/CD8/TCRalphabeta on thymocytes and thymocyte proliferative and apoptotic indexes were analyzed in sexually immature 30-day-old and in sexually mature 60-day-old female rats neonatally androgenized (NA) by subcutaneous injection of 500 microg testosterone propionate/day on days 1-3 and in their vehicle-administered counterparts. The treatment affected normal thymus development. Thus, at 30 days of age, there was a reduction in the thymus weight, reflecting a decrease in the main thymic compartments. However, at 60 days of age, thymus weight did not significantly differ from that in age-matched controls, since the cortical volume enlargement was followed by a proportional decrease in the medullary volume. In rats of both ages, the changes in thymic compartments most likely reflected alterations in the size of both lymphoid and nonlymphoid components. Furthermore, in NA rats, substantial changes in thymocyte phenotypic characteristics were registered, in spite of their age. In both groups of NA rats, a decrease in the relative proportion of the least mature CD4-8-TCRalphabeta- cells and in that of CD4+8- TCRalphabeta-/TCRalphabeta(low) cells followed by an increase in the percentage of their successor CD4+8+TCRalphabeta-/TCRalphabeta(low) cells was detected. In addition, in 30-day-old NA rats, the relative proportions of CD4+8+TCRalphabeta(high) cells (just positively selected) and that of mature single positive (CD4+8- and CD4-8+) and CD4-8- double negative TCRalphabeta(high) cells, were reduced, while in 60-day-old NA rats only the percentage of CD4+8+TCRalphabeta(high) thymocytes was decreased. Thus, the study showed that the changes in the development of the hypothalamo-pituitary-gonadal axis induced by neonatal androgenization may affect the thymus development and intrathymic T-cell maturation.

[Thymus function and autoimmunity]

PURPOSE

Thymus is the site of T-cell development and is essential for the induction of self-tolerance, by deletion of autoreactive T lymphocytes (negative selection) and by generation of regulatory T cells. Defect of the selection mechanism of both types of lymphocytes lead to autoimmune diseases.

CURRENT KNOWLEDGE AND KEY POINTS

Elimination of potentially self-reactive T cells in the thymus requires the intrathymic expression of ubiquitous and "tissue-specific" antigens. Some thymic antigen expressions are dependent on AIRE expression. Mutations in the AIRE gene that are associated with the absence of autoantigen expression in the thymus, defects in the peptide presentation or in apoptosis can allow autoreactive T cells to escape negative selection, and are associated with autoimmune diseases. Recent data are now available concerning the thymic selection of autoreactive regulatory T cells. The Foxp3 gene was recently shown to be predominantly expressed in regulatory T cells and could be a more specific marker of regulatory T cells than phenotypic markers.

FUTURE PROSPECTS AND PROJECTS

Animal models show that regulatory T cells injection or intrathymic inoculation of antigen lead to immunological tolerance in autoimmunity and transplantation. These novel strategies could be used in human.

Tolerance mechanisms and recent progress

There has been tremendous progress in our understanding of the mechanisms mediating allograft tolerance, which have been revealed to be far more complex and regulated than hitherto suspected. New results have enriched our understanding of the relative contributions of the direct and indirect pathways to immunity and tolerance over time. The role of central tolerance has been expanded with the surprising discovery of "ectopic" or "promiscuous" antigens expressed by medullary thymic epithelial cells, and the function of the thymus in generating naturally occurring CD4+ CD25+ regulatory T cells. In the periphery, it is increasingly appreciated that tolerance is a highly active process, with tolerogenic dendritic cells and regulatory T cells being the major players. However, the challenge of understanding the complex interactions regulating the dynamic balance between immunity and tolerance are formidable, and new tools from the more formal disciplines of nonlinear dynamics and systems engineering may help provide insight. Although many hurdles remain, the progress in elucidating the basic mechanisms of tolerance is rapidly being translated into clinical trials and provides grounds for optimism that clinical tolerance will eventually become a reality.

Absence of programmed death receptor 1 alters thymic development and enhances generation of CD4/CD8 double-negative TCR-transgenic T cells

Programmed death receptor 1 (PD-1) is expressed on thymocytes in addition to activated lymphocyte cells. Its ligation is thought to negatively regulate T cell activation, and PD-1(-/-) mice develop autoimmunity. To study the role of PD-1 on the development and function of a monoclonal CD8(+) T cell population, 2C TCR-transgenic/recombination-activating gene 2(-/-)/PD-1(-/-) mice were generated. Unexpectedly, approximately 30% of peripheral T cells in these mice were CD4/CD8 double negative (DN). Although the DN cells were not activated by Ag-expressing APCs, they functioned normally in response to anti-CD3/anti-CD28. These cells had a naive surface phenotype and lacked expression of NK1.1, B220, and gammadelta TCR; and the majority did not up-regulate CD8alphaalpha expression upon activation, arguing that they are not predominantly diverted gammadelta-lineage cells. The thymus was studied in detail to infer the mechanism of generation of DN peripheral T cells. Total thymus cellularity was reduced in 2C TCR-transgenic/recombination-activating gene 2(-/-)/PD-1(-/-) mice, and a relative increase in DN cells and decrease in double-positive (DP) cells were observed. Increased annexin V(+) cells among the DP population argued for augmented negative selection in PD-1(-/-) mice. In addition, an increased fraction of the DN thymocytes was HSA negative, suggesting that they had undergone positive selection. This possibility was supported by decreased emergence of DN PD-1(-/-) 2C cells in H-2(k) bone marrow chimera recipients. Our results are consistent with a model in which absence of PD-1 leads to greater negative selection of strongly interacting DP cells as well as increased emergence of DN alphabeta peripheral T cells.

Positive and negative selection of T cells

A functional immune system requires the selection of T lymphocytes expressing receptors that are major histocompatibility complex restricted but tolerant to self-antigens. This selection occurs predominantly in the thymus, where lymphocyte precursors first assemble a surface receptor. In this review we summarize the current state of the field regarding the natural ligands and molecular factors required for positive and negative selection and discuss a model for how these disparate outcomes can be signaled via the same receptor. We also discuss emerging data on the selection of regulatory T cells. Such cells require a high-affinity interaction with self-antigens, yet differentiate into regulatory cells instead of being eliminated.

Self-specific MHC class II-restricted CD4-CD8- T cells that escape deletion and lack regulatory activity

In the presence of the I-Ealpha protein, transgenic (Tg) mice expressing the 1H3.1 alphabeta TCR that is specific for the Ealpha52-68:I-A(b) complex display drastic intrathymic deletion. Although peripheral T cells from these mice remained unresponsive to the Ealpha52-68:I-A(b) complex, they contained a subpopulation able to specifically react to this complex in the presence of exogenous IL-2, indicating that some 1H3.1 alphabeta TCR Tg T cells have escaped clonal deletion and efficiently populated the periphery. IL-2-dependent, Ealpha52-68:I-A(b) complex-responsive T cells were CD4-CD8- and expressed the 1H3.1 alphabeta TCR. Such T cells could develop intrathymically, did not show sign of regulatory/suppressor activity, displayed a typical naive phenotype, and seemed to persist in vivo over time. CD4-CD8- TCR Tg T cells were also detected when the surface density of the deleting ligand was increased on MHC class II+ cells. In addition, the development of CD4-CD8- 1H3.1 alphabeta TCR Tg T cells could be supported by I-A(b) molecules. These observations indicate that CD4 surface expression neither specifies, nor is required for, the thymic export of mature thymocytes expressing a MHC class II-restricted alphabeta TCR. The data also show that, although the avidity of the interaction involved in intrathymic deletion is significantly lower than that involved in mature T cell activation, its range can be large enough to be influenced by the presence or absence of coreceptors. Finally, the margin created by the absence of CD4 coreceptor was substantial because it could accommodate various amounts of the deleting ligand on thymic stromal cells.

Regulatory T cells in transplantation

There has recently been an explosion of renewed interest in regulatory T cells, particularly those within the CD4(+)CD25(+) population. It is becoming increasingly apparent that these cells exist not only as naturally occurring cells that may contribute to the maintenance of self-tolerance, but they also have the potential to prevent rejection of allografts in experimental models. Such cells have now been identified in humans as well as in rodents.