B7-CD28 Interaction Promotes Proliferation and Survival but Suppresses Differentiation of CD4−CD8− T Cells in the Thymus

Co-Stimulatory Blockade of the CD28/CD80-86/CTLA-4 Balance in Transplantation: Impact on Memory T Cells?

CD28 and CTLA-4 are prototypal co-stimulatory and co-inhibitory cell surface signaling molecules interacting with CD80/86, known to be critical for immune response initiation and regulation, respectively. Initial “bench-to-beside” translation, two decades ago, resulted in the development of CTLA4-Ig, a biologic that targets CD80/86 and prevents T-cell costimulation. In spite of its proven effectiveness in inhibiting allo-immune responses, particularly in murine models, clinical experience in kidney transplantation with belatacept (high-affinity CTLA4-Ig molecule) reveals a high incidence of acute, cell-mediated rejection. Originally, the etiology of belatacept-resistant graft rejection was thought to be heterologous immunity, i.e., the cross-reactivity of the pool of memory T cells from pathogen-specific immune responses with alloantigens. Recently, the standard view that memory T cells arise from effector cells after clonal contraction has been challenged by a “developmental” model, in which less differentiated memory T cells generate effector cells. This review delineates how this shift in paradigm, given the differences in co-stimulatory and co-inhibitory signal depending on the maturation stage, could profoundly affect our understanding of the CD28/CD80-86/CTLA-4 blockade and highlights the potential advantages of selectively targeting CD28, instead of CD80/86, to control post-transplant immune responses.

γδ T cells acquire effector fates in the thymus and differentiate into cytokine-producing effectors in a Listeria model of infection independently of CD28 costimulation

Both antigen recognition and CD28 costimulation are required for the activation of naïve αβ T cells and their subsequent differentiation into cytokine-producing or cytotoxic effectors. Notably, this two-signal paradigm holds true for all αβ T cell subsets, regardless of whether they acquire their effector function in the periphery or the thymus. Because of contradictory results, however, it remains unresolved as to whether CD28 costimulation is necessary for γδ T cell activation and differentiation. Given that γδ T cells have been recently shown to acquire their effector fates in the thymus, it is conceivable that the contradictory results may be explained, in part, by a differential requirement for CD28 costimulation in the development or differentiation of each γδ T cell effector subset. To test this, we examined the role of CD28 in γδ T cell effector fate determination and function. We report that, although IFNγ-producing γδ T (γδ-IFNγ) cells express higher levels of CD28 than IL-17-producing γδ T (γδ-17) cells, CD28-deficiency had no effect on the thymic development of either subset. Also, following Listeria infection, we found that the expansion and differentiation of γδ-17 and γδ-IFNγ effectors were comparable between CD28^+/+ and CD28^−/− mice. To understand why CD28 costimulation is dispensable for γδ T cell activation and differentiation, we assessed glucose uptake and utilization by γδ T cells, as CD28 costimulation is known to promote glycolysis in αβ T cells. Importantly, we found that γδ T cells express higher surface levels of glucose transporters than αβ T cells and, when activated, exhibit effector functions over a broader range of glucose concentrations than activated αβ T cells. Together, these data not only demonstrate an enhanced glucose metabolism in γδ T cells but also provide an explanation for why γδ T cells are less dependent on CD28 costimulation than αβ T cells.

CTLA-4 controls the thymic development of both conventional and regulatory T cells through modulation of the TCR repertoire

Cytotoxic T lymphocyte-associated antigen-4 (CTLA-4; CD152) is of pivotal importance for self-tolerance, with deficiency or unfavorable polymorphisms leading to autoimmune disease. Tolerance to self-antigens is achieved through thymic deletion of highly autoreactive conventional T (Tconv) cells and generation of FoxP3(+) regulatory T (Treg) cells. The main costimulatory molecule, CD28, augments the negative selection of Tconv cells and promotes the generation of FoxP3(+) Treg cells. The role of its antagonistic homolog CTLA-4, however, remains a topic of debate. To address this topic, we investigated the thymic development of T cells in the presence and absence of CTLA-4 in a T-cell receptor (TCR) transgenic mouse model specific for the myelin basic protein peptide Ac1-9. We reveal that CTLA-4 is expressed in the corticomedullary region of the thymus. Its absence alters the response of CD4(+)CD8(-) thymocytes to self-antigen recognition, which affects the quantity of the Treg cells generated and broadens the repertoire of peripheral Tconv cells. T-cell repertoire alteration after deletion of CTLA-4 results from changes in TCR Vα and Jα segment selection as well as CDR3α composition in Tconv and Treg cells. CTLA-4, therefore, regulates the early development of self-reactive T cells in the thymus and plays a key role in central tolerance.

CD86 is expressed on murine hematopoietic stem cells and denotes lymphopoietic potential

A unique subset of CD86(-) HSCs was previously discovered in mice that were old or chronically stimulated with lipopolysaccharide. Functionally defective HSCs were also present in those animals, and we now show that CD86(-) CD150(+) CD48(-) HSCs from normal adult mice are particularly poor at restoring the adaptive immune system. Levels of the marker are high on all progenitors with lymphopoietic potential, and progressive loss helps to establish relations between progenitors corresponding to myeloid and erythroid lineages. CD86 represents an important tool for subdividing HSCs in several circumstances, identifying those unlikely to generate a full spectrum of hematopoietic cells.

Systemic effector and regulatory immune responses to chlamydial antigens in trachomatous trichiasis

Trachomatous trichiasis (TT) caused by repeated or chronic ocular infection with Chlamydia trachomatis is the result of a pro-fibrotic ocular immune response. At the conjunctiva, the increased expression of both inflammatory (IL1B, TNF) and regulatory cytokines (IL10) have been associated with adverse clinical outcomes. We measured in vitro immune responses of peripheral blood to a number of chlamydial antigens. Peripheral blood effector cells (CD4, CD69, IFNγ, IL-10) and regulatory cells (CD4, CD25, FOXP3, CTLA4/GITR) were readily stimulated by C. trachomatis antigens but neither the magnitude (frequency or stimulation index) or the breadth and amount of cytokines produced in vitro [IL-5, IL-10, IL-12 (p70), IL-13, IFNγ, and TNFα] were significantly different between TT cases and their non-diseased controls. Interestingly we observed that CD4+ T cells account for <50% of the IFNγ positive cells induced following stimulation. Further investigation in individuals selected from communities where exposure to ocular infection with C. trachomatis is endemic indicated that CD3-CD56+ (classical natural killer cells) were a major early source of IFNγ production in response to C. trachomatis elementary body stimulation and that the magnitude of this response increased with age. Future efforts to unravel the contribution of the adaptive immune response to conjunctival fibrosis should focus on the early events following infection and the interaction with innate immune mediated mechanisms of inflammation in the conjunctiva.

Trichloroethylene alters central and peripheral immune function in autoimmune-prone MRL(+/+) mice following continuous developmental and early life exposure

Trichloroethylene (TCE) is a widespread environmental toxicant known to promote CD4(+) T-lymphocyte activation, IFNgamma production, and autoimmunity in adult MRL(+/+) mice. Because developing tissues may be more sensitive to toxicant exposure, it was hypothesized that continuous TCE exposure beginning at conception might induce even more pronounced CD4(+) T-lymphocyte effects and exacerbate the development of autoimmunity in MRL(+/+) mice. In the current study, MRL(+/+) mice were exposed to occupationally-relevant doses of TCE from conception until adulthood (i.e., 7-8 wk-of-age). The CD4(+) T-lymphocyte effects in the thymus and periphery were evaluated, as well as serum antibody levels. TCE exposure altered the number of thymocyte subsets, and reduced the capacity of the most immature CD4-/CD8- thymocytes to undergo apoptosis in vitro. In the periphery, T-lymphocyte IFN(gamma) production was monitored in the blood prior to sacrifice by intracellular cytokine staining and flow cytometry. TCE induced a dose-dependent increase in T-lymphocyte IFN(gamma) as early as 4-5-week-of-age. However, these effects were transient, and not observed in splenic T-lymphocytes in 7-8-week-old mice. In contrast, the serum levels of anti-histone autoantibodies and total IgG(2a) were significantly elevated in the TCE-exposed offspring. The data illustrated that occupationally-relevant doses of TCE administered throughout development until adulthood affected central and peripheral immune function in association with early signs of autoimmunity. Future studies will address the possibility that early-life exposure to TCE may alter some aspect of self tolerance in the thymus, leading to autoimmune disease later in life.

CTLA-4 on alloreactive CD4 T cells interacts with recipient CD80/86 to promote tolerance

Although the inhibitory receptor CTLA-4 (CD152) has been implicated in peripheral CD4 T-cell tolerance, its mechanism of action remains poorly defined. We analyzed mechanisms of CD4 cell tolerance in a model of tolerance induction involving establishment of mixed hematopoietic chimerism in recipients of fully MHC-mismatched allogeneic bone marrow cells with anti-CD154 mAb. Animals lacking CD80 and CD86 failed to achieve chimerism. We detected no T cell-intrinsic requirement for CD28 for chimerism induction. However, a CD4 T cell-intrinsic signal through CTLA-4 was shown to be essential within the first 48 hours of exposure to alloantigen for the establishment of tolerance and mixed chimerism. This signal must be provided by a recipient CD80/86(+) non-T-cell population. Donor CD80/86 expression was insufficient to achieve tolerance. Together, our findings demonstrate a surprising role for interactions of CTLA-4 expressed by alloreactive peripheral CD4 T cells with CD80/86 on recipient antigen-presenting cells (APCs) in the induction of early tolerance, suggesting a 3-cell tolerance model involving directly alloreactive CD4 cells, donor antigen-expressing bone marrow cells, and recipient antigen-presenting cells. This tolerance is independent of regulatory T cells and culminates in the deletion of directly alloreactive CD4 T cells.

The magnitude of thymic output is genetically determined through controlled intrathymic precursor T cell proliferation

The thymus plays a crucial role in providing the immune system with naive T cells showing a diverse TCR repertoire. Whereas the diversity of thymic production is mainly ensured by TCR rearrangement at both the TRA and TRB loci, the number of cells reaching the double-positive differentiation stage defines the extent of thymic output. A quantitative analysis of TCR excision circles (TREC; signal-joint TRECs and DJbetaTRECs) produced at different stages of thymopoiesis was performed in nine laboratory mouse strains. The results clearly demonstrate that the magnitude of thymic output is directly proportional to the extent of proliferation in the double-negative 4 thymocyte subset. Strikingly, intrathymic precursor T cell proliferation was found to be strain dependent, thus suggesting a genetic regulation of thymic output. The inherited character of thymic output was further confirmed by the transmission of the phenotype in a recessive fashion in F(1) progeny of the different parental strains. Our results provide the first demonstration of the genetic regulation of thymic output.

Notch signaling is required for proliferation but not for differentiation at a well-defined beta-selection checkpoint during human T-cell development

Notch signaling is absolutely required for beta-selection during mouse T-cell development, both for differentiation and proliferation. In this report, we investigated whether Notch has an equally important role during human T-cell development. We show that human CD34(+) thymocytes can differentiate into CD4(+)CD8beta(+) double positive (DP) thymocytes in the absence of Notch signaling. While these DP cells phenotypically resemble human beta-selected cells, they lack a T-cell receptor (TCR)-beta chain. Therefore, we characterized the beta-selection checkpoint in human T-cell development, using CD28 as a differential marker at the immature single positive CD4(+)CD3(-)CD8alpha(-) stage. Through intracellular TCR-beta staining and gene expression analysis, we show that CD4(+)CD3(-)CD8alpha(-)CD28(+) thymocytes have passed the beta-selection checkpoint, in contrast to CD4(+)CD3(-)CD8alpha(-)CD28(-) cells. These CD4(+)CD3(-)CD8alpha(-)CD28(+) thymocytes can efficiently differentiate into CD3(+)TCRalphabeta(+) human T cells in the absence of Notch signaling. Importantly, preselection CD4(+)CD3(-)CD8alpha(-)CD28(-) thymocytes can also differentiate into CD3(+)TCRalphabeta(+) human T cells without Notch activation when provided with a rearranged TCR-beta chain. Proliferation of human thymocytes, however, is clearly Notch-dependent. Thus, we have characterized the beta-selection checkpoint during human T-cell development and show that human thymocytes require Notch signaling for proliferation but not for differentiation at this stage of development.

Modulation of NKT cell development by B7-CD28 interaction: an expanding horizon for costimulation

It has been demonstrated that the development of NKT cells requires CD1d. The contribution of costimulatory molecules in this process has not been studied. Here we show that in mice with targeted mutations of B7-1/2 and CD28, the TCRbeta(+)alpha-Galcer/CD1d(+) (iValpha14 NKT) subset is significantly reduced in the thymus, spleen and liver. This is mainly due to decreased cell proliferation; although increased cell death in the thymi of CD28-deficient mice was also observed. Moreover, in the B7-1/2- and CD28-deficient mice, we found a decreased percentage of the CD4(-)NK1.1(+) subset and a correspondingly increased portion of the CD4(+)NK1.1(-) subset. In addition, the mice with a targeted mutation of either B7 or CD28 had a reduced susceptibility to Con A induced hepatitis, which is known to be mediated by NKT cells. Our results demonstrate that the development, maturation and function of NKT cell are modulated by the costimulatory pathway and thus expand the horizon of costimulation into NKT, which is widely viewed as a bridge between innate and adaptive immunity. As such, costimulation may modulate all major branches of cell-mediated immunity, including T cells, NK cells and NKT cells.

Immunology

The concept of forbidden foods that should not be eaten goes back to the Garden of Eden and apart from its religious meanings it may also have foreshadowed the concept of foods that can provoke adverse reactions. Thus we could say that allergic diseases have plagued mankind since the beginning of life on earth. The prophet Job was affected by a condition that following the rare symptoms described by the Holy Bible might be identified as a severe form of atopic dermatitis (AD). The earliest record of an apparently allergic reaction is 2621 B.C., when death from stinging insects was first described by hieroglyphics carved into the walls of the tomb of Pharaoh Menes depicting his death following the sting of a wasp. In 79 A.D., the death of the Roman admiral Pliny the Elder was ascribed to the SO₂-rich gases emanating from the eruption of Mount Vesuvius. Hippocrates (460–377 B.C.) was probably the first to describe how cow’s milk (CM) could cause gastric upset and hives, proposing dietetic measures including both treatment and prevention for CM allergy.

Memory CD8+ T cells require CD28 costimulation

CD8(+) T cells are a critical component of the adaptive immune response against infections and tumors. A current paradigm in immunology is that naive CD8(+) T cells require CD28 costimulation, whereas memory CD8(+) T cells do not. We show here, however, that during viral infections of mice, costimulation is required in vivo for the reactivation of memory CD8(+) T cells. In the absence of CD28 costimulation, secondary CD8(+) T cell responses are greatly reduced and this impairs viral clearance. The failure of CD8(+) T cells to expand in the absence of CD28 costimulation is CD4(+) T cell help independent and is accompanied by a failure to down-regulate Bcl-2 and by cell cycle arrest. This requirement for CD28 costimulation was shown in both influenza A and HSV infections. Thus, contrary to current dogma, memory CD8(+) T cells require CD28 costimulation to generate maximal secondary responses against pathogens. Importantly, this CD28 requirement was shown in the context of real infections were multiple other cytokines and costimulators may be up-regulated. Our findings have important implications for pathogens, such as HIV and measles virus, and tumors that evade the immune response by failing to provide CD28 costimulation. These findings also raise questions about the efficacy of CD8(+) T cell-based vaccines against such pathogens and tumors.

CD28 and Cd27 Costimulation of Cd8+ T Cells: A Story of Survival

Although the requirement of CD28 and CD27 costimulation has been clearly demonstrated during primary CD8+ T cell responses and this costimulation acts by providing proliferation and survival cues to naive CD8+ T cells, a number of questions also arise from these studies. Is the requirement for CD28 and CD27 costimulation restricted to the initiation of the immune response in the lymph nodes, where presumably the initial contact between naive CD8+ T cell and DC occurs? What is the purpose of the dramatic influx of DC to sites of inflammation such as the lung during influenza virus infection and the formation of inflammatory BALT (iBALT)?(104) Are such DC at the site of inflammation and at later stages of the immune response providing cytokines or costimulation to effector CD8+ T cells? If DC are required for optimal secondary responses (100), is CD28 costimulation the missing signal or is it other members of the B7:CD28 family or TNF family? Given that a number of investigators are actively addressing these questions, the answers we expect will be soon to come and open exciting new opportunities for immune enhancement or dampening strategies and vaccine adjuvants.

Gads-/- mice reveal functionally distinct subsets of TCRbeta+ CD4-CD8- double-negative thymocytes

TCRbeta expression in CD4(-)CD8(-) double-negative (DN) thymocytes induces signaling pathways that promote survival and proliferation, as well as differentiation into CD4(+)CD8(+) double-positive thymocytes. The signaling pathways that regulate survival, proliferation, and differentiation remain unclear. We used Gads-deficient mice to investigate the signaling pathways that regulate these cell fates. During this investigation, we focused on TCRbeta(+) DN thymocytes and found that there are at least three functionally distinct subsets of TCRbeta(+) DN thymocytes: TCRbeta(+) DN3E, TCRbeta(+) DN3L, and TCRbeta(+) DN4. Survival and proliferation of TCRbeta(+) DN3E were independent of Gads, but survival and proliferation of TCRbeta(+) DN3L cells were Gads dependent. Likewise, expression of Bcl-2 in TCRbeta(+) DN3E cells was Gads independent, but Gads was necessary for Bcl-2 expression in TCRbeta(+) DN3L cells. Bcl-2 expression was not dependent on Gads in TCRbeta(+) DN4 cells, but proliferation of TCRbeta(+) DN4 cells was Gads dependent. Gads was not required for the differentiation of DN thymocytes into DP thymocytes. In fact, Gads(-/-) DN3E cells differentiated into DP thymocytes more readily than wild-type cells. We conclude that signaling pathways required to initiate TCRbeta-induced survival and proliferation are distinct from the pathways that maintain survival and proliferation. Furthermore, signaling pathways that promote survival and proliferation may slow differentiation.

Thymic development and peripheral homeostasis of regulatory T cells

The development and maintenance of regulatory T (T-reg) cells is crucial for determining the level of reactivity in the immune system. Until recently, however, surprisingly little was known about the factors involved in the development of these cells in the thymus or the mechanisms that maintain them in the periphery. Studies have now demonstrated that thymic development of T-reg cells is facilitated by TCRs with increased affinity for self-peptide-MHC complexes. Increased TCR affinity alone, however, is not sufficient to support the development of T-reg cells, and external factors such as CD80 and CD86, ligands for co-stimulatory receptor CD28, and interleukin 2 are required. These factors are also needed to maintain the T-reg cell subset in the periphery.

The role of CD28 and cytotoxic T-lymphocyte antigen-4 (CTLA-4) in regulatory T-cell biology

The profound influence of CD28 and cytotoxic T-lymphocyte antigen-4 (CTLA-4) on T-cell immunity has been known for over a decade, yet the precise roles played by these molecules still continue to emerge. Initially viewed as molecules that provide cell-intrinsic costimulatory and inhibitory signals, recent evidence suggests that both CD28 and CTLA-4 are also important in the homeostasis and function of a population of suppressive cells, termed regulatory T cells (Tregs). Here we review the main features of the CD28 and CTLA-4 system and examine how these impact upon Treg biology.

The Scurfy mutation of FoxP3 in the thymus stroma leads to defective thymopoiesis

The Scurfy mutation of the FoxP3 gene (FoxP3^sf) in the mouse and analogous mutations in human result in lethal autoimmunity. The mutation of FoxP3 in the hematopoietic cells impairs the development of regulatory T cells. In addition, development of the Scurfy disease also may require mutation of the gene in nonhematopoietic cells. The T cell–extrinsic function of FoxP3 has not been characterized. Here we show that the FoxP3^sf mutation leads to defective thymopoiesis, which is caused by inactivation of FoxP3 in the thymic stromal cells. FoxP3 mutation also results in overexpression of ErbB2 in the thymic stroma, which may be involved in defective thymopoiesis. Our data reveal a novel T cell–extrinsic function of FoxP3. In combination, the T cell–intrinsic and –extrinsic defects provide plausible explanation for the severity of the autoimmune diseases in the scurfy mice and in patients who have immunodysregulation, polyendocrinopathy, enteropathy, and X-linked syndrome.

Regulated costimulation in the thymus is critical for T cell development: dysregulated CD28 costimulation can bypass the pre-TCR checkpoint

Expression of CD28 is highly regulated during thymic development, with CD28 levels extremely low on immature thymocytes but increasing dramatically as CD4- CD8- cells initiate expression of TCRbeta. B7-1 and B7-2, the ligands for CD28, have a restricted distribution in the thymic cortex where immature thymocytes reside and are more highly expressed in the medulla where the most mature thymocytes are located. To determine the importance of this regulated CD28/B7 expression for T cell development, we examined the effect of induced CD28 signaling of immature thymocytes in CD28/B7-2 double-transgenic mice. Strikingly, we found that differentiation to the CD4+ CD8+ stage in CD28/B7-2 transgenics proceeds independent of the requirement for TCRbeta expression manifest in wild-type thymocytes, occurring even in Rag- or CD3epsilon- knockouts. These findings indicate that signaling of immature thymocytes through CD28 in the absence of TCR- or pre-TCR-derived signals can promote an aberrant pathway of T cell differentiation and highlight the importance of finely regulated physiologic expression of CD28 and B7 in maintaining integrity of the "beta" checkpoint for pre-TCR/TCR-dependent thymic differentiation.