Ageing, autoimmunity and arthritis: T-cell senescence and contraction of T-cell repertoire diversity – catalysts of autoimmunity and chronic inflammation

Rheumatoid arthritis (RA), like many other autoimmune syndromes, is a disease of adults, with the highest incidence rates reported in the elderly. The immune system undergoes profound changes with advancing age that are beginning to be understood and that need to be incorporated into the pathogenetic models of RA. The age-related decline in thymic function causes extensive remodeling of the T-cell system. Age-dependent changes in T-cell homeostasis are accelerated in patients with RA. The repertoire of naive and memory T cells is less diverse, possibly as a result of thymic insufficiency, and it is biased towards autoreactive cells. Presenescent T cells emerge that are resistant to apoptosis and that often expand to large clonal populations. These cells are under the regulatory control of nonconventional costimulatory molecules, display potent effector functions, and appear to be critical in the synovial and extra-articular manifestations of RA.     

Normal T-cell turnover in sooty mangabeys harboring active simian immunodeficiency virus infection

Sooty mangabeys naturally infected with simian immunodeficiency virus (SIV) remain healthy though they harbor viral loads comparable to those in rhesus macaques that progress to AIDS. To assess the immunologic basis of disease resistance in mangabeys, we compared the effect of SIV infection on T-cell regeneration in both monkey species. Measurement of the proliferation marker Ki-67 by flow cytometry showed that mangabeys harbored proliferating T cells at a level of 3 to 4% in peripheral blood irrespective of their infection status. In contrast, rhesus macaques demonstrated a naturally high fraction of proliferating T cells (7%) that increased two- to threefold following SIV infection. Ki-67(+) T cells were predominantly CD45RA(-), indicating increased proliferation of memory cells in macaques. Quantitation of an episomal DNA product of T-cell receptor alpha rearrangement (termed alpha1 circle) showed that the concentration of recent thymic emigrants in blood decreased with age over a 2-log unit range in both monkey species, consistent with age-related thymic involution. SIV infection caused a limited decrease of alpha1 circle numbers in mangabeys as well as in macaques. Dilution of alpha1 circles by T-cell proliferation likely contributed to this decrease, since alpha1 circle numbers and Ki-67(+) fractions correlated negatively. These findings are compatible with immune exhaustion mediated by abnormal T-cell proliferation, rather than with early thymic failure, in SIV-infected macaques. Normal T-cell turnover in SIV-infected mangabeys provides an explanation for the long-term maintenance of a functional immune system in these hosts.     

Thymic function and allogeneic T-cell responses in stem-cell transplantation

The thymus is the primary site of T-cell production early in life, and has now been shown to continue to function in both healthy and immunocompromised individuals late into life. Positive and negative selection occurring in the thymus are two of the most important processes that govern the development and specificity of peripheral T cells, including their restriction to self HLA and their ability to respond in an alloreactive manner. In the chimeric state that follows successful allogeneic stem-cell transplants, the specificity of alloreactive cells may be governed by either host- or recipient-derived cellular elements, as well as maturing lymphoid cells, which are, in turn, derived from donor stem cells or host cells surviving transplant conditioning. The ability to measure recent thymic emigrants via the detection of T-cell receptor excision circles has facilitated studies of thymic function in immunodeficient individuals, including HIV-1 infected subjects and recipients of autologous or allogeneic stem-cell transplant (SCT). These studies have now demonstrated that thymic function is likely to play a beneficial role in immune reconstitution in these settings, but have yet to clearly demonstrate what clinical variables are the most important determinants of thymic persistence. It is also not yet clear how much the degree of thymic function following allogeneic SCT influences the alloreactive T-cell repertoire, although studies in animal models and early clinical studies suggest that GvHD results in thymic injury and dysfunction. Future studies will further clarify how thymic function shapes the repertoire of T cells that mediate alloreactivity, as well as protective pathogen-specific immune responses, following SCT. Finally, these studies will also demonstrate whether endogenous mediators of thymic function could be selectively applied to regulate post-SCT thymic function and alloreactivity.     

Thymic T-cell tolerance of neuroendocrine functions: physiology and pathophysiology.

Intimate interactions between the two major systems of cell-to-cell communication, the neuroendocrine and immune systems, play a pivotal role in homeostasis and developmental biology. During phylogeny as well as during ontogeny, the molecular foundations of the neuroendocrine system emerge before the generation of diversity within the system of immune defenses. Before reacting against non-self infectious agents, the immune system has to be educated in order to tolerate the host molecular structure (self). The induction of self-tolerance is a multistep process that begins in the thymus during fetal ontogeny (central tolerance) and also involves anergizing mechanisms outside the thymus (peripheral tolerance). The thymus is the primary lymphoid organ implicated in the development of competent and self-tolerant T-cells. During ontogeny, T-cell progenitors originating from hemopoietic tissues (yolk sac, fetal liver, then bone marrow) enter the thymus and undergo a program of proliferation, T-cell receptor (TCR) gene rearrangement, maturation and selection. Intrathymic T-cell maturation proceeds through discrete stages that can be traced by analysis of their cluster differentiation (CD) surface antigens. It is well established that close interactions between thymocytes (pre-T-cells) and the thymic cellular environment are crucial both for T-cell development and for induction of central self-tolerance. Particular interest has focused on the ability of thymic stromal cells to synthesize polypeptides belonging to various neuroendocrine families. The thymic repertoire of neuroendocrine-related precursors recapitulates at the molecular level the dual role of the thymus in T-cell negative and positive selection. Thymic precursors not only constitute a source of growth factors for cryptocrine signaling between thymic stromal cells and pre-T-cells, but are also processed in a way that leads to the presentation of self-antigens by (or in association with) thymic major histocompatibility complex (MHC) proteins. Thymic neuroendocrine self-antigens usually correspond to peptide sequences highly conserved during the evolution of their corresponding family. The thymic presentation of some neuroendocrine self-antigens does not seem to be restricted by MHC alleles. Through the presentation of neuroendocrine self-antigens by thymic MHC proteins, the T-cell system might be educated to tolerate main hormone families. More and more recent experiments support the concept that a defect in thymic tolerogenic function is implicated as an important factor in the pathophysiology of autoimmunity.     

Specific and nonspecific T-cell recruitment in viral meningitis: Possible implications for autoimmunity

Specific and nonspecific T-cell invasion into cerebrospinal fluid has been investigated in the nonfatal viral meningoencephalitis induced by intracerebral inoculation of mice with vaccinia virus. At the peak of the inflammatory process on Day 7 approximately 5 to 10% of the Lyt 2⁺ T cells present are apparently specific for vaccinia virus. Concurrently, in mice primed previously with influenza virus, 0.5 to 1.0% of the appropriate T-cell set located in cerebrospinal fluid is reactive to influenza-infected target cells. This vaccinia virus-induced inflammatory exudate may thus contain as many as 500 influenza-immune memory T cells. These findings are discussed from the aspect that such nonspecific T-cell invasion into the central nervous system during aseptic viral meningitis could result in exposure of potentially brain-reactive T cells to central nervous system components.     

Functional uncoupling of T-cell receptor engagement and Lck activation in anergic human thymic CD4+ T cells

Human thymic CD1a-CD4+ T cells in the final stage of thymic maturation are susceptible to anergy induced by a superantigen, toxic shock syndrome toxin-1 (TSST-1). Thymic CD4+ T-cell blasts, established by stimulating human thymic CD1a-CD4+ T cells with TSST-1 in vitro, produce a low level of interleukin-2 after restimulation with TSST-1, whereas TSST-1-induced adult peripheral blood (APB) CD4+ T-cell blasts produce high levels of interleukin-2. The extent of tyrosine phosphorylation of the T-cell receptor zeta chain induced after restimulation with TSST-1 was 2-4-fold higher in APB CD4+ T-cell blasts than in thymic CD4+ T-cell blasts. The tyrosine kinase activity of Lck was low in both thymic and APB CD4+ T-cell blasts before restimulation with TSST-1. After restimulation, the Lck kinase activity increased in APB CD4+ T-cell blasts but not in thymic CD4+ T-cell blasts. Surprisingly, Lck was highly tyrosine-phosphorylated in both thymic and APB CD4+ T-cell blasts before restimulation with TSST-1. After restimulation, it was markedly dephosphorylated in APB CD4+ T-cell blasts but not in thymic CD4+ T-cell blasts. Lck from APB CD4+ T-cell blasts bound the peptide containing the phosphotyrosine at the negative regulatory site of Lck-505 indicating that the site of dephosphorylation in TSST-1-activated T-cell blasts is Tyr-505. Confocal microscopy demonstrated that colocalization of Lck and CD45 was induced after restimulation with TSST-1 in APB CD4+ T-cell blasts but not in thymic CD4+ T-cell blasts. Further, remarkable accumulation of Lck in the membrane raft was observed in restimulated APB CD4+ T-cell blasts but not in thymic CD4+ T-cell blasts. These data indicate that interaction between Lck and CD45 is suppressed physically in thymic CD4+ T-cell blasts and plays a critical role in sustaining an anergic state.     

Generating intrathymic microenvironments to establish T-cell tolerance

Alphabeta T cells pass through a series of lymphoid tissue stromal microenvironments to acquire self tolerance and functional competence. In the thymus, positive selection of the developing T-cell receptor repertoire occurs in the cortex, whereas events in the medulla purge the system of self reactivity. T cells that survive are exported to secondary lymphoid organs where they direct first primary and then memory immune responses. This Review focuses on the microenvironments that nurture T-cell development rather than on T cells themselves. We summarize current knowledge on the formation of thymic epithelial-cell microenvironments, and highlight similarities between the environments that produce T cells and those that select and maintain them during immune responses.     

Role of thymic output in regulating CD8 T-cell homeostasis during acute and chronic viral infection

Although it is well documented that CD8 T cells play a critical role in controlling chronic viral infections, the mechanisms underlying the regulation of CD8 T-cell responses are not well understood. Using the mouse model of an acute and chronic lymphocytic choriomeningitis virus (LCMV) infection, we have examined the relative importance of peripheral T cells and thymic emigrants in the elicitation and maintenance of CD8 T-cell responses. Virus-specific CD8 T-cell responses were compared between mice that were either sham thymectomized or thymectomized (Thx) at approximately 6 weeks of age. In an acute LCMV infection, thymic deficiency did not affect either the primary expansion of CD8 T cells or the proliferative renewal and maintenance of virus-specific lymphoid and nonlymphoid memory CD8 T cells. Following a chronic LCMV infection, in Thx mice, although the initial expansion of CD8 T cells was normal, the contraction phase of the CD8 T-cell response was exaggerated, which led to a transient but striking CD8 T-cell deficit on day 30 postinfection. However, the virus-specific CD8 T-cell response in Thx mice rebounded quickly and was maintained at normal levels thereafter, which indicated that the peripheral T-cell repertoire is quite robust and capable of sustaining an effective CD8 T-cell response in the absence of thymic output during a chronic LCMV infection. Taken together, these findings should further our understanding of the regulation of CD8 T-cell homeostasis in acute and chronic viral infections and might have implications in the development of immunotherapy.     

Expression and modulation of C3 receptors during early T-cell ontogeny.

Using a corosette assay, optimal conditions were established for the detection of C3 receptors on T lymphocytes. E⁺-C3⁺ corosetting cells were demonstrated in four T-cell lines and six patients with E-rosetting acute lymphoblastic leukemia. Small numbers were detected in normal lymphoid tissues whereas thoracic duct lymph contained a large number of these cells. Following incubation of these tissues with thymic humoral factors, there was a decrease in corosetting cells with an increase in cells rosetting SRBC exclusively. Similar results were observed in vivo in a patient with severe combined immunodeficiency following a thymic epithelial cell transplant. Our data suggest that C3 receptor-bearing T lymphocytes occur early in T-cell ontogeny and can be modulated by thymic humoral factors.     

Increased cell division but not thymic dysfunction rapidly affects the T-cell receptor excision circle content of the naive T cell population in HIV-1 infection

Recent thymic emigrants can be identified by T cell receptor excision circles (TRECs) formed during T-cell receptor rearrangement. Decreasing numbers of TRECs have been observed with aging and in human immunodeficiency virus (HIV)-1 infected individuals, suggesting thymic impairment. Here, we show that in healthy individuals, declining thymic output will affect the TREC content only when accompanied by naive T-cell division. The rapid decline in TRECs observed during HIV-1 infection and the increase following HAART are better explained not by thymic impairment, but by changes in peripheral T-cell division rates. Our data indicate that TREC content in healthy individuals is only indirectly related to thymic output, and in HIV-1 infection is mainly affected by immune activation.     

Sclerodermatous chronic graft-versus-host disease induced by host T-cell-mediated autoimmunity

Despite a long-standing hypothesis that chronic graft-versus-host disease (cGVHD) is an autoimmune disorder, most mouse models of cGVHD have been developed on the assumption that donor T cells are essential for its development. Here we show that cGVHD may be caused by autoreactive host T cells in mice that have been lethally irradiated and grafted with T-cell-depleted allogeneic bone marrow cells. In this chimera, host T cells derived from radioresistant intrathymic T-cell precursors caused dermal fibrosis and periportal inflammation, without the requirement for donor T cells. The lack of host DCs within the thymus after high-dose irradiation allowed autoreactive host T cells to escape thymic negative selection. Moreover, the homeostatic expansion of these T cells may augment their autoreactivity. These findings indicate that host T-cell-mediated cGVHD is an autoimmune process that occurs following the grafting of T-cell-depleted BM cells into hosts with functioning thymuses. We propose, based on the present data, that host T-cell-dependent autoimmunity is a potential mechanism by which cGVHD is induced.     

Prss16 is not required for T-cell development

PRSS16 is a serine protease expressed exclusively in cortical thymic epithelial cells (cTEC) of the thymus, suggesting that it plays a role in the processing of peptide antigens during the positive selection of T cells. Moreover, the human PRSS16 gene is encoded in a region near the class I major histocompatibility complex (MHC) that has been linked to type 1 diabetes mellitus susceptibility. The mouse orthologue Prss16 is conserved in genetic structure, sequence, and pattern of expression. To study the role of Prss16 in thymic development, we generated a deletion mutant of Prss16 and characterized T-lymphocyte populations and MHC class II expression on cortical thymic epithelial cells. Prss16-deficient mice develop normally, are fertile, and show normal thymic morphology, cellularity, and anatomy. The total numbers and frequencies of thymocytes and splenic T-cell populations did not differ from those of wild-type controls. Surface expression of MHC class II on cTEC was also similar in homozygous mutant and wild-type animals, and invariant chain degradation was not impaired by deletion of Prss16. These findings suggest that Prss16 is not required for quantitatively normal T-cell development.     

Studies of T-cell activation in chronic inflammation

The strong association between specific alleles encoded within the MHC class II region and the development of rheumatoid arthritis (RA) has provided the best evidence to date that CD4+ T cells play a role in the pathogenesis of this chronic inflammatory disease. However, the unusual phenotype of synovial T cells, including their profound proliferative hyporesponsiveness to TCR ligation, has challenged the notion that T-cell effector responses are driven by cognate cartilage antigens in inflamed synovial joints. The hierarchy of T-cell dysfunction from peripheral blood to inflamed joint suggests that these defects are acquired through prolonged exposure to proinflammatory cytokines such as tumour necrosis factor (TNF)-alpha. Indeed, there are now compelling data to suggest that chronic cytokine activation may contribute substantially to the phenotype and effector function of synovial T cells. Studies reveal that chronic exposure of T cells to TNF uncouples TCR signal transduction pathways by impairing the assembly and stability of the TCR/CD3 complex at the cell surface. Despite this membrane-proximal effect, TNF selectively uncouples downstream signalling pathways, as is shown by the dramatic suppression of calcium signalling responses, while Ras/ERK activation is spared. On the basis of these data, it is proposed that T-cell survival and effector responses are driven by antigen-independent, cytokine-dependent mechanisms, and that therapeutic strategies that seek to restore T-cell homeostasis rather than further depress T-cell function should be explored in the future.     

Negative selection--clearing out the bad apples from the T-cell repertoire

Dead cells are a prominent feature of the thymic landscape as only 5% of developing thymocytes are exported as mature T cells. The remaining thymocytes die by one of two mechanisms; most thymocytes die because they are not positively selected and do not receive a survival signal, whereas a minority of thymocytes undergo T-cell receptor (TCR)-mediated apoptosis, a process known as negative selection. Negative selection is extremely important for establishing a functional immune system, as it provides an efficient mechanism for ridding the T-cell repertoire of self-reactive and potentially autoimmune lymphocytes. This review discusses several cellular and molecular aspects of negative selection.     

Sonic hedgehog signalling in T-cell development and activation

The production of mature functional T cells in the thymus requires signals from the thymic epithelium. Here, we review recent experiments showing that one way in which the epithelium controls the production of mature T cells is by the secretion of sonic hedgehog (SHH). We consider the increasing evidence that SHH-induced signalling is not only important for the differentiation and proliferation of early thymocyte progenitors, but also for modulating T-cell receptor signalling during repertoire selection, with implications for positive selection, CD4 versus CD8 lineage commitment, and clonal deletion of autoreactive cells. We also review the influence of hedgehog signalling in peripheral T-cell activation.     

Age-related hyperplasia of the thymus and T-cell system in the Buffalo rat

This report describes the development of hyperplasia of both the thymus and the peripheral T-cell system with advancing age in the Buffalo rat. Buffalo/Mna rats do not show age-related thymic involution, but rather develop thymic hyperplasia with advancing age. This thymic growth is expansile and there is no infiltration of the surrounding tissues. Because the enlarging thymus occupies the thoracic cavity, most of the rats die of respiratory failure by the age of 24 months. Thymic enlargement is due to primary hyperplasia of cortical epithelial cells and the large number of proliferating lymphocytes. The hyperplastic epithelial cells are bizarre in shape and strongly positive when stained with Th-3 monoclonal antibody (MoAb), anti-thymosin antibody and anti-EGF antibody, but negative with Th-4 MoAb. The patterns of distribution of CD-5+, CD-4+ and CD-8+ lymphocytes within the hyperplastic thymus are similar to those seen in young rats of other species. The high level of T-cell emigration from the thymus to the periphery appears to persist throughout life, since the percentage of normal splenic T-cells also increase with advancing age and exceed 70% of the total by 24 months of age. This thymic enlargement with abnormal hyperplasia of cortical epithelial cells can be prevented by hypophysectomy.     

T-cell anergy and peripheral T-cell tolerance

The discovery that T-cell recognition of antigen can have distinct outcomes has advanced understanding of peripheral T-cell tolerance, and opened up new possibilities in immunotherapy. Anergy is one such outcome, and results from partial T-cell activation. This can arise either due to subtle alteration of the antigen, leading to a lower-affinity cognate interaction, or due to a lack of adequate co-stimulation. The signalling defects in anergic T cells are partially defined, and suggest that T-cell receptor (TCR) proximal, as well as downstream defects negatively regulate the anergic T cell's ability to be activated. Most importantly, the use of TCR-transgenic mice has provided compelling evidence that anergy is an in vivo phenomenon, and not merely an in vitro artefact. These findings raise the question as to whether anergic T cells have any biological function. Studies in rodents and in man suggest that anergic T cells acquire regulatory properties; the regulatory effects of anergic T cells require cell to cell contact, and appear to be mediated by inhibition of antigen-presenting cell immunogenicity. Close similarities exist between anergic T cells, and the recently defined CD4+ CD25+ population of spontaneously arising regulatory cells that serve to inhibit autoimmunity in mice. Taken together, these findings suggest that a spectrum of regulatory T cells exists. At one end of the spectrum are cells, such as anergic and CD4+ CD25+ T cells, which regulate via cell-to-cell contact. At the other end of the spectrum are cells which secrete antiinflammatory cytokines such as interleukin 10 and transforming growth factor-beta. The challenge is to devise strategies that reliably induce T-cell anergy in vivo, as a means of inhibiting immunity to allo- and autoantigens.     

Molecular and genetic parameters defining T-cell clonal selection

Clonal selection of T cells occurs in the thymus and is responsible for generating a useful and functional repertoire of T cells. Aberrations in clonal selection result in altered T-cell homeostasis in the secondary lymphoid organs ranging from an absence of T cells to an overabundance of autoreactive T cells. The advent of new technologies facilitating the manipulation of the mouse genome has helped refine our understanding of the molecular and genetic pathways involved in clonal selection and has also revealed a high degree of complexity. Herein, we attempt to review recent advances in thymic selection processes, achieved mostly through genetic manipulations.     

Impaired T-cell differentiation in the thymus at the early stages of acute pathogenic chimeric simian-human immunodeficiency virus (SHIV) infection in contrast to less pathogenic SHIV infection

One of the mechanisms by which HIV infection induces the depletion of CD4+ T cells has been suggested to be impairment of T-cell development in the thymus, although there is no direct evidence that this occurs. To examine this possibility, we compared T-cell maturation in the intrathymic progenitors between macaques infected with an acute pathogenic chimeric simian-human immunodeficiency virus (SHIV), which causes profound and irreversible CD4+ T-cell depletion, and macaques infected with a less pathogenic SHIV, which causes only a transient CD4+ T-cell decline. Within 27 days post-inoculation (dpi), the two virus infections caused similar increases in plasma viral loads and similar decreases in CD4+ T-cell counts. However, in the thymus, the acute pathogenic SHIV resulted in increased thymic involution, atrophy and the depletion of immature T cells including CD4(+)CD8(+) double-positive (DP) cells, whereas the less pathogenic SHIV did not have these effects. Ex vivo differentiation of CD3(-)CD4(-)CD8(-) triple-negative (TN) intrathymic progenitors to DP cells was assessed by a monkey-mouse xenogenic fetal thymus organ culture system. Differentiation was impaired in the TN intrathymic progenitors of the acute pathogenic SHIV-infected monkeys, while differentiation was not impaired in the TN intrathymic progenitors of the less pathogenic SHIV-infected monkeys. These differences suggest that dysfunction of thymic maturation makes an important contribution to the irreversible depletion of circulating CD4+ T cells in vivo.