Conversion of naive T cells to a memory-like phenotype in lymphopenic hosts is not related to a homeostatic mechanism that fills the peripheral naive T cell pool

Suppression of the Immune Response by Syngeneic Splenocytes Adoptively Transferred to Sublethally Irradiated Mice

The peripheral T-cell pool consists of several, functionally distinct populations of CD8⁺ T cells. CD44 and CD62L are among the major surface markers that allow us to define T-cell populations. The expression of these molecules depends on the functional status of a T lymphocyte. Under lymphopenic conditions, peripheral T cells undergo homeostatic proliferation and acquire the memory-like surface phenotype CD44hiCD62Lhi. However, the data on the functional activity of these cells remains controversial. In this paper, we analyzed the effects of the adoptive transfer of syngeneic splenocytes on the recovery of CD8⁺ T cells in sublethally irradiated mice. Our data demonstrate that under lymphopenia, donor lymphocytes form a population of memory-like CD8⁺ T cells with the phenotype CD122+CD5+CD49dhiCXCR3+ that shares the phenotypic characteristics of true memory cells and suppressive CD8⁺ T cells. Ex vivo experiments showed that after adoptive transfer in irradiated mice, T cells lacked the functions of true effector or memory cells; the allogeneic immune response and immune response to pathogens were greatly suppressed in these mice.

Self-reactivity as the necessary cost of maintaining a diverse memory T-cell repertoire

The adaptive immune system is expected to protect the host from infectious agents and malignancies, while avoiding robust activation against self-peptides. However, T cells are notoriously inept at protection whenever the pathogen or tumor is persistent in the body for longer periods of time. While this has been thought of as an adaptation to limit the immunopathology from continued effector T-cell responses, it is also likely an extension of the T cell's intrinsic mechanisms which evolved to tolerate self-peptides. Here we deliberate on how the need to tolerate self-peptides might stem from a paradoxical requirement-the utility of such molecules in maintaining a diverse repertoire of pathogen-specific memory T cells in the body. Understanding the mechanisms underlying this intriguing nexus, therefore, has the potential to reveal therapeutic strategies not only for improving immune responses to chronic infections and tumors but also the long-term efficacy of vaccines aimed at cellular immune responses.

Post-transplant repopulation of naïve and memory T cells in blood and lymphoid tissue after alemtuzumab-mediated depletion in heart-transplanted cynomolgus monkeys

Repopulation of memory T cells (Tmem) in allograft recipients after lymphodepletion is a major barrier to transplant tolerance induction. Ineffective depletion of naïve T cells (Tn) and Tmem may predispose to repopulation of Tmem after transplantation. Cynomolgus macaque monkeys given heart allografts were lymphodepleted using Alemtuzumab (Campath-1H; anti-CD52). Peripheral blood (PB) and lymph nodes (LN) were analyzed for CD95(-) (Tn) and CD95(+) cells (Tmem), one day, one month and up to three months after Alemtuzumab infusion. CD52 expression, susceptibility to Alemtuzumab cytotoxicity and pro-apoptotic caspase-3 were evaluated in Tn and Tmem. In vivo, Alemtuzumab induction profoundly depleted lymphocytes in PB (99% reduction) but exerted a lesser effect in LN (70% reduction), with similar depletion of Tn and Tmem subsets. After transplantation, Tmem comprised the majority of lymphocytes in PB and LN. In vitro, LN T cells were more resistant to Alemtuzumab-mediated cytotoxicity than PB lymphocytes. CD4(+) Tn and Tmem were equally susceptible to Alemtuzumab-mediated cytotoxicity, whereas CD8(+) Tn were more resistant than CD8(+) Tmem. However, no significant differences in CD52 expression between lymphocyte subsets in PB and LN were observed. Caspase-3 expression was higher in PB than LN T cells. CD4(+) and CD8(+) Tn expressed lower levels of Caspase-3 than Tmem, in both PB and LN. Thus, after Alemtuzumab infusion, residual Tn in secondary lymphoid tissue may predispose to rapid recovery of Tmem in allograft recipients.

Safety and immunological responses to human mesenchymal stem cell therapy in difficult-to-treat HIV-1-infected patients

OBJECTIVE

HAART largely decreases morbidity and mortality in chronic HIV-1-infected patients, but immune nonresponders (INRs) with full viral suppression still fail to reverse the immune deficiency. This study evaluated the safety and immunological responses of human umbilical cord mesenchymal stem cell (MSC) therapy in HIV-1-infected INRs.

DESIGN AND METHODS

A total of 13 HIV-1-infected INRs were enrolled in this pilot prospectively open-labeled controlled clinical trial. Seven patients were administered three umbilical cord-MSC transfusions at 1-month interval during 12-months of follow-up, whereas six control patients were treated with saline in parallel. Immunological parameters were monitored in these patients throughout the trial.

RESULTS

All patients tolerated the umbilical cord-MSC transfusions well throughout the trial. The umbilical cord-MSC transfusions preferentially increased circulating naive and central memory CD4 T-cell counts and restored HIV-1-specific IFN-γ and IL-2 production in the INRs. These enhancements in immune reconstitution were also associated with the reduction of systemic immune activation and inflammation in vivo.

CONCLUSIONS

umbilical cord-MSC transfusions are well tolerated and can efficiently improve host immune reconstitution in INRs, suggesting that such treatments may be used as a novel immunotherapeutic approach to reversing immune deficiency in HIV-1-infected INRs (ClinicalTrials.gov identifier: NCT01213186).

Lymphodepletion and homeostatic proliferation: implications for transplantation

Control of the alloimmune response requires elimination and/or suppression of alloreactive immune cells. Lymphodepleting induction therapies are increasingly used to accomplish this goal, both as part of tolerance induction protocols or to reduce the requirements for maintenance immunosuppression in the peritransplant setting. However, it is well recognized that lymphopenia induces compensatory proliferation of immune cells, generally termed ``homeostatic proliferation,'' which favors the emergence of memory T cells. Paradoxically therefore, the result may be a situation that favors graft rejection and/or makes tolerance difficult to achieve or sustain. Yet all depletion is not alike, particularly with respect to the timing of reconstitution and the types of cells that repopulate the host. Thus, to design more effective induction strategies it is important to understand the homeostatic mechanisms, which exist to maintain a balanced repertoire of naïve and memory T and B cells in the periphery and how they respond to lymphodepletion. Here we will review the biology of homeostatic proliferation stimulated by lymphopenia, the effects of specific depleting agents on reconstitution of the T- and B-cell immune repertoire, drawing from both from animal models and human experience, and potential strategies to enhance allodepletion while minimizing the adverse effects of homeostatic proliferation.

A deficiency in nucleoside salvage impairs murine lymphocyte development, homeostasis, and survival

The homeostasis of the immune system is tightly controlled by both cell-extrinsic and -intrinsic mechanisms. These regulators, not all known to date, drive cells in and out of quiescence when and where required to allow the immune system to function. In this article, we describe a deficiency in deoxycytidine kinase (DCK), one of the major enzymes of the nucleoside salvage pathway, which affects peripheral T cell homeostatic proliferation and survival. As a result of an N-ethyl-N-nitrosourea-induced mutation in the last α helix of DCK, a functionally null protein has been generated in the mouse and affects the composition of the hematopoietic system. Both B and T lymphocyte development is impaired, leading to a state of chronic lymphopenia and to a significant increase in the number of myeloid cells and erythrocytes. In the periphery, we found that mutant lymphocytes adopt a CD44(high)CD62L(low) memory phenotype, with high levels of proliferation and apoptosis. These phenotypes are notably the result of a cell-extrinsic-driven lymphopenia-induced proliferation as wild-type cells transferred into DCK-deficient recipients adopt the same profile. In addition, DCK also regulates lymphocyte quiescence in a cell-intrinsic manner. These data establish dCK as a new regulator of hematopoietic integrity and lymphocyte quiescence and survival.

Premature aging of circulating T cells in patients with end-stage renal disease

Progressive loss of renal function is associated with a dysregulation of circulating T cells that may underlie their impaired T-cell immunity. Here we tested whether end-stage renal disease (ESRD)-related T-cell alterations are compatible with the concept of premature immunological aging. Younger patients (25-45 years old) with ESRD were found to resemble older healthy controls (60-80 years old) as they had a significant loss of naive T cells and a relative increase of memory T cells showing progressive terminal differentiation. A significant decrease in the content of T-cell receptor excision circles and telomere length in patients with ESRD confirmed these phenotypic data. The loss of naive T cells in patients with ESRD was associated with an excessive age-related decrease of recent thymic emigrants, indicating a premature decline in thymic function. Additionally, increased homeostatic proliferation of naive T cells was found in patients with ESRD, similar to that of older healthy individuals, with an increased susceptibility for activation-induced apoptosis. Therefore, both decreased thymic output and increased susceptibility of naive T cells for apoptosis may play a role in the loss of naive T cells in ESRD patients. Thus, our results are compatible with premature aging of the T-cell system of patients with ESRD comparable with that of healthy individuals 20-30 years older.

Cell-to-cell interactions and signals involved in the reconstitution of peripheral CD8 T(CM) and T(EM) cell pools

We here describe novel aspects of CD8⁺ and CD4⁺ T cell subset interactions that may be clinically relevant and provide new tools for regulating the reconstitution of the peripheral CD8⁺ T cell pools in immune-deficient states. We show that the reconstitution capacity of transferred isolated naïve CD8⁺ T cells and their differentiation of effector functions is limited, but both dramatically increase upon the co-transfer of CD4⁺ T cells. This helper effect is complex and determined by multiple factors. It was directly correlated to the number of helper cells, required the continuous presence of the CD4⁺ T cells, dependent on host antigen-presenting cells (APCs) expressing CD40 and on the formation of CD4/CD8/APC cell clusters. By comparing the recovery of (CD44⁺CD62L^high) T_CM and (CD44⁺CD62L^low) T_EM CD8⁺ T cells, we found that the accumulation of T_CM and T_EM subsets is differentially regulated. T_CM-cell accumulation depended mainly on type I interferons, interleukin (IL)-6, and IL-15, but was independent of CD4⁺ T-cell help. In contrast, T_EM-cell expansion was mainly determined by CD4⁺ T-cell help and dependent on the expression of IL-2Rβ by CD8 cells, on IL-2 produced by CD4⁺ T-cells, on IL-15 and to a minor extent on IL-6.

CD4+ CD25+ Treg regulate the contribution of CD8+ T-cell subsets in repopulation of the lymphopenic environment

Peripheral T-cell expansion is of major relevance for immune function after lymphopenia. In order to promote regeneration, the process should result in a peripheral T-cell pool with a similar subpopulation structure as before lymphopenia. We investigated the repopulation of the CD8(+) central-memory T cells (T(CM) ) and effector-memory T cells (T(EM)) pools after adoptive transfer of sorted CD8(+) T cells from naïve, T(CM) and T(EM) subsets into T-cell-deficient hosts. We show that the initial kinetics of expansion are distinct for each subset and that the contribution to the repopulation of the CD8(+) T-cell pool by the progeny of each subset is not a mere function of its initial expansion. We demonstrate that CD4(+) CD25(+) Treg play a major role in the repopulation of the CD8(+) T-cell pool and that CD8(+) T-cell subsets impact on each other. In the absence of CD4(+) CD25(+) Treg, a small fraction of naïve CD8(+) T cells strongly proliferates, correlating with further expansion and differentiation of co-expanding CD8(+) T cells. CD4(+) CD25(+) Treg suppress these responses and lead to controlled repopulation, contributing decisively to the maintenance of recovered T(CM) and T(EM) fractions, and leading to repopulation of each pool with progeny of its own kind.

Immunosuppressive therapy exacerbates autoimmunity in NOD mice and diminishes the protective activity of regulatory T cells

Mounting evidence indicates that immunosuppressive therapy and autologous bone marrow transplantation are relatively inefficient approaches to treat autoimmune diabetes. In this study we assessed the impact of immunosuppression on inflammatory insulitis in NOD mice, and the effect of radiation on immunomodulation mediated by adoptive transfer of various cell subsets. Sublethal radiation of NOD females at the age of 14 weeks (onset of hyperglycemia) delayed the onset of hyperglycemia, however two thirds of the mice became diabetic. Adoptive transfer of splenocytes into irradiated NON and NOD mice precipitated disease onset despite increased contents of CD25(+)FoxP3(+) T cells in the pancreas and regional lymphatics. Similar phenotypic changes were observed when CD25(+) T cells were infused after radiation, which also delayed disease onset without affecting its incidence. Importantly, irradiation increased the susceptibility to diabetes in NOD and NON mice (71-84%) as compared to immunomodulation with splenocytes and CD25(+) T cells in naïve recipients (44-50%). Although irradiation had significant and durable influence on pancreatic infiltrates and the fractions of functional CD25(+)FoxP3(+) Treg cells were elevated by adoptive cell transfer, this approach conferred no protection from disease progression. Irradiation was ineffective both in debulking of pathogenic clones and in restoring immune homeostasis, and the consequent homeostatic expansion evolves as an unfavorable factor in attempts to restore self-tolerance and might even provoke uncontrolled proliferation of pathogenic clones. The obstacles imposed by immunosuppression on abrogation of autoimmune insulitis require replacement of non-specific immunosuppressive therapy by selective immunomodulation that does not cause lymphopenia.

On the composition of the preimmune repertoire of T cells specific for Peptide-major histocompatibility complex ligands

Millions of T cells are produced in the thymus, each expressing a unique alpha/beta T cell receptor (TCR) capable of binding to a foreign peptide in the binding groove of a host major histocompatibility complex (MHC) molecule. T cell-mediated immunity to infection is due to the proliferation and differentiation of rare clones in the preimmune repertoire that by chance express TCRs specific for peptide-MHC (pMHC) ligands derived from the microorganism. Here we review recent findings that have altered our understanding of how the preimmune repertoire is established. Recent structural studies indicate that a germline-encoded tendency of TCRs to bind MHC molecules contributes to the MHC bias of T cell repertoires. It has also become clear that the preimmune repertoire contains functionally heterogeneous subsets including recent thymic emigrants, mature naive phenotype cells, memory phenotype cells, and natural regulatory T cells. In addition, sensitive new detection methods have revealed that the repertoire of naive phenotype T cells consists of distinct pMHC-specific populations that consistently vary in size in different individuals. The implications of these new findings for the clonal selection theory, self-tolerance, and immunodominance are discussed.

p66Shc links alpha1-adrenergic receptors to a reactive oxygen species-dependent AKT-FOXO3A phosphorylation pathway in cardiomyocytes

p66Shc is an adapter protein that is induced by hypertrophic stimuli and has been implicated as a major regulator of reactive oxygen species (ROS) production and cardiovascular oxidative stress responses. This study implicates p66Shc in an alpha(1)-adrenergtic receptor (alpha(1)-AR) pathway that requires the cooperative effects of protein kinase (PK)Cepsilon and PKCdelta and leads to AKT-FOXO3a phosphorylation in cardiomyocytes. alpha(1)-ARs promote p66Shc-YY(239/240) phosphorylation via a ROS-dependent mechanism that is localized to caveolae and requires epidermal growth factor receptor (EGFR) and PKCepsilon activity. alpha(1)-ARs also increase p66Shc-S(36) phosphorylation via an EGFR transactivation pathway involving PKCdelta. p66Shc links alpha(1)-ARs to an AKT signaling pathway that selectively phosphorylates/inactivates FOXO transcription factors and downregulates the ROS-scavenging protein manganese superoxide dismutase (MnSOD); the alpha(1)-AR-p66Shc-dependent pathway involving AKT does not regulate GSK3. Additional studies show that RNA interference-mediated downregulation of endogenous p66Shc leads to the derepression of FOXO3a-regulated genes such as MnSOD, p27Kip1, and BIM-1. p66Shc downregulation also increases proliferating cell nuclear antigen expression and induces cardiomyocyte hypertrophy, suggesting that p66Shc exerts an antihypertrophic action in neonatal cardiomyocytes. The novel alpha(1)-AR- and ROS-dependent pathway involving p66Shc identified in this study is likely to contribute to cardiomyocyte remodeling and the evolution of heart failure.

Life after the thymus: CD31+ and CD31- human naive CD4+ T-cell subsets

Early in life, thymic export establishes the size and the diversity of the human naive T-cell pool. Yet, on puberty thymic activity drastically decreases. Because the overall size of the naive T-cell pool decreases only marginally during ageing, peripheral postthymic expansion of naive T cells has been postulated to account partly for the maintenance of T-cell immunity in adults. So far, the analysis of these processes had been hampered by the inability to distinguish recent thymic emigrants from proliferated, peripheral, naive T cells. However, recently, CD31 has been introduced as a marker to distinguish 2 subsets of naive CD4(+) T cells with distinct T-cell receptor excision circle (TREC) content in the peripheral blood of healthy humans. Here, we review studies that have characterized TREC(hi) CD31(+ thymic)naive CD4(+) T cells and have accordingly used the assessment of this distinct subset of naive CD4(+) T cells as a correlate of thymic activity. We will discuss further potential clinical applications and how more research on CD31(+ thymic)naive and CD31(- central)naive CD4(+) T cells may foster our knowledge of the impact of thymic involution on immune competence.

Ablation of thymic export causes accelerated decay of naive CD4 T cells in the periphery because of activation by environmental antigen

A model of chemical thymectomy by inducible Rag ablation was used to study peripheral T cell homeostasis. Induction of Rag ablation was efficient and complete, leading to cessation of thymic T cell production within 3-4 weeks. The decay of peripheral T cells became apparent with a delay of an additional 2-3 weeks and was entirely accounted for by loss of naïve T cells, whereas numbers of memory phenotype and regulatory T cells were not decreased. Naïve CD4 T cells decayed with an average half-life of 50 days, whereas naïve CD8 T cells exhibited a considerably longer half-life. The rapid decay of naïve CD4 T cells was not caused by intrinsic survival differences compared with naïve CD8 T cells, but was caused by changes in the lymphopenic environment resulting in higher microbial load and consequential activation. This finding suggests that in lymphopenic conditions involving compromised thymic function replenishment and survival of a naïve CD4 T cell repertoire may be severely curtailed because of chronic activation. Such a scenario might play a role in the aging immune system and chronic viral infection, such as HIV infection, and contribute to loss of CD4 T cells and impaired immune function. As our data show, continued replenishment with cells from the thymus seems to be required to maintain efficient gut mucosal defense.

Immune system derived from homeostatic proliferation generates normal CD8 T-cell memory but altered repertoires and diminished heterologous immune responses

The host responds to lymphopenic environments by acute homeostatic proliferation of T lymphocytes, which acquire phenotypes similar to memory cells. Using T-cell knockout (KO) mice adoptively reconstituted with splenocytes from immunologically naive mice, we examined the immune responses of an immune system derived from homeostatically proliferating (HP) T cells. HP cells mounted relatively normal acute CD8 T-cell responses to lymphocytic choriomeningitis virus (LCMV), but with altered T-cell receptor (TCR) repertoires, and they became functional memory cells capable of recall responses. Although homeostatic proliferation does not normally fully restore T-cell numbers, the CD8(+) T-cell pool was completely restored in T-cell KO mice after LCMV infection. CD4 T-cell responses were lower and not fully restored but seemed sufficient to allow for complete differentiation of CD8 memory T cells. The LCMV-immune HP mouse had an immune repertoire heavily biased with LCMV epitope-specific T cells with oligoclonal expansions. LCMV-immune HP mice had reduced cross-reactive and non-cross-reactive CD8 T-cell responses when challenged with a T cell-cross-reactive virus. Thus, whereas an HP immune system is capable of mounting relatively normal acute and memory CD8 T-cell responses, the narrowing of the T-cell repertoire may reduce immune responses to subsequently encountered pathogens.

Augmentation of anti-tumor responses of adoptively transferred CD8+T cells in the lymphopenic setting by HSV amplicon transduction

Treatment of cancer with cytotoxic agents may induce lymphopenia. Adoptively transferred T cells have been reported to display enhanced anti-tumor efficacy in the lymphopenic setting. We reasoned that the anti-tumor effects of adoptively transferred cells in the lymphopenic host could be further augmented through local provision of an innate stimulus in the tumor bed. Utilizing a model in which mice were irradiated to induce lymphopenia, with limited shielding to allow tumor growth, we demonstrate that "triple" therapy consisting of radiation-induced lymphopenia, adoptive transfer of naïve CD8+ T cells, and intra-tumoral HSV amplicon injection resulted in reduced tumor growth compared to the combination of any two of the aforementioned interventions. To gain insight into the mechanism underlying this effect we studied the effects of HSV amplicon transduction into tumors on cytokine expression and on anti-tumor specific T cells. HSV amplicon transduction specifically induced several cytokine mRNAs including IFN-gamma, and IP-10. Adoptively transferred transgenic OT-1 T cells directed against Ovalbumin were more effective against Ovalbumin-expressing tumors when combined with intra-tumoral HSV amplicon injections in the lymphopenic host. Following intra-tumoral HSV-amplicon injections, anti-tumor T cells secreted higher levels of interferon-gamma in response to in-vitro re-stimulation with tumor cells, implying that HSV amplicon injection provided a strong signal for T cell activation. Combining adoptive transfer of naïve T cells in the lymphopenic setting with local T cell stimulation may facilitate expansion and activation of anti-tumor T cell populations in vivo, resulting in enhanced anti-tumor responses without the need to resort to prolonged in vitro T cell culture and/or manipulation.

Sparse production but preferential incorporation of recently produced naive T cells in the human peripheral pool

In mice, recent thymic emigrants (RTEs) make up a large part of the naïve T cell pool and have been suggested to be a distinct short-lived pool. In humans, however, the life span and number of RTEs are unknown. Although (2)H(2)O labeling in young mice showed high thymic-dependent daily naïve T cell production, long term up- and down-labeling with (2)H(2)O in human adults revealed a low daily production of naïve T cells. Using mathematical modeling, we estimated human naïve CD4 and CD8 T cell half-lives of 4.2 and 6.5 years, respectively, whereas memory CD4 and CD8 T cells had half-lives of 0.4 and 0.7 year. The estimated half-life of recently produced naïve T cells was much longer than these average half-lives. Thus, our data are incompatible with a substantial short-lived RTE population in human adults and suggest that the few naïve T cells that are newly produced are preferentially incorporated in the peripheral pool.

Mathematical modeling reveals the biological program regulating lymphopenia-induced proliferation

Recognition of peptide-MHC by the TCR induces T lymphocytes to undergo cell division. Although recognition of foreign peptide induces a program of cellular division and differentiation by responding T cells, stimulation by self-peptide MHC complexes in lymphopenic conditions induces a slower burst of divisions that may or may not be accompanied by effector differentiation. Although both responses are triggered by signals from the TCR, it is not known whether they represent distinct programs of cell cycle control. In this study, we use a mathematical modeling approach to analyze the proliferative response of TCR transgenic F5 T cells to lymphopenia. We tested two fundamentally different models of cell division: one in which T cells are triggered into an "autopilot" deterministic burst of divisions, a model successfully used elsewhere to describe T cell responses to cognate Ag, and a second contrasting model in which cells undergo independent single stochastic divisions. Whereas the autopilot model provided a very poor description of the F5 T cell responses to lymphopenia, the model of single stochastic divisions fitted the experimental data remarkably closely. Furthermore, this model proved robust because specific predictions of cellular behavior made by this model concerning the onset, rate, and nature of division were successfully validated experimentally. Our results suggest cell division induced by lymphopenia involves a process of single stochastic divisions, which is best suited to a homeostatic rather than differentiation role.

Th2 lymphoproliferative disorder of LatY136F mutant mice unfolds independently of TCR-MHC engagement and is insensitive to the action of Foxp3+ regulatory T cells

Mutant mice where tyrosine 136 of linker for activation of T cells (LAT) was replaced with a phenylalanine (Lat(Y136F) mice) develop a fast-onset lymphoproliferative disorder involving polyclonal CD4 T cells that produce massive amounts of Th2 cytokines and trigger severe inflammation and autoantibodies. We analyzed whether the Lat(Y136F) pathology constitutes a bona fide autoimmune disorder dependent on TCR specificity. Using adoptive transfer experiments, we demonstrated that the expansion and uncontrolled Th2-effector function of Lat(Y136F) CD4 cells are not triggered by an MHC class II-driven, autoreactive process. Using Foxp3EGFP reporter mice, we further showed that nonfunctional Foxp3(+) regulatory T cells are present in Lat(Y136F) mice and that pathogenic Lat(Y136F) CD4 T cells were capable of escaping the control of infused wild-type Foxp3(+) regulatory T cells. These results argue against a scenario where the Lat(Y136F) pathology is primarily due to a lack of functional Foxp3(+) regulatory T cells and suggest that a defect intrinsic to Lat(Y136F) CD4 T cells leads to a state of TCR-independent hyperactivity. This abnormal status confers Lat(Y136F) CD4 T cells with the ability to trigger the production of Abs and of autoantibodies in a TCR-independent, quasi-mitogenic fashion. Therefore, despite the presence of autoantibodies causative of severe systemic disease, the pathological conditions observed in Lat(Y136F) mice unfold in an Ag-independent manner and thus do not qualify as a genuine autoimmune disorder.

Modulation of T cell homeostasis and alloreactivity under continuous FTY720 exposure

The immunomodulator FTY720 inhibits lymph node (LN) and thymic egress, thereby constraining T cell circulation and reducing peripheral T cell numbers. Here, we analyzed in mouse models the as yet scarcely characterized impact of long-term (up to 6 months) FTY720 exposure on T cell homeostasis and possible consequences for alloreactivity. In green fluorescent protein (GFP) hemopoietic chimeras, the turnover of (initially GFP(-)) peripheral T cell pools was markedly delayed under FTY720, while normal homeostatic differences between CD4 and CD8 T cell sub-populations were retained or amplified further. Homeostatic proliferation was enhanced, and within shrinking T cell pools, the proportions of effector memory phenotype CD4 T cells (CD4T(PEM)) increased in spleens and LNs and of central memory phenotype CD8 T cells (CD8T(PCM)) in LNs. By contrast, the fractions of CD8T(PEM) and CD4T(PCM) remained stably small under FTY720. The enrichment for CD4T(PEM) and CD8T(PCM) correlated with larger proportions of IFNgamma-producing T cells upon nonspecific but not allospecific stimulation. Splenic CD4 T cells from FTY720-treated mice proliferated more strongly upon transfer to semi-allogeneic hosts. However, heart allograft survival was not compromised in FTY720 pre-treated recipients. It correlated with reduced intra-graft CD8 T cells, and the longest surviving transplants contained the highest numbers of CD4 T cells. Thus, continuous FTY720 exposure reveals differential homeostatic responses by memory phenotype CD4 and CD8 T cell sub-populations, and it may enhance alloreactive CD4 T cell proliferation and tissue infiltration without accelerating allograft rejection.

Transgenic CD4 T cells (DO11.10) are destroyed in MHC-compatible hosts by NK cells and CD8 T cells

During an immune response a small number of rare Ag-specific clones proliferate extensively and decline, leaving a residual population for long-term memory. TCR transgenic (tg) CD4 T cells have been used widely to study the primary and memory response in vivo. We show here that naive TCR tg CD4 T cells from the DO11.10 strain transferred into wild type (wt) BALB/c recipients and not stimulated declined rapidly at the same rate as those primed in vivo by Ag. In the same recipients wt CD4 T cells survived. There was no evidence of an inherent defect in the tg T cells, which survived well when returned to DO11.10 recipients. Surprisingly, wt CD4 T cells declined rapidly in the same DO11.10 hosts. By depleting wt recipients of NK cells or CD8+ cells, the speed of reduction was slowed by half; rapid destruction was prevented completely by combing the two treatments. In contrast, preimmunization accelerated the loss of tg T cells. The results suggested that tg CD4 T cells were actively rejected by both NK and CD8 T cell responses. We consider whether, despite extensive backcrossing, tg T cells may retain genetic material (minor histocompatibility Ags) flanking the construct that compromises their survival in wt recipients.

Quantitative analysis of T cell homeostatic proliferation

T cell homeostatic proliferation occurs on transfer of T cells into lymphopenic recipients; transferred cells undergo several rounds of division in the absence of specific antigen stimulation. For a quantitative analysis of this phenomenon, we applied a mathematical method to describe proliferating T cells to match peak distributions from actual CFSE dilution data. For in vitro stimulation of T cells with anti-CD3/anti-CD28, our simulation confirmed a high proportion of cells entering cell cycle with a low proportion undergoing apoptosis. When applied to homeostatic proliferation, it described striking differences in CD4 and CD8 T cell proliferation rates, and accurately predicted that successive divisions were accompanied by higher rates of apoptosis, limiting the accumulation of proliferating cells. Thus, the presence of multiple CFSE dilution peaks cannot be considered equivalent to lymphocyte expansion. Finally, genetic effects were identified that may help explain links between homeostatic proliferation and autoimmunity.

Lymphopenia-induced proliferation of donor T cells reduces their capacity for causing acute graft-versus-host disease

OBJECTIVE

T cells that undergo lymphopenia-induced proliferation (LIP) are characterized by greater effector and anti-tumor function than naïve T cells. But the ability of these T cells in causing graft-versus-host disease (GVHD) is not known.

METHODS

We tested the hypothesis that donor T cells that had undergone LIP would cause more severe GVHD than naïve T cells by utilizing well-characterized murine experimental models of allogeneic bone marrow transplantation (BMT).

RESULTS

Contrary to our hypothesis, LIP of donor T cells under either noninflammatory or irradiated conditions caused significantly reduced GVHD as determined by survival, clinical, pathologic, and biochemical parameters than naïve T cells. Compared to naïve donor T cells, LIP T cells demonstrated reduced expansion in vivo and in vitro after allogeneic BMT. The reduction in GVHD mortality and severity was observed across multiple strains after allogeneic BMT. In vivo mechanistic studies by cell depletion demonstrated an increase in the CD44(hi) "memory" phenotype T cells and not the CD4(+)CD25(+) T cell subset to be critical for the reduction in GVHD.

CONCLUSIONS

These data demonstrate that LIP of T cells regulates acute GVHD severity in contrast to their ability to cause increased allograft rejection, autoimmunity, or anti-tumor immunity.

No synergy between ATG induction and costimulation blockade induced kidney allograft survival in rhesus monkeys

BACKGROUND

Costimulation blockade with antibodies directed against human CD40 and CD86 leads to prolonged kidney allograft survival in rhesus monkeys, but fails to induce permanent graft acceptance. We have tested whether costimulation blockade is more effective after peripheral T-cell ablation with antithymocyte globulin (ATG), with the aim to remove already primed autoreactive cells present in the normal repertoire.

METHODS

Rhesus monkeys were transplanted with a mismatched kidney allograft. ATG was given around the time of transplantation (day -1 and 0). Costimulation blockade with anti-CD40+anti-CD86 was given at tapering dosages from day -1 to 56. Cyclosporin A (CsA) was given from day 42 onwards and first rejections occurring after day 42 were treated with prednisone.

RESULTS

We observed accelerated rejection in ATG-treated monkeys, compared to animals receiving only costimulation blockade. The accelerated rejection of the kidney allograft occurred despite the application of rejection therapy with steroids and CsA. Three of the five ATG-treated animals were found seropositive for donor-specific alloantibodies. Early biopsies (day 21) from animals treated with ATG and anti-CD40+anti-CD86 show substantially reduced expression of cytotoxic T lymphocyte-associated antigen-4 (CTLA-4) and forkhead box P3 (FOXP3) in focal infiltrates as compared to animals treated with only costimulation blockade. Furthermore, we observed the rapid reappearance of CD8 T-cells with a memory phenotype (disappearance of naive CD95/CD11a T-cells) in peripheral blood.

CONCLUSION

We conclude that (subtotal) T-cell depletion using ATG does not add to costimulation blockade induced kidney allograft survival.

Severely Impaired Clonal Deletion of CD4+T Cells in Low-Dose Irradiated Mice: Role of T Cell Antigen Receptor and IL-7 Receptor Signals

Systemic administration of high doses of soluble Ag induces peripheral CD4+ T cell tolerance in unmanipulated hosts. To test whether tolerance is modified under conditions of transient lymphopenia, we tracked the response of 5C.C7 TCR-transgenic CD4+ T cells to i.v. moth cytochrome c peptide in mice that received low-dose gamma irradiation 10 days previously. This model was chosen because it does not support spontaneous lymphopenia-induced proliferation of 5C.C7 cells, allowing the study of Ag-specific responses without interference from simultaneous spontaneous proliferation. Clonal expansion in response to i.v. peptide was increased in irradiated mice, while clonal deletion was severely impaired in comparison with untreated animals. Amplified TCR triggering was observed in irradiated hosts, consistent with dendritic cell activation leading to enhanced Ag presentation. Failure of deletion was accompanied by persistent T cell activation and accumulation of Th1 effector cells. Up-regulated expression of IL-7R and the prosurvival protein Bcl-x(L) was associated with clonal persistence. Cells with memory and naive phenotypes were both represented within persistent clones, but no Th1 function could be demonstrated within the long-term memory population. Failure of clonal deletion in irradiated hosts represents a novel mechanism limiting TCR diversity in a lymphopenic environment and may contribute to subsequent autoimmunity.

Loss of naïve cells accompanies memory CD4+ T-cell depletion during long-term progression to AIDS in Simian immunodeficiency virus-infected macaques

Human immunodeficiency virus and simian immunodeficiency virus (SIV) induce a slow progressive disease, characterized by the massive loss of memory CD4+ T cells during the acute infection followed by a recovery phase in which virus replication is partially controlled. However, because the initial injury is so severe and virus production persists, the immune system eventually collapses and a symptomatic fatal disease invariably occurs. We have assessed CD4+ T-cell dynamics and disease progression in 12 SIV-infected rhesus monkeys for nearly 2 years. Three macaques exhibiting a rapid progressor phenotype experienced rapid and irreversible loss of memory, but not naïve, CD4+ T lymphocytes from peripheral blood and secondary lymphoid tissues and died within the first 6 months of virus inoculation. In contrast, SIV-infected conventional progressor animals sustained marked but incomplete depletions of memory CD4+ T cells and continuous activation/proliferation of this T-lymphocyte subset. This was associated with a profound loss of naïve CD4+ T cells from peripheral blood and secondary lymphoid tissues, which declined at rates that correlated with disease progression. These data suggest that the persistent loss of memory CD4(+)T cells, which are being eliminated by direct virus killing and activation-induced cell death, requires the continuous differentiation of naïve into memory CD4+ T cells. This unrelenting replenishment process eventually leads to the exhaustion of the naïve CD4+T-cell pool and the development of disease.

CD25+CD4+ regulatory T cells and memory T cells prevent lymphopenia-induced proliferation of naive T cells in transient states of lymphopenia

Lymphopenia has been associated with autoimmune pathology and it has been suggested that lymphopenia-induced proliferation of naive T cells may be responsible for the development of immune pathology. In this study we demonstrate that lymphopenia-induced proliferation is restricted to conditions of extreme lymphopenia, because neither naive nor memory T cells transferred into T cell-depleted hosts proliferate unless the depletion exceeds 90% of the peripheral repertoire. Memory CD4 T cells as well as regulatory CD4 T cells proved to be relatively resistant to depletion regimes, and both subsets restrict the expansion and phenotypic conversion of naive T cells by an IL-7R-dependent mechanism. It therefore seems unlikely that lymphopenia-induced proliferation of peripheral T cells causes deleterious side effects that result in immune pathology in states of partial and transient lymphopenia.

CD52 ligation induces CD4 and CD8 down modulation in vivo and in vitro

To successfully induce donor-specific tolerance after immune depletion, it is essential to understand the residual and recovering immune system in the context of the depleting agent because the properties of such a recovering immune system differ based on the depleting agent used. In this study, we investigate the phenotypic and functional characteristics of T cells exposed to Campath-1H in vivo and in vitro. Recovering T cells demonstrated down modulated surface CD4 and CD8 (by flow cytometry) for up to 45 days after Campath-1H administration. Additionally, these T cells had an activated phenotype. To determine whether this CD4/8 down modulation was due to T-cell activation only or in part due to Campath-1H, whole blood from healthy volunteers was exposed to Campath-1H and the surviving lymphocytes isolated. Flow cytometry revealed a dose-dependent down modulation of CD4/8 without T-cell activation. Additionally, these Campath-1H-treated T cells were immunocompetent as indicated by increased surface CD69 and interleukin-2 (IL-2) production following stimulation by soluble anti-CD3 mAb. In conclusion, Campath-1H by itself down modulates surface CD4 and CD8 without activating T cells.

Renewing the T cell repertoire to arrest autoimmune aggression

There is now evidence that high-dose immune ablation and autologous hematopoietic stem cell transplantation in humans triggers a reconstitution program that leads to the comprehensive renewal of the T cell repertoire. We argue here that several features of this program help to explain how autologous hematopoietic stem cell transplantation can induce long-term clinical remission from organ-specific-, as well as systemic, autoimmune diseases. We propose a model envisioning a coordinated sequence of events, rebuilding an immune system that is competent against infection but that is substantially reconfigured in a way that is less likely to redevelop autoimmunity.

The death receptor Fas (CD95/APO-1) mediates the deletion of T lymphocytes undergoing homeostatic proliferation

Murine T cells adoptively transferred into syngeneic lymphopenic recipients undergo proliferation. Despite continued cell division, this lymphopenia-induced or homeostatic proliferation of a limited number of transferred T cells does not fill the T cell compartment. The continued expansion of the transferred T cells, even after stable T cell numbers have been reached, suggests that active cell death prevents further increase in T cell number. In this study, we show that wild-type T cells undergoing homeostatic proliferation are sensitive to Fas-mediated cell death. In the absence of Fas, T cells accumulate to significantly higher levels after transfer into lymphopenic recipients. Fas is, thus, a principal regulator of the expansion of peripheral T cells in response to self-peptide/MHC during T cell homeostasis. As Fas-deficient lpr mice manifest no significant abnormalities in thymic negative selection or in foreign Ag-induced peripheral T cell deletion, their lymphadenopathy may result from unrestrained homeostatic proliferation.

Do regulatory T cells play a role in the control of homeostatic proliferation?

The control of peripheral lymphocyte numbers is a fundamental aspect of the immune system. Regulatory T cells are involved in the suppression of autoimmune, antitumor, allergic, and other inflammatory responses, as well as in facilitating graft acceptance. In this paper, we discuss whether the control of homeostatic proliferation is another facet of the immune system that is controlled by regulatory T cells. A review of the published data connecting regulatory T cells with the control of homeostatic proliferation indicates that several key questions remain open. One of these relates to the stage at which regulatory T cells could play a role (i.e., T-cell proliferation vs. survival).

Effects of increasing IL-7 availability on lymphocytes during and after lymphopenia-induced proliferation

IL-7 is critically involved in regulating peripheral T cell homeostasis. To investigate the role of IL-7 on lymphopenia-induced proliferation of polyclonal lymphocytes, we have transferred CFSE-labeled cells into a novel T-lymphopenic, IL-7-transgenic mouse line. Results obtained indicate that T and B cells do not respond in the same way to IL-7-homeostatic signals. Overexpression of IL-7 enhances proliferation of both CD4(+) and CD8(+) T cells but with distinctly temporal effects. Expansion of naturally arising CD4(+)-regulatory T cells was like that of conventional CD4(+) T cells. IL-7 had no effect on B cell proliferation. By immunohistology, transferred T cells homed to T cell areas of spleen lymphoid follicles. Increasing IL-7 availability enhanced T cell recovery by promoting cell proliferation and reducing apoptosis during early stages of lymphopenia-induced proliferation. Taken together, these results provide new insights into the pleiotropic effects of IL-7 on lymphopenia-induced T cell proliferation.

A major role for memory CD4 T cells in the control of lymphopenia-induced proliferation of naive CD4 T cells

In a state of lymphopenia, naive and memory CD4 T cells compete with each other for expansion at the expense of naive T cells. This competition prevents the proliferation as well as the phenotypic and functional conversion of naive T cells to "memory-like" T cells and may consequently prevent immune pathology frequently associated with lymphopenia-induced proliferation of naive cells. However, in T cell replete mice, memory T cells do not compete with naive T cells, indicating independent homeostatic control of naive and memory CD4 T cells in conditions that do not involve profound lymphopenia. Moreover, within the memory compartment, subsequent generation of new memory T cells precludes the survival of memory-like T cells. Thus, memory T cells have a major role in the control of lymphopenia-induced proliferation of naive cells because they inhibit both the generation of memory-like T cells and their persistence within the memory compartment.

T cell homeostasis: Keeping useful T cells alive and live T cells useful

It has become clear that the regulation of T cell numbers is under peripheral homeostatic control. However, the rules for homeostasis vary with the T cells' differentiation state and the overall T cell number in the animal. Furthermore, homeostatic pressures can cause unexpected changes in T cell differentiation and function which might promote or dampen T cell reactivity. In this review, we focus on the role of peptide/MHC and cytokine interactions in regulating the size and composition of the T cell pool.

Age-related thymic activity in adults following chemotherapy-induced lymphopenia

BACKGROUND

The potential role of the adult thymus in T-cell homeostasis subsequent to lymphopenia remains the subject of debate. We examined whether thymic activity contributes to reconstitution of the peripheral T-cell pool, a critical process for patients recovering from antineoplastic therapy.

METHODS

In selected patients with various neoplastic diseases we assessed peripheral blood lymphocyte subsets by flow-cytometry, including thymus-derived, CD4+ T cells expressing the CD45RA molecule, and thymic size rebound by CT scan before, and 3, 6 and 12 months after completion of cytotoxic therapy.

RESULTS

Adult patients (n = 21, mean age of 30 years, range 18-49) had higher baseline numbers of B and lower numbers of NK cells than elderly patients (n = 15, mean age of 79 years, range 70-91), while total T-cell numbers did not differ. Despite the reduction of lymphocyte counts being comparable in the adult (mean of 45%) and elderly (mean of 49%) groups, occurring at, or near, completion of treatment, an enlargement of the previously atrophic thymus was evident in 63% of the adult, but in none of the elderly, subjects. In 22 patients who remained active disease-free during the following year, B cells and NK cells recovered to pretreatment levels as soon as at 3 months, whereas overall T-cell recovery occurred at 6 months post-treatment. Thymic rebound, observed in 11 of 22 patients who were of younger age, correlated significantly with a faster and more complete recovery of CD45RA+ CD4+ (mainly helper-naïve) T cells.

CONCLUSION

The adult thymus appears capable of regeneration, at least up to middle age, contributing significantly to the reconstitution of the peripheral T-cell pool following chemotherapy-induced lymphopenia. In advanced age, however, although peripheral homeostatic pathways appear intact, regeneration of the naïve repertoire is incomplete.

Homeostasis of T cell numbers: from thymus production to peripheral compartmentalization and the indexation of regulatory T cells

A system under homeostatic control tends to maintain its structure and functions by establishing dynamic equilibriums controlled by multiple regulatory mechanisms. We have shown that this is the case for immune system. Several different mechanisms seem to participate in the homeostatic control of T cell numbers and population distribution. In other words, besides a quantitative dimension, there is also a qualitative dimension in T cell homeostasis. This is achieved through competition by driving the specialization of sub-populations of lymphocytes to occupy specific niches in the peripheral pool and by developing independent homeostatic mechanisms for each particular cell sub-set. Thus, the sizes of the naïve and memory T cell compartments are governed by independent homeostatic mechanisms, which preserve the capacity to deal with any novel infection (conferred by the presence of naïve T cells) whilst ensuring the efficacy of memory responses when dealing with recurring antigens. Peripheral T cell homeostasis also depends on the integrity of sub-population structure and the presence of regulatory CD4+ CD25+ T cells. The indexation of regulatory CD4+ CD25+ T cell numbers to the numbers of peripheral activated CD4+ T cells is another mechanism of homeostasis that has major advantages in the control of immune responses. It ensures continuous regulation of T cell numbers throughout immune responses, allowing for increases in cell numbers as long as the proportion of CD4+ CD25+ regulatory T cells is kept.

Homeostasis and T cell regulation

Homeostatic regulation of cell numbers is an important principle in biology. Mechanisms that function to maintain or re-establish homeostasis in the immune system include interactions among antigen-presenting cells, regulatory T cells and cytokines. The vital role that homeostatic regulation plays in maintaining a functionally intact immune system is illustrated by the perturbation of the peripheral T cell repertoire that occurs after lymphopenic incidents, which frequently provoke either exacerbated immune or autoimmune responses. Recent studies show that transient states of lymphopenia occur in viral infections and in the neonatal state and might be involved in the development of autoimmune diseases. On the positive side, lymphopenia-provoked T cell expansion might enhance weak immune responses and thereby aid the rejection of tumours or the elimination of parasites.

Thymic output generates a new and diverse TCR repertoire after autologous stem cell transplantation in multiple sclerosis patients

Clinical trials have indicated that autologous hematopoietic stem cell transplantation (HSCT) can persistently suppress inflammatory disease activity in a subset of patients with severe multiple sclerosis (MS), but the mechanism has remained unclear. To understand whether the beneficial effects on the course of disease are mediated by lympho-depletive effects alone or are sustained by a regeneration of the immune repertoire, we examined the long-term immune reconstitution in patients with MS who received HSCT. After numeric recovery of leukocytes, at 2-yr follow-up there was on average a doubling of the frequency of naive CD4⁺ T cells at the expense of memory T cells. Phenotypic and T cell receptor excision circle (TREC) analysis confirmed a recent thymic origin of the expanded naive T cell subset. Analysis of the T cell receptor repertoire showed the reconstitution of an overall broader clonal diversity and an extensive renewal of clonal specificities compared with pretherapy. These data are the first to demonstrate that long-term suppression of inflammatory activity in MS patients who received HSCT does not depend on persisting lymphopenia and is associated with profound qualitative immunological changes that demonstrate a de novo regeneration of the T cell compartment.

Homeostatic proliferation of a Qa-1b-restricted T cell: a distinction between the ligands required for positive selection and for proliferation in lymphopenic hosts

Naive T cells proliferate in response to self MHC molecules after transfer into lymphopenic hosts, a process that has been termed homeostatic proliferation (HP). Previous studies have demonstrated that HP is driven by low level signaling induced by interactions with the same MHC molecules responsible for positive selection in the thymus. Little is known about the homeostatic regulation of T cells specific for class Ib molecules, including Qa-1 and H2-M3, though it has been suggested that their capacity to undergo homeostatic expansion may be inherently limited. In this study, we demonstrate that naive 6C5 TCR transgenic T cells with specificity for Qa-1(b) have a capacity similar to conventional T cells to undergo HP after transfer into sublethally irradiated mice. Proliferation was largely dependent on the expression of beta(2)-microglobulin, and experiments with congenic recipients expressing Qa-1(a) instead of Qa-1(b) demonstrated that HP is specifically driven by Qa-1(b) and not through cross-recognition of classical class I molecules. Thus, the same MHC molecule that mediates positive selection of 6C5 T cells is also required for HP. Homeostatic expansion, like positive selection, occurs in the absence of a Qa-1 determinant modifier, the dominant self-peptide bound to Qa-1 molecules. However, experiments with TAP(-/-) recipients demonstrate a clear distinction between the ligand requirements for thymic selection and HP. Positive selection of 6C5 T cells is dependent on TAP function, thus selection is presumably mediated by TAP-dependent peptides. By contrast, HP occurs in TAP(-/-) recipients, providing an example where the ligand requirements for HP are less stringent than for thymic selection.

Study of Oral Tolerance and Its Indirect Effects in Adoptive Cell Transfer Experiments

Parenteral exposure to antigens to which oral tolerance had been previously induced results in the inhibition of immune responses to other unrelated antigens. Herein we tested whether indirect effects of oral tolerance could be adoptively transferred. Anti-Ova- and antihemoglobin-specific responsiveness as well as oral tolernace to Ova were transferred to irradiated, but not to normal, nonirradiated recepients. Irradiation, thus, facilitated adoptive transfer of oral tolerance. However, the inhibitory (indirect) effects upon the unrelated immunogen were not adoptively transferred, even to irradiated recepients. In addition, we studied adoptively transferred CFSE-labeled spleen cells by flow cytometry in recipient spleen, inguinal lymph nodes, and bone marrow, both in irradiated and nonirradiated recipients, 1, 3, or 5 days after cell transfer. Comparing the percent and absolute number of CFSE-labeled cells in each organ displayed significant differences in the dynamics of decay of adoptively transferred cells from tolerant or immune donors.

Recombinant interleukin-7 induces proliferation of naive macaque CD4+ and CD8+ T cells in vivo

Interleukin-7 (IL-7) regulates T-cell homeostasis, and its availability is augmented in lymphopenic hosts. Naive CD8+ T cells transferred to lymphopenic mice acquire a memory-like phenotype, raising the possibility that IL-7 is the biological mediator of this effect. Here, we provide direct evidence that IL-7 induces the acquisition of memory-cell markers not only in CD8+ T cells but also in CD4+ T-cell subsets in immune-competent Indian rhesus macaques. The increase of these memory-like populations was dependent on the dose of the cytokine, and these cells were found in the blood as well as secondary lymphoid organs. Memory-like CD4+ and CD8+ T cells acquired the ability to secrete tumor necrosis factor alpha and, to a lesser extent, gamma interferon following stimulation with a cognate antigen. The phenotypic change observed in naive T cells was promptly reversed after discontinuation of IL-7. Importantly, IL-7 induced cycling of both CD4+ and CD8+ central memory and effector memory T cells, demonstrating its contribution to the maintenance of the entire T-cell pool. Thus, IL-7 may be of benefit in the treatment of iatrogenic or virus-induced T-cell depletion.

Generation of Anergic and Regulatory T Cells following Prolonged Exposure to a Harmless Antigen

Regulatory CD4(+) T cells are known to develop during the induction of donor-specific peripheral tolerance to transplanted tissues; it is proposed that such tolerance is a consequence of persistent, danger-free stimulation by Ag. To test this hypothesis, male RAG-1(-/-) mice were recolonized with small numbers of monospecific CD4(+) T cells specific for the male H-2E(k)-restricted Ag Dby. After 6 wk in the male environment, the monospecific CD4(+) T cells, having recolonized the host, had become anergic to stimulation in vitro and had acquired a regulatory capacity. CD4(+) T cells in these mice expressed higher levels of CTLA-4 and glucocorticoid-induced TNF-related receptor than naive CD4(+) T cells, but only 3% of the recolonizing cells were CD25(+) and did not express significant foxP3 mRNA. In vivo, these tolerant T cells could censor accumulation of, and IFN-gamma production by, naive T cells, with only a slight inhibition of proliferation. This suppressive effect was not reversed by the addition of fresh bone marrow-derived male dendritic cells. These results suggest that persistent exposure to Ag in conditions that fail to evoke proinflammatory stimuli leads to the development of T cells that are both anergic and regulatory.