Naive CD4(+) lymphocytes convert to anergic or memory-like cells in T cell-deprived recipients

Innate memory T cells

Memory T cells are usually considered to be a feature of a successful immune response against a foreign antigen, and such cells can mediate potent immunity. However, in mice, alternative pathways have been described, through which naïve T cells can acquire the characteristics and functions of memory T cells without encountering specific foreign antigen or the typical signals required for conventional T cell differentiation. Such cells reflect a response to the internal rather the external environment, and hence such cells are called innate memory T cells. In this review, we describe how innate memory subsets were identified, the signals that induce their generation and their functional properties and potential role in the normal immune response. The existence of innate memory T cells in mice raises questions about whether parallel populations exist in humans, and we discuss the evidence for such populations during human T cell development and differentiation.

T cells generated in the absence of a thoracic thymus fail to establish homeostasis

Cervical thymus mimics the thoracic thymus in supporting T-cell development and exists in a subset of mice and humans. Importantly, it remains unknown whether the cervical thymus can generate T cells that are self-tolerant in the complete absence of signals from the thoracic thymus. Using a fetal liver reconstitution model in thoracic thymectomized RAG(-/-) mice, we found that T cells could be generated without contribution from the thoracic thymus. However, these mice had decreased T cells, increased proportions of effector memory T cells and Treg phenotype cells, increased serum IgG1/2b, and increased frequency of T cells expressing IFN-γ, IL-17 or IL-10. Half of the mice that received a thoracic thymectomy and fetal liver cells, unlike sham surgery controls, developed substantial morbidity with age. Disease was associated with lymphopenia-driven activation rather than inherent defects in the cervical thymus, as both thoracic and cervical thymocytes could generate disease in lymphopenic recipients. Administration of the homeostatic cytokine IL-7 caused a rapid, transient increase in T-cell numbers and reduced the time to disease onset. Together the data suggests that the cervical thymus can function in the complete absence of the thoracic thymus; however, the T cells generated do not establish homeostasis.

Impact of T cell selection methods in the success of clinical adoptive immunotherapy

Chemotherapy and/or radiotherapy regular regimens used for conditioning of recipients of hematopoietic stem cell transplantation (SCT) induce a period of transient profound immunosuppression. The onset of a competent immunological response, such as the appearance of viral-specific T cells, is associated with a lower incidence of viral infections after haematopoietic transplantation. The rapid development of immunodominant peptide virus screening together with advances in the design of genetic and non-genetic viral- and tumoural-specific cellular selection strategies have opened new strategies for cellular immunotherapy in oncologic recipients who are highly sensitive to viral infections. However, the rapid development of cellular immunotherapy in SCT has disclosed the role of the T cell selection method in the modulation of functional cell activity and of in vivo secondary effects triggered following immunotherapy.

Depletion and recovery of lymphoid subsets following morphine administration

BACKGROUND AND PURPOSE

Opioid use and abuse has been linked to significant immunosuppression, which has been attributed, in part, to drug-induced depletion of lymphocytes. We sought to define the mechanisms by which lymphocyte populations are depleted and recover following morphine treatment in mice.

EXPERIMENTAL APPROACH

Mice were implanted with morphine pellets and B- and T-cell subsets in the bone marrow, thymus, spleen and lymph nodes were analysed at various time points. We also examined the effects of morphine on T-cell development using an ex vivo assay.

KEY RESULTS

The lymphocyte populations most susceptible to morphine-induced depletion were the precursor cells undergoing selection. As the lymphocytes recovered, more lymphocyte precursors proliferated than in control mice. In addition, peripheral T-cells displayed evidence that they had undergone homeostatic proliferation during the recovery phase of the experiments.

CONCLUSIONS AND IMPLICATIONS

The recovery of lymphocytes following morphine-induced depletion occurred in the presence of morphine and via increased proliferation of lymphoid precursors and homeostatic proliferation of T-cells.

Antigen-dependent and -independent mechanisms of T and B cell hyperactivation during chronic HIV-1 infection

Continuous loss of CD4(+) T lymphocytes and systemic immune activation are hallmarks of untreated chronic HIV-1 infection. Chronic immune activation during HIV-1 infection is characterized by increased expression of activation markers on T cells, elevated levels of proinflammatory cytokines, and B cell hyperactivation together with hypergammaglobulinemia. Importantly, hyperactivation of T cells is one of the best predictive markers for progression toward AIDS, and it is closely linked to CD4(+) T cell depletion and sustained viral replication. Aberrant activation of T cells is observed mainly for memory CD4(+) and CD8(+) T cells and is documented, in addition to increased expression of surface activation markers, by increased cell cycling and apoptosis. Notably, the majority of these activated T cells are neither HIV specific nor HIV infected, and the antigen specificities of hyperactivated T cells are largely unknown, as are the exact mechanisms driving their activation. B cells are also severely affected by HIV-1 infection, which is manifested by major changes in B cell subpopulations, B cell hyperactivation, and hypergammaglobulinemia. Similar to those of T cells, the mechanisms underlying this aberrant B cell activation remain largely unknown. In this review, we summarized current knowledge about proposed antigen-dependent and -independent mechanisms leading to lymphocyte hyperactivation in the context of HIV-1 infection.

Lck regulates IL-10 expression in memory-like Th1 cells

The Src family kinase Lck is thought to facilitate Th2 differentiation; however, its role in Th1 cells has not been well explored. Using mice that lack Lck in mature T cells, we find that lck(-/-) Th1 skewed cells have normal expression of T-bet and produce IFN-γ at WT levels. However, there is a 3-fold increase in IL-10 producing cells in the mutant cultures. These cells do not have elevated levels of IL-4, GATA3, IL-17 or Foxp3, indicating that they are not Th2, Th17, or Foxp3(+) T regulatory cells (Treg). Nor do these cells behave in a similar manner as the type 1 Treg. Most of the IL-10 in the lck(-/-) Th1 cultures is derived from the memory/activated subset, as the cytokine profile from Th1 cultures established from purified CD62L(+) (naïve) cells are similar to WT cells. Furthermore, this IL-10 expression appears to be dependent on IL-12 and correlates with elevated c-Maf. These data highlight a previously unappreciated role for Lck in regulating IL-10 in Th1 cells.

Naive CD4+ T lymphocytes circulate through lymphoid organs to interact with endogenous antigens and upregulate their function

Naive T lymphocytes recirculate through the lymph-vascular system and enter and exit lymphoid organs. Using mice expressing the photoconvertible fluorescence protein Kaede, we demonstrated that naive T cells seek to interact with endogenous Ags after migrating to the lymphoid organs. The interaction with endogenous Ags transiently induces CD69 expression on T cells, which prolongs retention in the lymphoid organs. Cells that fail to express CD69 or lose CD69 expression migrate to other lymphoid organs. Functionally, CD69(+)-naive CD4(+) T cells exhibit faster and greater cytokine production than do CD69(-) naive CD4(+) T cells. These results indicate that CD4(+) T cells continuously migrate to interact with endogenous Ags, and such an interaction plays an important role in the Ag reactivity of naive CD4(+) T cells.

Beta(2) integrin deficiency yields unconventional double-negative T cells distinct from mature classical natural killer T cells in mice

Expressed on leucocytes, beta(2) integrins (CD11/CD18) are specifically involved in leucocyte function. Using a CD18-deficient (CD18(-/-)) mouse model, we here report on their physiological role in lymphocyte differentiation and trafficking. CD18(-/-) mice present with a defect in the distribution of lymphocytes with highly reduced numbers of naïve B and T lymphocytes in inguinal and axillary lymph nodes. In contrast, cervical lymph nodes were fourfold enlarged harbouring unconventional T-cell receptor-alphabeta (TCR-alphabeta) and TCR-gammadelta CD3(+) CD4(-) CD8(-) (double-negative; DN) T cells that expanded in situ. Using adoptive transfer experiments, we found that these cells did not home to peripheral lymph nodes of CD18(wt) recipients but, like antigen-experienced T or natural killer (NK) T cells, recirculated through non-lymphoid organs. Lacking regulatory functions in vitro, CD18(-/-) TCR-alphabeta DN T cells did not suppress the proliferation of polyclonally activated CD4(+) or CD8(+) (single-positive; SP) T cells. Most interestingly, CD18(-/-) TCR-alphabeta DN T cells showed intermediate TCR expression levels, an absent activation through allogeneic major histocompatibility complex and a strong proliferative dependence on interleukin-2, hence, closely resembling NKT cells. However, our data oppose former reports, clearly showing that, because of an absent reactivity with CD1d-alphaGalCer dimers, these cells are not mature classical NKT cells. Our data indicate that CD18(-/-) TCR-alphabeta DN T cells, like NKT and TCR-gammadelta T cells, share characteristics of both adaptive and innate immune cells, and may accumulate as a compensatory mechanism to the functional defect of adaptive immunity in CD18(-/-) mice.

A conserved CXXC motif in CD3epsilon is critical for T cell development and TCR signaling

Structural integrity of the extracellular membrane-proximal stalk region of CD3ε is required for efficient signaling by the T cell antigen receptor complex. The results in this article suggest that receptor aggregation may not be sufficient for a complete T cell receptor signal and that some type of direct allosteric signal may be involved.

Virtually all T cell development and functions depend on its antigen receptor. The T cell receptor (TCR) is a multi-protein complex, comprised of a ligand binding module and a signal transmission module. The signal transmission module includes proteins from CD3 family (CD3ε, CD3δ, CD3γ) as well as the ζ chain protein. The CD3 proteins have a short extracellular stalk connecting their Ig-like domains to their transmembrane regions. These stalks contain a highly evolutionarily conserved CXXC motif, whose function is unknown. To understand the function of these two conserved cysteines, we generated mice that lacked endogenous CD3ε but expressed a transgenic CD3ε molecule in which these cysteines were mutated to serines. Our results show that the mutated CD3ε could incorporate into the TCR complex and rescue surface TCR expression in CD3ε null mice. In the CD3ε mutant mice, all stages of T cell development and activation that are TCR-dependent were impaired, but not eliminated, including activation of mature naïve T cells with the MHCII presented superantigen, staphylococcal enterotoxin B, or with a strong TCR cross-linking antibody specific for either TCR-Cβ or CD3ε. These results argue against a simple aggregation model for TCR signaling and suggest that the stalks of the CD3 proteins may be critical in transmitting part of the activation signal directly through the membrane.

The T cells of the immune system have surface receptors that detect unique features (called antigens) of foreign invaders such as viruses, bacteria and toxins. An encounter between an antigen and the T cell receptor sets off a chain of events that activates the T cell to proliferate and thus call to action the various arms of the immune response that ultimately eliminate the invader. A set of proteins, called CD3, associates with the T cell receptor, spanning the cell membrane. Their function is to deliver a signal to the inside of T cell that its receptor has encountered antigen on the outside of the cell. Two general ideas have been proposed to explain how the CD3 proteins accomplish this: That the engagement of the T cell receptor outside the cell directly causes a change in conformation in the intracellular portion of the associated CD3 proteins that is recognized by the intracellular signaling machinery; and that engagement of the T cell receptor causes clustering of multiple receptor and CD3 proteins such that interactions among the cytoplasmic portions of the many CD3 proteins now attract other proteins to start the chain of intercellular signaling. These two ideas are not mutually exclusive. We show here that mutations in a highly conserved extracellular portion of one of the CD3 proteins can impair the transmission of the activation signal without preventing receptor clustering. These results suggest that direct transmission of a conformational change across the membrane may constitute part of the CD3-mediated activation signal.

Lymphopenia-induced spontaneous T-cell proliferation as a cofactor for autoimmune disease development

Lymphopenia is thought to be a major cause of tolerance breakdown. In a lymphopenic environment, self-recognition events induce some T cells to expand strongly (a mechanism known as spontaneous proliferation). In this study, we show that in C57BL/6 mice, the repertoire resulting from lymphopenia-induced spontaneous CD4(+) T-cell proliferation included a proportion of regulatory T cells as large as that observed in a normal mouse, and no autoimmune disorder was observed. By contrast, in nonobese diabetic mice, differences in the ability of conventional and regulatory T cells to expand in response to lymphopenia led to an unbalance between these 2 T-cell compartments at the expense of regulatory T cells, resulting in the onset of autoimmune diseases. Notably, this accounted for the rapid transfer of diabetes with small numbers of BDC2.5 CD4(+) T cells. Thus, lymphopenia does not itself induce autoimmunity, but it should be considered as a cofactor for the development of autoimmune disorders.

RTEs: lazy T-cell teenagers

In this issue of Blood, Opiela and colleagues analyze the phenotype and function of the lymphoid periphery's youngest T cells, RTEs.

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.

Comprehensive assessment and mathematical modeling of T cell population dynamics and homeostasis

Our current view of T cell differentiation and population dynamics is assembled from pieces of data obtained from separate experimental systems and is thus patchy. We reassessed homeostasis and dynamics of T cells 1) by generating a mathematical model describing the spatiotemporal features of T cell differentiation, and 2) by fitting this model to experimental data generated by disturbing T cell differentiation through transient depletion of dividing T cells in mice. This specific depletion was obtained by administration of ganciclovir to mice expressing the conditional thymidine kinase suicide gene in T cells. With this experimental approach, we could derive quantitative parameters describing the cell fluxes, residence times, and rates of import, export, proliferation, and death across cell compartments for thymocytes and recent thymic emigrants (RTEs). Among other parameters, we show that 93% of thymocytes produced before single-positive stages are eliminated through the selection process. Then, a postselection peripheral expansion of naive T cells contributes three times more to naive T cell production than the thymus, with half of the naive T cells consisting of dividing RTEs. Altogether, this work provides a quantitative population dynamical framework of thymocyte development, RTEs, and naive T cells.

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.

T cell activation and proliferation characteristic for HIV-Nef transgenic mice is lymphopenia induced

The HIV-Nef protein has been implicated in generating high viral loads and T cell activation. Transgenic (tg) mice with constitutive T cell-specific Nef expression show a dramatic reduction in T cell number and highly increased T cell turnover. Previous studies in Nef tg mice attributed this T cell activation to a direct effect of Nef at the cellular level. Given the strongly reduced peripheral T cell numbers, we examined whether this enhanced T cell division might instead be lymphopenia induced. Adoptively transferred naive wild-type T cells into lymphopenic Nef tg mice showed high T cell turnover and obtained the same effector/memory phenotype as the autologous Nef tg T cells, supporting the idea that the microenvironment determines the phenotype of the T cells present. Moreover, in bone marrow chimeras from mixtures of wild-type and Nef tg bone marrow, with a full T cell compartment containing a small proportion of Nef tg T cells, Nef tg T cells kept a naive phenotype. These results demonstrate that T cell activation in the Nef tg mice is lymphopenia induced rather than due to a direct T cell-activating effect of Nef.

Environmental and intrinsic factors lead to antigen unresponsiveness in CD4(+) recent thymic emigrants from aged mice

Naive CD4 cells from aged mice respond inefficiently to Ag, but the factors that underlie the age-associated defects remain unclear. We have used two approaches to isolate recent thymic emigrants (RTE) in young and aged mice and have compared their capacity to respond to antigenic stimulation ex vivo. An in situ intrathymic CFSE injection labeled developing thymocytes and allowed the identification of RTE in secondary lymphoid tissues. Analysis of CFSE-labeled RTE and control unlabeled naive CD4 cells indicated that cells from aged mice were defective in their ability to increase intracellular Ca(2+) concentration following TCR cross-linking. Aged naive and RTE CD4 also secreted less IL-2 and proliferated less than that of comparable young CD4 populations. Defects in effector generation in aged RTE were overcome by the addition of IL-2 to cultures. RTE from both polyclonal and TCR transgenic mice were compromised, indicating that defects were independent of TCR specificity. In the second model, the cotransfer of congenic marker-labeled young and aged BM cells into young and aged syngeneic hosts revealed that hyporesponsiveness in aged RTE was caused by a combination of defects intrinsic to CD4 progenitors and defects induced by the aged environment. Depletion of peripheral CD4 cells in aged mice led to production of new RTE that were not defective. The results of this study suggest that defects induced by environmental and lineage intrinsic factors act together to reduce responses to Ag in aged naive CD4 cells and that these defects can be overcome in aged CD4 cells produced during recovery from 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.

T cell homeostasis in steady state and lymphopenic conditions

Homeostasis in the immune system is an important principle ensuring that the numbers of peripheral lymphocytes are kept more or less constant despite numerous disturbances in the immune system during the lifetime of an organism. Mechanisms relating to maintenance of homeostasis have mainly been investigated in experimental systems exhibiting extreme lymphopenic conditions in which the behavior of adoptively transferred lymphocytes is assessed in the absence of endogenous lymphocytes. While these experimental systems have yielded important insight into mechanisms that shape the survival and expansion potential of T cells, their extrapolation to normal steady state conditions that do not involve extreme lymphopenia has sometimes been difficult. We review here the different scenarios of homeostatic control mechanisms in steady state as well as severely or partially lymphopenic environments.

Reshaping the past: Strategies for modulating T-cell memory immune responses

Memory T cells are generated following an initial encounter with antigen, persist over the lifetime of an individual, and mediate rapid and robust functional responses upon antigenic recall. While immune memory is generally associated with protective immune response to pathogens, memory T cells can be generated to diverse types of antigens including autoantigens and alloantigens through homologous or crossreactive priming and comprise the majority of circulating T cells during adulthood. Memory T cells can therefore play critical roles in propagating and perpetuating autoimmune disease and in mediating allograft rejection, although the precise pathways for regulation of memory immune responses remain largely undefined. Moreover, evaluating and designing strategies to modulate memory T-cell responses are challenging given the remarkable heterogeneity of memory T cells, with different subsets predominating in lymphoid versus non-lymphoid tissue sites. In this review, we discuss what is presently known regarding the effect of current immunomodulation strategies on the memory T-cell compartment and potential strategies for controlling immunological recall.

The Thymus Plays a Role in Oral Tolerance in Experimental Autoimmune Encephalomyelitis

The oral administration of myelin proteins has been used for the successful prevention and treatment of experimental autoimmune encephalomyelitis (EAE). We questioned whether the thymus was involved in oral tolerance. In this study, euthymic myelin basic protein (MBP) TCR transgenic mice are protected from EAE when fed MBP but are not protected when thymectomized. Similarly, in a cell transfer system, T cell responses to OVA measured in vivo were suppressed significantly only in the OVA-fed euthymic mice but not in the thymectomized mice. We observed that the absence of the thymus dramatically enhanced the Th1 response. We explored three alternatives to determine the role of the thymus in oral tolerance: 1) as a site for the induction of regulatory T cells; 2) a site for deletion of autoreactive T cells; or 3) a site for the dissemination of naive T cells. We found that Foxp3(+)CD4(+)CD25(+) T cells are increased in the periphery but not in the thymus after Ag feeding. These CD4(+)CD25(+) T cells also express glucocorticoid-induced TNFR and intracellular CTLA4 and suppress Ag-specific proliferation of CD4(+)CD25(-) cells in vitro. The thymus also plays a role in deletion of autoreactive T cells in the periphery following orally administered MBP. However, thymectomy does not result in homeostatic proliferation and the generation of memory cells in this system. Overall, the oral administration of MBP has a profound effect on systemic immune responses, mediated largely by the generation of regulatory T cells that act to prevent or suppress EAE.

The generation of protective memory-like CD8+ T cells during homeostatic proliferation requires CD4+ T cells

Antigen-specific memory T cells are a critical component of protective immunity because of their increased frequency and enhanced reactivity after restimulation. However, it is unclear whether 'memory-like' T cells generated during lymphopenia-induced homeostatic proliferation can also offer protection against pathogens. Here we show that homeostatic proliferation-induced memory (HP-memory) CD8(+) T cells controlled bacterial infection as effectively as 'true' memory CD8(+) T cells, but their protective capacity required the presence of CD4(+) T cells during homeostatic proliferation. The necessity for CD4 help was overcome, however, if the HP-memory CD8(+) T cells lacked expression of TRAIL (tumor necrosis factor-related apoptosis-inducing ligand; also called Apo-2L). Thus, like conventional CD8(+) memory T cells, the protective function of HP-memory CD8(+) T cells shows dependence on CD4(+) T cell help.

Homeostasis of the memory T cell pool

Memory T cells are critical for the establishment of long-term immunity. The number of memory T cells formed at the conclusion of the primary response is strongly influenced by the number of effector T cells generated in the response, but some factors can additionally enhance the efficiency and quality of memory cell recruitment. Homeostasis of the memory T cell pool depends on cytokine-mediated regulation of cell survival and proliferation. This review discusses factors that influence both the development and the maintenance of the memory T cell pool.

Control of homeostatic proliferation by regulatory T cells

Homeostatic proliferation of T cells leads to the generation of effector/memory cells, which have the potential to cause harm to the host. The role of Tregs in the control of homeostatic proliferation is unclear. In this study we utilized mice that either harbor or lack Tregs as recipients of monoclonal or polyclonal T cells. We observed that while Tregs completely prevented cell division of T cells displaying low affinity for self ligands, they had a less marked, albeit significant, effect on cell cycle entry of T cells displaying higher affinity. The presence of Tregs resulted in a lower accumulation of T cells, enhanced apoptosis, and impaired differentiation to a cytokine-producing state. We conclude that Tregs play a major role in the control of homeostatic proliferation.

Regulation of mature T cell homeostasis

The overall size and the composition of the mature T cell pool are regulated by homeostatic mechanisms. Recent work has revealed that homeostatic signals are received from contact with two members of the common gamma chain family of cytokines, IL-7 and IL-15, and from self-MHC/peptide ligands. In essence, homeostasis of naïve T cells is regulated by IL-7 and self-MHC/peptide ligands and homeostasis of memory CD8 cells is controlled by IL-7 and IL-15. All of these signals also appear to be important to a varying degree for homeostasis of memory CD4 cells, but the details are less well understood than for other cell type.

In vivo expansion of CD4CD45RO-CD25 T cells expressing foxP3 in IL-2-treated HIV-infected patients

Administration of IL-2 to HIV-infected patients leads to expansion of a unique subset of CD4CD45ROCD25 cells. In this study, the origin, clonality, and function of these cells were investigated. Analysis of TCR excision circles revealed that the CD4CD45ROCD25 cells were the product of peripheral expansion but remained polyclonal as determined by TCR repertoire analysis. Phenotypically, these cells were distinct from naturally occurring Tregs; they exhibited intermediate features, between those of memory and naive cells, and had lower susceptibility to apoptosis than CD45ROCD25 or memory T cells. Studies of intracellular cytokine production and proliferation revealed that cytokine-expanded naive CD25 cells had low IL-2 production and required costimulation for proliferation. Despite elevated expression of forkhead transcription factor P3 (foxP3), they exerted only weak suppression compared with CD45ROCD25 cells (Tregs). In summary, in vivo IL-2 administration to HIV-infected patients leads to peripheral expansion of a population of long-lived CD4CD45ROCD25 cells that express high levels of foxP3 but exert weak suppressive function. These CD4CD25 cytokine-expanded naive cells, distinct from antigen-triggered cells and Tregs, play a role in the maintenance of a state of low turnover and sustained expansion of the CD4 T cell pool.

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.

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.

T cell homeostasis in tolerance and immunity

The size of the peripheral T cell pool is remarkably stable throughout life, reflecting precise regulation of cellular survival, proliferation, and apoptosis. Homeostatic proliferation refers to the process by which T cells spontaneously proliferate in a lymphopenic host. The critical signals driving this expansion are "space," contact with self-major histocompatibility complex (MHC)/peptide complexes, and cytokine stimulation. A number of studies have delineated an association between T cell lymphopenia, compensatory homeostatic expansion, and the development of diverse autoimmune syndromes. In the nonobese diabetic mouse model of type 1 diabetes, lymphopenia-induced homeostatic expansion fuels the generation of islet-specific T cells. Excess interleukin-21 facilitates T cell cycling but limited survival, resulting in recurrent stimulation of T cells specific for self-peptide/MHC complexes. Indeed, data from several experimental models of autoimmunity indicate that a full T cell compartment restrains homeostatic expansion of self-reactive cells that could otherwise dominate the repertoire. This review describes the mechanisms that govern T cell homeostatic expansion and outlines the evidence that lymphopenia presents a risk for development of autoimmune disease.

The Gads (GrpL) adaptor protein regulates T cell homeostasis

Little is known about the role of the Gads (GrpL) adaptor protein in mature T cell populations. In this study we show that the effects of Gads deficiency on murine CD4(+) and CD8(+) T cells are markedly different. Gads(-/-) CD4(+) T cells were markedly deficient in the spleen and had an activated phenotype and a rapid turnover rate. When transferred into a wild-type host, Gads(-/-) CD4(+) T cells continued to proliferate at a higher rate than wild-type CD4(+) T cells, demonstrating a defect in homeostatic proliferation. Gads(-/-) CD8(+) T cells had a memory-like phenotype, produced IFN-gamma in response to ex vivo stimulation, and underwent normal homeostatic proliferation in wild-type hosts. Gads(-/-) T cells had defective TCR-mediated calcium responses, but had normal activation of ERK. Gads(-/-) CD4(+) T cells, but not CD8(+) T cells, had a severe block of TCR-mediated proliferation and a high rate of spontaneous cell death and were highly susceptible to CD95-induced apoptosis. This suggests that the rapid turnover of Gads(-/-) CD4(+) T cells is due to a defect in cell survival. The intracellular signaling pathways that regulate homeostasis in CD4(+) and CD8(+) T cells are clearly different, and the Gads adaptor protein is critical for homeostasis of CD4(+) T cells.

CD4 T-cell memory

CD4 T-cell memory is in some ways more enigmatic than CD8 T-cell memory. This is mostly due to the fact that CD4 T cells tend to expand far less in response to antigenic stimuli, thereby thwarting attempts at their detection during the course of an immune response. Nevertheless, there is a wide range of experimental models that have provided information regarding the survival and maintenance of CD4 memory cells, their functional capacities, their differentiation states and program of development following activation. The emerging picture is one of great versatility and functional heterogeneity as befits their central position within the immune system.

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.

Are major histocompatibility complex molecules involved in the survival of naive CD4+ T cells?

The exact role of major histocompatibility complex (MHC) molecules in the peripheral survival of naive T cells is controversial, as some studies have suggested that they are critically required whereas others have suggested that they are not. Here we controlled for some of the features that differed among the earlier studies, and analyzed both the survival and expansion of naive CD4⁺ T cells transferred into MHC syngeneic, allogeneic, or MHC negative environments. We found that naive T cells transferred into MHC negative or allogeneic environments often fail to survive because of rejection and/or competition by natural killer (NK) cells, rather than failure to recognize a particular MHC allele. In the absence of NK cells, naive CD4⁺ T cells survived equally well regardless of the MHC type of the host. There was, however, an MHC requirement for extensive space-induced “homeostatic” expansion. Although the first few divisions occurred in the absence of MHC molecules, the cells did not continue to divide or transit to a CD44^hi phenotype. Surprisingly, this MHC requirement could be satisfied by alleles other than the restricting haplotype. Therefore, space-induced expansion and survival are two different phenomena displaying different MHC requirements. Memory CD4⁺ T cells, whose survival and expansion showed no requirements for MHC molecules at all, dampened the space-induced expansion of naive cells, showing that the two populations are not independent in their requirements for peripheral niches.

Host stem cells can selectively reconstitute missing lymphoid lineages in irradiation bone marrow chimeras

The regulatory elements governing the process of lymphopoiesis from pluripotential stem cells to mature lymphocytes are not well understood. In this study we found that in bone marrow chimeras made by reconstituting lethally irradiated normal mice with bone marrow taken from genetically B-cell-deficient animals (microMT.B6 --> F1) the B-cell compartment is reconstituted with host-derived B cells. Similarly, in animals reconstituted with bone marrow taken from mice with genetic deficiencies in the development of T cells (TCR-/- --> F1) or both B and T cells (RAG-/- --> F1), the missing lymphocyte lineage(s) was specifically reconstituted from host-derived cells. In all chimeras, all other blood lineages were generated from donor-derived stem cells. Control chimeras (B6 --> F1) had only donor-derived hematopoietic cells as expected. The reconstituted, host-derived lymphoid compartments contained normal functional cell populations as determined by the presence of T cells expressing all 16 common TCR Vbeta families, and the presence of all antibody isotypes in the serum. Reconstituted TCR-/- --> F1 chimeras were also able to mount T-cell proliferative responses to foreign antigens equal to those of control animals. This observation would seem to suggest that during lymphopoietic reconstitution, missing lymphoid lineages can dictate their own reconstitution.

On the role of MHC class II molecules in the survival and lymphopenia-induced proliferation of peripheral CD4+ T cells

CD4(+) T cells expand after transfer into lymphopenic H-2(b) A(beta)-/- mice (I-A(beta)-, I-E(alpha)-deficient mice) but not after transfer into lymphopenic MHC II(Delta/Delta) mice (I-A(alpha)-, I-A(beta)-, I-E(alpha)-, and I-E(beta)-deficient mice), implying that in H-2(b) A(beta)-/- mice, A(alpha) chain and E(beta) chain associate to form a hybrid A(alpha)E(beta) MHC class II molecule. In light of this unexpected result, we reexamined the MHC class II requirement in the survival and lymphopenia-induced proliferation of CD4(+) T cells. Here we show that expansion, but not short-term survival, of CD4(+) T cells depends on interactions with MHC class II molecules in lymphopenic mice. Nevertheless, interactions with classical MHC class II molecules are required for CD4(+) T cells to survive in CD8(+) T-cell-containing mice.

Involvement of avidity for major histocompatibility complex in homeostasis of naive and memory T cells

The requirements for survival and self-renewal of peripheral T cells and the nature of mechanisms controlling the size of the naive and memory pool are not completely understood. Here, we examine the involvement of the major histocompatibility complex (MHC) in survival and homeostatic expansion of naive and memory T cells. We show that the homeostatic behavior of naive T cell receptor (TCR)-transgenic T cells can be deduced by the expression levels of TCR and CD5, a negative regulator of TCR signaling. Both these factors determine the strength of TCR stimulation by MHC-derived signals. We further show that, similarly to naive T cells, MHC-derived signals influence the homeostatic expansion capacity of memory T cells under lymphopenic conditions. In contrast to naive T cells, however, memory T cells can reach a homeostatic equilibrium, in which survival/self-renewal of each clone is dissociated from their avidity for MHC-derived signals.

Long-term CD4+ and CD8+ memory T cells developed in severe combined immunodeficiency mice during homoeostasis exhibit differences in sensitivity to antigen

T cells transferred in small numbers to lymphopenic hosts proliferate spontaneously, and naïve T cells turn into memory cells without complete cellular reconstitution of the lymphoid compartment. In this study, neonatal severe combined immunodeficiency mice were treated with peripheral CD4+ or CD8+ T cells purified from the spleen of syngeneic C.B-17 mice. At 2 weeks and more pronounced at 10 weeks post treatment, a majority of the residing donor T cells showed memory phenotype, with high expression of CD44 and an early onset of proliferation and cytokine production upon stimulation. These memory type of donor cells were sustained in numbers for at least 1.5 years post treatment in a homoeostatic fashion, recognized by normal CD4/CD8 ratio and no bias towards type 1 or type 2 immune response. Furthermore, amongst the memory type of cells, there was a striking difference in their response, where the CD8+ donor cells had higher threshold for stimulation than the CD4+ donor cells.

Suppressor of cytokine signaling-1 is a critical regulator of interleukin-7-dependent CD8+ T cell differentiation

To determine the tissue-specific functions of SOCS-1, mice were generated in which the SOCS-1 gene could be deleted in individual tissues. A reporter gene of SOCS-1 promoter activity was also inserted. Using the reporter, high SOCS-1 expression was found at the CD4(+)CD8(+) stage in thymocyte development. To investigate the function of this expression, the SOCS-1 gene was specifically deleted throughout the thymocyte/T/NKT cell compartment. Unlike SOCS-1(-/-) mice, these mice did not develop lethal multiorgan inflammation but developed multiple lymphoid abnormalities, including enhanced differentiation of thymocytes toward CD8(+) T cells and very high percentages of peripheral CD8(+) T cells with a memory phenotype (CD44(hi)CD25(lo)CD69(lo)). These phenotypes were found to correlate with hypersensitivity to the gamma-common family of cytokines.

T cell regulation as a side effect of homeostasis and competition

We have previously hypothesized that maintaining a balanced peripheral immune system may not be the sole responsibility of a specialized subset of T cells dedicated to immune regulation, but also a side effect of normal competition for shared resources within an intact immune system. Here we show that regulatory activity is correlated with high homeostatic expansion potential, reflecting the avidity for self-peptide:MHC complexes. Monoclonal transgenic T cells with high homeostatic expansion potential and lacking characteristics previously associated with regulatory function were able to regulate wasting disease induced by transfer of a small number of naive CD45RB(hi) CD4 T cells into lymphopenic hosts. Self-regulatory function is also found in the naive polyclonal T cell repertoire depleted of CD25(+) T cells. T cells capable of preventing immune pathology, like the transgenic T cells, express higher than average levels of CD5, an indicator of avidity for self:MHC peptide complexes. We therefore propose that dysregulated expansion of potentially pathogenic T cells in a lymphopenic environment can be prevented by members of the naive T cell repertoire, irrespective of their specificity, as a side effect of their response to homeostatic and antigenic stimulation.

CD40 ligand in pathogenesis of autoimmune ovarian disease of day 3-thymectomized mice: implication for CD40 ligand antibody therapy

The blockade of CD40 ligand (CD40L) is effective in autoimmune disease prevention. Recently, a brief period of CD40L mAb treatment was reported to induce tolerance and enhancement of CD4(+)CD25(+) regulatory T cell activity. We therefore determined the efficacy of CD40L mAb treatment in autoimmunity that resulted from CD4(+)CD25(+) regulatory T cell deficiency. Autoimmune ovarian disease (AOD) and oocyte autoantibody response of day 3-thymectomized (d3tx) mice were inhibited by continuous CD40L mAb treatment from day 3, or from days 10-14, whereas CD40L mAb treatment confined to the neonatal week was ineffective. The enhanced expression of memory markers (CD44 and CD62L(low)) on CD4(+) T cells of the d3tx mice was unaffected by CD40L mAb treatment. In contrast, their increased T cell activation markers (CD69 and CD25) were eliminated by CD40L mAb treatment. Moreover, ex vivo activated T cells of d3tx mice expressed elevated intracellular IFN-gamma, and this was also blocked by CD40L mAb. The memory T cells, although nonpathogenic in CD40L mAb-positive environment, transferred severe AOD to CD40L mAb(-) neonatal recipients. Most importantly, CD40L mAb treatment inhibited AOD in recipients of T cells from d3tx donors with severe AOD and led to regression of AOD in d3tx mice documented at 4 wk. Therefore, 1) the continuous presence of CD40L mAb both prevents and causes regression of AOD in the d3tx mice; and 2) the multiple steps of the d3tx autoimmune disease, including T cell activation, cytokine production, T cell-mediated inflammation, and tissue injury, are CD40L dependent.

Cutting edge: homeostatic proliferation of peripheral T lymphocytes is regulated by clonal competition

Homeostatic proliferation functions to maintain peripheral T cell numbers and is regulated by cytokines. In this study, we provide evidence that T cell homeostasis is also regulated by clonal competition. Naive polyclonal T cells divided when transferred to TCR transgenic hosts, as did monoclonal T cells when transferred to TCR transgenic hosts of differing clonotype. However, T cells did not divide in hosts of identical clono-type. Transgenic T cell proliferation was inhibited in irradiated hosts of the same clonotype, while cotransferred nontransgenic T cells proliferated extensively. These results show that clonal competition is a component of homeostasis that may contribute to selection of the peripheral T cell repertoire.

Murine neonatal CD4+ lymph node cells are highly deficient in the development of antigen-specific Th1 function in adoptive adult hosts

It is well established that murine neonates are biased toward Th2 responses. Th2-dominant responses are observed following immunization with a variety of Ags, using different carrier/adjuvant systems, and are seen in both BALB/c and C57BL/6 mice. Therefore, Th2 skewing appears to be a universal phenomenon unique to the neonatal period. One important question about this phenomenon is whether these responses are due to T cell intrinsic properties or are regulated by the neonatal environment. Here we have addressed this issue by transferring neonatal or adult CD4(+) lymph node cells to adoptive adult recombinase-activating gene 2(-/-) hosts and studied the development of Th responses. Neonatal CD4(+) cells were highly deficient in the development of both primary and secondary Ag-specific Th1 responses. This did not appear to be due to anergy of a developed population, since exogenous IL-2 only marginally increased production of the Th1 cytokine IFN-gamma. This profound Th1 deficiency was observed despite similar proliferation by neonatal and adult cells within the recombinase-activating gene 2(-/-) hosts. Moreover, neonatal CD4(+) cells up-regulated activation markers in a manner similar to adult CD4(+) cells. Therefore, although their proliferation and phenotypic maturation proceeded normally, neonatal CD4(+) cells appeared to be intrinsically deficient in the functional maturation of Th1 lineage cells. These results offer a candidate explanation for the reduced graft-vs-host responses observed following transplantation of cord blood cells or murine neonatal lymphoid cells to allogeneic adult hosts.

Prolonged islet allograft survival by adenovirus-mediated transfer of sICAM-1/Ig immunoadhesin gene

Administration of monoclonal antibodies directed against the leukocyte function-associated antigen 1 (LFA-1)-intercellular adhesion molecule 1 (ICAM-1) pathway showed that these costimulatory molecules play a key role in allograft rejection. Here, adenoviral gene transfer of an immunoadhesin, sICAM-1/Ig, was used to prolong islet allograft survival in a mouse model, and was compared with anti-LFA-1 antibody administration. A replication-deficient recombinant adenoviral vector encoding a chimeric protein, in which the extracellular domain of ICAM-1 is covalently linked to the C(H)2-C(H)3 domains of an IgG1, was used for gene transfer. C3H murine islets were transplanted under the kidney capsule of streptozotocin-induced diabetic BALB/c mice. Experimental groups underwent adenovirus vector administration either in vivo (intravenous injection) or ex vivo (gene transfer to the graft), and control groups received either an empty vector (Ad.null) or an anti-LFA-1 monoclonal antibody. Graft survival was significantly prolonged by in vivo sICAM-1/Ig gene transfer as compared with both Ad.null and anti-LFA-1 groups, but not by ex vivo gene transfer. Histological examination of the grafts showed the presence of a mononuclear infiltrate within functioning grafts, suggesting that the homing of alloreactive T cells was not altered. In vitro T cell proliferation experiments indicated that sICAM-1/Ig exerted agonist effects on both CD4(+) and CD8(+) T cells.

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

To examine directly whether a limited number of naive T cells transferred to lymphopenic hosts can truly fill the peripheral naive T cell pool, we compared the expansion and phenotype of naive T cells transferred to three different hosts, namely recombination-activating gene-deficient mice, CD3epsilon-deficient mice, and irradiated normal mice. In all three recipients, the absolute number of recovered cells was much smaller than in normal mice. In addition, transferred naive T cells acquired a memory-like phenotype that remained stable with time. Finally, injected cells were rapidly replaced by host thymic migrants in irradiated normal mice. Only continuous output of naive T cells by the thymus can generate a full compartment of truly naive T cells. Thus, 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.