CD4+CD25+ regulatory T cells are activated in vivo by recognition of self

T cell responses at diagnosis of amyotrophic lateral sclerosis predict disease progression

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease, involving neuroinflammation and T cell infiltration in the central nervous system. However, the contribution of T cell responses to the pathology of the disease is not fully understood. Here we show, by flow cytometric analysis of blood and cerebrospinal fluid (CSF) samples of a cohort of 89 newly diagnosed ALS patients in Stockholm, Sweden, that T cell phenotypes at the time of diagnosis are good predictors of disease outcome. High frequency of CD4⁺FOXP3^- effector T cells in blood and CSF is associated with poor survival, whereas high frequency of activated regulatory T (Treg) cells and high ratio between activated and resting Treg cells in blood are associated with better survival. Besides survival, phenotypic profiling of T cells could also predict disease progression rate. Single cell transcriptomics analysis of CSF samples shows clonally expanded CD4⁺ and CD8⁺ T cells in CSF, with characteristic gene expression patterns. In summary, T cell responses associate with and likely contribute to disease progression in ALS, supporting modulation of adaptive immunity as a viable therapeutic option.

Self-Recognition Sensitizes Mouse and Human Regulatory T Cells to Low-Dose CD28 Superagonist Stimulation

In rodents, low doses of CD28-specific superagonistic monoclonal antibodies (CD28 superagonists, CD28SA) selectively activate regulatory T cells (Treg). This observation has recently been extended to humans, suggesting an option for the treatment of autoimmune and inflammatory diseases. However, a mechanistic explanation for this phenomenon is still lacking. Given that CD28SA amplify T cell receptor (TCR) signals, we tested the hypothesis that the weak tonic TCR signals received by conventional CD4⁺ T cells (Tconv) in the absence of cognate antigen require more CD28 signaling input for full activation than the stronger TCR signals received by self-reactive Treg. We report that in vitro, the response of mouse Treg and Tconv to CD28SA strongly depends on MHC class II expression by antigen-presenting cells. To separate the effect of tonic TCR signals from self-peptide recognition, we compared the response of wild-type Treg and Tconv to low and high CD28SA doses upon transfer into wild-type or H-2M knockout mice, which lack a self-peptide repertoire. We found that the superior response of Treg to low CD28SA doses was lost in the absence of self-peptide presentation. We also tested if potentially pathogenic autoreactive Tconv would benefit from self-recognition-induced sensitivity to CD28SA stimulation by transferring TCR transgenic OVA-specific Tconv into OVA-expressing mice and found that low-dose CD28SA application inhibited, rather than supported, their expansion, presumably due to the massive concomitant activation of Treg. Finally, we report that also in the in vitro response of human peripheral blood mononuclear cells to CD28SA, HLA II blockade interferes with the expansion of Treg by low-dose CD28SA stimulation. These results provide a rational basis for the further development of low-dose CD28SA therapy for the improvement of Treg activity.

TCR Signaling and CD28/CTLA-4 Signaling Cooperatively Modulate T Regulatory Cell Homeostasis

Foxp3⁺ T regulatory cells (Tregs), conventional CD4⁺Foxp3^- T cells, and CD8⁺ T cells represent heterogeneous populations composed of naive phenotype (NP, CD44^low) and memory phenotype (MP, CD44^high) subpopulations. NP and MP subsets differ in their activation state, contribution to immune function, and capacity to proliferate in vivo. To further understand the factors that contribute to the differential homeostasis of NP/MP subsets, we examined the differential effects of CD28 and CTLA-4 interaction with CD80/CD86, as well as MHC class II-TCR interaction within mouse Treg pools and CD4⁺ and CD8⁺ T cell pools. Blockade of CD80/CD86 with CTLA-4-Ig markedly reduced the cycling and absolute numbers of MP Tregs and MP CD4⁺ T cells, with minimal effect on the NP T cell subpopulations. Blockade of MHC class II-TCR interaction led to selective expansion of MP Tregs and MP CD4⁺ and CD8⁺ T cells that was reversed upon cotreatment with CTLA-4-Ig. Treatment with anti-CTLA-4 mAb altered MP Treg and MP CD4⁺ and CD8⁺ T cell homeostasis in a manner similar to that observed with anti-MHC class II. We postulate a complex pathway in which CD28 is the primary driver of Treg proliferation and CTLA-4 functions as the main brake but is likely dependent on TCR signals and CD80/CD86. These findings have important implications for the use of biologic agents targeting such pathways to modulate autoimmune and neoplastic disease.

Conventional and Regulatory CD4+ T Cells That Share Identical TCRs Are Derived from Common Clones

Results from studies comparing the diversity and specificity of the TCR repertoires expressed by conventional (Tconv) and regulatory (Treg) CD4⁺ T cell have varied depending on the experimental system employed. We developed a new model in which T cells express a single fixed TCRα chain, randomly rearranged endogenous TCRβ chains, and a Foxp3-GFP reporter. We purified CD4⁺Foxp3^- and CD4⁺Foxp3⁺ cells, then performed biased controlled multiplex PCR and high throughput sequencing of endogenous TCRβ chains. We identified >7,000 different TCRβ sequences in the periphery of 5 individual mice. On average, ~12% of TCR sequences were expressed by both conventional and regulatory populations within individual mice. The CD4⁺ T cells that expressed shared TCR sequences were present at higher frequencies compared to T cells expressing non-shared TCRs. Furthermore, nearly all (>90%) of the TCR sequences that were shared within mice were identical at the DNA sequence level, indicating that conventional and regulatory T cells that express shared TCRs are derived from common clones. Analysis of TCR repertoire overlap in the thymus reveals that a large proportion of Tconv and Treg sharing observed in the periphery is due to clonal expansion in the thymus. Together these data show that there are a limited number of TCR sequences shared between Tconv and Tregs. Also, Tconv and Tregs sharing identical TCRs are found at relatively high frequencies and are derived from common progenitors, of which a large portion are generated in the thymus.

CD28 co-stimulation in T-cell homeostasis: a recent perspective

T-cells play a key role within the adaptive immune system mediating cellular immunity and orchestrating the immune response as a whole. Their activation requires not only recognition of antigen/major histocompatibility complexes by the T-cell receptor but in addition co-stimulation via the CD28 molecule through binding to CD80, CD86, or as recently discovered, inducible co-stimulator ligand expressed by antigen-presenting cells. Apart from tight control of the co-stimulatory signal by the T-cell receptor complex, expression of the inhibitory receptor cytotoxic T-lymphocyte antigen-4 (CTLA-4) sharing its ligands with CD28 is required to avoid inappropriate or prolonged T-cell activation. CD4(+) Foxp3(+) regulatory T (Treg) cells, which are crucial inhibitors of autoimmunity, add another level of complexity in that they differ from conventional non-regulatory CD4(+) T-cells by strongly depending on CD28 signaling for their generation and homeostasis. Moreover, CTLA-4 is constitutively expressed by Treg cells where it serves as a key mediator of suppression, while conventional CD4(+) T-cells express CTLA-4 only after activation. Here, we discuss recent insights into the molecular events underlying CD28-mediated co-stimulation, its impact on gene regulation, and the differential role of CD28 expression on Treg cells versus conventional CD4(+) and CD8(+) T-cells. Moreover, we summarize the exciting therapeutic options which have arisen from our current understanding of T-cell co-stimulation. Some of these have already been translated into the clinic, while others are expected to follow soon due to promising preclinical results. In particular, we discuss the failed 2006 trial of the CD28 superagonist TGN1412, and the return of this potent T-cell activator to clinical development.

MHC Class II Polymorphisms, Autoreactive T-Cells, and Autoimmunity

Major histocompatibility complex (MHC) genes, also known as human leukocyte antigen genes (HLA) in humans, are the prevailing contributors of genetic susceptibility to autoimmune diseases such as Type 1 Diabetes (T1D), multiple sclerosis, and rheumatoid arthritis, among others (1–3). Although the pathways through which MHC molecules afford autoimmune risk or resistance remain to be fully mapped out, it is generally accepted that they do so by shaping the central and peripheral T-cell repertoires of the host toward autoimmune proclivity or resistance, respectively. Disease-predisposing MHC alleles would both spare autoreactive thymocytes from central tolerance and bias their development toward a pathogenic phenotype. Protective MHC alleles, on the other hand, would promote central deletion of autoreactive thymocytes and skew their development toward non-pathogenic phenotypes. This interpretation of the data is at odds with two other observations: that in MHC-heterozygous individuals, resistance is dominant over susceptibility; and that it is difficult to understand how deletion of one or a few clonal autoreactive T-cell types would suffice to curb autoimmune responses driven by hundreds if not thousands of autoreactive T-cell specificities. This review provides an update on current advances in our understanding of the mechanisms underlying MHC class II-associated autoimmune disease susceptibility and/or resistance and attempts to reconcile these seemingly opposing concepts.

Cloning, expression, and functional characterization of TL1A-Ig

TNF superfamily member 15 (TL1A) is the ligand for TNFR superfamily (TNFRSF)25. We previously reported that TNFRSF25 stimulation with an agonist Ab, 4C12, expands pre-existing CD4(+)Foxp3(+) regulatory T cells (Tregs) in vivo. To determine how the physiological ligand differs from the Ab, we generated a soluble mouse TL1A-Ig fusion protein that forms a dimer of TL1A trimers in solution with an apparent molecular mass of 516 kDa. In vitro, TL1A-Ig mediated rapid proliferation of Foxp3(+) Tregs and a population of CD4(+)Foxp3(-) conventional T cells. TL1A-Ig also blocked de novo biogenesis of inducible Tregs and it attenuated the suppressive function of Tregs. TNFRSF25 stimulation by TL1A-Ig in vivo induced expansion of Tregs such that they increased to 30-35% of all CD4(+) T cells in the peripheral blood within 5 d of treatment. Treg proliferation in vivo was dependent on TCR engagement with MHC class II. Elevated Treg levels can be maintained for at least 20 d with daily injections of TL1A-Ig. TL1A-Ig-expanded Tregs expressed high levels of activation/memory markers KLRG1 and CD103 and were highly suppressive ex vivo. TL1A-Ig-mediated Treg expansion in vivo was protective against allergic lung inflammation, a mouse model for asthma, by reversing the ratio of conventional T cells to Tregs in the lung and blocking eosinophil exudation into the bronchoalveolar fluid. Thus, TL1A-Ig fusion proteins are highly active and tightly controllable agents to stimulate Treg proliferation in vivo, and they are uniquely able to maintain high levels of expanded Tregs by repeated administration.

Cellular therapies supplement: the role of granulocyte macrophage colony-stimulating factor and dendritic cells in regulatory T-cell homeostasis and expansion

Regulatory T cells are a subset of T cells with inhibitory function that are critical for protection against autoimmunity and immunopathology. A failure to maintain adequate regulatory T-cell numbers in the periphery results in autoimmune manifestations, highlighting the importance of the continuous maintenance of peripheral regulatory T cells. The cellular and molecular requirements for regulatory T-cell homeostasis and expansion are not fully understood but involve a complex interplay among dendritic cells, conventional T cells, and regulatory T cells. In addition, soluble factors such as the cytokine granulocyte macrophage colony-stimulating factor may play a role in enhancing these interactions. In this review, we discuss our National Blood Foundation-funded studies relating to the role of granulocyte macrophage colony-stimulating factor and dendritic cells in controlling regulatory T-cell homeostasis and expansion.

Functional analysis and gene expression profile of umbilical cord blood regulatory T cells

The aim of the study was to analyze and compare the functional properties and the gene expression profile of regulatory T cells (Tregs) isolated from cord blood (CB) units (n = 23) and from the peripheral blood (PB) of adult normal donors (n = 13). Tregs were purified from mononuclear cells and expanded for 6 days with anti-CD3, anti-CD28, and IL-2. CB and PB Tregs presented similar immunophenotypic features. However, Tregs isolated from CB presented a much higher expansion capacity; this was confirmed by the genomic characterization that showed in CB-derived Tregs significant enrichments of genes involved in cell proliferation, chromatin modification, and regulation of gene expression. All samples were positive for the FoxP3 gene and protein after expansion. CB and PB expanded Tregs exerted a comparable and potent suppressive function on the proliferative reaction of autologous T cells stimulated by allogeneic dendritic cells and presented a high in vitro IL-10 production capacity. Gene profile analysis also revealed for PB Tregs significant enrichments of genes involved in the adaptive immune response. These data offer further insights into the understanding of the biology of CB transplantation indicating a possible role played by CB Tregs in the suppression of the allogeneic T cell response.

Antigen-non-specific regulation centered on CD25+Foxp3+ Treg cells

CD4(+)CD25(+)Foxp3(+) regulatory T cells (Tregs) are of special interest in immunology because of their potent inhibitory function. Many fundamental aspects of Tregs, including their antigenic profile, development and peripheral homeostasis, remain highly controversial. Here, we propose a Treg-centered antigen-non-specific immunoregulation model focused on the T-cell system, particularly on CD4(+) T cells. The T-cell pool consists of naive T cells (Tnais), Tregs and effector T cells (Teffs). Regardless of antigen specificity, the ratio of the activated T-cell subsets (Treg/Teff/Tnai) and their temporal and spatial uniformity dictate the differentiation of Tnais. Activated Tregs inhibit the activation, proliferation, induction and activity of Teffs; in contrast, activated Teffs inhibit the induction of Tregs from Tnais but cooperate with Treg-specific antigens to promote the proliferation and activity of Tregs. In many cases, these interactions are antigen-non-specific, whereas the activation of both Tregs and Teffs is antigen-specific. Memory T-cell subsets are essential for the maintenance of adaptive immune responses, but the antigen-non-specific interactions among T-cell subsets may be more important during the establishment of the adaptive immune system to a newly encountered antigen. This is especially important when new and memory antigens are presented closely-both temporally and spatially-to T cells, because there are always baseline levels of activated Tregs, which are usually higher than levels of memory T cells for new antigens. Based on this hypothesis, we further infer that, under physiological conditions, Tregs in lymph nodes mainly recognize antigens frequently released from draining tissues, and that these self-reactive Tregs are commonly involved in the establishment of adaptive immunity to new antigens and in the feedback control of excessive responses to pathogens.

Regulatory T cells: roles of T cell receptor for their development and function

Naturally arising CD4(+)CD25(+) regulatory T cells (Treg cells), which specifically express the forkhead family transcription factor Foxp3, are essential for the maintenance of immunological self-tolerance and immune homeostasis. Stimulation of the T cell antigen receptor (TCR) via recognizing self-peptide/major histocompatibility complex (MHC) is required for their expression of Foxp3 in the course of their development in the thymus. The TCR repertoires displayed by Treg cells and naïve T cells are apparently distinct, suggesting that Treg cells with high reactivity to self-peptide/MHC ligands are somehow driven to Treg cell lineage in the thymus. Treg cells also require stimulation via TCR to exert suppression in the periphery. At the molecular level, assembly of Foxp3, Foxp3-interacting factors, and chromatin-remodeling factors is in part under the control of TCR signaling, and TCR stimulation alters Foxp3-dependent transcriptional regulation, protein-protein interaction, and Foxp3 recruitment to the specific genomic loci. These findings collectively indicate that the TCR signaling is essential for suppressive function of Treg cells and that TCR has a determinant role for driving developing T cells to the Foxp3(+)CD4(+)CD25(+) Treg cell lineage and differentiation.

T cell receptor CDR3 sequence but not recognition characteristics distinguish autoreactive effector and Foxp3(+) regulatory T cells

The source, specificity, and plasticity of the forkhead box transcription factor 3 (Foxp3)(+) regulatory T (Treg) and conventional T (Tconv) cell populations active at sites of autoimmune pathology are not well characterized. To evaluate this, we combined global repertoire analyses and functional assessments of isolated T cell receptors (TCR) from TCRalpha retrogenic mice with autoimmune encephalomyelitis. Treg and Tconv cell TCR repertoires were distinct, and autoantigen-specific Treg and Tconv cells were enriched in diseased tissue. Autoantigen sensitivity and fine specificity of these cells intersected, implying that differences in responsiveness were not responsible for lineage specification. Notably, autoreactive Treg and Tconv cells could be fully distinguished by an acidic versus aliphatic variation at a single TCR CDR3 residue. Our results imply that ontogenically distinct Treg and Tconv cell repertoires with convergent specificities for autoantigen respond during autoimmunity and argue against more than limited plasticity between Treg and Tconv cells during autoimmune inflammation.

Dominant role of antigen dose in CD4+Foxp3+ regulatory T cell induction and expansion

The definitions of tolerogenic vs immunogenic dendritic cells (DC) remain controversial. Immature DC have been shown to induce T regulatory cells (Treg) specific for foreign and allogeneic Ags. However, we have previously reported that mature DC (mDC) prevented the onset of autoimmune diabetes, whereas immature DC (iDC) were therapeutically ineffective. In this study, islet-specific CD4(+) T cells from BDC2.5 TCR-transgenic mice were stimulated in the absence of exogenous cytokine with iDC or mDC pulsed with high- or low-affinity antigenic peptides and examined for Treg induction. Both iDC and mDC presenting low peptide doses induced weak TCR signaling via the Akt/mammalian target of rapamycin (mTOR) pathway, resulting in significant expansion of Foxp3(+) Treg. Furthermore, unpulsed mDC, but not iDC, also induced Treg. High peptide doses induced strong Akt/mTOR signaling and favored the expansion of Foxp3(neg) Th cells. The inverse correlation of Foxp3 and Akt/mTOR signaling was also observed in DO11.10 and OT-II TCR-transgenic T cells and was recapitulated with anti-CD3/CD28 stimulation in the absence of DC. IL-6 production in these cultures correlated positively with Ag dose and inversely with Treg expansion. Studies with T cells or DC from IL-6(-/-) mice revealed that IL-6 production by T cells was more important in the inhibition of Treg induction at low Ag doses. These studies indicate that the strength of Akt/mTOR signaling, a critical T cell-intrinsic determinant for Treg vs Th induction, can be controlled by adjusting the dose of antigenic peptide. Furthermore, this operates in a dominant fashion over DC phenotype and cytokine production.

Self heat-shock protein 65-mediated regulation of autoimmune arthritis

Heat-shock proteins (Hsps) have been invoked in the pathogenesis of a variety of autoimmune diseases. The mycobacterial heat-shock protein 65 (Bhsp65) has been studied extensively as one of the antigenic triggers of autoimmunity in experimental models of, as well as patients with, rheumatoid arthritis. As Hsps are highly conserved and immunogenic, it is generally anticipated that self Hsps might serve as the endogenous targets of the immune response initiated by the homologous foreign Hsps. Contrary to this expectation, studies in the rat adjuvant arthritis (AA) model have revealed that priming of the self (rat) hsp65 (Rhsp65)-directed T cells in the Lewis rat leads to protection against AA instead of disease induction or aggravation. The arthritis-protective attribute of the self hsp65 is also evident following spontaneous priming of the anti-Rhsp65 T cells during the natural course of AA. Furthermore, immunization of rats with human hsp60, or with Bhsp65 peptides that are crossreactive with the corresponding self hsp65 peptides, leads to protection against AA. Importantly, high levels of T cell reactivity against self hsp60 in patients with juvenile idiopathic arthritis positively correlate with a favorable outcome of the disease. Thus, immune response against self hsp65 in autoimmune arthritis is protective rather than being pathogenic.

The T-cell receptor repertoire of regulatory T cells

The CD4(+) CD25(+) regulatory population of T cells (Treg cells), which expresses the forkhead family transcription factor (Foxp3), is the key component of the peripheral tolerance mechanism that protects us from a variety of autoimmune diseases. Experimental evidence shows that Treg cells recognize a wide range of antigenic specificities with increased reactivity to self antigens, although the affinity of these interactions remains to be further defined. The Treg repertoire is highly diverse with a distinct set of T-cell receptors (TCRs), and yet is overlapping to some extent with the repertoire of conventional T cells (Tconv cells). The majority of Treg cells are generated in the thymus. However, the role of the TCR specificity in directing thymic precursors to become Treg or Tconv cells remains unclear. On the one hand, the higher self reactivity of Treg cells and utilization of different TCRs in Treg and Tconv repertoires suggest that in TCR interactions an initial decision is made about the 'suitability' of a developing thymocyte to become a Treg cell. On the other hand, as Treg cells can recognize a wide range of foreign antigens, have a diverse TCR repertoire, and show some degree of overlap with Tconv cells, the signals through the TCR may be complementary to the TCR-independent process that generates precursors of Treg cells. In this review, we discuss how different features of the Treg repertoire influence our understanding of Treg specificities and the role of self reactivity in the generation of this population.

Regulation of autoimmune arthritis by self-heat-shock proteins

Heat-shock proteins (hsps) are highly conserved and immunogenic, and they are generally perceived to be attractive initiators or targets of a pathogenic immune response, and as such, have been implicated in the pathogenesis of autoimmune arthritis. However, studies in animal models and arthritis patients have unraveled the disease-regulating attributes of self-hsp65. We propose that the self-hsp65 induces a protective and beneficial immune response because of its ubiquitous distribution, stress inducibility and participation in tolerogenic processes. By contrast, the foreign hsp65 that does not influence the above processes and that resides admixed with microbial ligands for innate receptors generates an inflammatory pathogenic response. The regulatory properties of self-hsps need be fully explored and might be used for therapeutic purposes.

Regulatory T cells prevent transfer of type 1 diabetes in NOD mice only when their antigen is present in vivo

Regulatory T cells (Tregs) can potentially be used as tools to suppress pathogenic T cells in autoimmune diseases such as type 1 diabetes. For use in therapy it is critically important to determine whether suppression by Tregs requires a population specific for the target of autoimmunity, such as pancreatic beta cells in type 1 diabetes. Current reports in the NOD mouse model of type 1 diabetes are in conflict as to whether suppression of disease by Tregs is Ag-dependent. We have addressed this question by evaluating the effects of islet-specific TGF-beta-induced Tregs in recipient mice in which the Treg Ag is either present or absent. Our data show that Treg numbers in pancreas are reduced in the absence of Ag and that there are Ag-dependent differences in the effects of Tregs on pathogenic T cells in the pancreas. By examining protection from diabetes induced by T cell transfer, we have clearly demonstrated that Tregs suppress only in the presence of their Ag and not in mice in which the islets lack the Treg Ag. Our results also suggest that in sufficiently large populations of polyclonal Tregs, there will be adequate numbers of islet-specific Tregs to suppress diabetes.

CD4+ FoxP3+ regulatory T cells confer infectious tolerance in a TGF-beta-dependent manner

CD4⁺FoxP3⁺ regulatory T (T reg) cells comprise a separate lineage of T cells that are essential for maintaining immunological tolerance to self. The molecular mechanism(s) by which T reg cells mediate their suppressive effects remains poorly understood. One molecule that has been extensively studied in T reg cell suppression is transforming growth factor (TGF)-β, but its importance remains controversial. We found that TGF-β complexed to latency-associated peptide (LAP) is expressed on the cell surface of activated but not resting T reg cells. T reg cell LAP–TGF-β plays an important role in the suppression of the proliferation of activated T cells, but it is not required for the suppression of naive T cell activation. More importantly, T reg cell–derived TGF-β could generate de novo CD4⁺FoxP3⁺ T cells in vitro from naive precursors in a cell contact–dependent, antigen-presenting cell–independent and α_V integrin–independent manner. The newly induced CD4⁺FoxP3⁺ T cells are suppressive both in vitro and in vivo. Transfer of activated antigen-specific T reg cells with naive antigen-specific responder T cells to normal recipients, followed by immunization, also results in induction of FoxP3 expression in the responder cells. T reg cell–mediated generation of functional CD4⁺FoxP3⁺ cells via this TGF-β–dependent pathway may represent a major mechanism as to how T reg cells maintain tolerance and expand their suppressive abilities.

Multi-faceted control of autoaggression: Foxp3+ regulatory T cells in murine models of organ-specific autoimmune disease

The discovery of forkhead box p3 (Foxp3) as the critical transcriptional controller of suppressive function in murine CD4(+) T regulatory (Treg) cells has allowed precise analyses of these cells in a range of immunopathological models. Recent data have revealed key roles for Foxp3+ Tregs in murine models of human organ-specific autoimmune conditions. Do these Tregs target the same autoantigens recognized by the autoaggressive T cells that need to be controlled? Under steady state conditions there may not be a need for such a shared recognition to dampen spontaneous anti-self priming in the lymphoid organs. However, when they are needed to control ongoing inflammation, Tregs recognizing autoantigens found in the diseased organ appear to have significantly stronger suppressive powers. We reflect on these observations that clearly have relevance for the translation of Treg-targeting immune therapies to human disease.