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

Lipid-Polymer Hybrid Nanoparticles Utilize B Cells and Dendritic Cells to Elicit Distinct Antigen-Specific CD4+ and CD8+ T Cell Responses

Antigen-presenting cells (APCs) are widely studied for treating immune-mediated diseases, and dendritic cells (DCs) are potent APCs that uptake and present antigens (Ags). However, DCs face several challenges that hinder their clinical translation due to their inability to control Ag dosing and low abundance in peripheral blood. B cells are a potential alternative to DCs, but their poor nonspecific Ag uptake capabilities compromise controllable priming of T cells. Here, we developed phospholipid-conjugated Ags (L-Ags) and lipid-polymer hybrid nanoparticles (L/P-Ag NPs) as delivery platforms to expand the range of accessible APCs for use in T cell priming. These delivery platforms were evaluated using DCs, CD40-activated B cells, and resting B cells to understand the impacts of various Ag delivery mechanisms for generation of Ag-specific T cell responses. L-Ag delivery (termed depoting) of MHC class I- and II-restricted Ags successfully loaded all APC types in a tunable manner and primed both Ag-specific CD8+ and CD4+ T cells, respectively. Incorporating L-Ags and polymer-conjugated Ags (P-Ag) into NPs can direct Ags to different uptake pathways to engineer the dynamics of presentation and shape T cell responses. DCs were capable of processing and presenting Ag delivered from both L- and P-Ag NPs, yet B cells could only utilize Ag delivered from L-Ag NPs, which led to differential cytokine secretion profiles in coculture studies. Altogether, we show that L-Ags and P-Ags can be rationally paired within a single NP to leverage distinct delivery mechanisms to access multiple Ag processing pathways in two APC types, offering a modular delivery platform for engineering Ag-specific immunotherapies.

Deciphering the role of protein kinase A in the control of FoxP3 expression in regulatory T cells in health and autoimmunity

The molecular mechanisms that govern differential T cell development from CD4+CD25-conventional T (Tconv) into CD4+CD25+ forkhead-box-P3+ (FoxP3+) inducible regulatory T (iTreg) cells remain unclear. Herein, we investigated the relative contribution of protein kinase A (PKA) in this process. Mechanistically, we found that PKA controlled the efficiency of human iTreg cell generation through the expression of different FoxP3 splicing variants containing or not the exon 2. We found that transient PKA inhibition reduced the recruitment of cAMP-responsive element-binding protein (CREB) on regulatory regions of the FoxP3 gene, a condition that is associated with an impaired acquisition of their suppressive capacity in vitro. To corroborate our findings in a human model of autoimmunity, we measured CREB phosphorylation and FoxP3 levels in iTreg cells from treatment-naïve relapsing-remitting (RR)-multiple sclerosis (MS) subjects. Interestingly, both phospho-CREB and FoxP3 induction directly correlated and were significantly reduced in RR-MS patients, suggesting a previously unknown mechanism involved in the induction and function of human iTreg cells.

The regulation and differentiation of regulatory T cells and their dysfunction in autoimmune diseases

The discovery of FOXP3+ regulatory T (Treg) cells as a distinct cell lineage with a central role in regulating immune responses provided a deeper understanding of self-tolerance. The transcription factor FOXP3 serves a key role in Treg cell lineage determination and maintenance, but is not sufficient to enable the full potential of Treg cell suppression, indicating that other factors orchestrate the fine-tuning of Treg cell function. Moreover, FOXP3-independent mechanisms have recently been shown to contribute to Treg cell dysfunction. FOXP3 mutations in humans cause lethal fulminant systemic autoinflammation (IPEX syndrome). However, it remains unclear to what degree Treg cell dysfunction is contributing to the pathophysiology of common autoimmune diseases. In this Review, we discuss the origins of Treg cells in the periphery and the multilayered mechanisms by which Treg cells are induced, as well as the FOXP3-dependent and FOXP3-independent cellular programmes that maintain the suppressive function of Treg cells in humans and mice. Further, we examine evidence for Treg cell dysfunction in the context of common autoimmune diseases such as multiple sclerosis, inflammatory bowel disease, systemic lupus erythematosus and rheumatoid arthritis.

Cellular and molecular signaling towards T cell immunological self-tolerance

The binding of a cognate antigen to T cell receptor (TCR) complex triggers a series of intracellular events controlling T cell activation, proliferation, and differentiation. Upon TCR engagement, different negative regulatory feedback mechanisms are rapidly activated to counterbalance T cell activation, thus preventing excessive signal propagation and promoting the induction of immunological self-tolerance. Both positive and negative regulatory processes are tightly controlled to ensure the effective elimination of foreign antigens while limiting surrounding tissue damage and autoimmunity. In this context, signals deriving from co-stimulatory molecules (i.e., CD80, CD86), co-inhibitory receptors (PD-1, CTLA-4), the tyrosine phosphatase CD45 and cytokines such as IL-2 synergize with TCR-derived signals to guide T cell fate and differentiation. The balance of these mechanisms is also crucial for the generation of CD4+ Foxp3+ regulatory T cells, a cellular subset involved in the control of immunological self-tolerance. This review provides an overview of the most relevant pathways induced by TCR activation combined with those derived from co-stimulatory and co-inhibitory molecules implicated in the cell-intrinsic modulation of T cell activation. In addition to the latter, we dissected mechanisms responsible for T cell-mediated suppression of immune cell activation through regulatory T cell generation, homeostasis, and effector functions. We also discuss how imbalanced signaling derived from TCR and accessory molecules can contribute to autoimmune disease pathogenesis.

Phosphorylation of hnRNP A1-Serine 199 Is Not Required for T Cell Differentiation and Function

hnRNP A1 is an important RNA-binding protein that influences many stages of RNA processing, including transcription, alternative splicing, mRNA nuclear export, and RNA stability. However, the role of hnRNP A1 in immune cells, specifically CD4+ T cells, remains unclear. We previously showed that Akt phosphorylation of hnRNP A1 was dependent on TCR signal strength and was associated with Treg differentiation. To explore the impact of hnRNP A1 phosphorylation by Akt on CD4+ T cell differentiation, our laboratory generated a mutant mouse model, hnRNP A1-S199A (A1-MUT) in which the major Akt phosphorylation site on hnRNP A1 was mutated to alanine using CRISPR Cas9 technology. Immune profiling of A1-MUT mice revealed changes in the numbers of Tregs in the mesenteric lymph node. We found no significant differences in naive CD4+ T cell differentiation into Th1, Th2, Th17, or T regulatory cells (Tregs) in vitro. In vivo, Treg differentiation assays using OTII-A1-Mut CD4+ T cells exposed to OVA food revealed migration and homing defects in the A1-MUT but no change in Treg induction. A1-MUT mice were immunized with NP- keyhole limpet hemocyanin, and normal germinal center development, normal numbers of NP-specific B cells, and no change in Tfh numbers were observed. In conclusion, Akt phosphorylation of hnRNP A1 S199 does not play a role in CD4+ T cell fate or function in the models tested. This hnRNP A1-S199A mouse model should be a valuable tool to study the role of Akt phosphorylation of hnRNP A1-S199 in different cell types or other mouse models of human disease.

Stimulation Strength Determined by Superantigen Dose Controls Subcellular Localization of FOXP3 Isoforms and Suppressive Function of CD4+CD25+FOXP3+ T Cells

Staphylococcal superantigens induce massive activation of T cells and inflammation, leading to toxic shock syndrome. Paradoxically, increasing evidence indicates that superantigens can also induce immunosuppression by promoting regulatory T cell (Treg) development. In this study, we demonstrate that stimulation strength plays a critical role in superantigen-mediated induction of immunosuppressive human CD4+CD25+FOXP3+ T cells. Suboptimal stimulation by a low dose (1 ng/ml) of staphylococcal enterotoxin C1 (SEC1) led to de novo generation of Treg-like CD4+CD25+FOXP3+ T cells with strong suppressive activity. In contrast, CD4+CD25+ T cells induced by optimal stimulation with high-dose SEC1 (1 µg/ml) were not immunosuppressive, despite high FOXP3 expression. Signal transduction pathway analysis revealed differential activation of the PI3K signaling pathway and expression of PTEN in optimal and suboptimal stimulation with SEC1. Additionally, we identified that FOXP3 isoforms in Treg-like cells from the suboptimal condition were located in the nucleus, whereas FOXP3 in nonsuppressive cells from the optimal condition localized in cytoplasm. Sequencing analysis of FOXP3 isoform transcripts identified five isoforms, including a FOXP3 isoform lacking partial exon 3. Overexpression of FOXP3 isoforms confirmed that both an exon 2-lacking isoform and a partial exon 3-lacking isoform confer suppressive activity. Furthermore, blockade of PI3K in optimal stimulation conditions led to induction of suppressive Treg-like cells with nuclear translocation of FOXP3, suggesting that PI3K signaling impairs induction of Tregs in a SEC1 dose-dependent manner. Taken together, these data demonstrate that the strength of activation signals determined by superantigen dose regulates subcellular localization of FOXP3 isoforms, which confers suppressive functionality.

Immunoregulation: the interplay between metabolism and redox homeostasis

Regulatory T cells are fundamental for the induction and maintenance of immune homeostasis, with their dysfunction resulting in uncontrolled immune responses and tissue destruction predisposing to autoimmunity, transplant rejection and several inflammatory and metabolic disorders. Recent discoveries have demonstrated that metabolic processes and mitochondrial function are critical for the appropriate functioning of these cells in health, with their metabolic adaptation, influenced by microenvironmental factors, seen in several pathological processes. Upon activation regulatory T cells rearrange their oxidation-reduction (redox) system, which in turn supports their metabolic reprogramming, adding a layer of complexity to our understanding of cellular metabolism. Here we review the literature surrounding redox homeostasis and metabolism of regulatory T cells to highlight new mechanistic insights of these interlinked pathways in immune regulation.

Combined Treatment With TGF-β1, Retinoic Acid, and Lactoferrin Robustly Generate Inducible Tregs (iTregs) Against High Affinity Ligand

Forkhead box P3-positive (Foxp3+)-inducible Tregs (iTregs) are readily generated by TGF-β1 at low TCR signaling intensity. TGF-β1-mediated Foxp3 expression is further enhanced by retinoic acid (RA) and lactoferrin (LF). However, the intensity of TCR signaling required for induction of Foxp3 expression by TGF-β1 in combination with RA and LF is unknown. Here, we found that either RA or LF alone decreased TGF-β1-mediated Foxp3 expression at low TCR signaling intensity. In contrast, at high TCR signaling intensity, the addition of either RA or LF strongly increased TGF-β1-mediated Foxp3 expression. Moreover, decreased CD28 stimulation was more favorable for TGF-β1/LF-mediated Foxp3 expression. Lastly, we found that at high signaling intensities of both TCR and CD28, combined treatment with TGF-β1, RA, and LF induced robust expression of Foxp3, in parallel with powerful suppressive activity against responder T cell proliferation. Our findings that TGFβ/RA/LF strongly generate high affinity Ag-specific iTreg population would be useful for the control of unwanted hypersensitive immune reactions such as various autoimmune diseases.

Phosphatase Ssu72 Is Essential for Homeostatic Balance Between CD4+ T Cell Lineages

Ssu72, a dual-specificity protein phosphatase, not only participates in transcription biogenesis, but also affects pathophysiological functions in a tissue-specific manner. Recently, it has been shown that Ssu72 is required for T cell differentiation and function by controlling multiple immune receptor-mediated signals, including TCR and several cytokine receptor signaling pathways. Ssu72 deficiency in T cells is associated with impaired fine-tuning of receptor-mediated signaling and a defect in CD4+ T cell homeostasis, resulting in immune-mediated diseases. However, the mechanism by which Ssu72 in T cells integrates the pathophysiology of multiple immune-mediated diseases is still poorly elucidated. In this review, we will focus on the immunoregulatory mechanism of Ssu72 phosphatase in CD4+ T cell differentiation, activation, and phenotypic function. We will also discuss the current understanding of the correlation between Ssu72 in T cells and pathological functions which suggests that Ssu72 might be a therapeutic target in autoimmune disorders and other diseases.

T cell receptor signaling in the differentiation and plasticity of CD4+ T cells

CD4⁺ T cells are critical components of the adaptive immune system. The T cell receptor (TCR) and co-receptor signaling cascades shape the phenotype and functions of CD4⁺ T cells. TCR signaling plays a crucial role in T cell development, antigen recognition, activation, and differentiation upon recognition of foreign- or auto-antigens. In specific autoimmune conditions, altered TCR repertoire is reported and can predispose autoimmunity with organ-specific inflammation and tissue damage. TCR signaling modulates various signaling cascades and regulates epigenetic and transcriptional regulation during homeostasis and disease conditions. Understanding the mechanism by which coreceptors and cytokine signals control the magnitude of TCR signal amplification will aid in developing therapeutic strategies to treat inflammation and autoimmune diseases. This review focuses on the role of the TCR signaling cascade and its components in the activation, differentiation, and plasticity of various CD4⁺ T cell subsets.

Recent insights into the role of Akt in CD4 T-cell activation and differentiation: alternative splicing and beyond

The activation and differentiation of CD4⁺ T cells is a complex process that is controlled by many factors. A critical component of the signaling pathway triggered following T-cell receptor (TCR) engagement is the serine threonine kinase Akt. Akt is involved in the control of many cellular processes including proliferation, metabolism, and differentiation of specific T_H-cell subsets. Recent work has shown that, depending on the nature or strength of the TCR activation, Akt may activate different sets of substrates which then lead to differential cellular outcomes. Akt plays an important role in controlling the strength of the TCR signal and several recent studies have identified novel mechanisms including control of the expression of negative regulators of TCR signaling, and the influence on regulatory T cells (Treg) and T_H17 differentiation. Many of these functions are mediated via control of the FoxO family of transcription factors, that play an important role in metabolism and Th cell differentiation. A theme that is emerging is that Akt does not function in the same way in all T-cell types. We highlight differences between CD4 and CD8 T cells as well as between Treg, T_H17, and T_FH cells. While Akt activity has been implicated in the control of alternative splicing in tumor cells, recent studies are emerging that indicate that similar functions may exist in CD4 T cells. In this mini review, we highlight some of the recent advances in these areas of Akt function that demonstrate the varied role that Akt plays in the function of CD4 T cells.

Lactoferrin-derived chimeric peptide (LFch) strongly boosts TGFβ1-mediated inducible Treg differentiation possibly through downregulating TCR/CD28 signalling

We recently reported that lactoferrin (LF) induces Foxp3⁺ Treg differentiation through binding to TGFβ receptor III (TβRIII), and this activity was further enhanced by TGFβ1. Generally, a low T-cell receptor (TCR) signal strength is favourable for Foxp3⁺ Treg differentiation. In the present study, we explored the effect of lactoferrin chimera (LFch, containing lactoferricin [aa 17-30] and lactoferrampin [aa 265-284]), along with TGFβ1 on Foxp3⁺ Treg differentiation. LFch alone did not induce Foxp3 expression, yet LFch dramatically enhanced TGFβ1-induced Foxp3 expression. LFch had little effect on the phosphorylation of Smad3, a canonical transcriptional factor of TGFβ1. Instead, LFch attenuated the phosphorylation of S6 (a target of mTOR), IκB and PI3K. These activities of LFch were completely abrogated by pretreatment of LFch with soluble TGFβ1 receptor III (sTβRIII). Consistent with this, the activity of LFch on TGFβ1-induced Foxp3 expression was also abrogated by treatment with sTβRIII. Finally, the TGFβ1/LFch-induced T cell population substantially suppressed the proliferation of responder CD4⁺ T cells. These results indicate that LFch robustly enhances TGFβ1-induced Foxp3⁺ Treg differentiation by diminishing TCR/CD28 signal intensity.

Pancreatic islet-specific engineered Tregs exhibit robust antigen-specific and bystander immune suppression in type 1 diabetes models

Adoptive transfer of regulatory T cells (T_regs) is therapeutic in type 1 diabetes (T1D) mouse models. T_regs that are specific for pancreatic islets are more potent than polyclonal T_regs in preventing disease. However, the frequency of antigen-specific natural T_regs is extremely low, and ex vivo expansion may destabilize T_regs, leading to an effector phenotype. Here, we generated durable, antigen-specific engineered T_regs (EngT_regs) from primary human CD4⁺ T cells by combining FOXP3 homology-directed repair editing and lentiviral T cell receptor (TCR) delivery. Using TCRs derived from clonally expanded CD4⁺ T cells isolated from patients with T1D, we generated islet-specific EngT_regs that suppressed effector T cell (T_eff) proliferation and cytokine production. EngT_regs suppressed T_effs recognizing the same islet antigen in addition to bystander T_effs recognizing other islet antigens through production of soluble mediators and both direct and indirect mechanisms. Adoptively transferred murine islet-specific EngT_regs homed to the pancreas and blocked diabetes triggered by islet-specific T_effs or diabetogenic polyclonal T_effs in recipient mice. These data demonstrate the potential of antigen-specific EngT_regs as a targeted therapy for preventing T1D.

Droplet Microfluidic Technology for the Early and Label-Free Isolation of Highly-Glycolytic, Activated T-Cells

A label-free, fixation-free and passive sorting method is presented to isolate activated T-cells shortly after activation and prior to the display of activation surface markers. It uses a recently developed sorting platform dubbed "Sorting by Interfacial Tension" (SIFT) that sorts droplets based on pH. After polyclonal (anti-CD3/CD28 bead) activation and a brief incubation on chip, droplets containing activated T-cells display a lower pH than those containing naive cells due to increased glycolysis. Under specific surfactant conditions, a change in pH can lead to a concurrent increase in droplet interfacial tension. The isolation of activated T-cells on chip is hence achieved as flattened droplets are displaced as they encounter a micro-fabricated trench oriented diagonally with respect to the direction of flow. This technique leads to an enrichment of activated primary CD4+ T-cells to over 95% from an initial mixed population of naive cells and cells activated for as little as 15 min. Moreover, since the pH change is correlated to successful activation, the technique allows the isolation of T-cells with the earliest activation and highest glycolysis, an important feature for the testing of T-cell activation modulators and to determine regulators and predictors of differentiation outcomes.

Research Progress on the Immunogenicity and Regeneration of Acellular Adipose Matrix: A Mini Review

Acellular adipose matrix (AAM) has received increasing attention for soft tissue reconstruction, due to its abundant source, high long-term retention rate and in vivo adipogenic induction ability. However, the current decellularization methods inevitably affect native extracellular matrix (ECM) properties, and the residual antigens can trigger adverse immune reactions after transplantation. The behavior of host inflammatory cells mainly decides the regeneration of AAM after transplantation. In this review, recent knowledge of inflammatory cells for acellular matrix regeneration will be discussed. These advancements will inform further development of AAM products with better properties.

Lycopene intake induces colonic regulatory T cells in mice and suppresses food allergy symptoms

BACKGROUND

Food allergy (FA) is a common disease in children; thus, a high level of safety is required for its prevention and treatment. Colonic regulatory T cells (Tregs) have been suggested to attenuate FA. We investigated the Treg-inducing ability and anti-FA effects of carotenoids, a pigment contained in vegetables and fruits.

METHODS

C57BL/6N mice were fed a diet containing 0.01% (w/w) of lycopene, β-carotene, astaxanthin or lutein for 4 weeks, and the population of colonic Tregs was assessed. Subsequently, to evaluate the Treg-inducing ability of lycopene, splenic naïve CD4⁺ T cells from BALB/c mice were cultured with anti-CD3/CD28 antibody, TGF-β and lycopene, and the frequencies of Tregs were examined. The effect of 0.1% (w/w) lycopene containing diet on FA was investigated in OVA-induced FA model BALB/c mice.

RESULTS

In screening, only lycopene significantly increased the frequency and number of colonic Tregs. Lycopene also increased Treg differentiation in splenic naïve CD4⁺ T cells. In FA mice, lycopene feeding significantly increased the number of colonic Tregs and attenuated allergic symptoms. The expression levels of IL-4, IL-9 and IL-13 mRNA in colonic mucosa were also significantly reduced by lycopene. IL-9 is known to induce proliferation of mast cells, and we found that lycopene feeding significantly reduced the number of mast cells in the colonic mucosa of FA mice.

CONCLUSION

Our results suggest that lycopene, a carotenoid present in many common foods on the market, may have the potential to induce colonic Tregs and suppress FA symptoms.

Methoxy polyethylene glycol modification promotes adipogenesis by inducing the production of regulatory T cells in xenogeneic acellular adipose matrix

Acellular adipose matrix (AAM) has emerged as an important biomaterial for adipose tissue regeneration. Current decellularization methods damage the bioactive components of the extracellular matrix (ECM), and the residual immunogenic antigens may induce adverse immune responses. Here, we adopted a modified decellularization method which can protect more bioactive components with less immune reaction by methoxy polyethylene glycol (mPEG). Then, we determined the adipogenic mechanisms of mPEG-modified AAM after xenogeneic transplantation. AAM transplantation caused significantly lesser adipogenesis in the wild-type group than in the immune-deficient group. The mPEG-modified AAM showed significantly lower immunogenicity and higher adipogenesis than the AAM alone after xenogeneic transplantation. Furthermore, mPEG modification increased regulatory T (Treg) cell numbers in the AAM grafts, which in turn enhanced the M2/M1 macrophage ratio by secreting IL-10, IL-13, and TGF-β1. These findings suggest that mPEG modification effectively reduces the immunogenicity of xenogeneic AAM and promotes adipogenesis in the AAM grafts. Hence, mPEG-modified AAM can serve as an ideal biomaterial for xenogeneic adipose tissue engineering.

Synergistic and additive interactions between receptor signaling networks drive the regulatory T cell versus T helper 17 cell fate choice

CD4+ T cells differentiate into subsets that promote immunity or minimize damage to the host. T helper 17 cells (Th17) are effector cells that function in inflammatory responses. T regulatory cells (Tregs) maintain tolerance and prevent autoimmunity by secreting immunosuppressive cytokines and expressing check point receptors. While the functions of Th17 and Treg cells are different, both cell fate trajectories require T cell receptor (TCR) and TGF-β receptor (TGF-βR) signals, and Th17 polarization requires an additional IL-6 receptor (IL-6R) signal. Utilizing high-resolution phosphoproteomics, we identified that both synergistic and additive interactions between TCR, TGF-βR, and IL-6R shape kinase signaling networks to differentially regulate key pathways during the early phase of Treg versus Th17 induction. Quantitative biochemical analysis revealed that CD4+ T cells integrate receptor signals via SMAD3, which is a mediator of TGF-βR signaling. Treg induction potentiates the formation of the canonical SMAD3/4 trimer to activate a negative feedback loop through kinases PKA and CSK to suppress TCR signaling, phosphatidylinositol metabolism, and mTOR signaling. IL-6R signaling activates STAT3 to bind SMAD3 and block formation of the SMAD3/4 trimer during the early phase of Th17 induction, which leads to elevated TCR and PI3K signaling. These data provide a biochemical mechanism by which CD4+ T cells integrate TCR, TGF-β, and IL-6 signals via generation of alternate SMAD3 complexes that control the development of early signaling networks to potentiate the choice of Treg versus Th17 cell fate.

Crosstalk of Microorganisms and Immune Responses in Autoimmune Neuroinflammation: A Focus on Regulatory T Cells

Regulatory T cells (Tregs) are the major determinant of peripheral immune tolerance. Many Treg subsets have been described, however thymus-derived and peripherally induced Tregs remain the most important subpopulations. In multiple sclerosis, a prototypical autoimmune disorder of the central nervous system, Treg dysfunction is a pathogenic hallmark. In contrast, induction of Treg proliferation and enhancement of their function are central immune evasion mechanisms of infectious pathogens. In accordance, Treg expansion is compartmentalized to tissues with high viral replication and prolonged in chronic infections. In friend retrovirus infection, Treg expansion is mainly based on excessive interleukin-2 production by infected effector T cells. Moreover, pathogens seem also to enhance Treg functions as shown in human immunodeficiency virus infection, where Tregs express higher levels of effector molecules such as cytotoxic T-lymphocyte-associated protein 4, CD39 and cAMP and show increased suppressive capacity. Thus, insights into the molecular mechanisms by which intracellular pathogens alter Treg functions might aid to find new therapeutic approaches to target central nervous system autoimmunity. In this review, we summarize the current knowledge of the role of pathogens for Treg function in the context of autoimmune neuroinflammation. We discuss the mechanistic implications for future therapies and provide an outlook for new research directions.

In Vivo Expansion of Antigen-Specific Regulatory T Cells through Staggered Fc.IL-2 Mutein Dosing and Antigen-Specific Immunotherapy

In mice, Ag administration in the absence of adjuvant typically elicits tolerogenic immune responses through the deletion or inactivation of conventional CD4 T cells and the formation or expansion of regulatory CD4 T cells (Treg). Although these "Ag-specific immunotherapy" (ASI) approaches are currently under clinical development to treat autoinflammatory conditions, efficacy and safety may be variable and unpredictable because of the diverse activation states of immune cells in subjects with autoimmune and allergic diseases. To reliably induce Ag-specific tolerance in patients, novel methods to control T cell responses during ASI are needed, and strategies that permanently increase Treg frequencies among Ag-specific CD4 T cells may provide long-lasting immunosuppression between treatments. In this study, we present an approach to durably increase the frequency of Ag-specific Treg in mice by administering ASI when Treg numbers are transiently increased with individual doses of a half-life-extended Treg-selective IL-2 mutein. Repeated weekly cycles of IL-2 mutein doses (day 0) followed by ASI (day 3) resulted in a 3- to 5-fold enrichment in Treg among Ag-responsive CD4 T cells. Expanded Ag-specific Treg persisted for more than 3 wk following treatment cessation, as well as through an inflammatory T cell response to an Ag-expressing virus. Combining Treg enrichment with ASI has the potential to durably treat autoimmune disease or allergy by increasing the Treg/conventional CD4 T cell ratio among autoantigen- or allergen-specific T cells.

Kynurenines as a Novel Target for the Treatment of Malignancies

Malignancies are unquestionably a significant public health problem. Their effective treatment is still a big challenge for modern medicine. Tumors have developed a wide range of mechanisms to evade an immune and therapeutic response. As a result, there is an unmet clinical need for research on solutions aimed at overcoming this problem. An accumulation of tryptophan metabolites belonging to the kynurenine pathway can enhance neoplastic progression because it causes the suppression of immune system response against cancer cells. They are also involved in the development of the mechanisms responsible for the resistance to antitumor therapy. Kynurenine belongs to the most potent immunosuppressive metabolites of this pathway and has a significant impact on the development of malignancies. This fact prompted researchers to assess whether targeting the enzymes responsible for its synthesis could be an effective therapeutic strategy for various cancers. To date, numerous studies, both preclinical and clinical, have been conducted on this topic, especially regarding the inhibition of indoleamine 2,3-dioxygenase activity and their results can be considered noteworthy. This review gathers and systematizes the knowledge about the role of the kynurenine pathway in neoplastic progression and the findings regarding the usefulness of modulating its activity in anticancer therapy.

Strength and Numbers: The Role of Affinity and Avidity in the 'Quality' of T Cell Tolerance

The ability of T cells to identify foreign antigens and mount an efficient immune response while limiting activation upon recognition of self and self-associated peptides is critical. Multiple tolerance mechanisms work in concert to prevent the generation and activation of self-reactive T cells. T cell tolerance is tightly regulated, as defects in these processes can lead to devastating disease; a wide variety of autoimmune diseases and, more recently, adverse immune-related events associated with checkpoint blockade immunotherapy have been linked to a breakdown in T cell tolerance. The quantity and quality of antigen receptor signaling depend on a variety of parameters that include T cell receptor affinity and avidity for peptide. Autoreactive T cell fate choices (e.g., deletion, anergy, regulatory T cell development) are highly dependent on the strength of T cell receptor interactions with self-peptide. However, less is known about how differences in the strength of T cell receptor signaling during differentiation influences the 'function' and persistence of anergic and regulatory T cell populations. Here, we review the literature on this subject and discuss the clinical implications of how T cell receptor signal strength influences the 'quality' of anergic and regulatory T cell populations.

TCR Affinity Controls the Dynamics but Not the Functional Specification of the Antimycobacterial CD4+ T Cell Response

The quality of T cell responses depends on the lymphocytes' ability to undergo clonal expansion, acquire effector functions, and traffic to the site of infection. Although TCR signal strength is thought to dominantly shape the T cell response, by using TCR transgenic CD4⁺ T cells with different peptide:MHC binding affinity, we reveal that TCR affinity does not control Th1 effector function acquisition or the functional output of individual effectors following mycobacterial infection in mice. Rather, TCR affinity calibrates the rate of cell division to synchronize the distinct processes of T cell proliferation, differentiation, and trafficking. By timing cell division-dependent IL-12R expression, TCR affinity controls when T cells become receptive to Th1-imprinting IL-12 signals, determining the emergence and magnitude of the Th1 effector pool. These findings reveal a distinct yet cooperative role for IL-12 and TCR binding affinity in Th1 differentiation and suggest that the temporal activation of clones with different TCR affinity is a major strategy to coordinate immune surveillance against persistent pathogens.

CAR- and TRuC-redirected regulatory T cells differ in capacity to control adaptive immunity to FVIII

Regulatory T cells (Tregs) control immune responses in autoimmune disease, transplantation, and enable antigen-specific tolerance induction in protein-replacement therapies. Tregs can exert a broad array of suppressive functions through their T cell receptor (TCR) in a tissue-directed and antigen-specific manner. This capacity can now be harnessed for tolerance induction by "redirecting" polyclonal Tregs to overcome low inherent precursor frequencies and simultaneously augment suppressive functions. With the use of hemophilia A as a model, we sought to engineer antigen-specific Tregs to suppress antibody formation against the soluble therapeutic protein factor (F)VIII in a major histocompatibility complex (MHC)-independent fashion. Surprisingly, high-affinity chimeric antigen receptor (CAR)-Treg engagement induced a robust effector phenotype that was distinct from the activation signature observed for endogenous thymic Tregs, which resulted in the loss of suppressive activity. Targeted mutations in the CD3ζ or CD28 signaling motifs or interleukin (IL)-10 overexpression were not sufficient to restore tolerance. In contrast, complexing TCR-based signaling with single-chain variable fragment (scFv) recognition to generate TCR fusion construct (TRuC)-Tregs delivered controlled antigen-specific signaling via engagement of the entire TCR complex, thereby directing functional suppression of the FVIII-specific antibody response. These data suggest that cellular therapies employing engineered receptor Tregs will require regulation of activation thresholds to maintain optimal suppressive function.

TLR9 Sensing of Self-DNA Controls Cell-Mediated Immunity to Listeria Infection via Rapid Conversion of Conventional CD4+ T Cells to Treg

CD4⁺ T lymphocytes are crucial for controlling a range of innate and adaptive immune effectors. For CD8⁺ cytotoxic T lymphocyte (CTL) responses, CD4⁺ T cells can function as helpers (T_H) to amplify magnitude and functionality or as regulatory cells (T_reg) capable of profound inhibition. It is unclear what determines differentiation to these phenotypes and whether pathogens provoke alternate programs. We find that, depending on the size of initial dose, Listeria infection drives CD4⁺ T cells to act as T_H or induces rapid polyclonal conversion to immunosuppressive T_reg. Conversion to T_reg depends on the TLR9 and IL-12 pathways elicited by CD8a⁺ dendritic cell (DC) sensing of danger-associated neutrophil self-DNA. These findings resolve long-standing questions regarding the conditional requirement for T_H amongst pathogens and reveal a remarkable degree of plasticity in the function of CD4⁺ T cells, which can be quickly converted to T_regin vivo by infection-mediated immune modulation.

Dolina et al. show that Listeria infectious dose drives conventional CD4⁺ T cells to act as T_H or mediates conversion to T_reg. Differentiation to T_reg dominates heightened doses and is promoted by CD8α⁺ DC TLR9 engagement of neutrophil self-DNA and IL-12 production, revealing plasticity in the function of CD4⁺ T cells.

Human dendritic cells adenovirally-engineered to express three defined tumor antigens promote broad adaptive and innate immunity

Dendritic cell (DC) immunotherapy has shown a promising ability to promote anti-tumor immunity in vitro and in vivo. Many trials have tested single epitopes and single antigens to activate single T cell specificities, and often CD8(+) T cells only. We previously found that determinant spreading and breadth of antitumor immunity correlates with improved clinical response. Therefore, to promote activation and expansion of polyclonal, multiple antigen-specific CD8(+) T cells, as well as provide cognate help from antigen-specific CD4(+) T cells, we have created an adenovirus encoding three full length melanoma tumor antigens (tyrosinase, MART-1 and MAGE-A6, "AdVTMM"). We previously showed that adenovirus (AdV)-mediated antigen engineering of human DC is superior to peptide pulsing for T cell activation, and has positive biological effects on the DC, allowing for efficient activation of not only antigen-specific CD8(+) and CD4(+) T cells, but also NK cells. Here we describe the cloning and testing of "AdVTMM2," an E1/E3-deleted AdV encoding the three melanoma antigens. This novel three-antigen virus expresses mRNA and protein for all antigens, and AdVTMM-transduced DC activate both CD8(+) and CD4(+) T cells which recognize melanoma tumor cells more efficiently than single antigen AdV. Addition of physiological levels of interferon-α (IFNα) further amplifies melanoma antigen-specific T cell activation. NK cells are also activated, and show cytotoxic activity. Vaccination with multi-antigen engineered DC may provide for superior adaptive and innate immunity and ultimately, improved antitumor responses.

Diverse Roles of Akt in T cells

Akt kinases translate various external cues into intracellular signals that control cell survival, proliferation, metabolism and differentiation. This review discusses the requirement for Akt and its targets in determining the fate and function of T cells. We discuss the importance of Akt at various stages of T cell development including β-selection during which Akt fulfills the energy requirements of highly proliferative DN3 cells. Akt also plays an integral role in CD8 T cell biology where its regulation of Foxo transcription factors and mTORC1 metabolic activity controls effector versus memory CD8 T cell differentiation. Finally, Akt promotes the differentiation of naïve CD4 T cells into Th1, Th17 and Tfh cells but inhibits the development of Treg cells. We also highlight how modulating Akt in T cells is a promising avenue for enhancing cell-based cancer immunotherapy.

Hyaluronan synthesis inhibition impairs antigen presentation and delays transplantation rejection

A coat of pericellular hyaluronan surrounds mature dendritic cells (DC) and contributes to cell-cell interactions. We asked whether 4-methylumbelliferone (4MU), an oral inhibitor of HA synthesis, could inhibit antigen presentation. We find that 4MU treatment reduces pericellular hyaluronan, destabilizes interactions between DC and T-cells, and prevents T-cell proliferation in vitro and in vivo. These effects were observed only when 4MU was added prior to initial antigen presentation but not later, consistent with 4MU-mediated inhibition of de novo antigenic responses. Building on these findings, we find that 4MU delays rejection of allogeneic pancreatic islet transplant and allogeneic cardiac transplants in mice and suppresses allogeneic T-cell activation in human mixed lymphocyte reactions. We conclude that 4MU, an approved drug, may have benefit as an adjunctive agent to delay transplantation rejection.

Lactoferrin Induces Tolerogenic Bone Marrow-Derived Dendritic Cells

Dendritic cells (DCs) are professional antigen-presenting cells (APCs) that initiate both T-cell responses and tolerance. Tolerogenic DCs (tDCs) are regulatory DCs that suppress immune responses through the induction of T-cell anergy and Tregs. Because lactoferrin (LF) was demonstrated to induce functional Tregs and has a protective effect against inflammatory bowel disease, we explored the tolerogenic effects of LF on mouse bone marrow-derived DCs (BMDCs). The expression of CD80/86 and MHC class II was diminished in LF-treated BMDCs (LF-BMDCs). LF facilitated BMDCs to suppress proliferation and elevate Foxp3⁺ induced Treg (iTreg) differentiation in ovalbumin-specific CD4⁺ T-cell culture. Foxp3 expression was further increased by blockade of the B7 molecule using CTLA4-Ig but was diminished by additional CD28 stimulation using anti-CD28 Ab. On the other hand, the levels of arginase-1 and indoleamine 2,3-dioxygenase-1 (known as key T-cell suppressive molecules) were increased in LF-BMDCs. Consistently, the suppressive activity of LF-BMDCs was partially restored by inhibitors of these molecules. Collectively, these results suggest that LF effectively causes DCs to be tolerogenic by both the suppression of T-cell proliferation and enhancement of iTreg differentiation. This tolerogenic effect of LF is due to the reduction of costimulatory molecules and enhancement of suppressive molecules.

TRI microparticles prevent inflammatory arthritis in a collagen-induced arthritis model

Despite recent progress in the treatment of rheumatoid arthritis (RA), many patients still fail to achieve remission or low disease activity. An imbalance between auto-reactive effector T cells (Teff) and regulatory T cells (Treg) may contribute to joint inflammation and damage in RA. Therefore, restoring this balance is a promising approach for the treatment of inflammatory arthritis. Accordingly, our group has previously shown that the combination of TGF-β-releasing microparticles (MP), rapamycin-releasing MP, and IL-2-releasing MP (TRI MP) can effectively increase the ratio of Tregs to Teff in vivo and provide disease protection in several preclinical models. In this study TRI MP was evaluated in the collagen-induced arthritis (CIA) model. Although this formulation has been tested previously in models of destructive inflammation and transplantation, this is the first model of autoimmunity for which this therapy has been applied. In this context, TRI MP effectively reduced arthritis incidence, the severity of arthritis scores, and bone erosion. The proposed mechanism of action includes not only reducing CD4+ T cell proliferation, but also expanding a regulatory population in the periphery soon after TRI MP administration. These changes were reflected in the CD4+ T cell population that infiltrated the paws at the onset of arthritis and were associated with a reduction of immune infiltrate and inflammatory myeloid cells in the paws. TRI MP administration also reduced the titer of collagen antibodies, however the contribution of this reduced titer to disease protection remains uncertain since there was no correlation between collagen antibody titer and arthritis score.

Human CD8+ T Cells Exhibit a Shared Antigen Threshold for Different Effector Responses

CD8⁺ T cells produce TNF-α, IL-2, and IFN-γ with similar Ag thresholds.

Costimulation decreases Ag thresholds similarly for different cytokines.

A common rate-limiting switch downstream of the TCR can explain these findings.

T cells recognizing cognate pMHC Ags become activated to elicit a myriad of cellular responses, such as target cell killing and the secretion of different cytokines, that collectively contribute to adaptive immunity. These effector responses have been hypothesized to exhibit different Ag dose and affinity thresholds, suggesting that pathogen-specific information may be encoded within the nature of the Ag. In this study, using systematic experiments in a reductionist system, in which primary human CD8⁺ T cell blasts are stimulated by recombinant peptides presented on MHC Ag alone, we show that different inflammatory cytokines have comparable Ag dose thresholds across a 25,000-fold variation in affinity. Although costimulation by CD28, CD2, and CD27 increased cytokine production in this system, the Ag threshold remained comparable across different cytokines. When using primary human memory CD8⁺ T cells responding to autologous APCs, equivalent thresholds were also observed for different cytokines and killing. These findings imply a simple phenotypic model of TCR signaling in which multiple T cell responses share a common rate-limiting threshold and a conceptually simple model of CD8⁺ T cell Ag recognition, in which Ag dose and affinity do not provide any additional response-specific information.

Hepatitis B envelope antigen increases Tregs by converting CD4+CD25- T cells into CD4+CD25+Foxp3+ Tregs

Hepatitis B virus (HBV) can establish a lifelong chronic infection in humans, leading to liver cirrhosis, liver failure and hepatocellular carcinoma. Patients with chronic hepatitis B (CHB) exhibit a weak virus-specific immune response. Regulatory T cells (Tregs) play a key role in regulating the immune response in patients with CHB. Patients with hepatitis B envelope antigen (HBeAg)-positive CHB harbored a higher percentage of Tregs in their peripheral blood than those with HBeAg-negative CHB. However, whether and how HBeAg manipulates the host immune system to increase the population of Tregs remains to be elucidated. The present manuscript describes a preliminary immunological study of HBeAg in a mouse model. Multiple potential CD4⁺ T cell epitopes in HBeAg were identified using Immune Epitope Database consensus binding prediction. It was demonstrated that HBeAg treatment increased the numbers of Tregs in mouse spleens in vitro and in vivo. Furthermore, it was indicated that the HBeAg-mediated increase in Tregs occurred through the conversion of CD4⁺CD25^- T cells into CD4⁺CD25⁺Foxp3⁺ Tregs. Additionally, in vitro study illustrated that HBeAg stimulated murine spleen cells to produce increased transforming growth factor-β, which is required to enable HBeAg to convert T cells into Tregs. The results of the present study may provide further evidence of the effect of HBeAg on Tregs and aid in the development of novel HBeAg-based immunotherapy for CHB.

Transcriptomic profiling of bovine blood dendritic cells and monocytes following TLR stimulation

Dendritic cells (DC) and monocytes are vital for the initiation of innate and adaptive immune responses. Recently, we identified bona fide DC subsets in blood of cattle, revealing subset- and species-specific transcription of toll-like receptors (TLR). In the present study, we analyzed phenotypic and transcriptional responses of bovine DC subsets and monocytes to in vitro stimulation with four to six different TLR ligands. Bovine DC subsets, especially plasmacytoid DC (pDC), showed a clear increase of CCR7, CD25, CD40, CD80, CD86, and MHC-II expression both on mRNA and protein level. Flow cytometric detection of p38 MAPK phosphorylation 15 min after stimulation confirmed activation of DC subsets and monocytes in accordance with TLR gene expression. Whole-transcriptome sequencing of sorted and TLR-stimulated subsets revealed potential ligand- and subset-specific regulation of genes associated with inflammation, T-cell co-stimulation, migration, metabolic reprogramming, and antiviral activity. Gardiquimod was found to evoke strong responses both in DC subsets and monocytes, while Poly(I:C) and CpG preferentially triggered responses in cDC1 and pDC, respectively. This in-depth analysis of ligand responsiveness is essential for the rational design of vaccine adjuvants in cattle, and provides a solid basis for comparative studies on DC and monocyte biology across species.

Allergen alters IL-2/αIL-2-based Treg expansion but not tolerance induction in an allergen-specific mouse model

Background

Regulatory T lymphocytes (Treg) play an important role in preventing allergic diseases. We characterized Treg expansion kinetics, marker profiles, and recirculation behavior in allergen‐challenged mice, which had been pretreated with IL‐2/αIL‐2 complexes in the presence or absence of allergen. Moreover, the ability of induced Treg to control airway hyperreactivity and effector functions of lung T cells was determined.

Methods

Humanized TCR/HLA‐transgenic allergy mice were treated in vivo with recombinant IL‐2 complexed to the anti‐IL‐2 mAb JES6‐1 in the presence or absence of mugwort pollen extract (MPE) on days 0‐2. Afterward, they were intranasally challenged with MPE (days 13‐15) followed by determination of airway hyperreactivity and lung T cell effector functions. Multiparametric flow cytometry on peripheral blood T cells was performed on a daily basis.

Results

IL‐2/αIL‐2 complexes highly efficiently expanded peripheral Treg cells, while concomitant allergen exposure altered the phenotype of expanded Treg cells. Notably, application of allergen together with IL‐2/αIL‐2 complexes induced expression of Treg marker molecules CTLA4, NRP1, Helios, and GITR on conventional T cells. Apart from CD25, GARP was identified as the most reliable surface‐expressed lineage discrimination marker of Treg expanded in the presence of IL‐2/αIL‐2 complexes and allergen. Finally, IL‐2/αIL‐2 complex‐expanded Treg cells could be recalled upon allergen challenge, which was associated with suppression of lung‐specific Th2 responses long after initial treatment.

Conclusion

The characterization of reliable surface and transcription markers of IL‐2/αIL‐2 complex‐expanded Treg along with their expansion kinetics and function will help to identify protocols for their long‐term expansion in vivo.

Regulation of T Helper Cell Fate by TCR Signal Strength

T cells are critical in orchestrating protective immune responses to cancer and an array of pathogens. The interaction between a peptide MHC (pMHC) complex on antigen presenting cells (APCs) and T cell receptors (TCRs) on T cells initiates T cell activation, division, and clonal expansion in secondary lymphoid organs. T cells must also integrate multiple T cell-intrinsic and extrinsic signals to acquire the effector functions essential for the defense against invading microbes. In the case of T helper cell differentiation, while innate cytokines have been demonstrated to shape effector CD4⁺ T lymphocyte function, the contribution of TCR signaling strength to T helper cell differentiation is less understood. In this review, we summarize the signaling cascades regulated by the strength of TCR stimulation. Various mechanisms in which TCR signal strength controls T helper cell expansion and differentiation are also discussed.

mTOR signaling at the crossroads of environmental signals and T-cell fate decisions

The evolutionarily conserved serine/threonine kinase mTOR (mechanistic target of rapamycin) forms the distinct protein complexes mTORC1 and mTORC2 and integrates signals from the environment to coordinate downstream signaling events and various cellular processes. T cells rely on mTOR activity for their development and to establish their homeostasis and functional fitness. Here, we review recent progress in our understanding of the upstream signaling and downstream targets of mTOR. We also provide an updated overview of the roles of mTOR in T-cell development, homeostasis, activation, and effector-cell fate decisions, as well as its important impacts on the suppressive activity of regulatory T cells. Moreover, we summarize the emerging roles of mTOR in T-cell exhaustion and transdifferentiation. A better understanding of the contribution of mTOR to T-cell fate decisions will ultimately aid in the therapeutic targeting of mTOR in human disease.

The Immune Response to the fVIII Gene Therapy in Preclinical Models

Neutralizing antibodies to factor VIII (fVIII), referred to as "inhibitors," remain the most challenging complication post-fVIII replacement therapy. Preclinical development of novel fVIII products involves studies incorporating hemophilia A (HA) and wild-type animal models. Though immunogenicity is a critical aspect of preclinical pharmacology studies, gene therapy studies tend to focus on fVIII expression levels without major consideration for immunogenicity. Therefore, little clarity exists on whether preclinical testing can be predictive of clinical immunogenicity risk. Despite this, but perhaps due to the potential for transformative benefits, clinical gene therapy trials have progressed rapidly. In more than two decades, no inhibitors have been observed. However, all trials are conducted in previously treated patients without a history of inhibitors. The current review thus focuses on our understanding of preclinical immunogenicity for HA gene therapy candidates and the potential indication for inhibitor treatment, with a focus on product- and platform-specific determinants, including fVIII transgene sequence composition and tissue/vector biodistribution. Currently, the two leading clinical gene therapy vectors are adeno-associated viral (AAV) and lentiviral (LV) vectors. For HA applications, AAV vectors are liver-tropic and employ synthetic, high-expressing, liver-specific promoters. Factors including vector serotype and biodistribution, transcriptional regulatory elements, transgene sequence, dosing, liver immunoprivilege, and host immune status may contribute to tipping the scale between immunogenicity and tolerance. Many of these factors can also be important in delivery of LV-fVIII gene therapy, especially when delivered intravenously for liver-directed fVIII expression. However, ex vivo LV-fVIII targeting and transplantation of hematopoietic stem and progenitor cells (HSPC) has been demonstrated to achieve durable and curative fVIII production without inhibitor development in preclinical models. A critical variable appears to be pre-transplantation conditioning regimens that suppress and/or ablate T cells. Additionally, we and others have demonstrated the potential of LV-fVIII HSPC and liver-directed AAV-fVIII gene therapy to eradicate pre-existing inhibitors in murine and canine models of HA, respectively. Future preclinical studies will be essential to elucidate immune mechanism(s) at play in the context of gene therapy for HA, as well as strategies for preventing adverse immune responses and promoting immune tolerance even in the setting of pre-existing inhibitors.

Tuning T helper cell differentiation by ITK

CD4+ effector T cells effectuate T cell immune responses, producing cytokines to orchestrate the nature and type of immune responses. The non-receptor tyrosine kinase IL-2 inducible T cell kinase (ITK), a mediator of T cell Receptor signaling, plays a critical role in tuning the development of these effector cells. In this review we discussed the role that signals downstream of ITK, including the Ras/MAPK pathway, play in differentially controlling the differentiation of TH17, Foxp3+ T regulatory (Treg) cells, and Type 1 regulatory T (Tr1) cells, supporting a model of ITK signals controlling a decision point in the effector T cell differentiation process.

Attenuation of TCR-induced transcription by Bach2 controls regulatory T cell differentiation and homeostasis

Differentiation and homeostasis of Foxp3+ regulatory T (Treg) cells are strictly controlled by T-cell receptor (TCR) signals; however, molecular mechanisms that govern these processes are incompletely understood. Here we show that Bach2 is an important regulator of Treg cell differentiation and homeostasis downstream of TCR signaling. Bach2 prevents premature differentiation of fully suppressive effector Treg (eTreg) cells, limits IL-10 production and is required for the development of peripherally induced Treg (pTreg) cells in the gastrointestinal tract. Bach2 attenuates TCR signaling-induced IRF4-dependent Treg cell differentiation. Deletion of IRF4 promotes inducible Treg cell differentiation and rescues pTreg cell differentiation in the absence of Bach2. In turn, loss of Bach2 normalizes eTreg cell differentiation of IRF4-deficient Treg cells. Mechanistically, Bach2 counteracts the DNA-binding activity of IRF4 and limits chromatin accessibility, thereby attenuating IRF4-dependent transcription. Thus, Bach2 balances TCR signaling induced transcriptional activity of IRF4 to maintain homeostasis of thymically-derived and peripherally-derived Treg cells.

Cutting Edge: TCR Signal Strength Regulates Acetyl-CoA Metabolism via AKT

TCR signaling activates kinases including AKT/mTOR that engage metabolic networks to support the energetic demands of a T cell during an immune response. It is realized that CD4⁺ T cell subsets have different metabolic requirements. Yet, how TCR signaling is coupled to the regulation of intermediate metabolites and how changes in metabolite flux contribute to T cell differentiation are less established. We find that TCR signaling regulates acetyl-CoA metabolism via AKT in murine CD4⁺ T cells. Weak TCR signals promote AKT-catalyzed phosphorylation and inhibition of citrate synthase, elevated acetyl-CoA levels, and hyperacetylation of mitochondrial proteins. Genetic knockdown of citrate synthase promotes increased nuclear acetyl-CoA levels, increased histone acetylation at the FOXP3 promotor and induction of FOXP3 transcription. These data identify a circuit between AKT signaling and acetyl-CoA metabolism regulated via TCR signal strength and that transient fluctuations in acetyl-CoA levels function in T cell fate decisions.

Immunometabolic Checkpoints of Treg Dynamics: Adaptation to Microenvironmental Opportunities and Challenges

In the last decades, immunologists have started to consider intracellular metabolism in relation with the dynamics and functions of immune cells, especially when it became clear that microenvironmental alterations were associated with immune dysfunctions. Regulatory T cells (Tregs) are equipped with a variety of immunological and metabolic sensors, and encompass circulating as well as tissue-resident cells, being therefore particularly susceptible to microenvironmental cues. Moreover, Tregs undergo metabolic reprogramming over the course of an immune response, allowing the use of alternate substrates and engaging different metabolic pathways for energetic demands. The study of metabolic mechanisms supporting Treg dynamics has led to puzzling results, due to several limitations, including the heterogeneity of population in the same tissues and between different tissues, the difficulty in considering all the interconnected metabolic pathways during a cellular process, and the differences between in vitro and in vivo conditions. Therefore, Treg reliance on different metabolic routes (oxidation rather than glycolysis) has been a matter of controversy in recent years. Metabolic reprogramming and altered bioenergetics are now identified as hallmarks in cancer, and are employed by cancer cells to determine the availability of metabolites and molecules, thus affecting the fate of tumor-infiltrating immune cells. In particular, the tumor microenvironment forces a metabolic restriction and a plethora of synergistic intrinsic and extrinsic stresses, leading to an impaired anti-tumor immunity and favoring Treg generation, expansion, and suppressive function. This leads to the understanding that Tregs and conventional T cells have different capability to adapt to metabolic hurdles. Considering the role of Tregs in dictating the outcome of tumor-specific responses, it would be important to understand the specific Treg metabolic profile that provides an advantage at the tumor site, to finally identify new targets for therapy. In this review, we will report and discuss the major recent findings about the metabolic pathways required for Treg development, expansion, migration and functions, in relation to tissue-derived signals. We will focus on the adipose tissue and the liver, where Tregs are exposed to a variety of metabolites, and on the tumor microenvironment as the context where Tregs develop the ability to adapt to perturbations in nutrient accessibility.

Optimal Tolerogenic Dendritic Cells in Type 1 Diabetes (T1D) Therapy: What Can We Learn From Non-obese Diabetic (NOD) Mouse Models?

Tolerogenic dendritic cells (tolDCs) are explored as a promising standalone or combination therapy in type 1 diabetes (T1D). The therapeutic application of tolDCs, including in human trials, has been tested also in other autoimmune diseases, however, T1D displays some unique features. In addition, unlike in several disease-induced animal models of autoimmune diseases, the prevalent animal model for T1D, the NOD mouse, develops diabetes spontaneously. This review compares evidence of various tolDCs approaches obtained from animal (mainly NOD) models of T1D with a focus on parameters of this cell-based therapy such as protocols of tolDC preparation, antigen-specific vs. unspecific approaches, doses of tolDCs and/or autoantigens, application schemes, application routes, the migration of tolDCs as well as their preventive, early pre-onset intervention or curative effects. This review also discusses perspectives of tolDC therapy and areas of preclinical research that are in need of better clarification in animal models in a quest for effective and optimal tolDC therapies of T1D in humans.

T cells transduce T-cell receptor signal strength by generating different phosphatidylinositols

T-cell receptor (TCR) signaling strength is a dominant factor regulating T-cell differentiation, thymic development, and cytokine signaling. The molecular mechanisms by which TCR signal strength is transduced to downstream signaling networks remains ill-defined. Using computational modeling, biochemical assays, and imaging flow cytometry, we found here that TCR signal strength differentially generates phosphatidylinositol species. Weak TCR signals generated elevated phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) and reduced phosphatidylinositol (3,4,5)-trisphosphate (PIP3) levels, whereas strong TCR signals reduced PI(4,5)P2 and elevated PIP3 levels. A proteomics screen revealed that focal adhesion kinase bound PI(4,5)P2, biochemical assays disclosed that focal adhesion kinase is preferentially activated by weak TCR signals and is required for optimal Treg induction, and further biochemical experiments revealed how TCR signaling strength regulates AKT activation. Low PIP3 levels generated by weak TCR signals were sufficient to activate phosphoinositide-dependent kinase-1 to phosphorylate AKT on Thr-308 but insufficient to activate mTOR complex 2 (mTORC2), whereas elevated PIP3 levels generated by a strong TCR signal were required to activate mTORC2 to phosphorylate Ser-473 on AKT. Our results provide support for a model that links TCR signaling to mTORC2 activation via phosphoinositide 3-kinase signaling. Together, the findings in this work establish that T cells measure TCR signal strength by generating different levels of phosphatidylinositol species that engage alternate signaling networks to control cell fate decisions.

Protein Tyrosine Phosphatases: Regulators of CD4 T Cells in Inflammatory Bowel Disease

Protein tyrosine phosphatases (PTPs) play a critical role in co-ordinating the signaling networks that maintain lymphocyte homeostasis and direct lymphocyte activation. By dephosphorylating tyrosine residues, PTPs have been shown to modulate enzyme activity and both mediate and disrupt protein-protein interactions. Through these molecular mechanisms, PTPs ultimately impact lymphocyte responses to environmental cues such as inflammatory cytokines and chemokines, as well as antigenic stimulation. Mouse models of acute and chronic intestinal inflammation have been shown to be exacerbated in the absence of PTPs such as PTPN2 and PTPN22. This increase in disease severity is due in part to hyper-activation of lymphocytes in the absence of PTP activity. In accordance, human PTPs have been linked to intestinal inflammation. Genome wide association studies (GWAS) identified several PTPs within risk loci for inflammatory bowel disease (IBD). Therapeutically targeting PTP substrates and their associated signaling pathways, such as those implicated in CD4⁺ T cell responses, has demonstrated clinical efficacy. The current review focuses on the role of PTPs in controlling CD4⁺ T cell activity in the intestinal mucosa and how disruption of PTP activity in CD4⁺ T cells can contribute to intestinal inflammation.

An altered CD8+ T cell epitope of insulin prevents type 1 diabetes in humanized NOD mice

Autoreactive CD8⁺ T cells, which play an indispensable role in β cell destruction, represent an emerging target for the prevention of type 1 diabetes (T1D). Altered peptide ligands (APLs) can efficiently induce antigen-specific T cells anergy, apoptosis or shifts in the immune response. Here, we found that HLA-A*0201-restricted CD8⁺ T cell responses against a primary β-cell autoantigen insulin epitope InsB1_5-14 were present in both NOD.β2m^null.HHD NOD mice and T1D patients. We generated several APL candidates for InsB1_5-14 by residue substitution at the p6 position. Only H6F exhibited an inhibitory effect on mInsB1_5-14-specific CD8⁺ T cell responses in vitro. H6F treatment significantly reduced the T1D incidence, which was accompanied by diminished autoreactive CD8⁺ T cell responses to mInsB1_5-14, inhibited infiltration of CD8⁺ and CD4⁺ T cells in the pancreas and reduced pro-inflammatory cytokine production in pancreatic and splenic T cells in NOD.β2m^null.HHD mice. Mechanistically, H6F treatment significantly augmented a tiny portion of CD8⁺CD25⁺Foxp3⁺ T cells in the spleen and especially in the pancreas. This subset exhibited typical Treg phenotypes and required peptide-specific restimulation to exert immunosuppressive activity. Therefore, this APL H6F may be a promising candidate with potential clinical application value for antigen-specific prevention of T1D.

Effect of TGF-β1 on blood CD4+CD25high regulatory T cell proliferation and Foxp3 expression during non-small cell lung cancer blood metastasis

Metastatic circulating tumor cells in non-small cell lung cancer (NSCLC) metastasis have been reported to be associated with an immune response. The present study aimed to provide a theoretical basis for the immunomodulatory processes during NSCLC blood metastasis. NSCLC blood and normal peripheral blood mononuclear cells (PBMCs) were collected. The quantity of cluster of differentiation (CD)4⁺CD25^high regulatory T (Treg) cells and the intracellular forkhead box protein 3 (Foxp3) expression in CD4⁺CD25^high Treg cells were determined by flow cytometry. Furthermore, the effect of transforming growth factor β1 (TGF-β1) on NSCLC blood CD4⁺CD25⁺ Treg cell proliferation was explored by activating blood mononuclear cells with an anti-CD3 monoclonal antibody, interleukin-2 and different doses of TGF-β1. Reverse transcription-quantitative polymerase chain reaction assays were used to detect the mRNA expression of Foxp3. Carboxyfluorescein succinimidyl ester staining was used to analyze the proliferation dynamics of lymphocyte subsets. Results indicate that the proportion of CD4⁺ T cells in the blood of patients with NSCLC was significantly higher compared with normal peripheral blood (P<0.01). Foxp3 expression in NSCLC blood Treg cells was significantly decreased compared with normal peripheral blood (P<0.01). NSCLC blood mononuclear cells treated with TGF-β1 at 1, 5 and 25 ng/ml significantly induced Foxp3 expression in CD4⁺CD25⁺ Treg cells compared with the control group (P<0.05). The proportion of CD4⁺CD25⁺ Treg and CD8⁺ T cells were elevated in generation 6, 7, 8 after 6 days of TGF-β1 treatment compared with untreated cells. The proportion of CD4⁺CD25⁺ Treg and CD8⁺ T cells were elevated in generation 8, 9 and with TGF-β1 treatment after 8 days compared with untreated cells. These results indicate that CD4⁺CD25⁺ Treg cells proliferate at a greater rate compared with CD8⁺ T cells after 4, 6 or 8 days of treatment. The proportion of CD4⁺CD25^high Treg cells in NSCLC blood was significantly higher (P<0.05) compared with normal peripheral blood. The number of Foxp3⁺ T cells was significantly lower (P<0.05) compared with normal peripheral blood. The data presented in this study suggest that NSCLC blood CD4⁺CD25^high Treg cells are functionally immature and that TGF-β1 may promote maturation.

mTOR Complex 1 Signaling Regulates the Generation and Function of Central and Effector Foxp3+ Regulatory T Cells

The mechanistic/mammalian target of rapamycin (mTOR) has emerged as a critical integrator of signals from the immune microenvironment capable of regulating T cell activation, differentiation, and function. The precise role of mTOR in the control of regulatory T cell (Treg) differentiation and function is complex. Pharmacologic inhibition and genetic deletion of mTOR promotes the generation of Tregs even under conditions that would normally promote generation of effector T cells. Alternatively, mTOR activity has been observed to be increased in Tregs, and the genetic deletion of the mTOR complex 1 (mTORC1)-scaffold protein Raptor inhibits Treg function. In this study, by employing both pharmacologic inhibitors and genetically altered T cells, we seek to clarify the role of mTOR in Tregs. Our studies demonstrate that inhibition of mTOR during T cell activation promotes the generation of long-lived central Tregs with a memory-like phenotype in mice. Metabolically, these central memory Tregs possess enhanced spare respiratory capacity, similar to CD8⁺ memory cells. Alternatively, the generation of effector Tregs (eTregs) requires mTOR function. Indeed, genetic deletion of Rptor leads to the decreased expression of ICOS and PD-1 on the eTregs. Overall, our studies define a subset of mTORC1^hi eTregs and mTORC1^lo central Tregs.

Differential T-cell receptor signals for T helper cell programming

Upon encounter with their cognate antigen, naive CD4 T cells become activated and are induced to differentiate into several possible T helper (Th) cell subsets. This differentiation depends on a number of factors including antigen-presenting cells, cytokines and co-stimulatory molecules. The strength of the T-cell receptor (TCR) signal, related to the affinity of TCR for antigen and antigen dose, has emerged as a dominant factor in determining Th cell fate. Recent studies have revealed that TCR signals of high or low strength do not simply induce quantitatively different signals in the T cells, but rather qualitatively distinct pathways can be induced based on TCR signal strength. This review examines the recent literature in this area and highlights important new developments in our understanding of Th cell differentiation and TCR signal strength.

Egr2-independent, Klf1-mediated induction of PD-L1 in CD4+ T cells

Programmed death ligand 1 (PD-L1)-mediated induction of immune tolerance has been vigorously investigated in autoimmunity and anti-tumor immunity. However, details of the mechanism by which PD-L1 is induced in CD4+ T cells are unknown. Here, we revealed the potential function of Klf1 and Egr2-mediated induction of PD-L1 in CD4+ T cells. We focused on the molecules specifically expressed in CD4+CD25-LAG3+ regulatory T cells (LAG3+ Tregs) highly express of PD-L1 and transcription factor Egr2. Although ectopic expression of Egr2 induced PD-L1, a deficiency of Egr2 did not affect its expression, indicating the involvement of another PD-L1 induction mechanism. Comprehensive gene expression analysis of LAG3+ Tregs and in silico binding predictions revealed that Krüppel-like factor 1 (Klf1) is a candidate inducer of the PD-L1 gene (Cd274). Klf1 is a transcription factor that promotes β-globin synthesis in erythroid progenitors, and its role in immunological homeostasis is unknown. Ectopic expression of Klf1 induced PD-L1 in CD4+ T cells through activation of the PI3K-mTOR signaling pathway, independent of STATs signaling and Egr2 expression. Our findings indicate that Klf1 and Egr2 are modulators of PD-L1-mediated immune suppression in CD4+ T cells and might provide new insights into therapeutic targets for autoimmune diseases and malignancies.