T cell receptor signal strength in Treg and iNKT cell development demonstrated by a novel fluorescent reporter mouse

Adjusting to self in the thymus: CD4 versus CD8 lineage commitment and regulatory T cell development

During thymic development, thymocytes adjust their TCR response based on the strength of their reactivity to self-peptide MHC complexes. This tuning process allows thymocytes with a range of self-reactivities to survive positive selection and contribute to a diverse T cell pool. In this review, we will discuss recent advances in our understanding of how thymocytes tune their responsiveness during positive selection, and we present a "sequential selection" model to explain how MHC specificity influences lineage choice. We also discuss recent evidence for cell type diversity in the medulla and discuss how this heterogeneity may contribute to medullary niches for negative selection and regulatory T cell development.

An IGHG1 variant exhibits polarized prevalence and confers enhanced IgG1 antibody responses against life-threatening organisms

Evolutionary pressures sculpt population genetics, whereas immune adaptation fortifies humans against life-threatening organisms. How the evolution of selective genetic variation in adaptive immune receptors orchestrates the adaptation of human populations to contextual perturbations remains elusive. Here, we show that the G396R coding variant within the human immunoglobulin G1 (IgG1) heavy chain presents a concentrated prevalence in Southeast Asian populations. We uncovered a 190-kb genomic linkage disequilibrium block peaked in close proximity to this variant, suggestive of potential Darwinian selection. This variant confers heightened immune resilience against various pathogens and viper toxins in mice. Mechanistic studies involving severe acute respiratory syndrome coronavirus 2 infection and vaccinated individuals reveal that this variant enhances pathogen-specific IgG1+ memory B cell activation and antibody production. This G396R variant may have arisen on a Neanderthal haplotype background. These findings underscore the importance of an IGHG1 variant in reinforcing IgG1 antibody responses against life-threatening organisms, unraveling the intricate interplay between human evolution and immune adaptation.

DOCK2 Deficiency Causes Defects in Antiviral T-Cell Responses and Impaired Control of Herpes Simplex Virus Infection

The expanding number of rare immunodeficiency syndromes offers an opportunity to understand key genes that support immune defense against infectious diseases. However, analysis of these in patients is complicated by their treatments and comorbid infections, requiring the use of mouse models for detailed investigations. We developed a mouse model of DOCK2 immunodeficiency and herein demonstrate that these mice have delayed clearance of herpes simplex virus type 1 (HSV-1) infections. We also uncovered a critical, cell-intrinsic role of DOCK2 in the priming of antiviral CD8+ T cells and in particular their initial expansion, despite apparently normal early activation of these cells. When this defect was overcome by priming in vitro, DOCK2-deficient CD8+ T cells were surprisingly protective against HSV-1 disease, albeit not as effectively as wild-type cells. These results shed light on a cellular deficiency that is likely to impact antiviral immunity in DOCK2-deficient patients.

Non-classical monocytes scavenge the growth factor CSF1 from endothelial cells in the peripheral vascular tree to ensure survival and homeostasis

Unlike sessile macrophages that occupy specialized tissue niches, non-classical monocytes (NCMs)-circulating phagocytes that patrol and cleanse the luminal surface of the vascular tree-are characterized by constant movement. Here, we examined the nature of the NCM's nurturing niche. Expression of the growth factor CSF1 on endothelial cells was required for survival of NCMs in the bloodstream. Lack of endothelial-derived CSF1 did not affect blood CSF1 concentration, suggesting that NCMs rely on scavenging CSF1 present on endothelial cells. Deletion of the transmembrane chemokine and adhesion factor CX3CL1 on endothelial cells impaired NCM survival. Mechanistically, endothelial-derived CX3CL1 and integrin subunit alpha L (ITGAL) facilitated the uptake of CSF1 by NCMs. CSF1 was produced by all tissular endothelial cells, and deletion of Csf1 in all endothelial cells except bone marrow sinusoids impaired NCM survival, arguing for a model where the full vascular tree acts as a niche for NCMs and where survival and patrolling function are connected.

Lack of canonical thyroid hormone receptor α signaling changes regulatory T cell phenotype in female mice

The immune system has emerged as an important target of thyroid hormones (THs); however, the role of TH in T cells has so far remained elusive. In this study, we assessed the effect of TH receptor α (TRα) signaling on activation and function of T cells. Our findings show that lack of canonical TRα action not only increased the frequency of regulatory T cells (Treg) but propelled an activated and migratory Treg phenotype and nuclear factor κB (NF-κB) activation in Treg. Conversely, canonical TRα action reduced activation of the NF-κB pathway previously shown to play a pivotal role in Treg differentiation and function. Taken together, our findings demonstrate that TRα impacts T cell differentiation and phenotype. Given the well-known interaction of inflammation, immune responses, and TH axis in e.g., severe illness, altered TH-TRα signaling may have an important role in regulating T cell responses during disease.

Lymphotoxin limits Foxp3+ regulatory T cell development from Foxp3lo precursors via IL-4 signaling

Regulatory T cells (Treg) are critical players of immune tolerance that develop in the thymus via two distinct developmental pathways involving CD25+Foxp3- and CD25-Foxp3lo precursors. However, the mechanisms regulating the recently identified Foxp3lo precursor pathway remain unclear. Here, we find that the membrane-bound lymphotoxin α1β2 (LTα1β2) heterocomplex is upregulated during Treg development upon TCR/CD28 and IL-2 stimulation. We show that Lta expression limits the maturational development of Treg from Foxp3lo precursors by regulating their proliferation, survival, and metabolic profile. Transgenic reporter mice and transcriptomic analyses further reveal that medullary thymic epithelial cells (mTEC) constitute an unexpected source of IL-4. We demonstrate that LTα1β2-lymphotoxin β receptor-mediated interactions with mTEC limit Treg development by down-regulating IL-4 expression in mTEC. Collectively, our findings identify the lymphotoxin axis as the first inhibitory checkpoint of thymic Treg development that fine-tunes the Foxp3lo Treg precursor pathway by limiting IL-4 availability.

Lymphatic-localized Treg-mregDC crosstalk limits antigen trafficking and restrains anti-tumor immunity

The tumor microenvironment (TME) has a significant impact on tumor growth and immunotherapy efficacies. However, the precise cellular interactions and spatial organizations within the TME that drive these effects remain elusive. Using advanced multiplex imaging techniques, we have discovered that regulatory T cells (Tregs) accumulate around lymphatic vessels in the peripheral tumor stroma. This localized accumulation is facilitated by mature dendritic cells enriched in immunoregulatory molecules (mregDCs), which promote chemotaxis of Tregs, establishing a peri-lymphatic Treg-mregDC niche. Within this niche, mregDCs facilitate Treg activation, which in turn restrains the trafficking of tumor antigens to the draining mesenteric lymph nodes, thereby impeding the initiation of anti-tumor adaptive immune responses. Disrupting Treg recruitment to mregDCs inhibits tumor progression. Our study provides valuable insights into the organization of TME and how local crosstalk between lymphoid and myeloid cells suppresses anti-tumor immune responses.

Lrp10 suppresses IL7R limiting CD8 T cell homeostatic expansion and anti-tumor immunity

Signals emanating from the T-cell receptor (TCR), co-stimulatory receptors, and cytokine receptors each influence CD8 T-cell fate. Understanding how these signals respond to homeostatic and microenvironmental cues can reveal new ways to therapeutically direct T-cell function. Through forward genetic screening in mice, we discover that loss-of-function mutations in LDL receptor-related protein 10 (Lrp10) cause naive and central memory CD8 T cells to accumulate in peripheral lymphoid organs. Lrp10 encodes a conserved cell surface protein of unknown immunological function. T-cell activation induces Lrp10 expression, which post-translationally suppresses IL7 receptor (IL7R) levels. Accordingly, Lrp10 deletion enhances T-cell homeostatic expansion through IL7R signaling. Lrp10-deficient mice are also intrinsically resistant to syngeneic tumors. This phenotype depends on dense tumor infiltration of CD8 T cells, which display increased memory cell characteristics, reduced terminal exhaustion, and augmented responses to immune checkpoint inhibition. Here, we present Lrp10 as a new negative regulator of CD8 T-cell homeostasis and a host factor that controls tumor resistance with implications for immunotherapy.

Intestinal cDC1s provide cues required for CD4+ T cell-mediated resistance to Cryptosporidium

Cryptosporidium is an enteric pathogen and a prominent cause of diarrheal disease worldwide. Control of Cryptosporidium requires CD4+ T cells, but how protective CD4+ T cell responses are generated is poorly understood. Here, Cryptosporidium parasites that express MHCII-restricted model antigens were generated to understand the basis for CD4+ T cell priming and effector function. These studies revealed that parasite-specific CD4+ T cells are primed in the draining mesenteric lymph node but differentiate into Th1 cells in the gut to provide local parasite control. Although type 1 conventional dendritic cells (cDC1s) were dispensable for CD4+ T cell priming, they were required for CD4+ T cell gut homing and were a source of IL-12 at the site of infection that promoted local production of IFN-γ. Thus, cDC1s have distinct roles in shaping CD4+ T cell responses to an enteric infection: first, to promote gut homing from the mesLN, and second, to drive effector responses in the intestine.

NR4A1 transcriptionally regulates the differentiation of stem-like CD8+ T cells in the tumor microenvironment

CD8+ T cells are rendered exhausted in tumor and chronic infection. Among heterogeneous exhausted T cells, a subpopulation of progenitor-like (Tpex) cells have been found important for long-term tumor or pathogen control and are also the main responders in immunotherapy. Using an RFP reporter mouse for the orphan nuclear receptor NR4A1, originally characterized as critical in T cell dysfunction, we discover that the reporter is highly expressed in Tpex cells in tumor and chronic infection. Enforced expression of Nr4a1 promotes Tpex cell accumulation, whereas tumor control is improved after Nr4a1 deletion, associated with increased effector function but decreased long-term maintenance of CD8+ T cells. Integrating chromatin immunoprecipitation sequencing (ChIP-seq) and RNA sequencing (RNA-seq) analysis, NR4A1 is found to bind and promote the expression of Tpex-related genes, as well as suppress terminal differentiation-associated genes. This study therefore has identified a key role of NR4A1 in Tpex regulation and provides a promising target for immunotherapy.

Decoding changes in tumor-infiltrating leukocytes through dynamic experimental models and single-cell technologies

The ability to characterize immune cells and explore the molecular interactions that govern their functions has never been greater, fueled in recent years by the revolutionary advance of single-cell analysis platforms. However, precisely how immune cells respond to different stimuli and where differentiation processes and effector functions operate remain incompletely understood. Inferring cellular fate within single-cell transcriptomic analyses is now omnipresent, despite the assumptions typically required in such analyses. Recently developed experimental models support dynamic analyses of the immune response, providing insights into the temporal changes that occur within cells and the tissues in which such transitions occur. Here we will review these approaches and discuss how these can be combined with single-cell technologies to develop a deeper understanding of the immune responses that should support the development of better therapeutic options for patients.

Cellular architecture shapes the naïve T cell response

After antigen stimulation, naïve T cells display reproducible population-level responses, which arise from individual T cells pursuing specific differentiation trajectories. However, cell-intrinsic predeterminants controlling these single-cell decisions remain enigmatic. We found that the subcellular architectures of naïve CD8 T cells, defined by the presence (TØ) or absence (TO) of nuclear envelope invaginations, changed with maturation, activation, and differentiation. Upon T cell receptor (TCR) stimulation, naïve TØ cells displayed increased expression of the early-response gene Nr4a1, dependent upon heightened calcium entry. Subsequently, in vitro differentiation revealed that TØ cells generated effector-like cells more so compared with TO cells, which proliferated less and preferentially adopted a memory-precursor phenotype. These data suggest that cellular architecture may be a predeterminant of naïve CD8 T cell fate.

Lymphatic vessel transit seeds cytotoxic resident memory T cells in skin draining lymph nodes

Lymphatic transport shapes the homeostatic immune repertoire of lymph nodes (LNs). LN-resident memory T cells (TRMs) play an important role in site-specific immune memory, yet how LN TRMs form de novo after viral infection remains unclear. Here, we tracked the anatomical distribution of antiviral CD8+ T cells as they seeded skin and LN TRMs using a model of vaccinia virus-induced skin infection. LN TRMs localized to the draining LNs (dLNs) of infected skin, and their formation depended on the lymphatic egress of effector CD8+ T cells from the skin, already poised for residence. Effector CD8+ T cell transit through skin was required to populate LN TRMs in dLNs, a process reinforced by antigen encounter in skin. Furthermore, LN TRMs were protective against viral rechallenge in the absence of circulating memory T cells. These data suggest that a subset of tissue-infiltrating CD8+ T cells egress from tissues during viral clearance and establish a layer of regional protection in the dLN basin.

Dendritic cell-mediated responses to secreted Cryptosporidium effectors promote parasite-specific CD8+ T cell responses

Cryptosporidium causes debilitating diarrheal disease in patients with primary and acquired defects in T cell function. However, it has been a challenge to understand how this infection generates T cell responses and how they mediate parasite control. Here, Cryptosporidium was engineered to express a parasite effector protein (MEDLE-2) that contains the major histocompatibility complex-I restricted SIINFEKL epitope which is recognized by T cell receptor transgenic OT-I(OVA-TCR-I) clusters of differentiation (CD)8+ T cells. These modified parasites induced expansion of endogenous SIINFEKL-specific and OT-I CD8+ T cells that were a source of interferon-gamma (IFN-γ) that could restrict growth of Cryptosporidium. This T cell response was dependent on the translocation of the effector and similar results were observed with another secreted parasite effector (rhoptry protein 1). Although infection and these translocated effector proteins are restricted to intestinal epithelial cells, type 1 conventional dendritic cells were required to generate CD8+ T cell responses to these model antigens. These data sets highlight Cryptosporidium effectors as potential targets of the immune system and suggest that crosstalk between enterocytes and type 1 conventional dendritic cells is crucial for CD8+ T cell responses to Cryptosporidium.

CD8 is down(regulated) for tolerance

Activated CD8+ T cells directly kill target cells. Therefore, the regulation of their function is central to avoiding immunopathology. Mechanisms that curb effector functions in CD4+ and CD8+ T cells are mostly shared, yet important differences occur. Here, we focus on the control of CD8+ T cell activity and discuss the importance of a poorly understood aspect of tolerance that directly impairs engagement of target cells: the downregulation of CD8. We contextualize this process and propose that it represents a key element during CD8+ T cell modulation.

Antitumor progenitor exhausted CD8+ T cells are sustained by TCR engagement

The durability of an antitumor immune response is mediated in part by the persistence of progenitor exhausted CD8+ T cells (Tpex). Tpex serve as a resource for replenishing effector T cells and preserve their quantity through self-renewal. However, it is unknown how T cell receptor (TCR) engagement affects the self-renewal capacity of Tpex in settings of continued antigen exposure. Here we use a Lewis lung carcinoma model that elicits either optimal or attenuated TCR signaling in CD8+ T cells to show that formation of Tpex in tumor-draining lymph nodes and their intratumoral persistence is dependent on optimal TCR engagement. Notably, attenuated TCR stimulation accelerates the terminal differentiation of optimally primed Tpex. This TCR-reinforced Tpex development and self-renewal is coupled to proximal positioning to dendritic cells and epigenetic imprinting involving increased chromatin accessibility at Egr2 and Tcf1 target loci. Collectively, this study highlights the critical function of TCR engagement in sustaining Tpex during tumor progression.

Defining Human Regulatory T Cells beyond FOXP3: The Need to Combine Phenotype with Function

Regulatory T cells (Tregs) are essential to maintain immune homeostasis by promoting self-tolerance. Reduced Treg numbers or functionality can lead to a loss of tolerance, increasing the risk of developing autoimmune diseases. An overwhelming variety of human Tregs has been described, based on either specific phenotype, tissue compartment, or pathological condition, yet the bulk of the literature only addresses CD25-positive and CD127-negative cells, coined by naturally occurring Tregs (nTregs), most of which express the transcription factor Forkhead box protein 3 (FOXP3). While the discovery of FOXP3 was seminal to understanding the origin and biology of nTregs, there is evidence in humans that not all T cells expressing FOXP3 are regulatory, and that not all Tregs express FOXP3. Namely, the activation of human T cells induces the transient expression of FOXP3, irrespective of whether they are regulatory or inflammatory effectors, while some induced T cells that may be broadly defined as Tregs (e.g., Tr1 cells) typically lack demethylation and do not express FOXP3. Furthermore, it is unknown whether and how many nTregs exist without FOXP3 expression. Several other candidate regulatory molecules, such as GITR, Lag-3, GARP, GPA33, Helios, and Neuropilin, have been identified but subsequently discarded as Treg-specific markers. Multiparametric analyses have uncovered a plethora of Treg phenotypes, and neither single markers nor combinations thereof can define all and only Tregs. To date, only the functional capacity to inhibit immune responses defines a Treg and distinguishes Tregs from inflammatory T cells (Teffs) in humans. This review revisits current knowledge of the Treg universe with respect to their heterogeneity in phenotype and function. We propose that it is unavoidable to characterize human Tregs by their phenotype in combination with their function, since phenotype alone does not unambiguously define Tregs. There is an unmet need to align the expression of specific markers or combinations thereof with a particular suppressive function to coin functional Treg entities and categorize Treg diversity.

Intratumoral antigen signaling traps CD8+ T cells to confine exhaustion to the tumor site

Immunotherapy advances have been hindered by difficulties in tracking the behaviors of lymphocytes after antigen signaling. Here, we assessed the behavior of T cells active within tumors through the development of the antigen receptor signaling reporter (AgRSR) mouse, fate-mapping lymphocytes responding to antigens at specific times and locations. Contrary to reports describing the ready egress of T cells out of the tumor, we find that intratumoral antigen signaling traps CD8+ T cells in the tumor. These clonal populations expand and become increasingly exhausted over time. By contrast, antigen-signaled regulatory T cell (Treg) clonal populations readily recirculate out of the tumor. Consequently, intratumoral antigen signaling acts as a gatekeeper to compartmentalize CD8+ T cell responses, even within the same clonotype, thus enabling exhausted T cells to remain confined to a specific tumor tissue site.

Streptococcus agalactiae and Escherichia coli induce distinct effector γδ T cell responses during neonatal sepsis

Neonates born prematurely are vulnerable to life-threatening conditions such as bacterial sepsis. Streptococcus agalactiae (GBS) and Escherichia coli are frequent causative pathogens of neonatal sepsis, however, it remains unclear if these pathogens induce differential immune responses. We find that γδ T cells rapidly respond to single-organism GBS and E. coli bloodstream infections in neonatal mice. Furthermore, GBS and E. coli induce distinct cytokine production from IFN-γ and IL-17 producing γδ T cells, respectively. We also find that IL-17 production during E. coli infection is driven by γδTCR signaling, whereas IFN-γ production during GBS infection occurs independently of γδTCR signaling. The divergent effector responses of γδ T cells during GBS and E. coli infections impart distinctive neuroinflammatory phenotypes on the neonatal brain. Thus, the neonatal adaptive immune system differentially responds to distinct bacterial stimuli, resulting in unique neuroinflammatory phenotypes.

Long-lasting mRNA-encoded interleukin-2 restores CD8+ T cell neoantigen immunity in MHC class I-deficient cancers

Major histocompatibility complex (MHC) class I antigen presentation deficiency is a common cancer immune escape mechanism, but the mechanistic implications and potential strategies to address this challenge remain poorly understood. Studying β2-microglobulin (B2M) deficient mouse tumor models, we find that MHC class I loss leads to a substantial immune desertification of the tumor microenvironment (TME) and broad resistance to immune-, chemo-, and radiotherapy. We show that treatment with long-lasting mRNA-encoded interleukin-2 (IL-2) restores an immune cell infiltrated, IFNγ-promoted, highly proinflammatory TME signature, and when combined with a tumor-targeting monoclonal antibody (mAB), can overcome therapeutic resistance. Unexpectedly, the effectiveness of this treatment is driven by IFNγ-releasing CD8+ T cells that recognize neoantigens cross-presented by TME-resident activated macrophages. These macrophages acquire augmented antigen presentation proficiency and other M1-phenotype-associated features under IL-2 treatment. Our findings highlight the importance of restoring neoantigen-specific immune responses in the treatment of cancers with MHC class I deficiencies.

The intestinal microbiota modulates the transcriptional landscape of iNKT cells at steady-state and following antigen exposure

Invariant Natural Killer T (iNKT) cells are unconventional T cells that respond to microbe-derived glycolipid antigens. iNKT cells exert fast innate effector functions that regulate immune responses in a variety of contexts, including during infection, cancer, or inflammation. The roles these unconventional T cells play in intestinal inflammation remain poorly defined and vary based on the disease model and species. Our previous work suggested that the gut microbiota influenced iNKT cell functions during dextran sulfate sodium-induced colitis in mice. This study, shows that iNKT cell homeostasis and response following activation are altered in germ-free mice. Using prenatal fecal transplant in specific pathogen-free mice, we show that the transcriptional signatures of iNKT cells at steady state and following αGC-mediated activation in vivo are modulated by the microbiota. Our data suggest that iNKT cells sense the microbiota at homeostasis independently of their T cell receptors. Finally, iNKT cell transcriptional signatures are different in male and female mice. Collectively, our findings suggest that sex and the intestinal microbiota are important factors that regulate iNKT cell homeostasis and responses. A deeper understanding of microbiota-iNKT cell interactions and the impact of sex could improve the development of iNKT cell-based immunotherapies.

Immune microniches shape intestinal Treg function

The intestinal immune system is highly adapted to maintaining tolerance to the commensal microbiota and self-antigens while defending against invading pathogens1,2. Recognizing how the diverse network of local cells establish homeostasis and maintains it in the complex immune environment of the gut is critical to understanding how tolerance can be re-established following dysfunction, such as in inflammatory disorders. Although cell and molecular interactions that control T regulatory (Treg) cell development and function have been identified3,4, less is known about the cellular neighbourhoods and spatial compartmentalization that shapes microorganism-reactive Treg cell function. Here we used in vivo live imaging, photo-activation-guided single-cell RNA sequencing5-7 and spatial transcriptomics to follow the natural history of T cells that are reactive towards Helicobacter hepaticus through space and time in the settings of tolerance and inflammation. Although antigen stimulation can occur anywhere in the tissue, the lamina propria-but not embedded lymphoid aggregates-is the key microniche that supports effector Treg (eTreg) cell function. eTreg cells are stable once their niche is established; however, unleashing inflammation breaks down compartmentalization, leading to dominance of CD103+SIRPα+ dendritic cells in the lamina propria. We identify and validate the putative tolerogenic interaction between CD206+ macrophages and eTreg cells in the lamina propria and identify receptor-ligand pairs that are likely to govern the interaction. Our results reveal a spatial mechanism of tolerance in the lamina propria and demonstrate how knowledge of local interactions may contribute to the next generation of tolerance-inducing therapies.

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.

Deciphering the developmental trajectory of tissue-resident Foxp3+ regulatory T cells

Foxp3+ TREG cells have been at the focus of intense investigation for their recognized roles in preventing autoimmunity, facilitating tissue recuperation following injury, and orchestrating a tolerance to innocuous non-self-antigens. To perform these critical tasks, TREG cells undergo deep epigenetic, transcriptional, and post-transcriptional changes that allow them to adapt to conditions found in tissues both at steady-state and during inflammation. The path leading TREG cells to express these tissue-specialized phenotypes begins during thymic development, and is further driven by epigenetic and transcriptional modifications following TCR engagement and polarizing signals in the periphery. However, this process is highly regulated and requires TREG cells to adopt strategies to avoid losing their regulatory program altogether. Here, we review the origins of tissue-resident TREG cells, from their thymic and peripheral development to the transcriptional regulators involved in their tissue residency program. In addition, we discuss the distinct signalling pathways that engage the inflammatory adaptation of tissue-resident TREG cells, and how they relate to their ability to recognize tissue and pathogen-derived danger signals.

Tonic TCR and IL-1β signaling mediate phenotypic alterations of naive CD4+ T cells

Inert naive CD4+ T (TN) cells differentiate into functional T helper (Th) or regulatory T (Treg) cell subsets upon encountering antigens, mediating properly directed immune responses. Although all TN cells can differentiate into any of the Th and Treg cell subsets, heterogeneity exists among TN cells. By constructing reporter mice to detect ongoing T cell receptor (TCR) signaling, we identify that interleukin (IL)-1β signaling affects TN cell characteristics, independent of tonic TCR signaling, which also alters TN cell phenotypes. IL-1β reversibly attenuates the differentiation potential of TN cells toward Treg cells. IL-1β signaling is elevated in the splenic TN cells, consequently attenuating their differentiation potential toward Treg cells. Aberrant elevation of IL-1β signaling augments colitogenic activities of TN cells. TN cells in patients with colitis exhibited elevated IL-1β signaling. We demonstrate that phenotypic alteration in TN cells by IL-1β is an important mechanism in the regulation of immune responses.

Tipping the balance in autoimmunity: are regulatory t cells the cause, the cure, or both?

Regulatory T cells (Tregs) are a specialized subgroup of T-cell lymphocytes that is crucial for maintaining immune homeostasis and preventing excessive immune responses. Depending on their differentiation route, Tregs can be subdivided into thymically derived Tregs (tTregs) and peripherally induced Tregs (pTregs), which originate from conventional T cells after extrathymic differentiation at peripheral sites. Although the regulatory attributes of tTregs and pTregs partially overlap, their modes of action, protein expression profiles, and functional stability exhibit specific characteristics unique to each subset. Over the last few years, our knowledge of Treg differentiation, maturation, plasticity, and correlations between their phenotypes and functions has increased. Genetic and functional studies in patients with numeric and functional Treg deficiencies have contributed to our mechanistic understanding of immune dysregulation and autoimmune pathologies. This review provides an overview of our current knowledge of Treg biology, discusses monogenetic Treg pathologies and explores the role of Tregs in various other autoimmune disorders. Additionally, we discuss novel approaches that explore Tregs as targets or agents of innovative treatment options.

Streptococcus agalactiae and Escherichia coli Induce Distinct Effector γδ T Cell Responses During Neonatal Sepsis

Neonates born prematurely are highly vulnerable to life-threatening conditions such as bacterial sepsis. Streptococcus agalactiae, also known as group B Streptococcus (GBS) and Escherichia coli are frequent causative pathogens of neonatal sepsis, however, it remains unclear if distinct sepsis pathogens induce differential adaptive immune responses. In the present study, we find that γδ T cells in neonatal mice rapidly respond to single-organism GBS and E. coli bloodstream infections and that these pathogens induce distinct activation and cytokine production from IFN-γ and IL-17 producing γδ T cells, respectively. We also report differential reliance on γδTCR signaling to elicit effector cytokine responses during neonatal sepsis, with IL-17 production during E. coli infection being driven by γδTCR signaling, and IFN-γ production during GBS infection occurring independently of γδTCR signaling. Furthermore, we report that the divergent effector responses of γδ T cells during GBS and E. coli infections impart distinctive neuroinflammatory phenotypes on the neonatal brain. The present study reveals that the neonatal adaptive immune system differentially responds to distinct bacterial stimuli, resulting in unique neuroinflammatory phenotypes.

Kidins220 and Aiolos promote thymic iNKT cell development by reducing TCR signals

Development of T cells is controlled by the signal strength of the TCR. The scaffold protein kinase D-interacting substrate of 220 kilodalton (Kidins220) binds to the TCR; however, its role in T cell development was unknown. Here, we show that T cell-specific Kidins220 knockout (T-KO) mice have strongly reduced invariant natural killer T (iNKT) cell numbers and modest decreases in conventional T cells. Enhanced apoptosis due to increased TCR signaling in T-KO iNKT thymocytes of developmental stages 2 and 3 shows that Kidins220 down-regulates TCR signaling at these stages. scRNA-seq  indicated that the transcription factor Aiolos is down-regulated in Kidins220-deficient iNKT cells. Analysis of an Aiolos KO demonstrated that Aiolos is a downstream effector of Kidins220 during iNKT cell development. In the periphery, T-KO iNKT cells show reduced TCR signaling upon stimulation with α-galactosylceramide, suggesting that Kidins220 promotes TCR signaling in peripheral iNKT cells. Thus, Kidins220 reduces or promotes signaling dependent on the iNKT cell developmental stage.

Type I interferon induces TCR-dependent and -independent antimicrobial responses in γδ intraepithelial lymphocytes

Intraepithelial lymphocytes (IEL) expressing the γδ T cell receptor (TCR) survey the intestinal epithelium to limit the invasion of microbial pathogens. The production of type I interferon (IFN) is a central component of an antiviral immune response, yet how these pro-inflammatory cytokines contribute to γδ IEL effector function remains unclear. Based on the unique activation status of IELs, and their ability to bridge innate and adaptive immunity, we investigated the extent to which type I IFN signaling modulates γδ IEL function. Using an ex vivo culture model, we find that type I IFN alone is unable to drive IFNγ production, yet low level TCR activation synergizes with type I IFN to induce IFNγ production in murine γδ IELs. Further investigation into the underlying molecular mechanisms of co-stimulation revealed that TCRγδ-mediated activation of NFAT and JNK is required for type I IFN to promote IFNγ expression in a STAT4- dependent manner. Whereas type I IFN rapidly upregulates antiviral gene expression independent of a basal TCRγδ signal, neither tonic TCR triggering nor the presence of a TCR agonist was sufficient to elicit type I IFN-induced IFNγ production in vivo . However, bypassing proximal TCR signaling events synergized with IFNAR/STAT4 activation to induce γδ IEL IFNγ production. These findings indicate that γδ IELs contribute to host defense in response to type I IFN by mounting a rapid antimicrobial response independent of TCRγδ signaling, and under permissive conditions, produce IFNγ in a TCR-dependent manner.

PD-1 Limits IL-2 Production and Thymic Regulatory T Cell Development

Inhibitory proteins, such as programmed cell death protein 1 (PD-1), have been studied extensively in peripheral T cell responses to foreign Ags, self-Ags, and neoantigens. Notably, these proteins are first expressed during T cell development in the thymus. Reports suggest that PD-1 limits regulatory T cell (Treg) development, but the mechanism by which PD-1 exerts this function remains unknown. The present study expands the evaluation of murine PD-1 and its ligands in the thymus, demonstrating that some of the highest expressers of PD-1 and programmed death-ligand 1 are agonist selected cells. Surprisingly, we reveal a selective role for PD-1 in regulating the developmental niche only for Tregs because other agonist selected cell populations, such as NK T cells, remain unchanged. We also ruled out PD-1 as a regulator of proliferation or cell death of agonist selected Tregs and further demonstrated that PD-1-deficient Tregs have reduced TCR signaling. Unexpectedly, the data suggest that PD-1-deficient thymocytes produce elevated levels of IL-2, a Treg niche-limiting cytokine. Collectively, these data suggest a novel role for PD-1 in regulating IL-2 production and the concurrent agonist selection of murine thymic Tregs. This observation has implications for the use of checkpoint blockade in the context of cancer and infection.

Assembling the thymus medulla: Development and function of epithelial cell heterogeneity

The thymus is a unique primary lymphoid organ that supports the production of self-tolerant T-cells essential for adaptive immunity. Intrathymic microenvironments are microanatomically compartmentalised, forming defined cortical, and medullary regions each differentially supporting critical aspects of thymus-dependent T-cell maturation. Importantly, the specific functional properties of thymic cortical and medullary compartments are defined by highly specialised thymic epithelial cells (TEC). For example, in the medulla heterogenous medullary TEC (mTEC) contribute to the enforcement of central tolerance by supporting deletion of autoreactive T-cell clones, thereby counterbalancing the potential for random T-cell receptor generation to contribute to autoimmune disease. Recent advances have further shed light on the pathways and mechanisms that control heterogeneous mTEC development and how differential mTEC functionality contributes to control self-tolerant T-cell development. Here we discuss recent findings in relation to mTEC development and highlight examples of how mTEC diversity contribute to thymus medulla function.

IL-27 regulates the differentiation of follicular helper NKT cells via metabolic adaptation of mitochondria

Invariant natural killer T (iNKT) cells are innate-like T lymphocytes that express an invariant T cell receptor α chain and contribute to bridging innate and acquired immunity with rapid production of large amounts of cytokines after stimulation. Among effecter subsets of iNKT cells, follicular helper NKT (NKTFH) cells are specialized to help B cells. However, the mechanisms of NKTFH cell differentiation remain to be elucidated. In this report, we studied the mechanism of NKTFH cell differentiation induced by pneumococcal surface protein A and α-galactosylceramide (P/A) vaccination. We found that Gr-1+ cells helped iNKT cell proliferation and NKTFH cell differentiation in the spleen by producing interleukin-27 (IL-27) in the early phase after vaccination. The neutralization of IL-27 impaired NKTFH cell differentiation, which resulted in compromised antibody production and diminished protection against Streptococcus pneumoniae infection by the P/A vaccine. Our data indicated that Gr-1+ cell-derived IL-27 stimulated mitochondrial metabolism, meeting the energic demand required for iNKT cells to differentiate into NKTFH cells. Interestingly, Gr-1+ cell-derived IL-27 was induced by iNKT cells via interferon-γ production. Collectively, our findings suggest that optimizing the metabolism of iNKT cells was essential for acquiring specific effector functions, and they provide beneficial knowledge on iNKT cell-mediated vaccination-mediated therapeutic strategies.

Patients with ankylosing spondylitis present a distinct CD8 T cell subset with osteogenic and cytotoxic potential

OBJECTIVES

Ankylosing spondylitis (AS) is a chronic inflammatory rheumatic disease affecting mainly the axial skeleton. Peripheral involvement (arthritis, enthesitis and dactylitis) and extra-musculoskeletal manifestations, including uveitis, psoriasis and bowel inflammation, occur in a relevant proportion of patients. AS is responsible for chronic and severe back pain caused by local inflammation that can lead to osteoproliferation and ultimately spinal fusion. The association of AS with the human leucocyte antigen-B27 gene, together with elevated levels of chemokines, CCL17 and CCL22, in the sera of patients with AS, led us to study the role of CCR4+ T cells in the disease pathogenesis.

METHODS

CD8+CCR4+ T cells isolated from the blood of patients with AS (n=76) or healthy donors were analysed by multiparameter flow cytometry, and gene expression was evaluated by RNA sequencing. Patients with AS were stratified according to the therapeutic regimen and current disease score.

RESULTS

CD8+CCR4+ T cells display a distinct effector phenotype and upregulate the inflammatory chemokine receptors CCR1, CCR5, CX3CR1 and L-selectin CD62L, indicating an altered migration ability. CD8+CCR4+ T cells expressing CX3CR1 present an enhanced cytotoxic profile, expressing both perforin and granzyme B. RNA-sequencing pathway analysis revealed that CD8+CCR4+ T cells from patients with active disease significantly upregulate genes promoting osteogenesis, a core process in AS pathogenesis.

CONCLUSIONS

Our results shed light on a new molecular mechanism by which T cells may selectively migrate to inflammatory loci, promote new bone formation and contribute to the pathological ossification process observed in AS.

Unraveling T-cell dynamics using fluorescent timer: Insights from the Tocky system

Understanding the temporal dynamics of T-cell transcription is crucial for insights into immune cell function and development. In this study, we show the features of the Timer-of-Cell-Kinetics-and-Activity (Tocky) system, which enables analysis of temporal dynamics of cell activities and differentiation, leveraging Fluorescent Timer protein, which spontaneously changes its emission spectrum from blue to red fluorescence in known kinetics, as reporters. The current study examines the properties of the Tocky system, highlighting the Timer-Angle approach, which is a core algorithm of Tocky analysis and converts Timer Blue and Red fluorescence into Timer Angle and Intensity by trigonometric transformation. Importantly, Tocky analyzes time-related events within individual cells by the two phases of measurements, distinguishing between (1) the temporal sequence of cellular activities and differentiation within the time domain, and (2) the transcription frequency within the frequency domain. The transition from time measurement to frequency analysis, particularly at the Persistent locus that bridges these domains, highlights that system's unique property in what is measured and analyzed by Tocky. Intriguingly, the sustained transcriptional activities observed in cells at the Persistent locus may have unique biological features as demonstrated in activated regulatory T-cells (Treg) and pathogenic T-cells, respectively, using Foxp3-Tocky and Nr4a3-Tocky models. In conclusion, the Tocky system can provide crucial data for advancing our understanding of T-cell dynamics and function.

Early generation of a precursor CD8 T cell that can adapt to acute or chronic viral infection

Virus specific PD-1+ TCF-1+ TOX+ stem-like CD8+ T cells are essential for maintaining T cell responses during chronic infection and are also critical for PD-1 directed immunotherapy. In this study we have used the mouse model of chronic LCMV infection to examine when these virus specific stem-like CD8+ T cells are generated during the course of chronic infection and what is the role of antigen in maintaining the stem-like program. We found that these stem-like CD8+ T cells are generated early (day 5) during chronic infection and that antigen is essential for maintaining their stem-like program. This early generation of stem-like CD8+ T cells suggested that the fate commitment to this cell population was agnostic to the eventual outcome of infection and the immune system prepares a priori for a potential chronic infection. Indeed, we found that an identical virus specific stem-cell like CD8+ T cell population was also generated during acute LCMV infection but these cells were lost once the virus was cleared. To determine the fate of these early PD-1+TCF-1+TOX+ stem-like CD8+ T cells that are generated during both acute and chronic LCMV infection we set up two reciprocal adoptive transfer experiments. In the first experiment we transferred day 5 stem-like CD8+ T cells from chronically infected into acutely infected mice and examined their differentiation after viral clearance. We found that these early stem-like CD8+ T cells downregulated canonical markers of the chronic stem-like CD8+ T cells and expressed markers (CD127 and CD62L) associated with central memory CD8+ T cells. In the second experiment, we transferred day 5 stem-like cells from acutely infected mice into chronically infected mice and found that these CD8+ T cells could function like resource cells after transfer into a chronic environment by generating effector CD8+ T cells in both lymphoid and non-lymphoid tissues while also maintaining the number of stem-like CD8+ T cells. These findings provide insight into the generation and maintenance of virus specific stem-like CD8+ T cells that play a critical role in chronic viral infection. In particular, our study highlights the early generation of stem-like CD8+ T cells and their ability to adapt to either an acute or chronic infection. These findings are of broad significance since these novel stem-like CD8+ T cells play an important role in not only viral infections but also in cancer and autoimmunity.

The type 1 diabetes susceptibility locus Idd5 favours robust neonatal development of highly autoreactive regulatory T cells in the NOD mouse

Regulatory T lymphocytes expressing the transcription factor Foxp3 (Tregs) play an important role in the prevention of autoimmune diseases and other immunopathologies. Aberrations in Treg-mediated immunosuppression are therefore thought to be involved in the development of autoimmune pathologies, but few have been documented. Recent reports indicated a central role for Tregs developing during the neonatal period in the prevention of autoimmune pathology. We therefore investigated the development of Tregs in neonatal NOD mice, an important animal model for autoimmune type 1 diabetes. Surprisingly, we found that, as compared with seven other commonly studied inbred mouse strains, in neonatal NOD mice, exceptionally large proportions of developing Tregs express high levels of GITR and PD-1. The latter phenotype was previously associated with high Treg autoreactivity in C57BL/6 mice, which we here confirm for NOD animals. The proportions of newly developing GITRhighPD-1+ Tregs rapidly drop during the first week of age. A genome-wide genetic screen indicated the involvement of several diabetes susceptibility loci in this trait. Analysis of a congenic mouse strain confirmed that Idd5 contributes to the genetic control of GITRhighPD-1+ Treg development in neonates. Our data thus demonstrate an intriguing and paradoxical correlation between an idiosyncrasy in Treg development in NOD mice and their susceptibility to type 1 diabetes.

Cbl-b mitigates the responsiveness of naive CD8+ T cells that experience extensive tonic T cell receptor signaling

Naive T cells experience tonic T cell receptor (TCR) signaling in response to self-antigens presented by major histocompatibility complex (MHC) in secondary lymphoid organs. We investigated how relatively weak or strong tonic TCR signals influence naive CD8+ T cell responses to stimulation with foreign antigens. The heterogeneous expression of Nur77-GFP, a transgenic reporter of tonic TCR signaling, in naive CD8+ T cells suggests variable intensities or durations of tonic TCR signaling. Although the expression of genes associated with acutely stimulated T cells was increased in Nur77-GFPHI cells, these cells were hyporesponsive to agonist TCR stimulation compared with Nur77-GFPLO cells. This hyporesponsiveness manifested as diminished activation marker expression and decreased secretion of IFN-γ and IL-2. The protein abundance of the ubiquitin ligase Cbl-b, a negative regulator of TCR signaling, was greater in Nur77-GFPHI cells than in Nur77-GFPLO cells, and Cbl-b deficiency partially restored the responsiveness of Nur77-GFPHI cells. Our data suggest that the cumulative effects of previously experienced tonic TCR signaling recalibrate naive CD8+ T cell responsiveness. These changes include gene expression changes and negative regulation partially dependent on Cbl-b. This cell-intrinsic negative feedback loop may enable the immune system to restrain naive CD8+ T cells with higher self-reactivity.

Implications of stress-induced gene expression for hematopoietic stem cell aging studies

A decline in hematopoietic stem cell (HSC) function is believed to underlie hematological shortcomings with age; however, a comprehensive molecular understanding of these changes is currently lacking. Here we provide evidence that a transcriptional signature reported in several previous studies on HSC aging is linked to stress-induced changes in gene expression rather than aging. Our findings have strong implications for the design and interpretation of HSC aging studies.

Dynamic Multiplex Tissue Imaging in Inflammation Research

Inflammation is a highly dynamic process with immune cells that continuously interact with each other and parenchymal components as they migrate through tissue. The dynamic cellular responses and interaction patterns are a function of the complex tissue environment that cannot be fully reconstructed ex vivo, making it necessary to assess cell dynamics and changing spatial patterning in vivo. These dynamics often play out deep within tissues, requiring the optical focus to be placed far below the surface of an opaque organ. With the emergence of commercially available two-photon excitation lasers that can be combined with existing imaging systems, new avenues for imaging deep tissues over long periods of time have become available. We discuss a selected subset of studies illustrating how two-photon microscopy (2PM) has helped to relate the dynamics of immune cells to their in situ function and to understand the molecular patterns that govern their behavior in vivo. We also review some key practical aspects of 2PM methods and point out issues that can confound the results, so that readers can better evaluate the reliability of conclusions drawn using this technology.

Establishment of CD8+ T Cell Thymic Central Tolerance to Tissue-Restricted Antigen Requires PD-1

Highly self-reactive T cells are censored from the repertoire by both central and peripheral tolerance mechanisms upon receipt of high-affinity TCR signals. Clonal deletion is considered a major driver of central tolerance; however, other mechanisms such as induction of regulatory T cells and functional impairment have been described. An understanding of the interplay between these different central tolerance mechanisms is still lacking. We previously showed that impaired clonal deletion to a model tissue-restricted Ag did not compromise tolerance. In this study, we determined that murine T cells that failed clonal deletion were rendered functionally impaired in the thymus. Programmed cell death protein 1 (PD-1) was induced in the thymus and was required to establish cell-intrinsic tolerance to tissue-restricted Ag in CD8+ thymocytes independently of clonal deletion. In bone marrow chimeras, tolerance was not observed in PD-L1-deficient recipients, but tolerance was largely maintained following adoptive transfer of tolerant thymocytes or T cells to PD-L1-deficient recipients. However, CRISPR-mediated ablation of PD-1 in tolerant T cells resulted in broken tolerance, suggesting different PD-1 signaling requirements for establishing versus maintaining tolerance. Finally, we showed that chronic exposure to high-affinity Ag supported the long-term maintenance of tolerance. Taken together, our study identifies a critical role for PD-1 in establishing central tolerance in autoreactive T cells that escape clonal deletion. It also sheds light on potential mechanisms of action of anti-PD-1 pathway immune checkpoint blockade and the development of immune-related adverse events.

H2-O deficiency promotes regulatory T cell differentiation and CD4 T cell hyperactivity

Regulatory T cells (Treg) are crucial immune modulators, yet the exact mechanism of thymic Treg development remains controversial. Here, we present the first direct evidence for H2-O, an MHC class II peptide editing molecular chaperon, on selection of thymic Tregs. We identified that lack of H2-O in the thymic medulla promotes thymic Treg development and leads to an increased peripheral Treg frequency. Single-cell RNA-sequencing (scRNA-seq) analysis of splenic CD4 T cells revealed not only an enrichment of effector-like Tregs, but also activated CD4 T cells in the absence of H2-O. Our data support two concepts; a) lack of H2-O expression in the thymic medulla creates an environment permissive to Treg development and, b) that loss of H2-O drives increased basal auto-stimulation of CD4 T cells. These findings can help in better understanding of predispositions to autoimmunity and design of therapeutics for treatment of autoimmune diseases.

γδ T cells respond directly and selectively to the skin commensal yeast Malassezia for IL-17-dependent fungal control

Stable microbial colonization of the skin depends on tight control by the host immune system. The lipid-dependent yeast Malassezia typically colonizes skin as a harmless commensal and is subject to host type 17 immunosurveillance, but this fungus has also been associated with diverse skin pathologies in both humans and animals. Using a murine model of Malassezia exposure, we show that Vγ4+ dermal γδ T cells expand rapidly and are the major source of IL-17A mediating fungal control in colonized skin. A pool of memory-like Malassezia-responsive Vγ4+ T cells persisted in the skin, were enriched in draining lymph nodes even after fungal clearance, and were protective upon fungal re-exposure up to several weeks later. Induction of γδT17 immunity depended on IL-23 and IL-1 family cytokine signalling, whereas Toll-like and C-type lectin receptors were dispensable. Furthermore, Vγ4+ T cells from Malassezia-exposed hosts were able to respond directly and selectively to Malassezia-derived ligands, independently of antigen-presenting host cells. The fungal moieties detected were shared across diverse species of the Malassezia genus, but not conserved in other Basidiomycota or Ascomycota. These data provide novel mechanistic insight into the induction and maintenance of type 17 immunosurveillance of skin commensal colonization that has significant implications for cutaneous health.

Regulatory T Cells for Control of Autoimmunity

Regulatory T (Treg) cells, which specifically express the master transcription factor FoxP3, are indispensable for the maintenance of immunological self-tolerance and homeostasis. Their functional or numerical anomalies can be causative of autoimmune and other inflammatory diseases. Recent advances in the research of the cellular and molecular basis of how Treg cells develop, exert suppression, and maintain their function have enabled devising various ways for controlling physiological and pathological immune responses by targeting Treg cells. It is now envisaged that Treg cells as a "living drug" are able to achieve antigen-specific immune suppression of various immune responses and reestablish immunological self-tolerance in the clinic.

Topical corticosteroids inhibit allergic skin inflammation but are ineffective in impeding the formation and expansion of resident memory T cells

BACKGROUND

Tissue-resident memory T (TRM ) cells are detrimental in allergic contact dermatitis (ACD), in which they contribute to the chronicity and severity of the disease.

METHODS

We assessed the impact of a standard topical corticosteroid (TCS) treatment, triamcinolone acetonide (TA), on the formation, maintenance and reactivation of epidermal TRM cells in a preclinical model of ACD to 2,4-dinitrofluorobenzene. TA 0.01% was applied at different time points of ACD response and we monitored skin inflammation and tracked CD8+ CD69+ CD103+ TRM by flow cytometry and RNA sequencing.

RESULTS

The impact of TA on TRM formation depended on treatment regimen: (i) in a preventive mode, that is, in sensitized mice before challenge, TA transiently inhibited the infiltration of effector T cells and the accumulation of TRM upon hapten challenge. In contrast, (ii) in a curative mode, that is, at the peak of the ACD response, TA blocked skin inflammation but failed to prevent the formation of TRM . Finally, (iii) in a proactive mode, that is, on previous eczema lesions, TA had no effect on the survival of skin TRM , but transiently inhibited their reactivation program upon allergen reexposure. Indeed, specific TRM progressively regained proliferative functions upon TA discontinuation and expanded in the tissue, leading to exaggerated iterative responses. Interestingly, TRM re-expansion correlated with the decreased clearance of hapten moieties from the skin induced by repeated TA applications.

CONCLUSIONS

Our results demonstrate that TCS successfully treat ACD inflammation, but are mostly ineffective in impeding the formation and expansion of allergen-specific TRM , which certainly restricts the induction of lasting tolerance in patients with chronic dermatitis.

Machine learning analysis of the T cell receptor repertoire identifies sequence features of self-reactivity

The T cell receptor (TCR) determines specificity and affinity for both foreign and self-peptides presented by the major histocompatibility complex (MHC). Although the strength of TCR interactions with self-pMHC impacts T cell function, it has been challenging to identify TCR sequence features that predict T cell fate. To discern patterns distinguishing TCRs from naive CD4+ T cells with low versus high self-reactivity, we used data from 42 mice to train a machine learning (ML) algorithm that identifies population-level differences between TCRβ sequence sets. This approach revealed that weakly self-reactive T cell populations were enriched for longer CDR3β regions and acidic amino acids. We tested our ML predictions of self-reactivity using retrogenic mice with fixed TCRβ sequences. Extrapolating our analyses to independent datasets, we predicted high self-reactivity for regulatory T cells and slightly reduced self-reactivity for T cells responding to chronic infections. Our analyses suggest a potential trade-off between TCR repertoire diversity and self-reactivity. A record of this paper's transparent peer review process is included in the supplemental information.

Recently activated CD4 T cells in tuberculosis express OX40 as a target for host-directed immunotherapy

After Mycobacterium tuberculosis (Mtb) infection, many effector T cells traffic to the lungs, but few become activated. Here we use an antigen receptor reporter mouse (Nur77-GFP) to identify recently activated CD4 T cells in the lungs. These Nur77-GFPHI cells contain expanded TCR clonotypes, have elevated expression of co-stimulatory genes such as Tnfrsf4/OX40, and are functionally more protective than Nur77-GFPLO cells. By contrast, Nur77-GFPLO cells express markers of terminal exhaustion and cytotoxicity, and the trafficking receptor S1pr5, associated with vascular localization. A short course of immunotherapy targeting OX40+ cells transiently expands CD4 T cell numbers and shifts their phenotype towards parenchymal protective cells. Moreover, OX40 agonist immunotherapy decreases the lung bacterial burden and extends host survival, offering an additive benefit to antibiotics. CD4 T cells from the cerebrospinal fluid of humans with HIV-associated tuberculous meningitis commonly express surface OX40 protein, while CD8 T cells do not. Our data thus propose OX40 as a marker of recently activated CD4 T cells at the infection site and a potential target for immunotherapy in tuberculosis.

Selective ablation of thymic and peripheral Foxp3+ regulatory T cell development

Foxp3+ regulatory T (Treg) cells of thymic (tTreg) and peripheral (pTreg) developmental origin are thought to synergistically act to ensure immune homeostasis, with self-reactive tTreg cells primarily constraining autoimmune responses. Here we exploited a Foxp3-dependent reporter with thymus-specific GFP/Cre activity to selectively ablate either tTreg (ΔtTreg) or pTreg (ΔpTreg) cell development, while sparing the respective sister populations. We found that, in contrast to the tTreg cell behavior in ΔpTreg mice, pTreg cells acquired a highly activated suppressor phenotype and replenished the Treg cell pool of ΔtTreg mice on a non-autoimmune C57BL/6 background. Despite the absence of tTreg cells, pTreg cells prevented early mortality and fatal autoimmunity commonly observed in Foxp3-deficient models of complete Treg cell deficiency, and largely maintained immune tolerance even as the ΔtTreg mice aged. However, only two generations of backcrossing to the autoimmune-prone non-obese diabetic (NOD) background were sufficient to cause severe disease lethality associated with different, partially overlapping patterns of organ-specific autoimmunity. This included a particularly severe form of autoimmune diabetes characterized by an early onset and abrogation of the sex bias usually observed in the NOD mouse model of human type 1 diabetes. Genetic association studies further allowed us to define a small set of autoimmune risk loci sufficient to promote β cell autoimmunity, including genes known to impinge on Treg cell biology. Overall, these studies show an unexpectedly high functional adaptability of pTreg cells, emphasizing their important role as mediators of bystander effects to ensure self-tolerance.

Impaired T cell IRE1α/XBP1 signaling directs inflammation in experimental heart failure with preserved ejection fraction

Heart failure with preserved ejection fraction (HFpEF) is a widespread syndrome with limited therapeutic options and poorly understood immune pathophysiology. Using a 2-hit preclinical model of cardiometabolic HFpEF that induces obesity and hypertension, we found that cardiac T cell infiltration and lymphoid expansion occurred concomitantly with cardiac pathology and that diastolic dysfunction, cardiomyocyte hypertrophy, and cardiac phospholamban phosphorylation were T cell dependent. Heart-infiltrating T cells were not restricted to cardiac antigens and were uniquely characterized by impaired activation of the inositol-requiring enzyme 1α/X-box-binding protein 1 (IRE1α/XBP1) arm of the unfolded protein response. Notably, selective ablation of XBP1 in T cells enhanced their persistence in the heart and lymphoid organs of mice with preclinical HFpEF. Furthermore, T cell IRE1α/XBP1 activation was restored after withdrawal of the 2 comorbidities inducing HFpEF, resulting in partial improvement of cardiac pathology. Our results demonstrated that diastolic dysfunction and cardiomyocyte hypertrophy in preclinical HFpEF were T cell dependent and that reversible dysregulation of the T cell IRE1α/XBP1 axis was a T cell signature of HFpEF.

Lrp10 suppresses IL7R limiting CD8 T cell homeostatic expansion and anti-tumor immunity

Signals emanating from the T cell receptor (TCR), co-stimulatory receptors, and cytokine receptors each influence CD8 T cell fate. Understanding how these signals respond to homeostatic and microenvironmental cues can reveal new ways to therapeutically direct T cell function. Through forward genetic screening in mice, we discovered that loss-of-function mutations in LDL receptor related protein 10 ( Lrp10 ) caused naïve and central memory CD8 T cells to accumulate in peripheral lymphoid organs. Lrp10 encodes a conserved cell surface protein of unknown immunological function. Lrp10 was induced with T cell activation and its expression post-translationally suppressed IL7 receptor (IL7R) levels. Accordingly, Lrp10 deletion enhanced T cell homeostatic expansion through IL7R signaling. Lrp10 -deficient mice were also intrinsically resistant to syngeneic tumors. This phenotype depended on dense tumor infiltration of CD8 T cells that displayed increased memory cell characteristics, reduced terminal exhaustion, and augmented responses to immune checkpoint inhibition. Here, we present Lrp10 as a new negative regulator of CD8 T cell homeostasis and a host factor that controls tumor resistance with implications for immunotherapy.

Prior viral infection primes cross-reactive CD8+ T cells that respond to mouse heart allografts

Introduction

Significant evidence suggests a connection between transplant rejection and the presence of high levels of pre-existing memory T cells. Viral infection can elicit viral-specific memory T cells that cross-react with allo-MHC capable of driving allograft rejection in mice. Despite these advances, and despite their critical role in transplant rejection, a systematic study of allo-reactive memory T cells, their specificities, and the role of cross-reactivity with viral antigens has not been performed.

Methods

Here, we established a model to identify, isolate, and characterize cross-reactive T cells using Nur77 reporter mice (C57BL/6 background), which transiently express GFP exclusively upon TCR engagement. We infected Nur77 mice with lymphocytic choriomeningitis virus (LCMV-Armstrong) to generate a robust memory compartment, where quiescent LCMV-specific memory CD8+ T cells could be readily tracked with MHC tetramer staining. Then, we transplanted LCMV immune mice with allogeneic hearts and monitored expression of GFP within MHC-tetramer defined viral-specific T cells as an indicator of their ability to cross-react with alloantigens.

Results

Strikingly, prior LCMV infection significantly increased the kinetics and magnitude of rejection as well as CD8+ T cell recruitment into allogeneic, but not syngeneic, transplanted hearts, relative to non-infected controls. Interestingly, as early as day 1 after allogeneic heart transplant an average of ~8% of MHC-tetramer+ CD8+ T cells expressed GFP, in contrast to syngeneic heart transplants, where the frequency of viral-specific CD8+ T cells that were GFP+ was <1%. These data show that a significant percentage of viral-specific memory CD8+ T cells expressed T cell receptors that also recognized alloantigens in vivo. Notably, the frequency of cross-reactive CD8+ T cells differed depending upon the viral epitope. Further, TCR sequences derived from cross-reactive T cells harbored distinctive motifs that may provide insight into cross-reactivity and allo-specificity.

Discussion

In sum, we have established a mouse model to track viral-specific, allo-specific, and cross-reactive T cells; revealing that prior infection elicits substantial numbers of viral-specific T cells that cross-react to alloantigen, respond very early after transplant, and may promote rapid rejection.