Defective TCR expression in transgenic mice constructed using cDNA-based alpha- and beta-chain genes under the control of heterologous regulatory elements

A CARMIL2 gain-of-function mutation suffices to trigger most CD28 costimulatory functions in vivo

Naive T cell activation requires both TCR and CD28 signals. The CARMIL2 cytosolic protein enables CD28-dependent activation of the NF-κB transcription factor via its ability to link CD28 to the CARD11 adaptor protein. Here, we developed mice expressing a mutation named Carmil2QE and mimicking a mutation found in human T cell malignancies. Naive T cells from Carmil2QE mice contained preformed CARMIL2QE-CARD11 complexes in numbers comparable to those assembling in wild-type T cells after CD28 engagement. Such ready-made CARMIL2QE-CARD11 complexes also formed in CD28-deficient mice where they unexpectedly induced most of the functions that normally result from CD28 engagement in a manner that remains antigen-dependent. In turn, tumor-specific T cells expressing Carmil2QE do not require CD28 engagement and thereby escape to both PD-1 and CTLA-4 inhibition. In conclusion, we uncovered the overarching role played by CARMIL2-CARD11 signals among those triggered by CD28 and exploited them to induce potent solid tumor-specific T cell responses in the absence of CD28 ligands and immune checkpoint inhibitors.

Cholinergic regulation of thymocyte negative selection

The immune and nervous systems use a common chemical language for communication, namely, the cholinergic signaling involving acetylcholine (ACh) and its receptors (AChRs). Whether and how this language also regulates the development of the immune system is poorly understood. Here, we show that mouse CD4+CD8+ double-positive thymocytes express high levels of α9 nicotinic AChR (nAChR) and that this receptor controls thymic negative selection. α9 nAChR-deficient mice show an altered T cell receptor (TCR) repertoire and reduced CD4+ and CD8+ T cells in a mixed bone marrow chimera setting. α9 nAChR-mediated signaling regulates TCR strength and thymocyte survival. Thymic tuft cells, B cells and some T cells express choline acetyltransferase and are potential ACh sources, with ACh derived from T cells having the most important role. Furthermore, α9 nAChR deficiency during thymocyte development contributes to the altered development of autoimmune diseases in mice. Our results thus reveal a mechanism controlling immune cell development that involves cholinergic signaling.

A wave of Thetis cells imparts tolerance to food antigens early in life

Within the intestine, peripherally-induced regulatory T (pTreg) cells play an essential role in suppressing inflammatory responses to food proteins. However, the identity of antigen-presenting cells (APC) that instruct food-specific pTreg cells is poorly understood. Here, we found that a subset of Thetis cells, TC IV, is required for food-specific pTreg cell differentiation. TC IV were almost exclusively present within mesenteric lymph nodes suggesting that the presence of TC IV underlies the phenomenon of oral tolerance. A wave of TC IV differentiation in the peri-weaning period was associated with a window of opportunity for enhanced pTreg generation in response to food antigens. Our findings indicate that TC IV may represent a therapeutic target for the treatment of food-associated allergic and inflammatory diseases.

The inhibitory receptor Siglec-E controls antigen-presenting cell activation and T cell-mediated transplant rejection

After transplantation, inflammation and tissue injury release danger signals that activate myeloid cells, driving adaptive immune responses and acute rejection. Current immunosuppressants primarily target T cells but inadequately control innate immunity. Regulatory signals controlling innate responses in transplantation remain elusive. The sialic acid-binding immunoglobulin-like lectin-E (Siglec-E, or SigE) binds sialylated ligands to suppress inflammation. In mouse heart transplants, SigE is up-regulated in graft-infiltrating myeloid cells, including dendritic cells (DCs). SigE deficiency in recipients, but not donors, accelerates acute rejection by enhancing DC activation, nuclear factor κB (NF-κB) signaling, and tumor necrosis factor-α (TNF-α) production, thereby boosting alloreactive T cell responses. Conversely, SigE overexpression on DCs reduces activation by danger signals and their T cell allostimulatory capacity. The human homologs Siglecs-7 and -9 were up-regulated in rejecting allograft biopsies, and their higher expression correlated with improved allograft survival. Thus, SigE/7/9 is a crucial inhibitory receptor controlling antigen-presenting cell activation and T cell-mediated transplant rejection, offering therapeutic potential.

Syndecans and glycosaminoglycans influence B-cell development and activation

Syndecans (SDCs) are glycosaminoglycan-containing cell surface proteins with diverse functions in the immune system with SDC1 (CD138) and SDC4 expressed in B-lineage cells. Here, we show that stem cells lacking either molecule generate fewer B-cell progenitors but give rise to mature B cells in vivo. Deletion of the plasma cell "marker" CD138 has no effect on homeostatic or antigen-induced plasma cell formation. Naive B cells express high SDC4 and encounter with cognate antigen results in transient CD138 upregulation and SDC4 loss, both further modulated by IL-4, IL-21, and CD40 ligation. SDC4 is downregulated on germinal center B cells and absent on most memory B cells. Glycosaminoglycans such as those attached to SDCs, and heparin, a commonly used therapeutic, regulate survival and activation of naive B cells by limiting responsiveness to cognate antigen. Conversely, ablation of SDC4 results in increased baseline and antigen-induced B-cell activation. Collectively, our data reveal B-cell activation- and subset-dependent SDC expression and show that SDC4 and GAGs can limit antigen-induced activation to promote B-cell survival and expansion.

Macrophages direct location-dependent recall of B cell memory to vaccination

Vaccines generate long-lived plasma cells and memory B cells (Bmems) that may re-enter secondary germinal centers (GCs) to further mutate their B cell receptor upon boosting and re-exposure to antigen. We show in mouse models that lymph nodes draining the site of primary vaccination harbor a subset of Bmems that reside in the subcapsular niche, generate larger recall responses, and are more likely to re-enter GCs compared with circulating Bmems in non-draining lymph nodes. This location-dependent recall of Bmems into the GC in the draining lymph node was dependent on CD169+ subcapsular sinus macrophages (SSMs) in the subcapsular niche. In human participants, boosting of the BNT162b2 vaccine in the same arm generated more rapid secretion of broadly neutralizing antibodies, GC participation, and clonal expansion of SARS-CoV-2-specific B cells than boosting of the opposite arm. These data reveal an unappreciated role for primed draining lymph node SSMs in Bmem cell fate determination.

Transient Inflammation of Pancreatic Exocrine Tissue in Autoimmune Diabetes Follows Onset of Islet Damage and Utilizes Heparanase-1

Inflammation of the exocrine pancreas accompanies autoimmune diabetes in mouse models and humans. However, the relationship between inflammation in the exocrine and endocrine (islet) compartments has not been explored. To address this issue, we used a transgenic mouse model in which autoimmune diabetes is acutely induced after the transfer of islet beta cell-specific transgenic T cells. Histological analyses demonstrated that inflammation of the exocrine pancreas, which was initially mild, resulted in the transient but widespread disruption of acinar tissue. Islet inflammation preceded exacerbated exocrine pathology, progressed to T cell-induced islet damage/destruction and persisted when exocrine inflammation subsided. Heparanase-1 (HPSE-1), an endoglycosidase that degrades heparan sulfate in basement membranes (BMs), when preferentially expressed in recipient cells but not donor (HPSE-1-deficient (HPSE-KO)) T cells, played a critical role in both exocrine and islet inflammation. In this context, HPSE-1 facilitates the passage of autoimmune T cells across the sub-endothelial basement membrane (BM) of pancreatic blood vessels and initially into the exocrine tissue. Peak exocrine inflammation that preceded or accompanied the acute onset of diabetes and HPSE-1 potentially contributed to acinar damage. In contrast to inflammation, HPSE-1 expressed by donor T cells played a key role in the induction of diabetes by allowing autoimmune T cells to traverse peri-islet BMs in order to destroy insulin-producing beta cells. Overall, our findings suggest that major exocrine pancreas injury is not required for the initiation of autoimmune islet damage and is not essential at the time of diabetes onset.

T-B cell cooperation in ectopic lymphoid follicles propagates CNS autoimmunity

Meningeal ectopic lymphoid follicle (eLF)-like structures have been described in multiple sclerosis, but their role in central nervous system (CNS) autoimmunity is unclear. Here, we used a T helper 17 (TH17) adoptive transfer experimental autoimmune encephalomyelitis model featuring formation of eLFs. Single-cell RNA sequencing revealed that clusters of activated B cells and B1/marginal zone-like B cells were overrepresented in the CNS and identified B cells poised for undergoing germinal center reactions and clonal expansion in the CNS. Using intravital imaging to directly visualize TH17-B cell interactions, we demonstrated that T and B cells form long-lasting antigen-specific contacts in meningeal eLFs that result in reactivation of autoreactive T cells. CNS T cells depended on CNS B cells to maintain a proinflammatory cytokine profile. Our study reveals that extensive T-B cell cooperation occurs in meningeal eLFs, promoting both B cell differentiation and T cell reactivation, and may thereby propagate smoldering inflammation in the CNS.

PRDM16-dependent antigen-presenting cells induce tolerance to gut antigens

The gastrointestinal tract is continuously exposed to foreign antigens in food and commensal microorganisms with potential to induce adaptive immune responses. Peripherally induced T regulatory (pTreg) cells are essential for mitigating inflammatory responses to these agents1-4. Although RORγt+ antigen-presenting cells (APCs) have been shown to programme gut microbiota-specific pTreg cells5-7, their definition remains incomplete, and the APC responsible for food tolerance has remained unknown. Here we identify an APC subset that is required for differentiation of both food- and microbiota-specific pTreg cells and for establishment of oral tolerance. Development and function of these APCs require expression of the transcription factors PRDM16 and RORγt, as well as a unique Rorc(t) cis-regulatory element. Gene expression, chromatin accessibility, and surface marker analysis establish the pTreg-inducing APCs as myeloid in origin, distinct from type 3 innate lymphoid cells, and sharing epigenetic profiles with classical dendritic cells, and designate them PRDM16+RORγt+ tolerizing dendritic cells (tolDCs). Upon genetic perturbation of tolDCs, we observe a substantial increase in food antigen-specific T helper 2 cells in lieu of pTreg cells, leading to compromised tolerance in mouse models of asthma and food allergy. Single-cell analyses of freshly resected mesenteric lymph nodes from a human organ donor, as well as multiple specimens of human intestine and tonsil, reveal candidate tolDCs with co-expression of PRDM16 and RORC and an extensive transcriptome shared with tolDCs from mice, highlighting an evolutionarily conserved role across species. Our findings suggest that a better understanding of how tolDCs develop and how they regulate T cell responses to food and microbial antigens could offer new insights into developing therapeutic strategies for autoimmune and allergic diseases as well as organ transplant tolerance.

Regulatory T cells converted from Th1 cells in tumors suppress cancer immunity via CD39

Regulatory T (Treg) cells are known to impede antitumor immunity, yet the regulatory mechanisms and functional roles of these cells remain poorly understood. In this study, through the characterization of multiple cancer models, we identified a substantial presence of peripherally induced Treg cells in the tumor microenvironment (TME). Depletion of these cells triggered antitumor responses and provided potent therapeutic effects by increasing functional CD8+ T cells. Fate-mapping and transfer experiments revealed that IFN-γ-expressing T helper (Th) 1 cells differentiated into Treg cells in response to TGF-β signaling in tumors. Pseudotime trajectory analysis further revealed the terminal differentiation of Th1-like Treg cells from Th1 cells in the TME. Tumor-resident Treg cells highly expressed T-bet, which was essential for their functions in the TME. Additionally, CD39 was highly expressed by T-bet+ Treg cells in both mouse and human tumors, and was necessary for Treg cell-mediated suppression of CD8+ T cell responses. Our study elucidated the developmental pathway of intratumoral Treg cells and highlighted novel strategies for targeting them in cancer patients.

The Immune-Modulatory Function of Megakaryocytes in the Hematopoietic Niche of Myeloproliferative Neoplasms

Myeloproliferative neoplasms (MPNs) are clonal stem cell disorders characterized by dysregulated megakaryopoiesis and neoplastic hematopoietic stem cell (HSC) expansion. Using a murine model with MK-specific JAK2V617F expression, we establish an MPN aging model where mutant MKs drive HSC expansion and a progressive decline in wild-type HSC function. Compared to wild-type MKs, JAK2V617F MKs exhibit heightened inflammation and innate immune activation with aging, including increased antigen presentation, elevated pro-inflammatory cytokines, skewed T cell populations, and impaired T cell functions in the marrow niche. Enhanced MK immunomodulatory function is linked to mutant cell expansion and MPN progression in a chimeric murine model with co-existing wild-type and JAK2V617F mutant HSCs. LINE-1 (long-interspersed element-1), a retrotransposon linked to innate immune activation and aging, is upregulated in mutant MKs during aging in murine models. We validated that LINE-1-encoded protein ORF1p is expressed in marrow MKs in 12 of 13 MPN patients but absent in control samples from patients undergoing orthopedic surgery (n=5). These findings suggest that MKs reprogram the marrow immune microenvironment, impairing normal HSC function while promoting neoplastic expansion in MPNs. LINE-1 activation in mutant MKs may be a key driver of immune dysregulation in MPNs.

Key Points

JAK2V617F mutant MKs reprogram the marrow immune microenvironment to promote neoplastic HSC expansion in MPNs.LINE-1 activation in diseased MKs triggers chronic inflammation and immune dysfunction in MPNs.

IL-6 Signalling to Responding T Cells Is Key to Calcitonin Gene-Related Peptide-Exposed Endothelial Cell Enhancement of Th17 Immunity During Langerhans Cell Antigen Presentation

Calcitonin gene-related peptide (CGRP) biases Langerhans cell (LC) Ag presentation to CD4+ T cells towards Th17-type immunity through actions on endothelial cells (ECs). We now report further evidence that IL-6 signalling at responding T cells mediates this effect. This CGRP effect was absent with ECs from IL-6 KO mice. Exposure of LCs, but not T cells, to IL-6 enhanced IL-6 and IL-17A production and reduced IFN-γ in the T-cell response. Pretreatment of LCs with IL-6 receptor α-chain (IL-6Rα) antibodies prior to IL-6 exposure significantly inhibited these responses. However, T-cell pretreatment with an IL-6/IL-6Rα chimera mimicked the effect of IL-6 pretreatment of LCs on T-cell responses. When this experiment was performed in the presence of the ADAM17 and ADAM10 inhibitor TAPI-1 during LC pretreatment of LCs and during the Ag presentation culture, release of soluble IL-6Rα chains into the medium was very significantly reduced, but this did not affect levels of T-cell cytokine release. Interestingly, LC exposure to IL-6 significantly increased LC IL-6 expression. Furthermore, pretreatment of T cells with antibodies against the IL-6 receptor β-chain significantly inhibited the IL-6 effect. CGRP may stimulate ECs in lymphatics and/or lymph nodes to produce IL-6 which likely results in migrating LCs nonclassically presenting IL-6. Furthermore, we found that IL-6 induces IL-6 production by LCs, suggesting an autocrine amplification pathway for this effect.

Macrophage-derived Fgl2 dampens antitumor immunity through regulation of FcγRIIB+CD8+ T cells in melanoma

Cancer immunotherapy has emerged as a promising therapeutic modality but heterogeneity in patient responsiveness remains. Thus, greater understanding of the immunologic factors that dictate response to immunotherapy is critical to improve patient outcomes. Here, we show that fibrinogen-like protein 2 (Fgl2) is elevated in the setting of melanoma in humans and mice and plays a functional role in inhibiting the CD8+ T cell response. Surprisingly, the tumor itself is not the major cellular source of Fgl2. Instead, we found that macrophage-secreted Fgl2 dampens the CD8+ T cell response through binding and apoptosis of FcγRIIB+CD8+ T cells. This regulation was CD8+ T cell autonomous and not via an antigen-presenting cell intermediary, as absence of Fcgr2b from the CD8+ T cells rendered T cells insensitive to Fgl2 regulation. Fgl2 is robustly expressed by macrophages in 10 cancer types in humans and in 6 syngeneic tumor models in mice, underscoring the clinical relevance of Fgl2 as a therapeutic target to promote T cell activity and improve patient immunotherapeutic response.

Regulatory T cells constrain T cells of shared specificity to enforce tolerance during infection

During infections, CD4+ Foxp3+ regulatory T (Treg) cells must control autoreactive CD4+ conventional T (Tconv) cell responses against self-peptide antigens while permitting those against pathogen-derived "nonself" peptides. We defined the basis of this selectivity using mice in which Treg cells reactive to a single prostate-specific self-peptide were selectively depleted. We found that self-peptide-specific Treg cells were dispensable for the control of Tconv cells of matched specificity at homeostasis. However, they were required to control such Tconv cells and prevent autoimmunity toward the prostate after exposure to elevated self-peptide during infection. Notably, the Treg cell response to self-peptide did not affect protective Tconv cell responses to a pathogen-derived peptide. Thus, self-peptide-specific Treg cells promoted self-nonself discrimination during infection by selectively controlling Tconv cells of shared self-specificity.

Antigen-presenting cancer associated fibroblasts enhance antitumor immunity and predict immunotherapy response

Cancer-associated fibroblasts (CAF) play a crucial role in tumor progression and immune regulation. However, the functional heterogeneity of CAFs remains unclear. Here, we identify antigen-presenting CAFs (apCAF), characterized by high MHC II expression, in gastric cancer (GC) tumors and find that apCAFs are preferentially located near tertiary lymphoid structures. Both in vivo and in vitro experiments demonstrate that apCAFs promote T cell activation and enhances its cytotoxic and proliferative capacities, thereby strengthening T cell-mediated anti-tumor immunity. Additionally, apCAFs facilitate the polarization of macrophages toward a pro-inflammatory phenotype. These polarized macrophages, in turn, promote the formation of apCAFs, creating a positive feedback loop that amplifies anti-tumor immune responses. Notably, baseline tumors in immunotherapy responders across various cancer types exhibit higher levels of apCAFs infiltration. This study advances the understanding of CAFs heterogeneity in GC and highlights apCAFs as a potential biomarker for predicting immunotherapy response in pan-cancer.

Prdm16-dependent antigen-presenting cells induce tolerance to intestinal antigens

The gastrointestinal tract is continuously exposed to foreign antigens in food and commensal microbes with potential to induce adaptive immune responses. Peripherally induced T regulatory (pTreg) cells are essential for mitigating inflammatory responses to these agents1-4. While RORγt+ antigen-presenting cells (RORγt-APCs) were shown to program gut microbiota-specific pTreg5-7, their definition remains incomplete, and the APC responsible for food tolerance has remained elusive. Here, we identify a distinct subset of RORγt-APCs, designated tolerogenic dendritic cells (tDC), required for differentiation of both food- and microbiota-specific pTreg cells and for establishment of oral tolerance. tDC development and function require expression of the transcription factors Prdm16 and RORγt, as well as a unique Rorc(t) cis-regulatory element. Gene expression, chromatin accessibility, and surface marker analysis establish tDC as myeloid in origin, distinct from ILC3, and sharing epigenetic profiles with classical DC. Upon genetic perturbation of tDC, we observe a substantial increase in food antigen-specific T helper 2 (Th2) cells in lieu of pTreg, leading to compromised tolerance in mouse models of asthma and food allergy. Single-cell analyses of freshly resected mesenteric lymph nodes from a human organ donor, as well as multiple specimens of human intestine and tonsil, reveal candidate tDC with co-expression of PRDM16 and RORC and an extensive transcriptome shared with mice, highlighting an evolutionarily conserved role across species. Our findings suggest that a better understanding of how tDC develop and how they regulate T cell responses to food and microbial antigens could offer new insights into developing therapeutic strategies for autoimmune and allergic diseases as well as organ transplant tolerance.

CD301b+ dendritic cell-derived IL-2 dictates CD4+ T helper cell differentiation

T helper (Th) cell differentiation is fundamental to functional adaptive immunity. Different subsets of dendritic cells (DC) preferentially induce different types of Th cells, but the DC-derived mechanism for Th type 2 (Th2) differentiation is not fully understood. Here, we show that in mice, CD301b+ DCs, a major Th2-inducing DC subset, drive Th2 differentiation through cognate interaction by rapidly inducing IL-2 receptor signalling in CD4+ T cells. Mechanistically, CD40 engagement prompts IL-2 production selectively from CD301b+ DCs to maximize CD25 expression in CD4+ T cells, which instructs the Th2 fate decision, while simultaneously skewing CD4+ T cells away from the T follicular helper fate. Moreover, CD301b+ DCs utilize their own CD25 to facilitate directed action of IL-2 toward cognate CD4+ T cells, as genetic deletion of CD25 in CD301b+ DCs results in reduced IL-2-mediated signalling in antigen-specific CD4+ T cells and hence their Th2 differentiation. These results highlight the critical role of DC-intrinsic CD40-IL-2 axis in Th cell fate decision.

WNK1-dependent water influx is required for CD4+ T cell activation and T cell-dependent antibody responses

Signaling from the T cell antigen receptor (TCR) on CD4+ T cells plays a critical role in adaptive immune responses by inducing T cell activation, proliferation, and differentiation. Here we demonstrate that WNK1, a kinase implicated in osmoregulation in the kidney, is required in T cells to support T-dependent antibody responses. We show that the canonical WNK1-OXSR1-STK39 kinase signaling pathway is required for TCR signaling in CD4+ T cells, their subsequent entry into the cell cycle, and suppression of the ATR-mediated G2/M cell cycle checkpoint. We show that the WNK1 pathway regulates ion influx leading to water influx, potentially through AQP3, and that water influx is required for TCR-induced signaling and cell cycle entry. Thus, TCR signaling via WNK1, OXSR1, STK39 and AQP3 leads to water entry that is essential for CD4+ T cell proliferation and hence T cell-dependent antibody responses.

Tumor-derived lactic acid promotes acetylation of histone H3K27 and differentiation of IL-10-producing regulatory B cells through direct and indirect signaling pathways

Tumor cells are known to enhance glycolysis, even under normoxic conditions, via the Warburg effect, producing excess lactic acid in the tumor microenvironment. Lactic acid enhances the IL-23/IL-17 pathway and induces chronic inflammation. The acidic microenvironment formed by lactic acid suppresses immune cell proliferation and activation. In the present study, we clarified that lactic acid had two novel activities for immune cells. First, lactic acid specifically enhanced acetylation at lysine 27 of histone H3 (H3K27ac) in splenic B cells and monocytes/macrophages, and this epigenetically up-regulates the expression of genes. Acetylation and methylation of other residues of histone H3 were rarely induced. Second, lactic acid induced a particularly-marked enhancement of Il10 gene expression in B cells, leading to an increase in IL-10-producing regulatory B (Breg) cells. Furthermore, two pathways should be involved in both the enhancement of H3K27ac and the induction of Breg cells by lactic acid: a direct pathway that enhances the CD40 signal in B cells, and an indirect pathway that affects B cells by activating the exchange protein directly activated by cAMP (EPAC) 1/2 in non-B cells. In tumor-bearing mice, the levels of H3K27ac of tumor-infiltrating B cells were significantly higher than splenic B cells and were suppressed by intraperitoneal injection of the EPAC1/2 inhibitor. In conclusion, tumor-derived lactic acid increases H3K27ac and IL-10-producing Breg cells, causing the suppression of anti-tumor immunity.

A gut commensal protozoan determines respiratory disease outcomes by shaping pulmonary immunity

The underlying mechanisms used by the intestinal microbiota to shape disease outcomes of the host are poorly understood. Here, we show that the gut commensal protozoan, Tritrichomonas musculis (T.mu), remotely shapes the lung immune landscape to facilitate perivascular shielding of the airways by eosinophils. Lung-specific eosinophilia requires a tripartite immune network between gut-derived inflammatory group 2 innate lymphoid cells and lung-resident T cells and B cells. This network exacerbates the severity of allergic airway inflammation while hindering the systemic dissemination of pulmonary Mycobacterium tuberculosis. The identification of protozoan DNA sequences in the sputum of patients with severe allergic asthma further emphasizes the relevance of commensal protozoa in human disease. Collectively, these findings demonstrate that a commensal protozoan tunes pulmonary immunity via a gut-operated lung immune network, promoting both beneficial and detrimental disease outcomes in response to environmental airway allergens and pulmonary infections.

Advances in bacterial artificial chromosome (BAC) transgenic mice for gene analysis and disease research

Transgenic mice, including those created using Bacterial Artificial Chromosomes (BACs), are artificial manipulations that have become critical tools for studying gene function. While conventional transgenic techniques face challenges in achieving precise expression of foreign genes in specific cells and tissues, BAC transgenic mice offer a solution by incorporating large DNA segments that can include entire expression units with tissue-specific enhancers. This review provides a thorough examination of BAC transgenic mouse technology, encompassing both traditional and humanized models. We explore the benefits and drawbacks of BAC transgenesis compared to other techniques such as knock-in and CRISPR/Cas9 technologies. The review emphasizes the applications of BAC transgenic mice in various disciplines, including neuroscience, immunology, drug metabolism, and disease modeling. Additionally, we address crucial aspects of generating and analyzing BAC transgenic mice, such as position effects, copy number variations, and strategies to mitigate these challenges. Despite certain limitations, humanized BAC transgenic mice have proven to be invaluable tools for studying the pathogenesis of human diseases, drug development, and understanding intricate gene regulatory mechanisms. This review discusses current topics on BAC transgenic mice and their evolving significance in biomedical research.

Nr4a1 and Nr4a3 redundantly control clonal deletion and contribute to an anergy-like transcriptome in auto-reactive thymocytes to impose tolerance in mice

The Nr4a nuclear hormone receptors are transcriptionally upregulated in response to antigen recognition by the T cell receptor (TCR) in the thymus and are implicated in clonal deletion, but the mechanisms by which they operate are not clear. Moreover, their role in central tolerance is obscured by redundancy among the Nr4a family members and by their reported functions in Treg generation and maintenance. Here we take advantage of competitive bone marrow chimeras and the OT-II/RIPmOVA model to show that Nr4a1 and Nr4a3 are essential for the upregulation of Bcl2l11/BIM and thymic clonal deletion by self-antigen. Importantly, thymocytes lacking Nr4a1/3 acquire an anergy-like signature after escaping clonal deletion and Treg lineage diversion. We further show that the Nr4a family helps mediate a broad transcriptional program in self-reactive thymocytes that resembles anergy and may operate at the margins of canonical thymic tolerance mechanisms to restrain self-reactive T cells after thymic egress.

TCR transgenic clone selection guided by immune receptor analysis and single-cell RNA expression of polyclonal responders

Since the precursor frequency of naive T cells is extremely low, investigating the early steps of antigen-specific T cell activation is challenging. To overcome this detection problem, adoptive transfer of a cohort of T cells purified from T cell receptor (TCR) transgenic donors has been extensively used but is not readily available for emerging pathogens. Constructing TCR transgenic mice from T cell hybridomas is a labor-intensive and sometimes erratic process, since the best clones are selected based on antigen-induced CD69 upregulation or IL-2 production in vitro, and TCR chains are polymerase chain reaction (PCR)-cloned into expression vectors. Here, we exploited the rapid advances in single-cell sequencing and TCR repertoire analysis to select the best clones without hybridoma selection, and generated CORSET8 mice (CORona Spike Epitope specific CD8 T cell), carrying a TCR specific for the Spike protein of SARS-CoV-2. Implementing newly created DALI software for TCR repertoire analysis in single-cell analysis enabled the rapid selection of the ideal responder CD8 T cell clone, based on antigen reactivity, proliferation, and immunophenotype in vivo. Identified TCR sequences were inserted as synthetic DNA into an expression vector and transgenic CORSET8 donor mice were created. After immunization with Spike/CpG-motifs, mRNA vaccination or SARS-CoV-2 infection, CORSET8 T cells strongly proliferated and showed signs of T cell activation. Thus, a combination of TCR repertoire analysis and scRNA immunophenotyping allowed rapid selection of antigen-specific TCR sequences that can be used to generate TCR transgenic mice.

Protocol to study ex vivo T cell priming by conventional dendritic cells from the mouse spleen

Conventional dendritic cells (cDC) are professional antigen-presenting cells able to prime naive T cells. Here, we present a protocol for ex vivo T cell priming by murine splenic cDC. We describe the steps of injecting fluorescently labeled antigens to mice, purifying antigen-bearing cDC, and priming antigen-specific T cells ex vivo. This protocol is suitable for studying the T cell priming function of cDC in various murine models and helps factor in the effect of the microenvironment on cDC ability to uptake and process antigens. For complete details on the use and execution of this protocol, please refer to Ashayeripanah et al.1.

Airway-resident memory CD4 T cell activation accelerates antigen presentation and T cell priming in draining lymph nodes

Specialized memory CD4 T cells that reside long-term within tissues are critical components of immunity at portals of pathogen entry. In the lung, such tissue-resident memory (Trm) cells are activated rapidly after infection and promote local inflammation to control pathogen levels before circulating T cells can respond. However, optimal clearance of Influenza A virus can require Trm and responses by other virus-specific T cells that reach the lung only several days after their activation in secondary lymphoid organs. Whether local CD4 Trm sentinel activity can affect the efficiency of T cell activation in secondary lymphoid organs is not clear. Here, we found that recognition of antigen by influenza-primed Trm in the airways promoted more rapid migration of highly activated antigen-bearing DC to the draining lymph nodes. This in turn accelerated the priming of naive T cells recognizing the same antigen, resulting in newly activated effector T cells reaching the lungs earlier than in mice not harboring Trm. Our findings, thus, reveal a circuit linking local and regional immunity whereby antigen recognition by Trm improves effector T cell recruitment to the site of infection though enhancing the efficiency of antigen presentation in the draining lymph node.

RANKL treatment restores thymic function and improves T cell-mediated immune responses in aged mice

Age-related thymic involution, leading to reduced T cell production, is one of the major causes of immunosenescence. This results in an increased susceptibility to cancers, infections, and autoimmunity and in reduced vaccine efficacy. Here, we identified that the receptor activator of nuclear factor κB (RANK)-RANK ligand (RANKL) axis in the thymus is altered during aging. Using a conditional transgenic mouse model, we demonstrated that endothelial cells depend on RANK signaling for their cellularity and functional maturation. Decreased RANKL availability during aging resulted in a decline in cellularity and function of both endothelial cells and thymic epithelial cells, contributing to thymic involution. We then found that, whereas RANKL neutralization in young mice mimicked thymic involution, exogenous RANKL treatment in aged mice restored thymic architecture as well as endothelial cell and epithelial cell abundance and functional properties. Consequently, RANKL improved T cell progenitor homing to the thymus and boosted T cell production. This cascade of events resulted in peripheral T cell renewal and effective antitumor and vaccine responses in aged mice. Furthermore, we conducted a proof-of-concept study that showed that RANKL stimulates endothelial cells and epithelial cells in human thymic organocultures. Overall, our findings suggest that targeting the RANK-RANKL axis through exogenous RANKL administration could represent a therapeutic strategy to rejuvenate thymic function and improve T cell immunity during aging.

Aiolos promotes CXCR3 expression on Th1 cells via positive regulation of IFN-γ/STAT1 signaling

CD4+ T helper 1 (Th1) cells coordinate adaptive immune responses to intracellular pathogens, including viruses. Key to this function is the ability of Th1 cells to migrate within secondary lymphoid tissues, as well as to sites of inflammation, which relies on signals received through the chemokine receptor CXCR3. CXCR3 expression is driven by the Th1 lineage-defining transcription factor T-bet and the cytokine-responsive STAT family members STAT1 and STAT4. Here, we identify the Ikaros zinc finger (IkZF) transcription factor Aiolos (Ikzf3) as an additional positive regulator of CXCR3 both in vitro and in vivo using a murine model of influenza virus infection. Mechanistically, we found that Aiolos-deficient CD4+ T cells exhibited decreased expression of key components of the IFN-γ/STAT1 signaling pathway, including JAK2 and STAT1. Consequently, Aiolos deficiency resulted in decreased levels of STAT1 tyrosine phosphorylation and reduced STAT1 enrichment at the Cxcr3 promoter. We further found that Aiolos and STAT1 formed a positive feedback loop via reciprocal regulation of each other downstream of IFN-γ signaling. Collectively, our study demonstrates that Aiolos promotes CXCR3 expression on Th1 cells by propagating the IFN-γ/STAT1 cytokine signaling pathway.

Synergistic blockade of TIGIT and PD-L1 increases type-1 inflammation and improves parasite control during murine blood-stage Plasmodium yoelii non-lethal infection

Pro-inflammatory immune responses are rapidly suppressed during blood-stage malaria but the molecular mechanisms driving this regulation are still incompletely understood. In this study, we show that the co-inhibitory receptors TIGIT and PD-1 are upregulated and co-expressed by antigen-specific CD4+ T cells (ovalbumin-specific OT-II cells) during non-lethal Plasmodium yoelii expressing ovalbumin (PyNL-OVA) blood-stage infection. Synergistic blockade of TIGIT and PD-L1, but not individual blockade of each receptor, during the early stages of infection significantly improved parasite control during the peak stages (days 10-15) of infection. Mechanistically, this protection was correlated with significantly increased plasma levels of IFN-γ, TNF, and IL-2, and an increase in the frequencies of IFN-γ-producing antigen-specific T-bet+ CD4+ T cells (OT-II cells), but not antigen-specific CD8+ T cells (OT-I cells), along with expansion of the splenic red pulp and monocyte-derived macrophage populations. Collectively, our study identifies a novel role for TIGIT in combination with the PD1-PD-L1 axis in regulating specific components of the pro-inflammatory immune response and restricting parasite control during the acute stages of blood-stage PyNL infection.

A bispecific CD40 agonistic antibody allowing for antibody-peptide conjugate formation to enable cancer-specific peptide delivery, resulting in improved T proliferation and anti-tumor immunity in mice

Current antibody-based immunotherapy depends on tumor antigen shedding for proper T cell priming. Here we select a novel human CD40 agonistic drug candidate and generate a bispecific antibody, herein named BiA9*2_HF, that allows for rapid antibody-peptide conjugate formation. The format is designed to facilitate peptide antigen delivery to CD40 expressing cells combined with simultaneous CD40 agonistic activity. In vivo, the selected bispecific antibody BiA9*2_HF loaded with peptide cargos induces improved antigen-specific proliferation of CD8+ (10-15 fold) and CD4+ T cells (2-7 fold) over control in draining lymph nodes. In both virus-induced and neoantigen-based mouse tumor models, BiA9*2_HF demonstrates therapeutic efficacy and elevated safety profile, with complete tumor clearance, as well as measured abscopal impact on tumor growth. The BiA9*2_HF drug candidate can thus be utilized to tailor immunotherapeutics for cancer patients.

Paracrine regulations of IFN-γ secreting CD4 + T cells by lumican and biglycan are protective in allergic contact dermatitis

The extracellular matrix (ECM) is known to regulate innate immune cells but its role in T cell functions is poorly understood. Here, we show a protective role for ECM proteoglycans, lumican and biglycan in hapten-induced contact dermatitis that is achieved through limiting proinflammatory CD4 + T cells. Lumican and biglycan-null mice develop significant inflammation with greater numbers of CD4 + T cells in hapten-challenged ear pinnae, while their draining lymph nodes show increased T-bet-STAT1 signaling, Th1 commitment, and IFN-γ secreting CD4 + T cell proliferation. Wild type mouse lymph node fibroblastic reticular cells secrete lumican, biglycan and decorin, a related proteoglycan, while none are expressed by naive or activated T cells. In vitro , lumican and biglycan co-localize with LFA-1 on T cell surfaces, and all three proteoglycans suppress LFA-1 mediated T cell activation. Overall, this study elucidates a novel paracrine regulation of Th1 cells by ECM proteoglycans to limit inflammation and tissue damage.

Conversion of vaccines from low to high immunogenicity by antibodies with epitope complementarity

Existing antibodies (Abs) have varied effects on humoral immunity during subsequent infections. Here, we leveraged in vivo systems that allow precise control of antigen-specific Abs and B cells to examine the impact of Ab dose, affinity, and specificity in directing B cell activation and differentiation. Abs competing with the B cell receptor (BCR) epitope showed affinity-dependent suppression. By contrast, Abs targeting a complementary epitope, not overlapping with the BCR, shifted B cell differentiation toward Ab-secreting cells. Such Abs allowed for potent germinal center (GC) responses to otherwise poorly immunogenic sites by promoting antigen capture and presentation by low-affinity B cells. These mechanisms jointly diversified the B cell repertoire by facilitating the recruitment of high- and low-affinity B cells into Ab-secreting cell, GC, and memory B cell fates. Incorporation of small amounts of monoclonal Abs into protein- or mRNA-based vaccines enhanced immunogenicity and facilitated sustained immune responses, with implications for vaccine design and our understanding of protective immunity.

Proximity-dependent labeling identifies dendritic cells that drive the tumor-specific CD4+ T cell response

Dendritic cells (DCs) are uniquely capable of transporting tumor antigens to tumor-draining lymph nodes (tdLNs) and interact with effector T cells in the tumor microenvironment (TME) itself, mediating both natural antitumor immunity and the response to checkpoint blockade immunotherapy. Using LIPSTIC (Labeling Immune Partnerships by SorTagging Intercellular Contacts)-based single-cell transcriptomics, we identified individual DCs capable of presenting antigen to CD4+ T cells in both the tdLN and TME. Our findings revealed that DCs with similar hyperactivated transcriptional phenotypes interact with helper T cells both in tumors and in the tdLN and that checkpoint blockade drugs enhance these interactions. These findings show that a relatively small fraction of DCs is responsible for most of the antigen presentation in the tdLN and TME to both CD4+ and CD8+ tumor-specific T cells and that classical checkpoint blockade enhances CD40-driven DC activation at both sites.

Internal regulation between constitutively expressed T cell co-inhibitory receptors BTLA and CD5 and tolerance in recent thymic emigrants

Immunologic self-tolerance involves signals from co-inhibitory receptors. Several T cell co-inhibitors, including PD-1, are expressed upon activation, whereas CD5 and BTLA are expressed constitutively. The relationship between constitutively expressed co-inhibitors and when they are needed is unknown. Deletion of Btla demonstrated BTLA regulates CD5 expression. Loss of BTLA signals, but not signalling by its ligand, HVEM, leads to increased CD5 expression. Higher CD5 expression set during thymic selection is associated with increased self-recognition, suggesting that BTLA might be needed early to establish self-tolerance. We found that BTLA and PD-1 were needed post-thymic selection in recent thymic emigrants (RTE). RTE lacking BTLA caused a CD4 T cell and MHC class II dependent multi-organ autoimmune disease. Together, our findings identify a negative regulatory pathway between two constitutively expressed co-inhibitors, calibrating their expression. Expression of constitutive and induced co-inhibitory receptors is needed early to establish tolerance in the periphery for RTE.

β-Glucan-A promising immunocyte-targeting drug delivery vehicle: Superiority, applications and future prospects

Drug delivery technologies that could convert promising therapeutics into successful therapies have been under broad research for many years. Recently, β-glucans, natural-occurring polysaccharides extracted from many organism species such as yeast, fungi and bacteria, have attracted increasing attention to serve as drug delivery carriers. With their unique structure and innate immunocompetence, β-glucans are considered as promising carriers for targeting delivery especially when applied in the vaccine construction and oral administration of therapeutic agents. In this review, we focus on three types of β-glucans applied in the drug delivery system including yeast β-glucan, Schizophyllan and curdlan, highlighting the benefits of β-glucan based delivery system. We summarize how β-glucans as delivery vehicles have aided various therapeutics ranging from macromolecules including proteins, peptides and nucleic acids to small molecular drugs to reach desired cells or organs in terms of loading strategies. We also outline the challenges and future directions for developing the next generation of β-glucan based delivery systems.

Macrophage WDFY3, a protector against autoimmunity

Efficient efferocytosis is essential for maintaining homeostasis. Excessive apoptotic cell (AC) death and impaired macrophage efferocytosis lead to autoantigen release and autoantibody production, immune activation, and organ damage. It remains unclear whether these immunogenic autoantigens are the sole cause of increased autoimmunity or if efferocytosis of ACs directly influences macrophage function, impacting their ability to activate T cells and potentially amplifying autoimmune responses. Additionally, it has not been established if enhancing macrophage efferocytosis or modulating macrophage responses to AC engulfment can be protective in autoimmune-like disorders. Our previous work showed WDFY3 is crucial for efficient macrophage efferocytosis. This study reveals that myeloid knockout of Wdfy3 exacerbates autoimmunity in young mice with increased AC burden by systemic injections of ACs and in middle-aged mice developing spontaneous autoimmunity, whereas ectopic overexpression of WDFY3 suppresses autoimmunity in these models. Macrophages, as efferocytes, can activate T cells and the inflammasome upon engulfing ACs, which are suppressed by overexpressing WDFY3. This work uncovered the role of WDFY3 as a protector against autoimmunity by promoting macrophage efferocytosis thus limiting autoantigen production, as well as mitigating T cell activation and inflammasome activation.

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.

Evaluating in vivo approaches for studying the roles of thymic DCs in T cell development in mice

T cells express an enormous repertoire of T cell receptors, enabling them to recognize any potential antigen. This large repertoire undergoes stringent selections in the thymus, where receptors that react to self- or non-danger-associated- antigens are purged. We know that thymic tolerance depends on signals and antigens presented by the thymic antigen presenting cells, but we still do not understand precisely how many of these cells actually contribute to tolerance. This is especially true for thymic dendritic cells (DC), which are composed of diverse subpopulations that are derived from different progenitors. Although the importance of thymic DCs has long been known, the functions of specific DC subsets have been difficult to untangle. There remains insufficient systematic characterization of the ontogeny and phenotype of thymic APCs in general. As a result, validated experimental models for studying thymic DCs are limited. Recent technological advancement, such as multi-omics analyses, has enabled new insights into thymic DC biology. These recent findings indicate a need to re-evaluate the current tools used to study the function of these cells within the thymus. This review will discuss how thymic DC subpopulations can be defined, the models that have been used to assess functions in the thymus, and models developed for other settings that can be potentially used for studying thymic DCs.

The TCR assigns naive T cells to a preferred lymph node

Naive T cells recirculate between the spleen and lymph nodes where they mount immune responses when meeting dendritic cells presenting foreign antigen. As this may happen anywhere, naive T cells ought to visit all lymph nodes. Here, deep sequencing almost-complete TCR repertoires led to a comparison of different lymph nodes within and between individual mice. We find strong evidence for a deterministic CD4/CD8 lineage choice and a consistent spatial structure. Specifically, some T cells show a preference for one or multiple lymph nodes, suggesting that their TCR interacts with locally presented (self-)peptides. These findings are mirrored in TCR-transgenic mice showing localized CD69 expression, retention, and cell division. Thus, naive T cells intermittently sense antigenically dissimilar niches, which is expected to affect their homeostatic competition.

Correction of age-associated defects in dendritic cells enables CD4+ T cells to eradicate tumors

Defective host defenses later in life are associated with changes in immune cell activities, suggesting that age-specific considerations are needed in immunotherapy approaches. In this study, we found that PD-1 and CTLA4-based cancer immunotherapies are unable to eradicate tumors in elderly mice. This defect in anti-tumor activity correlated with two known age-associated immune defects: diminished abundance of systemic naive CD8+ T cells and weak migratory activities of dendritic cells (DCs). We identified a vaccine adjuvant, referred to as a DC hyperactivator, which corrects DC migratory defects in the elderly. Vaccines containing tumor antigens and DC hyperactivators induced T helper type 1 (TH1) CD4+ T cells with cytolytic activity that drive anti-tumor immunity in elderly mice. When administered early in life, DC hyperactivators were the only adjuvant identified that elicited anti-tumor CD4+ T cells that persisted into old age. These results raise the possibility of correcting age-associated immune defects through DC manipulation.

Opposing effects of pre-existing antibody and memory T cell help on the dynamics of recall germinal centers

Re-exposure to an antigen generates abundant antibody responses and drives the formation of secondary germinal centers (GCs). Recall GCs in mice consist almost entirely of naïve B cells, whereas recall antibodies derive overwhelmingly from memory B cells. Here, we examine this division between cellular and serum compartments. After repeated immunization with the same antigen, tetramer analyses of recall GCs revealed a marked decrease in the ability of B cells in these structures to bind the antigen. Boosting with viral variant proteins restored antigen binding in recall GCs, as did genetic ablation of primary-derived antibody-secreting cells through conditional deletion of Prdm1, demonstrating suppression of GC recall responses by pre-existing antibodies. In hapten-carrier experiments in which B and T cell specificities were uncoupled, memory T cell help allowed B cells with undetectable antigen binding to access GCs. Thus, antibody-mediated feedback steers recall GC B cells away from previously targeted epitopes and enables specific targeting of variant epitopes, with implications for vaccination protocols.

Constitutive Flt3 signaling impacts conventional dendritic cell function

The development of dendritic cells (DCs) depends on signaling via the FMS-like tyrosine kinase 3 (Flt3) receptor. How Flt3 signaling impacts terminally differentiated DC function is unknown. This is important given the increasing interest in exploiting Flt3 for vaccination and tumor immunotherapy. Here, we examined DCs in mice harboring constitutively activated Flt3 (Flt3-ITD). Flt3ITD/ITD mice possessed expanded splenic DC subsets including plasmacytoid DC, conventional DC (cDC)1, cDC2, double positive (DP) cDC1 (CD11c+ CD8+ CD11b- CD103+ CD86+), noncanonical (NC) cDC1 (CD11c+ CD8+ CD11b- CD103- CD86-) and single positive (SP) cDC1 (CD11c+ CD8+ CD11b- CD103- CD86+). Outcomes of constitutive Flt3 signaling differed depending on the cDC subset examined. In comparison with wild type (WT) DCs, all Flt3ITD/ITD cDCs displayed an altered surface phenotype with changes in costimulatory molecules, major histocompatibility complex class I (MHC I) and II (MHC II). Cytokine secretion patterns, antigen uptake, antigen proteolysis and antigen presenting function differed between WT and Flt3ITD/ITD subsets, particularly cDC2. In summary, Flt3 signaling impacts the function of terminally differentiated cDCs with important consequences for antigen presentation.

MAIT cells monitor intestinal dysbiosis and contribute to host protection during colitis

Intestinal inflammation shifts microbiota composition and metabolism. How the host monitors and responds to such changes remains unclear. Here, we describe a protective mechanism by which mucosal-associated invariant T (MAIT) cells detect microbiota metabolites produced upon intestinal inflammation and promote tissue repair. At steady state, MAIT ligands derived from the riboflavin biosynthesis pathway were produced by aerotolerant bacteria residing in the colonic mucosa. Experimental colitis triggered luminal expansion of riboflavin-producing bacteria, leading to increased production of MAIT ligands. Modulation of intestinal oxygen levels suggested a role for oxygen in inducing MAIT ligand production. MAIT ligands produced in the colon rapidly crossed the intestinal barrier and activated MAIT cells, which expressed tissue-repair genes and produced barrier-promoting mediators during colitis. Mice lacking MAIT cells were more susceptible to colitis and colitis-driven colorectal cancer. Thus, MAIT cells are sensitive to a bacterial metabolic pathway indicative of intestinal inflammation.

Innate phase production of IFN-γ by memory and effector T cells expressing early activation marker CD69 during infection with Cryptococcus deneoformans in the lungs

Cryptococcus deneoformans is a yeast-type fungus that causes fatal meningoencephalitis in immunocompromised patients and evades phagocytic cell elimination through an escape mechanism. Memory T (Tm) cells play a central role in preventing the reactivation of this fungal pathogen. Among these cells, tissue-resident memory T (TRM) cells quickly respond to locally invaded pathogens. This study analyzes the kinetics of effector T (Teff) cells and Tm cells in the lungs after cryptococcal infection. Emphasis is placed on the kinetics and cytokine expression of TRM cells in the early phase of infection. CD4+ Tm cells exhibited a rapid increase by day 3, peaked at day 7, and then either maintained their levels or exhibited a slight decrease until day 56. In contrast, CD8+ Tm cells reached their peak on day 3 and thereafter decreased up to day 56 post-infection. These Tm cells were predominantly composed of CD69+ TRM cells and CD69+ CD103+ TRM cells. Disruption of the CARD9 gene resulted in reduced accumulation of these TRM cells and diminished interferon (IFN) -γ expression in TRM cells. TRM cells were derived from T cells with T cell receptors non-specific to ovalbumin in OT-II mice during cryptococcal infection. In addition, TRM cells exhibited varied behavior in different tissues. These results underscore the importance of T cells, which produce IFN-γ in the lungs during the early stage of infection, in providing early protection against cryptococcal infection through CARD9 signaling.

The aged microenvironment impairs BCL6 and CD40L induction in CD4+ T follicular helper cell differentiation

Weakened germinal center responses by the aged immune system result in diminished immunity against pathogens and reduced efficacy of vaccines. Prolonged contacts between activated B cells and CD4+ T cells are crucial to germinal center formation and T follicular helper cell (Tfh) differentiation, but it is unclear how aging impacts the quality of this interaction. Peptide immunization confirmed that aged mice have decreased expansion of antigen-specific germinal center B cells and reduced antibody titers. Furthermore, aging was associated with accumulated Tfh cells, even in naïve mice. Despite increased numbers, aged Tfh had reduced expression of master transcription factor BCL6 and increased expression of the ectonucleotidase CD39. In vitro activation revealed that proliferative capacity was maintained in aged CD4+ T cells, but not the costimulatory molecule CD40L. When activated in vitro by aged antigen-presenting cells, young CD4+ naïve T cells generated reduced numbers of activated cells with upregulated CD40L. To determine the contribution of cell-extrinsic influences on antigen-specific Tfh induction, young, antigen-specific B and CD4+ T cells were adoptively transferred into aged hosts prior to peptide immunization. Transferred cells had reduced expansion and differentiation into germinal center B cell and Tfh and reduced antigen-specific antibody titers when compared to young hosts. Young CD4+ T cells transferred aged hosts differentiated into Tfh cells with reduced PD-1 and BCL6 expression, and increased CD39 expression, though they maintained their mitochondrial capacity. These results highlight the role of the lymphoid microenvironment in modulating CD4+ T cell differentiation, which contributes to impaired establishment and maintenance of germinal centers.

Thetis cells induce food-specific Treg cell differentiation and oral tolerance

The intestinal immune system must establish tolerance to food antigens to prevent onset of allergic and inflammatory diseases. Peripherally generated regulatory T (pTreg) cells play an essential role in suppressing inflammatory responses to allergens; however, the antigen-presenting cell (APC) that instructs food-specific pTreg cells is not known. Here, we show that antigen presentation and TGF-β activation by a subset of RORγt + antigen-presenting cells (APC), Thetis cells IV (TC IV), is required for food-induced pTreg cell differentiation and oral tolerance. By contrast, antigen presentation by dendritic cells (DCs) was dispensable for pTreg induction but required for T H 1 effector responses, highlighting a division of labor between tolerogenic TCs and pro-inflammatory DCs. While antigen presentation by TCs was required for food-specific pTreg generation both in early life and adulthood, the increased abundance of TCs in the peri-weaning period was associated with a window of opportunity for enhanced pTreg differentiation. These findings establish a critical role for TCs in oral tolerance and suggest that these cells may represent a key therapeutic target for the treatment of food-associated allergic and inflammatory diseases.

Iron regulates the quiescence of naive CD4 T cells by controlling mitochondria and cellular metabolism

In response to an immune challenge, naive T cells undergo a transition from a quiescent to an activated state acquiring the effector function. Concurrently, these T cells reprogram cellular metabolism, which is regulated by iron. We and others have shown that iron homeostasis controls proliferation and mitochondrial function, but the underlying mechanisms are poorly understood. Given that iron derived from heme makes up a large portion of the cellular iron pool, we investigated iron homeostasis in T cells using mice with a T cell-specific deletion of the heme exporter, FLVCR1 [referred to as knockout (KO)]. Our finding revealed that maintaining heme and iron homeostasis is essential to keep naive T cells in a quiescent state. KO naive CD4 T cells exhibited an iron-overloaded phenotype, with increased spontaneous proliferation and hyperactive mitochondria. This was evidenced by reduced IL-7R and IL-15R levels but increased CD5 and Nur77 expression. Upon activation, however, KO CD4 T cells have defects in proliferation, IL-2 production, and mitochondrial functions. Iron-overloaded CD4 T cells failed to induce mitochondrial iron and exhibited more fragmented mitochondria after activation, making them susceptible to ferroptosis. Iron overload also led to inefficient glycolysis and glutaminolysis but heightened activity in the hexosamine biosynthetic pathway. Overall, these findings highlight the essential role of iron in controlling mitochondrial function and cellular metabolism in naive CD4 T cells, critical for maintaining their quiescent state.

Derivation of functional thymic epithelial organoid lines from adult murine thymus

Thymic epithelial cells (TECs) orchestrate T cell development by imposing positive and negative selection on thymocytes. Current studies on TEC biology are hampered by the absence of long-term ex vivo culture platforms, while the cells driving TEC self-renewal remain to be identified. Here, we generate long-term (>2 years) expandable 3D TEC organoids from the adult mouse thymus. For further analysis, we generated single and double FoxN1-P2A-Clover, Aire-P2A-tdTomato, and Cldn4-P2A-tdTomato reporter lines by CRISPR knockin. Single-cell analyses of expanding clonal organoids reveal cells with bipotent stem/progenitor phenotypes. These clonal organoids can be induced to express Foxn1 and to generate functional cortical- and Aire-expressing medullary-like TECs upon RANK ligand + retinoic acid treatment. TEC organoids support T cell development from immature thymocytes in vitro as well as in vivo upon transplantation into athymic nude mice. This organoid-based platform allows in vitro study of TEC biology and offers a potential strategy for ex vivo T cell development.

Nrf2 regulates the activation-driven expansion of CD4 + T-cells by differentially modulating glucose and glutamine metabolism

Upon antigenic stimulation, CD4 + T-cells undergo clonal expansion, elevating their bioenergetic demands and utilization of nutrients like glucose and glutamine. The nuclear factor erythroid 2-related factor 2 (Nrf2) is a well-known regulator of oxidative stress, but its involvement in modulating the metabolism of CD4 + T-cells remains unexplored. Here, we elucidate the role of Nrf2 beyond the traditional antioxidation, in modulating activation-driven expansion of CD4 + T-cells by influencing their nutrient metabolism. T-cell-specific activation of Nrf2 enhances early activation and IL-2 secretion, upregulates TCR-signaling, and increases activation-driven proliferation of CD4 + T-cells. Mechanistically, high Nrf2 inhibits glucose metabolism through glycolysis but promotes glutamine metabolism via glutaminolysis to support increased T-cell proliferation. Further, Nrf2 expression is temporally regulated in activated CD4 + T-cells with elevated expression during the early activation, but decreased expression thereafter. Overall, our findings uncover a novel role of Nrf2 as a metabolic modulator of CD4 + T-cells, thus providing a framework for improving Nrf2-targeting therapies and T-cell immunotherapies.

Serine enrichment in tumors promotes regulatory T cell accumulation through sphinganine-mediated regulation of c-Fos

CD4+ regulatory T (Treg) cells accumulate in the tumor microenvironment (TME) and suppress the immune system. Whether and how metabolite availability in the TME influences Treg cell differentiation is not understood. Here, we measured 630 metabolites in the TME and found that serine and palmitic acid, substrates required for the synthesis of sphingolipids, were enriched. A serine-free diet or a deficiency in Sptlc2, the rate-limiting enzyme catalyzing sphingolipid synthesis, suppressed Treg cell accumulation and inhibited tumor growth. Sphinganine, an intermediate metabolite in sphingolipid synthesis, physically interacted with the transcription factor c-Fos. Sphinganine c-Fos interactions enhanced the genome-wide recruitment of c-Fos to regions near the transcription start sites of target genes including Pdcd1 (encoding PD-1), which promoted Pdcd1 transcription and increased inducible Treg cell differentiation in vitro in a PD-1-dependent manner. Thus, Sptlc2-mediated sphingolipid synthesis translates the extracellular information of metabolite availability into nuclear signals for Treg cell differentiation and limits antitumor immunity.

Tumor-targeted therapy with BRAF-inhibitor recruits activated dendritic cells to promote tumor immunity in melanoma

BACKGROUND

Tumor-targeted therapy causes impressive tumor regression, but the emergence of resistance limits long-term survival benefits in patients. Little information is available on the role of the myeloid cell network, especially dendritic cells (DC) during tumor-targeted therapy.

METHODS

Here, we investigated therapy-mediated immunological alterations in the tumor microenvironment (TME) and tumor-draining lymph nodes (LN) in the D4M.3A preclinical melanoma mouse model (harboring the V-Raf murine sarcoma viral oncogene homolog B (BRAF)V600E mutation) by using high-dimensional multicolor flow cytometry in combination with multiplex immunohistochemistry. This was complemented with RNA sequencing and cytokine quantification to characterize the immune status of the tumors. The importance of T cells during tumor-targeted therapy was investigated by depleting CD4+ or CD8+ T cells in tumor-bearing mice. Tumor antigen-specific T-cell responses were characterized by performing in vivo T-cell proliferation assays and the contribution of conventional type 1 DC (cDC1) to T-cell immunity during tumor-targeted therapy was assessed using Batf3-/- mice lacking cDC1.

RESULTS

Our findings reveal that BRAF-inhibitor therapy increased tumor immunogenicity, reflected by an upregulation of genes associated with immune activation. The T cell-inflamed TME contained higher numbers of activated cDC1 and cDC2 but also inflammatory CCR2-expressing monocytes. At the same time, tumor-targeted therapy enhanced the frequency of migratory, activated DC subsets in tumor-draining LN. Even more, we identified a cDC2 population expressing the Fc gamma receptor I (FcγRI)/CD64 in tumors and LN that displayed high levels of CD40 and CCR7 indicating involvement in T cell-mediated tumor immunity. The importance of cDC2 is underlined by just a partial loss of therapy response in a cDC1-deficient mouse model. Both CD4+ and CD8+ T cells were essential for therapy response as their respective depletion impaired therapy success. On resistance development, the tumors reverted to an immunologically inert state with a loss of DC and inflammatory monocytes together with the accumulation of regulatory T cells. Moreover, tumor antigen-specific CD8+ T cells were compromised in proliferation and interferon-γ-production.

CONCLUSION

Our results give novel insights into the remodeling of the myeloid landscape by tumor-targeted therapy. We demonstrate that the transient immunogenic tumor milieu contains more activated DC. This knowledge has important implications for the development of future combinatorial therapies.