Pathways and mechanisms of CD4+CD8αα+ intraepithelial T cell development.

The mammalian small intestine epithelium harbors a peculiar population of CD4+CD8αα+ T cells that are derived from mature CD4+ T cells through reprogramming of lineage-specific transcription factors. CD4+CD8αα+ T cells occupy a unique niche in T cell biology because they exhibit mixed phenotypes and functional characteristics of both CD4+ helper and CD8+ cytotoxic T cells. The molecular pathways driving their generation are not fully mapped. However, recent studies demonstrate the unique role of the commensal gut microbiota as well as distinct cytokine and chemokine requirements in the differentiation and survival of these cells. We review the established and newly identified factors involved in the generation of CD4+CD8αα+ intraepithelial lymphocytes (IELs) and place them in the context of the molecular machinery that drives their phenotypic and functional differentiation.

Natural intraepithelial lymphocyte populations rise during necrotic enteritis in chickens.

Intraepithelial lymphocytes (IEL) reside in the epithelium at the interface between the contents of the intestinal lumen and the sterile environment of the lamina propria. Because of this strategic location, IEL play a crucial role in various immunological processes, ranging from pathogen control to tissue stability. In mice and humans, IEL exhibit high diversity, categorized into induced IEL (conventional CD4 and CD8αß T cells) and natural IEL (TCRαßCD8αα, TCRγδ, and TCRneg IEL). In chickens, however, the subpopulations of IEL and their functions in enteric diseases remain unclear. Thus, we conducted this study to investigate the role of IEL populations during necrotic enteritis (NE) in chickens. At 14 days of age, sixty-three Specific-pathogen-free (SPF) birds were randomly assigned to three treatments: Control (sham challenge), Eimeria maxima challenge (EM), and Eimeria maxima + Clostridium Perfringens (C. Perfringens) co-challenge (EM/CP). The EM and EM/CP birds were infected with Eimeria maxima at day 14 of age, and EM/CP birds were additionally orally inoculated with C. perfringens at days 18 and 19 of age. Birds were weighed at days 18, 20, and 26 of age to assess body weight gain (BWG). At 20 days of age (1 day-post C. perfringens infection; dpi), and 26 days of age (7 dpi), 7 birds per treatment were euthanized, and jejunum was harvested for gross lesion scores, IEL isolation, and gene expression. The EM/CP birds exhibited subclinical NE disease, lower BWG and shorter colon length. The Most changes in the IEL populations were observed at 1 dpi. The EM/CP group showed substantial increases in the total number of natural IEL subsets, including TCRαß+CD4-CD8-, TCRαß+CD8αα+, TCRγδ+, TCRneg and innate CD8α (iCD8α) cells by at least two-fold. However, by 7 dpi, only the number of TCRαß+CD4-CD8- and TCRαß+CD8αα+ IEL maintained their increase in the EM/CP group. The EM/CP group had significantly higher expression of proinflammatory cytokines (IL-1ß and IFN-γ) and Osteopontin (OPN) in the jejunum at 1 dpi. These findings suggest that natural IEL with innate and innate-like functions might play a critical role in the host response during subclinical NE, potentially conferring protection against C. perfringens infection.

Toll-like receptor 2 deficiency promotes the generation of alloreactive γδT17 cells after cardiac transplantation in mice.

Homograft rejection is the main cause of heart transplantation failure. The role of TLR2, a major member of the toll-like receptor (TLR) family, in transplantation rejection is has yet to be elucidated. In this study, we used a mouse model of acute cardiac transplantation rejection to investigate whether the TLR2 signalling pathway can regulate cardiac transplantation rejection by regulating alloreactive IL-17+γδT (γδT17) cells. We found that the expression of TLR2 on the surface of dendritic cells (DCs) and macrophages increased during acute transplantation rejection. In addition, our investigation revealed that γδT17 cells exert a significant influence on acute cardiac transplantation rejection. TLR2 gene knockout resulted in an increase in alloreactive γδT17 cells in the spleen and heart grafts of recipient mice compared with wild-type recipient mice and an increase in the mRNA expression of IL-17, IL-1ß, CCR6, and CCL20 in the heart grafts. In an in vitro experiment, a mixed lymphocyte reaction was conducted to assess the impact of TLR2 deficiency on the generation of γδT17 cells, which further substantiated a significant increase compared to that in wild-type controls. Furthermore, the mixed lymphocyte reaction showed that TLR2 regulated the production of γδT17 cells by regulating the ability of DCs to secrete IL-1ß. These results suggest that TLR2 signalling is important for regulating the generation of γδT17 cells after cardiac allograft transplantation.

Extracellular CIRP induces CD4CD8αα intraepithelial lymphocyte cytotoxicity in sepsis.

BACKGROUND: In sepsis, intestinal barrier dysfunction is often caused by the uncontrolled death of intestinal epithelial cells (IECs). CD4CD8αα intraepithelial lymphocytes (IELs), a subtype of CD4+ T cells residing within the intestinal epithelium, exert cytotoxicity by producing granzyme B (GrB) and perforin (Prf). Extracellular cold-inducible RNA-binding protein (eCIRP) is a recently identified alarmin which stimulates TLR4 on immune cells to induce proinflammatory responses. Here, we hypothesized that eCIRP enhances CD4CD8αα IEL cytotoxicity and induces IEC death in sepsis. METHODS: We subjected wild-type (WT) and CIRP-/- mice to sepsis by cecal ligation and puncture (CLP) and collected the small intestines to isolate IELs. The expression of GrB and Prf in CD4CD8αα IELs was assessed by flow cytometry. IELs isolated from WT and TLR4-/- mice were challenged with recombinant mouse CIRP (eCIRP) and assessed the expression of GrB and Prf in CD4CD8αα by flow cytometry. Organoid-derived IECs were co-cultured with eCIRP-treated CD4CD8αα cells in the presence/absence of GrB and Prf inhibitors and assessed IEC death by flow cytometry. RESULTS: We found a significant increase in the expression of GrB and Prf in CD4CD8αα IELs of septic mice compared to sham mice. We found that GrB and Prf levels in CD4CD8αα IELs were increased in the small intestines of WT septic mice, while CD4CD8αα IELs of CIRP-/- mice did not show an increase in those cytotoxic granules after sepsis. We found that eCIRP upregulated GrB and Prf in CD4CD8αα IELs isolated from WT mice but not from TLR4-/- mice. Furthermore, we also revealed that eCIRP-treated CD4CD8αα cells induced organoid-derived IEC death, which was mitigated by GrB and Prf inhibitors. Finally, histological analysis of septic mice revealed that CIRP-/- mice were protected from tissue injury and cell death in the small intestines compared to WT mice. CONCLUSION: In sepsis, the cytotoxicity initiated by the eCIRP/TLR4 axis in CD4CD8αα IELs is associated with intestinal epithelial cell (IEC) death, which could lead to gut injury.

The immunopathological landscape of human pre-TCRα deficiency: From rare to common variants.

We describe humans with rare biallelic loss-of-function PTCRA variants impairing pre-α T cell receptor (pre-TCRα) expression. Low circulating naive αß T cell counts at birth persisted over time, with normal memory αß and high γδ T cell counts. Their TCRα repertoire was biased, which suggests that noncanonical thymic differentiation pathways can rescue αß T cell development. Only a minority of these individuals were sick, with infection, lymphoproliferation, and/or autoimmunity. We also report that 1 in 4000 individuals from the Middle East and South Asia are homozygous for a common hypomorphic PTCRA variant. They had normal circulating naive αß T cell counts but high γδ T cell counts. Although residual pre-TCRα expression drove the differentiation of more αß T cells, autoimmune conditions were more frequent in these patients compared with the general population.

Intraepithelial lymphocytes promote intestinal regeneration through CD160/HVEM signaling.

Chemotherapy and radiotherapy frequently lead to intestinal damage. The mechanisms governing the repair or regeneration of intestinal damage are still not fully elucidated. Intraepithelial lymphocytes (IELs) are the primary immune cells residing in the intestinal epithelial layer. However, whether IELs are involved in intestinal epithelial injury repair remains unclear. Here, we found that IELs rapidly infiltrated the intestinal crypt region and are crucial for the recovery of the intestinal epithelium post-chemotherapy. Interestingly, IELs predominantly promoted intestinal regeneration by modulating the proliferation of transit-amplifying (TA) cells. Mechanistically, the expression of CD160 on IELs allows for interaction with herpes virus entry mediator (HVEM) on the intestinal epithelium, thereby activating downstream nuclear factor kappa (NF-κB) signaling and further promoting intestinal regeneration. Deficiency in either CD160 or HVEM resulted in reduced proliferation of intestinal progenitor cells, impaired intestinal damage repair, and increased mortality following chemotherapy. Remarkably, the adoptive transfer of CD160-sufficient IELs rescued the Rag1 deficient mice from chemotherapy-induced intestinal inflammation. Overall, our study underscores the critical role of IELs in intestinal regeneration and highlights the potential applications of targeting the CD160-HVEM axis for managing intestinal adverse events post-chemotherapy and radiotherapy.

BCL6 is required for the thymic development of TCRαß+CD8αα+ intraepithelial lymphocyte lineage.

TCRαß+CD8αα+ intraepithelial lymphocytes (CD8αα+ αß IELs) are a specialized subset of T cells in the gut epithelium that develop from thymic agonist selected IEL precursors (IELps). The molecular mechanisms underlying the selection and differentiation of this T cell type in the thymus are largely unknown. Here, we found that Bcl6 deficiency in αß T cells resulted in the near absence of CD8αα+ αß IELs. BCL6 was expressed by approximately 50% of CD8αα+ αß IELs and by the majority of thymic PD1+ IELps after agonist selection. Bcl6 deficiency blocked early IELp generation in the thymus, and its expression in IELps was induced by thymic TCR signaling in an ERK-dependent manner. As a result of Bcl6 deficiency, the precursors of IELps among CD4+CD8+ double-positive thymocytes exhibited increased apoptosis during agonist selection and impaired IELp differentiation and maturation. Together, our results elucidate BCL6 as a crucial transcription factor during the thymic development of CD8αα+ αß IELs.

Hyperactivation and enhanced cytotoxicity of reduced CD8+ gamma delta T cells in the intestine of patients with Crohn's disease correlates with disease activity.

BACKGROUND AND AIMS: We aimed to investigate the immune characteristics of intestinal CD8+ gamma delta T (CD8+ γδ T) cells in Crohn's disease (CD) and their correlation with disease activity. METHODS: The study cohorts included 21 CD patients and 21 healthy individuals. CD8+ γδ T cells were isolated from human ileal mucosa for detection by flow cytometry. The activation or inhibition status of cells was detected by detecting the expression of activation marker HLA-DR and the immunosuppressive molecule PD-1 on cells. The cytotoxicity of cells was assessed by detecting the expression of cytotoxic molecules (Perforin, Granzyme B, and TRAIL) in cells. Ratios of investigated cells were calculated as prediction factors by receiver operating characteristic curve (ROC) analysis. RESULTS: The study revealed a reduction in intestinal CD8+ γδT cells among active CD patients, with a more pronounced reduction observed in moderately active patients compared to mildly active patients. Moreover, active CD patients exhibited heightened activation levels in their intestinal CD8+ γδT cells, whereas the activation was comparatively weakened in moderately active patients compared with mildly active patients. Additionally, the cytotoxicity of intestinal CD8+ γδT cells was enhanced solely in mildly active patients, while it was impaired in moderately active patients compared with mildly active patients. Furthermore, HLA-DR+ CD8+ γδT cell ratio, CD8+ γδT ratio, and CD8+ γδT count were identified as indicators in the diagnosis of active CD. Meanwhile, the ratios of Granzyme B+ CD8+ γδT cell and Perforin+ CD8+ γδT cell were identified as indicators that distinguish mildly moderately active CD cases. CONCLUSIONS: Intestinal CD8+ γδT was reduced in active CD patients, but their activation and cytotoxicity were enhanced. However, with increased disease activity, intestinal CD8+ γδ T cells became dysfunctional. CD-specific perturbations observed in various phenotypic markers in CD8+ γδ T cells can be used as indicators to assist in diagnosing CD patients.

Gamma delta T cells in acute myeloid leukemia: biology and emerging therapeutic strategies.

γδ T cells play an important role in disease control in acute myeloid leukemia (AML) and have become an emerging area of therapeutic interest. These cells represent a minor population of T lymphocytes with intrinsic abilities to recognize antigens in a major histocompatibility complex-independent manner and functionally straddle the innate and adaptive immunity interface. AML shows high expression of phosphoantigens and UL-16 binding proteins that activate the Vδ2 and Vδ1 subtypes of γδ T cells, respectively, leading to γδ T cell-mediated cytotoxicity. Insights from murine models and clinical data in humans show improved overall survival, leukemia-free survival, reduced risk of relapse, enhanced graft-versus-leukemia effect, and decreased graft-versus-host disease in patients with AML who have higher reconstitution of γδ T cells following allogeneic hematopoietic stem cell transplantation. Clinical trials leveraging γδ T cell biology have used unmodified and modified allogeneic cells as well as bispecific engagers and monoclonal antibodies. In this review, we discuss γδ T cells' biology, roles in cancer and AML, and mechanisms of immune escape and antileukemia effect; we also discuss recent clinical advances related to γδ T cells in the field of AML therapeutics.

Single cell analysis revealed that two distinct, unique CD4+ T cell subsets were increased in the small intestinal intraepithelial lymphocytes of aged mice.

Recent advances in research suggest that aging has a controllable chronic inflammatory disease aspect. Aging systemic T cells, which secrete pro-inflammatory factors, affect surrounding somatic cells, and accelerate the aging process through chronic inflammation, have attracted attention as potential therapeutic targets in aging. On the other hand, there are few reports on the aging of the intestinal immune system, which differs from the systemic immune system in many ways. In the current study, we investigated the age-related changes in the intestinal immune system, particularly in T cells. The most significant changes were observed in the CD4+ T cells in the small intestinal IEL, with a marked increase in this fraction in old mice and reduced expression of CD27 and CD28, which are characteristic of aging systemic T cells. The proliferative capacity of aging IEL CD4+ T cells was significantly more reduced than that of aging systemic T cells. Transcriptome analysis showed that the expression of inflammatory cytokines was not upregulated, whereas Cd8α, NK receptors, and Granzymes were upregulated in aging IEL CD4+ T cells. Functional analysis showed that aging IEL T cells had a higher cytotoxic function against intestinal tumor organoids in vitro than young IEL T cells. scRNAseq revealed that splenic T cells show a transition from naïve to memory T cells, whereas intestinal T cells show the emergence of a CD8αα+CD4+ T cell fraction in aged mice, which is rarely seen in young cells. Further analysis of the aging IEL CD4+ T cells showed that two unique subsets are increased that are distinct from the systemic CD4+ T cells. Subset 1 has a pro-inflammatory component, with expression of IFNγ and upregulation of NFkB signaling pathways. Subset 2 does not express IFNγ, but upregulates inhibitory molecules and nIEL markers. Expression of granzymes and Cd8a was common to both. These fractions were in opposite positions in the clustering by UMAP and had different TCR repertoires. They may be involved in the suppression of intestinal aging and longevity through anti-tumor immunity, elimination of senescent cells and stressed cells in the aging environment. This finding could be a breakthrough in aging research.

ATP citrate lyase (ACLY)-dependent immunometabolism in mucosal T cells drives experimental colitis in vivo.

OBJECTIVE: Mucosal T cells play a major role in inflammatory bowel disease (IBD). However, their immunometabolism during intestinal inflammation is poorly understood. Due to its impact on cellular metabolism and proinflammatory immune cell function, we here focus on the enzyme ATP citrate lyase (ACLY) in mucosal T cell immunometabolism and its relevance for IBD. DESIGN: ACLY expression and its immunometabolic impact on colitogenic T cell function were analysed in mucosal T cells from patients with IBD and in two experimental colitis models. RESULTS: ACLY was markedly expressed in colon tissue under steady-state conditions but was significantly downregulated in lamina propria mononuclear cells in experimental dextran sodium sulfate-induced colitis and in CD4+ and to a lesser extent in CD8+ T cells infiltrating the inflamed gut in patients with IBD. ACLY-deficient CD4+ T cells showed an impaired capacity to induce intestinal inflammation in a transfer colitis model as compared with wild-type T cells. Assessment of T cell immunometabolism revealed that ACLY deficiency dampened the production of IBD-relevant cytokines and impaired glycolytic ATP production but enriched metabolites involved in the biosynthesis of phospholipids and phosphatidylcholine. Interestingly, the short-chain fatty acid butyrate was identified as a potent suppressor of ACLY expression in T cells, while IL-36α and resolvin E1 induced ACLY levels. In a translational approach, in vivo administration of the butyrate prodrug tributyrin downregulated mucosal infiltration of ACLYhigh CD4+ T cells and ameliorated chronic colitis. CONCLUSION: ACLY controls mucosal T cell immunometabolism and experimental colitis. Therapeutic modulation of ACLY expression in T cells emerges as a novel strategy to promote the resolution of intestinal inflammation.

High-dimensional in situ proteomics imaging to assess γδ T cells in spatial biology.

This study presents a high-dimensional immunohistochemistry approach to assess human γδ T cell subsets in their native tissue microenvironments at spatial resolution, a hitherto unmet scientific goal due to the lack of established antibodies and required technology. We report an integrated approach based on multiplexed imaging and bioinformatic analysis to identify γδ T cells, characterize their phenotypes, and analyze the composition of their microenvironment. Twenty-eight γδ T cell microenvironments were identified in tissue samples from fresh frozen human colon and colorectal cancer where interaction partners of the immune system, but also cancer cells were discovered in close proximity to γδ T cells, visualizing their potential contributions to cancer immunosurveillance. While this proof-of-principle study demonstrates the potential of this cutting-edge technology to assess γδ T cell heterogeneity and to investigate their microenvironment, future comprehensive studies are warranted to associate phenotypes and microenvironment profiles with features such as relevant clinical characteristics.

Dynamics of The Γδtcr Repertoires During The Dedifferentiation Process and Pilot Implications for Immunotherapy of Thyroid Cancer.

γδ T cells are evolutionarily conserved T lymphocytes that manifest unique antitumor efficacy independent of tumor mutation burden (TMB) and conventional human leukocyte antigen (HLA) recognition. However, the dynamic changes in their T cell receptor (TCR) repertoire during cancer progression and treatment courses remain unclear. Here, a comprehensive characterization of γδTCR repertoires are performed in thyroid cancers with divergent differentiation states through cross-sectional studies. The findings revealed a significant correlation between the differentiation states and TCR repertoire diversity. Notably, highly expanded clones are prominently enriched in γδ T cell compartment of dedifferentiated patients. Moreover, by longitudinal investigations of the γδ T cell response to various antitumor therapies, it is found that the emergence and expansion of the Vδ2neg subset may be potentially associated with favorable clinical outcomes after post-radiotherapeutic immunotherapy. These findings are further validated at single-cell resolution in both advanced thyroid cancer patients and a murine model, underlining the importance of further investigations into the role of γδTCR in cancer immunity and therapeutic strategies.

The transcription factor Aiolos restrains the activation of intestinal intraepithelial lymphocytes.

Intestinal intraepithelial lymphocytes (IELs) exhibit prompt innate-like responses to microenvironmental cues and require strict control of effector functions. Here we showed that Aiolos, an Ikaros zinc-finger family member encoded by Ikzf3, acted as a regulator of IEL activation. Ikzf3-/- CD8αα+ IELs had elevated expression of NK receptors, cytotoxic enzymes, cytokines and chemokines. Single-cell RNA sequencing of Ikzf3-/- and Ikzf3+/+ IELs showed an amplified effector machinery in Ikzf3-/- CD8αα+ IELs compared to Ikzf3+/+ counterparts. Ikzf3-/- CD8αα+ IELs had increased responsiveness to interleukin-15, which explained a substantial part, but not all, of the observed phenotypes. Aiolos binding sites were close to those for the transcription factors STAT5 and RUNX, which promote interleukin-15 signaling and cytolytic programs, and Ikzf3 deficiency partially increased chromatin accessibility and histone acetylation in these regions. Ikzf3 deficiency in mice enhanced susceptibility to colitis, underscoring the relevance of Aiolos in regulating the effector function in IELs.

A tip-predominant distribution of villous intraepithelial lymphocytes is not specific to celiac disease in children with Marsh I lesions.

OBJECTIVES: To assess if the distribution of villous intraepithelial lymphocytes (IELs) in a pediatric cohort with Marsh I histopathology is specific to celiac disease (CeD). METHODS: Multicenter, retrospective case-control study between January 2001 and December 2019 in children (<18 years) with and without CeD with intraepithelial lymphocytosis and normal villous architecture. Pathology specimens were reviewed by 2 study pathologists who were blinded to the final diagnosis. Morphologic features (villous height to crypt depth ratio [Vh:Cd]) and IELs in the villous tip, top, or bottom half of the villus were quantified. RESULTS: Of the 97 children with Marsh I histopathology identified during the study period, 63 were excluded due to an insufficient number of well-oriented villous-crypt complexes or a Vh:Cd less than 2. Villous IELs were measured in 34 cases (14 CeD, 20 non-CeD controls). There was no difference between the non-CeD and CeD groups in the mean IELs at the villous tip (14.0 ± 7.1 vs 11.7 ± 6.0, P = .31), top (46.4 ± 18.4 vs 38.3 ± 10.8, P = .11), or bottom (29.8 ± 16.8 vs 28.5 ± 12.8, P = .80) half of each villus, respectively. CONCLUSIONS: The distribution of IELs in Marsh I lesions is not specific for CeD.

Intraepithelial lymphocytes are associated with epithelial injury in feline intestinal T-cell lymphoma.

Our previous study indicated that cytotoxicity of intraepithelial lymphocytes is a poor prognostic factor in feline intestinal T-cell lymphoma (FITL), but the effect of cytotoxic lymphocytes on mucosal epithelium is still unknown. Thus, we investigated the association between cytotoxic lymphocytes and mucosal epithelium in 71 cases of feline intestinal T-cell lymphoma (FITL): epithelial injury, basement membrane injury, cleaved-caspase-3 positivity of epithelial cells, and the number and Ki67 positivity of intraepithelial lymphocytes in granzyme B (GRB)+ and GRB- FITLs were evaluated. Epithelial injury score and the number of intraepithelial lymphocytes in granzyme B (GRB)+ FITL were significantly higher than those of GRB- FITL (P<0.05, P<0.05), but no significant differences were found in the basement membrane injury score, the percentage of cleaved-caspase-3+ epithelial cells, and the percentage of Ki67+ intraepithelial lymphocytes. There was a significant correlation between the epithelial injury score and the number of intraepithelial lymphocytes (P<0.05), but no significant correlation was observed between the epithelial injury score and Ki67+ percentage of intraepithelial lymphocytes. Because epithelial cell cleaved-caspase-3 positivity was observed in FITL, regardless of GRB expression in lymphocytes, GRB-mediated apoptosis may not contribute to epithelial injury in FITL. The association between increased number of intraepithelial lymphocytes and epithelial injury suggests that intraepithelial lymphocytes infiltration may contribute to epithelial injury in FITL.

Gamma Delta T Cells: Role in Immunotherapy of Hepatocellular Carcinoma.

The most typical type of liver cancer or hepatocellular carcinoma (HCC) develops from hepatocyte loss. Non-alcoholic fatty liver disease (NAFLD), viral hepatitis C and cirrhosis are the leading causes of HCC. With the Hepatitis B vaccine and medicines, there are several treatments for HCC, including liver resection, ablation, transplantation, immunotherapy, gene therapy, radiation embolization, and targeted therapy. Currently, a wide range of studies are carried out on gene therapy to identify biomarkers and pathways, which help us identify the exact stage of the disorder and reduce its effects. γδT cells have recently received much interest as a potential cancer treatment method in adaptive immunotherapy. γδT cells can quickly form connections between receptor and ligand activation. They can clonally expand and are a significant source of cytokines and chemokines. The present review provides a comprehensive understanding on the function of γδT cells in immunotherapies and how they are used to treat HCC.

A distinct topology of BTN3A IgV and B30.2 domains controlled by juxtamembrane regions favors optimal human γδ T cell phosphoantigen sensing.

Butyrophilin (BTN)-3A and BTN2A1 molecules control the activation of human Vγ9Vδ2 T cells during T cell receptor (TCR)-mediated sensing of phosphoantigens (PAg) derived from microbes and tumors. However, the molecular rules governing PAg sensing remain largely unknown. Here, we establish three mechanistic principles of PAg-mediated γδ T cell activation. First, in humans, following PAg binding to the intracellular BTN3A1-B30.2 domain, Vγ9Vδ2 TCR triggering involves the extracellular V-domain of BTN3A2/BTN3A3. Moreover, the localization of both protein domains on different chains of the BTN3A homo-or heteromers is essential for efficient PAg-mediated activation. Second, the formation of BTN3A homo-or heteromers, which differ in intracellular trafficking and conformation, is controlled by molecular interactions between the juxtamembrane regions of the BTN3A chains. Finally, the ability of PAg not simply to bind BTN3A-B30.2, but to promote its subsequent interaction with the BTN2A1-B30.2 domain, is essential for T-cell activation. Defining these determinants of cooperation and the division of labor in BTN proteins improves our understanding of PAg sensing and elucidates a mode of action that may apply to other BTN family members.