Intratumoral immune triads are required for immunotherapy-mediated elimination of solid tumors.

Tumor-specific CD8+ T cells are frequently dysfunctional and unable to halt tumor growth. We investigated whether tumor-specific CD4+ T cells can be enlisted to overcome CD8+ T cell dysfunction within tumors. We find that the spatial positioning and interactions of CD8+ and CD4+ T cells, but not their numbers, dictate anti-tumor responses in the context of adoptive T cell therapy as well as immune checkpoint blockade (ICB): CD4+ T cells must engage with CD8+ T cells on the same dendritic cell during the effector phase, forming a three-cell-type cluster (triad) to license CD8+ T cell cytotoxicity and cancer cell elimination. When intratumoral triad formation is disrupted, tumors progress despite equal numbers of tumor-specific CD8+ and CD4+ T cells. In patients with pleural mesothelioma treated with ICB, triads are associated with clinical responses. Thus, CD4+ T cells and triads are required for CD8+ T cell cytotoxicity during the effector phase and tumor elimination.

Intratumoral immune triads are required for adoptive T cell therapy-mediated elimination of solid tumors.

Tumor-reactive CD8 T cells found in cancer patients are frequently dysfunctional, unable to halt tumor growth. Adoptive T cell transfer (ACT), the administration of large numbers of in vitro-generated cytolytic tumor-reactive CD8 T cells, is an important cancer immune therapy being pursued. However, a limitation of ACT is that transferred CD8 T cells often rapidly lose effector function, and despite exciting results in certain malignancies, few ACT clinical trials have shown responses in solid tumors. Here, we developed preclinical cancer mouse models to investigate if and how tumor-specific CD4 T cells can be enlisted to overcome CD8 T cell dysfunction in the setting of ACT. In situ confocal microscopy of color-coded cancer cells, tumor-specific CD8 and CD4 T cells, and antigen presenting cells (APC), combined with functional studies, revealed that the spatial positioning and interactions of CD8 and CD4 T cells, but not their numbers, dictates ACT efficacy and anti-tumor responses. We uncover a new role of antigen-specific CD4 T cells in addition to the known requirement for CD4 T cells during priming/activation of naïve CD8 T cells. CD4 T cells must co-engage with CD8 T cells and APC cross-presenting CD8- and CD4-tumor antigens during the effector phase, forming a three-cell-cluster (triad), to license CD8 T cell cytotoxicity and mediate cancer cell elimination. Triad formation transcriptionally and epigenetically reprogram CD8 T cells, prevent T cell dysfunction/exhaustion, and ultimately lead to the elimination of large established tumors and confer long-term protection from recurrence. When intratumoral triad formation was disrupted, adoptively transferred CD8 T cells could not be reprogrammed, and tumors progressed despite equal numbers of tumor-infiltrating CD8 and CD4 T cells. Strikingly, the formation of CD4 T cell::CD8 T cell::APC triads in tumors of patients with lung cancers treated with immune checkpoint blockade was associated with clinical responses, but not CD4::APC dyads or overall numbers of CD8 or CD4 T cells, demonstrating the importance of triads in non-ACT settings in humans. Our work uncovers intratumoral triads as a key requirement for anti-tumor immunity and a new role for CD4 T cells in CD8 T cell cytotoxicity and cancer cell eradication.

An autoimmune stem-like CD8 T cell population drives type 1 diabetes.

CD8 T cell-mediated autoimmune diseases result from the breakdown of self-tolerance mechanisms in autoreactive CD8 T cells1. How autoimmune T cell populations arise and are sustained, and the molecular programmes defining the autoimmune T cell state, are unknown. In type 1 diabetes, ß-cell-specific CD8 T cells destroy insulin-producing ß-cells. Here we followed the fate of ß-cell-specific CD8 T cells in non-obese diabetic mice throughout the course of type 1 diabetes. We identified a stem-like autoimmune progenitor population in the pancreatic draining lymph node (pLN), which self-renews and gives rise to pLN autoimmune mediators. pLN autoimmune mediators migrate to the pancreas, where they differentiate further and destroy ß-cells. Whereas transplantation of as few as 20 autoimmune progenitors induced type 1 diabetes, as many as 100,000 pancreatic autoimmune mediators did not. Pancreatic autoimmune mediators are short-lived, and stem-like autoimmune progenitors must continuously seed the pancreas to sustain ß-cell destruction. Single-cell RNA sequencing and clonal analysis revealed that autoimmune CD8 T cells represent unique T cell differentiation states and identified features driving the transition from autoimmune progenitor to autoimmune mediator. Strategies aimed at targeting the stem-like autoimmune progenitor pool could emerge as novel and powerful immunotherapeutic interventions for type 1 diabetes.

TCR signal strength defines distinct mechanisms of T cell dysfunction and cancer evasion.

T cell receptor (TCR) signal strength is a key determinant of T cell responses. We developed a cancer mouse model in which tumor-specific CD8 T cells (TST cells) encounter tumor antigens with varying TCR signal strength. High-signal-strength interactions caused TST cells to up-regulate inhibitory receptors (IRs), lose effector function, and establish a dysfunction-associated molecular program. TST cells undergoing low-signal-strength interactions also up-regulated IRs, including PD1, but retained a cell-intrinsic functional state. Surprisingly, neither high- nor low-signal-strength interactions led to tumor control in vivo, revealing two distinct mechanisms by which PD1hi TST cells permit tumor escape; high signal strength drives dysfunction, while low signal strength results in functional inertness, where the signal strength is too low to mediate effective cancer cell killing by functional TST cells. CRISPR-Cas9-mediated fine-tuning of signal strength to an intermediate range improved anti-tumor activity in vivo. Our study defines the role of TCR signal strength in TST cell function, with important implications for T cell-based cancer immunotherapies.

Cytotoxic CD8+ T lymphocytes expressing ALS-causing SOD1 mutant selectively trigger death of spinal motoneurons.

Adaptive immune response is part of the dynamic changes that accompany motoneuron loss in amyotrophic lateral sclerosis (ALS). CD4+ T cells that regulate a protective immunity during the neurodegenerative process have received the most attention. CD8+ T cells are also observed in the spinal cord of patients and ALS mice although their contribution to the disease still remains elusive. Here, we found that activated CD8+ T lymphocytes infiltrate the central nervous system (CNS) of a mouse model of ALS at the symptomatic stage. Selective ablation of CD8+ T cells in mice expressing the ALS-associated superoxide dismutase-1 (SOD1)G93A mutant decreased spinal motoneuron loss. Using motoneuron-CD8+ T cell coculture systems, we found that mutant SOD1-expressing CD8+ T lymphocytes selectively kill motoneurons. This cytotoxicity activity requires the recognition of the peptide-MHC-I complex (where MHC-I represents major histocompatibility complex class I). Measurement of interaction strength by atomic force microscopy-based single-cell force spectroscopy demonstrated a specific MHC-I-dependent interaction between motoneuron and SOD1G93A CD8+ T cells. Activated mutant SOD1 CD8+ T cells produce interferon-γ, which elicits the expression of the MHC-I complex in motoneurons and exerts their cytotoxic function through Fas and granzyme pathways. In addition, analysis of the clonal diversity of CD8+ T cells in the periphery and CNS of ALS mice identified an antigen-restricted repertoire of their T cell receptor in the CNS. Our results suggest that self-directed immune response takes place during the course of the disease, contributing to the selective elimination of a subset of motoneurons in ALS.

Integrin ß1 Optimizes Diabetogenic T Cell Migration and Function in the Pancreas.

T cell search behavior is dictated by their need to encounter their specific antigen to eliminate target cells. However, mechanisms controlling effector T cell motility are highly tissue-dependent. Specifically, how diabetogenic T cells encounter their target beta cells in dispersed islets throughout the pancreas (PA) during autoimmune diabetes remains unclear. Using intra-vital 2-photon microscopy in a mouse model of diabetes, we found that CXCR3 chemokine downregulated CD8+ T cell motility specifically within islets, promoting effector cell confinement to their target sites. By contrast, T cell velocity and directionality in the exocrine tissue were enhanced along blood vessels and extracellular matrix fibers. This guided migration implicated integrin-dependent interactions, since integrin blockade impaired exocrine T cell motility. In addition, integrin ß1 blockade decreased CD4+ T cell effector phenotype specifically in the PA. Thus, we unveil an important role for integrins in the PA during autoimmune diabetes that may have important implications for the design of new therapies.

Gilz-Activin A as a Novel Signaling Axis Orchestrating Mesenchymal Stem Cell and Th17 Cell Interplay.

Mesenchymal stem cells (MSC) are highly immunosuppressive cells able to reduce chronic inflammation through the active release of mediators. Recently, we showed that glucocorticoid-induced leucine zipper (Gilz) expression by MSC is involved in their therapeutic effect by promoting the generation of regulatory T cells. However, the mechanisms underlying this pivotal role of Gilz remain elusive. Methods and Results In this study, we have uncovered evidence that Gilz modulates the phenotype and function of Th1 and Th17 cells likely by upregulating the level of Activin A and NO2 secreted by MSC. Adoptive transfer experiments sustained this Gilz-dependent suppressive effect of MSC on Th1 and Th17 cell functions. In immunoregulatory MSC, obtained by priming with IFN-γ and TNF-α, Gilz was translocated to the nucleus and bound to the promoters of inos and Activin ßA to induce their expression. The increased expression of Activin A directly impacted on Th17 cells fate by repressing their differentiation program through the activation of Smad3/2 and enhancing IL-10 production. Conclusion Our results reveal how Gilz controls inos and Activin ßA gene expression to ultimately assign immunoregulatory status to MSC able to repress the pathogenic Th17 cell differentiation program and uncover Activin A as a novel mediator of MSC in this process.

CD4+ T Helper Cells Play a Key Role in Maintaining Diabetogenic CD8+ T Cell Function in the Pancreas.

Autoreactive CD8+ and CD4+ T cells have been assigned independent key roles in the destruction of insulin-producing beta cells resulting in type 1 diabetes. Although CD4 help for the generation of efficient CD8+ T cell responses in lymphoid tissue has been extensively described, whether these two cell populations cooperate in islet destruction in situ remains unclear. By using intravital 2-photon microscopy in a mouse model of diabetes, we visualized both effector T cell populations in the pancreas during disease onset. CD4+ T helper cells displayed a much higher arrest in the exocrine tissue than islet-specific CD8+ T cells. This increased arrest was major histocompatibility complex (MHC) class II-dependent and locally correlated with antigen-presenting cell recruitment. CD8+ T cells deprived of continued CD4 help specifically in the pancreas, through blocking MHC class II recognition, failed to maintain optimal effector functions, which contributed to hamper diabetes progression. Thus, we provide novel insight in the cellular mechanisms regulating effector T cell functionality in peripheral tissues with important implications for immunotherapies.

Metabolism Regulates Exposure of Pancreatic Islets to Circulating Molecules In Vivo.

Pancreatic ß-cells modulate insulin secretion through rapid sensing of blood glucose and integration of gut-derived signals. Increased insulin demand during pregnancy and obesity alters islet function and mass and leads to gestational diabetes mellitus and type 2 diabetes in predisposed individuals. However, it is unclear how blood-borne factors dynamically access the islets of Langerhans. Thus, understanding the changes in circulating molecule distribution that accompany compensatory ß-cell expansion may be key to developing novel antidiabetic therapies. Here, using two-photon microscopy in vivo in mice, we demonstrate that islets are almost instantly exposed to peaks of circulating molecules, which rapidly pervade the tissue before clearance. In addition, both gestation and short-term high-fat-diet feeding decrease molecule extravasation and uptake rates in vivo in islets, independently of ß-cell expansion or islet blood flow velocity. Together, these data support a role for islet vascular permeability in shaping ß-cell adaptive responses to metabolic demand by modulating the access and sensing of circulating molecules.

Systemic LPS Translocation Activates Cross-Presenting Dendritic Cells but Is Dispensable for the Breakdown of CD8+ T Cell Peripheral Tolerance in Irradiated Mice.

Lymphodepletion is currently used to enhance the efficacy of cytotoxic T lymphocyte adoptive transfer immunotherapy against cancer. This beneficial effect of conditioning regimens is due, at least in part, to promoting the breakdown of peripheral CD8+ T cell tolerance. Lymphodepletion by total body irradiation induces systemic translocation of commensal bacteria LPS from the gastrointestinal tract. Since LPS is a potent activator of the innate immune system, including antigen presenting dendritic cells, we hypothesized that LPS translocation could be required for the breakdown of peripheral tolerance observed in irradiated mice. To address this issue, we have treated irradiated mice with antibiotics in order to prevent LPS translocation and utilized them in T cell adoptive transfer experiments. Surprisingly, we found that despite of completely blocking LPS translocation into the bloodstream, antibiotic treatment did not prevent the breakdown of peripheral tolerance. Although irradiation induced the activation of cross-presenting CD8+ dendritic cells in the lymphoid tissue, LPS could not solely account for this effect. Activation of dendritic cells by mechanisms other than LPS translocation is sufficient to promote the differentiation of potentially autoreactive CD8+ T cells into effectors in irradiated mice. Our data indicate that LPS translocation is dispensable for the breakdown of CD8+ T cell tolerance in irradiated mice.