The PD-1 Pathway Regulates Development and Function of Memory CD8+ T Cells following Respiratory Viral Infection.

The PD-1 pathway regulates dysfunctional T cells in chronic infection and cancer, but the role of this pathway during acute infection remains less clear. Here, we demonstrate that PD-1 signals are needed for optimal memory. Mice deficient in the PD-1 pathway exhibit impaired CD8+ T cell memory following acute influenza infection, including reduced virus-specific CD8+ T cell numbers and compromised recall responses. PD-1 blockade during priming leads to similar differences early post-infection but without the defect in memory formation, suggesting that timing and/or duration of PD-1 blockade could be tailored to modulate host responses. Our studies reveal a role for PD-1 as an integrator of CD8+ T cell signals that promotes CD8+ T cell memory formation and suggest PD-1 continues to fine-tune CD8+ T cells after they migrate into non-lymphoid tissues. These findings have important implications for PD-1-based immunotherapy, in which PD-1 inhibition may influence memory responses in patients.

Fibroblastic reticular cells enhance T cell metabolism and survival via epigenetic remodeling.

Lymph node fibroblastic reticular cells (FRCs) respond to signals from activated T cells by releasing nitric oxide, which inhibits T cell proliferation and restricts the size of the expanding T cell pool. Whether interactions with FRCs also support the function or differentiation of activated CD8+ T cells is not known. Here we report that encounters with FRCs enhanced cytokine production and remodeled chromatin accessibility in newly activated CD8+ T cells via interleukin-6. These epigenetic changes facilitated metabolic reprogramming and amplified the activity of pro-survival pathways through differential transcription factor activity. Accordingly, FRC conditioning significantly enhanced the persistence of virus-specific CD8+ T cells in vivo and augmented their differentiation into tissue-resident memory T cells. Our study demonstrates that FRCs play a role beyond restricting T cell expansion-they can also shape the fate and function of CD8+ T cells.

CD160 Stimulates CD8+ T Cell Responses and Is Required for Optimal Protective Immunity to Listeria monocytogenes.

CD160 promotes NK cell cytotoxicity and IFN-γ production, but the function of CD160 on CD8+ T cells remains unclear with some studies supporting a coinhibitory role and others a costimulatory role. In this study, we demonstrate that CD160 has a costimulatory role in promoting CD8+ T cell effector functions needed for optimal clearance of oral Listeria monocytogenes infection. CD160-/- mice did not clear oral L. monocytogenes as efficiently as wild type (WT) littermates. WT RAG-/- and CD160-/- RAG-/- mice similarly cleared L. monocytogenes, indicating that CD160 on NK cells does not contribute to impaired L. monocytogenes clearance. Defective L. monocytogenes clearance is due to compromised intraepithelial lymphocytes and CD8+ T cell functions. There was a reduction in the frequencies of granzyme B-expressing intraepithelial lymphocytes in L. monocytogenes-infected CD160-/- mice as compared with WT littermate controls. Similarly, the frequencies of granzyme B-expressing splenic CD8+ T cells and IFN-γ and TNF-α double-producer CD8+ T cells were significantly reduced in L. monocytogenes-infected CD160-/- mice compared with WT littermates. Adoptive transfer studies showed that RAG-/- recipients receiving CD160-/- CD8+ T cells had a higher mortality, exhibited more weight loss, and had a higher bacterial burden compared with RAG-/- recipients receiving WT CD8+ T cells. These findings demonstrate that CD160 provides costimulatory signals to CD8+ T cells needed for optimal CD8+ T cell responses and protective immunity during an acute mucosal bacterial infection.

Targeted reconstruction of T cell receptor sequence from single cell RNA-seq links CDR3 length to T cell differentiation state.

The T cell compartment must contain diversity in both T cell receptor (TCR) repertoire and cell state to provide effective immunity against pathogens. However, it remains unclear how differences in the TCR contribute to heterogeneity in T cell state. Single cell RNA-sequencing (scRNA-seq) can allow simultaneous measurement of TCR sequence and global transcriptional profile from single cells. However, current methods for TCR inference from scRNA-seq are limited in their sensitivity and require long sequencing reads, thus increasing the cost and decreasing the number of cells that can be feasibly analyzed. Here we present TRAPeS, a publicly available tool that can efficiently extract TCR sequence information from short-read scRNA-seq libraries. We apply it to investigate heterogeneity in the CD8+ T cell response in humans and mice, and show that it is accurate and more sensitive than existing approaches. Coupling TRAPeS with transcriptome analysis of CD8+ T cells specific for a single epitope from Yellow Fever Virus (YFV), we show that the recently described 'naive-like' memory population have significantly longer CDR3 regions and greater divergence from germline sequence than do effector-memory phenotype cells. This suggests that TCR usage is associated with the differentiation state of the CD8+ T cell response to YFV.

The microRNA miR-31 inhibits CD8+ T cell function in chronic viral infection.

During infection, antigen-specific T cells undergo tightly regulated developmental transitions controlled by transcriptional and post-transcriptional regulation of gene expression. We found that the microRNA miR-31 was strongly induced by activation of the T cell antigen receptor (TCR) in a pathway involving calcium and activation of the transcription factor NFAT. During chronic infection with lymphocytic choriomeningitis virus (LCMV) clone 13, miR-31-deficent mice recovered from clinical disease, while wild-type mice continued to show signs of disease. This disease phenotype was explained by the presence of larger numbers of cytokine-secreting LCMV-specific CD8+ T cells in miR-31-deficent mice than in wild-type mice. Mechanistically, miR-31 increased the sensitivity of T cells to type I interferons, which interfered with effector T cell function and increased the expression of several proteins related to T cell dysfunction during chronic infection. These studies identify miR-31 as an important regulator of T cell exhaustion in chronic infection.

Epigenetic stability of exhausted T cells limits durability of reinvigoration by PD-1 blockade.

Blocking Programmed Death-1 (PD-1) can reinvigorate exhausted CD8 T cells (TEX) and improve control of chronic infections and cancer. However, whether blocking PD-1 can reprogram TEX into durable memory T cells (TMEM) is unclear. We found that reinvigoration of TEX in mice by PD-L1 blockade caused minimal memory development. After blockade, reinvigorated TEX became reexhausted if antigen concentration remained high and failed to become TMEM upon antigen clearance. TEX acquired an epigenetic profile distinct from that of effector T cells (TEFF) and TMEM cells that was minimally remodeled after PD-L1 blockade. This finding suggests that TEX are a distinct lineage of CD8 T cells. Nevertheless, PD-1 pathway blockade resulted in transcriptional rewiring and reengagement of effector circuitry in the TEX epigenetic landscape. These data indicate that epigenetic fate inflexibility may limit current immunotherapies.

The epigenetic landscape of T cell exhaustion.

Exhausted T cells in cancer and chronic viral infection express distinctive patterns of genes, including sustained expression of programmed cell death protein 1 (PD-1). However, the regulation of gene expression in exhausted T cells is poorly understood. Here, we define the accessible chromatin landscape in exhausted CD8+ T cells and show that it is distinct from functional memory CD8+ T cells. Exhausted CD8+ T cells in humans and a mouse model of chronic viral infection acquire a state-specific epigenetic landscape organized into functional modules of enhancers. Genome editing shows that PD-1 expression is regulated in part by an exhaustion-specific enhancer that contains essential RAR, T-bet, and Sox3 motifs. Functional enhancer maps may offer targets for genome editing that alter gene expression preferentially in exhausted CD8+ T cells.

Binding of the cytoplasmic domain of CD28 to the plasma membrane inhibits Lck recruitment and signaling.

The T cell costimulatory receptor CD28 is required for the full activation of naïve T cells and for the development and maintenance of Foxp3(+) regulatory T (Treg) cells. We showed that the cytoplasmic domain of CD28 was bound to the plasma membrane in resting cells and that ligand binding to CD28 resulted in its release. Membrane binding by the CD28 cytoplasmic domain required two clusters of basic amino acid residues, which interacted with the negatively charged inner leaflet of the plasma membrane. These same clusters of basic residues also served as interaction sites for Lck, a Src family kinase critical for CD28 function. This signaling complex was further stabilized by the Lck-mediated phosphorylation of CD28 Tyr(207) and the subsequent binding of the Src homology 2 (SH2) domain of Lck to this phosphorylated tyrosine. Mutation of the basic clusters in the CD28 cytoplasmic domain reduced the recruitment to the CD28-Lck complex of protein kinase Cθ (PKCθ), which serves as a key effector kinase in the CD28 signaling pathway. Consequently, mutation of either a basic cluster or Tyr(207) impaired CD28 function in mice as shown by the reduced thymic differentiation of FoxP3(+) Treg cells. On the basis of these results, we propose a previously undescribed model for the initiation of CD28 signaling.

Compendium of Immune Signatures Identifies Conserved and Species-Specific Biology in Response to Inflammation.

Gene-expression profiling has become a mainstay in immunology, but subtle changes in gene networks related to biological processes are hard to discern when comparing various datasets. For instance, conservation of the transcriptional response to sepsis in mouse models and human disease remains controversial. To improve transcriptional analysis in immunology, we created ImmuneSigDB: a manually annotated compendium of ∼5,000 gene-sets from diverse cell states, experimental manipulations, and genetic perturbations in immunology. Analysis using ImmuneSigDB identified signatures induced in activated myeloid cells and differentiating lymphocytes that were highly conserved between humans and mice. Sepsis triggered conserved patterns of gene expression in humans and mouse models. However, we also identified species-specific biological processes in the sepsis transcriptional response: although both species upregulated phagocytosis-related genes, a mitosis signature was specific to humans. ImmuneSigDB enables granular analysis of transcriptomic data to improve biological understanding of immune processes of the human and mouse immune systems.

CD39 Expression Identifies Terminally Exhausted CD8+ T Cells.

Exhausted T cells express multiple co-inhibitory molecules that impair their function and limit immunity to chronic viral infection. Defining novel markers of exhaustion is important both for identifying and potentially reversing T cell exhaustion. Herein, we show that the ectonucleotidse CD39 is a marker of exhausted CD8+ T cells. CD8+ T cells specific for HCV or HIV express high levels of CD39, but those specific for EBV and CMV do not. CD39 expressed by CD8+ T cells in chronic infection is enzymatically active, co-expressed with PD-1, marks cells with a transcriptional signature of T cell exhaustion and correlates with viral load in HIV and HCV. In the mouse model of chronic Lymphocytic Choriomeningitis Virus infection, virus-specific CD8+ T cells contain a population of CD39high CD8+ T cells that is absent in functional memory cells elicited by acute infection. This CD39high CD8+ T cell population is enriched for cells with the phenotypic and functional profile of terminal exhaustion. These findings provide a new marker of T cell exhaustion, and implicate the purinergic pathway in the regulation of T cell exhaustion.

Acute myocardial infarction activates distinct inflammation and proliferation pathways in circulating monocytes, prior to recruitment, and identified through conserved transcriptional responses in mice and humans.

AIMS: Monocytes play critical roles in tissue injury and repair following acute myocardial infarction (AMI). Specifically targeting inflammatory monocytes in experimental models leads to reduced infarct size and improved healing. However, data from humans are sparse, and it remains unclear whether monocytes play an equally important role in humans. The aim of this study was to investigate whether the monocyte response following AMI is conserved between humans and mice and interrogate patterns of gene expression to identify regulated functions. METHODS AND RESULTS: Thirty patients (AMI) and 24 control patients (stable coronary atherosclerosis) were enrolled. Female C57BL/6J mice (n = 6/group) underwent AMI by surgical coronary ligation. Myocardial injury was quantified by magnetic resonance imaging (human) and echocardiography (mice). Peripheral monocytes were isolated at presentation and at 48 h. RNA from separated monocytes was hybridized to Illumina beadchips. Acute myocardial infarction resulted in a significant peripheral monocytosis in both species that positively correlated with the extent of myocardial injury. Analysis of the monocyte transcriptome following AMI demonstrated significant conservation and identified inflammation and mitosis as central processes to this response. These findings were validated in both species. CONCLUSIONS: Our findings show that the monocyte transcriptome is conserved between mice and humans following AMI. Patterns of gene expression associated with inflammation and proliferation appear to be switched on prior to their infiltration of injured myocardium suggesting that the specific targeting of inflammatory and proliferative processes in these immune cells in humans are possible therapeutic strategies. Importantly, they could be effective in the hours after AMI.

Inducible RNAi in vivo reveals that the transcription factor BATF is required to initiate but not maintain CD8+ T-cell effector differentiation.

The differentiation of effector CD8(+) T cells is critical for the development of protective responses to pathogens and for effective vaccines. In the first few hours after activation, naive CD8(+) T cells initiate a transcriptional program that leads to the formation of effector and memory T cells, but the regulation of this process is poorly understood. Investigating the role of specific transcription factors (TFs) in determining CD8(+) effector T-cell fate by gene knockdown with RNAi is challenging because naive T cells are refractory to transduction with viral vectors without extensive ex vivo stimulation, which obscures the earliest events in effector differentiation. To overcome this obstacle, we developed a novel strategy to test the function of genes in naive CD8(+) T cells in vivo by creating bone marrow chimera from hematopoietic progenitors transduced with an inducible shRNA construct. Following hematopoietic reconstitution, this approach allowed inducible in vivo gene knockdown in any cell type that developed from this transduced progenitor pool. We demonstrated that lentivirus-transduced progenitor cells could reconstitute normal hematopoiesis and develop into naive CD8(+) T cells that were indistinguishable from wild-type naive T cells. This experimental system enabled induction of efficient gene knockdown in vivo without subsequent manipulation. We applied this strategy to show that the TF BATF is essential for initial commitment of naive CD8(+) T cells to effector development but becomes dispensable by 72h. This approach makes possible the study of gene function in vivo in unperturbed cells of hematopoietic origin that are refractory to viral transduction.

Circulating T follicular regulatory and helper cells have memory-like properties.

Follicular Tregs (Tfr cells) inhibit antibody production, whereas follicular Th cells (Tfh cells) stimulate it. Tfr cells are found in blood; however, relatively little is known about the developmental signals for these cells or their functions. Here we demonstrated that circulating Tfr and Tfh cells share properties of memory cells and are distinct from effector Tfr and Tfh cells found within lymph nodes (LNs). Circulating memory-like Tfh cells were potently reactivated by DCs, homed to germinal centers, and produced more cytokines than did effector LN Tfh cells. Circulating memory-like Tfr cells persisted for long periods of time in vivo and homed to germinal centers after reactivation. Effector LN Tfr cells suppressed Tfh cell activation and production of cytokines, including IL-21, and inhibited class switch recombination and B cell activation. The suppressive function of this population was not dependent on specific antigen. Similar to LN effector Tfr cells, circulating Tfr cells also suppressed B and Tfh cells, but with a much lower capacity. Our data indicate that circulating memory-like Tfr cells are less suppressive than LN Tfr cells and circulating memory-like Tfh cells are more potent than LN effector Tfh cells; therefore, these circulating populations can provide rapid and robust systemic B cell help during secondary antigen exposure.

The transcription factor BATF operates as an essential differentiation checkpoint in early effector CD8+ T cells.

The transcription factor BATF is required for the differentiation of interleukin 17 (IL-17)-producing helper T cells (TH17 cells) and follicular helper T cells (TFH cells). Here we identified a fundamental role for BATF in regulating the differentiation of effector of CD8(+) T cells. BATF-deficient CD8(+) T cells showed profound defects in effector population expansion and underwent proliferative and metabolic catastrophe early after encountering antigen. BATF, together with the transcription factors IRF4 and Jun proteins, bound to and promoted early expression of genes encoding lineage-specific transcription-factors (T-bet and Blimp-1) and cytokine receptors while paradoxically repressing genes encoding effector molecules (IFN-γ and granzyme B). Thus, BATF amplifies T cell antigen receptor (TCR)-dependent expression of transcription factors and augments the propagation of inflammatory signals but restrains the expression of genes encoding effector molecules. This checkpoint prevents irreversible commitment to an effector fate until a critical threshold of downstream transcriptional activity has been achieved.

The transcription factors T-bet and Runx are required for the ontogeny of pathogenic interferon-γ-producing T helper 17 cells.

T helper 17 (Th17) cells can give rise to interleukin-17A (IL-17A)- and interferon (IFN)-γ-double-producing cells that are implicated in development of autoimmune diseases. However, the molecular mechanisms that govern generation of IFN-γ-producing Th17 cells are unclear. We found that coexpression of the Th1 and Th17 cell master transcription factors, T-bet and retinoid-related orphan receptor gamma-t (RORγt), respectively, did not generate Th cells with robust IL-17 and IFN-γ expression. Instead, development of IFN-γ-producing Th17 cells required T-bet and Runx1 or Runx3. IL-12-stimulated Th17 cells upregulated Runx1, which bound to the Ifng locus in a T-bet-dependent manner. Reciprocally, T-bet or Runx1 deficiency or inhibition of Runx transcriptional activity impaired the development of IFN-γ-producing Th17 cells during experimental autoimmune encephalomyelitis, which correlated with substantially ameliorated disease course. Thus, our studies identify a critical role for T-bet and Runx transcription factors in the generation of pathogenic IFN-γ-producing Th17 cells.

MYC, a downstream target of BRD-NUT, is necessary and sufficient for the blockade of differentiation in NUT midline carcinoma.

NUT midline carcinoma (NMC) is an aggressive type of squamous cell carcinoma that is defined by the presence of BRD-NUT fusion oncogenes, which encode chimeric proteins that block differentiation and maintain tumor growth. BRD-NUT oncoproteins contain two bromodomains whose binding to acetylated histones is required for the blockade of differentiation in NMC, but the mechanisms by which BRD-NUT act remain uncertain. Here, we provide evidence that MYC is a key downstream target of BRD4-NUT. Expression profiling of NMCs shows that the set of genes whose expression is maintained by BRD4-NUT is highly enriched for MYC upregulated genes, and MYC and BRD4-NUT protein expression is strongly correlated in primary NMCs. More directly, we find that BRD4-NUT associates with the MYC promoter and is required to maintain MYC expression in NMC cell lines. Moreover, both siRNA knockdown of MYC and a dominant-negative form of MYC, omomyc, induce differentiation of NMC cells. Conversely, differentiation of NMC cells induced by knockdown of BRD4-NUT is abrogated by enforced expression of MYC. Together, these findings suggest that MYC is a downstream target of BRD4-NUT that is required for maintenance of NMC cells in an undifferentiated, proliferative state. Our findings support a model in which dysregulation of MYC by BRD-NUT fusion proteins has a central role in the pathogenesis of NMC.