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Increasing evidence indicates close interaction between immune cells and the brain, revising the traditional view of the immune privilege of the brain. However, the specific mechanisms by which immune cells promote normal neural function are not entirely understood. Mucosal-associated invariant T cells (MAIT cells) are a unique type of innate-like T cell with molecular and functional properties that remain to be better characterized. In the present study, we report that MAIT cells are present in the meninges and express high levels of antioxidant molecules. MAIT cell deficiency in mice results in the accumulation of reactive oxidative species in the meninges, leading to reduced expression of junctional protein and meningeal barrier leakage. The presence of MAIT cells restricts neuroinflammation in the brain and preserves learning and memory. Together, our work reveals a new functional role for MAIT cells in the meninges and suggests that meningeal immune cells can help maintain normal neural function by preserving meningeal barrier homeostasis and integrity.

TCRαβ+ CD8α+ CD8β− intraepithelial lymphocytes (CD8αα IEL) differentiate from thymic IEL precursors (IELp) and contribute to gut homeostasis. However, their development remains poorly understood. Here we show that the zinc finger transcription factor LRF, encoded by Zbtb7a, is highly expressed in both CD8αα+ IELs and in IELp and serves in a cell-intrinsic manner to promote IEL differentiation. Mice genetically deficient for LRF (Cd4-Cre Zbtb7afl/fl) lack CD8αα+ IELs, contrasting with largely conserved numbers of conventional CD8αβ+ T cells in the thymus and the secondary lymphoid organs. Unlike their wild-type counterparts, LRF-deficient IELp failed to prevent colitis caused by adoptive transfer of CD4+ T cells in T cell-deficient mice. LRF disruption affects neither the development of thymic IELp, nor their in vitro expansion or differentiation in response to IL-15. However, Cd4-Cre Zbtb7afl/fl mice accumulate CD8αα+ T cells in the spleen, unlike wild-type controls, suggesting impaired migration of IELp to the gut. Single-cell RNA sequencing found LRF necessary for the expression of genes characteristic of the most mature IELp, including Itgb7, encoding the β7 subunit of α4β7. Chromatin immunoprecipitation and gene regulatory network analyses both defined Itgb7 as an LRF target. Our study identifies LRF as an essential transcriptional regulator of IELp maturation in the thymus and subsequent migration to the intestinal epithelium.

The central nervous system (CNS) has historically been viewed as an immunologically privileged site, but recent studies have uncovered a vast landscape of immune cells that reside primarily along its borders. While microglia are largely responsible for surveying the parenchyma, CNS barrier sites are inhabited by a plethora of different innate and adaptive immune cells that participate in everything from the defense against microbes to the maintenance of neural function. Static and dynamic imaging studies have revolutionized the field of neuroimmunology by providing detailed maps of CNS immune cells as well as information about how these cells move, organize, and interact during steady state and inflammatory conditions. These studies have also redefined our understanding of neural immune interactions at a cellular level and reshaped our conceptual view of immune privilege in this specialized compartment. This review will focus on insights gained using imaging techniques in the field of neuroimmunology, with an emphasis on anatomy and CNS immune dynamics during homeostasis, infectious diseases, injuries, and aging.

The surface of the central nervous system (CNS) is protected by the meninges, which contain a dense network of meningeal macrophages (MMs). Here, we examined the role of tissue-resident MM in viral infection. MHC-II� MM were abundant neonatally, whereas MHC-II+ MM appeared over time. These barrier macrophages differentially responded to in vivo peripheral challenges such as LPS, SARS-CoV-2, and lymphocytic choriomeningitis virus (LCMV). Peripheral LCMV infection, which was asymptomatic, led to a transient infection and activation of the meninges. Mice lacking macrophages but conserving brain microglia, or mice bearing macrophage-specific deletion of Stat1 or Ifnar, exhibited extensive viral spread into the CNS. Transcranial pharmacological depletion strategies targeting MM locally resulted in several areas of the meninges becoming infected and fatal meningitis. Low numbers of MHC-II+ MM, which is seen upon LPS challenge or in neonates, corelated with higher viral load upon infection. Thus, MMs protect against viral infection and may present targets for therapeutic manipulation., Authors affiliations: (1) Centre d'Immunologie de Marseille-Luminy, Aix Marseille Université, Inserm, CNRS, Marseille, France (2) Ecole Normale Supérieure de Lyon, Université Claude Bernard – Lyon I, Université de Lyon, Lyon, France (3)Centre d'Immunophénomique, Aix Marseille Université , Inserm, CNRS, Marseille, France (4) TERI (Tumor Escape, Resistance and Immunity) Department, Centre de Recherche en Cancérologie de Lyon, Centre Léon Bérard, Université Claude Bernard – Lyon 1, Université de Lyon, Inserm, CNRS, Lyon, France (5) Viral Immunology and Intravital Imaging Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD 20892, USA (6) Shanghai Institute of Immunology, Shanghai Jiao Tong University School of Medicine, Shanghai, China (7) University Paris-Saclay, Inserm U1015, Gustave Roussy Cancer Center, Villejuif, France (8) Singapore Immunology Network, A*STAR, Singapore, Singapore (9) Centre for Inflammation Biology and Cancer Immunology, Cancer Research UK King's Health Partners Centre, School of Immunology and Microbial Sciences, King's College London, London SE1 1UL, UK (10) Lead contact

Traumatic brain injury (TBI) and cerebrovascular injury are leading causes of disability and mortality worldwide. Systemic infections often accompany these disorders and can worsen outcomes. Recovery after brain injury depends on innate immunity, but the effect of infections on this process is not well understood. Here, we demonstrate that systemically introduced microorganisms and microbial products interfered with meningeal vascular repair after TBI in a type I interferon (IFN-I)-dependent manner, with sequential infections promoting chronic disrepair. Mechanistically, we discovered that MDA5-dependent detection of an arenavirus encountered after TBI disrupted pro-angiogenic myeloid cell programming via induction of IFN-I signaling. Systemic viral infection similarly blocked restorative angiogenesis in the brain parenchyma after intracranial hemorrhage, leading to chronic IFN-I signaling, blood–brain barrier leakage and a failure to restore cognitive–motor function. Our findings reveal a common immunological mechanism by which systemic infections deviate reparative programming after central nervous system injury and offer a new therapeutic target to improve recovery. Traumatic brain injury and stroke are commonly complicated by systemic infections, which impede recovery and lead to poor clinical outcomes. Using a mouse model, McGavern and colleagues show systemic microbial infections impair central nervous system revascularization and repair by a mechanism involving type I interferon signaling.

During chronic infection and cancer, a self-renewing CD8+ T cell subset maintains long-term immunity and is critical to the effectiveness of immunotherapy. These stem-like CD8+ T cells diverge from other CD8+ subsets early after chronic viral infection. However, pathways guarding stem-like CD8+ T cells against terminal exhaustion remain unclear. Here, we show that the gene encoding transcriptional repressor BACH2 is transcriptionally and epigenetically active in stem-like CD8+ T cells but not terminally exhausted cells early after infection. BACH2 overexpression enforced stem-like cell fate, whereas BACH2 deficiency impaired stem-like CD8+ T cell differentiation. Single-cell transcriptomic and epigenomic approaches revealed that BACH2 established the transcriptional and epigenetic programs of stem-like CD8+ T cells. In addition, BACH2 suppressed the molecular program driving terminal exhaustion through transcriptional repression and epigenetic silencing. Thus, our study reveals a new pathway that enforces commitment to stem-like CD8+ lineage and prevents an alternative terminally exhausted cell fate.

Costimulatory CD40 plays an essential role in autoimmune diseases, including experimental autoimmune encephalomyelitis (EAE), a murine model of human multiple sclerosis (MS). However, how CD40 drives autoimmune disease pathogenesis is not well defined. Here, we used a conditional knockout approach to determine how CD40 orchestrates a CNS autoimmune disease induced by recombinant human myelin oligodendrocyte glycoprotein (rhMOG). We found that deletion of CD40 in either dendritic cells (DCs) or B cells profoundly reduced EAE disease pathogenesis. Mechanistically, CD40 expression on DCs was required for priming pathogenic Th cells in peripheral draining lymph nodes and promoting their appearance in the CNS. By contrast, B cell CD40 was essential for class-switched MOG-specific Ab production, which played a crucial role in disease pathogenesis. In fact, passive transfer of MOG-immune serum or IgG into mice lacking CD40 on B cells but not DCs reconstituted autoimmune disease, which was associated with inundation of the spinal cord parenchyma by Ig and complement. These data demonstrate that CD40 supports distinct effector programs in B cells and DCs that converge to drive a CNS autoimmune disease and identify targets for intervention.

Neuroanatomical barriers with physical, chemical, and immunological properties play an essential role in preventing the spread of peripheral infections into the CNS. A failure to contain pathogens within these barriers can result in very serious CNS diseases. CNS barriers are inhabited by an elaborate conglomerate of innate and adaptive immune cells that are highly responsive to environmental challenges. The CNS and its barriers can also be protected by memory T and B cells elicited by prior infection or vaccination. Here, we discuss the different CNS barriers from a developmental, anatomical, and immunological standpoint and summarize our current understanding of how memory cells protect the CNS compartment. We then discuss a contemporary challenge to CNS-barrier system (SARS-CoV-2 infection) and highlight approaches to promote immunological protection of the CNS via vaccination.

T cell specificity and function are linked during development, as MHC-II-specific TCR signals generate CD4 helper T cells and MHC-I-specific TCR signals generate CD8 cytotoxic T cells, but the basis remains uncertain. We now report that switching coreceptor proteins encoded byCd4andCd8gene loci functionally reverses the T cell immune system, generating CD4 cytotoxic and CD8 helper T cells. Such functional reversal reveals that coreceptor proteins promote the helper-lineage fate when encoded byCd4, but promote the cytotoxic-lineage fate when encoded inCd8—regardless of the coreceptor proteins each locus encodes. Thus, T cell lineage fate is determined bycis-regulatory elements in coreceptor gene loci and is not determined by the coreceptor proteins they encode, invalidating coreceptor signal strength as the basis of lineage fate determination. Moreover, we consider that evolution selected the particular coreceptor proteins thatCd4andCd8gene loci encode to avoid generating functionally reversed T cells because they fail to promote protective immunity against environmental pathogens.

Progenitor-like CD8+ T cells mediate long-term immunity to chronic infection and cancer and respond potently to immune checkpoint blockade. These cells share transcriptional regulators with memory precursor cells, including T cell-specific transcription factor 1 (TCF1), but it is unclear whether they adopt distinct programs to adapt to the immunosuppressive environment. By comparing the single-cell transcriptomes and epigenetic profiles of CD8+ T cells responding to acute and chronic viral infections, we found that progenitor-like CD8+ T cells became distinct from memory precursor cells before the peak of the T cell response. We discovered a coexpression gene module containing Tox that exhibited higher transcriptional activity associated with more abundant active histone marks in progenitor-like cells than memory precursor cells. Moreover, thymocyte selection-associated high mobility group box protein TOX (TOX) promoted the persistence of antiviral CD8+ T cells and was required for the programming of progenitor-like CD8+ T cells. Thus, long-term CD8+ T cell immunity to chronic viral infection requires unique transcriptional and epigenetic programs associated with the transcription factor TOX.

Infiltration of the central nervous system (CNS) parenchyma by peripheral B cells provides an important defense against neurotropic infections but also contributes to different neuroinflammatory diseases. While the periphery is known to support CNS humoral immune responses, recent studies have demonstrated that the meninges receive a steady influx of B cells from the skull bone marrow and provide an important niche for B-cell development and tolerance. Meningeal vascular barriers like the dural venous sinuses are also inhabited by plasma cells that secrete antibodies locally and sequester pathogens, preventing their entry into the CNS. However, these beneficial aspects of CNS humoral immunity are sometimes offset by pathological responses such as the development of meningeal ectopic lymphoid structures that promote autoimmune damage in the underlying parenchyma. In this review, we discuss recent advances in our understanding of CNS humoral immunity, with an emphasis on anatomy, infections, and autoimmune disease.

During the immune response, CD4+ T cells differentiate into distinct effector subtypes, including follicular helper T (Tfh) cells that help B cells, and into memory cells. Tfh and memory cells are required for long-term immunity; both depend on the transcription factor Bcl6, raising the question whether they differentiate through similar mechanisms. Here, using single-cell RNA and ATAC sequencing, we show that virus-responding CD4+ T cells lacking both Bcl6 and Blimp1 can differentiate into cells with transcriptomic, chromatin accessibility, and functional attributes of memory cells but not of Tfh cells. Thus, Bcl6 promotes memory cell differentiation primarily through its repression of Blimp1. These findings demonstrate that distinct mechanisms underpin the differentiation of memory and Tfh CD4+ cells and define the Bcl6–Blimp1 axis as a potential target for promoting long-term memory T cell differentiation.

Summary The generation of high-affinity neutralizing antibodies, the objective of most vaccine strategies, occurs in B cells within germinal centers (GCs) and requires rate-limiting “help” from follicular helper CD4+ T (Tfh) cells. Although Tfh differentiation is an attribute of MHC II-restricted CD4+ T cells, the transcription factors driving Tfh differentiation, notably Bcl6, are not restricted to CD4+ T cells. Here, we identified a requirement for the CD4+-specific transcription factor Thpok during Tfh cell differentiation, GC formation, and antibody maturation. Thpok promoted Bcl6 expression and bound to a Thpok-responsive region in the first intron of Bcl6. Thpok also promoted the expression of Bcl6-independent genes, including the transcription factor Maf, which cooperated with Bcl6 to mediate the effect of Thpok on Tfh cell differentiation. Our findings identify a transcriptional program that links the CD4+ lineage with Tfh differentiation, a limiting factor for efficient B cell responses, and suggest avenues to optimize vaccine generation.

Intrinsic complement C3 activity is integral to human T helper type 1 (Th1) and cytotoxic T cell responses. Increased or decreased intracellular C3 results in autoimmunity and infections, respectively. The mechanisms regulating intracellular C3 expression remain undefined. We identified complement, including C3, as among the most significantly enriched biological pathway in tissue-occupying cells. We generated C3-reporter mice and confirmed that C3 expression was a defining feature of tissue-immune cells, including T cells and monocytes, occurred during transendothelial diapedesis, and depended on integrin lymphocyte-function-associated antigen 1 (LFA-1) signals. Immune cells from patients with leukocyte adhesion deficiency type 1 (LAD-1) had reduced C3 transcripts and diminished effector activities, which could be rescued proportionally by intracellular C3 provision. Conversely, increased C3 expression by T cells from arthritis patients correlated with disease severity. Our study defines integrins as key controllers of intracellular complement, demonstrates that perturbations in the LFA-1-C3-axis contribute to primary immunodeficiency, and identifies intracellular C3 as biomarker of severity in autoimmunity.

Patients with Activated-PI3Kd Syndrome (APDS) present with sinopulmonary infections, lymphadenopathy and CMV and/or EBV viremia, yet why patients fail to clear certain viral infections remains poorly understood. Using APDS patient samples and a mouse model (Pik3cdE1020K/+ mice), we demonstrate that, upon activation, Pik3cdE1020K/+ CD8+ T cells exhibit exaggerated features of short-lived effectors both in vitro and post-viral infection, associated with increased Fas-mediated apoptosis due to sustained phosphorylation of FoxO1 and derepression of FasL. In addition, Pik3cdE1020K/+ CD8+ T cells exhibit enhanced mTORC1 and c-Myc signatures; metabolic perturbations; and reorientation of their chromatin landscape. Conversely, Pik3cdE1020K/+ CD8+ T cells failed to sustain expression of proteins critical for maintenance of long-lived memory cells, including TCF1. Strikingly, activated Pik3cdE1020K/+ CD8+ T cells exhibit altered transcriptional and epigenetic circuits characterized by a pronounced IL-2/STAT5 signature associated with heightened IL-2 responses that prevented differentiation to memory-like cells in the presence of IL-15. Our data position PI3Kd as a central driver integrating multiple signaling circuits that promote terminal CD8+ T cell effector differentiation at the expense of memory and long-lived T cell responses. This work was funded in part by the Intramural Research Program of NIAID, NIH.

T follicular helper (Tfh) cells are a specialized population of CD4+ T cells that provide help to B cells for the formation and maintenance germinal centers, and the production of high affinity class-switched antibodies, long-lived plasma cells, and memory B cells. As such, Tfh cells are essential for the generation of successful long-term humoral immunity and memory responses to vaccination and infection. Conversely, overproduction of Tfh cells has been associated with the generation of autoantibodies and autoimmunity. Data from gene-targeted mice, pharmacological inhibitors, as well as studies of human and mice expressing activating mutants have revealed that PI3Kδ is a key regulator of Tfh cell differentiation, acting downstream of ICOS to facilitate inactivation of FOXO1, repression of Klf2 and induction of Bcl6. Nonetheless, here we show that after acute LCMV infection, WT and activated-PI3Kδ mice (Pik3cdE1020K/+) show comparable ratios of Tfh:Th1 viral specific CD4+ T cells, despite higher polyclonal Tfh cells in Pik3cdE1020K/+ mice. Thus, the idea that PI3K activity primarily drives Tfh cell differentiation may be an oversimplification and PI3K-mediated pathways are likely to integrate multiple signals to promote distinct effector T cell lineages. The consequences of dysregulated Tfh cell generation will be discussed in the context of the human primary immunodeficiency “Activated PI3K-delta Syndrome” (APDS), also known as “p110 delta-activating mutation causing senescent T cells, lymphadenopathy and immunodeficiency” (PASLI). Overall, these data underscore a major role for PI3K signaling in the orchestration of T lymphocyte responses.

International audience; The transcription factor T-bet has been associated with increased susceptibility to systemic and organ-specific autoimmunity, but the mechanism by which T-bet expression promotes neuroinflammation remains unknown. In this study, we demonstrate a cardinal role of T-bet-dependent NKp46(+) innate lymphoid cells (ILCs) in the initiation of CD4(+) T(H)17-mediated neuroinflammation. Loss of T-bet specifically in NKp46(+) ILCs profoundly impaired the ability of myelin-reactive T(H)17 cells to invade central nervous system (CNS) tissue and protected the mice from autoimmunity. T-bet-dependent NKp46(+) ILCs localized in the meninges and acted as chief coordinators of meningeal inflammation by inducing the expression of proinflammatory cytokines, chemokines and matrix metalloproteinases, which together facilitated T cell entry into CNS parenchyma. Our findings uncover a detrimental role of T-bet-dependent NKp46(+) ILCs in the development of CNS autoimmune disease.

Tissue macrophages have an embryonic origin and can be replenished in some tissues under steady-state conditions by blood monocytes. However, little is known about the residency and properties of infiltrating monocytes after an inflammatory challenge. The meninges of the central nervous system (CNS) are populated by a dense network of macrophages that act as resident immune sentinels. Here we show that, following lymphocytic choriomeningitis virus infection, resident meningeal macrophages (MMs) acquired viral antigen and interacted directly with infiltrating cytotoxic T lymphocytes, which led to macrophage depletion. Concurrently, the meninges were infiltrated by inflammatory monocytes that engrafted the meningeal niche and remained in situ for months after viral clearance. This engraftment led to interferon-γ-dependent functional changes in the pool of MMs, including loss of bacterial and immunoregulatory sensors. Collectively, these data indicate that peripheral monocytes can engraft the meninges after an inflammatory challenge, imprinting the compartment with long-term defects in immune function.

IL-7 is known to be vital for T cell homeostasis but has previously been presumed to be dispensable for TCR-induced activation. Here, we show that IL-7 is critical for the initial activation of CD4(+) T cells in that it provides some of the necessary early signaling components, such as activated STAT5 and Akt. Accordingly, short-term in vivo IL-7Rα blockade inhibited the activation and expansion of autoantigen-specific CD4(+) T cells and, when used to treat experimental autoimmune encephalomyelitis (EAE), prevented and ameliorated disease. Our studies demonstrate that IL-7 signaling is a prerequisite for optimal CD4(+) T cell activation and that IL-7R antagonism may be effective in treating CD4(+) T cell-mediated neuroinflammation and other autoimmune inflammatory conditions.

Cellular inhibitor of apoptosis proteins (c-IAP) 1 and 2 are widely expressed ubiquitin protein ligases that regulate a variety of cellular functions, including the sensitivity of T cells to costimulation. 4-1BB is a TNF receptor family member that signals via a complex that includes TRAF family members and the c-IAPs to upregulate NF-κB and ERK, and has been implicated in memory T-cell survival. Here, we show that effector and memory T cells from mice expressing a dominant negative E3-inactive c-IAP2 (c-IAP2(H570A)) have impaired signaling downstream of 4-1BB. When infected with lymphocytic choriomeningitis virus, unlike mice in which c-IAPs were acutely downregulated by c-IAP antagonists, the primary response of c-IAP2(H570A) mice was normal. However, the number of antigen-specific CD8(+) but not CD4(+) T cells declined more rapidly and to a greater extent in c-IAP2(H570A) mice than in WT controls. Studies with T-cell adoptive transfer demonstrated that the enhanced decay of memory cells was T-cell intrinsic. Thus, c-IAP E3 activity is required for 4-1BB coreceptor signaling and maintenance of CD8(+) T-cell memory.

Mild traumatic brain injury (mTBI) can cause meningeal vascular injury and cell death that spreads into the brain parenchyma and triggers local inflammation and recruitment of peripheral immune cells. The factors that dictate meningeal recovery after mTBI are unknown at present. Here we demonstrated that most patients who had experienced mTBI resolved meningeal vascular damage within 2–3 weeks, although injury persisted for months in a subset of patients. To understand the recovery process, we studied a mouse model of mTBI and found extensive meningeal remodeling that was temporally reliant on infiltrating myeloid cells with divergent functions. Inflammatory myelomonocytic cells scavenged dead cells in the lesion core, whereas wound-healing macrophages proliferated along the lesion perimeter and promoted angiogenesis through the clearance of fibrin and production of the matrix metalloproteinase MMP-2. Notably, a secondary injury experienced during the acute inflammatory phase aborted this repair program and enhanced inflammation, but a secondary injury experienced during the wound-healing phase did not. Our findings demonstrate that meningeal vasculature can undergo regeneration after mTBI that is dependent on distinct myeloid cell subsets.

Proper development and function of the mammalian central nervous system (CNS) depend critically on the activity of parenchymal sentinels referred to as microglia. Although microglia were first described as ramified brain-resident phagocytes, research conducted over the past century has expanded considerably upon this narrow view and ascribed many functions to these dynamic CNS inhabitants. Microglia are now considered among the most versatile cells in the body, possessing the capacity to morphologically and functionally adapt to their ever-changing surroundings. Even in a resting state, the processes of microglia are highly dynamic and perpetually scan the CNS. Microglia are in fact vital participants in CNS homeostasis, and dysregulation of these sentinels can give rise to neurological disease. In this review, we discuss the exciting developments in our understanding of microglial biology, from their developmental origin to their participation in CNS homeostasis and pathophysiological states such as neuropsychiatric disorders, neurodegeneration, sterile injury responses, and infectious diseases. We also delve into the world of microglial dynamics recently uncovered using real-time imaging techniques.

Stem-like CD8 T cells maintain long-term antiviral immunity during chronic infection and share regulatory pathways with memory precursor cells generated after acute infection. However, it is unclear whether stem-like CD8 T cells require distinct transcriptional and epigenetic regulation for their longevity and adaptation to the immunosuppressive environment of chronic infection. By comparing single-cell transcriptomes and epigenetic profiles of CD8 T cells responding to acute and chronic viral infections, we found that stem-like CD8 T cells became distinct from memory precursors prior to the termination of clonal expansion. We discovered a co-expression gene module containing Toxthat exhibited higher transcriptional activity associated with more active histone marks in stem-like T cells than memory precursors. Moreover, TOX promoted persistence of antiviral CD8 T cells and was required for stem-like CD8 differentiation. Thus, stem-like CD8 T cells require distinct transcriptional and epigenetic programs for their differentiation and persistence during chronic viral infection.

Virus-specific cytotoxic CD8+ T cells are in cell cycle as they transit from lymphoid tissues to sites of infection., After virus infection, cytotoxic T lymphocytes (CTLs) divide rapidly to eradicate the pathogen and prevent the establishment of persistence. The magnitude of an antiviral CTL response is thought to be controlled by the initiation of a cell cycle program within lymphoid tissues. However, it is presently not known whether this division program proceeds during migration or is influenced locally at sites of viral infection. We demonstrate that antiviral CTLs remain in cell cycle while transiting to infected tissues. Up to one third of virus-specific CTLs within blood were found to be in cell cycle after infection with lymphocytic choriomeningitis virus or vesicular stomatitis virus. Using two-photon microscopy, we found that effector CTL divided rapidly upon arrest in the virus-infected central nervous system as well as in meningeal blood vessels. We also observed that MHC I–dependent interactions, but not costimulation, influenced the division program by advancing effector CTL through stages of the cell cycle. These results demonstrate that CTLs are poised to divide in transit and that their numbers can be influenced locally at the site of infection through interactions with cells displaying cognate antigen.

In the version of this article initially published, in second paragraph of the second subsection of Results (‘Peripheral licensing of CD4+ TH17 cells in Tbx21−/− hosts’), the figure citation (‘Fig. 1c’) in the sentence that begins “In addition to” was incorrect. The correct citation is ‘Fig. 1d’.

Mild traumatic brain injury (mTBI) causes severe meningeal cell death and vascular injury, leading to a robust peripheral immune response featuring neutrophils and monocytes/macrophages. Complications such as repetitive head injuries or viral infections following mTBI have the potential to significantly alter the immune response and impact wound repair. Using a closed-skull compression model of mTBI, we found that meningeal vasculature can regenerate within 7 days of injury, coinciding with a dynamic, bifurcated myeloid cell response. Macrophages derived from inflammatory monocytes scavenged dead cells while non-classical monocyte-derived macrophages promoted angiogenesis. The CD206+ angiogenic macrophages were positioned around the lesion perimeter where they aided in fibrinogen clearance and matrix remodeling. A peripheral infection by lymphocytic choriomeningitis virus (LCMV) during peak vascular damage delayed wound repair and promoted a significant meningeal CD8+ T cell response. The infected environment also shifted the myeloid cells to a more inflammatory state, reducing the amount of CD206+ macrophages around the lesion. Interestingly, we found that a secondary injury also interrupted wound repair only when experienced during the acute “inflammatory” but not the “wound-healing” phase. In fact, the mTBI lesion and infiltrating macrophages were resistant to further damage when reinjured during the wound-healing phase. Our findings demonstrate that meningeal vasculature can undergo robust regeneration following mTBI orchestrated by distinct myeloid cell populations. However, this wound repair process can be significantly disrupted by both sterile and viral insults following the initial injury.

The central nervous system (CNS) is a finely tuned organ that participates in nearly every aspect of our day-to-day function. Neurons lie at the core of this functional unit and maintain an active dialogue with one another as well as their fellow CNS residents (e.g. astrocytes, oligodendrocytes, microglia). Because of this complex dialogue, it is essential that the CNS milieu be tightly regulated in order to permit uninterrupted and efficient neural chemistry. This is accomplished in part by anatomical barriers that segregate vascular components from the cerebral spinal fluid (CSF) and brain parenchyma. These barriers impede entry of noxious materials and enable the CNS to maintain requisite protein and ionic balances for constant electrochemical signaling. Under homeostatic conditions, the CNS is protected by the presence of specialized endothelium/epithelium, the blood brain barrier (BBB), and the blood-CSF barrier. However, following CNS infection these protective barriers can be comprised, sometimes resulting in severe neurological complications triggered by an imbalance or blockage of neural chemistry. In some instances, these disruptions are severe enough to be fatal. This review focuses on a selection of microbes (both viruses and parasites) that compromise vascular barriers and induce neurological complications upon gaining access to the CNS. Emphasis is placed on CNS diseases that result from a pathogenic interplay between host immune defenses and the invading microbe.

Persistence of even the stealthiest viruses can perturb immune function either to the benefit or detriment of the host. Lymphocytic choriomeningitis virus (LCMV) establishes lifelong, systemic persistence when introduced in utero or at birth. Despite a highly evolved host-pathogen relationship, LCMV cannot escape detection by the innate immune system, which results in chronic stimulation of the type 1 IFN pathway in adult carrier mice. In this study we demonstrate that IFN-β is chronically up-regulated in peripheral lymphoid and nonlymphoid tissues (but not the CNS) of mice persistently infected from birth with LCMV and that dendritic cells (DCs) represent at least one source of IFN-β. Interestingly, chronic stimulation of this innate pathway significantly elevated MHC class I expression in the CNS as well as the periphery. Elevated MHC I expression was dependent on IFN-αβ receptor but not MyD88-dependent signaling, as only genetic deletion of the former reduced MHC I to normal levels. An increase in circulating virus was also observed in the IFN-αβ receptor deficient carrier mice, signifying that type I IFN continually exerts anti-viral pressure during a LCMV carrier state. Finally, to determine whether heightened CNS MHC I could be therapeutically corrected, we purged LCMV carrier mice of their persistent infection using adoptive immunotherapy. This treatment significantly reduced CNS MHC I expression. Collectively, these data demonstrate that even a well adapted pathogen can chronically stimulate the innate immune system and consequently alter the expression of Ag presenting machinery in an immunologically specialized compartment like the CNS.

Restrictions in the diversity of an adaptive immune repertoire can facilitate viral persistence. Because a host afflicted with an immune deficiency is not likely to purge a persistent infection using endogenous mechanisms, it is important to explore adoptive therapies to supplement the host with a functional immune defense. In this study, we describe a virus carrier state that results from introducing lymphocytic choriomeningitis virus (LCMV) into adult mice possessing a restricted T cell repertoire. On infection of these mice, LCMV establishes systemic persistence, and within the CNS the virus infects astrocytes (and later oligodendrocytes) rather than its traditional parenchymal target neurons. To determine whether LCMV could be purged from a novel target selection in the absence of an endogenous immune repertoire, we adoptively transferred virus-specific memory cells into adult carrier mice. The memory cells purged virus from the periphery as well as the CNS, but they induced fatalities not typically associated with adoptive immunotherapy. When the repertoire of the recipient mice was examined, a deficiency in natural regulatory T cells was noted. We therefore supplemented carrier mice with regulatory T cells and simultaneously performed adoptive immunotherapy. Cotransfer of regulatory T cells significantly reduced mortality while still permitting the antiviral memory cells to purge the persistent infection. These data indicate that regulatory T cells can be used therapeutically to lessen the pathogenicity of virus-specific immune cells in an immunodeficient host. We also propose that the novel carrier state described herein will facilitate the study of immunotherapeutic regimens.

Therapeutic vaccination is a potentially powerful strategy to establish immune control and eradicate persistent viral infections. Large and multifunctional antiviral T cell responses are associated with control of viral persistence; however, for reasons that were mostly unclear, current therapeutic vaccination approaches to restore T cell immunity and control viral infection have been ineffective. Herein, we confirmed that neutralization of the immunosuppressive factor interleukin (IL)-10 stimulated T cell responses and improved control of established persistent lymphocytic choriomeningitis virus (LCMV) infection. Importantly, blockade of IL-10 also allowed an otherwise ineffective therapeutic DNA vaccine to further stimulate antiviral immunity, thereby increasing T cell responses and enhancing clearance of persistent LCMV replication. We therefore propose that a reason that current therapeutic vaccination strategies fail to resurrect/sustain T cell responses is because they do not alleviate the immunosuppressive environment. Consequently, blocking key suppressive factors could render ineffective vaccines more efficient at improving T cell immunity, and thereby allow immune-mediated control of persistent viral infection.

Monocytes and macrophages are a diverse population of innate immune cells that play a critical role in homeostasis and inflammation. These cells are surveillant by nature and closely monitor the vasculature and surrounding tissue during states of health and disease. Given their abundance and strategic positioning throughout the body, myeloid cells are among the first responders to any inflammatory challenge and are active participants in most immune-mediated diseases. Recent studies have shed new light on myeloid cell dynamics and function by use of an imaging technique referred to as intravital microscopy (IVM). This powerful approach allows researchers to gain real-time insights into monocytes and macrophages performing homeostatic and inflammatory tasks in living tissues. In this review, we will present a contemporary synopsis of how intravital microscopy has revolutionized our understanding of myeloid cell contributions to vascular maintenance, microbial defense, autoimmunity, tumorigenesis, and acute/chronic inflammatory diseases.

Given the global impact of persistent infections on the human population, it is of the utmost importance to devise strategies to noncytopathically purge tissues of infectious agents. The central nervous system (CNS) poses a unique challenge when considering such strategies, as it is an immunologically specialized compartment that contains a nonreplicative cell population. Administration of exogenously derived pathogen-specific memory T cells (referred to as adoptive immunotherapy) to mice burdened with a persistent lymphocytic choriomeningitis virus (LCMV) infection from birth results in eradication of the pathogen from all tissues, including the CNS. In this study, we sought mechanistic insights into this highly successful therapeutic approach. By monitoring the migration of traceable LCMV-specific memory CD8+ T cells after immunotherapy, it was revealed that cytotoxic T lymphocytes (CTLs) distributed widely throughout the CNS compartment early after immunotherapy, which resulted in a dramatic elevation in the activity of CNS antigen-presenting cells (APCs). Immunotherapy induced microglia activation as well as the recruitment of macrophages and dendritic cells (DCs) into the brain parenchyma. However, DCs emerged as the only CNS APC population capable of inducing memory CTLs to preferentially produce the antiviral cytokine tumor necrosis factor-α, a cytokine demonstrated to be required for successful immunotherapeutic clearance. DCs were also found to be an essential element of the immunotherapeutic process because in their absence, memory T cells failed to undergo secondary expansion, and viral clearance was not attained in the CNS. These experiments underscore the importance of DCs in the immunotherapeutic clearance of a persistent viral infection and suggest that strategies to elevate the activation/migration of DCs (especially within the CNS) may facilitate pathogen clearance.

Persistent viral infections continue to present major public health problems. Failure to achieve virus control confronts the immune system with a chronic viral burden that may involve immune cells themselves and directly compromise the functionality of effector lymphocytes and APCs. In this study we use the lymphocytic choriomeningitis virus system for persistent viral infection of its natural murine host and use analytical techniques for direct ex vivo visualization of virus-infected immune cells. We report that virtually all cells of the immune system can be infected, but the distribution of the viral burden is differentially allocated to lymphocyte and APC subsets of defined phenotypes. Importantly, the profile of immune cell infection found in the blood is broadly representative for the pattern of cellular infection in most organs and is independent of the presence of Abs or complement. By direct comparison of virus-infected and uninfected cell subsets, we demonstrate that lymphocytic choriomeningitis virus-infected T cells show preferential activation, skewed cytokine profiles, and increased apoptosis. In contrast, increased activation of APCs is generalized and independent of the presence of viral Ag. Our data indicate that specific patterns of immune cell infection are associated with distinct forms of immunostimulatory and immunosuppressive alterations that may provide insights into autoimmune processes associated with infectious disease and offer clues for therapeutic interventions aimed at restoration of complete immunity.

Infection of the pancreas with lymphocytic choriomeningitis virus results in rapid and differential expression among CXCR3 chemokines. IFN-γ-inducible protein of 10 kDa (IP-10), in contrast with monokine induced by IFN-γ and IFN-inducible T cell-α chemoattractant, is strongly expressed within 24 h postinfection. Blocking of IP-10, but not monokine induced by IFN-γ, aborts severity of Ag-specific injury of pancreatic β cells and abrogates type 1 diabetes. Mechanistically, IP-10 blockade impedes the expansion of peripheral Ag-specific T cells and hinders their migration into the pancreas. IP-10 expression was restricted to viruses infecting the pancreas and that are capable of causing diabetes. Hence, virus-induced organ-specific autoimmune diseases may be dependent on virus tropism and its ability to alter the local milieu by selectively inducing chemokines that prepare the infected tissue for the subsequent destruction by the adaptive immune response.

The central nervous system (CNS) is considered an “immunoprivileged” site with restricted access and a unique microenvironment that profoundly affects the capacity of T cells to exert their functions. The lymphocytic choriomeningitis virus model offers a unique system in which to evaluate the contrasting roles of specific T cells in causing lethal CNS disease or curing pervasive and life-long CNS infection. Specific T cell kinetics in the periphery is briefly discussed. The T cell–mediated mechanisms leading to fatal choriomeningitis are reviewed as are recent methodologic advances that will facilitate the study of antigen-specific T cells in disease pathogenesis. Understanding the specific constraints imposed by the CNS on local T cell activity has important consequences for the design of therapeutic strategies aimed at preventing or curing CNS infection.

Mild traumatic brain injury (mTBI) leads to a robust immune response featuring resident microglia/macrophages and recruitment of peripherally-derived monocytes/macrophages. We sought novel insights into the dynamics and function of innate myeloid cells following brain injury by studying a closed-skull focal contusion model of mTBI that permits real-time imaging by intravital two-photon microscopy (TPM). TPM studies of CX3CR1GFP/+ CCR2RFP/+ reporter mice revealed an early microglia response followed by CCR2+ monocytes and CX3CR1+ macrophages that accumulated 1 to 7 days post-TBI. This transition coincided with dynamic changes in gene expression that shifted from pro-inflammatory to wound-healing. Early cell death and vascular destruction led to an impressive re-vascularization of the lesioned meninges, with 80–100% wound closure observed within 7 days of injury. We used histo-cytometry to define the anatomical position of the different myeloid subpopulations within the injured meninges during the repair phase. This revealed the presence of non-classical and resident macrophages that highly expressed the remodeling enzyme matrix metalloproteinase 2 (Mmp2) in the lesion core. Depletion of neutrophils and classical monocytes increased pathology due to inefficient dead cell clearance, but re-vascularization of the wound was not affected. Interestingly, vascular regeneration was significantly impaired only when non-classical monocytes and resident meningeal macrophages were depleted, resulting in reduced numbers of CD206+macrophages in the lesion. These studies demonstrate that neutrophils, monocytes, and resident macrophages participate in coordinated yet divergent wound-healing responses to mTBI.

Studies have established a role for T cells in resolving persistent viral infections, yet emerging evidence indicates that both T and B cells are required to control some viruses. During persistent infection, a marked lag or failure to generate neutralizing antibodies is commonly observed and likely contributes to an inability to control certain pathogens. Using lymphocytic choriomeningitis virus (LCMV) as a model, we have examined how a persistent viral infection can suppress neutralizing humoral immunity. By tracking the fate of virus-specific B cells in vivo, we report that LCMV-specific B cells were rapidly deleted within a few days of persistent infection, and this deletion was completely reversed by blockade of type I interferon (IFN-I) signaling. Early interference with IFN-I signaling promoted survival and differentiation of LCMV-specific B cells, which accelerated the generation of neutralizing antibodies. This marked improvement in antiviral humoral immunity did not rely on the cessation of IFN-I signaling in B cells but on alterations in the virus-specific CD8+ T cell response. Using two-photon microscopy and in vivo calcium imaging, we observed that cytotoxic T lymphocytes (CTLs) productively engaged and killed LCMV-specific B cells in a perforin-dependent manner within the first few days of infection. Blockade of IFN-I signaling protected LCMV-specific B cells by promoting CTL dysfunction. Therapeutic manipulation of this pathway may facilitate efforts to promote humoral immunity during persistent viral infection in humans. Our findings illustrate how events that occur early after infection can disturb the resultant adaptive response and contribute to viral persistence.

Viral infections that enter the central nervous system (CNS) can drive immune-mediated neurological disorders such as meningitis, encephalitis, and meningoencephalitis. Developing therapeutic interventions to ameliorate these disorders is an important area of clinically relevant research. To gain insights into the immunopathogenesis of CNS viral infection, we study the fatal meningitis induced by lymphocytic choriomeningitis virus (LCMV). We have recently shown that this disease is induced by the pathogenic cooperation between virus-specific CD8+ T cells and innate myelomonocytic cells that damage CNS blood vessels. In addition, genomic studies revealed that the onset of disease is associated with a profound local type I interferon (IFN-I) signature that triggers massive immune extravasation into the meninges. In the present study, we explored approaches to therapeutically treat this disorder. Importantly, we observed that a single injection of a therapeutic antibody directed against the IFN-I receptor (IFNAR) prevented fatal disease as well as the development of symptoms in infected mice. Using a recombinant virus expressing iCre recombinase, we conducted in vivo fate mapping studies in IFNAR blocked fluorescent protein reporter mice. These studies revealed evidence of non-cytopathic viral clearance throughout meninges and glial limitans. Mechanistically, microarray studies uncovered that IFNAR blockade promoted noncytopathic viral clearance through widespread reduction of immune signaling within the CNS without altering the composition of the immune infiltrate. These studies demonstrate that it is possible to mitigate a fatal virus-induced CNS disease via modulation of innate immune signaling pathways.

Barrier immunity, particularly at mucosal surfaces, provides a first line of defense against invading pathogens. Unlike the more classical barrier surfaces in the gut, respiratory tract, genital tract, and skin, the olfactory epithelium within the nose is a specialized barrier structure containing layers of olfactory sensory neurons (OSNs) dedicated to our sense of smell. Because OSNs must contact the nasal airways, they are vulnerable to infection. Upper respiratory viral infection is known to damage the olfactory neuroepithelium and is associated with loss of olfaction. Neurotropic viruses, such as vesicular stomatitis virus (VSV), readily infect OSNs, yet the olfactory epithelial response to infection is unknown. Using a newly generated VSV expressing Cre recombinase, we tracked the fate of virus infected cells after clearance. Numerous cell types, including neural precursors and olfactory ensheathing cells, survived infection. In addition, several months after infection, we detected clonal patches of regenerated mature OSNs derived from previously infected neuroepithelial precursors. VSV infection drove a massive influx of CD45+ hematopoietic cells into the olfactory epithelium that established residence after viral clearance. This included antigen presenting cells and tissue resident CD4+ and CD8+ T cells that localized in and around the regenerating neuroepithelium. We postulate that these immune cells provide a barrier defense against viral rechallenge and might play a role in supporting neuronal regeneration. Collectively, these studies offer insights into how the nasal barrier remodels its neural-immune architecture to restore function and provide enhanced protection following a viral challenge.

T cells develop into a dysfunctional state, called T-cell exhaustion, during chronic viral infections and cancer. However, how antigen-specific T cells persist in these situations is poorly understood. Here, we show that CD8 T cells differentiate into a less exhausted TCF1highTim3low subset and a more exhausted TCF1lowTim3high subset in chronic viral infection as well as human and mouse solid tumors. The TCF1highTim3low CD8 T cells, generated early after chronic lymphocytic choriomeningitis virus (LCMV) infection, persist better and mount a far stronger secondary response than TCF1lowTim3high cells. Importantly, TCF1highTim3low cells act as progenitor cells that are capable of both self-renewal and terminally differentiating into TCF1lowTim3high cells. We have further shown that cell-intrinsic TCF1 signaling is necessary and sufficient to program these progenitors early after infection. Cell-intrinsic TCF1 deficiency led to a progressive loss of anti-viral CD4 and CD8 T cell responses during the chronic phase of infection. Several pro-exhaustion factors, including Tim3, Cish, and Blimp1, are suppressed by TCF1. Conversely, TCF1 up-regulates Bcl6, which strongly enforces the progenitor fate. Moreover, cell-intrinsic type I interferon signal suppresses the differentiation of progenitor-like CD8 T cells, and antibody-mediated blockade of IFNAR1 enhances the differentiation of these CD8 T cells in a TCF1-dependent manner. Thus, our study not only identifies a CD8 progenitor population, which is critical for long-term persistence of anti-viral CD8 responses during chronic viral infection, but also uncovers both positive and negative regulators of the early programming of this population.

Discovery of the meningeal lymphatic vessels in the brain meninges (coverings of the brain) led to the hypothesis that they carry macromolecules and immune cells from cerebrospinal fluid (CSF) to cervical lymph nodes (cLN). Our new findings suggest that the lymphatic system controls immune responses through guiding antigen-presenting cells, such as dendritic cells (DC) from the meninges to draining lymph nodes. By using surgical and pharmacological methods to interfere with meningeal lymphatic vessels, we found lymphatics to serve as a major conduit by which DC leave the meninges, traffic to cLN and promote the generation of antigen-specific T cells. Inflammation-induced migration of DC is primarily regulated through a CC-chemokine receptor 7 (CCR7) dependent pathway. Co-injection of equal parts of differentially labeled CCR7-deficient and wild-type (WT) DCs into the CSF of the brain did allow translocation of CCR7-competent DC into the T cell zone of the draining lymph nodes but not CCR7−/− DC. Subsequently, antigenic stimulation by CCR7−/− DC did not induce a T cells response in the cLN, whereas WT DC enforced the majority of antigen-specific T cells to proliferate and increase their effective trafficking through integrin expression. To gain insights into how T cell responses are elicited when the meninges are infected with a virus, we currently set out to understand the lymphatics function in a mouse model of virus-induced meningitis. Injection of Lymphocytic choriomeningitis virus into the CSF induces rapid, uniform and fatal inflammation that is dependent on the generation of viral specific CD8+ T cells and potentially the exit of migratory DC through meningeal lymphatic vessels.

PD-L1 decreases anti-viral CD8+ T cell motility and PD-1 blockade restores motility in the presence of high viral loads., Immune responses to persistent viral infections and cancer often fail because of intense regulation of antigen-specific T cells—a process referred to as immune exhaustion. The mechanisms that underlie the induction of exhaustion are not completely understood. To gain novel insights into this process, we simultaneously examined the dynamics of virus-specific CD8+ and CD4+ T cells in the living spleen by two-photon microscopy (TPM) during the establishment of an acute or persistent viral infection. We demonstrate that immune exhaustion during viral persistence maps anatomically to the splenic marginal zone/red pulp and is defined by prolonged motility paralysis of virus-specific CD8+ and CD4+ T cells. Unexpectedly, therapeutic blockade of PD-1–PD-L1 restored CD8+ T cell motility within 30 min, despite the presence of high viral loads. This result was supported by planar bilayer data showing that PD-L1 localizes to the central supramolecular activation cluster, decreases antiviral CD8+ T cell motility, and promotes stable immunological synapse formation. Restoration of T cell motility in vivo was followed by recovery of cell signaling and effector functions, which gave rise to a fatal disease mediated by IFN-γ. We conclude that motility paralysis is a manifestation of immune exhaustion induced by PD-1 that prevents antiviral CD8+ T cells from performing their effector functions and subjects them to prolonged states of negative immune regulation.

Type I interferons participate in the host defense against most viral infections through induction of innate antiviral proteins like tetherin (BST-2). In vitro studies have shown that tetherin can physically sequester viruses by linking them to infected cells, which in turn reduces virion release. It is not currently understood how this antiviral protein functions in vivo to control an immune response mounted against a persistent viral infection. We sought novel mechanistic insights into the function of tetherin by studying its role in the establishment of a persistent lymphocytic choriomeningitis virus (LCMV) infection. Following viral infection, tetherin was significantly upregulated in B cells, plasmacytoid dendritic cells (DCs), conventional DCs, and brain-resident microglia. Genetic deletion of tetherin resulted in an early failure to contain a persistence-prone strain of LCMV in the splenic marginal zone. This resulted in a rapid re-distribution of LCMV into the red and white pulp at early time points post-infection. This also interfered with the early priming of antiviral T cells by causing them to localize preferentially to the splenic red pulp where they proliferated less and acquired an impaired phenotype characterized by upregulation of exhaustion markers (PD-1, LAG-3, TIGIT), loss of cytokine production, and decreased motility. Importantly, these early alterations impeded the ability of the host to control persistence-prone LCMV. Viral control was delayed in peripheral tissues, and lifelong persistence was established in the brain. These data demonstrate that tetherin plays a critical in vivo role in shaping the anatomical distribution and adaptive immune response against a persistent viral infection.

Macrophage activation syndrome (MAS) is a life-threatening sepsis-like hyperinflammatory state arising in certain infectious and rheumatic diseases. Previous genetic associations have implicated genes critical for perforin-mediated cytotoxicity. However, we recently identified that activating mutations in inflammasome component NLRC4 lead to MAS and early-onset enterocolitis. Patients with NLRC4-MAS also have chronically and extraordinarily elevated serum IL-18. Notably, disease-causing mutations in other inflammasomes do not cause MAS or chronic IL-18 elevation. In vitro, IL-1b production by THP1 cells expressing various NLRC4 patient mutations correlated with disease severity and identified pathogenic clusters in the molecular structure. To further study the function of NLRC4, we created a mouse bearing the T337S point mutation. Like NLRC4-MAS patients, NLRC4-T337S mice demonstrate elevated serum IL-18 in the absence of systemic inflammation. However, macrophages from NLRC4-T337S mice did not demonstrate enhanced IL-1β or IL-18 production. Correspondingly, bone marrow chimeras demonstrated that IL-18 hyperproduction was derived from non-hematopoietic cells. NLRC4-T337S mice did not demonstrate more severe disease in response to repeated systemic stimulation through TLR9 or TLR4, nor upon LCMV infection, but developed colitis with IL-10 receptor blockade. These results suggest intestinal epithelia, which express both NLRC4 and proIL-18, may be a source of pathogenic IL-18 in this model. Future work will clarify the source(s) of excessive IL-18 and identify how chronic IL-18 effects on gut homeostasis and T and NK cell responses may promote systemic hyperinflammation.

Th1 cells and T follicular helper cells (Tfh cells), are generated early after viral infection. However, the molecular pathways that drive the differentiation of these two distinct subsets remain unclear. Here, we demonstrate that a molecular circuit involving TCF1, Blimp1, and IL-2 regulates the differentiation of viral-specific Tfh and Th1 cells. Tcf7 gene (encodes TCF1) is actively transcribed in naïve and Tfh cells but strongly repressed in Th1 cells. TCF1 expression was negatively correlated with CD25 and Blimp1 expression, and was suppressed by IL2 signaling and Blimp1. Notably, loss of Tcf7 in T cells led to defective Tfh-cell and GC responses after viral infection. Tcf7-deficient Tfh cells had reduced Tfh transcription signatures and increased Th1 transcription signatures. Moreover, Tcf7-deficient Tfh cells had lower mitochondrial mass and membrane potential, and showed reduced expression of genes related to oxidative metabolism. Il2ra and Prdm1 (encodes Blimp1) were ectopically expressed in Tcf7-deficient Tfh cells and were bound by TCF1, suggesting the existence of a negative feedback loop linking TCF1 to IL-2 and Blimp1. Over-expression of Bcl6 rescued Tfh defects caused by Tcf7 deficiency, demonstrating that TCF1 is upstream of the Bcl6-Blimp1 axis. Finally, we found that Tcf7-deficiency did not cause evident defect in Tfh responses in Th2 immunization models. Therefore, TCF1 specifically regulates antiviral Tfh responses.

Cerebral malaria (CM) is a severe complication of Plasmodium falciparum infection that results in thousands of deaths each year, mostly in African children. Pathological studies of CM victims reveal vascular hemorrhaging and edema within the brain, yet the underlying mechanisms that lead to this fatal condition are unknown. Using the animal model of experimental cerebral malaria (ECM), we sought mechanistic insights into the factors that drive the pathogenesis of CM. We found that ECM is associated with a large influx of immune cells into the central nervous system (CNS) at the peak of infection. By depleting individual innate and adaptive immune cell populations, we discovered that CD8 T cells alone were capable of mediating the fatal pathology associated with ECM, including breakdown of the blood brain barrier, edema, and extensive neuronal death in the brainstem. Using two-photon microscopy, we observed parasite-specific CD8 T cells slowly crawling and arresting along the CNS vasculature in vivo during ECM. This behavior was facilitated by the IFN-γ-dependent upregulation of antigen presentation and adhesion molecules on CNS endothelial cells (ECs). Antibody therapy against the adhesion molecules VLA-4 and LFA-1 disrupted CD8 T cell interactions with the CNS vasculature and prevented disease. Importantly, infected mice rendered genetically unable to present antigen on ECs had reduced CD8 T cell-brain EC interactions and were resistant to ECM. These results demonstrate that parasite-specific CD8 T cells interact with brain vascular ECs in an antigen-dependent manner to induce fatal vascular and neuronal pathology during ECM.

Persistent virus infection drives follicular T helper cell differentiation., CD4 T cell responses are crucial to prevent and control viral infection; however, virus-specific CD4 T cell activity is considered to be rapidly lost during many persistent viral infections. This is largely caused by the fact that during viral persistence CD4 T cells do not produce the classical Th1 cytokines associated with control of acute viral infections. Considering that CD4 T cell help is critical for both CD8 T cell and B cell functions, it is unclear how CD4 T cells can lose responsiveness but continue to sustain long-term control of persistent viral replication. We now demonstrate that CD4 T cell function is not extinguished as a result of viral persistence. Instead, viral persistence and prolonged T cell receptor stimulation progressively redirects CD4 T cell development away from the Th1 response induced during an acute infection toward T follicular helper cells. Importantly, this sustained CD4 T cell functionality is critical to maintain immunity and ultimately aid in the control of persistent viral infection.