Mural cells interact with macrophages in the dura mater to regulate CNS immune surveillance

The central nervous system (CNS) tightly regulates access of circulating immune cells. Immunosurveillance is therefore managed in the meninges at the borders of the CNS. Here, we demonstrated that mural cells, which include pericytes and smooth muscle cells, decreased coverage around blood vessels in the dura, the outermost layer of the meninges, and upregulated gene pathways involved in leukocyte migration in presymptomatic experimental autoimmune encephalomyelitis (EAE). Partially depleting mural cells promoted the trafficking of CNS antigen-specific T cells to the dura in a process that depended on resident antigen-presenting cells, thereby increasing susceptibility to passive EAE. Mechanistically, mural cells physically contacted macrophages in the dura and transferred cytoplasmic components, including processing bodies (RNA granules shown to reprogram transcriptomes), which were critical to suppress antigen-dependent T helper (TH) cell activation and TH17 differentiation. Our study revealed a mechanism by which mural cell-macrophage interactions regulate the trafficking of CNS antigen-specific T cells to the dura.

Olfactory immune response to SARS-CoV-2

Numerous pathogens can infect the olfactory tract, yet the pandemic caused by SARS-CoV-2 has strongly emphasized the importance of the olfactory mucosa as an immune barrier. Situated in the nasal passages, the olfactory mucosa is directly exposed to the environment to sense airborne odorants; however, this also means it can serve as a direct route of entry from the outside world into the brain. As a result, olfactotropic infections can have serious consequences, including dysfunction of the olfactory system, CNS invasion, dissemination to the lower respiratory tract, and transmission between individuals. Recent research has shown that a distinctive immune response is needed to protect this neuronal and mucosal tissue. A better understanding of innate, adaptive, and structural immune barriers in the olfactory mucosa is needed to develop effective therapeutics and vaccines against olfactotropic microbes such as SARS-CoV-2. Here, we summarize the ramifications of SARS-CoV-2 infection of the olfactory mucosa, review the subsequent immune response, and discuss important areas of future research for olfactory immunity to infectious disease.

Enolase inhibitors as therapeutic leads for Naegleria fowleri infection

Infections with the pathogenic free-living amoebae Naegleria fowleri can lead to life-threatening illnesses including catastrophic primary amebic meningoencephalitis (PAM). Efficacious treatment options for these infections are lacking and the mortality rate remains >95% in the US. Glycolysis is very important for the infectious trophozoite lifecycle stage and inhibitors of glucose metabolism have been found to be toxic to the pathogen. Recently, human enolase 2 (ENO2) phosphonate inhibitors have been developed as lead agents to treat glioblastoma multiforme (GBM). These compounds, which cure GBM in a rodent model, are well-tolerated in mammals because enolase 1 (ENO1) is the predominant isoform used systemically. Here, we describe findings that demonstrate that these agents are potent inhibitors of N. fowleri ENO ( Nf ENO) and are lethal to amoebae. In particular, (1-hydroxy-2-oxopiperidin-3-yl) phosphonic acid (HEX) was a potent enzyme inhibitor (IC 50 value of 0.14 ± 0.04 µM) that was toxic to trophozoites (EC 50 value of 0.21 ± 0.02 µM) while the reported CC 50 was >300 µM. Molecular docking simulation revealed that HEX binds strongly to the active site of Nf ENO with a binding affinity of -8.6 kcal/mol. Metabolomic studies of parasites treated with HEX revealed a 4.5 to 78-fold accumulation of glycolytic intermediates upstream of Nf ENO. Last, nasal instillation of HEX increased longevity of amoebae-infected rodents. Two days after infection, animals were treated for 10 days with 3 mg/kg HEX, followed by one week of observation. At the conclusion of the experiment, eight of 12 HEX-treated animals remained alive (resulting in an indeterminable median survival time) while one of 12 vehicle-treated rodents remained, yielding a median survival time of 10.9 days. Brains of six of the eight survivors were positive for amoebae, suggesting the agent at the tested dose suppressed, but did not eliminate, infection. These findings suggest that HEX is a promising lead for the treatment of PAM.

Protective human antibodies against a conserved epitope in pre- and postfusion influenza hemagglutinin

Phylogenetically and antigenically distinct influenza A and B viruses (IAV and IBV) circulate in human populations, causing widespread morbidity. Antibodies (Abs) that bind epitopes conserved in both IAV and IBV hemagglutinins (HAs) could protect against disease by diverse virus subtypes. Only one reported HA Ab, isolated from a combinatorial display library, protects against both IAV and IBV. Thus, there has been so far no information on the likelihood of finding naturally occurring human Abs that bind HAs of diverse IAV subtypes and IBV lineages. We have now recovered from several unrelated human donors five clonal Abs that bind a conserved epitope preferentially exposed in the postfusion conformation of IAV and IVB HA2. These Abs lack neutralizing activity in vitro but in mice provide strong, IgG subtype-dependent protection against lethal IAV and IBV infections. Strategies to elicit similar Abs routinely might contribute to more effective influenza vaccines.

Necroptosis Stimulates Interferon-Mediated Protective Anti-Tumor Immunity

Necroptosis is an inflammatory form of cell suicide that critically depends on the kinase activity of Receptor Interacting Protein Kinase 3 (RIPK3). Previous studies showed that immunization with necroptotic cells conferred protection against subsequent tumor challenge. Since RIPK3 can also promote apoptosis and NF-κB-dependent inflammation, it remains difficult to determine the contribution of necroptosis-associated release of damage-associated molecular patterns (DAMPs) in anti-tumor immunity. Here, we describe a system that allows us to selectively induce RIPK3-dependent necroptosis or apoptosis with minimal NF-κB-dependent inflammatory cytokine expression. In a syngeneic tumor challenge model, immunization with necroptotic cells conferred superior protection against subsequent tumor challenge. Surprisingly, this protective effect required CD4+ T cells rather than CD8+ T cells and is dependent on host type I interferon signaling. Our results provide evidence that death-dependent type I interferon production following necroptosis is sufficient to elicit protective anti-tumor immunity.

A broad antibody class engages the influenza virus hemagglutinin head at its stem interface

Influenza infection and vaccination impart strain-specific immunity that fails to protect against both seasonal antigenic variants and the next pandemic. However, antibodies directed to conserved sites can confer broad protection. We identify and characterize a class of human antibodies that engage a previously undescribed, conserved, epitope on the influenza hemagglutinin protein (HA). Prototype antibody S8V1-157 binds at the normally occluded interface between the HA head and stem. Antibodies to this HA head-stem interface epitope are non-neutralizing in vitro but protect against lethal infection in mice. Their breadth of binding extends across most influenza A serotypes and seasonal human variants. Antibodies to the head-stem interface epitope are present at low frequency in the memory B cell populations of multiple donors. The immunogenicity of the epitope warrants its consideration for inclusion in improved or "universal" influenza vaccines.

Olfactory immunology: the missing piece in airway and CNS defence

The olfactory mucosa is a component of the nasal airway that mediates the sense of smell. Recent studies point to an important role for the olfactory mucosa as a barrier to both respiratory pathogens and to neuroinvasive pathogens that hijack the olfactory nerve and invade the CNS. In particular, the COVID-19 pandemic has demonstrated that the olfactory mucosa is an integral part of a heterogeneous nasal mucosal barrier critical to upper airway immunity. However, our insufficient knowledge of olfactory mucosal immunity hinders attempts to protect this tissue from infection and other diseases. This Review summarizes the state of olfactory immunology by highlighting the unique immunologically relevant anatomy of the olfactory mucosa, describing what is known of olfactory immune cells, and considering the impact of common infectious diseases and inflammatory disorders at this site. We will offer our perspective on the future of the field and the many unresolved questions pertaining to olfactory immunity.

Pro-inflammatory cytokine responses to Naegleria fowleri infection

Naegleria fowleri, or the “brain-eating amoeba,” is responsible for a rare, but lethal, infection known as primary amoebic meningoencephalitis (PAM). Confirmed PAM cases have seen both a rise in numbers, as well as expansion of geographic range over the past several decades. There is no effective therapy for PAM and the clinical prognosis remains grim with a mortality rate over 95%. The role of the immune response in disease prevention and disease severity remains unclear. In this review, we explore potential roles of inflammatory immune responses to N. fowleri in disease pathogenesis with a primary focus on pro-inflammatory cytokines IL-1, IL-6, and TNFα. We also discuss modulating proinflammatory cytokines as an additional immune therapy in PAM treatment.

Identification of Immune Cells in Murine Olfactory Mucosa

Olfactory immunology is an emerging field in the context of infectious disease and neuroimmunology, yet characterization of immune cells within the murine olfactory mucosa remains sparse. This is partially due to the difficulty in distinguishing olfactory-resident immune cells from immune cells that reside within nasal turbinate bone marrow. Using techniques like intranasal antibody labeling, we have developed methods to definitively identify olfactory immune cells via flow cytometry and immunofluorescent confocal microscopy. This protocol will describe the best practices for these methods, as well as detail how intravenous antibody labeling can be used to study the blood-olfactory barrier, an important determinant of olfactory immunity. We also include validated markers for the identification of major olfactory immune populations.

Persistent post-COVID-19 smell loss is associated with immune cell infiltration and altered gene expression in olfactory epithelium

SARS-CoV-2 causes profound changes in the sense of smell, including total smell loss. Although these alterations are often transient, many patients with COVID-19 exhibit olfactory dysfunction that lasts months to years. Although animal and human autopsy studies have suggested mechanisms driving acute anosmia, it remains unclear how SARS-CoV-2 causes persistent smell loss in a subset of patients. To address this question, we analyzed olfactory epithelial samples collected from 24 biopsies, including from nine patients with objectively quantified long-term smell loss after COVID-19. This biopsy-based approach revealed a diffuse infiltrate of T cells expressing interferon-γ and a shift in myeloid cell population composition, including enrichment of CD207+ dendritic cells and depletion of anti-inflammatory M2 macrophages. Despite the absence of detectable SARS-CoV-2 RNA or protein, gene expression in the barrier supporting cells of the olfactory epithelium, termed sustentacular cells, appeared to reflect a response to ongoing inflammatory signaling, which was accompanied by a reduction in the number of olfactory sensory neurons relative to olfactory epithelial sustentacular cells. These findings indicate that T cell-mediated inflammation persists in the olfactory epithelium long after SARS-CoV-2 has been eliminated from the tissue, suggesting a mechanism for long-term post-COVID-19 smell loss.

Mucosal plasma cells are required to protect the upper airway and brain from infection

While blood antibodies mediate protective immunity in most organs, whether they protect nasal surfaces in the upper airway is unclear. Using multiple viral infection models in mice, we found that blood-borne antibodies could not defend the olfactory epithelium. Despite high serum antibody titers, pathogens infected nasal turbinates, and neurotropic microbes invaded the brain. Using passive antibody transfers and parabiosis, we identified a restrictive blood-endothelial barrier that excluded circulating antibodies from the olfactory mucosa. Plasma cell depletions demonstrated that plasma cells must reside within olfactory tissue to achieve sterilizing immunity. Antibody blockade and genetically deficient models revealed that this local immunity required CD4+ T cells and CXCR3. Many vaccine adjuvants failed to generate olfactory plasma cells, but mucosal immunizations established humoral protection of the olfactory surface. Our identification of a blood-olfactory barrier and the requirement for tissue-derived antibody has implications for vaccinology, respiratory and CNS pathogen transmission, and B cell fate decisions.

Early detection of cerebrovascular pathology and protective antiviral immunity by MRI

Central nervous system (CNS) infections are a major cause of human morbidity and mortality worldwide. Even patients that survive CNS infections can have lasting neurological dysfunction resulting from immune and pathogen induced pathology. Developing approaches to noninvasively track pathology and immunity in the infected CNS is crucial for patient management and development of new therapeutics. Here, we develop novel MRI-based approaches to monitor virus-specific CD8+ T cells and their relationship to cerebrovascular pathology in the living brain. We studied a relevant murine model in which a neurotropic virus (vesicular stomatitis virus) was introduced intranasally and then entered the brain via olfactory sensory neurons - a route exploited by many pathogens in humans. Using T2*-weighted high-resolution MRI, we identified small cerebral microbleeds as an early form of pathology associated with viral entry into the brain. Mechanistically, these microbleeds occurred in the absence of peripheral immune cells and were associated with infection of vascular endothelial cells. We monitored the adaptive response to this infection by developing methods to iron label and track individual virus specific CD8+ T cells by MRI. Transferred antiviral T cells were detected in the brain within a day of infection and were able to reduce cerebral microbleeds. These data demonstrate the utility of MRI in detecting the earliest pathological events in the virally infected CNS as well as the therapeutic potential of antiviral T cells in mitigating this pathology.

Reversal of the T cell immune system reveals the molecular basis for T cell lineage fate determination in the thymus

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 by Cd4 and Cd8 gene 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 by Cd4, but promote the cytotoxic-lineage fate when encoded in Cd8—regardless of the coreceptor proteins each locus encodes. Thus, T cell lineage fate is determined by cis-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 that Cd4 and Cd8 gene loci encode to avoid generating functionally reversed T cells because they fail to promote protective immunity against environmental pathogens.

To determine how T cell lineage fates are determined in the thymus, Singer and colleagues generated ‘FlipFlop’ mice with a functionally reversed T cell immune system that distinguishes TCR signal strength versus TCR signal duration.

Persistent post-COVID-19 smell loss is associated with inflammatory infiltration and altered olfactory epithelial gene expression

Most human subjects infected by SARS-CoV-2 report an acute alteration in their sense of smell, and more than 25% of COVID patients report lasting olfactory dysfunction. While animal studies and human autopsy tissues have suggested mechanisms underlying acute loss of smell, the pathophysiology that underlies persistent smell loss remains unclear. Here we combine objective measurements of smell loss in patients suffering from post-acute sequelae of SARS-CoV-2 infection (PASC) with single cell sequencing and histology of the olfactory epithelium (OE). This approach reveals that the OE of patients with persistent smell loss harbors a diffuse infiltrate of T cells expressing interferon-gamma; gene expression in sustentacular cells appears to reflect a response to inflammatory signaling, which is accompanied by a reduction in the number of olfactory sensory neurons relative to support cells. These data identify a persistent epithelial inflammatory process associated with PASC, and suggests mechanisms through which this T cell-mediated inflammation alters the sense of smell.

Aging-related olfactory loss is associated with olfactory stem cell transcriptional alterations in humans

BACKGROUNDPresbyosmia, or aging-related olfactory loss, occurs in a majority of humans over age 65 years, yet remains poorly understood, with no specific treatment options. The olfactory epithelium (OE) is the peripheral organ for olfaction and is subject to acquired damage, suggesting a likely site of pathology in aging. Adult stem cells reconstitute the neuroepithelium in response to cell loss under normal conditions. In aged OE, patches of respiratory-like metaplasia have been observed histologically, consistent with a failure in normal neuroepithelial homeostasis.MethodsAccordingly, we have focused on identifying cellular and molecular changes in presbyosmic OE. The study combined psychophysical testing with olfactory mucosa biopsy analysis, single-cell RNA-Sequencing (scRNA-Seq), and culture studies.ResultsWe identified evidence for inflammation-associated changes in the OE stem cells of presbyosmic patients. The presbyosmic basal stem cells exhibited increased expression of genes involved in response to cytokines or stress or the regulation of proliferation and differentiation. Using a culture model, we found that cytokine exposure drove increased TP63, a transcription factor acting to prevent OE stem cell differentiation.ConclusionsOur data suggest aging-related inflammatory changes in OE stem cells may contribute to presbyosmia via the disruption of normal epithelial homeostasis. OE stem cells may represent a therapeutic target for restoration of olfaction.FundingNIH grants DC018371, NS121067, DC016224; Office of Physician-Scientist Development, Burroughs-Wellcome Fund Research Fellowship for Medical Students Award, Duke University School of Medicine.

BACH2 enforces the transcriptional and epigenetic programs of stem-like CD8+ T cells

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.

Heterogeneity of Antiviral Responses in the Upper Respiratory Tract Mediates Differential Non-lytic Clearance of Influenza Viruses

Influenza viruses initiate infection in the upper respiratory tract (URT), but early viral tropism and the importance of cell-type-specific antiviral responses in this tissue remain incompletely understood. By infecting transgenic lox-stop-lox reporter mice with a Cre-recombinase-expressing influenza B virus, we identify olfactory sensory neurons (OSNs) as a major viral cell target in the URT. These cells become infected, then eliminate the virus and survive in the host post-resolution of infection. OSN responses to infection are characterized by a strong induction of interferon-stimulated genes and more rapid clearance of viral protein relative to other cells in the epithelium. We speculate that this cell-type-specific response likely serves to protect the central nervous system from infection. More broadly, these results highlight the importance of evaluating antiviral responses across different cell types, even those within the same tissue, to more fully understand the mechanisms of viral disease.

Diapedesis-Induced Integrin Signaling via LFA-1 Facilitates Tissue Immunity by Inducing Intrinsic Complement C3 Expression in Immune Cells

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.

T cell engagement of cross-presenting microglia protects the brain from a nasal virus infection

The neuroepithelium is a nasal barrier surface populated by olfactory sensory neurons that detect odorants in the airway and convey this information directly to the brain via axon fibers. This barrier surface is especially vulnerable to infection, yet respiratory infections rarely cause fatal encephalitis, suggesting a highly evolved immunological defense. Here, using a mouse model, we sought to understand the mechanism by which innate and adaptive immune cells thwart neuroinvasion by vesicular stomatitis virus (VSV), a potentially lethal virus that uses olfactory sensory neurons to enter the brain after nasal infection. Fate-mapping studies demonstrated that infected central nervous system (CNS) neurons were cleared noncytolytically, yet specific deletion of major histocompatibility complex class I (MHC I) from these neurons unexpectedly had no effect on viral control. Intravital imaging studies of calcium signaling in virus-specific CD8+ T cells revealed instead that brain-resident microglia were the relevant source of viral peptide-MHC I complexes. Microglia were not infected by the virus but were found to cross-present antigen after acquisition from adjacent neurons. Microglia depletion interfered with T cell calcium signaling and antiviral control in the brain after nasal infection. Collectively, these data demonstrate that microglia provide a front-line defense against a neuroinvasive nasal infection by cross-presenting antigen to antiviral T cells that noncytolytically cleanse neurons. Disruptions in this innate defense likely render the brain susceptible to neurotropic viruses like VSV that attempt to enter the CNS via the nose.

Single-cell RNA-seq reveals TOX as a key regulator of CD8+ T cell persistence in chronic infection

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 TCF1, but it is unclear whether they adopt distinct programs to adapt to the immunosuppressive environment. By comparing 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 precursors before the peak of the T-cell response. We discovered a co-expression gene module containing Tox that exhibited higher transcriptional activity associated with more abundant active histone marks in progenitor-like cells than memory precursors. Moreover, TOX promoted 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.

Mucosal immune mediated protection of the CNS following olfactory viral infection

Barrier immunity, particularly at mucosal surfaces, provides a first line of defense against invading pathogens. Unlike classical mucosal barrier surfaces in the gut and respiratory tract, the olfactory epithelium within the nose is a specialized barrier structure containing layers of olfactory sensory neurons (OSNs) dedicated to our sense of smell. Within nasal airways, OSNs are vulnerable to infection but also directly connected to the brain, thus anatomically, the olfactory epithelium functions as a mucosal barrier to the central nervous system (CNS). Upper respiratory viral infections can damage the olfactory neuroepithelium and are 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. VSV infection drove a massive influx of hematopoietic cells including antigen presenting cells and antigen specific CD4+ and CD8+ T cells that established residence within the olfactory epithelium after viral clearance. Upon rechallenge, neutralizing antibody completely protected the neuronal tissues of the olfactory mucosa and the CNS, however this protection appeared to be secretory IgA independent. Resident memory T cells within the olfactory mucosa rapidly produced effector cytokines and prevented heterotypic viral challenge from accessing the CNS. Olfactory VSV infection also resulted in widespread viral and immune mediated tissue damage that contributed to permanent changes in airway turbinate structure. Collectively, these studies offer insight into how the olfactory barrier remodels its neural-immune architecture to provide enhanced protection following a viral challenge.

T cells Protect the Brain from a Nasal Virus Infection by Engaging Local Myeloid Cells that Cross-Present Antigen

Pathogens that gain entry into the CNS elicit a uniquely tailored response from the immune compartment by modulating cytolytic and inflammatory functions to confer viral clearance with minimal brain damage. Neurotropic viruses such as vesicular stomatitis virus (VSV) infect neurons in the olfactory epithelial barrier and pass via sensory axons directly into the brain. Within the olfactory bulb, VSV-infected neurons contact a variety of CNS neurons; however, the infection rarely invades the brain. While type I interferon responses are crucial for initial viral control, little is known about how the local adaptive immune response prevents fatal viral neuroinvasion. In this study, we sought insights into how cytotoxic lymphocytes (CTL) prevent the distal spread of infection from the olfactory bulb. Without T cells, virus containment is compromised and VSV traverses caudally to hindbrain regions. Utilizing intravital two-photon microscopy and other techniques, we characterized the mechanism by which T cell interactions within the olfactory bulb facilitate barrier protection. Interestingly, chimeric mice deficient in MHC class I on brain-resident cells, as well as microglia-depleted mice, showed reduced antiviral T cell calcium signaling upon engagement of target cells, suggesting that microglia can cross-present viral antigen and indirectly facilitate viral clearance from adjacent virally infected neurons. This study has revealed a novel mechanism of T cell-mediated viral control in neurons that involves engagement of resident myeloid cells that acquire antigen but are not infected by the virus. This in turn provides barrier protection against a virus that attempts to enter the CNS via the nasal route.

Long term Immune Residence and Cellular Remodeling Follow Viral Infection of the Olfactory Neuroepithelial Barrier

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.

Dichotomous regulation of progenitor-like CD8 T cells during chronic viral infection by TCF1 and type I interferon

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.

Type I interferon suppresses neutralizing antiviral B cell responses by modulating CD8+ T cell differentiation

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.

The TCF1-Bcl6 axis counteracts type I interferon to repress exhaustion and maintain T cell stemness

During chronic viral infections and in cancer, T cells become dysfunctional, a state known as T cell exhaustion. Although it is well recognized that memory CD8 T cells account for the persistence of CD8 T cell immunity after acute infection, how exhausted T cells persist remains less clear. Using chronic infection with lymphocytic choriomeningitis virus clone 13 and tumor samples, we demonstrate that CD8 T cells differentiate into a less exhausted TCF1^high and a more exhausted TCF1^low population. Virus-specific TCF1^high CD8 T cells, which resemble T follicular helper (T_FH) cells, persist and recall better than do TCF1^low cells and act as progenitor cells to replenish TCF1^low cells. We show that TCF1 is both necessary and sufficient to support this progenitor-like CD8 subset, whereas cell-intrinsic type I interferon signaling suppresses their differentiation. Accordingly, cell-intrinsic TCF1 deficiency led to a loss of these progenitor CD8 T cells, sharp contraction of virus-specific T cells, and uncontrolled viremia. Mechanistically, TCF1 repressed several pro-exhaustion factors and induced Bcl6 in CD8 T cells, which promoted the progenitor fate. We propose that the TCF1-Bcl6 axis counteracts type I interferon to repress T cell exhaustion and maintain T cell stemness, which is critical for persistent antiviral CD8 T cell responses in chronic infection. These findings provide insight into the requirements for persistence of T cell immune responses in the face of exhaustion and suggest mechanisms by which effective T cell-mediated immunity may be enhanced during chronic infections and cancer.

Abstract A057: CX3CR1 distinguishes three distinct migratory memory CD8 T cell subsets

CD8 T cells are crucial for the protection against tumors and intracellular pathogens via their ability to generate long-term memory. Memory T cells (TMem) are however not a homogenous population. Traditionally, blood-borne TMem have been sub-divided into central memory (TCM) and effector memory (TEM) cells based on different migration behaviors and cytotoxic potential.

We recently discovered the existence of a novel subset of CD8 TMem that has unique migratory properties and a superior homeostatic self-renewal capacity. The hitherto unrecognized memory CD8 T cell subset is characterized by intermediate expression levels of CX3CR1, while classical TCM were CX3CR1−, and TEM were CX3CR1high. The fraction of CX3CR1int cells among antigen-experienced TMem was remarkably stable over the course of a year, while TEM gradually declined and TCM increased to be the largest subset by 10 months. Interestingly, CX3CR1int TMem had the highest proliferation rate of the three TMem subsets. While TCM and TEM were phenotypically stable over >10 weeks at steady state, CX3CR1int TMem not only self-renewed, but also gave rise to bonafide TCM. Consequently, CX3CR1int TMem contributed to the gradual increase in TCM.

The newly identified CX3CR1int TMem displayed unique migratory properties. Unlike CX3CR1high TEM, CX3CR1int gradually acquired lymph node homing capability. Moreover, parabiosis experiments and analysis of recirculating memory cells in thoracic duct lymph revealed that CX3CR1int TMem, not TEM, are the predominant T cell subset surveying non-lymphoid peripheral tissues.

Supported by a postdoctoral fellowship of the Cancer Research Institute Irvington Fellowship Program and a Rubicon fellowship (Netherlands Organization for Scientific Research, NWO) to CG, NIH T32 Training Grant in Hematology 5T32-HL07623-20 to EAM, NIH F31 grant CA171339 to SML, NIH T32 grant HL066987 to DA, the Ragon Institute of MGH, MIT and Harvard and NIH/NIAID RO1 AI069259, PO1 AI078897 and PO1 AI112521 to UHvA.

Citation Format: Carmen Gerlach, E. Ashley Moseman, Scott M. Loughhead, David Alvarez, Anthonie J. Zwijnenburg, Lisette Waanders, Rohit Garg, Juan C. de la Torre, Ulrich H. von Andrian. CX3CR1 distinguishes three distinct migratory memory CD8 T cell subsets [abstract]. In: Proceedings of the Second CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; 2016 Sept 25-28; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(11 Suppl):Abstract nr A057.

TCF1 regulates antiviral T follicular helper (Tfh) cell responses through a negative feedback loop with IL-2 and Blimp1

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.

TCF1 Is Required for the T Follicular Helper Cell Response to Viral Infection

T follicular helper (TFH) and T helper 1 (Th1) cells generated after viral infections are critical for the control of infection and the development of immunological memory. However, the mechanisms that govern the differentiation and maintenance of these two distinct lineages during viral infection remain unclear. We found that viral-specific TFH and Th1 cells showed reciprocal expression of the transcriptions factors TCF1 and Blimp1 early after infection, even before the differential expression of the canonical TFH marker CXCR5. Furthermore, TCF1 was intrinsically required for the TFH cell response to viral infection; in the absence of TCF1, the TFH cell response was severely compromised, and the remaining TCF1-deficient TFH cells failed to maintain TFH-associated transcriptional and metabolic signatures, which were distinct from those in Th1 cells. Mechanistically, TCF1 functioned through forming negative feedback loops with IL-2 and Blimp1. Our findings demonstrate an essential role of TCF1 in TFH cell responses to viral infection.

The great balancing act: regulation and fate of antiviral T-cell interactions

The fate of T lymphocytes revolves around a continuous stream of interactions between the T‐cell receptor (TCR) and peptide‐major histocompatibility complex (MHC) molecules. Beginning in the thymus and continuing into the periphery, these interactions, refined by accessory molecules, direct the expansion, differentiation, and function of T‐cell subsets. The cellular context of T‐cell engagement with antigen‐presenting cells, either in lymphoid or non‐lymphoid tissues, plays an important role in determining how these cells respond to antigen encounters. CD8⁺ T cells are essential for clearance of a lymphocytic choriomeningitis virus (LCMV) infection, but the virus can present a number of unique challenges that antiviral T cells must overcome. Peripheral LCMV infection can lead to rapid cytolytic clearance or chronic viral persistence; central nervous system infection can result in T‐cell‐dependent fatal meningitis or an asymptomatic carrier state amenable to immunotherapeutic clearance. These diverse outcomes all depend on interactions that require TCR engagement of cognate peptide‐MHC complexes. In this review, we explore the diversity in antiviral T‐cell behaviors resulting from TCR engagement, beginning with an overview of the immunological synapse and progressing to regulators of TCR signaling that shape the delicate balance between immunopathology and viral clearance.

Mannose receptor 1 mediates cellular uptake and endosomal delivery of CpG-motif containing oligodeoxynucleotides

Recognition of microbial components is critical for activation of TLRs, subsequent innate immune signaling, and directing adaptive immune responses. The DNA sensor TLR9 traffics from the endoplasmic reticulum to endolysosomal compartments where it is cleaved by resident proteases to generate a competent receptor. Activation of TLR9 by CpG-motif containing oligodeoxynucleotides (CpG ODNs) is preceded by agonist endocytosis and delivery into the endolysosomes. The events that dictate this process remain largely unknown; furthermore, it is unclear whether the receptors involved in mediating uptake of exogenous DNA are conserved for both naturally derived pathogenic DNA and synthetic ODNs. In this study, we report that peritoneal macrophages from a wild-derived inbred mouse strain, MOLF/Ei, are hyporesponsive to CpG ODN but are fully responsive to bacterial DNA, thus implying that microbial recognition is not fully recapitulated by a synthetic analog. To identify the gene responsible for the CpG ODN defect, we have performed genome-wide linkage analysis. Using N2 backcross mice, we mapped the trait with high resolution to a single locus containing Mrc1 as the gene conferring the trait. We show that mannose receptor 1 (MRC1; CD206) is involved in CpG ODN uptake and trafficking in wild-derived MOLF/Ei peritoneal macrophages. Furthermore, we show that other strains of wild-derived mice also require MRC1 for CpG-induced cytokine responses. These findings reveal novel functions for MRC1 and demonstrate that wild-derived mice are important and indispensable model for understanding naturally occurring regulators of inflammatory responses in innate immune pathways.

Bisphosphonates target B cells to enhance humoral immune responses

Bisphosphonates are a class of drugs that are widely used to inhibit loss of bone mass in patients. We show here that the administration of clinically relevant doses of bisphosphonates in mice increases antibody responses to live and inactive viruses, proteins, haptens, and existing commercial vaccine formulations. Bisphosphonates exert this adjuvant-like activity in the absence of CD4(+) and γδ T cells, neutrophils, or dendritic cells, and their effect does not rely on local macrophage depletion, Toll-like receptor signaling, or the inflammasome. Rather, bisphosphonates target directly B cells and enhance B cell expansion and antibody production upon antigen encounter. These data establish bisphosphonates as an additional class of adjuvants that boost humoral immune responses.

Chemokine guidance of central memory T cells is critical for antiviral recall responses in lymph nodes

A defining feature of vertebrate immunity is the acquisition of immunological memory, which confers enhanced protection against pathogens by mechanisms that are incompletely understood. Here, we compared responses by virus-specific naive T cells (T(N)) and central memory T cells (T(CM)) to viral antigen challenge in lymph nodes (LNs). In steady-state LNs, both T cell subsets localized in the deep T cell area and interacted similarly with antigen-presenting dendritic cells. However, upon entry of lymph-borne virus, only T(CM) relocalized rapidly and efficiently toward the outermost LN regions in the medullary, interfollicular, and subcapsular areas where viral infection was initially confined. This rapid peripheralization was coordinated by a cascade of cytokines and chemokines, particularly ligands for T(CM)-expressed CXCR3. Consequently, in vivo recall responses to viral infection by CXCR3-deficient T(CM) were markedly compromised, indicating that early antigen detection afforded by intranodal chemokine guidance of T(CM) is essential for efficient antiviral memory.

B cell maintenance of subcapsular sinus macrophages protects against a fatal viral infection independent of adaptive immunity

Neutralizing antibodies have been thought to be required for protection against acutely cytopathic viruses, such as the neurotropic vesicular stomatitis virus (VSV). Utilizing mice that possess B cells but lack antibodies, we show here that survival upon subcutaneous (s.c.) VSV challenge was independent of neutralizing antibody production or cell-mediated adaptive immunity. However, B cells were absolutely required to provide lymphotoxin (LT) α1β2, which maintained a protective subcapsular sinus (SCS) macrophage phenotype within virus draining lymph nodes (LNs). Macrophages within the SCS of B cell-deficient LNs, or of mice that lack LTα1β2 selectively in B cells, displayed an aberrant phenotype, failed to replicate VSV, and therefore did not produce type I interferons, which were required to prevent fatal VSV invasion of intranodal nerves. Thus, although B cells are essential for survival during VSV infection, their contribution involves the provision of innate differentiation and maintenance signals to macrophages, rather than adaptive immune mechanisms.

Endothelial heparan sulfate controls chemokine presentation in recruitment of lymphocytes and dendritic cells to lymph nodes

Heparan sulfate can bind several adhesion molecules involved in lymphocyte trafficking. However, the in vivo function of endothelial heparan sulfate in lymphocyte homing and stimulation of the immune response has not been elucidated. Here, we generated mutant mice deficient in the enzyme Ext1, which is required for heparan sulfate synthesis, in a Tek-dependent and inducible manner. Chemokine presentation was diminished in the mutant mice, causing the lack of appropriate integrin-mediated adhesion, and resulted in a marked decrease in lymphocyte sticking to high endothelial venules and in recruitment of resident dendritic cells through lymphatic vessels to the lymph nodes. As a consequence, mutant mice displayed a severe impairment in lymphocyte homing and a compromised contact hypersensitivity response. By contrast, lymphocyte rolling was increased because of loss of electrostatic repulsion by heparan sulfate. These results demonstrate critical roles of endothelial heparan sulfate in immune surveillance and immune response generation.

Critical role for the chemokine receptor CXCR6 in NK cell-mediated antigen-specific memory of haptens and viruses

Hepatic natural killer (NK) cells mediate antigen-specific contact hypersensitivity (CHS) in mice deficient in T cells and B cells. We report here that hepatic NK cells, but not splenic or naive NK cells, also developed specific memory of vaccines containing antigens from influenza, vesicular stomatitis virus (VSV) or human immunodeficiency virus type 1 (HIV-1). Adoptive transfer of virus-sensitized NK cells into naive recipient mice enhanced the survival of the mice after lethal challenge with the sensitizing virus but not after lethal challenge with a different virus. NK cell memory of haptens and viruses depended on CXCR6, a chemokine receptor on hepatic NK cells that was required for the persistence of memory NK cells but not for antigen recognition. Thus, hepatic NK cells can develop adaptive immunity to structurally diverse antigens, an activity that requires NK cell-expressed CXCR6.

Emerging nanotechnology approaches for HIV/AIDS treatment and prevention

Currently, there is no cure and no preventive vaccine for HIV/AIDS. Combination antiretroviral therapy has dramatically improved treatment, but it has to be taken for a lifetime, has major side effects and is ineffective in patients in whom the virus develops resistance. Nanotechnology is an emerging multidisciplinary field that is revolutionizing medicine in the 21st century. It has a vast potential to radically advance the treatment and prevention of HIV/AIDS. In this review, we discuss the challenges with the current treatment of the disease and shed light on the remarkable potential of nanotechnology to provide more effective treatment and prevention for HIV/AIDS by advancing antiretroviral therapy, gene therapy, immunotherapy, vaccinology and microbicides.

Immunosurveillance by hematopoietic progenitor cells trafficking through blood, lymph, and peripheral tissues

Constitutive egress of bone marrow (BM)-resident hematopoietic stem and progenitor cells (HSPCs) into the blood is a well-established phenomenon, but the ultimate fate and functional relevance of circulating HSPCs is largely unknown. We show that mouse thoracic duct (TD) lymph contains HSPCs that possess short- and long-term multilineage reconstitution capacity. TD-derived HSPCs originate in the BM, enter the blood, and traffic to multiple peripheral organs, where they reside for at least 36 hr before entering draining lymphatics to return to the blood and, eventually, the BM. HSPC egress from extramedullary tissues into lymph depends on sphingosine-1-phosphate receptors. Migratory HSPCs proliferate within extramedullary tissues and give rise to tissue-resident myeloid cells, preferentially dendritic cells. HSPC differentiation is amplified upon exposure to Toll-like receptor agonists. Thus, HSPCs can survey peripheral organs and can foster the local production of tissue-resident innate immune cells under both steady-state conditions and in response to inflammatory signals.

Profiling heparin-chemokine interactions using synthetic tools

Glycosaminoglycans (GAGs), such as heparin or heparan sulfate, are required for the in vivo function of chemokines. Chemokines play a crucial role in the recruitment of leukocyte subsets to sites of inflammation and lymphocytes trafficking. GAG-chemokine interactions mediate cell migration and determine which leukocyte subsets enter tissues. Identifying the exact GAC sequences that bind to particular chemokines is key to understand chemokine function at the molecular level and develop strategies to interfere with chemokine-mediated processes. Here, we characterize the heparin binding profiles of eight chemokines (CCL21, IL-8, CXCL12, CXCL13, CCL19, CCL25, CCL28, and CXCL16) by employing heparin microarrays containing a small library of synthetic heparin oligosaccharides. The chemokines differ significantly in their interactions with heparin oligosaccharides: While some chemokines, (e.g., CCL21) strongly bind to a hexasaccharide containing the GlcNSO3(6-OSO3)-IdoA(2-OSO3) repeating unit, CCL19 does not bind and CXCL12 binds only weakly. The carbohydrate microarray binding results were validated by surface plasmon resonance experiments. In vitro chemotaxis assays revealed that dendrimers coated with the fully sulfated heparin hexasaccharide inhibit lymphocyte migration toward CCL21. Migration toward CXCL12 or CCL19 was not affected. These in vitro homing assays indicate that multivalent synthetic heparin dendrimers inhibit the migration of lymphocytes toward certain chemokine gradients by blocking the formation of a chemokine concentration gradient on GAG endothelial chains. These findings are in agreement with preliminary in vivo measurements of circulating lymphocytes. The results presented here contribute to the understanding of GAG-chemokine interactions, a first step toward the design of novel drugs that modulate chemokine activity.

Natural antibody and complement mediate neutralization of influenza virus in the absence of prior immunity

Early control of virus replication by the innate immune response is essential to allow time for the generation of a more effective adaptive immune response. As an important component of innate immunity, complement has been shown to be necessary for protection against numerous microbial infections. This study was undertaken to investigate the role of complement in neutralizing influenza virus. Results demonstrated that the classical pathway of complement mediated serum neutralization of influenza virus. Although nonimmune serum neutralized influenza virus, the mechanism of virus neutralization (VN) required antibody, as sera from RAG1-deficient mice lacked VN activity; moreover, purified natural immunoglobulin M (IgM) restored VN activity to antibody-deficient sera. The mechanism of VN by natural IgM and complement was associated with virion aggregation and coating of the viral hemagglutinin receptor; however, viral lysis did not significantly contribute to VN. Additionally, reconstitution of RAG1-deficient mice with natural IgM resulted in delayed morbidity during influenza virus infection. Collectively, these results provide evidence that natural IgM and the early components of the classical pathway of complement work in concert to neutralize influenza virus and that this interaction may have a significant impact on the course of influenza viral pneumonia.

Cells of cutaneous immunity in Xenopus: studies during larval development and limb regeneration

The anuran Xenopus laevis is an experimental model for vertebrate development, immunology, and regenerative biology. Using histochemistry and immunohistochemistry (IHC) we examined embryonic, larval, and postmetamorphic Xenopus skin for the presence of dendritic cells (DCs), Langerhans cells (LCs), and dendritic epidermal T cells (DETCs), all components of cutaneous immunity that have been implicated in skin repair and regeneration. Cells expressing three markers for dendritic and Langerhans cells (formalin-resistant ATPase activity, major histocompatibility complex [MHC] class II antigens, and vimentin) and having morphology like that of these cells first appeared during late embryonic stages, becoming abundant by prometamorphosis. Cells positive for these markers were also numerous in the wound epithelia of regenerating hindlimbs at both early and late larval stages. Cells tentatively identified as DETCs were found, beginning at early larval stages, using IHC with antibodies against heterologous CD3epsilon chain and T-cell receptor delta. Further characterization and work with the putative DCs, LCs, and DETCs demonstrated here will allow not only greater understanding of the amphibian immune system, but also further elucidation of regenerative growth and scarring.

Human Plasmacytoid Dendritic Cells Activated by CpG Oligodeoxynucleotides Induce the Generation of CD4+CD25+Regulatory T Cells

Plasmacytoid dendritic cells (PDCs) are key effectors in host innate immunity and orchestrate adaptive immune responses. CpG oligodeoxynucleotides (ODN) have potent immunostimulatory effects on PDCs through TLR9 recognition and signaling. Little is known about the effects of CpG ODN on human PDC-mediated T cell priming. Here we show that type B CpG ODN effectively promotes PDCs to prime allogeneic naive CD4(+)CD25(-) T cells to differentiate into CD4(+)CD25(+) regulatory T (Treg) cells. The CD4(+)CD25(+) T cells induced by CpG ODN-activated PDCs express forkhead transcription factor 3 and produce IL-10, TGF-beta, IFN-gamma, and IL-6, but low IL-2 and IL-4. These CD4(+)CD25(+) T cells are hyporesponsive to secondary alloantigen stimulation and strongly inhibit proliferation of autologous or allogeneic naive CD4(+) T cells in an Ag-nonspecific manner. CpG ODN-activated PDCs require direct contact with T cells to induce CD4(+)CD25(+) Treg cells. Interestingly, IL-10 and TGF-beta were undetectable in the supernatants of CpG ODN-stimulated PDC cultures. Both CpG-A and CpG-C ODN-activated PDCs similarly induced the generation of CD4(+)CD25(+) Treg cells with strong immune suppressive function. This study demonstrates that TLR9 stimulation can promote PDC-mediated generation of CD4(+)CD25(+) Treg cells and suggests PDCs may play an important role in the maintenance of immunological tolerance.