Strategically Localized Dendritic Cells Promote Rapid T Cell Responses to Lymph-Borne Particulate Antigens

Controlling timing and location in vaccines

Vaccines are one of the most powerful technologies supporting public health. The adaptive immune response induced by immunization arises following appropriate activation and differentiation of T and B cells in lymph nodes. Among many parameters impacting the resulting immune response, the presence of antigen and inflammatory cues for an appropriate temporal duration within the lymph nodes, and further within appropriate subcompartments of the lymph nodes– the right timing and location– play a critical role in shaping cellular and humoral immunity. Here we review recent advances in our understanding of how vaccine kinetics and biodistribution impact adaptive immunity, and the underlying immunological mechanisms that govern these responses. We discuss emerging approaches to engineer these properties for future vaccines, with a focus on subunit vaccines.

Programmable multistage drug delivery to lymph nodes

Therapeutic delivery selectively to lymph nodes has the potential to address a variety of unmet clinical needs. However, owing to the unique structure of the lymphatics and the size-restrictive nature of the lymph node reticular network, delivering cargo to specific cells in the lymph node cortex and paracortex is difficult. Here, we describe a delivery system to overcome lymphatic and intra-lymph node transport barriers by combining nanoparticles that are rapidly conveyed to draining lymph nodes after administration in peripheral tissues with programmable degradable linkers. This platform enables the controlled release of intra-lymph-mobile small-molecular cargo, which can reach vastly more immune cells throughout the lymph node than either the particles or free compounds alone. The release rate can be programmed, allowing access to different lymph node structures and therefore specific lymphocyte subpopulations. We are thus able to alter the subtypes of drugged lymph node cells to improve immunotherapeutic effects.

Antigen presentation by dendritic cells and their instruction of CD4+ T helper cell responses

Dendritic cells are powerful antigen-presenting cells that are essential for the priming of T cell responses. In addition to providing T-cell-receptor ligands and co-stimulatory molecules for naive T cell activation and expansion, dendritic cells are thought to also provide signals for the differentiation of CD4+ T cells into effector T cell populations. The mechanisms by which dendritic cells are able to adapt and respond to the great variety of infectious stimuli they are confronted with, and prime an appropriate CD4+ T cell response, are only partly understood. It is known that in the steady-state dendritic cells are highly heterogenous both in phenotype and transcriptional profile, and that this variability is dependent on developmental lineage, maturation stage, and the tissue environment in which dendritic cells are located. Exposure to infectious agents interfaces with this pre-existing heterogeneity by providing ligands for pattern-recognition and toll-like receptors that are variably expressed on different dendritic cell subsets, and elicit production of cytokines and chemokines to support innate cell activation and drive T cell differentiation. Here we review current information on dendritic cell biology, their heterogeneity, and the properties of different dendritic cell subsets. We then consider the signals required for the development of different types of Th immune responses, and the cellular and molecular evidence implicating different subsets of dendritic cells in providing such signals. We outline how dendritic cell subsets tailor their response according to the infectious agent, and how such transcriptional plasticity enables them to drive different types of immune responses.

Dual-targeting nanoparticle vaccine elicits a therapeutic antibody response against chronic hepatitis B

Chronic hepatitis B is caused by prolonged infection with the hepatitis B virus (HBV), which can substantially increase the risk of developing liver disease. Despite the development of preventive vaccines against HBV, a therapeutic vaccine inducing an effective antibody response still remains elusive. The preS1 domain of the large HBV surface protein is the major viral attachment site on hepatocytes and thus offers a therapeutic target; however, its poor immunogenicity limits clinical translation. Here, we design a ferritin nanoparticle vaccine that can deliver preS1 to specific myeloid cells, including SIGNR1⁺ dendritic cells (which activate T follicular helper cells) and lymphatic sinus-associated SIGNR1⁺ macrophages (which can activate B cells). This nanoparticle vaccine induces a high-level and persistent anti-preS1 response that results in efficient viral clearance and partial serological conversion in a chronic HBV mouse model, offering a promising translatable vaccination strategy for the functional cure of chronic hepatitis B.

B Cell Activation and Response Regulation During Viral Infections

Acute viral infections are characterized by rapid increases in viral load, leading to cellular damage and the resulting induction of complex innate and adaptive antiviral immune responses that cause local and systemic inflammation. Successful antiviral immunity requires the activation of many immune cells, including T cells, natural killer cells, and macrophages. B cells play a unique part through their production of antibodies that can both neutralize and clear viral particles before virus entry into a cell. Protective antibodies are produced even before the first exposure of a pathogen, through the regulated secretion of so-called natural antibodies that are generated even in the complete absence of prior microbial exposure. An early wave of rapidly secreted antibodies from extrafollicular (EF) responses draws on the preexisting naive or memory repertoire of B cells to induce a strong protective response that in kinetics tightly follows the clearance of acute infections, such as with influenza virus. Finally, the generation of germinal centers (GCs) provides long-term protection through production of long-lived plasma cells and memory B cells, which shape and broaden the B cell repertoire for more effective responses following repeat exposures. In this study, we review B cell responses to acute viral infections, primarily influenza virus, from the earliest nonspecific B-1 cell to early, antigen-specific EF responses and finally to GC responses. Throughout, we address known factors that lead to distinct B cell response outcomes and discuss how their functions effect viral clearance, highlighting the critical contributions of each response type to the induction of highly protective antiviral humoral immunity.

CytoMAP: A Spatial Analysis Toolbox Reveals Features of Myeloid Cell Organization in Lymphoid Tissues

Recently developed approaches for highly multiplexed imaging have revealed complex patterns of cellular positioning and cell-cell interactions with important roles in both cellular- and tissue-level physiology. However, tools to quantitatively study cellular patterning and tissue architecture are currently lacking. Here, we develop a spatial analysis toolbox, the histo-cytometric multidimensional analysis pipeline (CytoMAP), which incorporates data clustering, positional correlation, dimensionality reduction, and 2D/3D region reconstruction to identify localized cellular networks and reveal features of tissue organization. We apply CytoMAP to study the microanatomy of innate immune subsets in murine lymph nodes (LNs) and reveal mutually exclusive segregation of migratory dendritic cells (DCs), regionalized compartmentalization of SIRPα- dermal DCs, and preferential association of resident DCs with select LN vasculature. The findings provide insights into the organization of myeloid cells in LNs and demonstrate that CytoMAP is a comprehensive analytics toolbox for revealing features of tissue organization in imaging datasets.

Shaping humoral immunity to vaccines through antigen-displaying nanoparticles

Strategies to qualitatively and quantitatively enhance the humoral response to immunizations with protein and polysaccharide antigens are of broad interest for development of new and more effective vaccines. A strategy of increasing importance is the formulation of antigens into a particulate format, mimicking the physical form of viruses. The potential benefits of enhanced B cell receptor engagement by nanoparticles have been long been appreciated, but recent studies are defining additional important factors governing how nanoparticle immunogens interact with the immune system in the context of lymphoid organs. This review will discuss findings about how nanoparticles enhance humoral immunity in vivo and factors governing the fate of nanoparticle immunogens in lymph nodes.

The lymph node at a glance - how spatial organization optimizes the immune response

A hallmark of the mammalian immune system is its ability to respond efficiently to foreign antigens without eliciting an inappropriate response to self-antigens. Furthermore, a robust immune response requires the coordination of a diverse range of cells present at low frequencies within the host. This problem is solved, in part, by concentrating antigens, antigen-presenting cells and antigen-responsive cells in lymph nodes (LNs). Beyond housing these cell types in one location, LNs are highly organized structures consisting of pre-positioned cells within well-defined microanatomical niches. In this Cell Science at a Glance article and accompanying poster, we outline the key cellular populations and components of the LN microenvironment that are present at steady state and chronicle the dynamic changes in these elements following an immune response. This review highlights the LN as a staging ground for both innate and adaptive immune responses, while providing an elegant example of how structure informs function.

Lymphatic endothelial cells of the lymph node

The influx and efflux of cells and antigens to and from the draining lymph nodes largely take place through the subcapsular, cortical and medullary sinus systems. Recent analyses in mice and humans have revealed unexpected diversity in the lymphatic endothelial cells, which form the distinct regions of the sinuses. As a semipermeable barrier, the lymphatic endothelial cells regulate the sorting of lymph-borne antigens to the lymph node parenchyma and can themselves serve as antigen-presenting cells. The leukocytes entering the lymph node via the sinus system and the lymphocytes egressing from the parenchyma migrate through the lymphatic endothelial cell layer. The sinus lymphatic endothelial cells also orchestrate the organogenesis of lymph nodes, and they undergo bidirectional signalling with other sinus-resident cells, such as subcapsular sinus macrophages, to generate a unique lymphatic niche. In this Review, we consider the structural and functional basis of how the lymph node sinus system coordinates immune responses under physiological conditions, and in inflammation and cancer.

Laser adjuvant for vaccination

The use of an immunologic adjuvant to augment the immune response is essential for modern vaccines which are relatively ineffective on their own. In the past decade, researchers have been consistently reporting that skin treatment with a physical parameter, namely laser light, augments the immune response to vaccine and functions as an immunologic adjuvant. This "laser adjuvant" has numerous advantages over the conventional chemical or biological agents; it is free from cold chain storage, hypodermic needles, biohazardous sharp waste, irreversible formulation with vaccine antigen, undesirable biodistribution in vital organs, or unknown long-term toxicity. Since vaccine formulations are given to healthy populations, these characteristics render the "laser adjuvant" significant advantages for clinical use and open a new developmental path for a safe and effective vaccine. In addition, laser technology has been used in the clinic for more than three decades and is therefore technically matured and has been proved to be safe. Currently, four classes of laser adjuvant have been reported; ultrashort pulsed, non-pulsed, non-ablative fractional, and ablative fractional lasers. Since each class of the laser adjuvant shows a distinct mechanism of action, a proper choice is necessary to craft an effective vaccine formulation toward a desired clinical benefit for a clinical vaccine to maximize protection. In addition, data also suggest that further improvement in the efficacy is possible when a laser adjuvant is combined with chemical or biological adjuvant(s). To realize these goals, further efforts to uncover the molecular mechanisms of action of the laser adjuvants is warranted. This review provides a summary and comments of the recent updates in the laser adjuvant technology.

Lymphatic endothelial cells prime naïve CD8+ T cells into memory cells under steady-state conditions

Lymphatic endothelial cells (LECs) chemoattract naïve T cells and promote their survival in the lymph nodes, and can cross-present antigens to naïve CD8+ T cells to drive their proliferation despite lacking key costimulatory molecules. However, the functional consequence of LEC priming of CD8+ T cells is unknown. Here, we show that while many proliferating LEC-educated T cells enter early apoptosis, the remainders comprise a long-lived memory subset, with transcriptional, metabolic, and phenotypic features of central memory and stem cell-like memory T cells. In vivo, these memory cells preferentially home to lymph nodes and display rapid proliferation and effector differentiation following memory recall, and can protect mice against a subsequent bacterial infection. These findings introduce a new immunomodulatory role for LECs in directly generating a memory-like subset of quiescent yet antigen-experienced CD8+ T cells that are long-lived and can rapidly differentiate into effector cells upon inflammatory antigenic challenge.

Regulators of T-cell fate: Integration of cell migration, differentiation and function

A fundamental question in immunology is how cells decide between distinct T helper, effector or memory differentiation fates. These decisions are paramount to overcome infection and establish long-lasting protection. The impact of cell location for the determination of T-cell fate decisions is an emerging field. This review will discuss our current understanding of the migration path that T cells follow, within draining lymph nodes, to steer differentiation down distinct paths of either effector or memory fates. In particular, the regulation of migration and cellular encounters mediated by the chemokine receptor CXCR3 and its ligands will be discussed. The combination of increased antigen density and unique cellular partners play a central role in facilitating the site-specific differentiation of effector T cells, within the interfollicular regions of draining lymph nodes. Recent advances have applied this knowledge to optimize vaccine design to target antigen to lymph nodes. Increased understanding of the regulation of CXCR3 ligands and how T cells integrate multiple chemokine cues will help further progress in this field and allow additional applications to direct cell differentiation outside the lymph node, to enhance memory residency in peripheral tissues and effector anti-tumor responses.

Force Measurement of Living Professional Phagocytes of the Immune System

In higher organisms, the professional phagocytes of the immune system (dendritic cells, neutrophils, monocytes, and macrophages) are responsible for pathogen clearance, the development of immune responses via cytokine secretion and presentation of antigens derived from internalized material, and the normal turnover and remodelling of tissues and disposal of dead cells. These functions rely on the ability of phagocytes to migrate and adhere to sites of infection, dynamically probe their environments to make contact with phagocytic targets, and perform phagocytosis, a mechanism of internalization of large particles, microorganisms, and cellular debris for intracellular degradation. The cell-generated forces that are necessary for the professional phagocytes to act in their roles as ‘first responders’ of the immune system have been the subject of mechanical studies in recent years. Methods of force measurement such as atomic force microscopy, traction force microscopy, micropipette aspiration, magnetic and optical tweezers, and exciting new variants of these have accompanied classical biological methods to perform mechanical investigations of these highly dynamic immune cells.

Germinal center B cell initiation, GC maturation, and the coevolution of its stromal cell niches

Throughout the developing GC response, B cell survival and fate choices made at the single cell level are dependent on signals received largely through interactions with other cells, often with cognate T cells. The type of signals that a given B cell can encounter is dictated by its location within tissue microarchitecture. The focus of this review is on the initiation and evolution of the GC response at the earliest time points. Here, we review the key factors influencing the progression of GC B cell differentiation that are both stage and context dependent. Finally, we describe the coevolution of niches within and surrounding the GC that influence the outcome of the GC response.

High-resolution 3D imaging and topological mapping of the lymph node conduit system

The conduit network is a hallmark of lymph node microanatomy, but lack of suitable imaging technology has prevented comprehensive investigation of its topology. We employed an extended-volume imaging system to capture the conduit network of an entire murine lymph node (comprising over 280,000 segments). The extensive 3D images provide a comprehensive overview of the regions supplied by conduits, including perivascular sleeves and distinctive “follicular reservoirs” within B cell follicles, surrounding follicular dendritic cells. A 3D topology map of conduits within the T-cell zone showed homogeneous branching, but conduit density was significantly higher in the superficial T-cell zone compared with the deep zone, where distances between segments are sufficient for T cells to lose contact with fibroblastic reticular cells. This topological mapping of the conduit anatomy can now aid modeling of its roles in lymph node function, as we demonstrate by simulating T-cell motility in the different T-cell zones.

Extended-volume confocal imaging allowed 3D visualisation of the fine network of conduits within lymph nodes; the resulting map of conduit topology underscores structural differences between the deep and superficial T cell zone and identifies "follicular reservoirs" within B cell follicles that concentrate lymphoid fluid around follicular dendritic cells.

Dendritic cell subsets in T cell programming: location dictates function

Dendritic cells (DCs) can be viewed as translators between innate and adaptive immunity. They integrate signals derived from tissue infection or damage and present processed antigen from these sites to naive T cells in secondary lymphoid organs while also providing multiple soluble and surface-bound signals that help to guide T cell differentiation. DC-mediated tailoring of the appropriate T cell programme ensures a proper cascade of immune responses that adequately targets the insult. Recent advances in our understanding of the different types of DC subsets along with the cellular organization and orchestration of DC and lymphocyte positioning in secondary lymphoid organs over time has led to a clearer understanding of how the nature of the T cell response is shaped. This Review discusses how geographical organization and ordered sequences of cellular interactions in lymph nodes and the spleen regulate immunity.

Design of polymeric nanocapsules to improve their lympho-targeting capacity

Aim: To design lympho-targeted nanocarriers with the capacity to enhance the activity of associated drugs/antigens whose target is within the lymphatic system. Materials & methods: Inulin (INU)-based nanocapsules (NCs), negatively charged and positively charged chitosan NCs were prepared by the solvent displacement techniques. The NCs were produced in two sizes: small (70 nm) and medium (170-250 nm). Results: In vitro results indicated that small NCs interacted more efficiently with dendritic cells than the larger ones. The study of the NCs biodistribution in mice, using 3D reconstruction of the popliteal lymph node, showed that small INU NCs have the greatest access and uniform accumulation in different subsets of resident immune cells. Conclusion: Small and negatively charged INU NCs have a potential as lympho-targeted antigen/drug nanocarriers.

The Biophysics of Lymphatic Transport: Engineering Tools and Immunological Consequences

Lymphatic vessels mediate fluid flows that affect antigen distribution and delivery, lymph node stromal remodeling, and cell-cell interactions, to thus regulate immune activation. Here we review the functional role of lymphatic transport and lymph node biomechanics in immunity. We present experimental tools that enable quantitative analysis of lymphatic transport and lymph node dynamics in vitro and in vivo. Finally, we discuss the current understanding for how changes in lymphatic transport and lymph node biomechanics contribute to pathogenesis of conditions including cancer, aging, neurodegeneration, and infection.

How to dissect the plasticity of antigen-specific immune response: a tissue perspective

Generation of antigen-specific humoral responses following vaccination or infection requires the maturation and function of highly specialized immune cells in secondary lymphoid organs (SLO), such as lymph nodes or tonsils. Factors that orchestrate the dynamics of these cells are still poorly understood. Currently, experimental approaches that enable a detailed description of the function of the immune system in SLO have been mainly developed and optimized in animal models. Conversely, methodological approaches in humans are mainly based on the use of blood-associated material because of the challenging access to tissues. Indeed, only few studies in humans were able to provide a discrete description of the complex network of cytokines, chemokines and lymphocytes acting in tissues after antigenic challenge. Furthermore, even fewer data are currently available on the interaction occurring within the complex micro-architecture of the SLO. This information is crucial in order to design particular vaccination strategies, especially for patients affected by chronic and immune compromising medical conditions who are under-vaccinated or who respond poorly to immunizations. Analysis of immune cells in different human tissues by high-throughput technologies, able to obtain data ranging from gene signature to protein expression and cell phenotypes, is needed to dissect the peculiarity of each immune cell in a definite human tissue. The main aim of this review is to provide an in-depth description of the current available methodologies, proven evidence and future perspectives in the analysis of immune mechanisms following immunization or infections in SLO.

Phenotypically distinct neutrophils patrol uninfected human and mouse lymph nodes

Neutrophils play a key role in innate immunity. As the dominant circulating phagocyte, they are rapidly recruited from the bloodstream to sites of infection or injury to internalize and destroy microbes. More recently, neutrophils have been identified in uninfected organs, challenging the classical view of their function. Here we show that neutrophils were present in lymph nodes (LNs) in homeostasis. Using flow cytometry and confocal imaging, we identified neutrophils within LNs in naive, unchallenged mice, including LNs draining the skin, lungs, and gastrointestinal tract. Neutrophils were enriched within specific anatomical regions, in the interfollicular zone, a site of T cell activation. Intravital two-photon microscopy demonstrated that LN neutrophils were motile, trafficked into LNs from both blood and tissues via high endothelial venules and afferent lymphatics, respectively, and formed interactions with dendritic cells in LNs. Murine and human LN neutrophils had a distinct phenotype compared with circulating neutrophils, with higher major histocompatibility complex II (MHCII) expression, suggesting a potential role in CD4 T cell activation. Upon ex vivo stimulation with IgG immune complex (IC), neutrophils up-regulated expression of MHCII and costimulatory molecules and increased T cell activation. In vivo, neutrophils were capable of delivering circulating IC to LNs, suggesting a broader functional remit. Overall, our data challenge the perception that neutrophil patrol is limited to the circulation in homeostasis, adding LNs to their routine surveillance territory.

Distributed Adaptive Search in T Cells: Lessons From Ants

There are striking similarities between the strategies ant colonies use to forage for food and immune systems use to search for pathogens. Searchers (ants and cells) use the appropriate combination of random and directed motion, direct and indirect agent-agent interactions, and traversal of physical structures to solve search problems in a variety of environments. An effective immune response requires immune cells to search efficiently and effectively for diverse types of pathogens in different tissues and organs, just as different species of ants have evolved diverse search strategies to forage effectively for a variety of resources in a variety of habitats. Successful T cell search is required to initiate the adaptive immune response in lymph nodes and to eradicate pathogens at sites of infection in peripheral tissue. Ant search strategies suggest novel predictions about T cell search. In both systems, the distribution of targets in time and space determines the most effective search strategy. We hypothesize that the ability of searchers to sense and adapt to dynamic targets and environmental conditions enhances search effectiveness through adjustments to movement and communication patterns. We also suggest that random motion is a more important component of search strategies than is generally recognized. The behavior we observe in ants reveals general design principles and constraints that govern distributed adaptive search in a wide variety of complex systems, particularly the immune system.

Engineering immunomodulatory biomaterials for type 1 diabetes

A cure for type 1 diabetes (T1D) would help millions of people worldwide, but remains elusive thus far. Tolerogenic vaccines and beta cell replacement therapy are complementary therapies that seek to address aberrant T1D autoimmune attack and subsequent beta cell loss. However, both approaches require some form of systematic immunosuppression, imparting risks to the patient. Biomaterials-based tools enable localized and targeted immunomodulation, and biomaterial properties can be designed and combined with immunomodulatory agents to locally instruct specific immune responses. In this Review, we discuss immunomodulatory biomaterial platforms for the development of T1D tolerogenic vaccines and beta cell replacement devices. We investigate nano- and microparticles for the delivery of tolerogenic agents and autoantigens, and as artificial antigen presenting cells, and highlight how bulk biomaterials can be used to provide immune tolerance. We examine biomaterials for drug delivery and as immunoisolation devices for cell therapy and islet transplantation, and explore synergies with other fields for the development of new T1D treatment strategies.

Beyond cDC1: Emerging Roles of DC Crosstalk in Cancer Immunity

Dendritic cells (DCs) efficiently process and present antigens to T cells, and by integrating environmental signals, link innate and adaptive immunity. DCs also control the balance between tolerance and immunity, and are required for T-cell mediated anti-tumor immunity. One subset of classical DCs, cDC1, are particularly important for eliciting CD8 T cells that can kill tumor cells. cDC1s are superior in antigen cross-presentation, a process of presenting exogenous antigens on MHC class I to activate CD8⁺ T cells. Tumor-associated cDC1s can transport tumor antigen to the draining lymph node and cross-present tumor antigens, resulting in priming and activation of cytotoxic T cells. Although cross-presenting cDC1s are critical for eliciting anti-tumor T cell responses, the role and importance of other DC subsets in anti-tumor immunity is not as well-characterized. Recent literature in other contexts suggests that critical crosstalk between DC subsets can significantly alter biological outcomes, and these DC interactions likely also contribute significantly to tumor-specific immune responses. Therefore, antigen presentation by cDC1s may be necessary but not sufficient for maximal immune responses against cancer. Here, we discuss recent advances in the understanding of DC subset interactions to maximize anti-tumor immunity, and propose that such interactions should be considered for the development of better DC-targeted immunotherapies.

Antigen presentation by dendritic cells for B cell activation

B cells are efficiently activated by antigens presented on cell membranes, which provide opportunity for receptor cross-linking and antigen capture. The two main cell types implicated in native antigen presentation to B cells are follicular dendritic cells (FDC), which reside in B cell follicles, and CD169⁺ macrophages, which line the antigen-exposed surfaces of these follicles in both the lymph nodes and the spleen. There is mounting evidence, however, that conventional dendritic cells (cDC) can also participate in native antigen presentation to B cells. This underappreciated role, largely hidden by the simultaneous need for cDC to participate in T cells priming, appears to be primarily mediated by the type 2 subset of cDC (cDC2), but may also be a function of cDC1. Better understanding of how cDC participate in B cell priming is likely to improve our capacity to develop effective humoral vaccines.

The Chemoattractant Receptor Ebi2 Drives Intranodal Naive CD4+ T Cell Peripheralization to Promote Effective Adaptive Immunity

Lymph nodes (LNs) play critical roles in adaptive immunity by concentrating in one location the antigens, antigen-presenting cells, and antigen-responsive lymphocytes involved in such responses. Recent studies have revealed nonrandom localization of innate and adaptive immune cells within these organs, suggesting that microanatomical positioning optimizes responses involving sparse cooperating cells. Here, we report that the peripheral localization of LN cDC2 dendritic cells specialized for MHC-II antigen presentation is matched by a similarly biased paracortical distribution of CD4⁺ T cells directed by the chemoattractant receptor Ebi2. In the absence of Ebi2, CD4⁺ T cells lose their location bias and are delayed in antigen recognition, proliferative expansion, differentiation, direct effector activity, and provision of help for CD8⁺ T cell-mediated memory responses, limiting host defense and vaccine responses. These findings demonstrate evolutionary selection for distinct niches within the LN that promote cellular responses, emphasizing the critical link between fine-grained tissue organization and host defense.

Capturing the Fantastic Voyage of Monocytes Through Time and Space

Monocytes are a subset of cells that are categorized together with dendritic cells (DCs) and macrophages in the mononuclear phagocyte system (MPS). Despite sharing several phenotypic and functional characteristics with MPS cells, monocytes are unique cells with the ability to function as both precursor and effector cells in their own right. Before the development of hematopoietic stem cells (HSCs) in utero, monocytes are derived from erythro-myeloid precursors (EMPs) in the fetal liver that are important for populating the majority of tissue resident macrophages. After birth, monocytes arise from bone marrow (BM)-derived HSCs and are released into the circulation upon their maturation, where they survey peripheral tissues and maintain endothelial integrity. Upon sensing of microbial breaches or inflammatory stimuli, monocytes migrate into tissues where their plasticity allows them to differentiate into cells that resemble macrophages or DCs according to the environmental niche. Alternatively, they may also migrate into tissues in the absence of inflammation and remain in an undifferentiated state where they perform homeostatic roles. As monocytes are typically on the move, the availability of intravital imaging approaches has provided further insights into their trafficking patterns in distinct tissue compartments. In this review, we outline the importance of understanding their functional behavior in the context of tissue compartments, and how these studies may contribute towards improved vaccine and future therapeutic strategies.

T Follicular Regulatory Cells and Antibody Responses in Transplantation

De novo donor-specific antibody (DSA) formation is a major problem in transplantation, and associated with long-term graft decline and loss as well as sensitization, limiting future transplant options. Forming high-affinity, long-lived antibody responses involves a process called the germinal center (GC) reaction, and requires interaction between several cell types, including GC B cells, T follicular helper (Tfh) and T follicular regulatory (Tfr) cells. T follicular regulatory cells are an essential component of the GC reaction, limiting its size and reducing nonspecific or self-reactive responses.An imbalance between helper function and regulatory function can lead to excessive antibody production. High proportions of Tfh cells have been associated with DSA formation in transplantation; therefore, Tfr cells are likely to play an important role in limiting DSA production. Understanding the signals that govern Tfr cell development and the balance between helper and regulatory function within the GC is key to understanding how these cells might be manipulated to reduce the risk of DSA development.This review discusses the development and function of Tfr cells and their relevance to transplantation. In particular how current and future immunosuppressive strategies might allow us to skew the ratio between Tfr and Tfh cells to increase or decrease the risk of de novo DSA formation.

Lymph node conduits transport virions for rapid T cell activation

Despite intense interest in antiviral T cell priming, the routes of virion movement in lymph nodes (LNs) are imperfectly understood. Current models fail to explain how virus-infected cells rapidly appear within the LN interior after viral infection. To better understand virion trafficking in the LN, we determined virion and infected-cell locations after vaccinia and Zika virus administration. Notably, many rapidly infected cells in the LN interior were adjacent to LN conduits. Using confocal and electron microscopy, we clearly visualized virions within conduits. Functionally, CD8⁺ T cells rapidly and preferentially associated with vaccinia virus-infected cells deeper in the LN, leading to T cell activation in the LN interior. These results reveal that it is possible for even large virions to flow through LN conduits and infect dendritic cells within the T cell zone to prime CD8⁺ T cells.

Virions can access lymph node conduits.

Studying interactions between dendritic cells and T cells in vivo

Antigen presentation is the key first step in the establishment of an antigen-specific T cell response. Among professional antigen presenting cells (APCs), dendritic cells (DCs) are the major population responsible for the priming of both CD4⁺ and CD8⁺ naïve T cells. This priming requires physical interaction between the DC and the T cell; during which signals are exchanged that determine both the magnitude and the quality of the ensuing response. The nature of these signals varies widely depending on the nature of the antigen, the anatomical site in which they take place, and the phenotype of the antigen-presenting DC, making the study of the dynamics, microanatomical distribution and phenotypic variation of DCs a key part of our understanding of adaptive immunity. Here, we provide a brief survey of how our view of T cell activation by DCs has evolved over recent years as intravital multiphoton microscopy and other emerging technologies have expanded our ability to study these events in vivo.

Lymph Node Subcapsular Sinus Macrophages as the Frontline of Lymphatic Immune Defense

Lymphatic vessels collect and transport lymph and pathogens to the draining lymph node (LN) to generate proper immune protection. A layer of macrophages that strategically line the LN subcapsular sinus (SCS) is directly exposed to the afferent lymph and are denoted as SCS macrophages. These macrophages are the frontline of immune defense that interact with lymph-borne antigens. The importance of these macrophages in limiting the spread of pathogens has been demonstrated in both viral and bacterial infection. In anti-microbial responses, these macrophages can directly or indirectly activate other LN innate immune cells to fight against pathogens, as well as activate T cells or B cells for adaptive immunity. As the first layer of immune cells embracing the tumor-derived antigens, SCS macrophages also actively participate in cancer immune regulation. Recent studies have shown that the LNs' SCS macrophage layer is interrupted in disease models. Despite their importance in fighting the spread of pathogens and in activating anti-tumor immunity, the mechanism and the immunological functional consequences for their disruption are not well-understood. Understanding the mechanism of these macrophages will enhance their capability for therapeutic targeting.

Single-Cell Analysis of Diverse Pathogen Responses Defines a Molecular Roadmap for Generating Antigen-Specific Immunity

The immune system generates pathogen-tailored responses. The precise innate immune cell types and pathways that direct robust adaptive immune responses have not been fully characterized. By using fluorescent pathogens combined with massively parallel single-cell RNA-seq, we comprehensively characterized the initial 48 h of the innate immune response to diverse pathogens. We found that across all pathogens tested, most of the lymph node cell types and states showed little pathogen specificity. In contrast, the rare antigen-positive cells displayed pathogen-specific transcriptional programs as early as 24 h after immunization. In addition, mycobacteria activated a specific NK-driven IFNγ response. Depletion of NK cells and IFNγ showed that IFNγ initiated a monocyte-specific signaling cascade, leading to the production of major chemokines and cytokines that promote Th1 development. Our systems immunology approach sheds light on early events in innate immune responses and may help further development of safe and efficient vaccines.

Harnessing the lymph node microenvironment

PURPOSE OF REVIEW

To evaluate role of the lymph node in immune regulation and tolerance in transplantation and recent advances in the delivery of antigen and immune modulatory signals to the lymph node.

RECENT FINDINGS

Lymph nodes are a primary site of immune cell priming, activation, and modulation, and changes within the lymph node microenvironment have the potential to induce specific regulation, suppression, and potentially tolerance. Antigen enters the lymph node either from tissues via lymphatics, from blood via high endothelial venules, or directly via injection. Here we review different techniques and materials to deliver antigen to the lymph node including microparticles or nanoparticles, ex-vivo antigen presenting cell manipulation, and use of receptor conjugation for specific intralymph node targeting locations.

SUMMARY

The promising results point to powerful techniques to harness the lymph node microenvironment and direct systemic immune regulation. The materials, techniques, and approaches suggest that translational and clinical trials in nonhuman primate and patients may soon be possible.

Impact of Polymer-TLR-7/8 Agonist (Adjuvant) Morphology on the Potency and Mechanism of CD8 T Cell Induction

Small molecule Toll-like receptor-7 and -8 agonists (TLR-7/8a) can be used as vaccine adjuvants to induce CD8 T cell immunity but require formulations that prevent systemic toxicity and focus adjuvant activity in lymphoid tissues. Here, we covalently attached TLR-7/8a to polymers of varying composition, chain architecture and hydrodynamic behavior (∼300 nm submicrometer particles, ∼10 nm micelles and ∼4 nm flexible random coils) and evaluated how these parameters of polymer-TLR-7/8a conjugates impact adjuvant activity in vivo. Attachment of TLR-7/8a to any of the polymer compositions resulted in a nearly 10-fold reduction in systemic cytokines (toxicity). Moreover, both lymph node cytokine production and the magnitude of CD8 T cells induced against protein antigen increased with increasing polymer-TLR-7/8a hydrodynamic radius, with the submicrometer particle inducing the highest magnitude responses. Notably, CD8 T cell responses induced by polymer-TLR-7/8a were dependent on CCR2+ monocytes and IL-12, whereas responses by a small molecule TLR-7/8a that unexpectedly persisted in vaccine-site draining lymph nodes (T1/2 = 15 h) had less dependence on monocytes and IL-12 but required Type I IFNs. This study shows how modular properties of synthetic adjuvants can be chemically programmed to alter immunity in vivo through distinct immunological mechanisms.

Distinct Signaling Thresholds Distinguish Friend from Foe

How do macrophages discriminate between homeostatic and pathogenic microbes using a single receptor? Distinct thresholds for NF-κB and MAPK signaling activation are key.

Re-Emergence of Dendritic Cell Vaccines for Cancer Treatment

Dendritic cells (DCs) are essential in immunity owing to their role in activating T cells, thereby promoting antitumor responses. Tumor cells, however, hijack the immune system, causing T cell exhaustion and DC dysfunction. Tumor-induced T cell exhaustion may be reversed through immune checkpoint blockade (ICB); however, this treatment fails to show clinical benefit in many patients. While ICB serves to reverse T cell exhaustion, DCs are still necessary to prime, activate, and direct the T cells to target tumor cells. In this review we provide a brief overview of DC function, describe mechanisms by which DC functions are disrupted by the tumor microenvironment, and highlight recent developments in DC cancer vaccines.

Chrysalis: A New Method for High-Throughput Histo-Cytometry Analysis of Images and Movies

Advances in imaging have led to the development of powerful multispectral, quantitative imaging techniques, like histo-cytometry. The utility of this approach is limited, however, by the need for time consuming manual image analysis. We therefore developed the software Chrysalis and a group of Imaris Xtensions to automate this process. The resulting automation allowed for high-throughput histo-cytometry analysis of three-dimensional confocal microscopy and two-photon time-lapse images of T cell-dendritic cell interactions in mouse spleens. It was also applied to epi-fluorescence images to quantify T cell localization within splenic tissue by using a "signal absorption" strategy that avoids computationally intensive distance measurements. In summary, this image processing and analysis software makes histo-cytometry more useful for immunology applications by automating image analysis.

Fingolimod can act as a facilitator to establish the primary T-cell response with reduced need of adjuvants

The CD8⁺ T-cell response is an essential part of the adaptive immunity. Adjuvants are routinely required for priming of T cells against antigens encountered in lymph nodes (LNs) to generate antigen-specific immunity but may concomitantly trigger unexpected inflammatory responses. Sphingosine-1-phosphate (S1P) induces transient desensitization of S1P receptors on LN T cells and temporarily blocks their egress, leading to prolonged intranodal retention that allows effective immunosurveillance and increases the chance of priming. In light of the regulatory role of S1P in T-cell migration, we here develop a strategic approach to the T-cell priming with protein vaccine containing low-dose TLR-based adjuvants (LDAV) to induce antigen-specific CD8⁺ T cell responses as efficiently as using regular dose adjuvants in vaccine (RDAV). We found that when combined with one low dose of the S1P analog fingolimod administered into the same vaccination site posteriorly at a specific time, LDAV can elicit a primary response that reaches the level of that induced by RDAV with respect to the response magnitude and functionality. Time-course studies indicate that LDAV and fingolimod in combination act to mimic the expansion kinetics of RDAV-primed antigen-specific CD8⁺ T cells. Further, intranodal accumulation of cDC1 is markedly enhanced in mice receiving the combination vaccination despite the decrease in adjuvant use. Of particular note is the marginal cutaneous inflammation at the injection site, indicating an added benefit of using fingolimod. Therefore, fingolimod as a nonadjuvant agent essentially facilitates antigen-specific T-cell priming with reduced need of adjuvants and minimized adverse reactions.

Subcapsular Sinus Macrophages: The Seat of Innate and Adaptive Memory in Murine Lymph Nodes

Subcapsular sinus (SCS) macrophages are strategically positioned at the lymph-tissue interface in the lymph node to trap and present antigen to B cells. Recent murine data has shown that SCS macrophages also prevent the systemic spread of lymph-borne pathogens and are capable of activating a diverse range of innate effector and adaptive memory cells, including follicular memory T cells and memory B cells (B_mems), that are either pre-positioned or rapidly recruited to the subcapsular niche following infection and inflammation. Furthermore, B_mems are rapidly reactivated to differentiate into plasma cells in subcapsular proliferative foci (SPF). Thus, understanding how SCS macrophages coordinate both innate and adaptive memory responses in the subcapsular niche can provide new opportunities to bolster immunity against pathogens and cancer.

A Novel Cellular Pathway of Antigen Presentation and CD4 T Cell Activation in vivo

Dendritic cell activation of CD4 T cells in the lymph node draining a site of infection or vaccination is widely considered the central event in initiating adaptive immunity. The accepted dogma is that this occurs by stimulating local activation and antigen acquisition by dendritic cells, with subsequent lymph node migration, however the generalizability of this mechanism is unclear. Here we show that in some circumstances antigen can bypass the injection site inflammatory response, draining freely and rapidly to the lymph nodes where it interacts with subcapsular sinus (SCS) macrophages resulting in their death. Debris from these dying SCS macrophages is internalized by monocytes recruited from the circulation. This coordinated response leads to antigen presentation by monocytes and interactions with naïve CD4 T cells that can drive the initiation of T cell and B cell responses. These studies demonstrate an entirely novel pathway leading to initiation of adaptive immune responses in vivo.

Determination of T Follicular Helper Cell Fate by Dendritic Cells

T follicular helper (Tfh) cells are a specialized subset of CD4⁺ T cells that collaborate with B cells to promote and regulate humoral responses. Unlike other CD4⁺ effector lineages, Tfh cells require interactions with both dendritic cells (DCs) and B cells to complete their differentiation. While numerous studies have assessed the potential of different DC subsets to support Tfh priming, the conclusions of these studies depend heavily on the model and method of immunization used. We propose that the location of different DC subsets within the lymph node (LN) and their access to antigen determine their potency in Tfh priming. Finally, we provide a three-step model that accounts for the ability of multiple DC subsets and related lineages to support the Tfh differentiation program.

New insights into the development of B cell responses: Implications for solid organ transplantation

A resurgent interest in the role of B cells following solid organ transplantation is being driven by clinical data suggesting that antibody mediated rejection (AMR) is a major cause of dysfunction and organ transplant failure. These observations suggest that, in a subset of patients, current immunotherapies are failing to control the development of alloantibody responses, and/or failing to reverse the production or the effects of alloantibodies. Quantification of donor-specific antibodies (DSA) has proven to be an imperfect predictor of AMR, and efforts to improve DSA quantification anticipate that this will result in improved predictive power. At the same time, attempts to control of ABMR have focused on the non-specific elimination of B cells, plasma cells (PCs) or circulating antibodies. In the past decade, there has been an improvement in our understanding of the processes that drive B cell differentiation into germinal center (GC)-dependent or GC-independent memory B cells and antibody-secreting PC. These insights are suggesting new ways to more specifically target the DSA response, which may lead to better long-term allograft survival outcomes while preserving protective immunity. In this review, new insights into processes that lead to antibody production upon primary and secondary antigen encounter are discussed, and the potential implications to DSA production as well as future areas of investigation to control AMR are discussed.

Recording the wild lives of immune cells

Intravital microscopic imaging can uncover fundamental aspects of immune cell behavior in real time in both healthy and pathological states. Here, we discuss approaches for single-cell imaging of adaptive and innate immune cells to explore how they migrate, communicate, and mediate regulatory or effector functions in various tissues throughout the body. We further review how intravital single-cell imaging can be used to study drug effects on immune cells.

Streamlining volumetric multi-channel image cytometry using hue-saturation-brightness-based surface creation

Image cytometry is the process of converting image data to flow cytometry-style plots, and it usually requires computer-aided surface creation to extract out statistics for cells or structures. One way of dealing with structures stained with multiple markers in three-dimensional images, is carrying out multiple rounds of channel co-localization and image masking before surface creation, which is cumbersome and laborious. We propose the application of the hue-saturation-brightness color space to streamline this process, which produces complete surfaces, and allows the user to have a global view of the data before flexibly defining cell subsets. Spectral compensation can also be performed after surface creation to accurately resolve different signals. We demonstrate the utility of this workflow in static and dynamic imaging datasets of a needlestick injury on the mouse ear, and we believe this scalable and intuitive approach will improve the ease of performing histocytometry on biological samples.

Yingrou Tan et al. present a method streamlining surface creation in 3D imaging by applying the hue-saturation-brightness transformed channels simultaneously. They show the utility of this approach by imaging ear skin following needlestick injury, observing immune cell infiltration.

Hyperactivated PI3Kδ promotes self and commensal reactivity at the expense of optimal humoral immunity

Gain-of-function mutations in the gene encoding the phosphatidylinositol-3-OH kinase catalytic subunit p110δ (PI3Kδ) result in a human primary immunodeficiency characterized by lymphoproliferation, respiratory infections and inefficient responses to vaccines. However, what promotes these immunological disturbances at the cellular and molecular level remains unknown. We generated a mouse model that recapitulated major features of this disease and used this model and patient samples to probe how hyperactive PI3Kδ fosters aberrant humoral immunity. We found that mutant PI3Kδ led to co-stimulatory receptor ICOS-independent increases in the abundance of follicular helper T cells (TFH cells) and germinal-center (GC) B cells, disorganized GCs and poor class-switched antigen-specific responses to immunization, associated with altered regulation of the transcription factor FOXO1 and pro-apoptotic and anti-apoptotic members of the BCL-2 family. Notably, aberrant responses were accompanied by increased reactivity to gut bacteria and a broad increase in autoantibodies that were dependent on stimulation by commensal microbes. Our findings suggest that proper regulation of PI3Kδ is critical for ensuring optimal host-protective humoral immunity despite tonic stimulation from the commensal microbiome.

Quantitative Measurement of Naïve T Cell Association With Dendritic Cells, FRCs, and Blood Vessels in Lymph Nodes

T cells play a vital role in eliminating pathogenic infections. To activate, naïve T cells search lymph nodes (LNs) for dendritic cells (DCs). Positioning and movement of T cells in LNs is influenced by chemokines including CCL21 as well as multiple cell types and structures in the LNs. Previous studies have suggested that T cell positioning facilitates DC colocalization leading to T:DC interaction. Despite the influence chemical signals, cells, and structures can have on naïve T cell positioning, relatively few studies have used quantitative measures to directly compare T cell interactions with key cell types. Here, we use Pearson correlation coefficient (PCC) and normalized mutual information (NMI) to quantify the extent to which naïve T cells spatially associate with DCs, fibroblastic reticular cells (FRCs), and blood vessels in LNs. We measure spatial associations in physiologically relevant regions. We find that T cells are more spatially associated with FRCs than with their ultimate targets, DCs. We also investigated the role of a key motility chemokine receptor, CCR7, on T cell colocalization with DCs. We find that CCR7 deficiency does not decrease naïve T cell association with DCs, in fact, CCR7-/- T cells show slightly higher DC association compared with wild type T cells. By revealing these associations, we gain insights into factors that drive T cell localization, potentially affecting the timing of productive T:DC interactions and T cell activation.