The microanatomy of T-cell responses

Microneedle-Mediated Immunization Promotes Lung CD8+ T-Cell Immunity

Microneedle array has proven more efficient in stimulating humoral immunity than intramuscular vaccination. However, its effectiveness in inducing pulmonary CD8+ T cells remains elusive, which is essential to the frontline defense against pulmonary viral infections such as influenza and COVID-19 viruses. The current investigation reveals that superior CD8+ T-cell responses are elicited by immunization with a microneedle array over intradermal or intramuscular immunization using the model antigen ovalbumin, irrespective of whether or not the antigen is provided in the lung. Mechanistically, microneedle array-mediated immunization targeted the epidermal layer and stimulated predominantly Langerhans cells, resulting in increased expression of α4β1 adhesion molecules on the CD8+ T-cell surface, which may play a role in T-cell homing to the lung, whereas CD8+ T cells induced by intramuscular immunization did not express the adhesion molecule sufficiently. CD8+ T cells with a lung-homing propensity were also seen after intradermal vaccination, yet to a much lesser extent. Accordingly, microneedle array immunization provided stronger protection against influenza viral infection than intradermal or intramuscular immunization. The observations offer insights into a strong cross-talk between epidermal immunization and lung immunity and are valuable for designing and delivering vaccines against respiratory viral infections.

Post-injury hydraulic fracturing drives fissure formation in the zebrafish basal epidermal cell layer

The skin epithelium acts as the barrier between an organism's internal and external environments. In zebrafish and other freshwater organisms, this barrier function requires withstanding a large osmotic gradient across the epidermis. Wounds breach this epithelium, causing a large disruption to the tissue microenvironment due to the mixing of isotonic interstitial fluid with the external hypotonic fresh water. Here, we show that, following acute injury, the larval zebrafish epidermis undergoes a dramatic fissuring process that resembles hydraulic fracturing, driven by the influx of external fluid. After the wound has sealed-preventing efflux of this external fluid-fissuring starts in the basal epidermal layer at the location nearest to the wound and then propagates at a constant rate through the tissue, spanning over 100 μm. During this process, the outermost superficial epidermal layer remains intact. Fissuring is completely inhibited when larvae are wounded in isotonic external media, suggesting that osmotic gradients are required for fissure formation. Additionally, fissuring partially depends on myosin II activity, as myosin II inhibition reduces the distance of fissure propagation away from the wound. During and after fissuring, the basal layer forms large macropinosomes (with cross-sectional areas ranging from 1 to 10 μm²). We conclude that excess external fluid entry through the wound and subsequent closure of the wound through actomyosin purse-string contraction in the superficial cell layer causes fluid pressure buildup in the extracellular space of the zebrafish epidermis. This excess fluid pressure causes tissue to fissure, and eventually the fluid is cleared through macropinocytosis.

Yin and yang roles of B lymphocytes in solid tumors: Balance between antitumor immunity and immune tolerance/immunosuppression in tumor-draining lymph nodes

The role of B cells in antitumor immunity has been reported to be either promotive or suppressive, but the specific mechanism remains to be comprehensively understood. However, this complicated situation likely depends on the temporal and spatial relationship between the developing tumor and B cells that recognize tumor antigens. Unlike responses against microbial or pathogenic infections, tumor cells are derived from autologous cells that have mutated and become aberrant; thus, elimination by the adaptive immune system is essentially inefficient. If tumor cells can evade immune attack at an early stage, non-destructive responses, such as tolerance and immunosuppression, are established over time. In tumor-draining lymph nodes (TDLNs), tumor antigen-reactive B cells potentially acquire immunoregulatory phenotypes and contribute to an immunosuppressive microenvironment. Therefore, triggering and enhancing antitumor responses by immunotherapies require selective control of these regulatory B cell subsets in TDLNs. In contrast, B cell infiltration and formation of tertiary lymphoid structures in tumors are positively correlated with therapeutic prognosis, suggesting that tumor antigen-specific activation of B cells and antibody production are advantageous for antitumor immunity in mid- to late-stage tumors. Given that the presence of B cells in tumor tissues may reflect the ongoing antitumor response in TDLNs, therapeutic induction and enhancement of these lymphocytes are expected to increase the overall effectiveness of immunotherapy. Therefore, B cells are promising targets, but the spatiotemporal balance of the subsets that exhibit opposite characteristics, that is, the protumor or antitumor state in TDLNs, should be understood, and strategies to separately control their functions should be developed to maximize the clinical outcome.

Multitier mechanics control stromal adaptations in the swelling lymph node

Lymph nodes (LNs) comprise two main structural elements: fibroblastic reticular cells that form dedicated niches for immune cell interaction and capsular fibroblasts that build a shell around the organ. Immunological challenge causes LNs to increase more than tenfold in size within a few days. Here, we characterized the biomechanics of LN swelling on the cellular and organ scale. We identified lymphocyte trapping by influx and proliferation as drivers of an outward pressure force, causing fibroblastic reticular cells of the T-zone (TRCs) and their associated conduits to stretch. After an initial phase of relaxation, TRCs sensed the resulting strain through cell matrix adhesions, which coordinated local growth and remodeling of the stromal network. While the expanded TRC network readopted its typical configuration, a massive fibrotic reaction of the organ capsule set in and countered further organ expansion. Thus, different fibroblast populations mechanically control LN swelling in a multitier fashion.

Sixt and colleagues show that different fibroblast populations in the lymph node mechanically control its swelling in a multitier fashion.

Integrins in the Immunity of Insects: A Review

Integrins are a large group of cell-surface proteins that are classified as transmembrane proteins. Integrins are classified into different types based on sequence variations, leading to structural and functional diversity. They are broadly distributed in animals and have a wide range of biological functions such as cell-to-cell communication, intracellular cytoskeleton organization, cellular signaling, immune responses, etc. Integrins are among the most abundant cell surface proteins in insects, exhibiting their indispensability in insect physiology. Because of their critical biological involvement in physiological processes, they appear to be a novel target for designing effective pest control strategies. In the current literature review, we first discuss the discovery and expression responses of integrins against various types of pathogens. Secondly, we examine the specific biological roles of integrins in controlling microbial pathogens, such as phagocytosis, encapsulation, nodulation, immune signaling, and so on. Finally, we describe the possible uses of integrins to control agricultural insect pests.

Nanomaterial-Based Drug Delivery System Targeting Lymph Nodes

The lymphatic system plays an indispensable role in humoral balance, lipid metabolism, and immune regulation. The lymph nodes (LNs) are known as the primary sites of tumor metastasis and the metastatic LNs largely affected the prognosis of the patiens. A well-designed lymphatic-targeted system favors disease treatment as well as vaccination efficacy. In recent years, development of nanotechnologies and emerging biomaterials have gained increasing attention in developing lymph-node-targeted drug-delivery systems. By mimicking the endogenous macromolecules or lipid conjugates, lymph-node-targeted nanocarries hold potential for disease diagnosis and tumor therapy. This review gives an introduction to the physiological functions of LNs and the roles of LNs in diseases, followed by a review of typical lymph-node-targeted nanomaterial-based drug-delivery systems (e.g., liposomes, micelles, inorganic nanomaterials, hydrogel, and nanocapsules). Future perspectives and conclusions concerned with lymph-node-targeted drug-delivery systems are also provided.

Micro- and Macro-Anatomical Frameworks of Lymph Nodes Indispensable for the Lymphatic System Filtering Function

In the lymphatic vascular system, lymph nodes (LNs) play a pivotal role in filtering and removing lymph-borne substances. The filtering function of LNs involves resident macrophages tightly associated with unique lymphatic sinus structures. Moreover, an intermittently arranged LN in the lymphatic pathway is considered to cooperatively prevent lymph-borne substances from entering blood circulation. However, the functional significance of tissue microarchitecture, cellular composition, and individual LNs in the “LN chain” system is not fully understood. To explore the mechanistic and histo-anatomical significance of LNs as lymph fluid filters, we subcutaneously injected fluorescent tracers into mice and examined the details of lymphatic transport to the LNs qualitatively and quantitatively. Lymph-borne tracers were selectively accumulated in the MARCO⁺ subcapsular-medullary sinus border (SMB) region of the LN, in which reticular lymphatic endothelial cells and CD169⁺F4/80⁺ medullary sinus macrophages construct a dense meshwork of the physical barrier, forming the main body to capture the tracers. We also demonstrated stepwise filtration via the LN chain in the lymphatic basin, which prevented tracer leakage into the blood. Furthermore, inflammatory responses that induce the remodeling of LN tissue as well as the lymphatic pathway reinforce the overall filtering capacity of the lymphatic basin. Taken together, specialized tissue infrastructure in the LNs and their systematic orchestration constitute an integrated filtering system for lymphatic recirculation.

Pre-incubation of corneal donor tissue with sCD83 improves graft survival via the induction of alternatively activated macrophages and tolerogenic dendritic cells

Immune responses reflect a complex interplay of cellular and extracellular components which define the microenvironment of a tissue. Therefore, factors that locally influence the microenvironment and re-establish tolerance might be beneficial to mitigate immune-mediated reactions, including the rejection of a transplant. In this study, we demonstrate that pre-incubation of donor tissue with the immune modulator soluble CD83 (sCD83) significantly improves graft survival using a high-risk corneal transplantation model. The induction of tolerogenic mechanisms in graft recipients was achieved by a significant upregulation of Tgfb, Foxp3, Il27, and Il10 in the transplant and an increase of regulatory dendritic cells (DCs), macrophages (Mφ), and T cells (Tregs) in eye-draining lymph nodes. The presence of sCD83 during in vitro DC and Mφ generation directed these cells toward a tolerogenic phenotype leading to reduced proliferation-stimulating activity in MLRs. Mechanistically, sCD83 induced a tolerogenic Mφ and DC phenotype, which favors Treg induction and significantly increased transplant survival after adoptive cell transfer. Conclusively, pre-incubation of corneal grafts with sCD83 significantly prolongs graft survival by modulating recipient Mφ and DCs toward tolerance and thereby establishing a tolerogenic microenvironment. This functional strategy of donor graft pre-treatment paves the way for new therapeutic options in the field of transplantation.

Imaging dendritic cell functions

Dendritic cells (DCs) are crucial for the appropriate initiation of adaptive immune responses. During inflammation, DCs capture antigens, mature, and migrate to lymphoid tissues to present foreign material to naïve T cells. These cells get activated and differentiate either into pathogen-specific cytotoxic CD8⁺ T cells that destroy infected cells or into CD4⁺ T helper cells that, among other effector functions, orchestrate antibody production by B cells. DC-mediated antigen presentation is equally important in non-inflammatory conditions. Here, DCs mediate induction of tolerance by presenting self-antigens or harmless environmental antigens and induce differentiation of regulatory T cells or inactivation of self-reactive immune cells. Detailed insights into the biology of DCs are, therefore, crucial for the development of novel vaccines as well as the prevention of autoimmune diseases. As in many other life science areas, our understanding of DC biology would be extremely restricted without bioimaging, a compilation of methods that visualize biological processes. Spatiotemporal tracking of DCs relies on various imaging tools, which not only enable insights into their positioning and migration within tissues or entire organs but also allow visualization of subcellular and molecular processes. This review aims to provide an overview of the imaging toolbox and to provide examples of diverse imaging techniques used to obtain fundamental insights into DC biology.

Immunotherapy for lung cancer: Focusing on chimeric antigen receptor (CAR)-T cell therapy

Besides traditional treatment strategies, including surgery, radiotherapy, and chemotherapy for lung cancer as the leading cause of cancer incidence and death, immunotherapy has also emerged as a new treatment strategy. The goal of immunotherapy is to stimulate the immune system responses against cancer, using various approaches such as therapeutic vaccines, monoclonal antibodies, immune checkpoint inhibitors, and T-cell therapy. Chimeric antigen receptor (CAR)-T cells, one of the most popular cancer immunotherapy approaches in the last decade, are genetically engineered T-cells to redirect patients' immune responses to recognize and eliminate tumor-associated antigens (TAA)-expressing tumor cells. CAR-T cell therapy provides promising benefits in lung tumors. In this review, we summarize different immunotherapy approaches for lung cancer, the structure of CAR-T cells, currently undergoing CARs in clinical trials, and various TAAs are being investigated as potential targets in designing CAR-T cells for lung cancer.

Mesenteric Lymph Node Transplantation in Mice to Study Immune Responses of the Gastrointestinal Tract

Mesenteric lymph nodes (mLNs) are sentinel sites of enteral immunosurveillance and immune homeostasis. Immune cells from the gastrointestinal tract (GIT) are constantly recruited to the mLNs in steady-state and under inflammatory conditions resulting in the induction of tolerance and immune cells activation, respectively. Surgical dissection and transplantation of lymph nodes (LN) is a technique that has supported seminal work to study LN function and is useful to investigate resident stromal and endothelial cell biology and their cellular interactions in experimental disease models. Here, we provide a detailed protocol of syngeneic mLN transplantation and report assays to analyze effective mLN engraftment in congenic recipients. Transplanted mLNs allow to study T cell activation and proliferation in preclinical mouse models. Donor mLNs proved viable and functional after surgical transplantation and regenerated blood and lymphatic vessels. Immune cells from the host completely colonized the transplanted mLNs within 7-8 weeks after the surgical intervention. After allogeneic hematopoietic cell transplantation (allo-HCT), adoptively transferred allogeneic CD4+ T cells from FVB/N (H-2q) mice homed to the transplanted mLNs in C57BL/6 (H-2b) recipients during the initiation phase of acute graft-versus-host disease (aGvHD). These CD4+ T cells retained full proliferative capacity and upregulated effector and gut homing molecules comparable to those in mLNs from unmanipulated wild-type recipients. Wild type mLNs transplanted into MHCII deficient syngeneic hosts sufficed to activate alloreactive T cells upon allogeneic hematopoietic cell transplantation, even in the absence of MHCII+ CD11c+ myeloid cells. These data support that orthotopically transplanted mLNs maintain physiological functions after transplantation. The technique of LN transplantation can be applied to study migratory and resident cell compartment interactions in mLNs as well as immune reactions from and to the gut under inflammatory and non-inflammatory conditions.

Integrins Modulate Extracellular Matrix Organization to Control Cell Signaling during Hematopoiesis

During hematopoiesis, progenitor cells receive and interpret a diverse array of regulatory signals from their environment. These signals control the maintenance of the progenitors and regulate the production of mature blood cells. Integrins are well known in vertebrates for their roles in hematopoiesis, particularly in assisting in the migration to, as well as the physical attachment of, progenitors to the niche. However, whether and how integrins are also involved in the signaling mechanisms that control hematopoiesis remains to be resolved. Here, we show that integrins play a key role during fly hematopoiesis in regulating cell signals that control the behavior of hematopoietic progenitors. Integrins can regulate hematopoiesis directly, via focal adhesion kinase (FAK) signaling, and indirectly, by directing extracellular matrix (ECM) assembly and/or maintenance. ECM organization and density controls blood progenitor behavior by modulating multiple signaling pathways, including bone morphogenetic protein (BMP) and Hedgehog (Hh). Furthermore, we show that integrins and the ECM are reduced following infection, which may assist in activating the immune response. Our results provide mechanistic insight into how integrins can shape the signaling environment around hematopoietic progenitors.

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.

Role of lymph node stroma and microenvironment in T cell tolerance

Lymph nodes (LNs) are at the cross roads of immunity and tolerance. These tissues are compartmentalized into specialized niche areas by lymph node stromal cells (LN SCs). LN SCs shape the LN microenvironment and guide immunological cells into different zones through establishment of a CCL19 and CCL21 gradient. Following local immunological cues, LN SCs modulate activity to support immune cell priming, activation, and fate. This review will present our current understanding of LN SC subsets roles in regulating T cell tolerance. Three major types of LN SC subsets, namely fibroblastic reticular cells, lymphatic endothelial cells, and blood endothelial cells, are discussed. These subsets serve as scaffolds to support and regulate T cell homeostasis. They contribute to tolerance by presenting peripheral tissue antigens to both CD4 and CD8 T cells. The role of LN SCs in regulating T cell migration and tolerance induction is discussed. Looking forward, recent advances in bioengineered materials and approaches to leverage LN SCs to induce T cell tolerance are highlighted, as are current clinical practices that allow for manipulation of the LN microenvironment to induce tolerance. Increased understanding of LN architecture, how different LN SCs integrate immunological cues and shape immune responses, and approaches to induce T cell tolerance will help further combat autoimmune diseases and graft rejection.

Active Lymphatic Drainage From the Eye Measured by Noninvasive Photoacoustic Imaging of Near-Infrared Nanoparticles

Purpose

To visualize and quantify lymphatic drainage of aqueous humor from the eye to cervical lymph nodes in the dynamic state.

Methods

A near-infrared tracer was injected into the right eye anterior chamber of 10 mice under general anesthesia. Mice were imaged with photoacoustic tomography before and 20 minutes, 2, 4, and 6 hours after injection. Tracer signal intensity was measured in both eyes and right and left neck lymph nodes at every time point and signal intensity slopes were calculated. Slope differences between right and left eyes and right and left nodes were compared using paired t-test. Neck nodes were examined with fluorescence optical imaging and histologically for the presence of tracer.

Results

Following right eye intracameral injection of tracer, an exponential decrease in tracer signal was observed from 20 minutes to 6 hours in all mice. Slope differences of the signal intensity between right and left eyes were significant (P < 0.001). Simultaneously, increasing tracer signal was observed in the right neck node from 20 minutes to 6 hours. Slope differences of the signal intensity between right and left neck nodes were significant (P = 0.0051). Ex vivo optical fluorescence imaging and histopathologic examination of neck nodes confirmed tracer presence within submandibular nodes.

Conclusions

Active lymphatic drainage of aqueous from the eye to cervical lymph nodes was measured noninvasively by photoacoustic imaging of near-infrared nanoparticles. This unique in vivo assay may help to uncover novel drugs that target alternative outflow routes to lower IOP in glaucoma and may provide new insights into lymphatic drainage in eye health and disease.

Time to Micromanage the Pathogen-Host-Vector Interface: Considerations for Vaccine Development

The current increase in vector-borne disease worldwide necessitates novel approaches to vaccine development targeted to pathogens delivered by blood-feeding arthropod vectors into the host skin. A concept that is gaining traction in recent years is the contribution of the vector or vector-derived components, like salivary proteins, to host-pathogen interactions. Indeed, the triad of vector-host-pathogen interactions in the skin microenvironment can influence host innate and adaptive responses alike, providing an advantage to the pathogen to establish infection. A better understanding of this "bite site" microenvironment, along with how host and vector local microbiomes immunomodulate responses to pathogens, is required for future vaccines for vector-borne diseases. Microneedle administration of such vaccines may more closely mimic vector deposition of pathogen and saliva into the skin with the added benefit of near painless vaccine delivery. Focusing on the 'micro'⁻from microenvironments to microbiomes to microneedles⁻may yield an improved generation of vector-borne disease vaccines in today's increasingly complex world.

CD169 macrophages regulate immune responses toward particulate materials in the circulating fluid

Tissue macrophages comprise heterogeneous subsets that differ in localization, phenotype and ontogeny. They acquire tissue-specific phenotype in order to maintain normal tissue physiology. This review summarizes the current knowledge about the functions of CD169-positive macrophage subset residing in the lymphoid organs and intestinal tract. Strategically positioned at the interface between tissue and circulating fluid, CD169+ macrophages in the lymphoid organs capture blood- and lymph-borne particulate materials. Antigen information relayed by CD169+ macrophages to neighbouring immune cells is important for enhancement of antimicrobial and antitumour immunity as well as induction of tolerance. In the intestinal tract, CD169+ macrophages localize distantly from epithelial border. Following mucosal injury, they exacerbate inflammation by producing CCL8 that recruits inflammatory monocytes. As such, a better understanding of CD169+ macrophage phenotypes may enable the design of tissue-specific therapies for both immunological and non-immunological diseases.

Mechanics of antigen extraction in the B cell synapse

B cell encounter with antigen displayed on antigen-presenting cells leads to B cell immune synapse formation, internalisation of the antigen, and stimulation of antibody responses. The sensitivity with which B cells detect antigen, and the quality and quantity of antigen that B cells acquire, depend upon mechanical properties of the immune synapse including interfacial tension, the strength of intermolecular bonds, and the compliance of the molecules and membranes that participate in antigen presentation. In this review, we discuss our current understanding of how these various physical parameters influence B cell antigen extraction in the immune synapse and how a more comprehensive understanding of B cell mechanics may promote the development of new approaches to stimulate the production of desired antibodies.

Synthetic Immunology: Hacking Immune Cells to Expand Their Therapeutic Capabilities

The ability of immune cells to survey tissues and sense pathologic insults and deviations makes them a unique platform for interfacing with the body and disease. With the rapid advancement of synthetic biology, we can now engineer and equip immune cells with new sensors and controllable therapeutic response programs to sense and treat diseases that our natural immune system cannot normally handle. Here we review the current state of engineered immune cell therapeutics and their unique capabilities compared to small molecules and biologics. We then discuss how engineered immune cells are being designed to combat cancer, focusing on how new synthetic biology tools are providing potential ways to overcome the major roadblocks for treatment. Finally, we give a long-term vision for the use of synthetic biology to engineer immune cells as a general sensor-response platform to precisely detect disease, to remodel disease microenvironments, and to treat a potentially wide range of challenging diseases.

Dendritic cell and antigen dispersal landscapes regulate T cell immunity

Gerner et al. show that spatial compartmentalization in lymph nodes of DCs specialized for MHC I versus MHC II presentation determines the amount of antigen these cells capture after immunization and regulates the relative generation of CD4⁺ versus CD8⁺ T cell responses.

Dendritic cell (DC) subsets with biased capacity for CD4⁺ and CD8⁺ T cell activation are asymmetrically distributed in lymph nodes (LNs), but how this affects adaptive responses has not been extensively studied. Here we used quantitative imaging to examine the relationships among antigen dispersal, DC positioning, and T cell activation after protein immunization. Antigens rapidly drained into LNs and formed gradients extending from the lymphatic sinuses, with reduced abundance in the deep LN paracortex. Differential localization of DCs specialized for major histocompatibility complex I (MHC I) and MHC II presentation resulted in preferential activation of CD8⁺ and CD4⁺ T cells within distinct LN regions. Because MHC I–specialized DCs are positioned in regions with limited antigen delivery, modest reductions in antigen dose led to a substantially greater decline in CD8⁺ compared with CD4⁺ T cell activation, expansion, and clonal diversity. Thus, the collective action of antigen dispersal and DC positioning regulates the extent and quality of T cell immunity, with important implications for vaccine design.

Bidirectional Crosstalk between Lymphatic Endothelial Cell and T Cell and Its Implications in Tumor Immunity

Lymphatic vessels have been traditionally considered as passive transporters of fluid and lipids. However, it is apparent from recent literature that the function of lymphatic vessels is not only restricted to fluid balance homeostasis but also extends to regulation of immune cell trafficking, antigen presentation, tolerance, and immunity, all which may impact the progression of inflammatory responses and diseases such as cancer. The lymphatic system and the immune system are intimately connected, and there is emergent evidence for a crosstalk between T cell and lymphatic endothelial cell (LEC). This review describes how LECs in lymph nodes can affect multiple functional properties of T cells and the impact of these LEC-driven effects on adaptive immunity and, conversely, how T cells can modulate LEC growth. The significance of such crosstalk between T cells and LECs in cancer will also be discussed.

The association of exosomes with lymph nodes

Cells produce extracellular nanovesicles known as exosomes that transport information between tissue microenvironments. Exosomes can engage and regulate the function of various immune cell types facilitating both normal and pathological processes. It follows that exosomes should also associate with lymph nodes containing immune cells. Herein, data derived from investigations that incorporate experiments pertaining to the trafficking of exosomes to lymph nodes is reviewed. Within lymph nodes, direct evidence demonstrates that exosomes associate with dendritic cells, subcapsular sinus macrophages, B lymphocytes and stromal cells. Interactions with endothelial cells are also likely. The functional significance of these associations depends on exosome type. Continued investigations into the relationship between exosomes and lymph nodes will further our understanding of how exosomes regulate immune cells subsets and may serve to inspire new exosome based therapeutics to treat a variety of diseases.

The intercell dynamics of T cells and dendritic cells in a lymph node-on-a-chip flow device

T cells play a central role in immunity towards cancer and infectious diseases. T cell responses are initiated in the T cell zone of the lymph node (LN), where resident antigen-bearing dendritic cells (DCs) prime and activate antigen-specific T cells passing by. In the present study, we investigated the T cell : DC interaction in a microfluidic device to understand the intercellular dynamics and physiological conditions in the LN. We show random migration of antigen-specific T cells onto the antigen-presenting DC monolayer independent of the flow direction with a mean T cell : DC dwell time of 12.8 min and a mean velocity of 6 μm min(-1). Furthermore, we investigated the antigen specific vs. unspecific attachment and detachment of CD8(+) and CD4(+) T cells to DCs under varying shear stress. In our system, CD4(+) T cells showed long stable contacts with APCs, whereas CD8(+) T cells presented transient interactions with DCs. By varying the shear stress from 0.01 to 100 Dyn cm(-2), it was also evident that there was a much stronger attachment of antigen-specific than unspecific T cells to stationary DCs up to 1-12 Dyn cm(-2). The mechanical force of the cell : cell interaction associated with the pMHC-TCR match under controlled tangential shear force was estimated to be in the range of 0.25-4.8 nN. Finally, upon performing attachment & detachment tests, there was a steady accumulation of antigen specific CD8(+) T cells and CD4(+) T cells on DCs at low shear stresses, which were released at a stress of 12 Dyn cm(-2). This microphysiological model provides new possibilities to recreate a controlled mechanical force threshold of pMHC-TCR binding, allowing the investigation of intercellular signalling of immune synapses and therapeutic targets for immunotherapy.

Engineering T Cells with Customized Therapeutic Response Programs Using Synthetic Notch Receptors

Redirecting T cells to attack cancer using engineered chimeric receptors provides powerful new therapeutic capabilities. However, the effectiveness of therapeutic T cells is constrained by the endogenous T cell response: certain facets of natural response programs can be toxic, whereas other responses, such as the ability to overcome tumor immunosuppression, are absent. Thus, the efficacy and safety of therapeutic cells could be improved if we could custom sculpt immune cell responses. Synthetic Notch (synNotch) receptors induce transcriptional activation in response to recognition of user-specified antigens. We show that synNotch receptors can be used to sculpt custom response programs in primary T cells: they can drive a la carte cytokine secretion profiles, biased T cell differentiation, and local delivery of non-native therapeutic payloads, such as antibodies, in response to antigen. SynNotch T cells can thus be used as a general platform to recognize and remodel local microenvironments associated with diverse diseases.

Modeling Lymph Flow and Fluid Exchange with Blood Vessels in Lymph Nodes

BACKGROUND

Lymph nodes (LNs) are positioned strategically throughout the body as critical mediators of lymph filtration and immune response. Lymph carries cytokines, antigens, and cells to the downstream LNs, and their effective delivery to the correct location within the LN directly impacts the quality and quantity of immune response. Despite the importance of this system, the flow patterns in LN have never been quantified, in part because experimental characterization is so difficult.

METHODS AND RESULTS

To achieve a more quantitative knowledge of LN flow, a computational flow model has been developed based on the mouse popliteal LN, allowing for a parameter sensitivity analysis to identify the important system characteristics. This model suggests that about 90% of the lymph takes a peripheral path via the subcapsular and medullary sinuses, while fluid perfusing deeper into the paracortex is sequestered by parenchymal blood vessels. Fluid absorption by these blood vessels under baseline conditions was driven mainly by oncotic pressure differences between lymph and blood, although the magnitude of fluid transfer is highly dependent on blood vessel surface area. We also predict that the hydraulic conductivity of the medulla, a parameter that has never been experimentally measured, should be at least three orders of magnitude larger than that of the paracortex to ensure physiologic pressures across the node.

CONCLUSIONS

These results suggest that structural changes in the LN microenvironment, as well as changes in inflow/outflow conditions, dramatically alter the distribution of lymph, cytokines, antigens, and cells within the LN, with great potential for modulating immune response.

Microenvironmental Control of High-Speed Interstitial T Cell Migration in the Lymph Node

T cells are highly concentrated in the lymph node (LN) paracortex, which serves an important role in triggering adoptive immune responses. Live imaging using two-photon laser scanning microscopy revealed vigorous and non-directional T cell migration within this area at average velocity of more than 10 μm/min. Active interstitial T cell movement is considered to be crucial for scanning large numbers of dendritic cells (DCs) to find rare cognate antigens. However, the mechanism by which T cells achieve such high-speed movement in a densely packed, dynamic tissue environment is not fully understood. Several new findings suggest that fibroblastic reticular cells (FRCs) and DCs control T cell movement in a multilateral manner. Chemokines and lysophosphatidic acid produced by FRCs cooperatively promote the migration, while DCs facilitate LFA-1-dependent motility via expression of ICAM-1. Furthermore, the highly dense and confined microenvironment likely plays a key role in anchorage-independent motility. We propose that T cells dynamically switch between two motility modes; anchorage-dependent and -independent manners. Unique tissue microenvironment and characteristic migration modality of T cells cooperatively generate high-speed interstitial movement in the LN.

Dendritic cells in remodeling of lymph nodes during immune responses

A critical hallmark of adaptive immune responses is the rapid and extensive expansion of lymph nodes. During this process, the complex internal structure of the organs is maintained revealing the existence of mechanisms able to balance lymph node integrity with structural flexibility. This article reviews the extensive architectural remodeling that occurs within lymph nodes during adaptive immune responses and how it is regulated by dendritic cells (DCs). In particular we focus on previously unappreciated functions of DCs in coordinating remodeling of lymph node vasculature, expansion of the fibroblastic reticular network and maintenance of lymphoid stromal phenotypes. Our increased understanding of these processes indicates that DCs need to be viewed not only as key antigen-presenting cells for lymphocytes but also as broad-acting immune sentinels that convey signals to lymphoid organ stroma and thereby facilitate immune response initiation at multiple levels.

Biomaterial Strategies for Immunomodulation

Strategies to enhance, suppress, or qualitatively shape the immune response are of importance for diverse biomedical applications, such as the development of new vaccines, treatments for autoimmune diseases and allergies, strategies for regenerative medicine, and immunotherapies for cancer. However, the intricate cellular and molecular signals regulating the immune system are major hurdles to predictably manipulating the immune response and developing safe and effective therapies. To meet this challenge, biomaterials are being developed that control how, where, and when immune cells are stimulated in vivo, and that can finely control their differentiation in vitro. We review recent advances in the field of biomaterials for immunomodulation, focusing particularly on designing biomaterials to provide controlled immunostimulation, targeting drugs and vaccines to lymphoid organs, and serving as scaffolds to organize immune cells and emulate lymphoid tissues. These ongoing efforts highlight the many ways in which biomaterials can be brought to bear to engineer the immune system.

Reverse-Phase Microarray Analysis Reveals Novel Targets in Lymph Nodes of Bacillus anthracis Spore-Challenged Mice

Anthrax is a frequently fatal infection of many animal species and men. The causative agent Bacillus anthracis propagates through the lymphatic system of the infected host; however, the specific interactions of the host and microbe within the lymphatics are incompletely understood. We report the first description of the phosphoprotein signaling in the lymph nodes of DBA/2 mice using a novel technique combining the reverse-phase microarray with the laser capture microdissesction. Mice were challenged into foot pads with spores of toxinogenic, unencapsulated Sterne strain. The spores quickly migrated to the regional popliteal lymph nodes and spread to the bloodstream as early as 3 h post challenge. All mice died before 72 h post challenge from the systemic disease accompanied by a widespread LN tissue damage by bacteria, including the hemorrhagic necrotizing lymphadenitis, infiltration of CD11b+ and CD3+ cells, and massive proliferation of bacteria in lymph nodes. A macrophage scavenger receptor CD68/macrosialin was upregulated and found in association with vegetative bacteria likely as a marker of their prior interaction with macrophages. The major signaling findings among the 65 tested proteins included the reduced MAPK signaling, upregulation of STAT transcriptional factors, and altered abundance of a number of pro- and anti-apoptotic proteins with signaling properties opposing each other. Downregulation of ERK1/2 was associated with the response of CD11b+ macrophages/dendritic cells, while upregulation of the pro-apoptotic Puma indicated a targeting of CD3+ T-cells. A robust upregulation of the anti-apoptotic survivin was unexpected because generally it is not observed in adult tissues. Taken together with the activation of STATs it may reflect a new pathogenic mechanism aimed to delay the onset of apoptosis. Our data emphasize a notion that the net biological outcome of disease is determined by a cumulative impact of factors representing the microbial insult and the protective capacity of the host.

Adenovirus Improves the Efficacy of Adoptive T-cell Therapy by Recruiting Immune Cells to and Promoting Their Activity at the Tumor

Despite the rapid progress in the development of novel adoptive T-cell therapies, the clinical benefits in treatment of established tumors have remained modest. Several immune evasion mechanisms hinder T-cell entry into tumors and their activity within the tumor. Of note, oncolytic adenoviruses are intrinsically immunogenic due to inherent pathogen-associated molecular patterns. Here, we studied the capacity of adenovirus to overcome resistance of chicken ovalbumin-expressing B16.OVA murine melanoma tumors to adoptive ovalbumin-specific CD8(+) T-cell (OT-I) therapy. Following intraperitoneal transfer of polyclonally activated OT-I lymphocytes, control of tumor growth was superior in mice given intratumoral adenovirus compared with control mice, even in the absence of oncolytic virus replication. Preexisting antiviral immunity against serotype 5 did not hinder the therapeutic efficacy of the combination treatment. Intratumoral adenovirus injection was associated with an increase in proinflammatory cytokines, CD45(+) leukocytes, CD8(+) lymphocytes, and F4/80(+) macrophages, suggesting enhanced tumor immunogenicity. The proinflammatory effects of adenovirus on the tumor microenvironment led to expression of costimulatory signals on CD11c(+) antigen-presenting cells and subsequent activation of T cells, thus breaking the tumor-induced peripheral tolerance. An increased number of CD8(+) T cells specific for endogenous tumor antigens TRP-2 and gp100 was detected in combination-treated mice, indicating epitope spreading. Moreover, the majority of virus/T-cell-treated mice rejected the challenge of parental B16.F10 tumors, suggesting that systemic antitumor immunity was induced. In summary, we provide proof-of-mechanism data on combining adoptive T-cell therapy and adenovirotherapy for the treatment of cancer.

Surface-mediated priming during in vitro generation of monocyte-derived dendritic cells

Ex vivo-generated human dendritic cells (DC) are most commonly generated from monocytes using standard cell culture dishes. To elucidate the effect of the plastic surface during the differentiation process, we compared a standard adhesive plastic dish with four different mainly non-adherent surfaces. Untouched monocytes were cultured for 3 days in the presence of IL-4 and GM-CSF. Time-lapse videos were recorded, and the phenotype of the cells was analysed by flow cytometry. The cytokine profiles were analysed using a 25-plex cytokine assay. The use of non-adherent surfaces led to a significant reduction in expression of CD14 and CD38, and a significant increase in expression of CD86 compared to standard culture dishes. Expression levels of DC-SIGN and PD-L2 were reduced significantly on cells cultured on non-adherent surfaces. The cytokine production was independent on the surface used. The surface-mediated priming should therefore be considered when aiming to induce specific immune responses. This is especially important with regard to DC-based immunotherapy, where an adjustment of the surface during the DC generation process might have highly beneficial effects.

Real-time tracking, retrieval and gene expression analysis of migrating human T cells

Dynamical analysis of single-cells allows assessment of the extent and role of cell-to-cell variability, however traditional dish-and-pipette techniques have hindered single-cell analysis in quantitative biology. We developed an automated microfluidic cell culture system that generates stable diffusion-based chemokine gradients, where cells can be placed in predetermined positions, monitored via single-cell time-lapse microscopy, and subsequently be retrieved based on their migration speed and directionality for further off-chip gene expression analysis, constituting a powerful platform for multiparameter quantitative studies of single-cell chemotaxis. Using this system we studied CXCL12-directed migration of individual human primary T cells. Spatiotemporally deterministic retrieval of T cell subsets in relation to their migration speed, and subsequent analysis with microfluidic droplet digital-PCR showed that the expression level of CXCR4 – the receptor of CXCL12 – underlies enhanced human T cell chemotaxis.

The endothelial protein PLVAP in lymphatics controls the entry of lymphocytes and antigens into lymph nodes

In the lymphatic sinuses of draining lymph nodes, soluble lymph-borne antigens enter the reticular conduits in a size-selective manner and lymphocytes transmigrate to the parenchyma. The molecular mechanisms that control these processes are unknown. Here we unexpectedly found that PLVAP, a prototypic endothelial protein of blood vessels, was synthesized in the sinus-lining lymphatic endothelial cells covering the distal conduits. In PLVAP-deficient mice, both small antigens and large antigens entered the conduit system, and the transmigration of lymphocytes through the sinus floor was augmented. Mechanistically, the filtering function of the lymphatic sinus endothelium was dependent on diaphragms formed by PLVAP fibrils in transendothelial channels. Thus, in the lymphatic sinus, PLVAP forms a physical sieve that regulates the parenchymal entry of lymphocytes and soluble antigens.

Spatiotemporal basis of innate and adaptive immunity in secondary lymphoid tissue

Secondary lymphoid tissues are the sites of both innate and adaptive host defense. Aside from the relatively static nonhematopoietic stromal elements and some macrophages and dendritic cells, most of the cells in these tissues are in constant movement, but the organs maintain a defined microanatomy with preferred locations for the bulk of T cells, B cells, and other lymphocytes and subsets of myeloid cells. Here we describe both the cell dynamics and spatial organization of lymph nodes and review how both physical features and molecular cues guide cell movement to optimize host defense. We emphasize the role of locality in improving the efficiency of a system requiring rare cells to find each other and interact productively through membrane-bound or short-range secreted mediators and highlight how changes in steady-state cell positioning during an infectious challenge contribute to rapid generation of productive responses.

Lymph node stromal cells negatively regulate antigen-specific CD4+ T cell responses

Lymph node (LN) stromal cells (LNSCs) form the functional structure of LNs and play an important role in lymphocyte survival and the maintenance of immune tolerance. Despite their broad spectrum of function, little is known about LNSC responses during microbial infection. In this study, we demonstrate that LNSC subsets display distinct kinetics following vaccinia virus infection. In particular, compared with the expansion of other LNSC subsets and the total LN cell population, the expansion of fibroblastic reticular cells (FRCs) was delayed and sustained by noncirculating progenitor cells. Notably, newly generated FRCs were preferentially located in perivascular areas. Viral clearance in reactive LNs preceded the onset of FRC expansion, raising the possibility that viral infection in LNs may have a negative impact on the differentiation of FRCs. We also found that MHC class II expression was upregulated in all LNSC subsets until day 10 postinfection. Genetic ablation of radioresistant stromal cell-mediated Ag presentation resulted in slower contraction of Ag-specific CD4(+) T cells. We propose that activated LNSCs acquire enhanced Ag-presentation capacity, serving as an extrinsic brake system for CD4(+) T cell responses. Disrupted function and homeostasis of LNSCs may contribute to immune deregulation in the context of chronic viral infection, autoimmunity, and graft-versus-host disease.

Toluene diisocyanate (TDI) disposition and co-localization of immune cells in hair follicles

Diisocyanates (dNCOs) are potent chemical allergens utilized in various industries. It has been proposed that skin exposure to dNCOs produces immune sensitization leading to work-related asthma and allergic disease. We examined dNCOs sensitization by using a dermal murine model of toluene diisocyanate (TDI) exposure to characterize the disposition of TDI in the skin, identify the predominant haptenated proteins, and discern the associated antigen uptake by dendritic cells. Ears of BALB/c mice were dosed once with TDI (0.1% or 4% v/v acetone). Ears and draining lymph nodes (DLNs) were excised at selected time points between 1 h and 15 days post-exposure and were processed for histological, immunohistochemical, and proteomic analyses. Monoclonal antibodies specific for TDI-haptenated protein (TDI-hp) and antibodies to various cell markers were utilized with confocal microscopy to determine co-localization patterns. Histopathological changes were observed following exposure in ear tissue of mice dosed with 4% TDI/acetone. Immunohistochemical staining demonstrated TDI-hp localization in the stratum corneum, hair follicles, and sebaceous glands. TDI-hp were co-localized with CD11b(+) (integrin αM/Mac-1), CD207(+) (langerin), and CD103(+) (integrin αE) cells in the hair follicles and in sebaceous glands. TDI-hp were also identified in the DLN 1 h post-exposure. Cytoskeletal and cuticular keratins along with mouse serum albumin were identified as major haptenated species in the skin. The results of this study demonstrate that the stratum corneum, hair follicles, and associated sebaceous glands in mice are dendritic cell accessible reservoirs for TDI-hp and thus identify a mechanism for immune recognition following epicutaneous exposure to TDI.

Emerging roles of lymphatic endothelium in regulating adaptive immunity

Emerging research on the roles of stromal cells in modulating adaptive immune responses has included a new focus on lymphatic endothelial cells (LECs). LECs are presumably the first cells that come into direct contact with peripheral antigens, cytokines, danger signals, and immune cells travelling from peripheral tissues to lymph nodes. LECs can modulate dendritic cell function, present antigens to T cells on MHC class I and MHC class II molecules, and express immunomodulatory cytokines and receptors, which suggests that their roles in adaptive immunity are far more extensive than previously realized. This Review summarizes the emergent evidence that LECs are important in maintaining peripheral tolerance, limiting and resolving effector T cell responses, and modulating leukocyte function.

The multiple faces of leukocyte interstitial migration

Spatiotemporal control of leukocyte dynamics within tissues is critical for successful innate and adaptive immune responses. Homeostatic trafficking and coordinated infiltration into and within sites of inflammation and infection rely on signaling in response to extracellular cues that in turn controls a variety of intracellular protein networks regulating leukocyte motility, migration, chemotaxis, positioning, and cell-cell interaction. In contrast to mesenchymal cells, leukocytes migrate in an amoeboid fashion by rapid cycles of actin polymerization and actomyosin contraction, and their migration in tissues is generally referred to as low adhesive and nonproteolytic. The interplay of actin network expansion, contraction, and adhesion shapes the exact mode of amoeboid migration, and in this review, we explore how leukocyte subsets potentially harness the same basic biomechanical mechanisms in a cell-type-specific manner. Most of our detailed understanding of these processes derives from in vitro migration studies in three-dimensional gels and confined spaces that mimic geometrical aspects of physiological tissues. We summarize these in vitro results and then critically compare them to data from intravital imaging of leukocyte interstitial migration in mouse tissues. We outline the technical challenges of obtaining conclusive mechanistic results from intravital studies, discuss leukocyte migration strategies in vivo, and present examples of mode switching during physiological interstitial migration. These findings are also placed in the context of leukocyte migration defects in primary immunodeficiencies. This overview of both in vitro and in vivo studies highlights recent progress in understanding the molecular and biophysical mechanisms that shape robust leukocyte migration responses in physiologically complex and heterogeneous environments.

Stromal and hematopoietic cells in secondary lymphoid organs: partners in immunity

Secondary lymphoid organs (SLOs), including lymph nodes, Peyer's patches, and the spleen, have evolved to bring cells of the immune system together. In these collaborative environments, lymphocytes scan the surfaces of antigen-presenting cells for cognate antigens, while moving along stromal networks. The cell-cell interactions between stromal and hematopoietic cells in SLOs are therefore integral to the normal functioning of these tissues. Not only do stromal cells physically construct SLO architecture but they are essential for regulating hematopoietic populations within these domains. Stromal cells interact closely with lymphocytes and dendritic cells, providing scaffolds on which these cells migrate, and recruiting them into niches by secreting chemokines. Within lymph nodes, stromal cell-ensheathed conduit networks transport small antigens deep into the SLO parenchyma. More recently, stromal cells have been found to induce peripheral CD8(+) T-cell tolerance and control the extent to which newly activated T cells proliferate within lymph nodes. Thus, stromal-hematopoietic crosstalk has important consequences for regulating immune cell function within SLOs. In addition, stromal cell interactions with hematopoietic cells, other stroma, and the inflammatory milieu have profound effects on key stromal functions. Here, we examine ways in which these interactions within the lymph node environment influence the adaptive immune response.

Macrophage heterogeneity in lymphoid tissues

Macrophages in lymphoid organs exhibit a wide variety of phenotypes and functions. These cells excel in the removal of apoptotic cells that arise during the generation of immune cells and are thereby essential for the prevention of auto-immune responses. In addition to this macrophages in the secondary lymphoid organs form an important barrier for spreading of infections by phagocytosis of pathogens and the activation of both innate and adaptive immune responses. Thus, the remarkable ability of macrophages to phagocytose and handle a wide range of self and non-self material and to produce immunomediators is effectively exploited within lymphoid organs to regulate immune activation.

Peripheral prepositioning and local CXCL9 chemokine-mediated guidance orchestrate rapid memory CD8+ T cell responses in the lymph node

After an infection, the immune system generates long-lived memory lymphocytes whose increased frequency and altered state of differentiation enhance host defense against reinfection. Recently, the spatial distribution of memory cells was found to contribute to their protective function. Effector memory CD8+ T cells reside in peripheral tissue sites of initial pathogen encounter, in apparent anticipation of reinfection. Here we show that within lymph nodes (LNs), memory CD8+ T cells were concentrated near peripheral entry portals of lymph-borne pathogens, promoting rapid engagement of infected sentinel macrophages. A feed-forward CXCL9-dependent circuit provided additional chemotactic cues that further increase local memory cell density. Memory CD8+ T cells also produced effector responses to local cytokine triggers, but their dynamic behavior differed from that seen after antigen recognition. These data reveal the distinct localization and dynamic behavior of naive versus memory T cells within LNs and how these differences contribute to host defense.

A spatially-organized multicellular innate immune response in lymph nodes limits systemic pathogen spread

The lymphatic network that transports interstitial fluid and antigens to lymph nodes constitutes a conduit system that can be hijacked by invading pathogens to achieve systemic spread unless dissemination is blocked in the lymph node itself. Here, we show that a network of diverse lymphoid cells (natural killer cells, γδ T cells, natural killer T cells, and innate-like CD8+ T cells) are spatially prepositioned close to lymphatic sinus-lining sentinel macrophages where they can rapidly and efficiently receive inflammasome-generated IL-18 and additional cytokine signals from the pathogen-sensing phagocytes. This leads to rapid IFNγ secretion by the strategically positioned innate lymphocytes, fostering antimicrobial resistance in the macrophage population. Interference with this innate immune response loop allows systemic spread of lymph-borne bacteria. These findings extend our understanding of the functional significance of cellular positioning and local intercellular communication within lymph nodes while emphasizing the role of these organs as highly active locations of innate host defense.

Lymph node vascular-stromal growth and function as a potential target for controlling immunity

Lymphadenopathy occurs in many autoimmune and inflammatory diseases, and vascular proliferation is a common feature in the enlarged lymph nodes. The lymph node vasculature plays critical roles in delivering immune cells as well as oxygen and micronutrients, and therefore represents a potential target for therapeutic manipulation of immunity. In this review, we discuss recent insights made in understanding the growth and function of the vascular and associated stromal compartment in immune-stimulated lymph nodes and the potential utility of altering this process in autoimmune diseases.

Stromal cells of the mouse spleen

The composition and function of stromal cells in the white pulp of the spleen resemble to a large extent the situation in other secondary lymphoid organs such as lymph nodes. The stromal cells play an important role in the support and guidance of lymphocytes and myeloid cells in the T and B cell zones of the spleen. Major differences of the spleen are found in the way cells enter the white pulp and the composition of stromal cells in the red pulp. In this review, the features of stromal cells of both white and red pulp will be described in light of the function of the spleen.