Dendritic cell behaviour in vivo : lessons learned from intravital two‐photon microscopy

Monitoring the Cascade of Tumor-specific Immune Response in vivo via Chemoenzymatic Proximity Labeling

Monitoring the highly dynamic and complex immune response remains a great challenge owing to the lack of reliable and specific approaches. Here, we develop a strategy to monitor the cascade of tumor immune response through the cooperation of pore-forming alginate gel with chemoenzymatic proximity-labeling. A macroporous gel containing tumor-associated antigens, adjuvants, and pro-inflammatory cytokines is utilized to recruit endogenous DCs and enhance their maturation in vivo. The mature DCs are then modified with GDP-fucose-fucosyltransferase (GDP-Fuc-Fuct) via the self-catalysis of fucosyltransferase (Fuct). Following the migration of the obtained Fuct-DCs to the draining lymph nodes (dLNs), the molecular recognition mediated interaction of DCs and T cells leads to the successful decoration of T cells with GDP-Fuc-azide through the Fuct catalyzed proximity-labeling. Therefore, the activated tumor-specific T cells in dLNs and tumors can be identified through bioorthogonal labeling, opening up a new avenue for studying the immune mechanism of tumors in situ.

Transcriptomic analysis reveal the responses of dendritic cells to VDBP

BACKGROUND

Vitamin D binding protein (VDBP) is an essential plasma carrier protein, which plays possible roles in reproductive health, disease and so on. However, the effects of VDBP on immunity have not been fully studied and the pertinent literatures remain very limited.

OBJECTIVE

In this study, we introduced the exogenous VDBP into DC2.4 and established a stable DC2.4/VDBP cell line to explore the role of this gene in immunity.

METHODS

Dendritic cells (DCs), as the most effective antigen presenting cells (APC) found so far, are directly involved in regulating some innate immunity. In order to evaluate the biological role of VDBP in DCs, we stably overexpressed VDBP in DCs, and conducted Cell Counting Kit‑8 (CCK-8 kit) and flow cytometry to detect changes in cell function. CCK-8 kit was used to monitor the viability of DCs after gene overexpression, and flow cytometry was used to detect changes in cell cycle distribution and apoptosis. Subsequently, in order to reveal the mechanism of VDBP regulating DCs, we adopted RNA sequencing (RNA-seq).

RESULTS

CCK-8 results revealed VDBP successfully inhibited viability of DCs. Besides, we found that overexpression of this gene greatly promoted apoptosis and obviously altered the cell cycle distribution of DCs in G1 and G2 phases. Moreover, RNA-seq was carried out and 151 differently expression genes (DEGs) were obtained. In addition, gene differential expression analysis showed that most of them were uniformly enriched in immunity-related pathways.

CONCLUSION

These results indicated that VDBP greatly repressed proliferation, facilitated apoptosis and changed cell cycle in DCs via altering the expression levels of gene associated with their cellular immunity.

De-risking clinical trial failure through mechanistic simulation

Drug development typically comprises a combination of pre-clinical experimentation, clinical trials, and statistical data-driven analyses. Therapeutic failure in late-stage clinical development costs the pharmaceutical industry billions of USD per year. Clinical trial simulation represents a key derisking strategy and combining them with mechanistic models allows one to test hypotheses for mechanisms of failure and to improve trial designs. This is illustrated with a T-cell activation model, used to simulate the clinical trials of IMA901, a short-peptide cancer vaccine. Simulation results were consistent with observed outcomes and predicted that responses are limited by peptide off-rates, peptide competition for dendritic cell (DC) binding, and DC migration times. These insights were used to hypothesise alternate trial designs predicted to improve efficacy outcomes. This framework illustrates how mechanistic models can complement clinical, experimental, and data-driven studies to understand, test, and improve trial designs, and how results may differ between humans and mice.

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.

An in silico model of cytotoxic T-lymphocyte activation in the lymph node following short peptide vaccination

Tumour immunotherapy is dependent upon activation and expansion of tumour-targetting immune cells, known as cytotoxic T-lymphocytes (CTLs). Cancer vaccines developed in the past have had limited success and the mechanisms resulting in failure are not well characterized. To elucidate these mechanisms, we developed a human-parametrized, in silico, agent-based model of vaccination-driven CTL activation within a clinical short-peptide vaccination context. The simulations predict a sharp transition in the probability of CTL activation, which occurs with variation in the separation rate (or off-rate) of tumour-specific immune response-inducing peptides (cognate antigen) from the major histocompatibility class I (MHC-I) receptors of dendritic cells (DCs) originally at the vaccination site. For peptides with MHC-I off-rates beyond this transition, it is predicted that no vaccination strategy will lead to successful expansion of CTLs. For slower off-rates, below the transition, the probability of CTL activation becomes sensitive to the numbers of DCs and T cells that interact subsequent to DC migration to the draining lymph node of the vaccination site. Thus, the off-rate is a key determinant of vaccine design.

Zigzag Generalized Lévy Walk: the In Vivo Search Strategy of Immunocytes

Immune responses are based on the coordinated searching behaviors of immunocytes that are aimed at tracking down specific targets. The search efficiency of immunocytes significantly affects the speed of initiation and development of immune responses. Previous studies have shown that not only the intermittent walk but also the zigzag turning preference of immunocytes contributes to the search efficiency. However, among existing models describing immunocytes' search strategy, none has captured both features. Here we propose a zigzag generalized Lévy walk model to describe the search strategy of immunocytes more accurately and comprehensively by considering both the intermittent and the zigzag-turning walk features. Based on the analysis of the searching behaviors of typical immune cell types, dendritic cells and leukocytes, in their native physiological environment, we demonstrate that the model can describe the in vivo search strategy of immunocytes well. Furthermore, by analyzing the search efficiency, we find that this type of search strategy enables immunocytes to capture rare targets in approximately half the time than the previously proposed generalized Lévy walk. This study sheds new light on the fundamental mechanisms that drive the efficient initiation and development of immune responses and in turn may lead to the development of novel therapeutic approaches for diseases ranging from infection to cancer.

Deficiency of lymph node-resident dendritic cells (DCs) and dysregulation of DC chemoattractants in a malnourished mouse model of Leishmania donovani infection

Malnutrition is thought to contribute to more than one-third of all childhood deaths via increased susceptibility to infection. Malnutrition is a significant risk factor for the development of visceral leishmaniasis, which results from skin inoculation of the intracellular protozoan Leishmania donovani. We previously established a murine model of childhood malnutrition and found that malnutrition decreased the lymph node barrier function and increased the early dissemination of L. donovani. In the present study, we found reduced numbers of resident dendritic cells (conventional and monocyte derived) but not migratory dermal dendritic cells in the skin-draining lymph nodes of L. donovani-infected malnourished mice. Expression of chemokines and their receptors involved in trafficking of dendritic cells and their progenitors to the lymph nodes was dysregulated. C-C chemokine receptor type 2 (CCR2) and its ligands (CCL2 and CCL7) were reduced in the lymph nodes of infected malnourished mice, as were CCR2-bearing monocytes/macrophages and monocyte-derived dendritic cells. However, CCR7 and its ligands (CCL19 and CCL21) were increased in the lymph node and CCR7 was increased in lymph node macrophages and dendritic cells. CCR2-deficient mice recapitulated the profound reduction in the number of resident (but not migratory dermal) dendritic cells in the lymph node but showed no alteration in the expression of CCL19 and CCL21. Collectively, these results suggest that the malnutrition-related reduction in the lymph node barrier to dissemination of L. donovani is related to insufficient numbers of lymph node-resident but not migratory dermal dendritic cells. This is likely driven by the altered activity of the CCR2 and CCR7 chemoattractant pathways.

Dendritic cell interactions with lymphatic endothelium

Afferent lymphatic vessels fulfill essential immune functions by transporting leukocytes and lymph-borne antigen to draining lymph nodes (dLNs). An important cell type migrating through lymphatic vessels are dendritic cells (DCs). DCs reside in peripheral tissues like the skin, where they take up antigen and transport it via the lymphatic vascular network to dLNs for subsequent presentation to T cells. As such, DCs play a key role in the induction of adaptive immune responses during infection and vaccination, but also for the maintenance of tolerance. Although the migratory pattern of DCs has been known for long time, interactions between DCs and lymphatic vessels are only now starting to be unraveled at the cellular level. In particular, new tools for visualizing lymphatic vessels in combination with time-lapse microscopy have recently generated valuable insights into the process of DC migration to dLNs. In this review we summarize and discuss current approaches for visualizing DCs and lymphatic vessels in tissues for imaging applications. Furthermore, we review the current state of knowledge about DC migration towards, into and within lymphatic vessels, particularly focusing on the cellular interactions that take place between DCs and the lymphatic endothelium.

Innate immunity in myasthenia gravis thymus: pathogenic effects of Toll-like receptor 4 signaling on autoimmunity

The thymus is the main site of immune sensitization to AChR in myasthenia gravis (MG). In our previous studies we demonstrated that Toll-like receptor (TLR) 4 is over-expressed in MG thymuses, suggesting its involvement in altering the thymic microenvironment and favoring autosensitization and autoimmunity maintenance processes, via an effect on local chemokine/cytokine network. Here, we investigated whether TLR4 signaling may favor abnormal cell recruitment in MG thymus via CCL17 and CCL22, two chemokines known to dictate immune cell trafficking in inflamed organs by binding CCR4. We also investigated whether TLR4 activation may contribute to immunodysregulation, via the production of Th17-related cytokines, known to alter effector T cell (Teff)/regulatory T cell (Treg) balance. We found that CCL17, CCL22 and CCR4 were expressed at higher levels in MG compared to normal thymuses. The two chemokines were mainly detected around medullary Hassall's corpuscles (HCs), co-localizing with TLR4(+) thymic epithelial cells (TECs) and CCR4(+) dendritic cells (DCs), that were present in higher number in MG thymuses compared to controls. TLR4 stimulation in MG TECs increased CCL17 and CCL22 expression and induced the production of Th17-related cytokines. Then, to study the effect of TLR4-stimulated TECs on immune cell interactions and Teff activation, we generated an in-vitro imaging model by co-culturing CD4(+) Th1/Th17 AChR-specific T cells, naïve CD4(+)CD25(+) Tregs, DCs and TECs from Lewis rats. We observed that TLR4 stimulation led to a more pronounced Teff activatory status, suggesting that TLR4 signaling in MG thymic milieu may affect cell-to-cell interactions, favoring autoreactive T-cell activation. Altogether our findings suggest a role for TLR4 signaling in driving DC recruitment in MG thymus via CCL17 and CCL22, and in generating an inflammatory response that might compromise Treg function, favoring autoreactive T-cell pathogenic responses.

Roles of ion transport in control of cell motility

Cell motility is an essential feature of life. It is essential for reproduction, propagation, embryonic development, and healing processes such as wound closure and a successful immune defense. If out of control, cell motility can become life-threatening as, for example, in metastasis or autoimmune diseases. Regardless of whether ciliary/flagellar or amoeboid movement, controlled motility always requires a concerted action of ion channels and transporters, cytoskeletal elements, and signaling cascades. Ion transport across the plasma membrane contributes to cell motility by affecting the membrane potential and voltage-sensitive ion channels, by inducing local volume changes with the help of aquaporins and by modulating cytosolic Ca(2+) and H(+) concentrations. Voltage-sensitive ion channels serve as voltage detectors in electric fields thus enabling galvanotaxis; local swelling facilitates the outgrowth of protrusions at the leading edge while local shrinkage accompanies the retraction of the cell rear; the cytosolic Ca(2+) concentration exerts its main effect on cytoskeletal dynamics via motor proteins such as myosin or dynein; and both, the intracellular and the extracellular H(+) concentration modulate cell migration and adhesion by tuning the activity of enzymes and signaling molecules in the cytosol as well as the activation state of adhesion molecules at the cell surface. In addition to the actual process of ion transport, both, channels and transporters contribute to cell migration by being part of focal adhesion complexes and/or physically interacting with components of the cytoskeleton. The present article provides an overview of how the numerous ion-transport mechanisms contribute to the various modes of cell motility.

Long-term time-lapse multimodal intravital imaging of regeneration and bone-marrow-derived cell dynamics in skin

A major challenge for translating cell-based therapies is understanding the dynamics of cells and cell populations in complex in vivo environments. Intravital microscopy has shown great promise for directly visualizing cell behavior in vivo. However, current methods are limited to relatively short imaging times (hours), by ways to track cell and cell population dynamics over extended time-lapse periods (days to weeks to months), and by relatively few imaging contrast mechanisms that persist over extended investigations. We present technology to visualize and quantify complex, multifaceted dynamic changes in natural deformable skin over long time periods using novel multimodal imaging and a non-rigid image registration method. These are demonstrated in green fluorescent protein (GFP) bone marrow (BM) transplanted mice to study dynamic skin regeneration. This technology provides a novel perspective for studying dynamic biological processes and will enable future studies of stem, immune, and tumor cell biology in vivo.

The malnutrition-related increase in early visceralization of Leishmania donovani is associated with a reduced number of lymph node phagocytes and altered conduit system flow

In a murine model of moderate childhood malnutrition we found that polynutrient deficiency led to a 4–5-fold increase in early visceralization of L. donovani (3 days post-infection) following cutaneous infection and a 16-fold decrease in lymph node barrier function (p<0.04 for all). To begin to understand the mechanistic basis for this malnutrition-related parasite dissemination we analyzed the cellularity, architecture, and function of the skin-draining lymph node. There was no difference in the localization of multiple cell populations in the lymph node of polynutrient deficient (PND) mice, but there was reduced cellularity with fewer CD11c⁺dendritic cells (DCs), fibroblastic reticular cells (FRCs), MOMA-2⁺ macrophages, and CD169⁺ subcapsular sinus macrophage (p<0.05 for all) compared to the well-nourished (WN) mice. The parasites were equally co-localized with DCs associated with the lymph node conduit network in the WN and PND mice, and were found in the high endothelial venule into which the conduits drain. When a fluorescent low molecular weight (10 kD) dextran was delivered in the skin, there was greater efflux of the marker from the lymph node conduit system to the spleens of PND mice (p<0.04), indicating that flow through the conduit system was altered. There was no evidence of disruption of the conduit or subcapsular sinus architecture, indicating that the movement of parasites into the subcortical conduit region was due to an active process and not from passive movement through a leaking barrier. These results indicate that the impaired capacity of the lymph node to act as a barrier to dissemination of L. donovani infection is associated with a reduced number of lymph node phagocytes, which most likely leads to reduced capture of parasites as they transit through the sinuses and conduit system.

The impact of malnutrition in the world is staggering. Malnutrition is thought to directly or indirectly contribute to more than half of all childhood deaths, most of them related to heightened susceptibility to infection. Visceral leishmaniasis (VL), caused by the intracellular protozoan Leishmania donovani, is a progressive, potentially fatal infection found in many resource-poor regions of the world. Most people who get infected with this parasite have only an asymptomatic latent infection, however, people who are malnourished have a greatly increased risk of developing severe VL. We initiated these studies of an experimental model that mimics human childhood malnutrition to better understand how malnutrition increases the susceptibility to VL at the molecular and cellular level. In this model we found that malnutrition led to failure of the skin-draining lymph node to act as a barrier to dissemination. This loss of lymph node barrier function was associated with a significant reduction in the numbers of dendritic cells and macrophages, phagocytic cells that capture and kill invading pathogens, and alteration of the flow of lymph through the lymph node.

Shedding light on cutaneous innate immune responses: the intravital microscopy approach

The skin is under constant assault by environmental factors and microbes. Innate immune cells in epidermis and dermis regulate immune responses against pathogens while maintaining tolerance against commensal bacteria and autoantigens. The introduction of intravital imaging approaches, in particular multiphoton microscopy, has enabled studying the cellular and molecular regulation of cutaneous immunity in real time within intact skin. Here, we discuss recent advances in our understanding of innate immune cell behaviour in the skin, as unravelled by intravital microscopy, with emphasis on the function of myeloid cells, including dendritic cells, neutrophils and monocytes.

Role of ion channels and transporters in cell migration

Cell motility is central to tissue homeostasis in health and disease, and there is hardly any cell in the body that is not motile at a given point in its life cycle. Important physiological processes intimately related to the ability of the respective cells to migrate include embryogenesis, immune defense, angiogenesis, and wound healing. On the other side, migration is associated with life-threatening pathologies such as tumor metastases and atherosclerosis. Research from the last ≈ 15 years revealed that ion channels and transporters are indispensable components of the cellular migration apparatus. After presenting general principles by which transport proteins affect cell migration, we will discuss systematically the role of channels and transporters involved in cell migration.

Over-expression of Hlx homeobox gene in DC2.4 dendritic cell enhances its maturation and antigen presentation

Hlx as a Th1-specific transcription factor, it appears to drive maturation of Th1 and IFN-γ secretion in cooperation with T-bet. In this study, we established a stable Hlx-over-expressed dendritic cell line (DC2.4/Hlx), and investigated the possible effect of Hlx gene on maturation of dendritic cell-line (DC2.4). Results shown that over-expressed Hlx in DC2.4 up-regulated the transcription and expression of IFN-γ, increased the expression of maturation makers including CD40, CD80, CD86, MHC-I and MHC-II. Functional assay for DC2.4/Hlx showed that over-expressed Hlx increased the expression level of interleukin-12 in the supernatant and decreased DC endocytosis when cells were incubated in vitro. Furthermore, using a syngeneic T cell activation model, we found that DC2.4/Hlx could obviously present ovalbumin (OVA) antigen to T cell in OVA pre-immunized mice.

Mechanisms of dendritic cell trafficking across the blood-brain barrier

Although the central nervous system (CNS) is considered to be an immunoprivileged site, it is susceptible to a host of autoimmune as well as neuroinflammatory disorders owing to recruitment of immune cells across the blood-brain barrier into perivascular and parenchymal spaces. Dendritic cells (DCs), which are involved in both primary and secondary immune responses, are the most potent immune cells in terms of antigen uptake and processing as well as presentation to T cells. In light of the emerging importance of DC traficking into the CNS, these cells represent good candidates for targeted immunotherapy against various neuroinflammatory diseases. This review focuses on potential physiological events and receptor interactions between DCs and the microvascular endothelial cells of the brain as they transmigrate into the CNS during degeneration and injury. A clear understanding of the underlying mechanisms involved in DC migration may advance the development of new therapies that manipulate these mechanistic properties via pharmacologic intervention. Furthermore, therapeutic validation should be in concurrence with the molecular imaging techniques that can detect migration of these cells in vivo. Since the use of noninvasive methods to image migration of DCs into CNS has barely been explored, we highlighted potential molecular imaging techniques to achieve this goal. Overall, information provided will bring this important leukocyte population to the forefront as key players in the immune cascade in the light of the emerging contribution of DCs to CNS health and disease.

Chemotaxis of cell populations through confined spaces at single-cell resolution

Cell migration is crucial for both physiological and pathological processes. Current in vitro cell motility assays suffer from various drawbacks, including insufficient temporal and/or optical resolution, or the failure to include a controlled chemotactic stimulus. Here, we address these limitations with a migration chamber that utilizes a self-sustaining chemotactic gradient to induce locomotion through confined environments that emulate physiological settings. Dynamic real-time analysis of both population-scale and single-cell movement are achieved at high resolution. Interior surfaces can be functionalized through adsorption of extracellular matrix components, and pharmacological agents can be administered to cells directly, or indirectly through the chemotactic reservoir. Direct comparison of multiple cell types can be achieved in a single enclosed system to compare inherent migratory potentials. Our novel microfluidic design is therefore a powerful tool for the study of cellular chemotaxis, and is suitable for a wide range of biological and biomedical applications.

DC mobilization from the skin requires docking to immobilized CCL21 on lymphatic endothelium and intralymphatic crawling

Dendritic cells (DCs) must travel through lymphatics to carry skin antigens into lymph nodes. The processes controlling their mobilization and migration have not been completely delineated. We studied how DCs in live mice respond to skin inflammation, transmigrate through lymphatic endothelium, and propagate in initial lymphatics. At steady state, dermal DCs remain sessile along blood vessels. Inflammation mobilizes them, accelerating their interstitial motility 2.5-fold. CCR7-deficient BMDCs crawl as fast as wild-type DCs but less persistently. We observed discrete depositions of CCL21 complexed with collagen-IV on the basement membrane of initial lymphatics. Activated DCs move directionally toward lymphatics, contact CCL21 puncta, and migrate through portals into the lumen. CCR7-deficient DCs arrive at lymphatics through random migration but fail to dock and transmigrate. Once inside vessels, wild-type DCs use lamellipodia to crawl along lymphatic endothelium and, sensing lymph flow, proceed downstream. DCs start drifting freely only in collecting lymphatics. These results demonstrate in vivo that the CCL21-CCR7 axis plays a dual role in DC mobilization: promoting both chemotaxis and arrest of DCs on lymphatic endothelium. Intralymphatic crawling, in which DCs combine active adhesion-based migration and directional cues from lymph flow, represents a new step in DC mobilization which may be amenable to regulation.

Analysis of behavior and trafficking of dendritic cells within the brain during toxoplasmic encephalitis

Under normal conditions the immune system has limited access to the brain; however, during toxoplasmic encephalitis (TE), large numbers of T cells and APCs accumulate within this site. A combination of real time imaging, transgenic reporter mice, and recombinant parasites allowed a comprehensive analysis of CD11c⁺ cells during TE. These studies reveal that the CNS CD11c⁺ cells consist of a mixture of microglia and dendritic cells (DCs) with distinct behavior associated with their ability to interact with parasites or effector T cells. The CNS DCs upregulated several chemokine receptors during TE, but none of these individual receptors tested was required for migration of DCs into the brain. However, this process was pertussis toxin sensitive and dependent on the integrin LFA-1, suggesting that the synergistic effect of signaling through multiple chemokine receptors, possibly leading to changes in the affinity of LFA-1, is involved in the recruitment/retention of DCs to the CNS and thus provides new insights into how the immune system accesses this unique site.

Toxoplasmic encephalitis (TE), caused by the protozoan parasite Toxoplasma gondii, can be potentially life threatening especially in immuno-compromised individuals. Immune cells including dendritic cells have been shown to accumulate in the brain during chronic toxoplasmosis; however, little is known about their function, their behavior in vivo, and the mechanisms by which they migrate into the brain. In the present studies, we utilize a combination of real time imaging, transgenic reporter mice, and recombinant parasites to reveal the distinct behavior and morphologies of dendritic cells within the brain and their ability to interact with parasites and effector T cells during TE. The CNS DCs were also found to exhibit a unique chemokine receptor expression pattern during infection, and the migration of DCs into the brain was mediated through a pertussis toxin (which blocks signaling downstream of several chemokine receptors) sensitive process and dependent on the integrin LFA-1. There is currently a poor understanding of the events that lead to DC recruitment to the CNS during inflammation in general, and our studies provide new insights into the mechanisms by which antigen-presenting cells gain access to the brain during infection.

Accelerated dendritic cell differentiation from migrating Ly6C(lo) bone marrow monocytes in early dermal West Nile virus infection

No study has investigated the participation of Ly6C(+) monocytes in the earliest phase of skin infection with the mosquito-borne West Nile virus. In a novel murine model mimicking natural dermal infection, CCL2-dependent bone marrow (BM)-derived monocyte migration, differentiation into Ly6C(+) dendritic cells (DC), and accumulation around dermal deposits of infected fibroblasts by day 1 postinfection were associated with increasing numbers of monocyte-derived TNF/inducible NO synthase-producing DC by day 2 postinfection in draining auricular lymph nodes (ALN). Adoptive transfer demonstrated simultaneous migration of bone marrow-derived Ly6C(lo) monocytes to virus-infected dermis and ALN, where they first become Ly6C(hi) DC within 24 h and then Ly6C(lo) DC by 72 h. DC migration from the infected dermis to the ALN derived exclusively from Ly6C(lo) BM monocytes. This demonstrates that Ly6C(hi) and Ly6C(lo) BM-derived monocytes have different fates in vivo and suggests that BM may be a reservoir of preinflammatory monocytes for rapid deployment as inflammatory DC during virus infection.

Accumulative difference image protocol for particle tracking in fluorescence microscopy tested in mouse lymphonodes

The basic research in cell biology and in medical sciences makes large use of imaging tools mainly based on confocal fluorescence and, more recently, on non-linear excitation microscopy. Substantially the aim is the recognition of selected targets in the image and their tracking in time. We have developed a particle tracking algorithm optimized for low signal/noise images with a minimum set of requirements on the target size and with no a priori knowledge of the type of motion. The image segmentation, based on a combination of size sensitive filters, does not rely on edge detection and is tailored for targets acquired at low resolution as in most of the in-vivo studies. The particle tracking is performed by building, from a stack of Accumulative Difference Images, a single 2D image in which the motion of the whole set of the particles is coded in time by a color level. This algorithm, tested here on solid-lipid nanoparticles diffusing within cells and on lymphocytes diffusing in lymphonodes, appears to be particularly useful for the cellular and the in-vivo microscopy image processing in which few a priori assumption on the type, the extent and the variability of particle motions, can be done.

Cell-autonomous and environmental contributions to the interstitial migration of T cells

A key to understanding the functioning of the immune system is to define the mechanisms that facilitate directed lymphocyte migration to and within tissues. The recent development of improved imaging technologies, most prominently multi-photon microscopy, has enabled the dynamic visualization of immune cells in real-time directly within intact tissues. Intravital imaging approaches have revealed high spontaneous migratory activity of T cells in secondary lymphoid organs and inflamed tissues. Experimental evidence points towards both environmental and cell-intrinsic cues involved in the regulation of lymphocyte motility in the interstitial space. Based on these data, several conceptually distinct models have been proposed in order to explain the coordination of lymphocyte migration both at the single cell and population level. These range from “stochastic” models, where chance is the major driving force, to “deterministic” models, where the architecture of the microenvironment dictates the migratory trajectory of cells. In this review, we focus on recent advances in understanding naïve and effector T cell migration in vivo. In addition, we discuss some of the contradictory experimental findings in the context of theoretical models of migrating leukocytes.

Dynamics of dendritic cell-T cell interactions: a role in T cell outcome

Antigen-specific dendritic cells (DC)-T cell encounters occur in lymph nodes (LNs) and are essential for the induction of both priming and tolerance. In both cases, T cells are rapidly activated and proliferate. However, the subsequent outcome of T cell activation depends on the modulation of different DC- and T cell-intrinsic signals. Recent advances in two-photon (2P) microscopy have furthered our understanding regarding the complex choreography of DCs and T cells in intact LNs, and established differences in the dynamics of DC-T cell contacts during priming and tolerance induction. The mechanisms that favour DC-T cell encounters, as well as the contribution of the frequency and the duration of such encounters in dictating the T cell response, are discussed in this review.

Alpha4beta1 integrin mediates the recruitment of immature dendritic cells across the blood-brain barrier during experimental autoimmune encephalomyelitis

Dendritic cells (DCs) within the CNS are recognized to play an important role in the effector phase and propagation of the immune response in experimental autoimmune encephalomyelitis (EAE), a mouse model for multiple sclerosis. However, the mechanisms regulating DC trafficking into the CNS still need to be characterized. In this study, we show by performing intravital fluorescence videomicroscopy of the inflamed spinal cord white-matter microvasculature in SJL mice with EAE that immature, and to a lesser extent, LPS-matured, bone marrow-derived DCs efficiently interact with the CNS endothelium by rolling, capturing, and firm adhesion. Immature but not LPS-matured DCs efficiently migrated across the wall of inflamed parenchymal microvessels into the CNS. Blocking alpha4 integrins interfered with the adhesion but not the rolling or capturing of immature and LPS-matured DCs to the CNS microvascular endothelium, inhibiting their migration across the vascular wall. Functional absence of beta1 integrins but not of beta7 integrins or alpha4beta7 integrin similarly reduced the adhesion of immature DCs to the CNS microvascular endothelium, demonstrating that alpha4beta1 but not alpha4beta7 integrin mediates this step of immature DCs interaction with the inflamed blood-brain barrier during EAE. Our study shows that during EAE, especially immature DCs migrate into the CNS, where they may be crucial for the perpetuation of the CNS-targeted autoimmune response. Thus therapeutic targeting of alpha4 integrins affects DC trafficking into the CNS and may therefore lead to the resolution of the CNS autoimmune inflammation by reducing the number of CNS professional APCs.

Imaging the immune response to monitor tumor immunotherapy

The goal of cancer immunotherapy is to promote antitumor immunity, and novel approaches include vaccination, adoptive transfer of tumor-reactive T cells, and administration of monoclonal antibodies and small molecules that target immune regulatory pathways. The molecular and cellular events responsible for tumor rejection are not completely defined and correlative studies have been used to help understand the mechanisms and extent of immune activation and tumor regression with these approaches. The real-time monitoring of immune responses to immunotherapy has been challenging as specific cell subsets may be difficult to define, and molecular pathways have evolved functionally diverse outcomes in different cells and in different tissues. Recently, improvements in optics and digital imaging have led to novel imaging techniques that make it possible to track the migration of individual immune cells ex vivo and in vivo, and to detect the dynamic interactions between T cells and antigen-presenting cells or tumor cells within complex microenvironments, including lymphoid tissue and established tumors. This review will explain some of the more established imaging techniques and discuss their role in monitoring the immune response in patients treated with various tumor immunotherapy approaches.

Development of an ex vivo human skin model for intradermal vaccination: tissue viability and Langerhans cell behaviour

The presence of resident Langerhans cells (LCs) in the epidermis makes the skin an attractive target for DNA vaccination. However, reliable animal models for cutaneous vaccination studies are limited. We demonstrate an ex vivo human skin model for cutaneous DNA vaccination which can potentially bridge the gap between pre-clinical in vivo animal models and clinical studies. Cutaneous transgene expression was utilised to demonstrate epidermal tissue viability in culture. LC response to the culture environment was monitored by immunohistochemistry. Full-thickness and split-thickness skin remained genetically viable in culture for at least 72 h in both phosphate-buffered saline (PBS) and full organ culture medium (OCM). The epidermis of explants cultured in OCM remained morphologically intact throughout the culture duration. LCs in full-thickness skin exhibited a delayed response (reduction in cell number and increase in cell size) to the culture conditions compared with split-thickness skin, whose response was immediate. In conclusion, excised human skin can be cultured for a minimum of 72 h for analysis of gene expression and immune cell activation. However, the use of split-thickness skin for vaccine formulation studies may not be appropriate because of the nature of the activation. Full-thickness skin explants are a more suitable model to assess cutaneous vaccination ex vivo.

Invited review article: Imaging techniques for harmonic and multiphoton absorption fluorescence microscopy

We review the current state of multiphoton microscopy. In particular, the requirements and limitations associated with high-speed multiphoton imaging are considered. A description of the different scanning technologies such as line scan, multifoci approaches, multidepth microscopy, and novel detection techniques is given. The main nonlinear optical contrast mechanisms employed in microscopy are reviewed, namely, multiphoton excitation fluorescence, second harmonic generation, and third harmonic generation. Techniques for optimizing these nonlinear mechanisms through a careful measurement of the spatial and temporal characteristics of the focal volume are discussed, and a brief summary of photobleaching effects is provided. Finally, we consider three new applications of multiphoton microscopy: nonlinear imaging in microfluidics as applied to chemical analysis and the use of two-photon absorption and self-phase modulation as contrast mechanisms applied to imaging problems in the medical sciences.

Dynamic Imaging of CD8(+) T cells and dendritic cells during infection with Toxoplasma gondii

To better understand the initiation of CD8⁺ T cell responses during infection, the primary response to the intracellular parasite Toxoplasma gondii was characterized using 2-photon microscopy combined with an experimental system that allowed visualization of dendritic cells (DCs) and parasite specific CD8⁺ T cells. Infection with T. gondii induced localization of both these populations to the sub-capsular/interfollicular region of the draining lymph node and DCs were required for the expansion of the T cells. Consistent with current models, in the presence of cognate antigen, the average velocity of CD8⁺ T cells decreased. Unexpectedly, infection also resulted in modulation of the behavior of non-parasite specific T cells. This TCR-independent process correlated with the re-modeling of the lymph node micro-architecture and changes in expression of CCL21 and CCL3. Infection also resulted in sustained interactions between the DCs and CD8⁺ T cells that were visualized only in the presence of cognate antigen and were limited to an early phase in the response. Infected DCs were rare within the lymph node during this time frame; however, DCs presenting the cognate antigen were detected. Together, these data provide novel insights into the earliest interaction between DCs and CD8⁺ T cells and suggest that cross presentation by bystander DCs rather than infected DCs is an important route of antigen presentation during toxoplasmosis.

Toxoplasma gondii is a protozoan parasite that can infect a wide range of hosts, including humans. Infection with T. gondii is potentially life threatening in immuno-compromised individuals and it can be detrimental during pregnancy, often leading to abortion of the fetus. Dendritic cells are thought to play a vital role in the development of protective immunity to Toxoplasma gondii through their ability to produce immunological signals such as cytokines and also process and present parasite derived peptides to T cells. However, little is known about the actual interactions between these cell types in an intact organ, such as the lymph node, during infection. Using the technology of live imaging by 2-photon microscopy we have identified a very early window of time during infection when dendritic cells and T cells make sustained contacts with one another, which appears crucial for the generation of protective responses. We also show that substantial changes are induced in the lymph node micro-architecture as a result of infection, which in turn could have effects on immune responses to secondary pathogens. Understanding the interaction between these immune cells in vivo that leads to resistance to active infection would help in the design of better strategies to develop protective immune responses against this pathogen in immuno-compromised individuals.

T-cell activation by dendritic cells in the lymph node: lessons from the movies

Interactions between T cells and dendritic cells (DCs) in the lymph nodes are crucial for initiating cell-mediated adaptive immune responses. With the help of two-photon imaging, the complexity of these cellular contacts in vivo has recently been captured in time-lapse movies in several immunological contexts. Well beyond the satisfaction of seeing a T-cell response as it happens, these experiments provide fundamental insights into the regulation and the biological meaning of T-cell-DC contact dynamics. This Review focuses on how this emerging field is changing our perception of T-cell activation by DCs.

Visualizing dendritic cell migration within the skin

Dendritic cells (DCs) within the skin are a heterogeneous population of cells, including Langerhans cells of the epidermis and at least three subsets of dermal DCs. Collectively, these DCs play important roles in the initiation of adaptive immune responses following antigen challenge of the skin as well as being mediators of tolerance to self-antigen. A key functional aspect of cutaneous DCs is their migration both within the skin and into lymphatic vessels, resulting in their emigration to draining lymph nodes. Here, we discuss our current understanding of the requirements for successful DC migration in and from the skin, and introduce some of the microscopic techniques developed in our laboratory to facilitate a better understanding of this process. In particular, we detail our current use of multi-photon excitation (MPE) microscopy of murine skin to dissect the migratory behavior of DCs in vivo.