In vivo imaging of lymphocyte trafficking

Spatiotemporal orchestration of macrophage activation trajectories by Vγ4 T cells during skin wound healing

Dysregulated macrophage polarization from pro-inflammatory M1 to anti-inflammatory M2 phenotypes underlies impaired cutaneous wound healing. This study reveals Vγ4+ γδ T cells spatiotemporally calibrate macrophage trajectories during skin repair via sophisticated interferon-γ (IFN-γ) conditioning across multiple interconnected tissues. Locally within wound beds, infiltrating Vγ4+ γδ T cells directly potentiate M1 activation and suppress M2 polarization thereby prolonging local inflammation. In draining lymph nodes, infiltrated Vγ4+ γδ T cells expand populations of IFN-γ-competent lymphocytes which disseminate systemically and infiltrate into wound tissues, further enforcing M1 macrophages programming. Moreover, Vγ4+γδ T cells flushed into bone marrow stimulate increased IFN-γ production, which elevates the output of pro-inflammatory Ly6C+monocytes. Mobilization of these monocytes continually replenishes the M1 macrophage pool in wounds, preventing phenotypic conversion to M2 activation. Thus, multi-axis coordination of macrophage activation trajectories by trafficking Vγ4+ γδ T cells provides a sophisticated immunological mechanism regulating inflammation timing and resolution during skin repair.

Integration of immune cells in organs-on-chips: a tutorial

Viral and bacterial infections continue to pose significant challenges for numerous individuals globally. To develop novel therapies to combat infections, more insight into the actions of the human innate and adaptive immune system during infection is necessary. Human in vitro models, such as organs-on-chip (OOC) models, have proven to be a valuable addition to the tissue modeling toolbox. The incorporation of an immune component is needed to bring OOC models to the next level and enable them to mimic complex biological responses. The immune system affects many (patho)physiological processes in the human body, such as those taking place during an infection. This tutorial review introduces the reader to the building blocks of an OOC model of acute infection to investigate recruitment of circulating immune cells into the infected tissue. The multi-step extravasation cascade in vivo is described, followed by an in-depth guide on how to model this process on a chip. Next to chip design, creation of a chemotactic gradient and incorporation of endothelial, epithelial, and immune cells, the review focuses on the hydrogel extracellular matrix (ECM) to accurately model the interstitial space through which extravasated immune cells migrate towards the site of infection. Overall, this tutorial review is a practical guide for developing an OOC model of immune cell migration from the blood into the interstitial space during infection.

Single-cell RNA sequencing depicts the local cell landscape in thyroid-associated ophthalmopathy

There is a specific reactivity and characteristic remodeling of the periocular tissue in thyroid-associated ophthalmopathy (TAO). However, local cell changes responsible for these pathological processes have not been sufficiently identified. Here, single-cell RNA sequencing is performed to characterize the transcriptional changes of cellular components in the orbital connective tissue in individuals with TAO. Our study shows that lipofibroblasts with RASD1 expression are highly involved in inflammation and adipogenesis during TAO. ACKR1⁺ endothelial cells and adipose tissue macrophages may engage in TAO pathogenesis. We find CD8⁺CD57⁺ cytotoxic T lymphocytes with the terminal differentiation phenotype to be another source of interferon-γ, a molecule actively engaging in TAO pathogenesis. Cell-cell communication analysis reveals increased activity of CXCL8/ACKR1 and TNFSF4/TNFRSF4 interactions in TAO. This study provides a comprehensive local cell landscape of TAO and may be valuable for future therapy investigation.

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A local transcriptional landscape of orbital connective tissue in TAO is developed

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RASD1-expressing lipofibroblasts are highly involved in adipogenesis and inflammation

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ACKR1⁺ endothelial cells contribute to inflammatory cell infiltration in TAO

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Adipose tissue macrophages engage in lipid metabolism and inflammatory response in TAO

Intravital three-photon microscopy allows visualization over the entire depth of mouse lymph nodes

Intravital confocal microscopy and two-photon microscopy are powerful tools to explore the dynamic behavior of immune cells in mouse lymph nodes (LNs), with penetration depth of ~100 and ~300 μm, respectively. Here, we used intravital three-photon microscopy to visualize the popliteal LN through its entire depth (600-900 μm). We determined the laser average power and pulse energy that caused measurable perturbation in lymphocyte migration. Long-wavelength three-photon imaging within permissible parameters was able to image the entire LN vasculature in vivo and measure CD8⁺ T cells and CD4⁺ T cell motility in the T cell zone over the entire depth of the LN. We observed that the motility of naive CD4⁺ T cells in the T cell zone during lipopolysaccharide-induced inflammation was dependent on depth. As such, intravital three-photon microscopy had the potential to examine immune cell behavior in the deeper regions of the LN in vivo.

The Critical Importance of Spatial and Temporal Scales in Designing and Interpreting Immune Cell Migration Assays

Intravital microscopy and other direct-imaging techniques have allowed for a characterisation of leukocyte migration that has revolutionised the field of immunology, resulting in an unprecedented understanding of the mechanisms of immune response and adaptive immunity. However, there is an assumption within the field that modern imaging techniques permit imaging parameters where the resulting cell track accurately captures a cell's motion. This notion is almost entirely untested, and the relationship between what could be observed at a given scale and the underlying cell behaviour is undefined. Insufficient spatial and temporal resolutions within migration assays can result in misrepresentation of important physiologic processes or cause subtle changes in critical cell behaviour to be missed. In this review, we contextualise how scale can affect the perceived migratory behaviour of cells, summarise the limited approaches to mitigate this effect, and establish the need for a widely implemented framework to account for scale and correct observations of cell motion. We then extend the concept of scale to new approaches that seek to bridge the current "black box" between single-cell behaviour and systemic response.

Aging of the Hematopoietic Stem Cell Niche: New Tools to Answer an Old Question

The hematopoietic stem cell (HSC) niche is a specialized microenvironment, where a complex and dynamic network of interactions across multiple cell types regulates HSC function. During the last years, it became progressively clearer that changes in the HSC niche are responsible for specific alterations of HSC behavior. The aging of the bone marrow (BM) microenvironment has been shown to critically contribute to the decline in HSC function over time. Interestingly, while upon aging some niche structures within the BM are degenerated and negatively affect HSC functionality, other niche cells and specific signals are preserved and essential to retaining HSC function and regenerative capacity. These new findings on the role of the aging BM niche critically depend on the implementation of new technical tools, developed thanks to transdisciplinary approaches, which bring together different scientific fields. For example, the development of specific mouse models in addition to coculture systems, new 3D-imaging tools, ossicles, and ex-vivo BM mimicking systems is highlighting the importance of new technologies to unravel the complexity of the BM niche on aging. Of note, an exponential impact in the understanding of this biological system has been recently brought by single-cell sequencing techniques, spatial transcriptomics, and implementation of artificial intelligence and deep learning approaches to data analysis and integration. This review focuses on how the aging of the BM niche affects HSCs and on the new tools to investigate the specific alterations occurring in the BM upon aging. All these new advances in the understanding of the BM niche and its regulatory function on HSCs have the potential to lead to novel therapeutical approaches to preserve HSC function upon aging and disease.

Heterogeneity in antiviral B cell responses: Lessons from the movies

Humoral and cellular responses to viral infections coexist in a dynamic equilibrium that often results in efficient viral clearance. However, in some infections one of the two responses prevails, for instance when an overactivation of cytotoxic T cells is accompanied by weak and insufficient antibody responses. Although the cellular response is usually sufficient to control a primary viral infection, in some cases clearance is not complete and persistent infections ensue. In order to design effective therapeutic or vaccination strategies aiming at inducing early and potent neutralizing antibody responses, a deep knowledge of the cellular and molecular determinants of antiviral immune responses is needed. Here, we review our understanding on the spatiotemporal dynamics of antiviral humoral immune responses, with a particular focus on recent studies using intravital imaging approaches as an insightful complement to more traditional techniques.

Expression of E-Cadherin in Epithelial Cancer Cells Increases Cell Motility and Directionality through the Localization of ZO-1 during Collective Cell Migration

Collective cell migration of epithelial tumor cells is one of the important factors for elucidating cancer metastasis and developing novel drugs for cancer treatment. Especially, new roles of E-cadherin in cancer migration and metastasis, beyond the epithelial–mesenchymal transition, have recently been unveiled. Here, we quantitatively examined cell motility using micropatterned free edge migration model with E-cadherin re-expressing EC96 cells derived from adenocarcinoma gastric (AGS) cell line. EC96 cells showed increased migration features such as the expansion of cell islands and straightforward movement compared to AGS cells. The function of tight junction proteins known to E-cadherin expression were evaluated for cell migration by knockdown using sh-RNA. Cell migration and straight movement of EC96 cells were reduced by knockdown of ZO-1 and claudin-7, to a lesser degree. Analysis of the migratory activity of boundary cells and inner cells shows that EC96 cell migration was primarily conducted by boundary cells, similar to leader cells in collective migration. Immunofluorescence analysis showed that tight junctions (TJs) of EC96 cells might play important roles in intracellular communication among boundary cells. ZO-1 is localized to the base of protruding lamellipodia and cell contact sites at the rear of cells, indicating that ZO-1 might be important for the interaction between traction and tensile forces. Overall, dynamic regulation of E-cadherin expression and localization by interaction with ZO-1 protein is one of the targets for elucidating the mechanism of collective migration of cancer metastasis.

Photonic technologies for liquid biopsies: recent advances and open research challenges

The recent development of sophisticated techniques capable of detecting extremely low concentrations of circulating tumor biomarkers in accessible body fluids, such as blood or urine, could contribute to a paradigm shift in cancer diagnosis and treatment. By applying such techniques, clinicians can carry out liquid biopsies, providing information on tumor presence, evolution, and response to therapy. The implementation of biosensing platforms for liquid biopsies is particularly complex because this application domain demands high selectivity/specificity and challenging limit-of-detection (LoD) values. The interest in photonics as an enabling technology for liquid biopsies is growing owing to the well-known advantages of photonic biosensors over competing technologies in terms of compactness, immunity to external disturbance, and ultra-high spatial resolution. Some encouraging experimental results in the field of photonic devices and systems for liquid biopsy have already been achieved by using fluorescent labels and label-free techniques and by exploiting super-resolution microscopy, surface plasmon resonance, surface-enhanced Raman scattering, and whispering gallery mode resonators. This paper critically reviews the current state-of-the-art, starting from the requirements imposed by the detection of the most common circulating biomarkers. Open research challenges are considered together with competing technologies, and the most promising paths of improvement are discussed for future applications.

Multiphoton Intravital Microscopy of Mandibular Draining Lymph Nodes: A Mouse Model to Study Corneal Immune Responses

Multiphoton intravital microscopy (MP-IVM) is a powerful tool to image cells in vivo. Its application in immunology research has opened new horizons, allowing intravital imaging of leukocytes at the single-cell level. A transparent cornea is vital to retain vision. As an immune privileged site, a rapid innate response to foreign antigens is crucial in clearing opportunistic bacterial and viral pathogens, and minimizing collateral structural damage to the cornea. Furthermore, dissecting the mechanisms and preventing the immunological rejection process after corneal transplantation is imperative to retain sight. Therefore, understanding the underlying mechanisms behind corneal immunity, specifically the process of antigen presentation and adaptive immunity in the mandibular draining lymph nodes (dLNs) in vivo, is crucial. Attempts of intravital imaging of mandibular dLNs have yielded little success to date, due to breathing artifacts and the location that is difficult to access. Herein, we present the first MP-IVM mouse model of the mandibular dLNs, utilizing transgenic mice in which CD11c⁺ cells are fluorescently labeled. Furthermore, we demonstrate that CD11c-YFP⁺ cells are localized mainly in the parafollicular cortex (T cell zone) and subcapsular area and are sparsely distributed in the follicular region (B cell zone) of mandibular dLNs during steady state. A significant increase in host CD11c-YFP⁺ cell density is noted at 14 and 21 days following allogeneic corneal transplantation, compared to steady state (p < 0.05). Moreover, allogeneic corneal transplantation results in increased host-derived CD11c-YFP⁺ cell mean speed and displacement in mandibular dLNs, compared to steady state (p < 0.001). The meandering index, an index for directionality, is significantly increased after allogeneic corneal transplantation at both 14 and 21 days, compared to steady state (p < 0.001). Taken together, our study demonstrates the necessary methodology required for intravital multiphoton imaging of the mandibular dLNs, allowing visualization of spatiotemporal kinetics of immune cells in vivo, and provides a window into the corneal immune reflex arc. This technique will be a powerful tool to investigate the pathogenesis of ocular immune and inflammatory diseases.

Age-Based Dynamics of a Stable Circulating Cd8 T Cell Repertoire Component

T-cell memory to pathogens can be envisioned as a receptor-based imprint of the pathogenic environment on the naïve repertoire of clonotypes. Recurrent exposures to a pathogen inform and reinforce memory, leading to a mature state. The complexity and temporal stability of this system in man is only beginning to be adequately described. We have been using a rank-frequency approach for quantitative analysis of CD8 T cell repertoires. Rank acts as a proxy for previous expansion, and rank-frequency, the number of clonotypes at a particular rank, as a proxy for abundance, with the relation of the two estimating the diversity of the system. Previous analyses of circulating antigen-experienced cytotoxic CD8 T-cell repertoires from adults have shown a complex two-component clonotype distribution. Here we show this is also the case for circulating CD8 T cells expressing the BV19 receptor chain from five adult subjects. When the repertoire characteristic of clonotype stability is added to the analysis, an inverse correlation between clonotype rank frequency and stability is observed. Clonotypes making up the second distributional component are stable; indicating that the circulation can be a depot of selected clonotypes. Temporal repertoire dynamics was further examined for influenza-specific T cells from children, middle-aged, and older adults. Taken together, these analyses describe a dynamic process of system development and aging, with increasing distributional complexity, leading to a stable circulating component, followed by loss of both complexity and stability.

Two-photon intravital imaging of leukocyte migration during inflammation in the respiratory system

Two-photon intravital imaging is a powerful method by which researchers are able to directly observe biological phenomena in live organisms. Researchers in various biomedical research fields have applied two-photon imaging to a variety of target organs by utilizing this technology’s ability to penetrate to significant depths with minimal phototoxicity. The mouse respiratory system in inflammation models is a good example, as two-photon intravital imaging can provide insights as to how the immune system is activated in response to inflammation within the respiratory system. Inflammation models can be generated via influenza viral, bacterial, or lipopolysaccharide injection. To exteriorize the lungs or trachea, thoracotomy or tracheotomy is performed, respectively; the appropriate combination of inflammation induction and organ exposure is selected depending on the study purpose. On the other hand, visualizing the movement of leukocytes is also an important component; to this end, immune cell populations of interest are either labeled via the genetic attachment of fluorescent proteins or stained with antibodies or dyes. With the proper selection of methods at each step, twophoton intravital imaging can yield visual evidence regarding immune responses to inflammation.

Imaging methods used to study mouse and human HSC niches: Current and emerging technologies

Bone marrow contains numerous different cell types arising from hematopoietic stem cells (HSCs) and non-hematopoietic mesenchymal/skeletal stem cells, in addition to other cell types such as endothelial cells- these non-hematopoietic cells are commonly referred to as stromal cells or microenvironment cells. HSC function is intimately linked to complex signals integrated by their niches, formed by combinations of hematopoietic and stromal cells. Studies of hematopoietic cells have been significantly advanced by flow cytometry methods, enabling the quantitation of each cell type in normal and perturbed situations, in addition to the isolation of these cells for molecular and functional studies. Less is known, however, about the specific niches for distinct developing hematopoietic lineages, or the changes occurring in the niche size and function in these distinct anatomical sites in the bone marrow under stress situations and ageing. Significant advances in imaging technology during the last decade have permitted studies of HSC niches in mice. Additional imaging technologies are emerging that will facilitate the study of human HSC niches in trephine BM biopsies. Here we provide an overview of imaging technologies used to study HSC niches, in addition to highlighting emerging technology that will help us to more precisely identify and characterize HSC niches in normal and diseased states.

Intravital Imaging of Vaccinia Virus-Infected Mice

Intravital multiphoton microscopy (MPM) allows the direct visualization of viral infections in real time as they occur in living animals. Here we describe the routes and considerations for murine infection with vaccinia virus (VACV) for imaging, and the preparation of the skin and inner lip (labial mucosa) of infected animals for MPM. Using different recombinant VACVs expressing fluorescent proteins in combination with transgenic fluorescent reporter mice, MPM imaging can be used to examine the movements, interactions, and functions of virus-infected cells or selected immune cell populations after infection.

Just Look! Intravital Microscopy as the Best Means to Study Kidney Cell Death Dynamics

Kidney cell death plays a key role in the progression of life-threatening renal diseases, such as acute kidney injury and chronic kidney disease. Injured and dying epithelial and endothelial cells take part in complex communication with the innate immune system, which drives the progression of cell death and the decrease in renal function. To improve our understanding of kidney cell death dynamics and its impact on renal disease, a study approach is needed that facilitates the visualization of renal function and morphology in real time. Intravital multiphoton microscopy of the kidney has been used for more than a decade and made substantial contributions to our understanding of kidney physiology and pathophysiology. It is a unique tool that relates renal structure and function in a time- and spatial-dependent manner. Basic renal function, such as microvascular blood flow regulation and glomerular filtration, can be determined in real time and homeostatic alterations, which are linked inevitably to cell death and can be depicted down to the subcellular level. This review provides an overview of the available techniques to study kidney dysfunction and inflammation in terms of cell death in vivo, and addresses how this novel approach can be used to improve our understanding of cell death dynamics in renal disease.

Intravital Imaging of B Cell Responses in Lymph Nodes

Humoral immune responses depend on B cells encountering antigen (Ag) in lymph nodes (LNs) draining infection sites, getting activated, interacting with different cells, proliferating and differentiating into antibody (Ab)-secreting cells. Each of these events occurs in distinct LN sub-compartments, requiring the migration of B cells from niche to niche in a fast and tightly coordinated fashion. While some of the rules that characterize B cell behavior in secondary lymphoid organs have been elucidated at the population level, we have only limited knowledge of the precise dynamics of B cell interactions with different kinds of LN cells at the single-cell level. Here, we describe in detail an intravital microscopy technique that allows the analysis of B cell dynamic behavior in the popliteal lymph node of anesthetized mice at high spatial and temporal resolution. A detailed understanding of the spatiotemporal dynamics of B cells within secondary lymphoid organs may lead to novel, rational vaccine strategies aimed at inducing rapid and long-lived humoral immune responses.

Intravital microscopy in historic and contemporary immunology

In this review, we discuss intravital microscopy of immune cells, starting from its historic origins to current applications in diverse organs. It is clear from a quantitative review of the literature that intravital microscopy is a key tool in both historic and contemporary immunological research, providing unique advances in our understanding of immune responses. We have chosen to focus this review on how intravital microscopy methodologies are used to image specific organs or systems and we present recent descriptions of fundamental immunological processes that could not have been achieved by other methods. The following target organs/systems are discussed in more detail: cremaster muscle, skin (ear and dorsal skin fold chamber), lymph node, liver, lung, mesenteric vessels, carotid artery, bone marrow, brain, spleen, foetus and lastly vessels of the knee joint.

Differential DARC/ACKR1 expression distinguishes venular from non-venular endothelial cells in murine tissues

BACKGROUND

Intravascular leukocyte recruitment in most vertebrate tissues is restricted to postcapillary and collecting venules, whereas capillaries and arterioles usually support little or no leukocyte adhesion. This segmental restriction is thought to be mediated by endothelial, rather than hemodynamic, differences. The underlying mechanisms are largely unknown, in part because effective tools to distinguish, isolate, and analyze venular endothelial cells (V-ECs) and non-venular endothelial cells (NV-ECs) have been unavailable. We hypothesized that the atypical chemokine receptor DARC (Duffy Antigen Receptor for Chemokines, a.k.a. ACKR1 or CD234) may distinguish V-ECs versus NV-ECs in mice.

METHODS

We generated a rat-anti-mouse monoclonal antibody (MAb) that specifically recognizes the erythroid and endothelial forms of native, surface-expressed DARC. Using this reagent, we characterized DARC expression and distribution in the microvasculature of murine tissues.

RESULTS

DARC was exquisitely restricted to post-capillary and small collecting venules and completely absent from arteries, arterioles, capillaries, veins, and most lymphatics in every tissue analyzed. Accordingly, intravital microscopy showed that adhesive leukocyte-endothelial interactions were restricted to DARC+ venules. DARC was detectable over the entire circumference of V-ECs, but was more concentrated at cell-cell junctions. Analysis of single-cell suspensions suggested that the frequency of V-ECs among the total microvascular EC pool varies considerably between different tissues.

CONCLUSIONS

Immunostaining of endothelial DARC allows the identification and isolation of intact V-ECs from multiple murine tissues. This strategy may be useful to dissect the mechanisms underlying segmental microvascular specialization in healthy and diseased tissues and to characterize the role of EC subsets in tissue-homeostasis, immune surveillance, infection, inflammation, and malignancies.

Graphene quantum dot based "switch-on" nanosensors for intracellular cytokine monitoring

The detection of cytokines in body fluids, cells, tissues and organisms continues to attract considerable attention due to the importance of these key cell signalling molecules in biology and medicine. We report a graphene quantum dot (GQD) based aptasensor able to specifically detect ultra-small amounts of cytokine molecules intracellularly. Graphene quantum dots modified with cytokine aptamers (Ap-GQDs) and epitope modified GQDs (Ep-GQDs) were prepared; both are normally fluorescent at sufficient dilution. However, the fluorescence of the conjugates of Ap-GQDs and Ep-GQDs is quenched due to aggregation between Ap-GQDs and Ep-GQDs. After incubation of the cytokine-secreting cells with the conjugates of Ap-GQDs and Ep-GQDs, the cytokines secreted in cells compete for binding with the epitope which is then displaced. The ensuing binding of cytokines with the aptamers results in the disaggregation of Ap-GQDs and Ep-GQDs, and the recovery of fluorescence. The conjugates of Ap-GQDs and Ep-GQDs were used as nanosensors for monitoring intracellular cytokine IFN-γ secretion with very high sensitivity (2 pg mL^-1). The disaggregation based sensing strategy in this nanosensor design is simple and universal; similar nanosensors can be used for the detection of a broad spectrum of cell-secreted molecules. Such nanosensors will serve as potential biomaterials for in vivo devices to monitor a variety of biological phenomena, in particular to understand cytokine secretion pathways in live cells.

A membrane-anchored fluorescent probe for detecting K(+) in the cell microenvironment

Cell-surface fluorescent probes are effective tools in cell biology and engineering. Here, we for the first time report a diacyllipid-aptamer conjugate-based fluorescent probe which could anchor on cell membrane for real-time tracking of potassium ions in the cell microenvironment.

Endotoxaemia-augmented murine venous thrombosis is dependent on TLR-4 and ICAM-1, and potentiated by neutropenia

Venous thromboembolism is a major cause of death during and immediately post-sepsis. Venous thrombosis (VT) is mediated by cell adhesion molecules and leukocytes, including neutrophil extracellular traps (NETs). Sepsis, or experimentally, endotoxaemia, shares similar characteristics and is modulated via toll like receptor 4 (TLR4). This study was undertaken to determine if endotoxaemia potentiates early stasis thrombogenesis, and secondarily to determine the role of VT TLR4, ICAM-1 and neutrophils (PMNs). Wild-type (WT), ICAM-1^-/- and TLR4^-/- mice underwent treatment with saline or LPS (10 mg/kg i. p.) alone, or followed by inferior vena cava (IVC) ligation to generate stasis VT. In vivo microscopy of leukocyte trafficking was performed in non-thrombosed mice, and tissue and plasma were harvested during early VT formation. Pre-thrombosis, circulating ICAM-1 was elevated and increased leukocyte adhesion and rolling occurred on the IVC of LPS-treated mice. Post-thrombosis, endotoxaemic mice formed larger, platelet-poor thrombi. Endotoxaemic TLR4^-/- mice did not have an augmented thrombotic response and exhibited significantly decreased circulating ICAM-1 compared to endotoxaemic WT controls. Endotoxaemic ICAM-1^-/- mice had significantly smaller thrombi compared to controls. Hypothesising that PMNs localised to the inflamed endothelium were promoting thrombosis, PMN depletion using anti-Ly6G antibody was performed. Paradoxically, VT formed without PMNs was amplified, potentially related to endotoxaemia induced elevation of PAI-1 and circulating FXIII, and decreased uPA. Endotoxaemia enhanced early VT occurs in a TLR-4 and ICAM-1 dependent fashion, and is potentiated by neutropenia. ICAM-1 and/or TLR-4 inhibition may be a unique strategy to prevent sepsis-associated VT.

Decoration of Reduced Graphene Oxide Nanosheets with Aryldiazonium Salts and Gold Nanoparticles toward a Label-Free Amperometric Immunosensor for Detecting Cytokine Tumor Necrosis Factor-α in Live Cells

In this study, a label-free electrochemical immunosensor was developed for detection of cytokine tumor necrosis factor-alpha (TNF-α). First, AuNPs loaded reduced graphene oxides nanocomposites (RGO-ph-AuNP) were prepared, and then, a mixed layer of 4-carbxyphenyl and 4-aminophenyl phosphorylcholine (PPC) was modified to the surface of AuNPs for the subsequent modification of anti-TNF-α capture antibody (Ab₁) to form the capture surface (Au-RGO-ph-AuNP-ph-PPC(-ph-COOH)) for the analyte TNF-α with the antifouling property. For reporting the presence of analyte, the anti-TNF-α detection antibody (Ab₂) was modified to the graphene oxides which have been modified with the 4-ferrocenylaniline through diazonium chemistry to form Ab₂-GO-ph-Fc. Then, a sandwich assay was formed on gold surfaces for the quantitative detection of TNF-α based on the electrochemical signal of ferrocene. X-ray photoelectron spectra (XPS), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), UV-vis, and electrochemistry were used for characterization of the stepwise fabrications on the interface. The prepared electrochemical immunosensor was successfully used for the detection of TNF-α over the range of 0.1-150 pg mL^-1. The lowest detection limit of this immunosensor is 0.1 pg mL^-1 TNF-α in 50 mM phosphate buffer at pH 7.0. The fabricated immunosensor provided high selectivity and stability and can be used to detect TNF-α secreted by live BV-2 cells with comparable accuracy to enzyme-linked immunosorbent assay (ELISA) but with lower limit of detection.

Microfluidic mazes to characterize T-cell exploration patterns following activation in vitro

The migration of T-cell subsets within peripheral tissues is characteristic of inflammation and immunoregulation. In general, the lymphocyte migratory response is assumed directional and guided by local gradients of chemoattractants and/or chemorepellents. However, little is known about how cells explore their tissue environment, and whether lymphocyte activation may influence speed and exploratory patterns of migration. To probe migration patterns by T-cells we designed a microfluidic maze device that replicates critical features of a tissue-like microenvironment. We quantified the migration patterns of unstimulated and mitogen-activated human T-cells at single cell resolution and found significant differences in exploration within microfluidic mazes. While unstimulated lymphocytes migrated in a directed manner, activated T-cells migrated through large areas of the mazes in an exploratory pattern in response to the chemoattractants RANTES (CCL5) and IP-10 (CXCL10). The analysis of migration enabled by the microfluidic devices help develop new methods for determining how human circulating T-cells function in vivo to seek out antigens in health and disease states.

Deep insights: intravital imaging with two-photon microscopy

Intravital multiphoton microscopy is widely used to assess the structure and function of organs in live animals. Although different tissues vary in their accessibility for intravital multiphoton imaging, considerable progress has been made in the imaging quality of all tissues due to substantial technical improvements in the relevant imaging components, such as optics, excitation laser, detectors, and signal analysis software. In this review, we provide an overview of the technical background of intravital multiphoton microscopy. Then, we note a few seminal findings that were made through the use of multiphoton microscopy. Finally, we address the technical limitations of the method and provide an outlook for how these limitations may be overcome through future technical developments.

Development of novel murine mammary imaging windows to examine wound healing effects on leukocyte trafficking in mammary tumors with intravital imaging

We developed mammary imaging windows (MIWs) to evaluate leukocyte infiltration and cancer cell dissemination in mouse mammary tumors imaged by confocal microscopy. Previous techniques relied on surgical resection of a skin flap to image the tumor microenvironment restricting imaging time to a few hours. Utilization of mammary imaging windows offers extension of intravital imaging of the tumor microenvironment. We have characterized strengths and identified some previously undescribed potential weaknesses of MIW techniques. Through iterative enhancements of a transdermal portal we defined conditions for improved quality and extended confocal imaging time for imaging key cell-cell interactions in the tumor microenvironment.

Video-rate in vivo fluorescence imaging with a line-scanned dual-axis confocal microscope

Video-rate optical-sectioning microscopy of living organisms would allow for the investigation of dynamic biological processes and would also reduce motion artifacts, especially for in vivo imaging applications. Previous feasibility studies, with a slow stage-scanned line-scanned dual-axis confocal (LS-DAC) microscope, have demonstrated that LS-DAC microscopy is capable of imaging tissues with subcellular resolution and high contrast at moderate depths of up to several hundred microns. However, the sensitivity and performance of a video-rate LS-DAC imaging system, with low-numerical aperture optics, have yet to be demonstrated. Here, we report on the construction and validation of a video-rate LS-DAC system that possesses sufficient sensitivity to visualize fluorescent contrast agents that are topically applied or systemically delivered in animal and human tissues. We present images of murine oral mucosa that are topically stained with methylene blue, and images of protoporphyrin IX-expressing brain tumor from glioma patients that have been administered 5-aminolevulinic acid prior to surgery. In addition, we demonstrate in vivo fluorescence imaging of red blood cells trafficking within the capillaries of a mouse ear, at frame rates of up to 30 fps. These results can serve as a benchmark for miniature in vivo microscopy devices under development.

Hepatic effector CD8(+) T-cell dynamics

CD8(+) T cells play a critical role in hepatitis B virus (HBV) pathogenesis. During acute, self-limited infections, these cells are instrumental to viral clearance; in chronic settings, they sustain repetitive cycles of hepatocellular necrosis that promote hepatocellular carcinoma development. Both CD8(+) T-cell defensive and destructive functions are mediated by antigen-experienced effector cells and depend on the ability of these cells to migrate to the liver, recognize hepatocellular antigens and perform effector functions. Understanding the signals that modulate the spatiotemporal dynamics of CD8(+) T cells in the liver, particularly in the context of antigen recognition, is therefore critical to gaining insight into the pathogenesis of acute and chronic HBV infection. Here, we highlight recent data on how effector CD8(+) T cells traffic within the liver, and we discuss the potential for novel imaging techniques to shed light on this important aspect of HBV pathogenesis.

Regulation of leucocyte homeostasis in the circulation

The functions of blood cells extend well beyond the immune functions of leucocytes or the respiratory and hemostatic functions of erythrocytes and platelets. Seen as a whole, the bloodstream is in charge of nurturing and protecting all organs by carrying a mixture of cell populations in transit from one organ to another. To optimize these functions, evolution has provided blood and the vascular system that carries it with various mechanisms that ensure the appropriate influx and egress of cells into and from the circulation where and when needed. How this homeostatic control of blood is achieved has been the object of study for over a century, and although the major mechanisms that govern it are now fairly well understood, several new concepts and mediators have recently emerged that emphasize the dynamism of this liquid tissue. Here we review old and new concepts that relate to the maintenance and regulation of leucocyte homeostasis in blood and briefly discuss the mechanisms for platelets and red blood cells.

An MHC II-dependent activation loop between adipose tissue macrophages and CD4+ T cells controls obesity-induced inflammation

An adaptive immune response triggered by obesity is characterized by the activation of adipose tissue CD4(+) T cells by unclear mechanisms. We have examined whether interactions between adipose tissue macrophages (ATMs) and CD4(+) T cells contribute to adipose tissue metainflammation. Intravital microscopy identifies dynamic antigen-dependent interactions between ATMs and T cells in visceral fat. Mice deficient in major histocompatibility complex class II (MHC II) showed protection from diet-induced obesity. Deletion of MHC II expression in macrophages led to an adipose tissue-specific decrease in the effector/memory CD4(+) T cells, attenuation of CD11c(+) ATM accumulation, and improvement in glucose intolerance by increasing adipose tissue insulin sensitivity. Ablation experiments demonstrated that the maintenance of proliferating conventional T cells is dependent on signals from CD11c(+) ATMs in obese mice. These studies demonstrate the importance of MHCII-restricted signals from ATMs that regulate adipose tissue T cell maturation and metainflammation.

Rapidly self-renewing human multipotent marrow stromal cells (hMSC) express sialyl Lewis X and actively adhere to arterial endothelium in a chick embryo model system

BACKGROUND

There have been conflicting observations regarding the receptors utilized by human multipotent mesenchymal bone marrow stromal cells (hMSC) to adhere to endothelial cells (EC). To address the discrepancies, we performed experiments with cells prepared with a standardized, low-density protocol preserving a sub-population of small cells that are rapidly self-renewing.

METHODS

Sialyl Lewis X (SLeX) and α4 integrin expression were determined by flow cytometry. Fucosyltransferase expression was determined by quantitative realtime RT-PCR. Cell adhesion assays were carried out with a panel of endothelial cells from arteries, veins and the microvasculature in vitro. In vivo experiments were performed to determine single cell interactions in the chick embryo chorioallantoic membrane (CAM). The CAM is a well-characterized respiratory organ allowing for time-lapse image acquisition of large numbers of cells treated with blocking antibodies against adhesion molecules expressed on hMSC.

RESULTS

hMSC expressed α4 integrin, SLeX and fucosyltransferase 4 and adhered to human EC from arteries, veins and the microvasculature under static conditions in vitro. In vivo, hMSC rolled on and adhered to arterioles in the chick embryo CAM, whereas control melanoma cells embolized. Inhibition of α4 integrin and/or SLeX with blocking antibodies reduced rolling and adhesion in arterioles and increased embolism of hMSC.

CONCLUSIONS

The results demonstrated that rapidly self-renewing hMSC were retained in the CAM because they rolled on and adhered to respiratory arteriolar EC in an α4 integrin- and SLeX-dependent manner. It is therefore important to select cells based on their cell adhesion receptor profile as well as size depending on the intended target of the cell and the injection route.

Comparative suitability of CFDA-SE and rhodamine 6G for in vivo assessment of leukocyte-endothelium interactions

Intravital fluorescence microscopy (IVM) is a predestined tool for investigating the fate of leukocytes during the process of leukocyte recruitment. In the present study, the commonly used dye for this purpose, rhodamine 6G, and carboxyfluorescein diacetate succinimidyl ester (CFDA-SE) were compared for leukocytes labelling with respect to suitability for IVM studies. Their potential in labelling different leukocytes subpopulations as well as their fluorescence intensities were assessed by flow cytometry revealing distinct differences between both dyes. These differences had a profound impact on their application for in vivo imaging of leukocyte-endothelium interactions. In summary, CFDA-SE revealed superior in labelling leukocytes for in vivo microscopy with respect to image quality. In addition, we could show the efficiency of CFDA-SE also under disease condition in an animal model of sepsis.

In vivo photoswitchable flow cytometry for direct tracking of single circulating tumor cells

Photoswitchable fluorescent proteins (PSFPs) that change their color in response to light have led to breakthroughs in studying static cells. However, using PSFPs to study cells in dynamic conditions is challenging. Here we introduce a method for in vivo ultrafast photoswitching of PSFPs that provides labeling and tracking of single circulating cells. Using in vivo multicolor flow cytometry, this method demonstrated the capability for studying recirculation, migration, and distribution of circulating tumor cells (CTCs) during metastasis progression. In tumor-bearing mice, it enabled monitoring of real-time dynamics of CTCs released from primary tumor, identifying dormant cells, and imaging of CTCs colonizing a primary tumor (self-seeding) or existing metastasis (reseeding). Integration of genetically encoded PSFPs, fast photoswitching, flow cytometry, and imaging makes in vivo single cell analysis in the circulation feasible to provide insights into the behavior of CTCs and potentially immune-related and bacterial cells in circulation.

Long-term intravital immunofluorescence imaging of tissue matrix components with epifluorescence and two-photon microscopy

Besides being a physical scaffold to maintain tissue morphology, the extracellular matrix (ECM) is actively involved in regulating cell and tissue function during development and organ homeostasis. It does so by acting via biochemical, biomechanical, and biophysical signaling pathways, such as through the release of bioactive ECM protein fragments, regulating tissue tension, and providing pathways for cell migration. The extracellular matrix of the tumor microenvironment undergoes substantial remodeling, characterized by the degradation, deposition and organization of fibrillar and non-fibrillar matrix proteins. Stromal stiffening of the tumor microenvironment can promote tumor growth and invasion, and cause remodeling of blood and lymphatic vessels. Live imaging of matrix proteins, however, to this point is limited to fibrillar collagens that can be detected by second harmonic generation using multi-photon microscopy, leaving the majority of matrix components largely invisible. Here we describe procedures for tumor inoculation in the thin dorsal ear skin, immunolabeling of extracellular matrix proteins and intravital imaging of the exposed tissue in live mice using epifluorescence and two-photon microscopy. Our intravital imaging method allows for the direct detection of both fibrillar and non-fibrillar matrix proteins in the context of a growing dermal tumor. We show examples of vessel remodeling caused by local matrix contraction. We also found that fibrillar matrix of the tumor detected with the second harmonic generation is spatially distinct from newly deposited matrix components such as tenascin C. We also showed long-term (12 hours) imaging of T-cell interaction with tumor cells and tumor cells migration along the collagen IV of basement membrane. Taken together, this method uniquely allows for the simultaneous detection of tumor cells, their physical microenvironment and the endogenous tissue immune response over time, which may provide important insights into the mechanisms underlying tumor progression and ultimate success or resistance to therapy.

Labour-efficient in vitro lymphocyte population tracking and fate prediction using automation and manual review

Interest in cell heterogeneity and differentiation has recently led to increased use of time-lapse microscopy. Previous studies have shown that cell fate may be determined well in advance of the event. We used a mixture of automation and manual review of time-lapse live cell imaging to track the positions, contours, divisions, deaths and lineage of 44 B-lymphocyte founders and their 631 progeny in vitro over a period of 108 hours. Using this data to train a Support Vector Machine classifier, we were retrospectively able to predict the fates of individual lymphocytes with more than 90% accuracy, using only time-lapse imaging captured prior to mitosis or death of 90% of all cells. The motivation for this paper is to explore the impact of labour-efficient assistive software tools that allow larger and more ambitious live-cell time-lapse microscopy studies. After training on this data, we show that machine learning methods can be used for realtime prediction of individual cell fates. These techniques could lead to realtime cell culture segregation for purposes such as phenotype screening. We were able to produce a large volume of data with less effort than previously reported, due to the image processing, computer vision, tracking and human-computer interaction tools used. We describe the workflow of the software-assisted experiments and the graphical interfaces that were needed. To validate our results we used our methods to reproduce a variety of published data about lymphocyte populations and behaviour. We also make all our data publicly available, including a large quantity of lymphocyte spatio-temporal dynamics and related lineage information.

Modeling leukocyte-leukocyte non-contact interactions in a lymph node

The interaction among leukocytes is at the basis of the innate and adaptive immune-response and it is largely ascribed to direct cell-cell contacts. However, the exchange of a number of chemical stimuli (chemokines) allows also non-contact interaction during the immunological response. We want here to evaluate the extent of the effect of the non-contact interactions on the observed leukocyte-leukocyte kinematics and their interaction duration. To this aim we adopt a simplified mean field description inspired by the Keller-Segel chemotaxis model, of which we report an analytical solution suited for slowly varying sources of chemokines. Since our focus is on the non-contact interactions, leukocyte-leukocyte contact interactions are simulated only by means of a space dependent friction coefficient of the cells. The analytical solution of the Keller-Segel model is then taken as the basis of numerical simulations of interactions between leukocytes and their duration. The mean field interaction force that we derive has a time-space separable form and depends on the chemotaxis sensitivity parameter as well as on the chemokines diffusion coefficient and their degradation rate. All these parameters affect the distribution of the interaction durations. We draw a successful qualitative comparison between simulated data and sets of experimental data for DC-NK cells interaction duration and other kinematic parameters. Remarkably, the predicted percentage of the leukocyte-leukocyte interactions falls in the experimental range and depends (≅25% increase) upon the chemotactic parameter indicating a non-negligible direct effect of the non-contact interaction on the leukocyte interactions.

Identification of cytolytic CD161- CD56+ regulatory CD8 T cells in human peripheral blood

We previously developed methods for establishing CD8 regulatory T cell (Treg) clones from normal human peripheral blood and demonstrated that these clones were capable of killing T cell receptor (TCR)-activated autologous CD4 T cells. Based on phenotypic and functional characterization of the CD8 Treg clones, we have identified a corresponding population of endogenous CD8 Treg in normal human peripheral blood. These cells appear morphologically as large lymphocytes with abundant cytoplasm and have the following unique phenotype: CD3⁺CD8⁺CD161⁻CD56⁺. The majority of CD8 Treg express CD45RA and CD62L with low or negative expression of CD45RO, CD25, CD27, CD28 and CCR7. The expression of CD94 and NKG2a on CD8 Treg was elevated compared to conventional CD8 T cells. Following in vitro activation, this T cell subset is capable of killing TCR-activated CD4 T cells. These studies identify an endogenous CD8 Treg population in humans and it will now be possible to characterize these cells in a variety of clinical conditions.

Intravital immunofluorescence for visualizing the microcirculatory and immune microenvironments in the mouse ear dermis

Visualizing the dynamic behaviors of immune cells in living tissue has dramatically increased our understanding of how cells interact with their surroundings, contributing important insights into mechanisms of leukocyte trafficking, tumor cell invasion, and T cell education by dendritic cells, among others. Despite substantial advances with various intravital imaging techniques including two-photon microscopy and the generation of multitudes of reporter mice, there is a growing need to assess cell interactions in the context of specific extracellular matrix composition and microvascular functions, and as well, simpler and more widely accessible methods are needed to image cell behaviors in the context of living tissue physiology. Here we present an antibody-based method for intravital imaging of cell interactions with the blood, lymphatic, and the extracellular matrix compartments of the living dermis while simultaneously assessing capillary permeability and lymphatic drainage function. Using the exposed dorsal ear of the anesthetized mouse and a fluorescence stereomicroscope, such events can be imaged in the context of specific extracellular matrix proteins, or matrix-bound chemokine stores. We developed and optimized the method to minimize tissue damage to the ear, rapidly immunostain for multiple extracellular or cell surface receptors of interest, minimize immunotoxicity with pre-blocking Fcγ receptors and phototoxicity with extracellular antioxidants, and highlight the major dermal tissue structures with basement membrane markers. We demonstrate differential migration behaviors of bone marrow-derived dendritic cells, blood-circulating leukocytes, and dermal dendritic cells, with the latter entering sparse CCL21-positive areas of pre-collecting lymphatic vessels. This new method allows simultaneous imaging of cells and tissue structures, microvascular function, and extracellular microenvironment in multiple skin locations for 12 hours or more, with the flexibility of immunolabeling in addition to genetic-based fluorescent reporters.

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

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

Cell-surface sensors: lighting the cellular environment

Cell-surface sensors are powerful tools to elucidate cell functions including cell signaling, metabolism, and cell-to-cell communication. These sensors not only facilitate our understanding in basic biology but also advance the development of effective therapeutics and diagnostics. While genetically encoded fluorescent protein/peptide sensors have been most popular, emerging cell surface sensor systems including polymer-, nanoparticle-, and nucleic acid aptamer-based sensors have largely expanded our toolkits to interrogate complex cellular signaling and micro- or nano-environments. In particular, cell-surface sensors that interrogate in vivo cellular microenvironments represent an emerging trend in the development of next generation tools which biologists may routinely apply to elucidate cell biology in vivo and to develop new therapeutics and diagnostics. This review focuses on the most recent development in areas of cell-surface sensors. We will first discuss some recently reported genetically encoded sensors that were used for monitoring cellular metabolites, proteins, and neurotransmitters. We will then focus on the emerging cell surface sensor systems with emphasis on the use of DNA aptamer sensors for probing cell signaling and cell-to-cell communication.

Morphological studies of lymphatic labyrinths in the rat mesenteric lymph node

To supplement and correct the morphological features of lymphatic labyrinths (LLs) in rat mesenteric lymph node, the distribution, morphology and origin of LLs, and cellular elements in LLs, particularly the organization and integrity of the wall of LLs were examined by silver impregnation, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and immunohistochemistry. LLs consisted of labyrinthine tubules and ran through not only the periphery of the deep cortical unit (DCU) but interfollicular cortex. LLs originated at the edge of the center of the DCU and of the follicle. At the site of their origin, the fibers in the wall of LL were continuous with the fibers located in the follicle and the center of DCU. The wall of LLs was a trilaminar membrane: a layer of flattened lymphatic endothelium; a layer of fibroblastic reticular cells; and amorphous substance and collagen fibers sandwiched between the above two layers. Under SEM and TEM, the whole amoeboid lymphocytes were moving through the pores in the wall of LL, which showed that lymphocytes end their journey through the paracortical cord by migrating into LLs. Immunohistochemical lymphatic vessel endothelial hyaluronan receptor-1 expression was present in cells lining the LLs and intraluminal stellate cells, which may belong to the "sinus endothelial/virgultar cells." LLs are specific channels that are different from lymphatic sinuses. LL may be regarded as a special part of lymphatic vascular system in lymph nodes. We confirm that LLs are important transport pathway of lymphocytes in lymph nodes. The structural framework of LLs facilitates the migration of lymphocytes.

Endogenous migration modulators as parent compounds for the development of novel cardiovascular and anti-inflammatory drugs

Development of novel cell migration modulators for anti-inflammatory and cardiovascular therapy is a complex task since any modulator will necessarily interfere with a balanced system of physiological regulators directing proper positioning of diverse immune cell types within the body. Whereas this shall serve efficient pathogen elimination, lack of proper control over these processes may result in counterproductive chronic inflammation and progressive tissue injury instead of healing. Prediction of the therapeutic potential or side effects of any migration modulator is not possible based on theoretical considerations alone but needs to be experimentally evaluated in preclinical disease models and by clinical studies. Here, we briefly summarize basic mechanism of cell migration, and groups of synthetic drugs currently in use for migration modulation. We then discuss one fundamental problem encountered with single-target approaches that arises from the complexity of any inflammation, with multiple interacting and often redundant factors being involved. This issue is likely to arise for any class of therapeutic agent (small molecules, peptides, antibodies, regulatory RNAs) addressing a single gene or protein. Against this background of studies on synthetic migration modulators addressing single targets, we then discuss the potential of endogenous proteins as therapeutic migration modulators, or as parent compounds for the development of mimetic drugs. Regulatory proteins of this type commonly address multiple receptors and signalling pathways and act upon the immune response in a phase-specific manner. Based on recent evidence, we suggest investigation of such endogenous migration modulators as novel starting points for anti-inflammatory and cardiovascular drug development.

Real-time differential labeling of blood, interstitium, and lymphatic and single-field analysis of vasculature dynamics in vivo

Lymph nodes are highly organized structures specialized for efficient regulation of adaptive immunity. The blood and lymphatic systems within a lymph node play essential roles by providing functionally distinct environments for lymphocyte entry and egress, respectively. Direct imaging and measurement of vascular microenvironments by intravital multiphoton microscopy provide anatomical and mechanistic insights into the essential events of lymphocyte trafficking. Lymphocytes, blood endothelial cells, and lymphatic endothelial cells express sphingosine 1-phosphate receptor 1, a key G protein-coupled receptor regulating cellular egress and a modulator of endothelial permeability. Here we report the development of a differential vascular labeling (DVL) technique in which a single intravenous injection of a fluorescent dextran, in combination with fluorescent semiconductor quantum dot particles, differentially labels multiple blood and lymphatic compartments in a manner dependent on the size of the fluorescent particle used. Thus DVL allows measurement of endothelial integrity in multiple vascular compartments and the affects or pharmacological manipulation in vascular integrity. In addition, this technique allows for real-time observation of lymphocyte trafficking across physiological barriers differentiated by DVL. Last, single-field fluid movement dynamics can be derived, allowing for the simultaneous determination of fluid flow rates in diverse blood and lymphatic compartments.

Intravital imaging of the mouse popliteal lymph node

Lymph nodes (LNs) are secondary lymphoid organs, which are strategically located throughout the body to allow for trapping and presentation of foreign antigens from peripheral tissues to prime the adaptive immune response. Juxtaposed between innate and adaptive immune responses, the LN is an ideal site to study immune cell interactions. Lymphocytes (T cells, B cells and NK cells), dendritic cells (DCs), and macrophages comprise the bulk of bone marrow-derived cellular elements of the LN. These cells are strategically positioned in the LN to allow efficient surveillance of self antigens and potential foreign antigens. The process by which lymphocytes successfully encounter cognate antigens is a subject of intense investigation in recent years, and involves an integration of molecular contacts including antigen receptors, adhesion molecules, chemokines, and stromal structures such as the fibro-reticular network. Prior to the development of high-resolution real-time fluorescent in vivo imaging, investigators relied on static imaging, which only offers answers regarding morphology, position, and architecture. While these questions are fundamental in our understanding of immune cell behavior, the limitations intrinsic with this technique does not permit analysis to decipher lymphocyte trafficking and environmental clues that affect dynamic cell behavior. Recently, the development of intravital two-photon laser scanning microscopy (2P-LSM) has allowed investigators to view the dynamic movements and interactions of individual cells within live LNs in situ. In particular, we and others have applied this technique to image cellular behavior and interactions within the popliteal LN, where its compact, dense nature offers the advantage of multiplex data acquisition over a large tissue area with diverse tissue sub-structures. It is important to note that this technique offers added benefits over explanted tissue imaging techniques, which require disruption of blood, lymph flow, and ultimately the cellular dynamics of the system. Additionally, explanted tissues have a very limited window of time in which the tissue remains viable for imaging after explant. With proper hydration and monitoring of the animal's environmental conditions, the imaging time can be significantly extended with this intravital technique. Here, we present a detailed method of preparing mouse popliteal LN for the purpose of performing intravital imaging.