Imaging the choreography of lymphocyte trafficking and the immune response

Synchronous or collision solid neoplasms and lymphomas: A systematic review of 308 case reports

BACKGROUND

The presence of a lymphoma associated with a solid synchronous neoplasm or collision neoplasm has been rarely in the literature, and a detailed characterization of these cases is lacking to date.

OBJECTIVE

To describe the main clinicopathological features of synchronous/collision tumors.

METHODS

A systematic search in PubMed, Scielo, and Virtual Health Library literature databases for cases or case series of synchronous or collision lymphoma and other solid neoplasms reported up to March 2021 was performed. Three reviewers independently screened the literature, extracted data, and assessed the quality of the included studies. The systematic review was performed following the Preferred Reporting Items for Systematic Meta-Analyses guidelines.

RESULTS

Mean age of patients was 62.9 years (52.9% men). A total of 308 cases were included (62% synchronous and 38% collision). The most frequent location of both synchronous and collision tumors was the gastrointestinal tract with the most common solid neoplasm being adenocarcinoma, and the most frequent lymphoma diffuse large B-cell lymphoma (21.7%) and mucosa-associated lymphoid tissue lymphoma (20.4%). Of the total number of mucosa-associated lymphoid tissue lymphomas and gastric adenocarcinomas, the presence of Helicobacter pylori infection was documented in 47.3% of them. Only 2% of all cases had a previous history of lymphoma. Thus, in most cases (98%), lymphoma was discovery incidentally. In addition, nodal lymphoma was associated with metastasis in 29 (9.4%) cases as collision tumor, most commonly (90%) in locoregional lymph nodes of the solid neoplasm.

CONCLUSIONS

The frequent association of some type of B-cell lymphoma and adenocarcinoma in synchronous/collision tumors of the gastrointestinal tract points to common pathogenic mechanisms in both neoplasia, particularly related to chronic inflammation in this location. In most cases, lymphoma identified in locoregional lymph nodes or distant of a carcinoma seems to represent an incidental finding during the carcinoma diagnostic/therapeutic approach. A synergy between carcinoma and lymphoma (involving inflammation and immunosuppression mechanisms) may favor tumor progression and dissemination. A better understating of the interactions lymphoma/carcinoma in the setting of synchronous/collision tumors may help to improve patient management and prognosis.

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-Related Cellular, Structural and Functional Changes in the Lymph Nodes: A Significant Component of Immunosenescence? An Overview

Aging affects all tissues and organs. Aging of the immune system results in the severe disruption of its functions, leading to an increased susceptibility to infections, an increase in autoimmune disorders and cancer incidence, and a decreased response to vaccines. Lymph nodes are precisely organized structures of the peripheral lymphoid organs and are the key sites coordinating innate and long-term adaptive immune responses to external antigens and vaccines. They are also involved in immune tolerance. The aging of lymph nodes results in decreased cell transport to and within the nodes, a disturbance in the structure and organization of nodal zones, incorrect location of individual immune cell types and impaired intercellular interactions, as well as changes in the production of adequate amounts of chemokines and cytokines necessary for immune cell proliferation, survival and function, impaired naïve T- and B-cell homeostasis, and a diminished long-term humoral response. Understanding the causes of these stromal and lymphoid microenvironment changes in the lymph nodes that cause the aging-related dysfunction of the immune system can help to improve long-term immune responses and the effectiveness of vaccines in the elderly.

Benign lymph node microenvironment is associated with response to immunotherapy

Introduction

Benign lymph nodes have been considered the hubs of immune surveillance in cancer patients. The microenvironment of these lymphoid tissues can be immune suppressed, hence allowing for tumor progression. Understanding the spectrum of benign findings in bystander lymph nodes in immune checkpoint blockade therapy could prove to be key to understanding the mechanism and assessing treatment response.

Methods

Benign lymph nodes and spleen were evaluated from patients treated with immunotherapy who subsequently received postmortem examination. We used quantitative immunofluorescence (QIF) to assess tumor infiltrating lymphocytes (TIL) and macrophage marker expression and characterized activation status using a novel multiplexed QIF assay including CD3, GranzymeB, and Ki67. We performed immunohistochemistry to correlate results of QIF.

Results

Benign lymph nodes from non-responders to immunotherapy showed significantly higher expression of cytotoxic markers and proliferation index (Ki67) in T cells compared to responders. Higher expression of PD-L1 in macrophages was also observed. There was no significant difference in CD3⁺ expression, but higher levels of CD8⁺ T cells as well as CD20⁺ B cells were seen in lymph nodes of non-responders. No significant differences were seen between responder and non-responder splenic tissue. Findings were supported by traditional immunostaining methods.

Conclusions

While most studies in biomarkers for immunotherapy focus on tumor microenvironment, we show that benign lymph node microenvironment may predict response to immunotherapy. In responding patients, bystander lymph nodes appear to have been mobilized, resulting in reduced cytotoxic T cells. Conversely, patients whose disease progressed on immunotherapy demonstrate higher levels of macrophages that express increased PD-L1, and activated T cells not recruited to the tumor site.

The Rho regulator Myosin IXb enables nonlymphoid tissue seeding of protective CD8+ T cells

Moalli et al. combine in vitro CD8⁺ T cell motility analysis with intravital imaging of mouse tissues to identify the actomyosin regulator Myo9b as a central player for nonlymphoid tissue infiltration during adaptive immune responses by facilitating crossing of tissue barriers.

T cells are actively scanning pMHC-presenting cells in lymphoid organs and nonlymphoid tissues (NLTs) with divergent topologies and confinement. How the T cell actomyosin cytoskeleton facilitates this task in distinct environments is incompletely understood. Here, we show that lack of Myosin IXb (Myo9b), a negative regulator of the small GTPase Rho, led to increased Rho-GTP levels and cell surface stiffness in primary T cells. Nonetheless, intravital imaging revealed robust motility of Myo9b^−/− CD8⁺ T cells in lymphoid tissue and similar expansion and differentiation during immune responses. In contrast, accumulation of Myo9b^−/− CD8⁺ T cells in NLTs was strongly impaired. Specifically, Myo9b was required for T cell crossing of basement membranes, such as those which are present between dermis and epidermis. As consequence, Myo9b^−/− CD8⁺ T cells showed impaired control of skin infections. In sum, we show that Myo9b is critical for the CD8⁺ T cell adaptation from lymphoid to NLT surveillance and the establishment of protective tissue–resident T cell populations.

Live-Animal Imaging of Renal Function by Multiphoton Microscopy

Intravital microscopy, microscopy of living animals, is a powerful research technique that combines the resolution and sensitivity found in microscopic studies of cultured cells with the relevance and systemic influences of cells in the context of the intact animal. The power of intravital microscopy has recently been extended with the development of multiphoton fluorescence microscopy systems capable of collecting optical sections from deep within the kidney at subcellular resolution, supporting high-resolution characterizations of the structure and function of glomeruli, tubules, and vasculature in the living kidney. Fluorescent probes are administered to an anesthetized, surgically prepared animal, followed by image acquisition for up to 3 hr. Images are transferred via a high-speed network to specialized computer systems for digital image analysis. This general approach can be used with different combinations of fluorescent probes to evaluate processes such as glomerular permeability, proximal tubule endocytosis, microvascular flow, vascular permeability, mitochondrial function, and cellular apoptosis/necrosis. © 2018 by John Wiley & Sons, Inc.

Live Imaging of Interstitial T Cell Migration Using Lymph Node Slices

Live imaging using various microscopic technologies is an indispensable tool for investigating the dynamic nature of immune cells. One of the most powerful techniques is the two-photon laser-scanning microscopy (TP-LSM), which has various advantages in observing deep tissues in vivo. Interstitial T cell migration in the lymph node (LN) is a phenomenon intensively examined using TP-LSM in the field of immunology. Intravital and explant methods have been standards for imaging T cell behaviors in the LN, though there are several limitations. Live imaging of LN slices, an LN explant sliced by a vibratome to expose tissue parenchyma, could provide an alternative approach with technical advantages for an in-depth analysis of interstitial T cell migration in vivo.

Thromboxane A2 acts as tonic immunoregulator by preferential disruption of low-avidity CD4+ T cell-dendritic cell interactions

Interactions between dendritic cells (DCs) and T cells control the decision between activation and tolerance induction. Thromboxane A2 (TXA2) and its receptor TP have been suggested to regulate adaptive immune responses through control of T cell-DC interactions. Here, we show that this control is achieved by selectively reducing expansion of low-avidity CD4(+) T cells. During inflammation, weak tetramer-binding TP-deficient CD4(+) T cells were preferentially expanded compared with TP-proficient CD4(+) T cells. Using intravital imaging of cellular interactions in reactive peripheral lymph nodes (PLNs), we found that TXA2 led to disruption of low- but not high-avidity interactions between DCs and CD4(+) T cells. Lack of TP correlated with higher expression of activation markers on stimulated CD4(+) T cells and with augmented accumulation of follicular helper T cells (TFH), which correlated with increased low-avidity IgG responses. In sum, our data suggest that tonic suppression of weak CD4(+) T cell-DC interactions by TXA2-TP signaling improves the overall quality of adaptive immune responses.

Multiphoton excited fluorescent materials for frequency upconversion emission and fluorescent probes

Recent progress in developing various strategies for exploiting efficient MPA fluorophores for two emerging technological MPA applications including frequency upconversion photoluminescence and lasing as well as 2PA fluorescence bioimaging and biosensing are presented. An intriguing application of MPA frequency-upconverted lasing offers opportunity for the fabrication of high-energy coherent light sources in the blue region which could create new advantages and breakthroughs in various laser-based applications. In addition, multiphoton excitation has led to considerable progress in the development of advanced diagnostic and therapeutic treatments; further advancement is anticipated with the emergence of various versatile 2PA fluorescence probes. It is widely appreciated that the two-photon excitation offers significant advantages for the biological fluorescence imaging and sensing which includes higher spatial resolution, less photobleaching and photodamage as well as deeper tissue penetration as compared to the one-photon excited microscopy. To be practically useful, the 2PA fluorescent probes for biological applications are required to have a site-specificity, a high fluorescence quantum yield, proper two-photon excitation and subsequent emission wavelengths, good photodecomposition stability, water solubility, and biocompatibility besides large 2PA action cross-sections.

Visualizing osteogenesis in vivo within a cell-scaffold construct for bone tissue engineering using two-photon microscopy

Tissue-engineering therapies have shown early success in the clinic, however, the cell-biomaterial interactions that result in successful outcomes are not yet well understood and are difficult to observe. Here we describe a method for visualizing bone formation within a tissue-engineered construct in vivo, at a single-cell resolution, and in situ in three dimensions using two-photon microscopy. First, two-photon microscopy and histological perspectives were spatially linked using fluorescent reporters for cells in the skeletal lineage. In the process, the tissue microenvironment that precedes a repair-focused study was described. The distribution and organization of type I collagen in the calvarial microenvironment was also described using its second harmonic signal. Second, this platform was used to observe in vivo, for the first time, host cells, donor cells, scaffold, and new bone formation within cell-seeded constructs in a bone defect. We examined constructs during bone repair 4 and 6 weeks after implantation. New bone formed on scaffolds, primarily by donor cells. Host cells formed a new periosteal layer that covered the implant. Scaffold resorption appeared to be site specific, where areas near the top were removed and deeper areas were completely embedded in new mineral. Visualizing the in vivo progression of the cell and scaffold microenvironment will contribute to our understanding of tissue-engineered regeneration and should lead to the development of more streamlined and therapeutically powerful approaches.

Live-animal imaging of renal function by multiphoton microscopy

Intravital microscopy, microscopy of living animals, is a powerful research technique that combines the resolution and sensitivity found in microscopic studies of cultured cells with the relevance and systemic influences of cells in the context of the intact animal. The power of intravital microscopy has recently been extended with the development of multiphoton fluorescence microscopy systems capable of collecting optical sections from deep within the kidney at subcellular resolution, supporting high-resolution characterizations of the structure and function of glomeruli, tubules, and vasculature in the living kidney. Fluorescent probes are administered to an anesthetized, surgically prepared animal, followed by image acquisition for up to 3 hr. Images are transferred via a high-speed network to specialized computer systems for digital image analysis. This general approach can be used with different combinations of fluorescent probes to evaluate processes such as glomerular permeability, proximal tubule endocytosis, microvascular flow, vascular permeability, mitochondrial function, and cellular apoptosis/necrosis.

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

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

Dynamic in situ cytometry uncovers T cell receptor signaling during immunological synapses and kinapses in vivo

Upon antigen recognition, T cells form either static (synapses) or migratory (kinapses) contacts with antigen-presenting cells. Addressing whether synapses and kinapses result in distinct T cell receptor (TCR) signals has been hampered by the inability to simultaneously assess T cell phenotype and behavior. Here, we introduced dynamic in situ cytometry (DISC), a combination of intravital multiphoton imaging and flow cytometry-like phenotypic analysis. Taking advantage of CD62L shedding as a marker of early TCR signaling, we examined how T cells sense TCR ligands of varying affinities in vivo. We uncovered three modes of antigen recognition: synapses with the strongest TCR signals, kinapses with robust signaling, and kinapses with weak signaling. As illustrated here, the DISC approach should provide unique opportunities to link immune cell behavior to phenotype and function in vivo.

The role of the lymphatic system in vaccine trafficking and immune response

The development and improvement of vaccines has been a significant endeavor on the part of the medical community for more than the last two centuries, and the success of these efforts is obvious when one considers the millions of lives that have been saved. Recent work in the field of vaccines, however, indicates that vaccines may be developed for even more challenging diseases than those previously addressed. It will be important in achieving this feat to account for the physical and chemical processes related to vaccine trafficking, rather than solely relying on our knowledge of the pathogen and our empirical experience. A thorough understanding of the lymphatic system is essential considering the role it plays in antigen trafficking and all immunological activity. This review describes the results of recent work that provides insight into the physiological processes of the lymphatic system and its various components with an emphasis on vaccine antigen trafficking from the administration site to secondary lymphoid tissues and the ensuing immune response. The review also discusses current challenges in designing vaccines and presents modern strategies for designing vaccines to better interface with the lymphatic system.

Detection of intra-tumor self antigen recognition during melanoma tumor progression in mice using advanced multimode confocal/two photon microscope

Determining how tumor immunity is regulated requires understanding the extent to which the anti-tumor immune response “functions” in vivo without therapeutic intervention. To better understand this question, we developed advanced multimodal reflectance confocal/two photon fluorescence intra-vital imaging techniques to use in combination with traditional ex vivo analysis of tumor specific T cells. By transferring small numbers of melanoma-specific CD8+ T cells (Pmel-1), in an attempt to mimic physiologic conditions, we found that B16 tumor growth alone was sufficient to induce naive Pmel-1 T cell proliferation and acquisition of effector phenotype. Tumor -primed Pmel-1 T cells, are capable of killing target cells in the periphery and secrete IFNγ, but are unable to mediate tumor regression. Within the tumor, Pmel-1 T cells have highly confined mobility, displaying long term interactions with tumor cells. In contrast, adoptively transferred non tumor-specific OT-I T cells show neither confined mobility, nor long term interaction with B16 tumor cells, suggesting that intra-tumor recognition of cognate self antigen by Pmel-1 T cells occurs during tumor growth. Together, these data indicate that lack of anti-tumor efficacy is not solely due to ignorance of self antigen in the tumor microenvironment but rather to active immunosuppressive influences preventing a protective immune response.

In vivo imaging of immune cell trafficking in cancer

Tumour establishment, progression and regression can be studied in vivo using an array of imaging techniques ranging from MRI to nuclear-based and optical techniques that highlight the intrinsic behaviour of different cell populations in the physiological context. Clinical in vivo imaging techniques and preclinical specific approaches have been used to study, both at the macroscopic and microscopic level, tumour cells, their proliferation, metastasisation, death and interaction with the environment and with the immune system. Fluorescent, radioactive or paramagnetic markers were used in direct protocols to label the specific cell population and reporter genes were used for genetic, indirect labelling protocols to track the fate of a given cell subpopulation in vivo. Different protocols have been proposed to in vivo study the interaction between immune cells and tumours by different imaging techniques (intravital and whole-body imaging). In particular in this review we report several examples dealing with dendritic cells, T lymphocytes and macrophages specifically labelled for different imaging procedures both for the study of their physiological function and in the context of anti-neoplastic immunotherapies in the attempt to exploit imaging-derived information to improve and optimise anti-neoplastic immune-based treatments.

B cell follicles and antigen encounters of the third kind

Defining where and in what form lymphocytes encounter antigen is fundamental to understanding how immune responses occur. Although knowledge of the recognition of antigen by CD4(+) and CD8(+) T cells has advanced greatly, understanding of the dynamics of B cell-antigen encounters has lagged. With the application of advanced imaging approaches, encounters of this third kind are now being brought into focus. Multiple processes facilitate these encounters, from the filtering functions of lymphoid tissues and migration paths of B cells to the antigen-presenting properties of macrophages and follicular dendritic cells. This Review will discuss how these factors work together in the lymph node to ensure efficient and persistent exposure of B cells to diverse forms of antigen and thus effective triggering of the humoral response.

Characterizing the dynamics of CD4+ T cell priming within a lymph node

Generating adaptive immunity postinfection or immunization requires physical interaction within a lymph node T zone between Ag-bearing dendritic cells (DCs) and rare cognate T cells. Many fundamental questions remain regarding the dynamics of DC-CD4+ T cell interactions leading to priming. For example, it is not known how the production of primed CD4+ T cells relates to the numbers of cognate T cells, Ag-bearing DCs, or peptide-MHCII level on the DC. To address these questions, we developed an agent-based model of a lymph node to examine the relationships among cognate T cell frequency, DC density, parameters characterizing DC-T cell interactions, and the output of primed T cells. We found that the output of primed CD4+ T cells is linearly related to cognate frequency, but nonlinearly related to the number of Ag-bearing DCs present during infection. This addresses the applicability of two photon microscopy studies to understanding actual infection dynamics, because these types of experiments increase the cognate frequency by orders of magnitude compared with physiologic levels. We found a trade-off between the quantity of peptide-major histocompatibility class II on the surface of individual DCs and number of Ag-bearing DCs present in the lymph node in contributing to the production of primed CD4+ T cells. Interestingly, peptide-major histocompatibility class II t(1/2) plays a minor, although still significant, role in determining CD4+ T cell priming, unlike the primary role that has been suggested for CD8+ T cell priming. Finally, we identify several pathogen-targeted mechanisms that, if altered in their efficiency, can significantly effect the generation of primed CD4+ T cells.

Immunopathogenesis of bronchial asthma

Bronchial asthma is a common immune-mediated disorder characterized by reversible airway inflammation, mucus production, and variable airflow obstruction with airway hyperresponsiveness. Allergen exposure results in the activation of numerous cells of the immune system, of which dendritic cells (DCs) and Th2 lymphocytes are of paramount importance. Although the epithelium was initially considered to function solely as a physical barrier, it is now evident that it plays a central role in the Th2-cell sensitization process due to its ability to activate DCs. Cytokines are inevitable factors in driving immune responses. To the list of numerous cytokines already known to be involved in the regulation of allergic reactions, new cytokines were added, such as TSLP, IL-25, and IL-33. IgE is also a central player in the allergic response. The activity of IgE is associated with a network of proteins, especially with its high- and low-affinity Fc receptors. Understanding the cellular and molecular mechanisms of allergic reactions helps us not only to understand the mechanisms of current treatments, but is also important for the identification of new targets for biological intervention. An IgE-specific monoclonal antibody, omalizumab, has already reached the clinic and similar biological agents will surely follow.

Sphingosine 1-phosphate receptor signaling

The sphingosine 1-phosphate (S1P) receptor signaling system is a productive model system. A hydrophobic zwitterionic lysophospholipid ligand with difficult physical properties interacts with five high-affinity G protein-coupled receptors to generate multiple downstream signals. These signals modulate homeostasis and pathology on a steep agonist concentration-response curve. Ligand presence is essential for vascular development and endothelial integrity, while acute increases in ligand concentrations result in cardiac death. Understanding this integrated biochemical system has exemplified the impact of both genetics and chemistry. Developing specific tools with defined biochemical properties for the reversible modulation of signals in real time has been essential to complement insights gained from genetic approaches that may be irreversible and compensated. Despite its knife-edge between life and death, this system, based in part on receptor subtype-selectivity and in part on differential attenuation of deleterious signals, now appears to be on the cusp of meaningful therapy for multiple sclerosis.

The who, how and where of antigen presentation to B cells

A functional immune system depends on the appropriate activation of lymphocytes following antigen encounter. In this Review, we summarize studies that have used high-resolution imaging approaches to visualize antigen presentation to B cells in secondary lymphoid organs. These studies illustrate that encounters of B cells with antigen in these organs can be facilitated by diffusion of the antigen or by the presentation of antigen by macrophages, dendritic cells and follicular dendritic cells. We describe cell-surface molecules that might be important in mediating antigen presentation to B cells and also highlight the key role of B cells themselves in antigen transport. Data obtained from the studies discussed here highlight the predominance, importance and variety of the cell-mediated processes that are involved in presenting antigen to B cells in vivo.

Dynamics of neutrophil migration in lymph nodes during infection

Although the signals that control neutrophil migration from the blood to sites of infection have been well characterized, little is known about their migration patterns within lymph nodes or the strategies that neutrophils use to find their local sites of action. To address these questions, we used two-photon scanning-laser microscopy to examine neutrophil migration in intact lymph nodes during infection with an intracellular parasite, Toxoplasma gondii. We found that neutrophils formed both small, transient and large, persistent swarms via a coordinated migration pattern. We provided evidence that cooperative action of neutrophils and parasite egress from host cells could trigger swarm formation. Neutrophil swarm formation coincided in space and time with the removal of macrophages that line the subcapsular sinus of the lymph node. Our data provide insights into the cellular mechanisms underlying neutrophil swarming and suggest new roles for neutrophils in shaping immune responses.

Imaging techniques: new insights into chemokine/chemokine receptor biology at the immune system

Our current knowledge of molecular and cellular responses in vivo is based mainly on event reconstruction from time-freeze observations. Conventional biochemical and genetic methods consider the cell as an individual entity and ligand/receptor pairs as isolated systems. In addition, the data refer to the average behavior of a pool of cells and/or receptors removed from their real-life context. The use of new technologies, particularly real-time imaging approaches, is showing us that biological responses are highly dynamic and extremely dependent on the context in which they take place, and therefore much more diverse than initially envisaged. This review focuses on the mechanistic insights that new imaging techniques, such as those based on resonance energy transfer and two-photon microscopy, contribute to our understanding of how receptors work within a single cell, and how cells work within a tissue. Cell movement is a complex and regulated process; it has a key role in embryogenesis, organogenesis, wound-healing and tumor invasion. Nonetheless, it is in immune system homeostasis and response that cell movement becomes essential. For this reason, immunology is being radically transformed and enriched by these new approaches. We will discuss the use of these techniques for studying chemokine/chemokine receptors and their role in the immune system function, and comment on the potential contribution to the design of therapeutic strategies.

Making friends in out-of-the-way places: how cells of the immune system get together and how they conduct their business as revealed by intravital imaging

A central characteristic of the immune system is the constantly changing location of most of its constituent cells. Lymphoid and myeloid cells circulate in the blood, and subsets of these cells enter, move, and interact within, then leave organized lymphoid tissues. When inflammation is present, various hematopoietic cells also exit the vasculature and migrate within non-lymphoid tissues, where they carry out effector functions that support host defense or result in autoimmune pathology. Effective innate and adaptive immune responses involve not only the action of these individual cells but also productive communication among them, often requiring direct membrane contact between rare antigen-specific or antigen-bearing cells. Here, we describe our ongoing studies using two-photon intravital microscopy to probe the in situ behavior of the cells of the immune system and their interactions with non-hematopoietic stromal elements. We emphasize the importance of non-random cell migration within lymphoid tissues and detail newly established mechanisms of traffic control that operate at multiple organizational scales to facilitate critical cell contacts. We also describe how the methods we have developed for imaging within lymphoid sites are being applied to other tissues and organs, revealing dynamic details of host-pathogen interactions previously inaccessible to direct observation.

The microanatomy of T-cell responses

The priming of a T cell results from its physical interaction with a dendritic cell (DC) that presents the cognate antigenic peptide. The success rate of such interactions is extremely low, because the precursor frequency of a naive T cell recognizing a specific antigen is in the range of 1:10(5)-10(6). To make this principle practicable, encounter frequencies between DCs and T cells are maximized within lymph nodes (LNs) that are compact immunological projections of the peripheral tissue they drain. But LNs are more than passive meeting places for DCs that immigrated from the tissue and lymphocytes that recirculated via the blood. The microanatomy of the LN stroma actively organizes the cellular encounters by providing preformed migration tracks that create dynamic but highly ordered movement patterns. LN architecture further acts as a sophisticated filtration system that sieves the incoming interstitial fluid at different levels and guarantees that immunologically relevant antigens are loaded on DCs or B cells while inert substances are channeled back into the blood circulation. This review focuses on the non-hematopoietic infrastructure of the lymph node. We describe the association between fibroblastic reticular cell, conduit, DC, and T cell as the essential functional unit of the T-cell cortex.

Transgenic Leishmania and the immune response to infection

Genetic manipulation of single-celled organisms such as the Leishmania parasite enables in depth analysis of the consequences of genotypic change on biological function. In probing the immune responses to infection, use of transgenic Leishmania has the potential to unravel both the contribution of the parasite to the infection process and the cellular interactions and mechanisms that characterize the innate and adaptive immune responses of the host. Here, we briefly review recent technical advances in parasite genetics and explore how these methods are being used to investigate parasite virulence factors, elucidate immune regulatory mechanisms and contribute to the development of novel therapeutics for the leishmaniases. Recent developments in imaging technology, such as bioluminescence and intravital imaging, combined with parasite transfection with fluorescent or enzyme-encoding marker genes, provides a rich opportunity for novel assessment of intimate, real-time host-parasite interactions at a previously unexplored level. Further advances in transgenic technology, such as the introduction of robust inducible gene cassettes for expression in intracellular parasite stages or the development of RNA interference methods for down-regulation of parasite gene expression in the host, will further advance our ability to probe host-parasite interactions and unravel disease-promoting mechanisms in the leishmaniases.

Lymphocyte cell motility: the twisting, turning tale of phosphoinositide 3-kinase

The PI3K (phosphoinositide 3-kinase) family of lipid kinases regulate cell motility in diverse organisms and cell types. In mammals, the main PI3K enzyme activated by chemokine receptor signalling is the class IB isoform, p110gamma. Studies of p110gamma-knockout mice have shown an essential function for this isoform in chemotaxis of neutrophils and macrophages both in vitro and in vivo. However, the roles of p110gamma and other PI3K enzymes and regulatory subunits in lymphocyte motility have been more difficult to discern. Recent studies of adoptively transferred, fluorescently labelled lymphocytes have revealed complex and unexpected functions for PI3K in lymphocyte migration in vivo. In this review we highlight cell-type-specific roles for PI3K catalytic and regulatory subunits in the homing and basal motility of lymphocytes in the intact lymph node.

Direct priming of antiviral CD8+ T cells in the peripheral interfollicular region of lymph nodes

It is uncertain how antiviral lymphocytes are activated in draining lymph nodes, the site where adaptive immune responses are initiated. Here, using intravital microscopy we show that after infection of mice with vaccinia virus (a large DNA virus) or vesicular stomatitis virus (a small RNA virus), virions drained to the lymph node and infected cells residing just beneath the subcapsular sinus. Naive CD8+ T cells rapidly migrated to infected cells in the peripheral interfollicular region and then formed tight interactions with dendritic cells, leading to complete T cell activation. Thus, antigen presentation at the lymph node periphery, not at lymphocyte exit sites in deeper lymph node venules, as dogma dictates, has a dominant function in antiviral CD8+ T cell activation.

Seeing is believing: a focus on the contribution of microscopic imaging to our understanding of immune system function

Many cells of the immune system do not occupy fixed tissue locations, but circulate in the blood, traffic through the lymph, and migrate within organized lymphoid organs and periphery tissues. Rare antigen-specific lymphocytes must find one another for productive adaptive immune responses and the different phases of cell-mediated and humoral immune response development take place in distinct sites. This historical feature examines how we have reached our current understanding of these aspects of immune system function. It emphasizes the critical role of ever-improving imaging techniques in determining where immune cells reside and interact and stresses the key past contribution of sequential static immunohistochemical analysis using monoclonal reagents. In combination with genetic studies, these imaging experiments resulted in our current paradigm that views activation-dependent changes in chemokine sensitivity as central to effective cell co-operation. We also highlight the very recent application of two-photon imaging to the direct observation of immune cell dynamics in a natural tissue environment, noting how the application of this technology has reinforced some existing ideas and is changing other long-held views. We conclude with some speculations about the opportunities for further advances using ever more powerful imaging methods.

Control of receptor internalization, signaling level, and precise arrival at the target in guided cell migration

Activation of the chemokine receptor CXCR4 by SDF1 controls a variety of biological processes in development, immune response, and disease [1-5]. The carboxyl-terminal region of CXCR4 is subject to phosphorylation that allows binding of regulatory proteins [5]; this results in downregulation of CXCR4 signaling and receptor internalization [6]. Notably, truncations of this part of CXCR4 have been implicated in WHIM syndrome, a dominantly inherited immunodeficiency disorder [7, 8]. Despite its importance in receptor signaling and the clinical relevance of its regulation, the precise function of regulating signaling level and internalization in controlling cell behavior is not known. Whereas a number of in vitro studies suggested that the carboxyl terminus of CXCR4 positively regulates chemotaxis (e.g., [9]), others reached the opposite conclusion [8, 10, 11]. These conflicting results highlight the importance of investigating this process under physiological conditions in the live animal. In this study, we demonstrate the significance of internalization and of controlling receptor signaling level for SDF-1-guided migration. We found that whereas internalization and the control over signaling intensity are dispensable for cell motility and directional sensing, they are essential for fine-tuning of migration in vivo, allowing precise arrival of zebrafish PGCs at their target, the region where the gonad develops.

Autoaggressive effector T cells in the course of experimental autoimmune encephalomyelitis visualized in the light of two-photon microscopy

Two photon microscopy (TPM) recently emerged as optical tool for the visualization of immune processes hundreds of micrometers deep in living tissue and organs. Here we summarize recent work on exploiting this technology to study brain antigen specific T cells. These cells are the cause of Experimental Autoimmune Encephalomyelitis (EAE) an autoimmune disease model of Multiple Sclerosis. TPM studies elucidated the dynamics of the autoaggressive effector T cells in peripheral immune milieus during preclinical EAE, where the cells become reprogrammed to enter their target organ. These studies revealed an unexpectedly lively locomotion behavior of the cells interrupted only by short-lasting contacts with the local immune stroma. Live T cell behavior was furthermore studied within the acutely inflamed CNS. Two distinct migratory patterns of the T cells were found: the majority of cells (60-70%) moved fast and seemingly unhindered through the compact CNS parenchyma. The motility of the other cell fraction was highly confined. The cells swung around a fixed cell pole forming long-lasting contacts to putative local antigen presenting cells.

A comparison of random vs. chemotaxis-driven contacts of T cells with dendritic cells during repertoire scanning

Generating adaptive immunity after infection or immunization requires physical interactions within a lymph node (LN) T-zone between antigen-bearing dendritic cells (DCs) that arrive from peripheral tissues and rare cognate T cells entering via high endothelial venules (HEVs). This interaction results in activation of cognate T cells, expansion of that T cell lineage and their exit from the LN T-zone via efferent lymphatics (ELs). How antigen-specific T cells locate DCs within this complex environment is controversial, and both random T cell migration and chemotaxis have been proposed. We developed an agent-based computational model of a LN that captures many features of T cell and DC dynamics observed by two-photon microscopy. Our simulations matched in vivo two-photon microscopy data regarding T cell speed, short-term directional persistence of motion and cell motility. We also obtained in vivo data regarding density of T cells and DCs within a LN and matched our model environment to measurements of the distance from HEVs to ELs. We used our model to compare chemotaxis with random motion and showed that chemotaxis increased total number of T cell DC contacts, but decreased unique contacts, producing fewer activated T cells. Our results suggest that, within a LN T-zone, a random search strategy is optimal for a rare cognate T cell to find its DC match and maximize production of activated T cells.

An adaptive route to innate immunity?

Recent notions of the immune response focus on the idea that innate pathogen recognition receptors (PRRs) lead to adaptive immune responses.

Ca2+ signals in CD4+ T cells during early contacts with antigen-bearing dendritic cells in lymph node

T cell activation by APC requires cytosolic Ca(2+) ([Ca(2+)](i)) elevation. Using two-photon microscopy, we visualized Ca(2+) signaling and motility of murine CD4(+) T cells within lymph node (LN) explants under control, inflammatory, and immunizing conditions. Without Ag under basal noninflammatory conditions, T cells showed infrequent Ca(2+) spikes associated with sustained slowing. Inflammation reduced velocities and Ca(2+) spiking in the absence of specific Ag. During early Ag encounter, most T cells engaged Ag-presenting dendritic cells in clusters, and showed increased Ca(2+) spike frequency and elevated basal [Ca(2+)](i). These Ca(2+) signals persisted for hours, irrespective of whether T cells were in contact with visualized dendritic cells. We propose that sustained increases in basal [Ca(2+)](i) and spiking frequency constitute a Ca(2+) signaling modality that, integrated over hours, distinguishes immunogenic from basal state in the native lymphoid environment.

Nanosystems for simultaneous imaging and drug delivery to T cells

The T-cell response defines the pathogenesis of many common chronic disease states, including diabetes, rheumatoid arthritis, and transplant rejection. Therefore, a diagnostic strategy that visualizes this response can potentially lead to early therapeutic intervention, avoiding catastrophic organ failure or prolonged sickness. In addition, the means to deliver a drug dose to those cells in situ with the same specificity used to image those cells would provide for a powerful therapeutic alternative for many disease states involving T cells. In this report, we review emerging nanosystems that can be used for simultaneous tracking and drug delivery to those cells. Because of their versatility, these systems--which combine specific receptor targeting with an imaging agent and drug delivery--are suited to both basic science and applications, from developing therapeutic strategies for autoimmune and alloimmune diseases, to noninvasive tracking of pathogenic T-cell migration.

In vivo imaging of cytotoxic T cell infiltration and elimination of a solid tumor

Although the immune system evolved to fight infections, it may also attack and destroy solid tumors. In most cases, tumor rejection is initiated by CD8⁺ cytotoxic T lymphocytes (CTLs), which infiltrate solid tumors, recognize tumor antigens, and kill tumor cells. We use a combination of two-photon intravital microscopy and immunofluorescence on ordered sequential sections to analyze the infiltration and destruction of solid tumors by CTLs. We show that in the periphery of a thymoma growing subcutaneously, activated CTLs migrate with high instantaneous velocities. The CTLs arrest in close contact to tumor cells expressing their cognate antigen. In regions where most tumor cells are dead, CTLs resume migration, sometimes following collagen fibers or blood vessels. CTLs migrating along blood vessels preferentially adopt an elongated morphology. CTLs also infiltrate tumors in depth, but only when the tumor cells express the cognate CTL antigen. In tumors that do not express the cognate antigen, CTL infiltration is restricted to peripheral regions, and lymphocytes neither stop moving nor kill tumor cells. Antigen expression by tumor cells therefore determines both CTL motility within the tumor and profound tumor infiltration.

Imaging of Germinal Center Selection Events During Affinity Maturation

The germinal center (GC) is an important site for the generation and selection of B cells bearing high-affinity antibodies, yet GC cell migration and interaction dynamics have not been directly observed. Using two-photon microscopy of mouse lymph nodes, we revealed that GC B cells are highly motile and extend long cell processes. They transited between GC dark and light zones and divided in both regions, although these B cells resided for only several hours in the light zone where antigen is displayed. GC B cells formed few stable contacts with GC T cells despite frequent encounters, and T cells were seen to carry dead B cell blebs. On the basis of these observations, we propose a model in which competition for T cell help plays a more dominant role in the selection of GC B cells than previously appreciated.