Optical projection tomography reveals dynamics of HEV growth after immunization with protein plus CFA and features shared with HEVs in acute autoinflammatory lymphadenopathy

L-selectin-dependent and -independent homing of naïve lymphocytes through the lung draining lymph node support T cell response to pulmonary Mycobacterium tuberculosis infection

Recruiting large numbers of naïve lymphocytes to lymph nodes is critical for mounting an effective adaptive immune response. While most naïve lymphocytes utilize homing molecule L-selectin to enter lymph nodes, some circulating cells can traffic to the lung-draining mediastinal lymph node (mLN) through lymphatics via the intermediate organ, lung. However, whether this alternative trafficking mechanism operates in infection and contributes to T cell priming are unknown. We report that in pulmonary Mycobacterium tuberculosis-infected mice, homing of circulating lymphocytes to the mLN is significantly less efficient than to non-draining lymph node. CD62L blockade only partially reduced the homing of naïve T lymphocytes, consistent with L-selectin-independent routing of naïve lymphocytes to the site. We further demonstrated that lymphatic vessels in infected mLN expanded significantly and inhibiting lymphangiogenesis with a vascular endothelial growth factor receptor 3 kinase inhibitor reduced the recruitment of intravenously injected naïve lymphocytes to the mLN. Finally, mycobacterium-specific T cells entering via the L-selectin-independent route were readily activated in the mLN. Our study suggests that both L-selectin-dependent and -independent pathways contribute to naïve lymphocyte entry into mLN during M. tuberculosis infection and the latter pathway may represent an important mechanism for orchestrating host defence in the lungs.

High endothelial venules in cancer: Regulation, function, and therapeutic implication

The lack of sufficient intratumoral CD8⁺ T lymphocytes is a significant obstacle to effective immunotherapy in cancer. High endothelial venules (HEVs) are organ-specific and specialized postcapillary venules uniquely poised to facilitate the transmigration of lymphocytes to lymph nodes (LNs) and other secondary lymphoid organs (SLOs). HEVs can also form in human and murine cancer (tumor HEVs [TU-HEVs]) and contribute to the generation of diffuse T cell-enriched aggregates or tertiary lymphoid structures (TLSs), which are commonly associated with a good prognosis. Thus, therapeutic induction of TU-HEVs may provide attractive avenues to induce and sustain the efficacy of immunotherapies by overcoming the major restriction of T cell exclusion from the tumor microenvironment. In this review, we provide current insight into the commonalities and discrepancies of HEV formation and regulation in LNs and tumors and discuss the specific function and significance of TU-HEVs in eliciting, predicting, and aiding anti-tumoral immunity.

Three-dimensional Imaging Reveals Immune-driven Tumor-associated High Endothelial Venules as a Key Correlate of Tumor Rejection Following Depletion of Regulatory T Cells

High endothelial venules (HEV) are specialized post capillary venules that recruit naïve T cells and B cells into secondary lymphoid organs (SLO) such as lymph nodes (LN). Expansion of HEV networks in SLOs occurs following immune activation to support development of an effective immune response. In this study, we used a carcinogen-induced model of fibrosarcoma to examine HEV remodeling after depletion of regulatory T cells (Treg). We used light sheet fluorescence microscopy imaging to visualize entire HEV networks, subsequently applying computational tools to enable topological mapping and extraction of numerical descriptors of the networks. While these analyses revealed profound cancer- and immune-driven alterations to HEV networks within LNs, these changes did not identify successful responses to treatment. The presence of HEV networks within tumors did however clearly distinguish responders from nonresponders. Finally, we show that a successful treatment response is dependent on coupling tumor-associated HEV (TA-HEV) development to T-cell activation implying that T-cell activation acts as the trigger for development of TA-HEVs which subsequently serve to amplify the immune response by facilitating extravasation of T cells into the tumor mass.

Lymph node swelling combined with temporary effector T cell retention aids T cell response in a model of adaptive immunity

Swelling of lymph nodes (LNs) is commonly observed during the adaptive immune response, yet the impact on T cell (TC) trafficking and subsequent immune response is not well known. To better understand the effect of macro-scale alterations, we developed an agent-based model of the LN paracortex, describing the TC proliferative response to antigen-presenting dendritic cells alongside inflammation-driven and swelling-induced changes in TC recruitment and egress, while also incorporating regulation of the expression of egress-modulating TC receptor sphingosine-1-phosphate receptor-1. Analysis of the effector TC response under varying swelling conditions showed that swelling consistently aided TC activation. However, subsequent effector CD8+ TC production was reduced in scenarios where swelling occurred too early in the TC proliferative phase or when TC cognate frequency was low due to increased opportunity for TC exit. Temporarily extending retention of newly differentiated effector TCs, mediated by sphingosine-1-phosphate receptor-1 expression, mitigated any negative effects of swelling by allowing facilitation of activation to outweigh increased access to exit areas. These results suggest that targeting temporary effector TC retention and egress associated with swelling offers new ways to modulate effector TC responses in, for example, immuno-suppressed patients and to optimize of vaccine design.

Characterizing perfusion defects in metastatic lymph nodes at an early stage using high-frequency ultrasound and micro-CT imaging

A perfusion defect in a metastatic lymph node (LN) can be visualized as a localized area of low contrast on contrast-enhanced CT, MRI or ultrasound images. Hypotheses for perfusion defects include abnormal hemodynamics in neovascular vessels or a decrease in blood flow in pre-existing blood vessels in the parenchyma due to compression by LN tumor growth. However, the mechanisms underlying perfusion defects in LNs during the early stage of LN metastasis have not been investigated. We show that tumor mass formation with very few microvessels was associated with a perfusion defect in a non-enlarged LN at the early stage of LN metastasis in a LN adenopathy mouse (LN size circa 10 mm). We found in a mouse model of LN metastasis, induced using non-keratinizing tumor cells, that during the formation of the perfusion defect in a non-enlarged LN, the number of blood vessels ≤ 50 μm in diameter decreased, while those of > 50 μm in diameter increased. The methods used were contrast-enhanced high-frequency ultrasound and contrast-enhanced micro-CT imaging systems, with a maximum spatial resolution of > 30 μm. Furthermore, we found no tumor angiogenesis or oxygen partial pressure (pO₂) changes in the metastatic LN. Our results demonstrate that the perfusion defect appears to be a specific form of tumorigenesis in the LN, which is a vascular-rich organ. We anticipate that a perfusion defect on ultrasound, CT or MRI images will be used as an indicator of a non-enlarged metastatic LN at an early stage.

A Straightforward Method for 3D Visualization of B Cell Clusters and High Endothelial Venules in Lymph Nodes Highlights Differential Roles of TNFRI and -II

Whole mount tissue immunolabeling and imaging of complete organs has tremendous benefits in characterizing organ morphology. Here, we present a straightforward method for immunostaining, clearing and imaging of whole murine peripheral lymph nodes (PLNs) for detailed analysis of their architecture and discuss all procedures in detail in a step-by-step approach. Given the importance of tumor necrosis factor receptor (TNFR) signaling in development of PLNs we used TNFRI^-/- and TNFRII^-/- mice models as proof-of-concept for this technique by visualizing and analyzing structural changes in PLN B cell clusters and high endothelial venules (HEVs). Samples were subjected to de- and rehydration with methanol, labeled with antibodies for B cells, T cells and high endothelial venules (HEVs) and optically cleared using benzyl alcohol-benzyl benzoate. Imaging was done using LaVision light sheet microscope and analysis with Imaris software. Using these techniques, we confirmed previous findings that TNFRI signaling is essential for formation of individual B cell clusters. In addition, Our data suggest that TNFRII signaling is also to some extent involved in this process as TNFRII-/- PLNs had a B cell cluster morphology reminiscent of TNFRI^-/- PLNs. Moreover, visualization and objective quantification of the complete PLN high endothelial vasculature unveiled reduced volume, length and branching points of HEVs in TNFRI^-/- PLNs, revealing an earlier unrecognized contribution of TNFRI signaling in HEV morphology. Together, these results underline the potential of whole mount tissue staining and advanced imaging techniques to unravel even subtle changes in lymphoid tissue architecture.

High resolution optical projection tomography platform for multispectral imaging of the mouse gut

Optical projection tomography (OPT) is a powerful tool for three-dimensional imaging of mesoscopic biological samples with great use for biomedical phenotyping studies. We present a fluorescent OPT platform that enables direct visualization of biological specimens and processes at a centimeter scale with high spatial resolution, as well as fast data throughput and reconstruction. We demonstrate nearly isotropic sub-28 µm resolution over more than 60 mm3 after reconstruction of a single acquisition. Our setup is optimized for imaging the mouse gut at multiple wavelengths. Thanks to a new sample preparation protocol specifically developed for gut specimens, we can observe the spatial arrangement of the intestinal villi and the vasculature network of a 3-cm long healthy mouse gut. Besides the blood vessel network surrounding the gastrointestinal tract, we observe traces of vasculature at the villi ends close to the lumen. The combination of rapid acquisition and a large field of view with high spatial resolution in 3D mesoscopic imaging holds an invaluable potential for gastrointestinal pathology research.

High endothelial venules (HEVs) in immunity, inflammation and cancer

High endothelial venules (HEVs) are specialized blood vessels mediating lymphocyte trafficking to lymph nodes (LNs) and other secondary lymphoid organs. By supporting high levels of lymphocyte extravasation from the blood, HEVs play an essential role in lymphocyte recirculation and immune surveillance for foreign invaders (bacterial and viral infections) and alterations in the body’s own cells (neoantigens in cancer). The HEV network expands during inflammation in immune-stimulated LNs and is profoundly remodeled in metastatic and tumor-draining LNs. HEV-like blood vessels expressing high levels of the HEV-specific sulfated MECA-79 antigens are induced in non-lymphoid tissues at sites of chronic inflammation in many human inflammatory and allergic diseases, including rheumatoid arthritis, Crohn’s disease, allergic rhinitis and asthma. Such vessels are believed to contribute to the amplification and maintenance of chronic inflammation. MECA-79⁺ tumor-associated HEVs (TA-HEVs) are frequently found in human tumors in CD3⁺ T cell-rich areas or CD20⁺ B-cell rich tertiary lymphoid structures (TLSs). TA-HEVs have been proposed to play important roles in lymphocyte entry into tumors, a process essential for successful antitumor immunity and lymphocyte-mediated cancer immunotherapy with immune checkpoint inhibitors, vaccines or adoptive T cell therapy. In this review, we highlight the phenotype and function of HEVs in homeostatic, inflamed and tumor-draining lymph nodes, and those of HEV-like blood vessels in chronic inflammatory diseases. Furthermore, we discuss the role and regulation of TA-HEVs in human cancer and mouse tumor models.

The Dual Role of High Endothelial Venules in Cancer Progression versus Immunity

Secondary lymphoid organs (SLOs) are important initiators and regulators of immunity. To carry out this function, the blood vasculature must deliver oxygen and nutrients and recruit circulating lymphocytes into the SLO parenchyma, where they encounter cognate antigen. High endothelial venules (HEVs) are specialised postcapillary venules that specifically serve this function and are found in all SLOs except spleen. It is becoming clear that alterations to HEV network density and/or morphology can result in immune activation or, as recently implicated, in providing an exit route for tumour cell dissemination and metastases. In this review, the structural plasticity of HEVs, the regulatory pathways underpinning this plasticity, and the relevance of these pathways to cancer progression will be discussed.

Development and Immunological Function of Lymph Node Stromal Cells

Stromal cells have for a long time been viewed as structural cells that support distinct compartments within lymphoid tissues and little more. Instead, an active cross-talk between endothelial and fibroblastic stromal cells drives the maturation of lymphoid niches, a relationship that is recapitulated during lymph node organogenesis, steady-state conditions, and following inflammation. In this review, we go over recent advances in genetic models and high-resolution transcriptomic analyses that have propelled the finer resolution of the stromal cell infrastructure of lymph nodes, revealing that the distinct subsets are strategically positioned to deliver a catered mixture of niche factors to interacting immune cell populations. Moreover, we discuss how changes in the activation state of poised stromal cell-underpinned niches rather than on-demand differentiation of new stromal cell subsets govern the efficient interaction of Ag, APC, and cognate B and T lymphocytes during adaptive immune responses.

High Endothelial Venules Accelerate Naive T Cell Recruitment by Tumor Necrosis Factor-Mediated R-Ras Upregulation

Recruitment of naive T cells to lymph nodes is essential for the development of adaptive immunity. Upon pathogen infection, lymph nodes promptly increase the influx of naive T cells from the circulation in order to screen and prime the T cells. The precise contribution of the lymph node vasculature to the regulation of this process remains unclear. Here we show a role for the Ras GTPase, R-Ras, in the functional adaptation of high endothelial venules to increase naive T cell trafficking to the lymph nodes. R-Ras is transiently up-regulated in the endothelium of high endothelial venules by the inflammatory cytokine tumor necrosis factor (TNF) within 24 hours of pathogen inoculation. TNF induces R-Ras upregulation in endothelial cells via JNK and p38 mitogen-activated protein kinase but not NF-κB. Studies of T cell trafficking found that the loss of function of endothelial R-Ras impairs the rapid acceleration of naive T cell recruitment to the lymph nodes upon inflammation. This defect diminished the ability of naive OT-1 T cells to develop antitumor activity against ovalbumin-expressing melanoma. Proteomic analyses suggest that endothelial R-Ras facilitates TNF-dependent transendothelial migration (diapedesis) of naive T cells by modulating molecular assembly the at T cell-endothelial cell interface. These findings give new mechanistic insights into the functional adaptation of high endothelial venules to accelerate naive T cell recruitment to the lymph nodes.

Three-dimensional bright-field microscopy with isotropic resolution based on multi-view acquisition and image fusion reconstruction

Optical Projection Tomography (OPT) is a powerful three-dimensional imaging technique used for the observation of millimeter-scaled biological samples, compatible with bright-field and fluorescence contrast. OPT is affected by spatially variant artifacts caused by the fact that light diffraction is not taken into account by the straight-light propagation models used for reconstruction. These artifacts hinder high-resolution imaging with OPT. In this work we show that, by using a multiview imaging approach, a 3D reconstruction of the bright-field contrast can be obtained without the diffraction artifacts typical of OPT, drastically reducing the amount of acquired data, compared to previously reported approaches. The method, purely based on bright-field contrast of the unstained sample, provides a comprehensive picture of the sample anatomy, as demonstrated in vivo on Arabidopsis thaliana and zebrafish embryos. Furthermore, this bright-field reconstruction can be implemented on practically any multi-view light-sheet fluorescence microscope without complex hardware modifications or calibrations, complementing the fluorescence information with tissue anatomy.

Systematic Evaluation of Chemically Distinct Tissue Optical Clearing Techniques in Murine Lymph Nodes

Activation of adaptive immunity is a complex process coordinated at multiple levels in both time and the three-dimensional context of reactive lymph nodes (LNs). Although microscopy-based visualization of its spatiotemporal dynamics unravels complexities of developing immune response, such approach is highly limited by light-obstructing nature of tissue components. Recently, tissue optical clearing (TOC) techniques were established to bypass this obstacle and now allow to image and quantify the entire murine organs with cellular resolution. However, the spectrum of TOC is represented by wide variety of chemically distinct methods, each having certain advantages and disadvantages that were unsatisfactorily compared for suitability to LNs clearing. In this study, we have systematically tested 13 typical TOC techniques and assessed their impact on a number of critical factors such as LN transparency, imaging depth, change in size, compatibility with proteinaceous fluorophores, immunostaining, H&E staining, and light-sheet fluorescence microscopy. Based on the detailed data specific to TOC process of murine LNs, we provide a reliable reference for most suitable methods in an application-dependent manner.

Remodeling of reactive lymph nodes: Dynamics of stromal cells and underlying chemokine signaling

Lymph nodes (LNs) are secondary immune organs dispersed throughout the body. They are primarily composed of lymphocytes, "transient passengers" that are only present for a few hours. During this time, they extensively interact with a meshwork of stromal cells. Although these cells constitute less than 5% of all LN cells, they are integral to LN function: Stromal cells create a three-dimensional network that provides a rigid backbone for the transport of lymph and generates "roads" for lymphocyte migration. Beyond structural support, the LN stroma also produces survival signals for lymphocytes and provides nutrients, soluble factors, antigens, and immune cells collectively required for immune surveillance and the generation of adaptive immune responses. A unique feature of LNs is their ability to considerably and rapidly change size: the volume and cellularity of inflamed LNs can increase up to 20-fold before returning to homeostatic levels. This cycle will be repeated many times during life and is accommodated by stromal cells. The dynamics underlying this dramatic remodeling are subject of this review. We will first introduce the main types of LN stromal cells and explain their known functions. We will then discuss how these cells enable LN growth during immune responses, with a particular focus on underlying cellular mechanisms and molecular cues. Similarly, we will elaborate on stromal dynamics mediating the return to LN homeostasis, a process that is mechanistically much less understood than LN expansion.

The Role of Endothelial Cells and TNF-Receptor Superfamily Members in Lymphoid Organogenesis and Function During Health and Inflammation

Lymph nodes (LNs) are crucial for the orchestration of immune responses. LN reactions depend on interactions between incoming and local immune cells, and stromal cells. To mediate these cellular interactions an organized vascular network within the LN exists. In general, the LN vasculature can be divided into two components: blood vessels, which include the specialized high endothelial venules that recruit lymphocytes from the bloodstream, and lymphatic vessels. Signaling via TNF receptor (R) superfamily (SF) members has been implicated as crucial for the development and function of LNs and the LN vasculature. In recent years the role of cell-specific signaling of TNFRSF members in different endothelial cell (EC) subsets and their roles in development and maintenance of lymphoid organs has been elucidated. Here, we discuss recent insights into EC-specific TNFRSF member signaling and highlight its importance in different EC subsets in LN organogenesis and function during health, and in lymphocyte activation and tertiary lymphoid structure formation during inflammation.

The Biophysics of Lymphatic Transport: Engineering Tools and Immunological Consequences

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

Angular restriction fluorescence optical projection tomography to localize micrometastases in lymph nodes

Lymph node biopsy is a primary means of staging breast cancer, yet standard pathological techniques are time-consuming and typically sample less than 1% of the total node volume. A low-cost fluorescence optical projection tomography (OPT) protocol is demonstrated for rapid imaging of whole lymph nodes in three dimensions. The relatively low scattering properties of lymph node tissue can be leveraged to significantly improve spatial resolution of lymph node OPT by employing angular restriction of photon detection. It is demonstrated through porcine lymph node metastases models that simple filtered-backprojection reconstruction is sufficient to detect and localize 200-μm-diameter metastases (the smallest clinically significant) in 1-cm-diameter lymph nodes.

Quantification of the Whole Lymph Node Vasculature Based on Tomography of the Vessel Corrosion Casts

Lymph nodes (LN) are crucial for immune function, and comprise an important interface between the blood and lymphatic systems. Blood vessels (BV) in LN are highly specialized, featuring high endothelial venules across which most of the resident lymphocytes crossed. Previous measurements of overall lymph and BV flow rates demonstrated that fluid also crosses BV walls, and that this is important for immune function. However, the spatial distribution of the BV in LN has not been quantified to the degree necessary to analyse the distribution of transmural fluid movement. In this study, we seek to quantify the spatial localization of LNBV, and to predict fluid movement across BV walls. MicroCT imaging of murine popliteal LN showed that capillaries were responsible for approximately 75% of the BV wall surface area, and that this was mostly distributed around the periphery of the node. We then modelled blood flow through the BV to obtain spatially resolved hydrostatic pressures, which were then combined with Starling’s law to predict transmural flow. Much of the total 10 nL/min transmural flow (under normal conditions) was concentrated in the periphery, corresponding closely with surface area distribution. These results provide important insights into the inner workings of LN, and provide a basis for further exploration of the role of LN flow patterns in normal and pathological functions.

3D Super-Resolution US Imaging of Rabbit Lymph Node Vasculature in Vivo by Using Microbubbles

Background Variations in lymph node (LN) microcirculation can be indicative of metastasis. The identification and quantification of metastatic LNs remains essential for prognosis and treatment planning, but a reliable noninvasive imaging technique is lacking. Three-dimensional super-resolution (SR) US has shown potential to noninvasively visualize microvascular networks in vivo. Purpose To study the feasibility of three-dimensional SR US imaging of rabbit LN microvascular structure and blood flow by using microbubbles. Materials and Methods In vivo studies were carried out to image popliteal LNs of two healthy male New Zealand white rabbits aged 6-8 weeks. Three-dimensional, high-frame-rate, contrast material-enhanced US was achieved by mechanically scanning with a linear imaging probe. Individual microbubbles were identified, localized, and tracked to form three-dimensional SR images and super-resolved velocity maps. Acoustic subaperture processing was used to improve image contrast and to generate enhanced power Doppler and color Doppler images. Vessel size and blood flow velocity distributions were evaluated and assessed by using Student paired t test. Results SR images revealed microvessels in the rabbit LN, with branches clearly resolved when separated by 30 µm, which is less than half of the acoustic wavelength and not resolvable by using power or color Doppler. The apparent size distribution of most vessels in the SR images was below 80 µm and agrees with micro-CT data, whereas most of those detected with Doppler techniques were larger than 80 µm in the images. The blood flow velocity distribution indicated that most of the blood flow in rabbit popliteal LN was at velocities lower than 5 mm/sec. Conclusion Three-dimensional super-resolution US imaging using microbubbles allows noninvasive nonionizing visualization and quantification of lymph node microvascular structures and blood flow dynamics with resolution below the wave diffraction limit. This technology has potential for studying the physiologic functions of the lymph system and for clinical detection of lymph node metastasis. Published under a CC BY 4.0 license. Online supplemental material is available for this article.

Imaging the Lymph Node Stroma

Lymph node (LN) stromal cells are being recognized as key organizers of the immune system. They assemble in complex 3D networks and hence, need to be studied in situ to fully understand their exact functions. Here, we describe two distinct but complementary procedures that allow analyzing LN stromal cells at high resolution by confocal imaging.

Attenuation artifacts in light sheet fluorescence microscopy corrected by OPTiSPIM

Light sheet fluorescence microscopy (LSFM) is rapidly becoming an essential technology for mesoscopic imaging of samples such as embryos and adult mouse organs. However, LSFM can suffer from optical artifacts for which there is no intrinsic solution. The attenuation of light due to absorbing material causes "shadow" artifacts along both the illumination and detection paths. Several approaches have been introduced to reduce this problem, including scanning illumination and multi-view imaging. However, neither of these approaches completely eliminates the problem. If the distribution of the absorbing material is complex, shadows cannot be avoided. We introduce a new approach that relies on multi-modal integration of two very different mesoscopic techniques. Unlike LSFM, optical projection tomography (OPT) can operate in transmission mode to create a voxel map of the 3D distribution of the sample's optical attenuation. Here, we demonstrate a hybrid instrument (OPTiSPIM) that can quantify this attenuation and use the information to correct the shadow artifacts of LSFM.

A Novel Computational Model Predicts Key Regulators of Chemokine Gradient Formation in Lymph Nodes and Site-Specific Roles for CCL19 and ACKR4

The chemokine receptor CCR7 drives leukocyte migration into and within lymph nodes (LNs). It is activated by chemokines CCL19 and CCL21, which are scavenged by the atypical chemokine receptor ACKR4. CCR7-dependent navigation is determined by the distribution of extracellular CCL19 and CCL21, which form concentration gradients at specific microanatomical locations. The mechanisms underpinning the establishment and regulation of these gradients are poorly understood. In this article, we have incorporated multiple biochemical processes describing the CCL19–CCL21–CCR7–ACKR4 network into our model of LN fluid flow to establish a computational model to investigate intranodal chemokine gradients. Importantly, the model recapitulates CCL21 gradients observed experimentally in B cell follicles and interfollicular regions, building confidence in its ability to accurately predict intranodal chemokine distribution. Parameter variation analysis indicates that the directionality of these gradients is robust, but their magnitude is sensitive to these key parameters: chemokine production, diffusivity, matrix binding site availability, and CCR7 abundance. The model indicates that lymph flow shapes intranodal CCL21 gradients, and that CCL19 is functionally important at the boundary between B cell follicles and the T cell area. It also predicts that ACKR4 in LNs prevents CCL19/CCL21 accumulation in efferent lymph, but does not control intranodal gradients. Instead, it attributes the disrupted interfollicular CCL21 gradients observed in Ackr4-deficient LNs to ACKR4 loss upstream. Our novel approach has therefore generated new testable hypotheses and alternative interpretations of experimental data. Moreover, it acts as a framework to investigate gradients at other locations, including those that cannot be visualized experimentally or involve other chemokines.

Lymph Node Stroma Dynamics and Approaches for Their Visualization

Lymphoid stromal cells are best known as the architectural cells of lymphoid organs. For decades, they have been considered as inert elements of the immune system but this view has changed dramatically in recent years, when it was discovered that they are endowed with critical immunoregulatory functions. It is now accepted that without them, the adaptive immune response would be compromised, if not abrogated entirely. Here, we review the function of the major lymphoid stromal cell types; the way they remodel upon inflammation; discuss the available tools to track their behavior; and introduce several methodological approaches that we believe will help improving our knowledge of these pivotal cell types.

Stromal Fibroblasts in Tertiary Lymphoid Structures: A Novel Target in Chronic Inflammation

Tertiary lymphoid structures (TLS) are organized aggregates of lymphocytes, myeloid, and stromal cells that provide ectopic hubs for acquired immune responses. TLS share phenotypical and functional features with secondary lymphoid organs (SLO); however, they require persistent inflammatory signals to arise and are often observed at target sites of autoimmune disease, chronic infection, cancer, and organ transplantation. Over the past 10 years, important progress has been made in our understanding of the role of stromal fibroblasts in SLO development, organization, and function. A complex and stereotyped series of events regulate fibroblast differentiation from embryonic life in SLOs to lymphoid organ architecture observed in adults. In contrast, TLS-associated fibroblasts differentiate from postnatal, locally activated mesenchyme, predominantly in settings of inflammation and persistent antigen presentation. Therefore, there are critical differences in the cellular and molecular requirements that regulate SLO versus TLS development that ultimately impact on stromal and hematopoietic cell function. These differences may contribute to the pathogenic nature of TLS in the context of chronic inflammation and malignant transformation and offer a window of opportunity for therapeutic interventions in TLS associated pathologies.

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

BACKGROUND

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

METHODS AND RESULTS

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

CONCLUSIONS

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

Regulation of Lymph Node Vascular-Stromal Compartment by Dendritic Cells

During normal and pathologic immune responses, lymph nodes can swell considerably. The lymph node vascular-stromal compartment supports and regulates the developing immune responses and undergoes dynamic expansion and remodeling. Recent studies have shown that dendritic cells (DCs), best known for their antigen presentation roles, can directly regulate the vascular-stromal compartment, pointing to a new perspective on DCs as facilitators of lymphoid tissue function. Here, we review the phases of lymph node vascular-stromal growth and remodeling during immune responses, discuss the roles of DCs, and discuss how this understanding can potentially be used for developing novel therapeutic approaches.

Implications of Lymphatic Transport to Lymph Nodes in Immunity and Immunotherapy

Adaptive immune response consists of many highly regulated, multistep cascades that protect against infection while preserving the health of autologous tissue. The proper initiation, maintenance, and resolution of such responses require the precise coordination of molecular and cellular signaling over multiple time and length scales orchestrated by lymphatic transport. In order to investigate these functions and manipulate them for therapy, a comprehensive understanding of how lymphatics influence immune physiology is needed. This review presents the current mechanistic understanding of the role of the lymphatic vasculature in regulating biomolecule and cellular transport from the interstitium, peripheral tissue immune surveillance, the lymph node stroma and microvasculature, and circulating lymphocyte homing to lymph nodes. This review also discusses the ramifications of lymphatic transport in immunity as well as tolerance and concludes with examples of how lymphatic-mediated targeting of lymph nodes has been exploited for immunotherapy applications.

Mouse mesenchymal stem cells inhibit high endothelial cell activation and lymphocyte homing to lymph nodes by releasing TIMP-1

Mesenchymal stem cells (MSC) represent a promising therapeutic approach in many diseases in view of their potent immunomodulatory properties, which are only partially understood. Here, we show that the endothelium is a specific and key target of MSC during immunity and inflammation. In mice, MSC inhibit activation and proliferation of endothelial cells in remote inflamed lymph nodes (LNs), affect elongation and arborization of high endothelial venules (HEVs) and inhibit T-cell homing. The proteomic analysis of the MSC secretome identified the tissue inhibitor of metalloproteinase-1 (TIMP-1) as a potential effector molecule responsible for the anti-angiogenic properties of MSC. Both in vitro and in vivo, TIMP-1 activity is responsible for the anti-angiogenic effects of MSC, and increasing TIMP-1 concentrations delivered by an Adeno Associated Virus (AAV) vector recapitulates the effects of MSC transplantation on draining LNs. Thus, this study discovers a new and highly efficient general mechanism through which MSC tune down immunity and inflammation, identifies TIMP-1 as a novel biomarker of MSC-based therapy and opens the gate to new therapeutic approaches of inflammatory diseases.

Organ-wide 3D-imaging and topological analysis of the continuous microvascular network in a murine lymph node

Understanding of the microvasculature has previously been limited by the lack of methods capable of capturing and modelling complete vascular networks. We used novel imaging and computational techniques to establish the topology of the entire blood vessel network of a murine lymph node, combining 63,706 confocal images at 2 μm pixel resolution to cover a volume of 3.88 mm(3). Detailed measurements including the distribution of vessel diameters, branch counts, and identification of voids were subsequently re-visualised in 3D revealing regional specialisation within the network. By focussing on critical immune microenvironments we quantified differences in their vascular topology. We further developed a morphology-based approach to identify High Endothelial Venules, key sites for lymphocyte extravasation. These data represent a comprehensive and continuous blood vessel network of an entire organ and provide benchmark measurements that will inform modelling of blood vessel networks as well as enable comparison of vascular topology in different organs.

Stromal infrastructure of the lymph node and coordination of immunity

The initiation of adaptive immune responses depends upon the careful maneuvering of lymphocytes and antigen into and within strategically placed lymph nodes (LNs). Non-hematopoietic stromal cells form the cellular infrastructure that directs this process. Once regarded as merely structural features of lymphoid tissues, these cells are now appreciated as essential regulators of immune cell trafficking, fluid flow, and LN homeostasis. Recent advances in the identification and in vivo targeting of specific stromal populations have resulted in striking new insights to the function of stromal cells and reveal a level of complexity previously unrealized. We discuss here recent discoveries that highlight the pivotal role that stromal cells play in orchestrating immune cell homeostasis and adaptive immunity.

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

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

Laminins affect T cell trafficking and allograft fate

Lymph nodes (LNs) are integral sites for the generation of immune tolerance, migration of CD4⁺ T cells, and induction of Tregs. Despite the importance of LNs in regulation of inflammatory responses, the LN-specific factors that regulate T cell migration and the precise LN structural domains in which differentiation occurs remain undefined. Using intravital and fluorescent microscopy, we found that alloreactive T cells traffic distinctly into the tolerant LN and colocalize in exclusive regions with alloantigen-presenting cells, a process required for Treg induction. Extracellular matrix proteins, including those of the laminin family, formed regions within the LN that were permissive for colocalization of alloantigen-presenting cells, alloreactive T cells, and Tregs. We identified unique expression patterns of laminin proteins in high endothelial venule basement membranes and the cortical ridge that correlated with alloantigen-specific immunity or immune tolerance. The ratio of laminin α4 to laminin α5 was greater in domains within tolerant LNs, compared with immune LNs, and blocking laminin α4 function or inducing laminin α5 overexpression disrupted T cell and DC localization and transmigration through tolerant LNs. Furthermore, reducing α4 laminin circumvented tolerance induction and induced cardiac allograft inflammation and rejection in murine models. This work identifies laminins as potential targets for immune modulation.

Multiple CD11c+ cells collaboratively express IL-1β to modulate stromal vascular endothelial growth factor and lymph node vascular-stromal growth

Lymphadenopathy in autoimmune and other lymphoproliferative diseases is in part characterized by immunoblasts and vascular proliferation. The lymph node vasculature, along with the nonvascular stromal compartment, supports lymphocyte function, and targeting vascular-stromal expansion in inflamed nodes may modulate lymphocyte function in disease. CD11c(+) cells are essential for vascular-stromal proliferation and the upregulation of vascular endothelial growth factor (VEGF) needed for vascular proliferation. However, targetable CD11c(+) cell-derived molecular mediators, the identity of relevant CD11c(+) cells, and whether CD11c(+) cells directly stimulate VEGF-expressing stromal cells are poorly understood. In this study we show that CD11c(+) CD11b(+) CCR2-dependent monocytes and CCR7-dependent dendritic cells express IL-1β. IL-1β blockade, IL-1β deficiency in radiosensitive cells, and CCR2/CCR7 double deficiency but not single deficiency all attenuate immunization-induced vascular-stromal proliferation. gp38(+) stromal fibroblastic reticular cells (FRCs) that express VEGF are enriched for Thy1(+) cells and partially overlap with CCL21-expressing FRCs, and FRC VEGF is attenuated with IL-1β deficiency or blockade. IL-1β localizes to the outer borders of the T zone, where VEGF-expressing cells are also enriched. Ex vivo, CD11b(+) cells enriched for IL-1β(+) cells can directly induce cultured gp38(+)Thy1(+) FRCs to upregulate VEGF. Taken together, these results suggest a mechanism whereby multiple recruited CD11c(+) populations express IL-1β and directly modulate FRC function to help promote the initiation of vascular-stromal growth in stimulated lymph nodes. These data provide new insight into how CD11c(+) cells regulate the lymph node vascular-stromal compartment, add to the evolving understanding of functional stromal subsets, and suggest a possible utility for IL-1β blockade in preventing inflammatory lymph node growth.

Role of the Lymphotoxin/LIGHT System in the Development and Maintenance of Reticular Networks and Vasculature in Lymphoid Tissues

Lymphoid organs are meeting zones where lymphocytes come together and encounter antigens present in the blood and lymph or as delivered by cells migrating from the draining tissue bed. The exquisite efficiency of this process relies heavily on highly specialized anatomy to direct and position the various players. Gated entry and exit control access to these theaters and reticular networks and associated chemokines guide cells into the proper sections. Lymphoid tissues are remarkably plastic, being able to expand dramatically and then involute upon resolution of the danger. All of the reticular scaffolds and vascular and lymphatic components adapt accordingly. As such, the lymph node (LN) is a wonderful example of a physiologic remodeling process and is potentially a guide to study such elements in pathological settings such as fibrosis, chronic infection, and tumor metastasis. The lymphotoxin/LIGHT axis delivers critical differentiation signals that direct and hone differentiation of both reticular networks and the vasculature. Considerable progress has been made recently in understanding the mesenchymal differentiation pathways leading to these specialized networks and in the remodeling that occurs in reactive LNs. In this article, we will review some new advances in the area in terms of developmental, differentiation, and maintenance events mediated by this axis.

Imaging of the spleen in malaria

Splenomegaly, albeit variably, is a hallmark of malaria; yet, the role of the spleen in Plasmodium infections remains vastly unknown. The implementation of imaging to study the spleen is rapidly advancing our knowledge of this so-called "blackbox" of the abdominal cavity. Not only has ex vivo imaging revealed the complex functional compartmentalization of the organ and immune effector cells, but it has also allowed the observation of major structural remodeling during infections. In vivo imaging, on the other hand, has allowed quantitative measurements of the dynamic passage of the parasite at spatial and temporal resolution. Here, we review imaging techniques used for studying the malarious spleen, from optical microscopy to in vivo imaging, and discuss the bright perspectives of evolving technologies in our present understanding of the role of this organ in infections caused by Plasmodium.