Intravital Immunofluorescence for Visualizing the Microcirculatory and Immune Microenvironments in the Mouse Ear Dermis

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

Tissue engineering complex and mechanically active microenvironments for studies in asthma and cancer

Alice Anna Tomei

The goals of tissue engineering include recapitulating specific tissue functions for regenerative medicine and developing in vitro models of human tissues to study human physiology and pathophysiology and for testing and screening drugs before expensive clinical trials. Success of these goals relies on the capability to recapitulate the complex interactions between cells and their microenvironments in vitro, which requires interdisciplinary approaches. Tissue engineering has evolved greatly in the past two decades, with notable progress in replacing function in tissues such as cartilage, bone, arteries, skin, and pancreatic islets. What is lacking, however, are examples of, and design principles for, engineering complex microenvironments to understand disease progression. The two particular examples addressed in this thesis are (1) airway remodeling in asthma and (2) tumor invasion and metastasis in cancer. These diseases involve complex interactions between different cell types as well as extracellular matrix remodeling, which, together with the mechanical environment that affects cell-cell and cell-matrix interactions and that can change in a diseased state (or in some cases induce the disease state), collectively cause the pathology. Such disease states are difficult to study in vivo because (a) one cannot easily tease out mechanisms or have much control over these complex interactions in an animal model, and (b) there are many differences between animal and human cells. The aim of this thesis was to engineer mechanically dynamic microenvironments for the study of two disease processes: viral infection in asthmatic remodeled airways and cancer metastasis to the lymph node. These involved multidisciplinary approaches and drew tools and methods from mechanobiology, materials science, cell biology, and molecular biology. First, we developed and characterized a relevant 3D in vitro model of the bronchial mucosa with a functional epithelial barrier, including improved epithelial differentiation, ciliogenesis, and mucociliary function and showed for the first time that dynamic compression, such as that seen in asthmatic bronchoconstriction, enhances viral infection of epithelial and subepithelial cells using lentiviral technology and may help explaining why asthmatic patients are more susceptible to viral infection. However, the strain device had deficiencies in reproducibility, ease of use, time to assemble, and most importantly, homogeneous distribution of strain (because collagen and other natural extracellular matrices are mechanically weak since they have >99% water and cannot retain residual strain). To address these problems, we engineered a completely different dynamic strain device, which includes an elastic porous superstructure to contain the collagen and which can modulate stress/strain to the entire culture system. We characterized this device and showed that we could impose dynamic strains to 3D soft, hydrated cultures over relatively long periods of time and with variable amplitude and frequency in a uniform way. The device has the potential to be used in a wide spectrum of applications, including the possibility to study the correlation between mechanostimulation and metastatic potential of tumor cells in the context of lung metastasis of many cancer types. Finally, we used this material as a supporting structure for tissue-generated stress such as that generated by myofibroblasts, and in particular the ones that build up the lymph node stroma in the paracortex. The importance of these cells relies on their ability to form conduits for chemokine transport to high endothelial venules, where lymphocytes enter the node, the ability to form a platform for directing immune cell migration and interactions, and the ability to secrete two chemokines that are important to regulate lymphocyte adhesion and migration: CCL19 and CCL21. Specifically, we found that lymph node stromal cells use this supporting composite to remodel and reorganize their surrounding matrix into physiologically relevant bundles. By giving these cells the relevant stimuli (matrix microenvironment and flow), we could recapitulate certain features of the lymph node microenvironment, including a physiological stromal structure and chemokine profile, that may be critical for understanding immune and cancer cell interactions there. Using this model we demonstrated for the first time that slow flow through the lymph node stroma can itself drive cell and stromal organization and upregulates CCL21 production. Finally, we incorporated this engineered lymph node model into a larger model of the tumor-lymphatic-lymph node pathway to model the relative modulation of tumor growth, migration within a lymphatic "channel" towards the lymph node, and behavior within this lymph node environment. We used human lymphatic endothelial cells and a mass of transduced tumor cells with engineered chemokine profiles. We showed how CCR7 signaling correlates with the ability of tumor cells to metastasize to the lymph node and to interact with lymph node stromal cells once they reach the node. Our findings demonstrate how a multidisciplinary tissue engineering approach allows creating complex microenvironments in vitro that include both physical and chemical cues and allows recapitulating tissue function in vitro and investigating dynamic pathophysiological diseases like asthma and cancer metastasis. It combines both the development of these model systems and some examples of their usefulness to gain insight into asthma pathophysiology and cancer biology.

EPFL2008

Dendritic cell chemotaxis in 3D

Ulrike Haessler

Dendritic cells (DCs) are crucial for the onset of adaptive immunity. Their outstanding migratory capacities in combination with their antigen presenting properties make them perfectly suited to pick up antigens in the periphery and transport them to the lymph nodes, where antigen presentation to T-cells is most efficient [1]. Their migration from the periphery to the lymph nodes, called DCs cell homing, is involved in both mounting effective adaptive immune responses as well as maintaining tolerance to self-antigens [2]. Both of them need to be balanced to maintain a healthy organism. So far two alternative mechanisms have been proposed to guide DCs from the interstitium through the extracellular matrix to the proximal lymphatic vessel [3]. One of these mechanisms is based on paracrine chemotaxis towards a CCL21 gradient created by the proximal lymphatic vessel. The second mechanism, called autologous chemotaxis, describes how cells can follow an autocrine gradient, created by the biasing of self-secreted chemokine by interstitial flow [4, 5]. To answer the question of how these two mechanisms contribute to DC homing, a quantitative understanding of how cells respond to gradients and interstitial flow present in their physiological microenvironment is needed [6]. To date, appropriate tools to study DC migration under defined gradients and in the presence of tunable interstitial flow rates in a 3D environment have been lacking. Within this thesis, we present two micro-scale devices that allow live cell observation on an inverted microscope. The first was optimized to expose cells to a steady-state chemokine gradient within a 3D microenvironment and is based on a microfluidic agarose scaffold which functions as a gradient buffer (Chapter 3). The novelty of a gradient buffer allows us to quickly establish stable gradients, which is an important feature in order to create stable and precisely defined gradients during tracking of fast-moving cells like DCs. We then performed an in-depth study of DC cell chemotactic responses towards the CCR7 ligands (CCL19 and CCL21) in 3D using this new agarose chamber (Chapter 4). With the help of an integrated computational model, we could precisely predict the gradient formation of matrix-bound CCL21 versus soluble CCL19 in the microcellular environment and found that DC answer differently to the two chemokines in response to gradient steepness. Furthermore, CCL21 was more chemoattractive than CCL19 when cells were exposed to competing gradients of both chemokines, and this was independent of the CCL21-binding properties. This represents the first quantitative study of CCR7-mediated DC chemotaxis in 3D in response to well-defined ligand gradients. The second device we developed for studying the effects of the physiological flow environment, which cells experience in capillary-fed tissues, on cell migration [6]. A 2-gel strategy allowed us to flexibly fine tune interstitial flow velocities and to precisely determine the actual flow rate for each position imaged on a microscope. We exposed DCs to a wide range of flow rates and observed an increase in persistence under flow compared to static (Chapter 6). In conclusion, this work presents quantitative insights into DC migration within precisely controlled 3D-microenvironments using new devices. It also highlights the role of a quantitative understanding of biological processes to distinguish between chemotactic mechanisms of DC cells.

EPFL2010

Long-term Intravital Immunofluorescence Imaging of Tissue Matrix Components with Epifluorescence and Two-photon Microscopy

Manuel Andreas Fankhauser, Witold Waldemar Kilarski, Amanda Waite Lund, Melody Swartz

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

Journal Of Visualized Experiments2014