- VISITING PROFESSOR Developmental and Regenerative Biology
My research laboratory carries out basic research on the differentiation and trafficking of monocytes and antigen-presenting dendritic cells. More recently, we have begun to study these cell types and their behavior in the context of atherosclerosis.
One of the fascinating properties of dendritic cells is their special capacity to efficiently emigrate out of tissues to lymph nodes via lymphatic vessels. Some of our research is directed to the study of basic mechanisms that regulate dendritic cell development from monocytes and the migration of dendritic cells to lymph nodes. These two fundamental lines of study now significantly influence our approach to studying atherosclerosis. We have observed in multiple model systems that monocytes that become dendritic cells readily emigrate out of tissues, usually via lymphatic vessels, whereas macrophages typically remain resident in the tissues where they form. To prevent the ongoing accumulation of macrophages in tissues and to maintain homeostasis, we hypothesize that it is important that some monocytes that enter a particular tissue continuously differentiate into dendritic cells that in turn subsequently emigrate from that tissue. We have proposed that this homeostatic process breaks down in atherosclerosis, such that monocyte-derived dendritic cells fail to emigrate and consequently aberrantly accumulate within plaques, contributing to plaque progression. Conversely, we propose that restoring the emigration of these cells from plaques facilitates plaque regression. This proposal is based in part on evidence from a model system that permitted us to trace whether and under what conditions cells emigrate from lesions. We have recently developed new techniques to trace monocyte fate within atherosclerotic plaques that will allow us to focus further on the differentiation of plaque-infiltrating monocytes to dendritic cells and macrophages and to identify mechanisms that affect their emigration from lesions. For the foreseeable future, the laboratory will remain connected to the study of monocyte biology and dendritic cell migration in a normal, non-diseased setting, while increasingly mobilizing our efforts to address these topics in the context of atherosclerosis.
Tracing monocytes. Monocytes are well known to differentiate into macrophages. Culture conditions of monocytes can also be modified to direct them toward dendritic cell differentiation. Our previous work has provided fundamental observations and models to study monocyte differentiation toward dendritic cells vs. macrophages. We have shown that human monocytes can receive cues from human endothelial cells and extracellular matrix to promote their differentiation to dendritic cells. In the same culture system, some of the monocytes become macrophages. This model provides a physiologic context to study how endogenous signals regulate and alter monocyte fate and moves beyond the use of exogenous cytokines to generate monocyte-derived dendritic cells. Currently, we are increasing the sophistication of this model to include both vascular and lymphatic endothelial surfaces that sandwich a human extracellular matrix. The matrix can be loaded with fibroblasts and mast cells as well. The addition of primary lymphatic endothelium permits us to study the mechanisms by which dendritic cells traverse lymphatic endothelium, a subject for which very little is known, after they earlier crossed blood endothelium as monocytes and began to differentiate. We are also employing this model to study the trafficking of nonmonocytic human blood dendritic cells across blood and lymphatic endothelium and to study the presentation of antigens that the nascent dendritic cells pick up in the extracellular matrix. An application of this work includes providing insight into how to optimize vaccines for maximal efficacy in the generation of potent antigen-presenting dendritic cells, and the model also has great potential for informing and shaping our studies in atherosclerosis. For example, we have found that this model parallels observations that we have made using in vivo models of atherosclerosis. It was also work in this model that led us to our current interest in monocyte subsets and the possibility that they have distinct roles in vivo, including in atherosclerotic plaques.
To parallel our in vitro work, we developed unique mouse models to trace monocyte fate in vivo. Although we know that monocytes differentiate to dendritic cells in vivo at least to some extent, we still do not how frequently a monocyte that leaves the blood becomes a dendritic cell vs. a macrophage in normal peripheral tissues. Nor do we know how many dendritic cells in lymph or in a lymph node derived from monocytes vs. arising from nonmonocytic precursors. These questions again are important to answer in order to interpret modifications to monocyte fate that may occur in diseases like atherosclerosis. In connection with tracing the fate of monocytes in general, we are closely monitoring the differences and distinct biological properties of monocyte subsets in vivo.
We have recently developed methods to label endogenous monocytes with nondegradable inert fluorescent particles. This labeling method also enables studies that examine trafficking patterns of monocytes and has already been useful in identifying a subpopulation of monocytes that can replenish epidermal dendritic cells. Furthermore, we have used the method to track monocyte subsets into atherosclerotic plaques. The greatest potential of the method, however, is its unique utility in allowing us to study the persistence within or emigration of monocyte-derived cells out of plaques, since the method will permit us for the first time to separate whether "disappearance" of macrophages and dendritic cells from plaques occurs via death or migratory egress from the plaque. This new method will permit analysis of molecular pathways that contribute to failed egress or pathways that facilitate egress.
Dendritic cell migration. Several years ago, we uncovered evidence, initially obtained using our in vitro model and then confirmed in vivo, that certain lipid mediators regulate dendritic cell migration. Since then, we have carried out projects to examine the roles of eicosanoids and selected other lipid mediators, including platelet activating factor and its oxidized mimetics, in regulating dendritic cell migration to lymph nodes, a process fundamental to the initiation of immune responses. Our interest in the role of lipid mediators in dendritic cell migration merges well with our studies in atherosclerosis. For example, in parallel to the study of monocyte-derived cell fate in atherosclerotic plaques, we characterized the biology of dendritic cells in the skin of apoE KO and LDLR KO mice. We believe that an understanding of the biology of immune cells like dendritic cells in atherosclerosis must consider how they are affected within plaques as well as systemically. Moreover, disease mediators may similarly alter plaque dendritic cells and those found in distal tissues. We observed that skin dendritic cells and their migration to lymph nodes was normal in young apoE and LDLR KO mice. However, in a disease-dependent manner, dendritic cells lost their capacity to migrate to lymph nodes and became trapped in skin (seemingly in parallel to plaques), even as the skin became inflamed and draining lymph nodes became hypertrophic. In order to characterize how dendritic cell migration to inflamed lymph nodes is normally regulated, we conducted additional and parallel studies in the absence of atherosclerotic disease. We observed that inflamed, hypertrophic lymph nodes normally permit increased dendritic cell entry into the nodal paracortex and facilitate this increased entry via an expanded lymphatic network. This expanded lymphatic network develops in mouse models of atherosclerosis as well, but quantification of lymphatic function reveals that apoE KO mice, in an age/disease-dependent manner, develop impaired hydraulic conductivity that limits lymphatic transport capacity. Thus, part of the impaired trafficking of dendritic cells in apoE KO mice may relate to the onset poor lymphatic function. Interestingly, one of the predicted outcomes of dysregulated lymphatic function is the entrapment of immune cells that promote tertiary lymphoid organ (TLO) formation in the absence of being able to mobilize to lymphoid organs efficiently. TLO-like structures form in the adventitia surrounding atherosclerotic lesions in humans and mice, consistent with the possibility that lymphatics within the adventitia are functioning suboptimally and fitting with the limited capacity of dendritic cells to mobilize from plaques to lymph nodes in apoE KO mice.
Llodra J, Angeli V, Liu J, Trogan E, Fisher EA, Randolph GJ. Emigration of monocyte-derived cells from atherosclerotic lesions of mice characterizes regressive, but not progressive, plaques. Proc. Natl. Acad. of Sci. USA 2004; 101: 11779-11784.
Angeli V, Llodra J, Rong JX, Satoh K, Ishii S, Shimizu T, Fisher EA, Randolph GJ. Dyslipidemia associated with atherosclerotic disease systemically alters dendritic cell mobilization. Immunity 2004; 21: 561-574.
Tacke F, Alvarez D, Kaplan TJ, Jakubzick C, Spanbroek R, Llodra J, Garin A, Liu J, Mack M, van Rooijen N, Lira SA, Habenicht AJ, Randolph GJ. Monocyte subsets differentially employ CCR2, CCR5, and CX3CR1 to accumulate within atherosclerotic plaques. J. Clinical Investigation 2007; 117: 185-194.
Jakubzick C, Tacke F, Llodra J, van Rooijen N, Randolph GJ. Modulation of DC trafficking to and from the airways. J. Immunol 2006; 176: 3578-3584.
Angeli V, Ginhoux F, Llodra J, Frenette PS, Skobe M, Merad M, Randolph GJ. B cell driven lymphangiogenesis in inflamed lymph nodes enhances dendritic cell mobilization. Immunity 2006; 24: 203-215.
Jakubzick C, Tacke F, Ginhoux F, Wagers AJ, van Rooijen N, Mack M, Merad M, Randolph GJ. Blood monocyte subsets differentially give rise to CD103+ and CD103- pulmonary dendritic cell populations. J. Immunol 2008; 180: 3019-3027.
Jakubzick C, Bonito A, Bogunovic M, Merad M, Randolph GJ. Lymph-migrating, tissue-derived dendritic cells are minor constituents within steady lymph nodes. J. Exp. Med 2008; 205: 2839-2850.
Randolph GJ, Ochando J, Partida-Sanchez S. Migration of dendritic cells and their precursors. Ann Rev Immunol 2008; 26: 293-316.
Randolph GJ. Emigration of monocyte-derived cells to lymph nodes during resolution of inflammation and its failure in atherosclerosis. Curr Opin Lipidol 2008; 19: 462-468.
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Dr.Randolph is not currently required to report Industry relationships.
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