Mononuclear phagocytes egress from an in vitro model of the vascular wall by migrating across endothelium in the basal to apical direction: role of intercellular adhesion molecule 1 and the CD11/CD18 integrins.

Little is known about how mononuclear phagocytes (MP) are cleared from sites of inflammation as inflammatory lesions resolve. In this study, the possibility that MP could be cleared from tissues by migrating across endothelium in the basal to apical direction was investigated. In an in vitro model of a blood vessel wall consisting of human umbilical vein endothelial cells (HUVEC) grown on amniotic tissue, a majority of MP that initially transmigrated into the amnion later exited by migrating back across the endothelium in the basal to apical direction. MP that egressed from these cultures adhered to the apical surface of the endothelium or were found nonadherent in the medium above the endothelium. Egression of MP continued throughout the 4-d period examined, displaying higher than first order kinetics and a t(1/2) of approximately 24 h. These kinetics were decreased by increasing the volume of medium bathing the cultures, suggesting that a soluble factor(s) regulates the rate of egression. In contrast, the kinetics were accelerated by pretreating the endothelium with IL-1. The initial phase of this increased rate of egression was inhibited by antibodies to inter- cellular adhesion molecule 1 (ICAM-1) or CD18 by 100 and 71%, respectively. Immunostaining revealed that ICAM-1 was present on the apical and basal surfaces of umbilical vein endothelium in vitro and in situ. These data demonstrate that MP can traverse endothelium in the basal to apical direction, and lend insight into the mechanisms by which this process occurs.

Differentiation of monocytes into dendritic cells in a model of transendothelial trafficking.

Essential to the dendritic cell system of antigen-presenting cells are the veiled dendritic cells that traverse afferent lymph to enter lymph nodes, where they initiate immune responses. The origin of veiled cells, which were discovered 20 years ago, is unclear. Monocytes cultured with endothelium differentiated into dendritic cells within 2 days, particularly after phagocytosing particles in subendothelial collagen. These nascent dendritic cells migrated across the endothelium in the ablumenal-to-lumenal direction, as would occur during entry into lymphatics. Monocytes that remained in the subendothelial matrix became macrophages. Therefore, monocytes have two potential fates associated with distinct patterns of migration.

Contribution of metabolic disease to bone fragility in MAGP1-deficient mice.

Microfibril-associated glycoprotein-1 (MAGP1) is an extracellular matrix protein that interacts with fibrillin and is involved in regulating the bioavailability of signaling molecules such as TGFß. Mice with germline MAGP1 deficiency (Mfap2-/-) develop increased adiposity, hyperglycemia, insulin resistance, bone marrow adipose tissue expansion, reduced cancellous bone mass, cortical bone thinning and bone fragility. The goal of this study was to assess whether the Mfap2-/- bone phenotypes were due to loss of MAGP1 locally or secondary to a change in whole body physiology (metabolic dysfunction). To do this, mice with conditional deletion of MAGP1 in the limb skeleton were generated by crossing MAGP1-flox mice (Mfap2lox/lox) with Prx1-Cre mice. Mfap2Prx-/- mice did not show any changes in peripheral adiposity, hyperglycemia or insulin sensitivity, but did have increased bone length and cancellous bone loss that was comparable to the germline Mfap2-/- knockout. Unlike the germline knockout, marrow adiposity, cortical bone thickness and bone strength in Mfap2Prx-/- mice were normal. These findings implicate systemic metabolic dysfunction in the development of bone fragility in germline Mfap2-/- mice. An unexpected finding of this study was the detection of MAGP1 protein in the Mfap2Prx-/- hematopoietic bone marrow, despite the absence of MAGP1 protein in osseous bone matrix and absent Mfap2 transcript expression at both sites. This suggests MAGP1 from a secondary site may accumulate in the bone marrow, but not be incorporated into the bone matrix, during times of regional MAGP1 depletion.

Migration of leukocytes across endothelium and beyond: molecules involved in the transmigration and fate of monocytes.

Passage of leukocytes across the endothelial lining into sites of inflammation has been shown to be regulated largely by platelet/endothelial cell adhesion molecule-1 (PECAM/CD31). We summarize recent work from our laboratory documenting that PECAM is involved both in diapedesis and the subsequent step of migration across the basal lamina. The former process involves a homophilic interaction between the amino-terminal extracellular domains of PECAM on the leukocyte and on the endothelial cell. The latter process involves a heterophilic interaction between the membrane-proximal extracellular domain of PECAM and an unknown ligand(s) in the subendothelial basal lamina. These findings are demonstrated in both in vitro and in vivo models. For monocytes, however, transmigration is just the first step in the next phase of their lives. In addition to their specific recruitment during the inflammatory response, many monocytes constitutively leave the bloodstream to enter tissues. However, only a fraction of these become tissue macrophages. In an in vitro model of monocyte emigration, approximately half of the leukocytes that initially transmigrate an endothelial monolayer migrate back out in the basal-to-apical direction within the next 2 days. This reverse transmigration cannot be blocked by anti-PECAM reagents, but involves p-glycoprotein and tissue factor expressed on the leukocytes. The reverse transmigrating cells are phenotypically dendritic cells (DC). Their maturation to mature DC can be accelerated by inclusion of inflammatory stimuli in the co-culture. The cells that remain behind in the subendothelial collagen are phenotypically macrophages. We postulate that a major source of DC in lymph nodes is cells derived from monocytes that enter a tissue via the blood and leave several days later via afferent lymphatic channels.

Genetic deletion of mPGES-1 abolishes PGE2 production in murine dendritic cells and alters the cytokine profile, but does not affect maturation or migration.

We undertook this study to determine the role of Microsomal PGE Synthase-1 (mPGES-1), and mPGES-1-generated Prostaglandin (PG) E2 on Dendritic Cell (DC) phenotype and function. Using mPGES-1 KnockOut (KO) mice, we generated bone marrow derived DCs and determined their eicosanoid production profile, cell surface marker expression, and cytokine production. We also assessed DC migratory and functional capacity in vivo. Compared to wild-type, mPGES-1 deficient DCs exhibited a markedly attenuated increase in PGE2 production upon LPS stimulation, and displayed preferential shunting towards PGD2 production. mPGES-1 KO DCs did not display deficiencies in maturation, migration or ability to sensitize T cells. However, mPGES-1 deficient DCs generated reduced amounts of the Th1 cytokine IL-12, which may in part be due to increased PGD2 rather than decreased PGE2. These findings provide useful information on the effects of inducible PGE2 on the innate immune system, and have important implications regarding potential consequences of pharmacologic mPGES-1 inhibition.

The fate of monocytes in atherosclerosis.

Monocytes are the primary inflammatory cell type that infiltrates early atherosclerotic plaques. Their recruitment into plaques drives disease progression. Disease interventions that target monocytes could act at several points: alteration in the phenotype of circulating monocyte subpopulations; reduced recruitment of monocytes into plaques; alterations in the survival of monocyte-derived cells in atherosclerosis; and promotion of migratory egress from plaques to bring about resolution of the plaque inflammatory response. All of these points of intervention will be briefly discussed in this article.

Dendritic cell migration to lymph nodes: cytokines, chemokines, and lipid mediators.

Mobilization of dendritic cells into lymphatic vessels requires cytokine stimulation and induction of the chemokine receptor CCR7. The respective roles of the CCR7 ligands CCL19 and CCL21 in mediating migration are not fully defined, but chemotaxis to CCL19 mediates Langerhans cell exit from the epidermis. Optimal chemotaxis to CCL19 occurs when DCs are triggered with exogenous leukotriene C(4), an eicosanoid transported out of the cell via the ATP binding cassette (ABC) transporter multidrug resistance related protein 1 (MRP1, ABCC1). Indeed, MRP1 and the related multidrug resistance protein 1 (MDR1, p-glycoprotein, ABCB1) may control the intracellular and extracellular accumulation of key signaling lipids that regulate dendritic cell migration.

Chemokines and tissue injury.

Accumulation of leukocytes at sites of inflammation is essential for host defense, yet secretory products of the white cells may augment injury by damaging surrounding healthy tissues. Members of the chemokine family of chemotactic cytokines play a fundamental role in this process by attracting and stimulating specific subsets of leukocytes. In vitro studies suggest that chemokines participate in at least three phases of leukocyte recruitment. First, they foster tight adhesion of circulating leukocytes to the vascular endothelium by activating leukocytic integrins. Second, because of their chemoattractant properties, chemokines guide leukocytes through the endothelial junctions and underlying tissue to the inflammatory focus. Finally, chemokines activate effector functions of leukocytes, including production of reactive oxygen intermediates and exocytosis of degradative enzymes. Animal studies in which antibodies are used to neutralize the activity of individual members of the chemokine family confirm that these mediators contribute to the development of both acute and chronic inflammatory conditions. A number of mechanisms may operate in vivo to limit the proinflammatory properties of chemokines. Therapies that target chemokines directly or enhance the body's mechanisms for controlling their activity may prove to be reasonable approaches for treatment of inflammatory diseases.

Immunolocalization of basal lamina components during development of chick otic and optic primordia.

Immunolocalization of laminin, fibronectin, and type IV collagen was examined during early morphogenetic shape changes of the avian inner ear and eye. The ear was studied from formation of the otic placode to invagination of the otic pit and the eye from the optic vesicle stage to formation of an optic cup. Distribution and intensity of immunoreactivity were compared in the two organ primordia and in adjacent epithelial layers. Laminin formed a continuous layer at the basal surface of the otic ectoderm and adjacent neural tube at all stages. The basal surfaces of the optic and lens epithelia also were continuously covered with laminin throughout development. The otic placode became attached to the neural ectoderm through a single layer of fibronectin and collagen IV between the layers of laminin. The ring-like attachment between the edges of the optic cup and lens primordium had the same structure. In addition, the central regions of the optic and lens primordia were attached by fibrils containing type IV collagen, whereas finer strands containing fibronectin and laminin also connected the otic epithelium and neural tube. The results are discussed in terms of models of invagination for the two primordia.

A soluble gradient of endogenous monocyte chemoattractant protein-1 promotes the transendothelial migration of monocytes in vitro.

Chemokines secreted by endothelium may promote diapedesis of leukocytes by a gradient-dependent chemotactic mechanism or by stimulating random motility so that leukocytes transmigrate in a gradient-independent manner. Alternatively, chemokines may bind to endothelium and extracellular matrix to stimulate haptotactic migration. We first analyzed the role of the chemokine monocyte chemoattractant protein-1 (MCP-1) in the migration of human monocytes across untreated or IL-1-stimulated HUVEC monolayers cultured on human amnion. Then we further examined whether MCP-1-dependent transmigration occurred through a chemokinetic, chemotactic, or haptotactic mechanism. A neutralizing mAb against MCP-1 inhibited passage of monocytes across untreated or IL-1-stimulated HUVEC by 74 +/- 3% and 45 +/- 4%, respectively. Addition of MCP-1 itself to the apical compartment of unstimulated HUVEC/amnion cultures also reduced the transmigration of monocytes, in this instance by 73 +/- 9%. MCP-1 suppressed diapedesis only when present above the endothelium at a concentration equal to or greater than that endogenously deposited beneath the endothelium, and its inhibitory action could be overcome by addition of more concentrated MCP-1 below the HUVEC cultures. As much as 90% of the MCP-1 secreted into the underlying basement membrane and connective tissue could be washed out of HUVEC/amnion cultures; this procedure decreased transmigration by 69 +/- 4%. These data indicate that MCP-1 promotes transmigration of monocytes, but only when present in a gradient across endothelial monolayers. They further suggest that this gradient is predominantly soluble, rather than haptotactic.

Role of tissue factor in adhesion of mononuclear phagocytes to and trafficking through endothelium in vitro.

An in vitro model consisting of endothelium grown on collagen was used to investigate how mononuclear phagocytes traverse endothelium in the basal-to-apical direction (reverse transmigration), a process that mimics their migration across vascular and/or lymphatic endothelium during atherosclerosis and resolution of inflammation, respectively. Monoclonal antibody (MoAb) VIC7 against tissue factor (TF) inhibited reverse transmigration by 77%. Recombinant tissue factor fragments containing at least six amino acids C-terminal to residue 202 also strongly inhibited reverse transmigration. TF was absent on resting monocytes but was induced on these cells after initial apical-to-basal transendothelial migration. Two additional observations suggest that TF is involved in adhesion between mononuclear phagocytes and endothelium: (1) when monocytes were incubated with lipopolysaccharide (LPS) to stimulate expression of TF before they were added to endothelium, VIC7 or soluble TF modestly inhibited their adhesion to the apical endothelial surface, each by about 35%; and (2) endothelial cells specifically bound to surfaces coated with TF fragments containing amino acids 202-219. This binding was blocked by anti-TF MoAb, suggesting that endothelial cells bear a receptor for TF. These data suggest that mononuclear phagocytes use TF, perhaps as an adhesive protein, to exit sites of inflammation.

Differentiation of phagocytic monocytes into lymph node dendritic cells in vivo.

We investigated the differentiation and trafficking of inflammatory monocytes that phagocytosed subcutaneously injected fluorescent microspheres. As expected, most of the monocytes became microsphere+ macrophages, which remained in subcutaneous tissue. However, about 25% of latex+ cells migrated to the T cell area of draining lymph nodes, where they expressed dendritic cell (DC)-restricted markers and high levels of costimulatory molecules. Microsphere-transporting cells were distinct from resident skin DCs, and this transport was reduced by more than 85% in monocyte-deficient osteopetrotic mice. Thus, a substantial minority of inflammatory monocytes carry phagocytosed particles to lymph nodes and differentiate into DCs.

The leukotriene C(4) transporter MRP1 regulates CCL19 (MIP-3beta, ELC)-dependent mobilization of dendritic cells to lymph nodes.

Adaptive immune responses begin after antigen-bearing dendritic cells (DCs) traffic from peripheral tissues to lymph nodes. Here, we show that DC migration from skin to lymph nodes utilizes the leukotriene C(4) (LTC(4)) transporter multidrug resistance-associated protein 1 (MRP1). DC mobilization from the epidermis and trafficking into lymphatic vessels was greatly reduced in MRP1(-/-) mice, but migration was restored by exogenous cysteinyl leukotrienes LTC(4) or LTD(4). In vitro, these cysteinyl leukotrienes promoted optimal chemotaxis to the chemokine CCL19, but not to other related chemokines. Antagonism of CCL19 in vivo prevented DC migration out of the epidermis. Thus, MRP-1 regulates DC migration to lymph nodes, apparently by transporting LTC(4), which in turn promotes chemotaxis to CCL19 and mobilization of DCs from the epidermis.

A physiologic function for p-glycoprotein (MDR-1) during the migration of dendritic cells from skin via afferent lymphatic vessels.

P-glycoprotein (MDR-1) is a well-known transporter that mediates efflux of chemotherapeutic agents from the intracellular milieu and thereby contributes to drug resistance. MDR-1 also is expressed by nonmalignant cells, including leukocytes, but physiologic functions for MDR-1 are poorly defined. Using an initial screening assay that included >100 mAbs, we observed that neutralizing mAbs MRK16, UIC2, and 4E3 against MDR-1 specifically and potently blocked basal-to-apical transendothelial migration of mononuclear phagocytes, a process that may mimic their migration into lymphatic vessels. Antagonists of MDR-1 then were used in a model of authentic lymphatic clearance. In this model, antigen-presenting dendritic cells (DC) migrate out of explants of cultured human skin and into the culture medium via dermal lymphatic vessels. DC and T cells derived from skin expressed MDR-1 on their surfaces. Addition of anti-MDR-1 mAbs MRK16, UIC2, or the MDR-1 antagonist verapamil to skin explants at the onset of culture inhibited the appearance of DC, and accompanying T cells, in the culture medium by approximately 70%. Isotype-matched control mAbs against other DC molecules including CD18, CD31, and major histocompatibility complex I did not block. In the presence of MDR-1 antagonists, epidermal DC were retained in the epidermis, in contrast to control conditions. In summary, this work identifies a physiologic function for MDR-1 during the mobilization of DC and begins to elucidate how these critical antigen-presenting cells migrate from the periphery to lymph nodes to initiate T lymphocyte-mediated immunity.