Endothelial cells of the blood and lymphatic vasculature are polarized cells with luminal surfaces specialized to interact with inflammatory cells upon the appropriate stimulation; they contain specialized transcellular transport systems, and their basal surfaces are attached to an extracellular basement membrane. In adult tissues the basement membrane forms a continuous sleeve around the endothelial tubes, and the interaction of endothelial cells with basement membrane components plays an important role in the maintenance of vessel wall integrity. During development, the basement membrane of endothelium provides distinct spatial and molecular information that influences endothelial cell proliferation, migration, and differentiation/maturation. Microvascular endothelium matures into phenotypically distinct types: continuous, fenestrated, and discontinuous, which also differ in their permeability properties. Development of these morphological and physiological differences is thought to be controlled by both soluble factors in the organ or tissue environment and by cell-cell and cell-matrix interactions. Basement membranes of endothelium, like those of other tissues, are composed of laminins, type IV collagens, heparan sulfate proteoglycans, and nidogens. However, isoforms of all four classes of molecules exist, which combine to form structurally and functionally distinct basement membranes. The endothelial cell basement membranes have been shown to be unique with respect to their laminin isoform composition. Laminins are a family of glycoprotein heterotrimers composed of an alpha, beta, and gamma chain. To date, 5alpha, 4beta, and 3gamma laminin chains have been identified that can combine to form 15 different isoforms. The laminin alpha-chains are considered to be the functionally important portion of the heterotrimers, as they exhibit tissue-specific distribution patterns and contain the major cell interaction sites. Vascular endothelium expresses only two laminin isoforms, and their expression varies depending on the developmental stage, vessel type, and the activation state of the endothelium. Laminin 8 (composed of laminin alpha4, beta1, and gamma1 chains) is expressed by all endothelial cells regardless of their stage of development, and its expression is strongly upregulated by cytokines and growth factors that play a role in inflammatory events. Laminin 10 (composed of laminin alpha5, beta1, and gamma1 chains) is detectable primarily in endothelial cell basement membranes of capillaries and venules commencing 3-4 wk after birth. In contrast to laminin 8, endothelial cell expression of laminin 10 is upregulated only by strong proinflammatory signals and, in addition, angiostatic agents such as progesterone. Other extracellular matrix molecules, such as BM40 (also known as SPARC/osteonectin), thrombospondins 1 and 2, fibronectin, nidogens 1 and 2, and collagen types VIII, XV, and XVIII, are also differentially expressed by endothelium, varying with the endothelium type and/or pathophysiological state. The da...

We describe a unique extracellular matrix (ECM) niche in the spleen, the marginal zone (MZ), characterized by the basement membrane glycoproteins, laminin α5 and agrin, that promotes formation of a specialized population of MZ B lymphocytes that respond rapidly to blood-borne antigens. Mice with reduced laminin α5 expression show reduced MZ B cells and increased numbers of newly formed (NF) transitional B cells that migrate from the bone marrow, without changes in other immune or stromal cell compartments. Transient integrin α6β1-mediated interaction of NF B cells with laminin α5 in the MZ supports the MZ B-cell population, their long-term survival, and antibody response. Data suggest that the unique 3D structure and biochemical composition of the ECM of lymphoid organs impacts on immune cell fate.B-cell development | immunology S econdary lymphoid organs are characterized by their unique patterns of immune cell compartmentalization, which is important for their function. Over recent years, the molecular basis for these distinct compartments within lymph nodes and the spleen has been intensely studied, revealing a complex interrelationship between cell-cell adhesion molecules, cytokines, and chemotactic factors. Most such adhesive and chemotactic factors are expressed by the stromal cells of the nonhematopoietic scaffold of secondary lymphoid organs, which constitute the reticular fiber network. However, little attention has been given to the acellular component of the reticular fiber network, and relatively little is known about the nature of this extracellular matrix (ECM) compartment and its function. In most tissues the ECM acts to separate cellular compartments and provides scaffolds for the adhesion and migration of cells, in the form of basement membranes or fibrillar interstitial matrices; it provides molecular cues that control differentiation and proliferation processes and determines cell survival vs. cell death (anoikis), either by direct interaction with cells or indirectly due to its large potential to bind and present cytokines and chemotactic factors (1). Whether similar functions exist in secondary lymphoid organs remains largely univestigated.The reticular fiber network consists of an inner core of fibrillar collagens, ensheathed by a basement membrane layer and microfibrillar proteins, and an outer layer of reticular fibroblasts (2-4). However, the molecular constituents of these layers vary in reticular fibers of different cellular compartments, and fundamental differences exist between the lymph node and spleen (4). Apart from acting as the structural backbone and potentially also a scaffold for immigrating cells (5), the reticular fiber network functions as a conduit for small-molecular-weight (<70 kDa) soluble factors (3, 6), which, in the lymph node, permit rapid delivery of antigens and/or chemokines (5) from the periphery to the surface of high endothelial venules for recruitment of T lymphocytes (2, 3). Whether other functions exist is not clear.Our work focuses on the ECM of the splee...

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