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Human B lymphocytes localize and differentiate within the microenvironment of lymphoid germinal centers. A frozen section binding assay was developed for the identification of those molecules involved in the adhesive interactions between B cells and lymphoid follicles. Activated human B cells and B cell lines were found to selectively adhere to germinal centers. The VLA-4 molecule on the lymphocyte and the adhesion molecule INCAM-110, expressed on follicular dendritic cells, supported this interaction. This cellular interaction model can be used for the study of how B cells differentiate.

Abstract. von Willebrand factor (VWF) is a large, adhesive glycoprotein that is biosynthesized and secreted by cultured endothelial cells (EC). Although these cells constitutively release VWF, they also contain a storage pool of this protein that can be rapidly mobilized. In this study, a dense organelle fraction was isolated from cultured umbilical vein endothelial cells by ceturifugation on a self-generated Percoll gradietu. Stimulation of EC by 4-phorbol 12-myristate 13-acetate (PMA) resulted in the disappearance of this organelle fraction and the synchronous loss of WeibelPalade bodies as judged by immunoelectron microscopy. Electrophoretic and serologic analyses of biosynthetically labeled dense organelle fraction revealed that it is comprised almost exclusively of VWF and its cleaved pro sequence. These two polypeptides were similarly localized exclusively to Weibel-Palade bodies by ultrastructural immunocytochemistry. The identity of the dense organelle as the Weibel-Palade body was further established by direct morphological examination of the dense organelle fraction. The VWF derived from this organelle is distributed among unusually high molecular weight multimers composed of fully processed monomeric subunits and is rapidly and quantitatively secreted in unmodified form after PMA stimulation. These studies: (a) establish that the Weibel-Palade body is the endothelial-specific storage organdie for regulated VWF secretion; (b) demonstrate that in cultured EC, the VWF concentrated in secretory organdies is of unusually high molecular weight and that this material may be rapidly mobilized in unmodified form; (c) imply that proteolytic processing of VWF involved in regulated secretion takes place after translocation to the secretory organelle; (d) provide a basis for further studies of intracellular protein trafficking in EC.