In vitro studies on the trafficking of dendritic cells through endothelial cells and extra-cellular matrix

Dendritic cells (DC) are antigen presenting cells (APC) with the unique ability to initiate an immune response. Immature DC are localized in peripheral tissues where they exert a sentinel function for incoming antigens (Ag). After Ag capture and exposure to inflammatory stimuli DC undergo maturation and migrate to regional lymph nodes where the presentation of antigenic peptides to T lymphocytes takes place. Thus their correct functioning as APC involves localization in tissues and trafficking via the lymph or blood to lymphoid organs. In the present study we have investigated the ability of DC to interact in vitro with human vascular endothelial cells (EC) and extracellular matrix (ECM). DC are differentiated from monocytes by in vitro exposure to GM-CSF and IL-13 for 7 days. In adhesion assays a considerable proportion of DC binds to resting EC monolayers and this adhesion is inhibited by anti-CD11a and CD11b, but not anti-CD11c mAbs. Binding to a natural ECM, derived from cultured EC involves VLA-4 and VLA-5 integrins. In a transmigration assay, 10% of input cells are able to cross the EC monolayer in the absence of exogenous stimuli. The amount of DC transmigrated through a monolayer of EC was increased of 2-3 fold by C-C chemokines RANTES, MIP1alpha, and MIP-1beta. Most importantly, in view of the trafficking pattern of these cells, a significant proportion of DC can migrate in a reverse transmigration assay, i.e. across the endothelial basement membrane and subsequently, across endothelial cells. Upon exposure to immune or inflammatory signals peripheral DC undergo maturation and migration to lymphoid organs. Functional maturation is associated with loss of responsiveness to chemokines present at sites of inflammation (e.g. MIP1alpha, MIP1beta and RANTES) and acquisition of a receptor repertoire which renders these cells responsive to signals which guide their localization in lymphoid organs (e.g. MIP3beta). A better understanding of the molecular basis of DC trafficking may provide molecular and conceptual tools to direct and modulate DC localization as a strategy to upregulate and orient specific immunity.