Abstract
Dendritic cells (DCs) link innate and adaptive immunity and are important for the induction of immune responses. Functional maturation resulting in migratory DCs is critical for the induction of efficient immune responses. In this thesis the cytoskeletal changes and the consequences for adhesive and migratory behavior that occur during DC maturation are studied. During maturation DCs lose cell adhesion structures and acquire migratory capacity. The specialized adhesion structures, podosomes, are dissolved in response to maturation stimuli and this is mediated by prostaglandins produced by the DCs. The prostaglandin PGE2 induces very rapid podosome loss. In addition to podosome loss, inactivation of 5 1-integrins are needed to acquire migration. Interestingly, Gram-negative bacteria are superior to Gram-positive bacteria in inducing podosome loss, migration and inducing expression of costimulatory molecules, CCR7 and cytokines in DCs. This is TLR4 and predominantly TRIF-dependent. Also these responses involve the production of prostaglandins (possibly PGE2) by DCs. PGE2 activates the EP2 and EP4 receptors on DCs, which raises cAMP levels. This results in activation of the small GTPase RhoA and inactivation of Rac1 and Cdc42. Active RhoA activates its downstream effector Rho kinase. Activation of the RhoA-Rho kinase axis leads to actomyosin contraction, which is necessary for podosome loss and leads to the (transient) formation of focal adhesions. These data suggest that podosomes are regulated by the contractile state of the cells. Interestingly, when DCs are seeded on substrates with varying topology, podosomes specifically form in those areas where height differences occur. These changes in substrate topology most probably induce a curvature of the cell membrane triggering local podosome formation. Thus, podosomes are mechanosensitive structures regulated by mechanical stress on the outside and cytoskeletal tension on the inside of the cell.
