Essential role of surface-bound chemoattractant in leukocyte migration

Abstract

MANY chemotactic factors, usually proteins or peptides, have been isolated and studied, but little is known about the basic mechanism of leukocyte migration. This movement is termed chemotaxis if its direction is determined by substances in the cells' environment¹. The chemotactic agent is assumed to convey information to the leukocytes by interaction with receptors. The subsequent sequence of events thus triggered in the cells is unknown but metabolic changes such as activation of an esterase have been reported as occurring as the cells move forward (for review see ref. 2). A role for surface-bound chemoattractant in cell locomotion was suggested by the observation that mouse fibroblasts move preferentially from substrates of lesser to those of greater capacity to bind to cells^3,4. A close relationship has also been suggested between the mechanism of chemotaxis and phagocytosis⁵. In both cases cells form protrusions of motile membrane which carry receptors for either chemoattractants or opsonins. For example, such membrane protrusions have been shown to bind bacteria and also to attach to opsonised particles, through Fc receptors to IgG-Fc or through C3 receptors to C3 fragments. Once initial contact is established during phagocytosis further membrane segments bind, enabling the cell to creep around the bacterium until it is ingested. We report here that using casein, a chemotactic agent in a standard chemotaxis assay⁶, we have found that binding of casein to a solid substratum is essential for its attracting capacity.