Abstract
Cell adhesion is a multistep, metabolically active process usually requiring several minutes or even hours to complete. This results in the formation of strong bonds that cannot be ruptured by mechanical forces encountered by living cells in their natural environment. However, the first seconds after contact formation are much more sensitive to external conditions and may be the critical step of adhesion. This step is very difficult to monitor without disturbing the observed system. We addressed this problem by studying the interaction between anti-CD8-coated or control surfaces and murine lymphoid cell lines bearing wild-type CD8 molecules, or genetically engineered molecules bearing extracellular CD8 domains and transmembranar and intracytoplasmic domains of class I histocompatibility molecules, or with extensive deletion of intracytoplasmic domains. We used a new method that consisted of monitoring the motion of cells driven along adhesive surfaces by a hydrodynamic force weaker than the reported strength of single ligand-receptor bonds, but sufficient to make free cells move with an easily detectable velocity of several micrometers per second. Cells exhibited short-term (< or = 0.5 s) adhesions to the surface with a frequency of about one event per 30-s period of contact. These events did not require specific antigen-antibody bonds. However, when anti-CD8 were present, strong adhesion was achieved within < 1 s, since most arrests were longer than a standard observation period of 1 min. This bond strengthening was not affected by cytochalasin, and it did not require intact intracellular domains on binding molecules. It is concluded that the initial step in strong adhesion may be viewed as a passive, diffusion-driven formation of a new specific bonds.