Quantification of the strength of cell-cell adhesion: the capture of tumor cells by activated murine macrophages proceeds through two distinct stages.

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Abstract
The binding of tumor cells by macrophages activated with Bacillus Calmette-Guerin is a necessary step toward destruction of those cells. Although several characteristics of the interaction have been defined, little is known of how the actual binding process develops. We used a technique to quantify the forces required to disrupt cell-cell interactions. Over a range of applied relative centrifugal forces, the majority of targets that bound to the activated macrophages fell on two distinct plateaus. Approximately 90% of added targets were bound to the monolayers of macrophages over the range of 1 to 100 X G; 25 to 30% remained bound from 1200 X G to 1500 X G. Two strengths of binding, termed weak and strong binding, respectively, were thus defined on the basis of these curves. Strong binding developed only between activated macrophages and tumor cells. By contrast, weak interactions occurred between either activated or nonactivated macrophages and neoplastic or non-neoplastic target cells. The strong binding required time (60 to 90 min), metabolic activity by the macrophages, and trypsin-sensitive surface structures on the macrophages for development, whereas the weak interaction occurred rapidly and required none of these. Additional evidence indicated the weak binding developed into strong when activated macrophages bound neoplastic cells. This stabilization increased the strength of force to separate tumor cells from the macrophages at least approximately 15 fold (i.e., from approximately 16 mu dynes/cell to approximately 240 mu dynes/cell). Of note, the development of strong binding of antibody-coated targets had distinct requirements for establishment. Taken together, the data suggest the stabilization of binding (i.e., the development of weak into strong binding) leading to effective cell-cell interaction is a complex and dynamic process that may vary depending upon the recognition system involved.