Dynamic morphology of metastatic mouse T-lymphoma cells invading through monolayers of 10T1/2 cells

We have used an in vitro model system to analyze cytomechanical aspects of tissue infiltration by T-lymphocytes. The interaction of metastatic T-lymphoma cells with a precultured monolayer of 10T1/2 fibroblast-like cells was recorded in time-lapse video with alternating phase contrast and reflection interference contrast microscopy. Sectioning of embedded specimens as well as cytoskeletal stainings have been performed on matching cocultures. The lymphoma cells did not strongly attach or spread on the dorsal surface of the monolayer cells. Invasion started with the protrusion of a pseudopodium through a narrow gap, and conspicuous constriction of the invading cell's body and nucleus was a consistent feature during the later steps. Overt retraction of the target cells was not seen, but the invading lymphoma cells elevated the fibroblasts over relatively large areas, thereby creating dome-shaped open spaces, allowing for further migration under the monolayer with minimal resistance. Invasion was not unidirectional but was readily reversible at any stage. Due to this wavering character, an invasion event could take more than 1 hour, although the shape alterations involved were fast. Even after the invasion process had been completed, the lymphoma cells could come out from below the monolayer again. Therefore we propose that invasion in this model should be considered as a dynamic equilibrium. Invading T-lymphoma cells displayed diffuse F-actin staining and a well-organized microtubular complex with the centrosomes behind the nucleus in the uropod, which also contained most vesicular organelles.