Differential Locomotion of Long‐ and Short‐term IL‐2‐activated Murine Natural Killer Cells in a Model Matrix Environment

Tumour infiltration by activated natural killer (A‐NK) cells is a pre‐requisite for tumour eradication by adoptive NK cell transfer. Extravasated A‐NK cells do not always succeed in reaching the crucial target cell conjugation. Therefore, we wished to study A‐NK cell locomotion and interactions with melanoma cells in a matrix environment (Matrigel) by electron, confocal and fluorescence microscopy. Two distinct patterns of A‐NK cell‐mediated matrix disintegration were revealed during incubation of tumour cells and A‐NK cells in Matrigel: (1) A‐NK cells pre‐cultured for 5 days altered the homogeneous texture of the Matrigel, an initial microporous appearance became a loose filamentous meshwork by 24 h. Matrix degrading protease inhibitors could not fully prevent this, but could delay the process; and (2) A‐NK cells pre‐cultured for 6 days or more, instead formed large excavations in the Matrigel leaving the remaining matrix less affected compared to the effects by the younger A‐NK cells. By histochemical staining with Cupromeronic Blue, the excavations were shown to contain proteoglycan material. Protease inhibitors had no discernable effect on the development of the excavations. The conspicuous capacity of A‐NK cells to disintegrate extracellular matrix and the formation of large excavations seems only partially to depend on matrix‐degrading proteases. Formation of extracellular proteoglycan material is suggested to facilitate A‐NK cell locomotion within a matrix environment.