Activated natural killer (A-NK) cells, defined by immunophenotype and selected by adherence to the plastic, were cultured from murine splenocytes for up to 10 days with the addition of 1000 U/ml of recombinant human IL-2 at 48 h intervals. During culture days 2-4 with high DNA synthesis the initially non-granulated small cells established large granular lymphocyte (LGL) morphology and then differentiated further into giant hypergranulated cells with huge accumulations of glycogen. Timed EM observations indicated that specific dual-compartment (lytic) granules arose by a sequence of events starting with neo-synthesis of small progenitors with a dense core and a few membranous lamellae at one pole. Core and vesicular regions probably expanded independently to give the mature organization of the granule. Eventually, the vesicular region of granules contained large amounts of multi-lamellar material and probable debris, and the dense core could be multiplied. Intracellular proteoglycans, visualized with Cupromeronic Blue cytochemistry, were organized in a three-dimensional network within the dense cores. In contrast with earlier reports, and in spite of several-fold increased granularity, the in vitro cytotoxicity of the A-NK cells against YAC-1 and B16 cells decreased after the third day of culture. A-NK cells with glycogen accumulations caused focal clearing in melanoma monolayers whereas younger effectors adhered to the targets. It is concluded that high dose IL-2 stimulation causes more far-going progressive morphological and functional differentiations of the A-NK cells than has previously been observed with bearing for the use of these cells in experimental adoptive immunotherapy.

An experimental set‐up for estimating a) cellular migration under agarose and b) response to chemoattractant gradients built up in the agarose was used in order to explore the behavior of adherent interleukin‐2 (IL‐2)‐activated natural killer (A‐NK) cells on cell culture plastic and after coating with extracellular matrix (ECM) constituents. A‐NK cells were deposited in wells in the agarose and directed migration, chemotaxis, towards aggregates and suspensions of B16F10 melanoma cells, suspensions of YAC‐1 cells, and tumor‐conditioned media, all deposited in wells at a 2.5 mm distance, was tested. A‐NK cell'chemotaxis was exclusively observed when B16F10 aggregates were used as attractants. The substrate influenced chemotaxis considerably, untreated plastic surface being most favorable for a chemotactic response, followed by laminin, fibronectin, and collagen IV pretreatments. Coating with reconstituted basement membrane matrix (Matrigel®) gave lesser random movements, chemokinesis, of A‐NK cells than coating with the purified components laminin and collagen IV, and the least motile response was obtained after collagen I pretreatment. These in vitro observations indicate that melanoma cell aggregates release humoral factors of a probably short‐lived nature with a chemoattractant effect on A‐NK cells, and that ECM composition influences migratory response, both conclusions with a bearing on the understanding of A‐NK cell infiltration into tumors in vivo.