Natural killer (NK) cells in the rat: "heterogeneity" as reflection of the activation status

Rat natural killer (NK)/natural cytotoxic (NC) cell activity was tested in a 4-h and a 20-h 51-Cr-release assay. In the 4-h assay, NK activity was high with peritoneal cells (PC), medium with peripheral blood lymphocytes (PBL) and spleen cells and low with bone marrow (BM), thymus and lymph node (LN) cells. The differences in lytic capacity were less pronounced in a 20-h assay. This led to the suggestion that NK/NC cells may get activated upon coculture with tumor cells during the long-term assay and that the short-term assay actually reflects the activation status of NK/NC cells in different organs rather than non-identity of cells with natural cytotoxic potential. This was supported by the following observations: 1. After in-vivo or in-vitro activation of NK cells, increased reactivity was observed in a 4-h assay especially with lymphoid cells of organs with low or medium NK activity and the differences to a priori highly active organs were significantly diminished. 2. Neither by biophysical (density) nor by biochemical (surface markers) methods differences between NK versus NC cells could be substantiated. 3. Closely related patterns of cytotoxicity were observed using adherently growing or lymphoid tumor target cells.