The infiltration of experimentally induced lung metastases of colon carcinoma CC531 by adoptively transferred interleukin‐2‐activated natural killer cells in wag rats

The number of IL‐2‐activated natural killer (A‐NK) cells reaching the tumor site in vivo may be crucial for their antitumor effect following adoptive immunotherapy. We investigated in a syngeneic rat model the infiltration of established lung metastases by adoptively transferred A‐NK cells. The Wag rat colon carcinoma CC531 was injected via a tail vein to induce pulmonary metastases. Syngeneic A‐NK cells were labeled with the fluorescent dye rhodamine (TRITC) and next injected via a tail vein in rats bearing day‐12 lung tumors. The number of A‐NK cells in tumor and in normal tissue per rat was counted in sections after administration of A‐NK cells. At all time points tested, a significant linear relationship between the cross‐section area of the tumor and the number of infiltrating cells was observed, but small tumor areas became fully infiltrated earlier than larger areas. At 24 hr after injection, approximately 10% of the injected cells were found in the tumor tissue and the average A‐NK‐cell‐to‐tumor‐cell ratio was estimated to be 1:3. A‐NK cells were found in the liver too, although the number of cells per mm² tissue was low compared with the pulmonary tumor tissue. Very low numbers of A‐NK cells were found in kidney, adrenal gland, spleen, and blood. We conclude that, in this syngeneic rat model, adoptively transferred A‐NK cells are able to find and specifically infiltrate pulmonary metastases in a time‐dependent fashion.