In vivo distribution and cytokine gene expression by enriched mouse LAK effector cells

Lymphokine activated killer (LAK) cells administered in combination with interleukin 2 (IL2) can mediate antitumor activity in tumor-bearing mice and advanced cancer patients. Relatively little is known about the mechanism by which adoptively transferred LAK cells plus IL2 mediate these antitumor effects in vivo, and it remains unclear to what extent the actual LAK effector cells can accumulate in tumors. In the present study, enriched cytolytic LAK effector cells were obtained by fractionation of bulk LAK cell cultures on Percoll density gradients. About 95% of the total lytic activity was recovered from the 55% of cells isolated in fraction 2 (Fr2). The cells recovered in Fr2 are mostly large, proliferating lymphoblasts that express either the NK-associated surface markers NK1.1 (38%) or LGL-1 (31%), or the cytotoxic T cell phenotype, Lyt2 (39%). The cytolytic lymphoblasts obtained from Fr2 were radiolabelled with either 111Indium-Oxine (111InOx) which labels all cells in the population, or with 125Iododeoxyuridine (125IUdR) which labels only proliferating cells, and injected iv into mice bearing murine renal cancer (Renca). 111InOx-labeled Fr2 cells migrated mostly to spleen (28%) and liver (35%), with approximately 5% of the injected label detectable in the Renca-bearing kidney by 24 hrs. In contrast, Fr2 cells labeled with 125IUdR, which labels only the proliferating blasts thought to include the actual effector cells, exhibited a very different localization pattern. 125IUdR-Fr2 cells were retained in the lungs at higher levels than were 111InOx-Fr2 cells and very little label was detectable in liver (6%), spleen (3%), or tumor bearing kidney (2%) at 24 hrs. These results suggest that most of the large, proliferating lymphoblasts are cleared from the body by 24 hrs and very few localize into even large tumors. Subsequently, Northern blot analyses performed on bulk LAK cells revealed a potent induction of mRNA for TNF alpha by 6 hrs and for IFN gamma by 48 hrs. The intensity of gene expression for both cytokines was increased in Fr2 as compared to the unfractionated bulk LAK cells or to non-cytolytic cells obtained from Fr3. Overall, these results suggest that at least some of the antitumor effects mediated by LAK cells occur by the release of cytokines that synergize with exogenous IL2 for the activation of host effector cells.