Interactions of Lymphokine-Activated Killer Cells and A549 Lung Carcinoma Nodules Maintained in Three-Dimensional Culture

Abstract

Lymphokine-activated killer (LAK) cells were cocultured in the presence of interleukin-2 (IL-2), with pulmonary surfactant secreting A549 lung carcinoma nodules and maintained in continuous three-dimensional culture for 2–6 days in an attempt to test the response of tumor cells which produce LAK cell inhibitory substances. The A549 nodules secrete mucus which envelops them. This mucus is also secreted inside pseudoalveolar structures characteristic of these nodules. The mucus contains the pulmonary surfactant and sialomu-cins, both LAK cell inhibitory substances. The spontaneous infiltration into the neoplastic tissue and the membrane contacts established between the two cell types were studied by means of histological, immunohistochemical and electron-microscopic methods. Free-floating LAK cells were allowed to sediment and adhere freely to the nodule surface. The cytostatic and cytolytic effects of LAK cells were tested using thymidine incorporation into DNA and flow cytometry. Despite the presence of a mucus envelope, LAK cells adhered to the A549 nodule surface and penetrated spontaneously into them in the presence of IL-2: they settled mainly in the pseudoalveolar structures where they became apoptotic. According to electron-microscopic observations performed on the second day of coculture, the LAK cells, which remained between the cancer cells, established mostly pinpoint contacts with the carcinoma cells, forming cytoplasmic fusions. These fusions indicate the induction of pores in both the cancer cell and the LAK cell membranes. Electron-microscopic observation also displayed LAK-cell-associated apoptotic and necrotic carcinoma cells. However, at this stage of the coculture (day 2), the DNA synthesis rate of the A549 nodules still remained unchanged; it diminished by approximately 3 times on day 4 and almost stopped on day 6: nodule disintegration was then complete. In the free-floating LAK cell component of the cocultures, DNA synthesis was already strongly inhibited (26 ×) by the second day. Nevertheless, their cytolytic effect remained unaltered, as was tested on A549 monolayer cells. The presence of tumor necrosis factor (TNF) in the coculture supernatant has been demonstrated, and when coculturing took place in the presence of monoclonal TNF antibody, nodule proliferation was significantly enhanced (up to 145%). Our results indicate that, despite the presence of pulmonary surfactant and sialomucins containing mucus, LAK cells were capable of killing lung carcinoma cells in three-dimensional culture at an early stage of coculture (day 2) by direct cell-to-cell contact. Total nodule disintegration, however, was complete much later (on day 6), and taking into account the low amount of LAK cells in the cancer tissue, this seemed to be the result of an indirect effect implying, in particular, the presence of soluble TNF.