Tumour-activated macrophages as effector cells in a tumour neutralization assay in vivo

Macrophages from C3D2 or C57B1/6 mice were activated in vitro by coculture with MC1M-AA sarcoma cells or by addition of cell free tumour ascites fluid from the same tumour. After 5-7 days of in vitro activation, macrophages were harvested, mixed with MC1M-AA or B-16 melanoma cells, and reinjected into C3D2 or C57B1/6 mice respectively. Mice were evaluated for tumour development, and the early histological appearance of the B16 melanomas was studied. Activated macrophages gave a significant delay and decrease in tumour take when mixed with B16 melanoma cells at a tumour cell: activated macrophage ratio of 1:20. When activated macrophages were mixed with MC1M-AA cells at a tumour cell: activated macrophage ratio of 1:50, a slight delay and a significant decrease in tumour take was observed. BCG-activated macrophages did exhibit a very weak effect on MC1M-AA tumour growth. When B16 melanoma cells were injected into mice together with activated macrophages, an acute inflammatory reaction was observed at the site of injection. The organization of the tumour was delayed, and necrotic tumour cells could be found. In some cases, small islands of tumour cells escaped killing, and gave rise to delayed tumour development.