Induction of malignant lymphoreticular cell line which displays spontaneous killer-cell activity

The origin and properties of a new malignant "spontaneous killer" cell line are described. The tumor, known as M9-78, was produced by treatment of tissue cultures consisting mostly of dividing DBA/2 mouse macrophages with the chemical carcinogen, 3-methylcholanthrene. Logarithmic phase cultures were treated for 6 days followed by further culture for 1 month in the absence of carcinogen. Subcutaneous injection of normal DBA/2 mice with 2.0 X 10(5) cells led to the formation of palpable tumors at the site of inoculation within 14 days; the tumors grew progressively and were highly metastatic. The malignant cells, though Fc-receptor-positive, did not possess many other properties normally associated with macrophages; morphologically, they bore a strong resemblance to bone-marrow-derived promonocytes. Because promonocytes have been shown previously to possess both antibody-dependent killer (K) cell as well as antibody-independent spontaneous killer-cell function, the M9-78 cells were tested for these biologic activities. The following results were obtained: (1) adherent, but not non-adherent, M9-78 tissue culture cells spontaneously killed a wide variety of (though not all) tumor-cell-line targets in long-term (18-24 h) 51Cr release, whereas no killing was seen in short (4 h) assays; (2) the cells were not capable of killing nucleated targets by antibody-dependent cell-mediated cytotoxicity; (3) a number of clones were obtained and tested for spontaneous killer-cell activity: some were highly active, whereas others were negative, with different patterns of specificity being evident; (4) the clones were unstable; (5) the pattern of target killing, when compared to normal spleen control NK cells, suggested the M9-78 cells were similar to activated macrophages or promonocytes in their target selectivity, although no definitive evidence was obtained which could implicate a promonocyte or monocyte origin for the cells.