Nonmalignancy of hybrids derived from two mouse malignant cells. II. Analysis of malignancy of LM (TK-) Cl 1D parental cells.

The possibility that Cl 1D cell line, an L-cell line derivative, was a mixture of malignant and nonmalignant cells was investigated because previous experiments showed that some hybrids derived from Cl 1D and tumor cell lines grew in vivo, whereas others apparently did not have that capacity. More highly malignant Cl 1D cells were not selected by animal passage of tumors obtained from inoculation of cultures. The tumor-producing capacity of this line (10%) was greatly increased (81%) in X-irradiated hosts. These observations suggested that the low capacity of Cl 1D cells to grow in vivo did not result from varying degrees of malignancy but mainly from interference of immunogenetic factors between cells and their inbred hosts of origin. Numerous hybrids between tumor and host cells were in all tumors examined.