Suppression of malignancy in mouse-man hybrid cells

Somatic cell hybrids were prepared by fusing malignant mouse melanoma cells with diploid human lymphocytes and fibroblasts in the presence of inactivated Sendai virus. Clones of hybrid cells differed from one another with respect to the number of chromosomes. In particular, human chromosomes were preferentially lost and mouse chromosomes retained. Hybrid clones containing a range of human chromosomes were selected and assayed for tumorigenicity in nude mice, and the presence or absence of specific human chromosomes was correlated with malignancy. Both human lymphocytes and human fibroblasts suppressed the malignancy of mouse melanoma cells. In most cases, suppression of malignancy was complete; in a few cases, tumours developed but the tumorigenicity of these hybrids was considerably less than that of the parent melanoma cells. Chromosome analyses failed to localize the suppressive effect to a specific chromosome.