Heterogeneous susceptibility of human melanoma clones to monocyte cytotoxicity: role of ICAM-1 defined by antibody blocking and gene transfer.

Five clones derived from the same human malignant melanoma lesion were studied for their susceptibility to killing by human monocytes activated by exposure to interferon (IFN)-gamma and lipopolysaccharide. Melanoma clones were heterogeneous in their susceptibility to human monocyte cytotoxicity, with one clone (2/21) exhibiting extremely low levels of lysis. The different levels of susceptibility to monocyte cytotoxicity were not accounted for by susceptibility or resistance to monokines [tumor necrosis factor (TNF), interleukin (IL)-1 and IL-6] because: (a) these effector molecules had little (TNF) or no (IL-1 and IL-6) cytolytic activity under these conditions; and (b) anti-TNF antibodies had marginal effects on cytotoxicity. Monocytes bound less to resistant than to susceptible melanoma cells. Monocyte-resistant 2/21 melanoma cells expressed substantially lower levels of ICAM-1 and VLA-4 than susceptible cells. Anti-CD18 and, to a lesser extent, anti-ICAM-1 mAb inhibited binding and cytotoxicity of human monocytes on malignant melanoma whereas anti-VLA-4 had no inhibitory action. Transfection of the ICAM-1 gene under the control of a constitutive promotor resulted in high levels of expression of ICAM-1 in 2/21 melanoma cells and, concomitantly, in augmented susceptibility to activated monocyte cytotoxicity. The augmented killing of ICAM-1 transfected 2/21 cells was inhibited by anti-ICAM-1 mAb. These results demonstrate that the CD18-ICAM-1 adhesion pathway can play an important role in the expression of human monocyte cytotoxicity on melanoma target cells and that heterogeneity in expression of ICAM-1 can underlie differences in susceptibility to tumoricidal activity.