Some characteristics of natural cytostatic mouse splenocytes.

Murine B16 melanoma cells and metastasis variants of this tumor are resistant to NK activity mediated by normal splenocytes. The B16 cells are, however, sensitive to splenocyte-mediated cytostasis. Cytostasis was measured by a [125I]UDR incorporation-inhibition [(125I]UDR I-I) assay. The main characteristics of the [125I[UDR I-I assay and of the cells mediating it are as follows: The activity is mediated by splenocytes but not by thymocytes, it is not syngeneically restricted and it does not decrease with age. The presence of effector cells is required as splenocyte supernatants or supernatants of effector-target cell mixtures do not cause [125I]UDR I-I. The activity is probably mediated by at least 2 populations of non-phagocytic splenocytes. The first population adheres to plastic surfaces and to Sephadex G-10 columns while the other does not. The sensitivity to [125I]UDR I-I of the high metastasis B16 variant was similar to that of the low metastasis variant.