The elaboration of a small molecular weight cytostatic factor by lymphoblastoid lines and activated lymphocytes

In this study, it was observed that the B lymphoid cell line RPMI 8392 produced a tumor cell cytostatic factor that is of lower molecular weight than previously described cytostatic proteins but of greater molecular weight than arachidonate metabolites. This low molecular weight factor was also found to be produced by normal murine lymphocytes upon mitogenic stimulation. Gel filtration chromatography revealed that the molecular weight of this molecule is approximately 6,500 daltons. This is the approximate molecular weight of another tumor cell directed lymphokine which we have previously described, Tumor Migration Inhibition Factor (TMIF). This lymphokine is defined by its ability to inhibit the in vitro migration of a variety of tumor cell types. It was not determined in this study whether or not the proliferation inhibitory activity and the migration inhibitory activity are due to separate molecules. However, experiments presented here demonstrate that these two activities are functionally distinct. In addition, it was found that the cytostatic factor described here, unlike higher molecular weight cytostatic factors, was not associated with cytotoxicity.