Inhibition of macrophage-mediated cytolysis by lipoproteins from cell-free tumorous ascites.

Cell-free ascites from syngeneic MM46 tumor-bearing C3H/He mice inhibited the antibody-dependent macrophage-mediated tumor lysis in vitro and the inhibitiory activity increased with the period of tumor bearing. Thus, the inhibitory activity was demonstrated at the effector cell level. Three fractions of lipoprotein and a protein-rich fraction were prepared from the cell-free ascites by sequential flotation. Among these, the very low density (p less than 1.006) and low density (1.006 less than p less than 1.063) lipoprotein fractions inhibited tumor lysis mediated by activated macrophages and syngeneic antitumor antibody, whereas the high density (1.063 less than p less than 1.21) lipoprotein and the protein-rich infranatant fractions did not. This inhibitory activity at the effector cell level may be due to functional depression of macrophages by lipoprotein fractions.