PROTECTION FROM TUMOR NECROSIS FACTOR-MEDIATED CYTOLYSIS BY PLATELETS

Infiltrating macrophages elicit tumor-destructive reactions by releasi ng cytolytic factors including tumor necrosis factor alpha (TNF-alpha) . Because platelets represent another major component of the cell infi ltrate in tumors, we examined whether they could affect TNFalpha-induc ed cell death. Exposure of L-929 fibrosarcoma cells to human platelets reduced TNFalpha-induced cytotoxicity and cytolysis, as determined by Cr-51 release assay and DNA fragmentation assay. This inhibitory effe ct, which depended on the concentration of platelets (0.1 to 10 x 10(6 )/0.1 ml), was as high as 50%. The decrease in responsiveness to TNF-a lpha reflected neither a degradation of TNF-alpha nor an inability of L-929 cells to bind TNF-alpha. Indeed, even though Scatchard analysis indicated the presence of 100 to 150 I-125-TNF-alpha binding sites/pla telet with a kd of 3.8 to 64 nM, addition of platelets up to 5 x 10(6) /0.1 ml did not compete with I-125-TNF-alpha binding to L-929 cells. F urthermore, addition of platelets 1 or 2 hours after that of TNF-alpha was still protective suggesting that platelets rather promoted hypore sponsiveness of L-929 cells to a postbinding effect of TNF-alpha. Plat elet-induced reduction of TNF-alpha response could be reproduced with supernatant fluids from platelets incubated at 37-degrees-C for 24 hou rs. The platelet-derived factor responsible for this effect was found to be a lipid of low molecular weight with high affinity for albumin a nd charcoal. A role for 12(S) hydroxyeicosatetraenoic acid is proposed because this metabolite reduced TNF-alpha-induced cytolysis in a dose -dependent manner, whereas other platelet-derived lipids including thr omboxane A2 and platelet activating factor were inactive. These observ ations indicate that the role of associated platelets has to be consid ered when analyzing the cytotoxic and cytolytic activity of macrophage -derived TNF-alpha on tumor cells.