We have previously proposed that fibrin deposition on tumor cells during their migration in the blood could protect them from elimination by natural killer (NK) or other cytotoxic cells. Anticoagulant drugs could prevent fibrin coagulation and increase the efficiency of cytotoxic effector cells in tumor cell elimination. To further investigate the protective roles of fibrin, we studied in vitro the susceptibility of various murine tumor cells to the cytotoxic activity of NK or lymphokine activated killer (LAK) cells in the presence of murine plasma or serum.

In the first set of experiments, tumor cells were incubated with plasma (at dilutions of 1:20–1:160) for 30 min before effector cells were added. Similarly, effector cells were first incubated with plasma before mixing with radiolabeled target cells for cytotoxicity assay. In some experiments target and effector cells and plasma were mixed simultaneously. The cytotoxic activity of both NK and LAK cells was inhibited if coagulation occurred around tumor-target or effector cells. Tumor cells were also protected when both target and effector cells were simultaneously mixed and trapped in the fibrin clot. Inhibition of the cytotoxic activity of effector cells against tumor cells was positively correlated with the level of fibrin clot formation. When the larger clot was formed and more radiolabeled tumor cells were trapped in the clot, the higher level of inhibition of cytotoxicity was observed. In contrast, serum did not affect the cytotoxic activity of NK or LAK cells. To exclude possible noncoagulation-related effects of plasma on LAK cells, a cytotoxicity series of experiments was performed using purified fibrinogen and thrombin. When fibrinogen and thrombin were preincubated with tumor cells or LAK cells or all components were admixed simultaneously, substantial protection of tumor cells from destruction by LAK cells was also observed. However, when heparin was added, fibrin coagulation was prevented and cytotoxic activity of LAK cells was restored.

Inhibition of LAK cytotoxicity and protection of tumor cells by fibrin coagulation were mostly due to the prevention of tumor-effector cell conjugate formation. Adding plasma at postbinding time periods (15–30 min after mixing effector and target cells) did not affect the ability of LAK cells to kill tumor cells confirming that fibrin coagulation influenced the binding rather than the lytic phase of cytotoxic cell activity.

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