Immunosuppressive Effect of Shedding Intercellular Adhesion Molecule 1 Antigen on Cell‐mediated Cytotoxicity against Tumor Cells

We have examined whether shedding intercellular adhesion molecule‐1 (ICAM‐1) antigen from cultured tumors is able to inhibit the leukocyte function‐associated antigen‐1 (LFA‐1)/ICAM‐1 interaction between cytotoxic effector cells and ICAM‐1(+) target tumor cells. The cytotoxic activity of lymphokine‐activated killer (LAK) cells incubated with spent media from ICAM‐1(+) tumor cells, especially interferon‐γ‐stimulated tumor cells, was significantly decreased as compared with that of LAK cells treated with fresh culture medium without ICAM‐1 antigen. Treatment of LAK cells with spent media from ICAM‐1(−) tumor cells did not cause a significant decrease of the cytolytic activity towards ICAM‐1(+) tumor cells. These findings suggest that shedding of ICAM‐1 antigen could be involved in binding of LFA‐1 to LAK cells, resulting in reduced cytolytic activity.