We investigated NK cell infiltration into tumor developing lesions at early stage of tumor development after intraperitoneal inoculation of 3LL lung carcinoma into syngeneic C57BL/6 mice. Natural killer (NK) cells, which were detected by anti‐NK 1.1 monoclonal antibody (mAb), remarkably increased in number in tumor‐developing lesions (peritoneal cavity) as early as day 3 after inoculation of 3LL. The tumor‐infiltrating NK cells from 3LL‐inoculated mice produced a high level of interferon‐γ by co‐culture with 3LL and showed enhanced cytotoxic activities against both NK‐sensitive (YAC‐1) and NK‐resistant (3LL and P815) tumors. Furthermore, mice depleted of NK cells by injection of anti‐NK 1.1 mAb or antiasialo GM1 antibody showed shorter survival times after intraperitoneal inoculation of 3LL when compared with control mice. These results suggest that NK cells infiltrate the tumor‐developing lesion at an early stage and may participate in the early protection against tumors through production of a high amount of interferon‐γ and enhanced cytotoxicity at tumor‐bearing sites.
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