Endogenous natural killer cells do not play a role in antitumor effects induced by interleukin-2 in a syngeneic rat colon tumor model

Previous experiments in a syngeneic rat liver tumor model using the colon adenocarcinoma CC531 demonstrated that injection of interleukin-2 (IL- 2) induced significant antitumor responses. Furthermore, it was found that this treatment strategy was accompanied by an increase in the number of natural killer (NK) cells in and around the tumor. In the present study, the role of endogenous NK cells in IL-2-mediated antitumor responses was further elucidated by depleting tumor-bearing rats of NK cells, using the anti-CD161A mouse IgG1 antibody 3.2.3. Rats were depleted either after or prior to tumor induction and subsequently treated with IL-2. The results demonstrated that depletion of NK cells in tumor-bearing rats did not influence IL-2-induced antitumor effects. In addition, injection of IL-2 in NK-cell-depleted rats induced repopulation of NK cells in the peripheral blood from 3 days on and further after the last injection with IL-2. Therefore, the possibility cannot be excluded that de novo recruited NK cells play a role in attaining IL-2 mediated antitumor effects, but NK cells, which were present before or during the start of IL-2 therapy, were not relevant.