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 a denocarcinoma CC531 demonstrated that injection of interleukin-2 (IL-2) ind uced significant antitumor responses. Furthermore, it was found that this t reatment strategy was accompanied by an increase in the number of natural k iller (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 eluci dated by depleting tumor-bearing rats of NK cells, using the anti-CD161A mo use 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 an d further after the last injection with IL-2. Therefore, the possibility ca nnot be excluded that de novo recruited NK cells play a role in attaining I L-2 mediated antitumor effects, but NK cells, which were present before or during the start of IL-2 therapy, were not relevant.