Intratumoral low-dose interleukin-2 induces rejection of distant solid tumour

This study shows that local tumor treatment with low-dose recombinant interleukin-2 (IL-2) can mediate rejection of a large distant solid tumour. When SL2 lymphoma cells were injected intraperitoneally (i.p.) in syngeneic DBA/2 mice on day 0.70% of these mice were cured by daily i.p. injections with 20,000 units IL-2 on days 10-14. After injecting mice with SL2 both i.p. and subcutaneously (s.c.) on the flank. 50% of the mice treated i.p. with low-dose IL-2 rejected both the i.p. tumour and the large distant s.c. tumour. In contrast, i.p. IL-2 treatment on days 10-14 cured fewer than 10% of the mice bearing only a s.c. SL2 tumour. The described IL-2 immunotherapy also caused systemic tumour rejection in mice bearing both ascitic and solid P815 mastocytoma. Thus it was shown that low-dose IL-2 can induce systemic tumour rejection, when injected at a site of tumour growth. Interleukin-2-induced rejection of s.c. SL2 tumour was highly specific, as mice that were rejecting i.p. and solid s.c. SL2 lymphoma did not reject solid P815 mastocytoma, which was injected s.c. simultaneously on the other flank. Furthermore, solid s.c. tumours consisting of mixtures of SL2 and P815 were not rejected in mice that rejected i.p. SL2 or P815. We conclude that intratumoral injections of low-dose IL-2 can enhance an ongoing weak immune reaction against the tumour resulting in systemic tumour rejection.