Background: IL-21 is an immune enhancing cytokine produced by T helper cells that showed promising results in pre-clinical and clinical cancer immunotherapy. We previously observed that the administration of anti-CD4 cell-depleting antibody strongly enhanced the anti-tumor effects of recombinant (r)IL-21immunotherapy in a syngeneic model of disseminated Neuroblastoma (NB). The removal of pre-existing and NB-induced CD4+CD25high Treg cells by the anti-CD4 mAb synergized with IL-21 in the induction of anti-NB CD8+ T cell responses. Murine NB cell lines express Programmed Death-Ligand (PD-L)1 and its interaction with PD-1 on immune cells may induce immune tolerance favoring tumor growth and spread. Here we studied the therapeutic effects of a combination of rIL-21 and PD-1/PD-L1 immune checkpoint blockers in disseminated syngeneic NB.
Methods: A/J mice were injected intravenously with either Neuro2a-Luc (GD2-) or NXS2-Luc (GD2+) NB cell lines to induce disseminated disease. rIL-21 therapy (1 μg/dose) was administered for 5 times. Immune checkpoint blocker, anti-PD-L1 mAb (200 μg/dose), was administered intra peritoneally for 5 times from intravenous injection of tumor cells. The tumor-free survival of treated mice was evaluated.
Results: Both anti-PD-L1 mAb and rIL-21 alone had a limited effect on NB development. The combination of anti-PD-L1 mAb and rIL-21 could not cure mice from disseminated NB. We then found that murine NB cell lines express PD-L2, which is an alternative ligand for PD-1 and thus NB could escape anti PD-L1 blockade.
Conclusion: Our data indicate that the use of anti PD-L1 mAb cannot cure disseminated NB even if associated with rIL-21. We are currently investigating the effects of anti-PD-1 mAb either alone or in combination with IL-21 or antibodies targeting Treg cells to further evaluate the role of this immune checkpoint in pre-clinical NB models.