Immune Checkpoint Inhibition Followed by Tumor Infiltration of Dendritic Cells in Murine Neuro-2a Neuroblastoma

Most tumors responding to immunotherapy with monoclonal antibodies targeting programmed cell death protein1 (PD1) and programmed death ligand-1 (PD-L1) show surface expression of PD-L1. Neuroblastoma has been reported to show low PD-L1 surface expression.

The effect of immune checkpoint inhibitor on mouse neuroblastoma was investigated, and host immune cells were analyzed in the tumor microenvironment. Expression of co-stimulatory molecules by Neuro-2a mouse neuroblastoma cells was analyzed using flow cytometer. Neuro-2a cells were inoculated subcutaneously into A/J mice, followed by intraperitoneal injection of antibodies targeting PD-1 and PD-L1. Mice were sacrificed for the measurement of tumor weights on day 14 following tumor inoculation, and tumor-infiltrating cells were analyzed using a flow cytometer.

Dim expression of PD-L1 was observed on the cell surface of cultured Neuro-2a cells. Growth of subcutaneous tumors was significantly suppressed, and PD-L1-expressing tumor cells were depleted by the antibody treatment. We confirmed that Neuro-2a cells opsonized by the anti-PD-L1 antibody were phagocytosed in the in vitro setting. In the treated tumor microenvironments, CD8α⁺ lymphocyte and CD11c⁺ MHC II⁺ cells were significantly accumulated in comparison with the control group. These CD11c⁺ MHC II⁺ cells expressed CD80, CD86, CD14, and CD40, but not CD205, PD-L1, or CTLA4. PD-1 expression was detected dimly. Immune suppressive effects of CD11b⁺Gr-1⁺ myeloid-derived suppressor cells by the administration of anti-PD-1 and PD-L1 antibodies were not observed in spleen, regional lymph nodes, or tumor microenvironment.

Our findings raise the possibility that co-administration of anti-PD-1 and anti-PD-L1 antibodies have a synergistic effect on inhibition of tumor growth and could be an effective therapy against neuroblastoma with dim expression of PD-L1.