ITOC2 – 032. Optimising active-specific immunotherapy in lymphodepleted mice with subcutaneous melanoma

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Background Lymphodepletion, reconstitution and active-specific tumour cell vaccination (LRAST) enhances the tumour-specific immune response in a murine melanoma model. Myeloid derived suppressor cells (MDSC, Gr-1+ CD11b+) may decrease the induction of T-cells and their therapeutic efficacy. Aim of the study was to evaluate the benefit of MDSC depletion in addition to LRAST, using anti-Gr-1 antibody (Ab). The Gr-1 epitope is also expressed on CD8+ T-cells. Methods Female C57BL/6 mice with subcutaneous (s.c.) D5 melanoma were lymphodepleted with cyclophosphamide and reconstituted with naive splenocytes. Vaccination was performed with irradiated syngeneic mGM-CSF-producing D5G6 melanoma cells. For MDSC depletion, anti-Gr-1 Ab (clone RB6-8C5, 230 μg) was injected every other day starting on the day of lymphodepletion. Induction of tumour-specific T-cells in tumour vaccine draining lymph nodes was evaluated using interferon (IFN)- γ assays. Immune cells were analysed by flow cytometry. D5 tumour growth was monitored. Results Anti-Gr-1 administration had no long-lasting effect on MDSC elimination. The antibody also bound to CD8+ Gr-1+ CD44+ T memory cells. The combination of anti-Gr-1 treatment and LRAST enhanced tumour-specific IFN- γ release. Nevertheless, this did not translate into improved survival. Conclusions Given an insufficient elimination of MDSC and the possible depletion of Gr-1+ memory T-cells, the use of anti-Gr-1 Ab in our tumour model seems questionable.