OP15. ENHANCEMENT OF MYELOID DENDRITIC CELLS THROUGH MAPK P38 INHIBITION PROMOTES T CELL PROLIFERATION AND RESTORES ADAPTIVE IMMUNITY IN GBM PATIENTS

Abstract

AbstractINTRODUCTION: Immunotherapy is an emerging backbone to oncology treatment in a number of cancers, mainly due to the advent of checkpoint blockade therapy. In Glioblastoma (GBM) immunotherapies such as peptide vaccines and laboratory generated monocyte-derived dendritic cells (moDC) are demonstrating promise in advanced stages of clinical trials. Recently however, a number of studies have highlighted the immune advantages circulating DC possess. We previously showed that the potential novel adoptive transfer of isolated circulating myeloid DC (MDC1) may be hampered in GBM patients due to reduced numbers of MDC1 and suppressed production of the key Th1-polarising cytokine IL-12. Importantly, IL-12 production by MDC1 was rescued and enhanced upon exposure to a MAPK p38 inhibitor (p38i). The increase in IL-12 with p38i was unique to MDC1s and in contrast to its effects on moDC. Here we tested the hypothesis that p38i enhances the capacity of MDC1 to promote T-cell proliferation. METHOD: Paramagnetic bead isolation kits were used to isolate circulating MDC1, CD4+ cells or peripheral blood lymphocytes (PBL) from mononuclear cells. A model of GBM:DC interaction was devised in which DC were co-cultured with tumour lysate (TL) from U87 (TL:DC ratio 50:1), or dexamethasone (Dex) 10-6M. The p38i, BIRB0796 1μM, was added to inhibit the MAPK pathway in MDC1s. The impact of GBM/Dex on DC function was determined in a mixed lymphocyte reaction (MLR) with PBL as responders. Further experiments, determined the impact of checkpoint inhibitors (Ipilimumab and Pembrolizumab) on the MLR. The MLR supernatants were analysed for IFN-γ and IL-10. RESULTS: Exposure to TL or Dex markedly inhibited the capacity of MDC1 to stimulate an MLR (p=0.001 and p<0.001 respectively). However, pre-treatment with p38i reversed this increasing proliferation above that of control cells (TL, p=0.001; DEX, p=0.021). Analysis of the supernatants of p38i-treated MDC1 stimulated PBL, showed a prominent increase in the Th1-cytokine IFN-γ. The addition of checkpoint inhibitors to TL or Dex treated MDC1 also restored T-cell proliferation. However, the greatest proliferation was observed with the combination of p38i MDC1s and checkpoint inhibitors. This combination was associated with further increases in IFN-γ secretion. CONCLUSION: MAPK p38 inhibited MDC1s stimulate an adaptive immune response greater than that of healthy MDC1s. Furthermore, this can reverse immunosuppression from Dex and TL on DCs, experienced by GBM patients. Interestingly, the combination of p38i MDC1 with checkpoint blockade inhibitors was most favourable at stimulating a lymphocyte response, postulating an advantageous immunotherapeutic strategy in patients.