LAUSR

Background: Preclinical models suggest that MAPK pathway is implicated in the immune-resistance of tumors and MEK-inhibition can increase the CD8+ T-cell infiltration and the efficacy of PD-1/PD-L1 blockade. Methods: First, we evaluated PD-L1 mRNA expression by Real Time qPCR and its protein production, togheter with MAPK proteins in a panel of non-small cell lung cancer (NSCLC) cell lines. Then, we studied the changes in PD-L1 and major histocompatibility complex class-I (MHC-I) expression and cytokines’ production, after inhibition with selumetinib or stimulation of MAPK signalling by phorbol 12-myristate 13-acetate (PMA). In addition, we explored the effect of MEK inhibition on T-cell function by using Peripheral blood mononuclear cells (PBMC) from healthy volunteers. Results: A consistent correlation between PD-L1 mRNA and protein expression across cell lines suggested that expression mainly depends on trascriptional regulation, and it is regulated by MAPK signal, through the bindng of p65 to the PD-L1 promoter. Moreover, MEK inhibition resulted in an increased expression of MHC-I on cancer cells and increased mRNA expression levels of IFN gamma, IL-6, IL-1B, and TNFalpha, all molecules involved in the activation and differentiation of TCD8+ cytotoxic lymphocytes (CTL) subset. In this scenario, we also tested the effect of MEK inhibitor on activated T-lymphocytes from PBMC of healthy volunteers. After five days of treatment, RT-qPCR analysis revealed a significant increase of mRNA expression of some typical CD8+ T cell pro-inflammatory cytokines, like IL-12, TNFalpha and IFNgamma. Conclusions: These results further support the idea that MEK inhibitor reduces PD-L1 expression and this allows the establishment of a pro inflammatory microenvironment. On the other side, pheripheral T cells, treated with selumetinib, produce pro inflammatory cytokines typical of CTL subset, that seems more involved in immune response against cancer. In this context, MEK inhibition may represent a potential mechanism to convert otherwise resistant cancers and suggest new potential treatment combination strategies of MEK-inhibitors with anti-PD-L1 antibodies in NSCLC. Legal entity responsible for the study: University of Campania “L. Vanvitelli” Funding: Has not received any funding Disclosure: All authors have declared no conflicts of interest.