Immunopharmacology of cancer vaccines: Restoration of NKG2D levels and functions in metastatic melanoma treated with exosomes

Abstract

2554 Background: The C-type lectin-like stimulatory immune receptor Natural Killer group 2 receptor (NKG2D) is expressed by NK and CD8+ T cells. NKG2D engagement is a natural mediator of immunosurveillance which can be compromised by locally sustained ligand expression. In cancer patients with NKG2D ligand- expressing tumors, NK and CD8+ T cells often express low levels of NKG2D and are functionally compromised. Dentritic cell-derived-exosomes (Dex) are nanomeric vesicles harboring functional MHC/peptide complexes capable of promoting T cell immune responses and tumor rejection in mice. Two Dex Phase I trials highlighted the and the safety of exosome administration. The observation of clinical regressions in the absence of detectable T cell responses prompted the search for alternate effector mechanisms. We study NKG2D expression and NK cells function in peripheral lymphocytes before and after Dex therapy. Methods: Exosomes were purified from day 7 autologous monocyte derived-DC cultures. Fifteen patients were enrolled and received exosome vaccinations. NK cells functions prior or following exosome vaccines were tested in standard Na251CrO4 chromium release assays. NKG2D and NKG2D ligand expression were studied using flow cytometry and/or western blot experiments. Results: In sharp contrast with DC, Dex bear functional NKG2D ligands leading to a selective downregulation of NKG2D activating receptors on autologous NK cells in vitro. Long term administration of Dex could enhance NKG2D expression levels on NK and CD8+ T cells in 50% of patients, restoring killing of NKG2D ligand expressing K562 and autologous tumour. In contrast to NKG2D ligands shed by tumors mediating immunosuppressive activity, exosomal NKG2D ligands promote NKG2D-dependent effector functions in vivo, that might account for the non MHC restricted-antitumor effects observed in the trial. Conclusions: NK cell activation represent a new pharmacodynamic for Dex leading to tumor recognition and regression in vivo. The Phase II trial using Dex shall include the monitoring of NKG2D ligands on Dex preparation and follow up NKG2D levels, functions and polymorphism in patients. No significant financial relationships to disclose.