IMMU-31. PROTEASOME REARRANGED PEPTIDES ARE A NOVEL SOURCE OF ANTIGENS FOR THE PEPTIDE VACCINE IMMUNOTHERAPY OF GLIOBLASTOMAS

Abstract

Abstract pepVIII, an anticancer vaccine targeting EGFRvIII, has been tested in several trials for glioblastoma including ACT IV. a randomized Phase III trial for newly diagnosed patients. While results from ACT IV were equivocal, available data suggests the elimination of EGFRvIII was not robust. Despite its essential role in antigen presentation, enhancing proteasomal processing is an unexploited strategy for improving vaccines. We examined 20 peptides in silico and experimentally, which showed that a tyrosine substitution (Y6-pepVIII) maximizes proteasome cleavage and survival in a subcutaneous tumor model. In an intracranial glioma model, Y6-pepVIII showed a 62 and 31% improvement in median survival compared to control animals and pepVIII-vaccinated mice. Y6-pepVIII vaccination altered tumor-infiltrating lymphocyte subsets and expression of PD-1 on intratumoral T cells. Combination with anti–PD-1 therapy cured 45% of the Y6-pepVIII–vaccinated mice but was ineffective for pepVIII-treated mice. LC-MS/MS analysis of proteasome-digested pepVIII and Y6-pepVIII revealed that most fragments were similar but more abundant inY6-pepVIII digests and 77% resulted from proteasome-catalyzed peptide splicing (PCPS). We identified 10 peptides that bound human and murine MHC class I. Nine were PCPS products and only one peptide was colinear with EGFRvIII, indicating that PCPS fragments may be a component of MHC class I recognition. Despite not being colinear with EGFRvIII, two of three PCPS products tested were capable of increasing survival when administered independently as vaccines. We hypothesize that the immune response to a vaccine represents the collective contribution from multiple PCPS and linear products. Our work suggests a strategy to increase proteasomal processing of a vaccine that results in an augmented immune response and enhanced survival in mice.