Research ArticleBRAIN TUMOR

Enhancing proteasomal processing improves survival for a peptide vaccine used to treat glioblastoma

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Science Translational Medicine  16 Jun 2021:
Vol. 13, Issue 598, eaax4100
DOI: 10.1126/scitranslmed.aax4100

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The value of optimization

Glioblastoma (GBM) is one of the most malignant types of tumors of the central nervous system with an average survival time of only 12 to 18 months. The synthetic peptide vaccine pepVIII, targeting EGFRvIII, has shown promising results in patients. Here, Fidanza et al. developed an optimization strategy and characterized several peptides derived from pepVIII in GBM murine models. The authors found that a tyrosine substitution (Y6-pepVIII) improved the efficacy of the vaccine in mice by increasing proteasome cleavage. Y6-pepVIII in combination with anti–PD-1 therapy increased survival compared to the checkpoint blockade therapy alone. The results suggest that similar optimization strategy could be effective in improving treatment efficacy of other peptide vaccines.

Abstract

Despite its essential role in antigen presentation, enhancing proteasomal processing is an unexploited strategy for improving vaccines. pepVIII, an anticancer vaccine targeting EGFRvIII, has been tested in several trials for glioblastoma. 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 mice. 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. Liquid chromatography–tandem mass spectrometry analysis of proteasome-digested pepVIII and Y6-pepVIII revealed that most fragments were similar but more abundant in Y6-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.

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