Peptide-based tumor-associated antigen vaccine in glioblastoma: Correlative results of the ETAPA Phase I clinical trial

Immunotherapy has yet to yield benefit in glioblastoma (GBM), in part due to inadequate T-cell priming and insufficient CD4⁺ helper support. To enhance coordinated CD4⁺ and CD8⁺ activation, the ETAPA phase 1 trial (NCT05283109) evaluated P30-EPS, a peptide vaccine linking three class I tumor-associated antigens (CMV pp65, Survivin, EphA2) to the universal class II tetanus toxoid epitope P30 (TVSFWLRVPKVSASHLE). This design aims to broadly recruit CD4⁺ T-cell help and augment cytotoxic responses.

Methods:

Adults with newly diagnosed, MGMT-unmethylated GBM (n = 18) received seven post-radiation intramuscular vaccinations with Hiltonol (20 µg/kg) and either 300 µg (n = 6) or 400 µg (n = 12) of P30-EPS. Peripheral blood mononuclear cells (PBMCs) were analyzed longitudinally by IFN-γ ELISpot, high-parameter spectral flow cytometry, and 5′ scRNA TCR sequencing to quantify antigen-specific cytokine production, helper and cytotoxic phenotypes, lineage distribution, clonal expansion, and repertoire diversity. Paired single-cell V(D)J TCR sequencing of PBMCs was used to resolve CD4⁺ and CD8⁺ subsets, define activation and memory-associated transcriptional programs, and map clonotype-specific trajectories. Baseline and on-treatment tumors are being profiled using spatial transcriptomics (ST) and spatial proteomics (SP) to localize vaccine-responsive T-cell populations within the tumor microenvironment.

Results:

The vaccine was well tolerated in all 18 patients, with all adverse events ≤ grade 2. P30-EPS induced antigen-specific IFN-γ responses in 11 patients, with stronger recall responses at the 400-µg dose. Patients with longer progression-free survival exhibited higher baseline frequencies of naïve CD4⁺ T cells and fewer terminally differentiated effector-memory (CD45RA⁺CCR7^–) cells, indicating a more plastic helper pool capable of sustaining CD8⁺ priming. Longitudinal TCR sequencing demonstrated expansion of vaccine-associated clonotypes after boosting, including emergence of new low-frequency clones and persistence of dominant clones, consistent with durable memory formation. Early single-cell transcriptomic and paired TCR analyses revealed coordinated activation signatures in helper and cytotoxic compartments that paralleled peripheral clonotype expansion. Pre- and post-vaccination tumor ST and SP analyses will be presented.

Conclusion:

ETAPA achieved its primary objectives, demonstrating safety and correlative findings indicating that a universal CD4⁺ helper epitope can enhance cytotoxic and memory responses in GBM. Integration of PBMC scRNA/TCRseq with tumor ST and SP profiling will define intratumoral correlates of systemic vaccine-induced immunity. These insights now support development of our next epitope-linked personalized vaccine for patients with glioblastoma.