(827) “Next-Generation Mpox Vaccine: Enhancing MVA-Based Immunity Through PD-1 Checkpoint Modulation "​

Introduction/Rationale: The 2022 Mpox outbreak exposed gaps in orthopoxvirus vaccine durability, as the licensed two-dose Modified Vaccinia Ankara (MVA) regimen provides only moderate and waning protection. Durable antiviral immunity requires coordinated humoral and CD8⁺ T-cell responses, with CD8⁺ T cells playing an important role in controlling infected cells at sites of viral replication. However, current vaccine strategies incompletely sustain these cellular responses. To address this limitation, MVA-X was engineered as an MVA-based vaccine in which a PD-1 antagonist peptide is encoded directly within the viral genome, enabling localized checkpoint modulation during antigen presentation.

Methods: Age- and sex-matched C57BL/6 mice received formulation buffer, single-dose MVA, two-dose MVA, or single-dose MVA-X. Mice were challenged with lethal vaccinia virus (VACV-WR) at days 55, 90, or 150 post-prime. Vaccine efficacy and immune responses were evaluated by survival, lung viral titers and genome burden, lung histopathology and immunohistochemistry, serum neutralization and binding antibody assays, and virus-specific CD8⁺ T-cell responses.

Results: A single dose of MVA-X conferred complete survival at all challenge time points, matching or exceeding two doses of MVA protection. At day 90 post-challenge, MVA-X vaccination resulted in >97% reductions in lung viral titers and genome copies, preservation of alveolar architecture, and localized peribronchiolar immune infiltrates, consistent with efficient tissue-level viral control. Despite declining binding antibody responses, animals remained protected, consistent with sustained virus-specific CD8⁺ T-cell responses detectable through day 150 post-prime.

Conclusion: These findings show that MVA-X supports durable cellular immune responses following a single immunization. This vaccine-intrinsic checkpoint modulation strategy offers a modular framework with broader applicability for enhancing cellular immunity in viral vaccine development.