Taconic Biosciences at the World Vaccine Congress

Reliable preclinical modeling of human mRNA vaccine responses remains a major bottleneck in vaccine development. Widely used humanized mouse platforms often fail to generate robust, MHC-restricted, antigen-specific T cell responses due to incomplete human thymic education. Cord blood-only CD34+ models rely on murine thymic selection, resulting in non-physiologic T cell repertoires, while BLT models employ human fetal tissue that can bias toward immature or regulatory-skewed phenotypes in addition to having ethical roadblocks. These limitations reduce confidence in translational immune readouts for industry vaccine pipelines. We developed and optimized the NeoThy™ humanized mouse model, a next-generation platform incorporating neonatal human thymus tissue and cord blood hematopoietic stem cells (HSCs). In this project, we assess its ability to mount functional responses to lipid nanoparticle (LNP)-encapsulated SARS-CoV-2 spike mRNA vaccination. NeoThy™ mice were generated in the NOG-IL6 background (Taconic), alongside matched cord blood-only controls using identical human donor HSC sources to eliminate genetic variability. Vaccination strategy mirrored human clinical regimens, including prime-boost scheduling and dose optimization. Immune chimerism and phenotypic profiling demonstrated comparable overall immune cell frequencies between NeoThy™ and cord-only cohorts, however, simple flow-based QC metrics alone are insufficient predictors of functional competence. Similarly, bulk antibody quantities and class switching remained modest across both platforms, consistent with historical limitations in humanized mouse humoral responses. These findings reinforce the need for deeper functional evaluation beyond standard phenotyping. Single-cell RNA sequencing of CD3+ T cells following vaccination revealed clear functional divergence between platforms. UMAP clustering demonstrated transcriptomic separation driven by thymic incorporation (i.e., NeoThy™ model). Vaccinated NeoThy™ mice upregulated antiviral and regulatory genes, including IFITM2, ZFP36, NFKBIA, and ANXA1, consistent with coordinated and moderated immune activation. In contrast, cord-only vaccinated mice exhibited upregulation of genes associated with cytotoxicity and dysfunctional or incompletely matured T cell states, including TOX, GZMK, CD74, and PECAM1. Importantly, TCR sequencing demonstrated increased clonal diversity in vaccinated NeoThy™ animals relative to cord-only counterparts, approaching patterns observed in human controls. Hill diversity analysis confirmed broader and less oligoclonal repertoires, suggesting improved adaptive immune modeling in the presence of human thymic education. Collectively, these data indicate that while antibody quantity alone may not distinguish platforms, functional T cell transcriptomic signatures and TCR diversity metrics reveal meaningful advantages of thymus-supported immune development in the NeoThy™ model. For vaccine developers, these findings suggest that NeoThy™ engraftment provides a more physiologically relevant environment for assessing mRNA vaccine-induced T cell responses, immune repertoire breadth, and potential correlates of protection. Ongoing work aims to expand sample size and integrate antigen-specific functional assays to further validate this system as a scalable translational bridge between rodent and early-phase clinical studies.