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The effective trafficking of dendritic cells (DCs) to the lymph nodes (LNs), orchestrated by CC‐chemokine receptor 7 (CCR7) and its ligand CCL21, is essential for the success of DC–based immunotherapies. This study explores the potential of C21TP, a naturally occurring basic peptide derived from the C‐terminal of CCL21, to enhance DC homing to the draining LNs in a murine model of DC migration. C21TP, containing three clusters of basic residues, significantly boosts CCL21‐mediated signaling and chemotaxis of DCs in vitro. In vivo, DCs formulated with C21TP prior to injection migrated more efficiently to the draining LNs than DCs alone or DCs formulated with a mutated version of C21TP, harboring substitutions in key basic residues. Further studies are needed to evaluate the impact of C21TP on T‐cell priming efficacy in the context of DC–based immunotherapies. Nonetheless, C21TP’s ability to enhance lymph node homing of adoptively transferred cells without additional cellular modifications could offer a practical and scalable approach for advancing future DC–based vaccines.

Personalized cancer vaccines (PCVs) largely leverage neoantigens arising from somatic mutations, limiting their application to patients with relatively high tumor mutational burden (TMB). This underscores the need for alternative antigens to design PCVs for low TMB cancers. To this end, we substantiate endogenous retroviral elements (EVEs) as tumor antigens through large-scale genomic analyses of healthy tissues and solid cancers. These analyses revealed that the breadth of EVE expression in tumors stratify checkpoint inhibitor-treated melanoma patients into groups with differential overall and progression-free survival. To enable the design of PCVs containing EVE-derived epitopes with therapeutic potential, we developed a computational pipeline, ObsERV. We show that EVE-derived peptides are presented as epitopes on tumors and can be predicted by ObsERV. Preclinical testing of ObsERV demonstrates induction of sustained poly-functional CD4+ and CD8+ T-cell responses as well as long-term tumor protection. As such, EVEs may facilitate and improve PCVs, especially for low-TMB patients.

Pharmacodynamic assessment of T-cell-based cancer immunotherapies often focus on detecting rare circulating T-cell populations. The therapy-induced immune cells in blood-derived clinical samples are often present in very low frequencies and with the currently available T-cell analytical assays, amplification of the cells of interest prior to analysis is often required. Current approaches aiming to enrich antigen-specific T cells from human Peripheral Blood Mononuclear Cells (PBMCs) depend on in vitro culturing in presence of their cognate peptides and cytokines. In the present work, we improved a standard, publicly available protocol for T-cell immune analyses based on the in vitro expansion of T cells. We used PBMCs from healthy subjects and well-described viral antigens as a model system for optimizing the experimental procedures and conditions. Using the standard protocol, we first demonstrated significant enrichment of antigen-specific T cells, even when their starting frequency ex vivo was low. Importantly, this amplification occurred with high specificity, with no or neglectable enrichment of irrelevant T-cell clones being observed in the cultures. Testing of modified culturing timelines suggested that the protocol can be adjusted accordingly to allow for greater cell yield with strong preservation of the functionality of antigen-specific T cells. Overall, our work has led to the refinement of a standard protocol for in vitro stimulation of antigen-specific T cells and highlighted its reliability and reproducibility. We envision that the optimized protocol could be applied for longitudinal monitoring of rare blood-circulating T cells in scenarios with limited sample material.