Fractalkine-Coated Nanoparticles Reprogram Killer T Cells In Vivo

Cytotoxic CD8+ T cells are central to immune defense against cancer and chronic infections, yet precisely modifying them inside a living organism — without removing, editing, and reinfusing them — has remained an unsolved challenge. This study introduces a targeted mRNA-LNP platform that uses fractalkine (CX3CL1), the natural ligand for the CX3CR1 receptor highly expressed on effector CD8+ T cells, to guide nanoparticles specifically to this immune subset in vivo.

The research team conjugated fractalkine to the surface of mRNA-loaded lipid nanoparticles and tested delivery specificity across cell culture systems, mouse models, and non-human primates (rhesus macaques). CX3CR1 expression was confirmed to be preferentially enriched on cytotoxic effector T cells compared to naive, memory, or regulatory T cell subsets, providing the molecular basis for selective targeting.

In mouse experiments, intravenous administration of fractalkine-LNPs achieved targeting of up to 90% of CX3CR1+ Teff cells in both blood and spleen. When loaded with IL-2-encoding mRNA, the nanoparticles prompted the targeted T cells to secrete exogenous IL-2, demonstrating functional protein production from delivered mRNA cargo. This is significant because IL-2 is a critical T cell survival and proliferation signal, and localized IL-2 production by Teff cells themselves could amplify immune responses without systemic toxicity.

In rhesus macaques — a model much closer to human immunology — fractalkine-conjugated mRNA-LNPs targeted approximately 100% of peripheral blood Teff cells. Delivery of mRNA encoding CD62L (L-selectin), a homing receptor not normally expressed on effector T cells, resulted in transient but clear CD62L surface expression, confirming successful mRNA translation in vivo in a primate system. The transient nature of expression is consistent with mRNA's inherent degradability, which is a safety feature for therapeutic applications.

The platform's key innovation is using a natural receptor-ligand interaction rather than synthetic antibodies or engineered peptides for targeting, which may improve tolerability and scalability. The findings collectively establish proof-of-concept for cell-type-specific in vivo mRNA delivery to T cells, with implications for cancer immunotherapy, antiviral strategies, and potentially autoimmune modulation.