Improving intradermal dosing consistency in mice using a hollow microneedle: A model study with a SARS-CoV-2 mRNA vaccine.

Intradermal vaccination can enhance immune responses, yet reproducible intradermal injection remains technically challenging, particularly for hollow microneedle delivery. Herein, using a mouse model, we aimed to establish a preclinical platform that enables more consistent intradermal vaccine delivery. We quantified age-dependent changes in mouse dorsal skin thickness and developed a pinch-type applicator, a hollow microneedle for mice, and an inserter to stabilize the skin and standardize effective skinfold thickness and insertion depth. Skinfold thickness measured using manual calipers varied with age, whereas cryosection-based measurements of the skin layer thickness from the outer surface to the dermal-subcutaneous adipose tissue interface showed only modest variations, indicating that visible age-related changes largely reflect skinfold structure. Skin-thickness-guided control of the pinching diameter markedly improved intradermal deliverability, and the feasibility of this process was confirmed in male C57BL/6J mice before vaccination. The mice were immunized twice at 2-week intervals with an mRNA-lipid nanoparticle (mRNA-LNP) vaccine encoding SARS-CoV-2 spike, delivered intramuscularly or intradermally at multiple dose levels; anti-receptor-binding domain (RBD) IgG responses measured at a fixed serum dilution and spike-specific cellular responses were evaluated 2 weeks post-boost. Intradermal administration induced measurable anti-RBD IgG responses and showed a tendency toward increased spike-specific cytokine-positive CD8a+ T cells compared with intramuscular dosing at the same dose, although this difference was not statistically significant. Collectively, this platform improved intradermal dosing consistency in mice and supported standard immunogenicity and immune-subset analyses, supporting incorporation of the intradermal route into small-animal proof-of-concept studies for the development of vaccines and therapeutics.