Intradermal Injection of a Protein Alone Without Additional Adjuvants Using a Needle-Free Pyro-Drive Jet Injector Induces Potent CD8+ T Cell-Mediated Antitumor Immunity

Vaccines usually contain an adjuvant that activates innate immunity to promote the acquisition of adaptive immunity. Aluminum and lipid nanoparticles have been used for this purpose, but their accumulation or widespread circulation in the body can lead to adverse effects. In contrast, physical adjuvants, which use physical energy to transiently stress tissues, do not persist in exposed tissues or cause lasting adverse effects. Herein, we investigate the effects of intradermal injection of endotoxin-free ovalbumin (OVA) protein alone without additional adjuvants using a needle-free pyro-drive jet injector (PJI) on tumor vaccination efficacy. Intradermal injection of OVA protein alone using PJI significantly increased OVA-specific CD8⁺ T cell expansion in the lymph node, although lymph node swelling was much less than when aluminum hydroxide was used. The injection also induced OVA-specific killing activity and antibody production and showed strong CD8⁺ T cell-dependent prophylactic antitumor effects against transplanted E.G7-OVA tumors. In particular, intradermal injection of the fluorescent OVA protein significantly enhanced its uptake by XCR1⁺ dendritic cells, which have a strong ability to cross-present extracellular proteins in the skin and draining lymph nodes. In addition, the injection increased the expression of HMGB1, one of the potent danger signals whose expression has been reported to increase in response to shear stress. Thus, intradermal injection of OVA protein alone without any additional adjuvants using PJI induces potent CD8⁺ T cell-mediated antitumor immunity by enhancing its uptake into XCR1⁺ dendritic cells, which have a high cross-presentation capacity accompanied by an increased expression of shear stress-induced HMGB1.