The generation of CTLs is crucial in the immunological fight against cancer and many infectious diseases. To achieve this, vaccine Ags need to be targeted to the cytosol of dendritic cells, which can activate CD8 T cells via MHC class I (MHCI). Therefore, such targeting has become one of the major objectives of vaccine research. In this study, we aimed to bypass the unwanted and default MHC class II Ag presentation and trigger MHCI presentation by using a photosensitizer that, upon light activation, would facilitate cytosolic targeting of codelivered Ag. Poly(lactide-co-glycolide) microparticles ∼1 μm size were loaded with OVA and the photosensitizer tetraphenyl chlorine disulphonate (TPCS2a) and administered intradermally in mice, which were illuminated 1 d later for activation of the photosensitizer. Immunization in the presence of TPCS2a significantly increased activation of CD8 T cells compared with immunization without TPCS2a and as measured by CD8 T cell proliferation, production of proinflammatory IFN-γ, TNF-α, and IL-2, and prevention of tumor growth. Cytotoxicity was demonstrated by granzyme B production in vitro and by in vivo killing of CFSE-labeled targets. CD4-dependent Ab responses were abrogated in mice immunized with TPCS2a-containing particles, suggesting that photosensitization facilitated a shift from default MHC class II toward MHCI Ag presentation. Hence, vaccine particles with Ag and photosensitizers proved an effective vehicle or adjuvant for stimulation of CTLs, and they may find potential application in therapeutic cancer vaccination and in prophylactic and therapeutic vaccination against intracellular infections.
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