CD301b+ cDC2 facilitate cytotoxic T lymphocytes activation within inducible skin-associated lymphoid tissue in contact dermatiti

Summary: Abstract Body: Dermal dendritic cells (dDCs) have been found to activate cytotoxic T lymphocytes (CTLs) in inducible skin-associated lymphoid tissue (iSALT) during the elicitation phase of contact hypersensitivity (CHS). To elucidate the molecular mechanism of CTL activation within iSALT, photoconvertible protein (KikGR)-expressing mice induced CHS with 2,4-dinitrofluorobenzene (DNFB) were used to selectively photoconvert cells within iSALT by laser irradiation. Analysis by scRNA-seq revealed that among multiple dDC subsets, CD301b+ cDC2, a subpopulation of cDC2 residing in non-lymphatic barrier tissues including skin, accumulated inside iSALT, while other DC subsets were comparable inside and outside iSALT. To investigate the role of CD301b+ cDC2, CD301b-diphtheria toxin (DT) receptor mice were used to selectively deplete CD301b+ cDC2. DT injection significantly suppressed ear swelling, iSALT formation, and IFN-γ+ CTLs. Traditionally, cDC1 has been considered the primary inducer of CTL activation due to its high cross-presentation ability, whereas the mechanism by which cDC2 activates CTLs has remained elusive. Co-culture of CTLs from DNFB-sensitized mice with CD301b+ cDC2 rapidly induced IFN-γ production after 72 hours of dinitrobenzene sulfonic acid stimulation, which was not suppressed by cross-presentation inhibitors, suggesting that CD301b+ cDC2 presents antigen directly to CTLs without processing. In addition, scRNA-seq analysis identified IL-15 and CCL5 as CTL activation factors derived from CD301b+ cDC2, with IL-15 neutralization suppressing ear swelling and IFN-γ+ CTLs during CHS and CCL5 neutralization attenuating iSALT formation. Finally, immunohistochemical analysis of human allergic contact dermatitis skin showed that CD301b+ cDC2 expressed IL-15 and adhered to CTLs. These results suggest that CD301b+ cDC2 recruits CTLs into iSALT via CCL5 and promotes their activation by IL-15. The novel concept of cross-talk between CD301b+ cDC2 and CTLs will advance the understanding of CTL-mediated skin diseases. Fuuka Minami¹, Ryota Asahina^{1, 2}, Kenji Kabashima¹ 1. Dermatology, Kyoto University Graduate School of Medicine, Kyoto, Japan. 2. Gifu University, Gifu, Japan. Adaptive and Auto-Immunity