CD4⁺ skin resident memory T cells preferentially colocalize with dermal Folr2^hi macrophages in contact hypersensitivity

In contact hypersensitivity (CHS), local immune memory is established in previously affected skin through the formation of CD4⁺ and CD8⁺ tissue-resident memory T (T_RM) cells. This memory contributes to disease recurrence by enhancing local antigen responsiveness and is maintained in the long term by T_RM cells, particularly CD4⁺ T_RM cells. However, the mechanisms underlying the maintenance and reactivation of CD4⁺ T_RM cells remain unclear. We herein examined the cellular niches persistently interacting with CD4⁺ T cells in naïve and CHS-healed mouse ear skin. Most CD4⁺ T cells were scattered in the dermis and colocalized with Folr2^hi macrophages, a previously unrecognized skin macrophage population, suggesting a physical interaction. In contrast, fewer than 20% of CD4⁺ T cells colocalized with dendritic cells (DCs) or other cell lineages. The administration of an anti-colony stimulating factor 1 receptor (CSF1R) antibody depleted nearly all Folr2^hi macrophages and several other myeloid cells, while the maintenance and reactivation of CD4⁺ T cells as well as other αβ T cells in healed skin remained unaffected. Moreover, in macrophage-depleted healed skin, CD4⁺ T cells did not establish new interactions with remaining antigen-presenting cells, and their contact rate with DCs remained unchanged. These results indicate that local immune memory in CHS-experienced skin is maintained and functions independently of CSF1R-dependent myeloid cells, including Folr2^hi macrophages, despite their predominant colocalization with skin CD4⁺ T_RM cells.