Inducible lymphoid clusters, iSALTs, in contact dermatitis: a new concept of acquired cutaneous immune responses.

Antigen presentation to peripheral memory T cells is a key step in the prompt elicitation of acquired immune responses. In the mucosa, specific sentinel lymphoid tissues called mucosa-associated lymphoid tissue serve as antigen presentation sites. Correspondingly, the concept of skin-associated lymphoid tissue (SALT) has been proposed in the 1980s. However, the details of SALT have not been clarified so far. Recently, the live imaging analysis using two photon microscopes are developed. Here, we have identified inducible lymphoid clusters in the skin, we called it inducible SALTs (iSALTs), using a murine contact hypersensitivity model. In the elicitation phase, dendritic cells (DCs) formed clusters and interacted for several hours with effector memory T cells in the dermis. This interaction was essential for proliferation and activation of effector memory T cells in situ in an antigen dependent manner. Interestingly, DC clusters were abrogated by depletion of skin macrophages. Furthermore, IL-1 treatment induced CXCL2 production from macrophages and DC clusters were suppressed with the blockade of IL-1R or CXCR2. Taken together, this sustained conjugation between DCs and memory T cells, iSALTs, is essential for establishment of the effector phase in acquired cutaneous immunity.