22–24 Conversely, skin-derived DCs were shown to induce E- and P-selectin ligands that are associated with homing to the skin.24,25 The capacity of DCs to instruct T-cell homing properties is related to their ability to produce active metabolites from tissue-derived factors.

Gut-derived DCs produce retinoic acid, which leads to imprinting of the gut-homing phenotype and suppression of the Opaganib skin-homing phenotype on T cells.26 Similarly, the active form of vitamin D3, 1,25(OH)(2)D(3), which is produced by skin DCs, induces T-cell expression of the skin-selective chemokine receptor CCR10, while inhibiting the expression of gut-homing receptors α4β7 integrin and CCR9.27 Interestingly, recent data also suggest that the DCs are not the starting point but are instructed by local stromal cells.28,29 Albeit the induction of a specific homing phenotype in primed T cells has been occasionally referred to as ‘imprinting’,23 recent data have rather challenged SRT1720 nmr the concept of permanent imprinting and

favour the assumption of flexibility in the expression of homing receptors.25 Hypothetically, organ-specific homing could also be explained by continuing selection or re-induction of a given receptor upon recirculation through selected tissues providing antigen-exposure and organ-specific co-signals.30 Efforts to demonstrate the stability of differentially expressed homing receptors in vivo have been made only recently. The expression of ligands for E/P-selectins that serve medroxyprogesterone as homing receptors for inflamed skin has been shown to persist for at least several weeks in vivo

only on a subfraction of T cells. However, upon repeated stimulation under ligand-inducing conditions (presence of IL-12), the stable fraction was increased, and ex vivo isolated selectin-ligand-positive effector/memory cells turned out to be almost completely stable.31 This shows that imprinting of a stable homing phenotype appears possible, but requires repeated stimulation under permissive conditions, similar to findings for the imprinting of a cytokine memory in T cells.32 The above-mentioned studies on the mucosal homing receptor α4β7 in CD8+ T cells suggested that expression of this receptor is not permanent after initial induction.25 In CD4+ T cells, repeated stimulations in the presence of retinoic acid were found to result in a largely persistent expression of α4β7, and, again, ex vivo isolated α4β7-high memory CD4+ cells remained positive for weeks after adoptive transfer (B. Szilagyi and A. Hamann, unpublished). In contrast, stable expression of the chemokine receptor CCR9, which is also induced on CD8+ cells by retinoic acid and considered to contribute to mucosal homing, was not observed (Mora et al.23 and B. Szilagyi and A. Hamann, unpublished).