The capacity of dendritic cells (DCs) to migrate from peripheral organs to lymph nodes (LNs) is important in the initiation of a T cell-mediated immune response. The ATP-binding cassette (ABC) transporters P-glycoprotein (P-gp; ABCB1) and the multidrug resistance protein 1 (MRP1; ABCC1) have been shown to play a role in both human and murine DC migration. Here we show that a more recently discovered family member, MRP4 (ABCC4), is expressed on both epidermal and dermal human skin DCs and contributes to the migratory capacity of DCs. Pharmacological inhibition of MRP4 activity or downregulation through RNAi in DCs resulted in reduced migration of DCs from human skin explants and of in vitro generated Langerhans cells. The responsible MRP4 substrate remains to be identified as exogenous addition of MRP4's known substrates prostaglandin E 2 , leukotriene B 4 and D 4 , or cyclic nucleotides (all previously implicated in DC migration) could not restore migration. This notwithstanding, our data show that MRP4 is an important protein, significantly contributing to human DC migration toward the draining lymph nodes, and therefore relevant for the initiation of an immune response and a possible target for immunotherapy.
IntroductionThe ATP-binding cassette (ABC) transporters were initially identified by their roles in clinical multidrug resistance (MDR) against a broad range of functionally and structurally unrelated anticancer agents. 1 Over the past 10 years, it has become apparent that several of the ABC transporters transport not only cytostatic agents but also inflammatory mediators such as platelet activating factor, 2 leukotrienes, 3 or prostaglandins. 4 Unraveling the specific roles of ABC transporters in the functioning of the immune system may therefore reveal new links for future immunotherapeutic approaches.Dendritic cells (DCs) are key initiators of the immune response, sampling their surroundings for foreign antigens. 5 They are also seen as a promising tool for targeted immunotherapeutic strategies. 6 We have shown that dendritic cell differentiation is impaired when the transporter activity of Multidrug Resistance Protein 1 (MRP1; ABCC1) is inhibited. 7 Randolph and coworkers previously reported that both P-glycoprotein (P-gp; ABCB1) and MRP1 are required for optimal DC migration. 8,9 The ABC transporter MRP4 (ABCC4) [10][11][12] is an organic anion transporter and has been described to transport prostaglandins such as PGA 2 , PGE 1 , and PGE 2 . 4 Because PGE 2 is believed to be crucial for DC migration, [13][14][15] we explored the role of this ABC transporter in DC migration. Moreover, a recent study showed that MRP4, like MRP1, can transport leukotrienes (eg, LTB 4 and LTC 4 ). 16 Thus, when expressed on DCs, MRP4 could have contributed to the original observations made for the role of MRP1 in DC migration, 9 because the used antagonist MK-571 can inhibit both transporters. In this manuscript we show that MRP4 is abundantly expressed in epidermal human skin DCs and at lower levels in dermal human ski...