Langerhans cell homeostasis in mice is dependent on mTORC1 but not mTORC2 function

The PI3K/Akt/mTOR pathway has emerged as a critical regulator of dendritic cell (DC) development and function. The kinase mTOR is found in 2 distinct complexes, mTORC1 and mTORC2. In this study, we show that mTORC1 but not mTORC2 is required for epidermal Langerhans cell (LC) homeostasis. Although the initial seeding of the epidermis with LCs is not affected, the lack of mTORC1 activity in DCs by conditional deletion of Raptor leads to a progressive loss of LCs in the skin of mice. Ablation of mTORC2 function by deletion of Rictor results in a modest reduction of LCs in skin draining lymph nodes. In young mice Raptor-deficient LCs show an increased tendency to leave the skin, leading to a higher frequency of migratory DCs in skin draining lymph nodes, indicating that the loss of LCs results from enhanced migration. LCs lacking Raptor are smaller and display reduced expression of Langerin, E-cadherin, β-catenin, and CCR7 but unchanged levels of MHC-II, ruling out enhanced spontaneous maturation. Ki-67 and annexin V stainings revealed a faster turnover rate and increased apoptosis of Raptor-deficient LCs, which might additionally affect the preservation of the LC network. Taken together our results show that the homeostasis of LCs strictly depends on mTORC1.