Birbeck granules are unusual rod-shaped structures specific to epidermal Langerhans cells, whose origin and function remain undetermined. We investigated the intracellular location and fate of Langerin, a protein implicated in Birbeck granule biogenesis, in human epidermal Langerhans cells. In the steady state, Langerin is predominantly found in the endosomal recycling compartment and in Birbeck granules. Langerin internalizes by classical receptor-mediated endocytosis and the first Birbeck granules accessible to endocytosed Langerin are those connected to recycling endosomes in the pericentriolar area, where Langerin accumulates. Drug-induced inhibition of endocytosis results in the appearance of abundant open-ended Birbeck granule-like structures appended to the plasma membrane, whereas inhibition of recycling induces Birbeck granules to merge with a tubular endosomal network. In mature Langerhans cells, Langerin traffic is abolished and the loss of internal Langerin is associated with a concomitant depletion of Birbeck granules. Our results demonstrate an exchange of Langerin between early endosomal compartments and the plasma membrane, with dynamic retention in the endosomal recycling compartment. They show that Birbeck granules are not endocytotic structures, rather they are subdomains of the endosomal recycling compartment that form where Langerin accumulates. Finally, our results implicate ADP-ribosylation factor proteins in Langerin trafficking and the exchange between Birbeck granules and other endosomal membranes. INTRODUCTIONLangerhans cells (LCs), the representatives of the dendritic cell lineage in the epidermis and mucosal tissues, capture antigen in the skin before migrating to the T-cell-dependent areas of draining lymph nodes. During this migration, they undergo a maturation process that allows them to present antigens to naive T cells (Kripke et al., 1990;Moll et al., 1993). Notably, Langerhans cells are the only epidermal cells to constitutively express major histocompatibility complex class II molecules (Klareskog et al., 1977;Rowden et al., 1977), CD1a molecules (Fithian et al., 1981), and Langerin (Valladeau et al., 2000) at their cell surface. In addition, Langerhans cells differ ultrastructurally from other dendritic cells through the presence of Birbeck granules (BGs), distinctive rod-shaped structures of variable length with a central, periodically striated lamella (Birbeck et al., 1961).Despite the use of "dynamic" electron microscope studies (reviewed in Schuler et al., 1991), Birbeck granules remain enigmatic. Specifically, conflicting theories exist regarding Article published online ahead of print. Mol. Biol. Cell 10.1091/ mbc.01-06-0300. Article and publication date are at www.molbiolcell.org/cgi/doi/10.1091/mbc.01-06-0300.† These authors contributed equally to this work and are listed in alphabetical order. # Corresponding author. E-mail address: daniel.hanau@efs-alsace.fr. Abbreviations used: Au, gold-labeled; BG, Birbeck granule; ERC, endosomal recycling compartment; Fi, freshl...

Previous work has suggested that the peptide‐carrier, heat‐shock protein (hsp)70, could directly activate APC. Here we show that this ability is related to endotoxin contamination of the humanrhsp70 produced in Escherichia coli. Hence, the ability of 1–3 μg/ml of rhsp70 to induce the maturation of human monocyte‐derived DC is abrogated in the presence of the LPS‐antagonist polymyxin B or when the rhsp70 contains less than 60 IU/mg endotoxin. Such a level of contamination of the rhsp70 is, however, sufficient — in the presence of soluble rCD14, the LPS co‐receptor — toinduce cytokine secretion from monocytes and DC, despite the presence of polymyxin B. However, when endotoxin contamination is below 10 IU/mg, rhsp70 does not induce cytokine secretion — even in the presence of soluble rCD14 — or activate p38 mitogen‐activated protein kinase signaling pathways, thus showing that an "endotoxin free" hsp70 does not activate APC.

Priming of CTL by means of heat shock proteins (hsp) is dependent on antigen‐presenting cells (APC), which present the hsp‐associated peptides, via their cell surface MHC class I molecules, toCD8+ T cells. It has not yet been established how human (hu) hsp70 interacts with the major (hu)APC, the dendritic cells (DC). Here we show that (hu)hsp70 is specifically internalized intoCD14–, Toll‐like receptor 4– monocyte‐derived (hu)DC by receptor‐mediated endocytosis. We further demonstrate that (hu)hsp70 and (hu)hsp60 share the same receptors on (hu)monocyte‐derived DC. Both molecules as well as MHC class I molecules are spontaneously internalized and reach the MHC class II‐enriched compartments. Finally, freshly isolated (hu) epidermal Langerhans cells (LC), the DC of the skin, as well as CD34+‐derived LC do not bind hsp60 or hsp70. Given the likely importance of the internalization of hsp70 by APC in the induction of the immune responses, the finding that hsp60 and hsp70 are internalized through the same receptor(s) may explain why microbial hsp60 represents a major T cell antigen. This may rationalize the use of microbial hsp60 to primeimmune responses against microbes. The lack of hsp60/70 receptors on epidermal LC raises the crucial question as to whether absence of priming of the skin and mucosal immune systems by hsp‐polypeptide complexes could account for some tissue‐specific diseases. This work also points to a potential advantage of using monocyte‐derived DC in human immunotherapeutic applications of hsp60/70.