RT Journal Article SR Electronic T1 Intracellular Dynamics and Fate of a Humanized Anti-Interleukin-6 Receptor Monoclonal Antibody, Tocilizumab (TCZ) JF Molecular Pharmacology JO Mol Pharmacol FD American Society for Pharmacology and Experimental Therapeutics SP mol.115.099184 DO 10.1124/mol.115.099184 A1 Keiko Fujimoto A1 Hiroaki Ida A1 Yuko Hirota A1 Masaki Ishigai A1 Jun Amano A1 Yoshitaka Tanaka YR 2015 UL http://molpharm.aspetjournals.org/content/early/2015/07/15/mol.115.099184.abstract AB Tocilizumab (TCZ), a humanized anti-IL-6 receptor (IL-6R) monoclonal antibody, abrogates signal transducer protein gp130-mediated IL-6 signaling by competitively inhibiting the binding of IL-6 to the receptor, and shows clinical efficacy in autoimmune and inflammatory diseases. Despite accumulating evidence for the therapeutic efficacy, the behavior and fate of TCZ at the cellular level remain largely unknown. To address this, we evaluated the endocytosis and intracellular trafficking of IL-6R in HeLa cells. The results of our study provide evidence that IL-6R is constitutively internalized from the cell surface by ligand- or TCZ-binding- and the independent expression of gp130 and is targeted via endosomes without being significantly directed to the recycling pathway to, and degraded in, lysosomes. Furthermore, the cytoplasmic tail of IL-6R is required for constitutive endocytosis of the receptor, which is mediated by the clathrin and adaptor protein (AP)-2 complex. We further demonstrate that a neonatal Fc receptor (FcRn) whose function is to regulate the serum persistence of IgG is confined primarily to early/recycling endosomes and rapidly transits between these compartments and late endosomes/lysosomes without being degraded. Importantly, the expression of FcRn induces the segregation of TCZ from IL-6R, resulting in extensive colocalization of TCZ and FcRn in IL-6R-depleted endosomal compartments. Collectively, our results suggest that FcRn can accelerate the retrieval of the internalized TCZ not only from endosomes, but also from lysosomes. Our findings provide a new insight into the mechanism by which the antibody internalized into cells is rescued from lysosomal degradation and its serum levels are maintained.