Fcα receptor cross-linking causes translocation of phosphatidylinositol-dependent protein kinase 1 and protein kinase Bα to MHC class II peptide-loading-like compartments

A20 IIA1.6 B cells cotransfected with Fc alphaR and wild-type gamma -chain (wt-ITAM (immunoreceptor tyrosine-based activation motif)) or Fc alphaR and gamma -chain, in which the wt-ITAM was substituted with the Fc gamma RIIA ITAM (IIA-ITAM), were used to investigate cell signaling events influencing presentation of Fc alphaR-targeted exogenous Ag in the context of MHC class II. wt-ITAM cells presented Fc alphaR-targeted OVA more efficiently than IIA-ITAM transfectants to OVA-specific T cell hybridomas. Phosphatidylinositol 3-kinase (PI 3-kinase) inhibition abrogated Ag presentation, suggesting that Fc alphaR may trigger a PI 3-kinase-dependent signal transduction pathway, and thus phosphatidylinositol-dependent protein kinase (PDK1) and protein kinase B alpha (PKB alpha) activation. Cross-linking Fc alphaR on wt-ITAM or IIA-ITAM cells triggered equivalent PI 3-kinase-dependent activation of PKBa. Furthermore, Fc alphaR cross-linking triggered recruitment of PDKI and serine-phosphorylated PKB alpha to capped cell surface FcaR irrespective of the gamma -chain ITAM. Although FcaR endocytosis was accompanied by translocation of PDK1 and phospho-PKB alpha to Fe alphaR-containing vesicles in both transfectants, this was decreased in IIA-ITAM cells, and a significant proportion of PDK1 and PKBa remained at the plasma membrane. In wt-ITAM cells, PDK1 and serine-phosphorylated PKBa translocated to lysosomal-associated membrane glycoprotein 1- and cathepsin B-containing vesicles, consistent with MHC class II peptide-loading compartments (MIIC) described by other groups. Our data indicate that translocation of signal transduction mediators to MIIC-like compartments accompanies efficient presentation of receptor-targeted Ag, and suggest a mechanism connecting signaling to the Ag-processing pathway.