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Receptor Cross-Linking Causes Translocation of Phosphatidylinositol-Dependent Protein Kinase 1 and Protein Kinase B to MHC Class II Peptide-Loading-Like Compartments1
Department of Microbiology, Dartmouth Medical School, Lebanon, NH 03756
A20 IIA1.6 B cells cotransfected with Fc
R and wild-type
-chain (wt-ITAM (immunoreceptor tyrosine-based activation motif)) or
FcR and -chain, in which the wt-ITAM was substituted with the
FcRIIA ITAM (IIA-ITAM), were used to investigate cell signaling
events influencing presentation of FcR-targeted exogenous Ag in the
context of MHC class II. wt-ITAM cells presented FcR-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 FcR may trigger a PI
3-kinase-dependent signal transduction pathway, and thus
phosphatidylinositol-dependent protein kinase (PDK1) and protein kinase
B (PKB) activation. Cross-linking FcR on wt-ITAM or IIA-ITAM
cells triggered equivalent PI 3-kinase-dependent activation of PKB.
Furthermore, FcR cross-linking triggered recruitment of PDK1 and
serine-phosphorylated PKB to capped cell surface FcR irrespective
of the -chain ITAM. Although FcR endocytosis was accompanied by
translocation of PDK1 and phospho-PKB to FcR-containing vesicles
in both transfectants, this was decreased in IIA-ITAM cells, and a
significant proportion of PDK1 and PKB remained at the plasma
membrane. In wt-ITAM cells, PDK1 and serine-phosphorylated PKB
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.
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