Composition of MHC class II-enriched lipid microdomains is modified during maturation of primary dendritic cells

Abstract: Dendritic cells (DCs) are the most po-tent antigen presenting cells. Major histocompati-bility complex (MHC) class II molecule expression changes with maturation; immature DCs concen-trate MHC class II molecules intracellularly, whereas maturation increases surface expression of MHC class II and costimulatory molecules to optimize antigen presentation. Signal transduction via MHC class II molecules localized in lipid mi-crodomains has been described in B lymphocytes and in the THP-1 monocyte cell line. We have characterized MHC class II molecules throughout human DC maturation with particular attention to their localization in lipid-rich microdomains. Only immature DCs expressed empty MHC class II mol-ecules, and maturation increased the level of pep-tide-bound heterodimers. Ligand binding to sur-face human leukocyte antigen (HLA)-DR induced rapid internalization in immature DCs. The propor-tion of cell-surface detergent-insoluble glycosphingo-lipid-enriched microdomain-clustered HLA-DR was higher in immature DCs despite the higher surface expression of HLA-DR in mature DCs. Constitu-ents of HLA-DR containing microdomains included the src kinase Lyn and the cytoskeletal protein tubulin in immature DCs. Maturation modified the composition of the HLA-DR-containing microdo-mains to include protein kinase C (PKC)-, Lyn, and the cytoskeletal protein actin, accompanied by the loss of tubulin. Signaling via HLA-DR redistrib-uted HLA-DR and-DM and PKC- as well as en-riching the actin content of mature DC microdo-mains. The increased expression of HLA-DR as a result of DC maturation was therefore accompa-nied by modification of the spatial organization of HLA-DR. Such regulation could contribute to the distinct responses induced by ligand binding to MHC class II molecules in immature versus matur