MHC class II signaling function is regulated during maturation of plasmacytoid dendritic cells

Dendritic cells (DC) play a central role in the immune response, linking innate and adaptative responses to pathogens. Myeloid DC (MDC) produce interleukin-12 in response to bacterial stimuli, whereas plasmacytoid DC (PDC) produce high levels of type I interferon upon viral infection. Human leukocyte antigen (HLA)-DR engagement has been shown to induce apoptosis in various antigen-presenting cells (APC). We now report the consequences of HLA-DR molecule engagement in human PDC, which had thus far not been studied as a result of the difficulty in isolating such cells. HLA-DR engagement on PDC, obtained using a two-step, immunomagnetic separation, led to recruitment of HLA-DR molecules at the site of engagement in mature but not immature PDC. In contrast, relocalization of protein kinase C (PKC) isoenzymes, indicating PKC activation, was observed at the site of HLA-DR engagement and was accompanied by relocalization of a lipid raft marker, the ganglioside M1 staining, in immature and mature PDC. Similar to MDC, HLA-DR-mediated apoptosis was regulated throughout PDC maturation. Freshly isolated PDC were resistant, whereas CD40 ligand-matured PDC were sensitive to HLA-DR-mediated apoptosis. Neither caspase activation nor PKC activation was required for HLA-DR-mediated apoptosis. However, the intrinsic pathway of apoptosis was implicated as mature PDC underwent mitochondrial depolarization in response to HLA-DR engagement. These data provide further arguments for considering HLA-DR-mediated apoptosis as a conserved mechanism of regulating survival of diverse APC and support the ongoing development of humanized ligands for HLA class II molecules as therapeutic tools for use in lymphoproliferative disease.