The transcription factor MITF is a critical regulator of GPNMB expression in dendritic cells

BACKGROUND: Dendritic cells (DC) are the most potent antigen-presenting cells (APC) with the unique ability to activate naïve T cells and to initiate and maintain primary immune responses. Immunosuppressive and anti-inflammatory stimuli on DC such as the cytokine IL-10 suppress the activity of the transcription factor NF-κB what results in downregulation of costimulatory molecules, MHC and cytokine production. Glycoprotein NMB (GPNMB) is a transmembrane protein, which acts as a coinhibitory molecule strongly inhibiting T cell responses if present on APC. Interestingly, its expression on human monocyte-derived dendritic cells (moDC) is dramatically upregulated upon treatment with IL-10 but also by the BCR-ABL tyrosine kinase inhibitors (TKI) imatinib, nilotinib or dasatinib used for the treatment of chronic myeloid leukemia (CML). However, the molecular mechanisms responsible for GPNMB overexpression are yet unknown. RESULTS: The immunosuppressive cytokine IL-10 and the BCR-ABL TKI imatinib or nilotinib, that were examined here, concordantly inhibit the PI3K/Akt signaling pathway, thereby activating the downstream serine/threonine protein kinase GSK3ß, and subsequently the microphthalmia-associated transcription factor (MITF) that is phosphorylated and translocated into the nucleus. Treatment of moDC with a small molecule inhibitor of MITF activity reduced the expression of GPNMB at the level of mRNA and protein, indicating that GPNMB expression is in fact facilitated by MITF activation. In line with these findings, PI3K/Akt inhibition was found to result in GPNMB overexpression accompanied by reduced stimulatory capacity of moDC in mixed lymphocyte reactions (MLR) with allogeneic T cells that could be restored by addition of the GPNMB T cell ligand syndecan-4 (SD-4). CONCLUSIONS: In summary, imatinib, nilotinib or IL-10 congruently inhibit the PI3K/Akt signaling pathway thereby activating MITF in moDC, resulting in a tolerogenic phenotype. These findings extend current knowledge on the molecular mechanisms balancing activating and inhibitory signals in human DC and may facilitate the targeted manipulation of T cell responses in the context of DC-based immunotherapeutic interventions. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s12964-015-0099-5) contains supplementary material, which is available to authorized users

Identification of a lysosomal peptide transport system induced during dendritic cell development

The delivery of protein fragments to major histocompatibility complex (MHC)-loading compartments of professional antigen-presenting cells is essential in the adaptive immune response against pathogens. Apart from the crucial role of the transporter associated with antigen processing (TAP) for peptide loading of MHC class I molecules in the endoplasmic reticulum, TAP-independent translocation pathways have been proposed but not identified so far. Based on its overlapping substrate specificity with TAP, we herein investigated the ABC transporter ABCB9, also named TAP-like (TAPL). Remarkably, TAPL expression is strongly induced during differentiation of monocytes to dendritic cells and to macrophages. TAPL does not, however, restore MHC class I surface expression in TAP-deficient cells, demonstrating that TAPL alone or in combination with single TAP subunits does not form a functional transport complex required for peptide loading of MHC I in the endoplasmic reticulum. In fact, by using quantitative immunofluorescence and subcellular fractionation, TAPL was detected in the lysosomal compartment co-localizing with the lysosome-associated membrane protein LAMP-2. By in vitro assays, we demonstrate a TAPL-specific translocation of peptides into isolated lysosomes, which strictly requires ATP hydrolysis. These results suggest a mechanism by which antigenic peptides have access to the lysosomal compartment in professional antigen-presenting cells