Error bars show SD

Error bars show SD. To determine whether the impaired stimulation of plasma cell differentiation by pDCs is a result of the reduced IFN secretion, we included an IFNAR blocking antibody in co-cultures of pDCs could be rescued by the addition of exogenous IFN (Fig.?4G,H). Introduction Plasmacytoid dendritic cells (pDCs) were initially discovered in human lymph nodes in the 1950s1. Their morphology proved to be controversial, with a lymphoid appearance that changes to a dendritic cell-like upon activation. The plasma cell-like appearance led to their nomenclature as Plasmacytoid dendritic cells2. The defining feature of pDCs is their ability to produce high levels of Type I Interferon (IFN) in response to viruses and viral components that are recognized by endosomal innate immune receptors such as the Toll-like receptors (TLR) 7 and TLR93,4. Upon stimulation of the TLRs with an agonist, 60% of the pDCs transcriptome is dedicated to the expression and secretion of IFN5, whereby each cell is capable of producing a staggering 1C3 picograms within 24?h5,6. Due to their high IFN production, pDCs play an important role in controlling the initial stage of viral infections. Secreted IFN enhances the activation of NK cells, B and T cells3. Moreover, pDCs have also been found to be a major source of IFN in inflammatory autoimmune diseases such as Systemic Lupus Erythematosus (SLE), psoriasis and Scleroderma7C9. As highly secretory cells, pDCs and plasma cells are the only hematopoietic cell type that depend on an augmented protein-folding ability of the endoplasmic reticulum (ER) for their survival10,11. Accumulation of unfolded proteins in the ER results in an ER stress that can be alleviated by the activation of MI-2 (Menin-MLL inhibitor 2) the unfolded protein response (UPR), which restores cellular proteostasis12. In particular, secretory cells require an enhanced protein-folding capacity of the ER which MI-2 (Menin-MLL inhibitor 2) involves the sensing of ER stress, an increased expression of protein folding chaperones and the expansion of the ER. The UPR consists of three pathways, the inositol-requiring transmembrane kinase/endonuclease (IRE1), the PKR-like ER protein kinase (PERK) and the activating transcription factor 6 (ATF6) pathways12C14. Upon accumulation of misfolded proteins, BiP dissociation leads to the activation of these three transmembrane proteins15. Activation of IRE1 generates an active isoform of the transcription factor XBP1 that can dimerize with the activated isoform of ATF6 which is generated by its translocation to the Golgi and proteolytic cleavage15C19. ATF6 plays a crucial role in increasing levels of the ER chaperones BiP and GRP94 to help cope with the increased folding demands. Moreover, ATF6 and XBP1 regulate ER expansion by increased lipid biosynthesis, whereby ER membrane expansion can alleviate ER stress independent of an accompanying increase in ER chaperone levels20,21. A fine balance is maintained between the different pathways, whereby IRE1 and ATF6 are the protective ones that are activated early upon ER stress and mediate protein folding22. Upon continued unmitigated stress and failure to restore protein homeostasis the PERK pathway plays a more prominent role, leads to the expression of CHOP, which is involved in ER associated protein degradation (ERAD)23,24. The ER resident protein MZB1 (pERP1) is abundantly expressed in innate-like B cells, including Marginal zone (Mz) B cells of the spleen and peritoneal B1 B cells, as well as in antibody-secreting cells25C27. Deletion of resulted in a reduced humoral response with plasma cells secreting lower levels of IgM following in vivo stimulation of B cells28,29. Proteomic analysis of MZB1 interaction partners identified many proteins involved in the UPR, including the WAF1 chaperones BiP, GRP94 (Hsp90B1), the calcium-binding proteins calnexin and calreticulin and the protein disulphide isomerase ERp57 and PDIA625. MZB1 MI-2 (Menin-MLL inhibitor 2) acts as a co-chaperone of GRP94 that is needed for its interaction with various client proteins, including immunoglobulins and integrins28C30. Recent transcriptome analysis of all hematopoietic cell types showed that mRNA is also expressed in the non-B cell lineage.