www.nature.com/natureimmunology • april 2003 • volume 4 no 4 • nature immunology Neal N. Iwakoshi 1 *,Ann-Hwee Lee 1 *, Prasanth Vallabhajosyula 1 , Kevin L. Otipoby 2 , Klaus Rajewsky 2 and Laurie H. Glimcher 1,3 Published online 3 March 2003; doi:10.1038/ni907 The transcription factor X-box binding protein 1 (XBP-1) is essential for the differentiation of plasma cells and the unfolded protein response (UPR). Here we show that UPR-induced splicing of XBP-1 by the transmembrane endonuclease IRE1 is required to restore production of immunoglobulin in XBP- 1 –/– mouse B cells, providing an integral link between XBP-1, the UPR and plasma cell differentiation. Signals involved in plasma cell differentiation, specifically interleukin-4, control the transcription of XBP-1, whereas its post-transcriptional processing is dependent on synthesis of immunoglobulins during B cell differentiation.We also show that XBP-1 is involved in controlling the production of interleukin-6, a cytokine that is essential for plasma cell survival. Thus, signals upstream and downstream of XBP-1 integrate plasma cell differentiation with the UPR. 1 Department of Immunology and Infectious Diseases, Harvard School of Public Health 651 Huntington Avenue, Boston, Massachusetts 02115-6017, USA. 2 Center for Blood Research, Harvard Medical School, Boston, Massachusetts 02115, USA. 3 Department of Medicine, Harvard Medical School, Boston, Massachusetts 02115, USA. *These authors contributed equally to this work. Correspondence should be addressed to L.H.G. (lglimche@hsph.harvard.edu). Plasma cell differentiation and the unfolded protein response intersect at the transcription factor XBP-1 During a primary humoral immune response, naive B cells interact with cognate antigens in the peripheral lymphoid organs. Coupled with appropriate costimulatory signals, this interaction leads to a complex series of events that ultimately results in the development of both anti- body-secreting plasma cells and antigen-specific memory B cells. Plasma cells are terminally differentiated effector cells that secrete large amounts of immunoglobulin (Ig) proteins. To handle this output, the plasma cell must greatly increase its secretory machinery. This increase can be seen by histological assessment, which shows that most of the cytoplasm of a terminally differentiated B cell comprises closely spaced cisternae of rough endoplasmic reticulum (ER) and secretory granules. The molecular mechanisms used by a plasma cell to accommodate this vast increase in Ig protein and to support terminal differentiation are largely unknown. The unfolded protein response (UPR) was first described in studies that examined the proximal signals responsible for inducing the stress proteins GRP78 (also known as BiP) and GRP94. Overexpression of misfolded proteins in the ER was found to be a primary signal for the increased production of these molecular chaperones 1 . Subsequently it became clear that the UPR exists in all eukaryotes. The molecular basis of this highly coordinated response strongly suggests that it is essential for the folding, processing, export and degradation of all proteins ema- nating from the ER during stressed and normal conditions. Delineation of the signaling pathway involved has been carried out mainly in the budding yeast Saccharomyces cerevisiae 2 . The most prox- imal signal from the lumen of the ER is received by a transmembrane endoribonuclease and kinase called Ire1p 3,4 . By mechanisms that are currently unclear, Ire1p senses the overabundance of unfolded proteins in the lumen of the ER. The oligomerization of this kinase leads to the activation of a C-terminal endoribonuclease by trans-autophosphoryla- tion of its cytoplasmic domains 5,6 . The only known substrate for Ire1p is the transcription factor Hac1. In yeast, HAC1 mRNA is not translat- ed under normal conditions owing to the presence of an intron that can attenuate translation by an unusual base-pairing mechanism 7,8 . On induction of the UPR, the intron is removed by the site-specific endori- bonuclease activity of Ire1p. On religation by the transfer RNA ligase Rlg1p, HAC1 i mRNA is translated into Hac1p, a potent transcriptional activator of UPR genes 9,10 . Studies examining the mammalian UPR have identified a more complex response that has additional components but shares the basic framework found in the yeast stress response. At least two Ire1p homologs in mammalian cells have been described: IRE1α, which is expressed ubiquitously 11 , and IRE1β, which is present only in the gut epithelium 12 . A second ER transmembrane component, protein kinase R–ER-related kinase (PERK), has been identified independently. This ER-localized kinase is a member of the eIF2α family of kinases. Phosphorylation of eIF2α is involved in attenuating translation in response to ER stress 13,14 . A third ER transmembrane component of the mammalian UPR, named ATF6, has been identified using a con- served region of 19 base pairs (bp) of the human GRP78 promoter as bait in a yeast one-hybrid approach 15 . ATF6, a basic leucine zipper (bZIP) transcription factor, is expressed constitutively in its inactive form in the membrane of the ER. Activation in response to ER stress results in proteolytic cleavage of its N-terminal cytoplasmic domain to produce a transcriptional activator that can induce genes involved in the UPR. A RTICLES 321 © 2003 Nature Publishing Group http://www.nature.com/natureimmunology