XBP1 and rheumatoid arthritis: Given prior publications demonstrating plasma cell infiltrates in the RA synovium38 and migration of plasma cells to inflamed tissue39,40, it is important to note that the subchondral bone marrow/synovium CD19+ B cells do not have a plasma cell phenotype and transcriptome analysis does not demonstrate a typical plasma cell signature, e.g., enrichment for genes like XBP1 or BLIMP1 or pathways, such as the unfolded protein response and intracellular protein transport41.