XBP1 and systemic lupus erythematosus: In contrast to findings regarding CD11c+T-bet+ B cells in SLE, we did not find evidence that FcRL4+ B cells are differentiating into antibody-secreting cells (ASC), as the required transcription factors (e.g. IRF4, PRDM1, XBP1 [33]) were either downregulated or not detected.