Novel Plasma Cell developmental intermediates produce graded expression of IgM secretory transcripts

Differentiation into plasma cells (PC) enables secretion of ~10,000 antibody molecules per second. This extraordinary capacity requires the upregulation of PC transcriptional determinants that increase immunoglobulin mRNA synthesis, coordinate alternative 3’end processing of the heavy chain transcript from the distal to proximal poly-adenylation site (PAS), and remodel the secretory pathway. Based on the difficulty of prospectively identifying secretory cells, we developed a dual-fluorescent protein reporter mouse to study the post-transcriptional-level transition from membrane-anchored to secretory immunoglobulin M (µM PAS and µS PAS, respectively), in single cells. We observed (i) dynamic acquisition of graded µS PAS usage during PC differentiation (ii) IRF4 and Blimp-1 functioned hierarchically to increase µ abundance as well as µS PAS usage and (iii) graded µS populations did or did not express Blimp-1. Interestingly, the low and high µS and Blimp-1-expressing populations arose from distinct developmental intermediates that exhibited dissimilar endoplasmic reticulum features revealing unpredicted complexity of PC differentiation. The novel cell and µS PAS fate trajectories may have implications for discrete specializations of the secretory pathway.