The Plasma Cell Proteome Initiates Early in B Cell Activation in Advance of Blimp-1.

Abstract Plasma cell (PC) differentiation is a highly regulated process that radically changes the morphology and function of a B lymphocyte to that of a professional protein secretor. It is becoming clear that this process precedes increase in immunoglobulin synthesis rather than reacting to it. We have shown previously that large segments of the PC transcriptome are initiated in an mTORC1-dependent manner prior to both Blimp-1 and Xbp1s expression. Still, the kinetics of induction of the PC proteome remain largely unknown. We performed a whole proteome kinetic analysis of activated Blimp-1-reporter follicular B cells in PC-inductive conditions every 12 hours for three days and observed distinct patterns in induction of the PC proteome. To elucidate transcriptional networks, we performed single cell RNA-seq with the same conditions and correlated transcription factor expression with genes coding for these proteome modules within single cells. We found distinct subgroups within proteome modules corresponding to transcriptional patterns each with a highly correlated transcription factor set. These included robust, transient, Myc-associated factors required for translation initiation; rapid, sustained expression of protein synthesis machinery; and gradual, sustained increases in proteins associated with ER function and secretion. Each of these preceded the induction of Blimp-1 and increase in immunoglobulin protein. Finally, using inducible deletion of Blimp-1 in conjunction with the reporter allele we show that these unique transcriptional and proteomic programs are initiated independently of Blimp-1 expression. Thus, activated B cells prepare for protein secretion by initiating the PC proteome prior to antibody secretion. Supported by grants from the NIH: R01 AI113545, AI139123, AI154932, R21 AI133998 (PI:Allman) and BTG was supported by training grant T32 CA009140 (PI:Pear)