The proteome of B cell maturation identifies PDCD4 as limiting the response of marginal zone B cells

During B cell maturation, transitional and mature B cells acquire cell-intrinsic features that determine their ability to exit quiescence and mount effective immune responses. We used high-resolution mass spectrometry to quantify the proteome of B cell subsets from the mouse spleen and map the differential expression of environmental sensing, transcription- and translation initiation-factors that define cellular identity and function. By comparing transcriptome and proteome, we identified mRNAs linked to B cell activation and differentiation that are expressed without detectable protein. These "poised" mRNAs might enable rapid protein production through increased translation or protein stability. In addition, we found that the translational repressor PDCD4 restrains the response of marginal zone B cells to a T-independent antigen. Our molecular characterization of B cell maturation is a valuable resource to further explore the mechanisms underpinning the specialised functions of B cell subsets.