A proteomic map of B cell activation and its shaping by mTORC1, MYC and iron

Using high resolution quantitative mass spectrometry, we have explored how immune activation and the metabolic checkpoint kinase mTORC1 (mammalian target of rapamycin complex 1) regulate the proteome of B lymphocytes. B cell activation via the B cell receptor, CD40 and the IL-4 receptor induced considerable re-modelling of the B cell protein landscape, with a 5-fold increase in total cellular protein mass within 24 hours of activation. Analysis of copy numbers per cell of >7,500 proteins revealed increases in the metabolic machinery that supports B cell activation and nutrient and amino acid transporters that fuel B cell biosynthetic capacity. We reveal that mTORC1 controls activation-induced cell growth and B cell proteome remodelling and inhibiting mTORC1 impairs the expression of amino acid transporters that fuel B cell protein production. We also show that mTORC1 activity regulates the expression of the transcription factor MYC and the transferrin receptor CD71. Blocking MYC activity phenocopied mTORC1 inhibition in many ways including impaired CD71 expression, while limiting iron availability during B cell activation impaired B cell growth and protein synthesis. This work provides a detailed map of naive and immune activated B cell proteomes and a greater understanding of the cellular machinery that direct B cell phenotypes. This work also provides new insights into the role of mTORC1, MYC and iron in regulating activation-induced proteome remodelling and protein production in B cells.