De novo pyrimidine synthesis controls lupus-prone germinal center B cell and plasma cell fates

Introduction/Rationale: The role of pyrimidine metabolites in SLE-prone germinal center (GC) and plasma cell (PC) responses is unknown.

Methods: A combination of steady-state metabolomics analysis and isotopic 15N-glutamine tracing were used to examine metabolic flux through de novo pyrimidine synthesis pathway in SLE-prone B cells. To examine role of the de novo pyrimidine synthesis pathway in the SLE autoimmune phenotype, global inhibition in SLE-prone mice was tested first, then UMPS conditional knockout SLE-prone mice were generated and phenotyped. Steady-state metabolomics, mitochondrial stress test, and metabolic flow cytometry were employed to elucidate a role for de novo pyrimidine synthesis in regulating metabolic pathways.

Results: Metabolomics and 15N-amide glutamine tracing reveal enhanced flux through pyrimidine synthesis in SLE-prone B cells. Global inhibition of de novo pyrimidine synthesis in SLE-prone mice dampened GC, PC and autoantibody responses, and kidney immune complex (IC) deposition. This inhibition did not overtly affect foreign antigen-driven GC, PC, and antibody responses. Newly generated UMPS conditional knockout SLE-prone mice revealed a B cell-intrinsic requirement of de novo pyrimidine synthesis in spontaneous GC, PC and autoantibody responses, and kidney IC deposition. Metabolomics, mitochondrial stress-test and metabolic flow cytometry helped delineate a crucial role for pyrimidine synthesis in B cell metabolic reprogramming that promotes autoimmunity. Mechanistically, mTORC1 and S6K1 downstream of TLR7 and CD40 signaling in B cells activate CAD, which catalyzes the first three steps of pyrimidine synthesis and upregulates de novo pyrimidine synthesis.

Conclusion: We identify a previously unknown B cell-intrinsic requirement of the pyrimidine pathway in autoimmune GC and PC responses, development of autoreactive B cells, and autoantibodies in SLE.