Graduate Student Yale University New Haven, Connecticut, United States
Disclosure(s):
Ruifeng Sun, MPhil: No financial relationships to disclose
Introduction/Rationale: Germinal center (GC) B cells and plasma cells strongly upregulate gene expression of SLC7A5, which is an amino acid antiporter that simultaneously mediates leucine uptake and glutamine export. While the importance of leucine uptake is established, the significance of glutamine export and its role in humoral immunity is unknown.
Methods: B-cell-specific Slc7a5-KO mice
Results: SLC7A5 was dispensable for B cell development. However, Slc7a5-/- GC-B cells failed to undergo IgG1 class switch and somatic hypermutation upon immunization. Moreover, Slc7a5-/- GC-B cells showed lower dark/light zone (DZ/LZ) cell ratio, cell cycle arrest in DZ, reduced AID expression, and impaired plasma cell differentiation and antibody production. In collagen induced arthritis model, Slc7a5 deletion decreased disease severity and reduced serum levels of anti-collagen IgG2a, IgG3, IgM, as well as IL6. To test whether impaired humoral immunity was caused by defective glutamine export, we created an alternative glutamine disposal route using 4-phenylbutyric acid (4-PBA). 4-PBA treatment significantly rescued IgG1 class switch of Slc7a5-/- cells ex vivo, supporting the critical role of glutamine export in GC-B cells. Metabolomics and isotope tracing revealed that Slc7a5-/- B-cells redirected glutamine from glutaminolysis to nucleotide salvage, prioritizing its role as nitrogen donor over carbon donor. We thus hypothesized that defective glutamine export led to cell defects by nitrogen (ammonia) accumulation. While only hepatic cells can use the urea cycle to detoxify ammonia, non-hepatic cells are known to transport ammonia to liver in the form of glutamine. To test if SLC7A5-mediated glutamine export indeed enables ammonia detoxification, we introduced the urea cycle enzyme CPS1 into Slc7a5-/- cells, which restored cell fitness, confirming the key role of SLC7A5-mediated glutamine export in ammonia detoxification.
