NK cell-derived IFNγ mobilizes free fatty acids from adipose tissue to promote early B cell activation during viral infection

The immune system plays a major role in the regulation of adipose tissue homeostasis. Viral infection often drives fat loss, but how and why this happens is unclear. Here, we show that visceral adipose tissue transiently decreases adiposity following viral infection. Upon pathogen encounter, adipose tissue upregulates surface expression of ligands for activating receptors on natural killer cells, which drives IFNγ secretion. This cytokine directly stimulates adipocytes to shift their balance from lipogenesis to lipolysis, which leads to release of lipids in circulation, most notably of free fatty acids. The free fatty acid oleic acid stimulates early-activated B cells by promoting oxidative phosphorylation. Oleic acid promoted expression of co-stimulatory B7 molecules on B cells and promoted their ability to prime CD8⁺ T cells. Inhibiting lipid uptake by activated B cells impaired CD8⁺ T cell responses, causing an increase of viral replication in vivo. Our findings uncover a previously unappreciated mechanism of metabolic adaptation to infection and provide a better understanding of the interactions between immune cells and adipose tissue in response to inflammation.