Mevalonate kinase deficiency (MKD) is a rare, inborn error of metabolism caused by biallelic loss-of-function mutations in mevalonate kinase (MVK). This leads to a clinical spectrum of autoinflammatory disease often triggered by infection. Mevalonate kinase is required for the synthesis of isoprenoid lipids necessary for post-translational prenylation of proteins, including Rho and Rab GTPases necessary for the formation and trafficking of cytolytic granules. We therefore examined whether granule trafficking and cytotoxicity are altered in NK cells from mouse models of MKD and patients.

Using an in vitro prenylation assay we confirmed that the prenylation of Rab GTPases was profoundly deficient in NK cells in Mvk mutant mice. Furthermore, Mvk mutant NK cells had higher side scatter (SSC/granularity) with a corresponding significant increase in granule number per cell. NK cells from Mvk mutant mice and from two MKD patients showed a ~40% reduction in target cell killing. To examine how cytotoxicity was altered, we used a dual-pipette aspiration assay and high-resolution live cell microscopy. As expected, in NK cells from control mice and healthy volunteers, cytolytic granules (stained with Lysotracker) consistently polarised to the immunological synapse with target cells. In contrast, granules failed to migrate to the interaction interface in NK cells from Mvk mutant mice and MKD patients, despite the formation of a microtubule organising centre. Furthermore, NK activation with PMA/ionomycin or co-culture with target cells also led to a higher proportion of IFNγ⁺ NK cells and significantly elevated IFNγ release.

We provide the first demonstration that MKD is associated with a defect in cytolytic granule trafficking in NK cells, likely caused by lack of prenylation of small GTPases. We propose that inflammatory flares in MKD may be largely the result of defective NK cytotoxicity following viral infection which, together with increased IFNγ production, results in excessive macrophage activation.