IFT20 regulates TFEB-dependent lytic granule biogenesis in cytotoxic T lymphocytes by orchestrating the MPR-dependent transport of granzyme B

Cytotoxic T lymphocytes (CTL) exploit specialized secretory lysosomes, the lytic granules (LG) to kill target cells. The LGs carry a battery of apoptosis-inducing molecules enriched in granzymes (GZM), perforin and FasL, which are released at the immune synapse formed by CTLs with their cognate targets. Recent studies have revealed an unexpected diversity among LGs, suggesting the existence of multiple vesicular trafficking pathways in their biogenesis and exocytosis. We have previously implicated the ciliary protein IFT20 in the retrograde trafficking of the cation-independent mannose-6-phosphate receptor (MPR), which is required for the lysosomal targeting of the acid hydrolases. Here we investigate the role of IFT20 in LG biogenesis in CTLs, showing that IFT20 is essential for MPR recycling to the trans-Golgi network and ensures proper granzyme B (GZMB) localization to LGs. As a result, IFT20 deficiency impairs the killing capability of CTLs. In turn, to rescue the lysosome and LG defects, IFT20-deficient CTLs expresses higher levels of lysosomal genes and of components of the cytotoxic machinery of LGs. Interestingly, an in silico analysis suggests a transcriptional co-regulation of lysosome and LG genes by the master regulator of lysosome biogenesis TFEB. Accordingly, modulation of TFEB results in alterations in the expression of LG-related genes and CTL-mediated cytotoxicity. Collectively, our results identify IFT20 as a new player in the trafficking pathways that regulate LG biogenesis and highlight the existence in CTLs of an extended gene expression program regulated by TFEB, downstream of IFT20.