An actin cytoskeletal barrier inhibits lytic granule release from Natural Killer cells in Chediak-Higashi syndrome.

Chediak-Higashi syndrome (CHS) is a rare disorder caused by biallelic mutations in the LYST gene, resulting in formation of giant lysosomes or lysosome-related organelles in several cell types. The disease is characterized by immunodeficiency and a fatal hemophagocytic lymphohistiocytosis due to impaired function of cytotoxic lymphocytes, mainly Natural Killer (NK) cells.We sought to determine the underlying biochemical cause of the impaired cytotoxicity of NK cells in CHS.We generated a human cell model of CHS, using CRISPR technology. We used a combination of classical techniques to evaluate lysosomal function and cell activity in the model system, and super-resolution microscopy to visualize filamentous (F-)actin and lytic granules in normal and LYST-deficient NK cells.Loss of LYST function in a human NK cell line, NK92mi, resulted in inhibition of NK cell cytotoxicity and reproduced other aspects of CHS cellular phenotype, including the presence of significantly enlarged lytic granules with defective exocytosis, and impaired integrity of endo-lysosomal compartments. The large granules had an acidic pH, normal activity of lysosomal enzymes, and were positive for the proteins essential for lytic granule exocytosis. Visualization of the actin meshwork openings at the immunological synapse revealed that the cortical actin acts as a barrier for secretion of such large granules at the cell-cell contact site. Decreasing the cortical actin density at the immunological synapse, or decreasing the lytic granule size, restored the ability of LYST-deficient NK cells to degranulate and kill target cells.The cortical actin and granule size play significant roles in NK cell cytotoxic function. The periodicity of sub-synaptic actin is an important factor limiting the release of large lytic granules from CHS NK cells, and could be a novel target for pharmaceutical intervention.