L is for lytic granules: lysosomes that kill

CTL are important cells in the immune system which are able to recognise and directly destroy virally infected, tumorigenic or foreign cells. The proteins which mediate this destruction are packaged into specialised secretory granules, termed lytic granules, which are secreted in response to target cell recognition. Curiously these specialised secretory granules also contain all the lysosomal hydrolases, and in CTL the lytic granules serve two separate functions: as a lysosome within the cell, and as a secretory granule when a target cell is recognised. These 'secretory lysosomes', which serve important roles in both protein degradation within the cells as well as regulated secretion of proteins from the cells, are also found in other cell types, all of which are derived from the hemopoietic lineage. This observation raises the possibility that cells of the hemopoietic lineage possess specialised sorting and secretory mechanisms which allow the lysosomes to be used as secretory organelles. Studies on Chediak Higashi syndrome support this idea, since in this naturally occurring genetic mutation, cells with secretory lysosomes are unable to secrete their granules while other conventional secretory cells are able to do so. Further studies on the mechanisms which regulate secretion of lytic proteins from CTL should identify the proteins involved in this unusual secretory pathway. Some aspects of the differences between conventional and 'secretory' lysosomes remain unresolved. How the biogenesis of the secretory lysosome differs from that of a conventional secretory granule is unclear. While conventional secretory cells sort proteins destined for the granule by a selective condensation in the TGN, the secretory lysosomes seem to use a combination of lysosomal and other sorting signals. Our preliminary studies suggest that haemopoietic cells possess specialised sorting mechanisms which allow the correct sorting of the secreted products to the lysosome, and that these signals are different from those found in conventional secretory (e.g. neurosecretory) cells. This finding and the observation that fibroblast lysosomes can undergo calcium-mediated exocytosis suggests that the unusual secretory system found in haemopoietic cells may be a result of specialised sorting mechanisms in these cells. In this case the Chediak lesion may turn out to be a sorting defect.