Normal lytic granule secretion by cytotoxic T lymphocytes deficient in BLOC-1, -2 and -3 and myosins Va, VIIa and XV

Melanocytes and cells of the immune system share an unusual secretory mechanism which uses the lysosome as a regulated secretory organelle. Recently, a number of the proteins required for these 'secretory lysosomes' to undergo exocytosis have been identified. These include Rab27a, Lyst, Rab geranyl geranyl transferase and the adapter protein complex AP-3. Patients lacking any of these proteins are characterized by the rare combination of albinism and immunodeficiency, revealing roles for these proteins in both melanocyte and immune cell secretion. In order to ask how far the link between albinism and immunodeficiency extends we have examined cytotoxic T-lymphocyte (CTL) secretion from two BLOC-3-deficient patients and seven different mouse models of Hermansky-Pudlak syndrome, all of which display defects in pigmentation and platelet function. We find that CTL function is normal in HPS patients and pale-ear mice deficient in BLOC-3, pallid, muted and sandy mice deficient in BLOC-1, ruby-eye mice deficient in BLOC-2 and buff mice deficient in Vps33a. Similarly, the unconventional myosins, Va, VIIa and XV, which can act as effectors for Rab27a in some cell types, are not required in CTL. These results reveal differences in the protein machinery required for biogenesis and/or secretion of lysosome-related organelles in CTL and melanocytes.