Regulated Secretion from Hemopoietic Cells

Address correspondence to Gillian M. Griffiths, Sir William Dunn School of Pathology, University of Oxford, South Parks Road, Oxford OX1 3RE, UK. Tel.: (44) 1865-275571. Fax: (44) 1865-275515. E-mail: gillian.griffiths @path.ox.ac.uk T HE process of regulated secretion (reviewed in Bugress and Kelly, 1987) is critical for the correct biological functioning of many different cells of the immune system, most of which are derived from the hemopoietic lineage (Table I). For example, T lymphocytes use regulated secretion to selectively destroy appropriate targets recognized by the T cell receptor, while mast cells degranulate in response to IgE cross-linking to counter parasitic infection. Unlike conventional secretory cells (e.g., exocrine and endocrine cells) which use a separate organelle for the storage and release of their secretory products (Fig. 1 a), cells of the hemopoietic lineage use lysosomes to store and release their secretory products (Fig. 1 b; Griffiths, 1996). These organelles have been termed secretory lysosomes. Although the lysosomal nature of the secretory granules found in several hemopoietic cells has been known for many years, recent evidence supports the idea that secretory lysosomes may use specialized mechanisms for sorting and secretion, which differ from those found in conventional secretory cells. Interestingly, a small number of nonhemopoietic cells also use these mechanisms, suggesting that secretory lysosomes may represent an early form of regulated secretion.