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Keywords:

  • electron microscopy;
  • human mast cells;
  • interleukin-4;
  • interleukin-5;
  • immunohistochemistry

Background

Human mast cells synthesize and secrete many cytokines of relevance to the pathogenesis of allergic diseases such as asthma and rhinitis. In particular, interleukin (IL) -4 and IL-5 are likely to play key roles in the development of the inflammatory response that characterizes these diseases. Immunohistochemical studies on human nasal and bronchial mucosal biopsies suggest that IL-4 and IL-5 may be stored preformed in mast cells.

Objective

To identify whether IL-4 and IL-5 are stored within mast cell secretory granules.

Methods

We used immunogold electron microscopic analysis on bronchial mucosa and lung parenchyma from resected lung specimens, and a nasal mucosal biopsy from a patient with active allergic rhinitis. Samples were fixed in 4% paraformaldehyde plus 0.5% glutaraldehyde and processed into Lowicryl K4M resin by the ‘Progressive Lowering of Temperature’ technique. Ultrathin sections were stained immunohistochemically by an indirect immunogold method.

Results

Immunoreactivity for IL-4, but not IL-5, was localized to the granules of mast cells in all tissue samples. IL-5 was localized to the matrix of eosinophil granules in these samples, but neither cytokine was detected in T cells. IL-4 immunoreactivity increased in the granules of mast cells 24 h after immunoglobulin (Ig) E-dependent activation (mean 17.5 ± 1.4 gold particles per granule) compared with nonactivated mast cells (mean 6.8 ± 0.8 gold particles per granule, P < 0.001), suggesting replenishment of stores by newly generated protein. Immunoreactive IL-5 remained undetectable in mast cells 24 h after activation, a time point at which they are known to secrete large quantities of this cytokine.

Conclusion

Human mast cells store IL-4 within the matrix of their granules. Very few, if any, lung or nasal mast cells store IL-5. A store of preformed IL-4 within mast cell granules is likely to have an important influence during the initiation and maintenance of the allergic immunological response.