Recombinant human alpha‐2a interferon promotes an atypical process of mast cell secretion with ultrastructural features suggestive for piecemeal degranulation

Purified mast cells (MC), isolated from the rat peritoneum, were stimulated in vitro with recombinant human IFN‐α 2a (rhIFN‐α 2a) and studied ultrastructurally. Quantitative determination of histamine release was also performed. The following ultrastructural features were observed: (1) dilation of single granules, leading to the formation of cytoplasmic cavities filled with dissolved or eroded matrices; (2) induction of partially empty, non‐fused granule containers close to unaltered resting granules, a process very suggestive of piecemeal degranulation; (3) focal exocytosis, characterized by opening of single granules to the cell exterior and/or fusion of a few granules into small secretory channels. Histamine release was slightly increased in rhIFN‐α 2a‐treated MC, although not to significant levels. These results indicate that rhIFN‐α 2a induces a characteristic pattern of degranulation in rat peritoneal MC and that a small proportion of rhIFN‐α 2a‐stimulated MC shows ultrastructural features suggestive for piecemeal degranulation.