Ferritin Particles Accumulate in Human Mast Cell Secretory Granules and Are Released upon FcεRI-mediated Activation
Monday, March 5, 2018
South Hall A2 (Convention Center)
Brant R Ward, MD PhD, Mark Hicks, Jaz-Munn Johnson, Yu Par A Myo
RATIONALE: Transcriptome analyses revealed that mast cell (MC) gene expression correlates better with macrophages than other granulocytes. Macrophages produce ferritin in inflammatory conditions like hemophagocytic lymphohistiocytosis. We hypothesized that MCs could be an alternate source of extracellular ferritin during inflammation.

METHODS: Primary human skin MCs were isolated and cultured from discarded surgical specimens. MC gene expression was evaluated by RNA expression array. Cellular ferritin light and heavy chains were assayed by Western blot. Ferritin content in culture supernatants was measured by ELISA after treating MCs with various stimuli. MC degranulation after stimulation was measured by β-hexosaminidase release. Ferritin localization within MCs was confirmed by superresolution microscopy.

RESULTS: MCs express ferritin light and heavy chain mRNA at similar levels to tryptase, among the genes with the highest expression in these cells. MCs do not release ferritin upon stimulation with pro-inflammatory cytokines, but release copious amounts after anti-FcεRI antibody treatment. Ferritin is detected within minutes of activation, suggesting it is released during degranulation. Western blots suggest ferritin particles released by MCs contain roughly equivalent amounts of heavy and light chains. Microscopy demonstrated ferritin within the cytoplasm of MCs, as well as accumulation within tryptase-containing granules.

CONCLUSIONS: Human skin MCs generate and store ferritin within secretory granules, and ferritin particles are released quickly upon FcεRI-mediated activation, similar to other mediators like tryptase, histamine, and TNFα. Ferritin is known to have direct modulatory activity on T cells and APCs, and ferritin particles released by MCs during degranulation may help to moderate inflammation at local sites within tissues.