Characterization of long-term mast cell lines established from HFs. (A) HF-derived leukocyte cultures were maintained and expanded in the presence of 5 added growth factors by repeated passages of loosely adherent cells (released from underlying fibroblast layers by gentle pipetting) to new culture plates. Cells harvested from 6-month-old cultures were stained for Lin, c-kit, and Sca-1. Data shown are the expression profiles for Lin and Sca-1 in nongated populations (left) and for Sca-1 and c-kit in the Lin^– populations (right). (B) The same preparations were examined for histamine release (means ± SD from triplicate samples) after pulsing with anti–DNP-IgE, followed by challenge with DNP-HSA or HSA alone. Statistically significant differences are indicated with asterisks (**P < .01). Data shown are representative of 2 different 6-month-old mast cell lines.