Human umbilical cord blood-derived mast cells cultured with NGF exhibit increased expression of chymase. Human umbilical cord blood-derived mast cells were cultured in SCF, IL-6, and PGE₂ for 19 or 20 weeks and were maintained for an additional 3 weeks in medium with or without NGF at 500 ng/mL (see text). Immunocytochemical analysis of the expression of chymase in these mast cells was performed as described in the Materials and Methods. The strength of chymase positivity was graded from negative (−) to strongly positive (+++) (as illustrated in Fig 10), and the distribution of mast cells exhibiting various intensities of chymase positivity in individual specimen was compared using the χ2 test. In each of the three independent experiments shown, the difference between the distribution of cells exhibiting various degrees of chymase positivity in cultures with or without exogenous NGF was significant (P < .0001, P = .0014, and P = .046 in Experiments 1, 2, and 3, respectively). In Experiment 3, we found that NGF which had been boiled (black bars in Experiment 3), when used at 500 ng/mL instead of native NGF in medium containing SCF and PGE₂ , had no detectable effect on chymase immunoreactivity (P = .93 v values for cells cultured without NGF in medium containing SCF, IL-6 and PGE₂ , P < .01 v values for cells cultured with native NGF ).