Regulatory system of human mast cell production

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We attempted to clarify the effects of IL-6 on the growth and properties of human mast cells using cultured mast cells selectively generated by stem cell factor (SCF) from CD34^+ cord blood cells. The addition of IL-6 to cultures containing mast cells resulted in a substantial reduction of the number of progenies grown by SCF in the liquid culture. This IL-6-mediated inhibition of mast cell growth may be due in part to the suppression at the precursor level, according to the results of a clonal cell culture assay. Moreover, a flow cytometric analysis showed that the cultured mast cells grown in the presence of SCF+IL-6 had decreased c-kit expression. The exposure of cultured mast cells to SCF+IL-6 also caused substantial increases in the cell size, frequency of chymase-positive cells and intracellular histamine level compared with the values obtained with SCF alone. The flow cytometric analysis revealed low but significant levels of expression of IL-6 receptor (IL-6R) and gp130 on the … More cultured mast cells grown with SCF.The addition of either anti-IL-6R antibody or anti-gp130 antibody abrogated the biological functions of IL-6. Although IL-4 exerted an effect similar to that of IL-6 on the cultured mast cells under stimulation with SCF, the results of comparative experiments suggest that the two cytokines use different regulatory mechanisms. Taken together, the present findings suggest that IL-6 modulates SCF-dependent human mast cell development directly via an IL-6R-gp130 system.Next, we examined the effects of retinoids on the human mast cell development using a serum-deprived culture system. When 10-week cultured mast cells derived from CD34^+ cord blood cells were used as target cells, both all-trans retinoic acid (ATRA) and 9-cis RA inhibited the progeny generation under stimulation with SCF in a dose-dependent manner. The early steps in mast cell development appear to be less sensitive to RA according to the single CD34^+c-kit^+ cord blood cell culture study. The optimal concentration of RAs also reduced the histamine concentration in the cultured mast cells (3.00±0.47 pg/cell in SCF alone, 1.44±0.18 pg/cell in SCF+ATRA, and 1.41±0.10 pg/cell in SCF+9-cis RA). RT-PCR analyses showed the expression of RARα, RARβ, RXRα and RXRβ mRNA in 10-week cultured mast cells. The addition of an RAR-selective agonist at 10^<-10> M to 10^<-7> M decreased the number of mast cells grown in SCF, whereas an RXR-selective agonist at up to 10^<-8> M was inactive. Among RAR subtype selective retinoids used at 10^<-9> M to 10^<-7> M, only the RARα agonist was equivalent to ATRA at 10^<-7> M in its ability to inhibit mast cell growth. Conversely, the addition of excess concentrations of a RARα antagonist profoundly counteracted the retinoid-mediated suppressive effects. These results suggest that RA inhibits SCF-dependent differentiation of human mast cell progenitors through a specific receptor. Less