MONOCYTES DO NOT MAKE MAST-CELLS WHEN CULTURED IN THE PRESENCE OF SCF- CHARACTERIZATION OF THE CIRCULATING MAST-CELL PROGENITOR AS A C-KIT-, CD14-, CD17-, COLONY-FORMING CELL(, CD34+, LY)

Mast cells (MC3) belong to the hemopoietic system and arise from hemop oietic precursor cells. Human MC progenitors can be detected in the bo ne marrow as well as in the peripheral blood (pb) and are responsive t o the mast cell growth factor SCF, the ligand of the c-kit tyrosine ki nase receptor. However, little is known about the subsets of cells tha t become committed to and differentiate into mature human MC. In this study, the identity of the circulating MC progenitor, previously felt to be a monocyte (Mo) or basophil (Ba), was investigated. For this pur pose, CD14+ pb monocytes, CD17+ pb basophils and CD34+ cord blood cell s were purified to homogeneity (> 95%) from mononuclear cells (normal adult donors, n = 17, cord blood, n = 2) by counter-flow centrifugatio n followed by cell sorting with mAb. In the presence of rhSCF, MC deve loped in long term suspension culture from pure CD34+ cells but not fr om pure Mo, pure Ba, or Ly (MC-tryptase levels on day 42: CD14+ Mo: 3. 7 +/- 0.8 vs CD17+ Ba: 3.2 +/- 0.5 vs Ly: 2.0 +/- 1.5 vs control: 196. 5 +/- 92.5 ng/ml, p < 0.001). Depletion of CD34+ cells from MNC result ed in a loss of MC in long term suspension culture, whereas depletion of either Mo, Ba, or Ly did not. In methyl-cellulose cultures in the p resence of rhSCF, MC and tryptase could be detected in pure (CFU-mast) and mixed (CFU-myeloid/mast) MC colonies. Together, MC do not origina te from circulating Mo, Ba, or Ly. The circulating MC progenitor is a CD34+, c-kit+, Ly-, CD14-, CD17- colony-forming cell. This is the firs t definitive demonstration that mast cells are replenished directly fr om early hemopoietic progenitors and thus form a unique cell lineage w ithin the hemopoietic system.