Human tissue mast cells (MCs) have the potential to express several neutral granule proteases, which are the most precise markers of the phenotypic heterogeneity of MCs. However, the full extent of such heterogeneity is limited by the fact that MCs containing either tryptase only or tryptase and chymase have long been considered to be the sole MC phenotypes. Moreover, the potential developmental relationship between human MCs of different protease phenotypes has remained a matter of dispute.

We attempted to define how human MCs with different protease phenotypes relate to their circulating progenitors.

MCs were generated from human peripheral blood–derived CD34⁺ progenitors in the presence of kit ligand (KITLG) and the cytokines IL-3, IL-9, and IL-6 under serum-free conditions, or by KITLG alone in the presence or absence of serum. The expression of chymase, carboxypeptidase A3, cathepsin G, granzyme B, and the tryptases derived from the TPSAB1, TPSB2, TPSD1, and TPSG1/PRSS31 genes were determined weekly at the mRNA and/or protein levels.

Incubation of CD34⁺ progenitors in the presence of KITLG and the cytokines IL-3, IL-9, and IL-6 promoted the development of a single population of MCs with a uniform tryptase⁺, chymase⁺, CPA3⁺, cathepsin G⁺, and granzyme B⁺ phenotype. Interestingly, the presence of KITLG alone was sufficient to induce the expression of all the above proteases.

All circulating human MC progenitors have the potential to differentiate into MCs expressing the complete panel of neutral granule proteases, implying that human MCs originate from a common MC-committed progenitor.