Adult mastocytosis is an incurable clonal disease associated with c-KIT mutations, mostly in exon 17 (D816V). In contrast, pediatric mastocytosis often spontaneously regresses and is considered a reactive disease. Previous studies on childhood mastocytosis assessed only a few patients and focused primarily on codon 816 mutations, with various results. In this study, we analyzed the entire c-KIT sequence from cutaneous biopsies of 50 children with mastocytosis (ages 0-16 years). A mutation of codon 816 (exon 17) was found in 42% of cases, and mutations outside exon 17 were observed in 44%. Unexpectedly, half of the mutations were located in the fifth Ig loop of c-KIT's extracellular domain, which is encoded by exons 8 and 9. All mutations identified in this study were somatic and caused a constitutive activation of c-KIT. There was no clear phenotype-genotype correlation, no clear relationship between the mutations and familial versus spontaneous disease, and no significant change in the relative expression of the c-KIT GNNK+ and GNNK isoforms. These findings strongly support the idea that, although pediatric mastocytosis can spontaneously regress, it is a clonal disease most commonly associated with activating mutations in c-KIT.

In the present report we have analyzed whether human normal cord blood-derived mast cells (CBMC) could interact with bacterial products, especially lipopolysaccharide (LPS) from Escherichia coli and peptidoglycan (PGN) from Staphylococcus aureus, known as Tolllike receptor (TLR) 4 and TLR2 agonists, respectively. We found that both LPS and PGN induced significant release of not only tumor necrosis factor- § (TNF- § ), but also IL-5, IL-10 and IL-13 by human mast cells (MC). We also established that the stimulation of CBMC with LPS or with PGN is mediated through interactions with TLR4 or with TLR2, respectively. Thus, our data indicate that activation of either TLR2 or TLR4 pathway may lead to a pro-Th2 immune response. However, the release of TNF- § induced by LPS, conversely to PGN, required the priming of CBMC by IL-4 and the presence of serum components, in particular soluble CD14. Of interest, stimulation by PGN, but not by LPS, induced release of histamine by human MC. Altogether, these findings provide the first evidence that human MC differentially respond towards bacterial components, and that their responses depend on TLR pathways and reveal human specificities in the pattern of cytokine production.