Despite the relevance of the c-kit/stem cell factor (SCF) signaling pathway in mast cell (MC) diseases, the exact frequency of KIT mutations in different compartments of bone marrow (BM) hematopoietic cells of individuals with systemic mastocytosis (SM), and its different diagnostic categories, remains unknown. In this study, we prospectively analyzed the presence of KIT mutations in fluorescence-activated cell-sorting (FACS)– purified populations of BM MCs (n = 113) and other BM cell compartments (n = 67) from adults with SM. Our results show the presence of D816V KIT mutation in virtually all adults (93%) with indolent and aggressive forms of SM, except well-differentiated SM (29%), while other KIT mutations were rarely (< 3%) detected. In around one-third of patients with mutated MCs, the KIT mutation was also detected in CD34+ hematopoietic cells and eosinophils, and, to a lesser extent, in monocytic, neutrophil-lineage BM precursor cells and lymphocytes. Most patient with poor-prognosis SM (81%) carried the KIT mutation in 2 or more BM myeloid cell populations, while this was detected in a smaller proportion (27%) of indolent cases. These results would support the notion that KIT mutation is a hallmark of adult SM where it targets a pluripotent hematopoietic stem cell, and may contribute to explaining previously observed discrepancies in the literature.

Monoclonal B-cell lymphocytosis (MBL) indicates the presence of less than 5 × 10⁹/L circulating monoclonal B cells in otherwise healthy subjects. Recently, it has been reported that circulating chronic lymphocytic leukemia (CLL)–like B cells can be detected using 4- or 5-multicolor flow cytometry in 5% to 7% of adults with normal lymphocyte counts. We investigated the frequency of circulating monoclonal B cells in 608 healthy subjects older than 40 years with normal blood counts, using a highly sensitive 8-color flow cytometry approach and systematic screening for total PB leukocyte count higher than 5 × 10⁶. We show that the frequency of PB monoclonal B cells is markedly higher than previously reported (12% for CLL-like B cells, found at frequencies of 0.17 ± 0.13 × 10⁹ cells/L), the incidence progressively increasing with age. Most cases (62%) showed clonal B-cell levels below the maximum sensitivity of the techniques described by others (< 0.01%), supporting the notion that detection of MBL may largely depend on the sensitivity of the flow cytometry approach used.

Background: A variable percentage of patients with systemic mast cell (MC) activation symptoms meet criteria for systemic mastocytosis (SM). We prospectively evaluated the clinical utility of the REMA score versus serum baseline tryptase (sBt) levels for predicting MC clonality and SM in 158 patients with systemic MC activation symptoms in the absence of mastocytosis in the skin (MIS). Methods: World Health Organization criteria for SM were applied in all cases. MC clonality was defined as the presence of KIT-mutated MC or by a clonal HUMARA test. The REMA score consisted of the assignment of positive or negative points as follows: male (+1), female (–1), sBt 25 µg/l (+2), presence (–2) or absence (+1) of pruritus, hives or angioedema and presence (+3) of presyncope or syncope. Efficiency of the REMA score for predicting MC clonality and SM was assessed by receiver operating characteristic (ROC) curve analyses and compared to those obtained by means of sBt levels alone. Results: Molecular studies revealed the presence of clonal MC in 68/80 SM cases and in 11/78 patients who did not meet the criteria for SM. ROC curve analyses confirmed the greater sensitivity and a similar specificity of the REMA score versus sBt levels (84 vs. 59% and 74 vs. 70% for MC clonality and 87 vs. 62% and 73 vs. 71% for SM, respectively). Conclusions: Our results confirm the clinical utility of the REMA score to predict MC clonality and SM in patients suffering from systemic MC activation symptoms without MIS.