ZAP-70 Expression in Human B cells: Analysis of Normal Cells and Acute Lymphoblastic Leukemia (ALL) Cells with Pro/Pre B Phenotype.

ZAP-70 is a protein tyrosine kinase of the Syk/ZAP-70 family that plays a critical role in the signal transduction from the T-cell receptor. In human lymphocytes, ZAP-70 gene has been reported to be expressed in T and NK derived cells, and in IgV_H unmutated B-chronic lymphocytic leukemia cells. More recently, ZAP-70 expression has been shown to be required for the development of pro-B cells to pre-B cells in mice. To ascertain the expression of ZAP-70 gene in human immature B-cell stages, we analyzed ZAP-70 protein and/or mRNA in normal human B cells at different stages of B cell maturation, including pro/pre-B cells and tumoral cells from 20 B-ALL. ZAP-70 expression was assessed by flow cytometry (FC), immunofluorescence (IF), and/or by quantitative real time RT-PCR (QRT-PCR). In normal bone marrow, ZAP-70 expression was found only in T and in immature B cells (CD19+/CD10+/CD20 −). Moreover, T cells -but no mature B cells- from normal tonsil expressed ZAP-70, as assessed by QRT-PCR and IF. In B-ALLs, a high ZAP-70 expression by FC was observed in 9/13 cases (mean, 82.6%, range 60–99%), whereas in 4 cases ZAP-70 was barely detectable (mean, 13%). By QRT-PCR, 10/16 B-ALLs showed levels of expression similar to ZAP-70 non-expressing cell lines and normal B-cells, whereas in the remaining cases ZAP-70 expression was 3–4 times higher than in normal mature B-cells. Taken together, a high expression of ZAP-70 was found in 11/21 (52%) B-ALLs. No relationship was observed between the level of ZAP-70 expression and the B-ALL maturation status. In conclusion, among normal B cell subsets ZAP-70 expression is restricted to B-cells with pro/pre phenotype. In addition, ZAP-70 is expressed in 52% of B-ALLs, probably as a reflection of their B-cell origin.