The expression of leukocyte alkaline phosphatase (LAP) in neutrophils is re
duced in some patients with myelodysplastic syndrome (MDS), We quantitative
ly assayed for LAP in MDS leukocytes by a flow cytometry based method using
a monoclonal antibody raised against human bone alkaline phosphatase. The
LAP expression was assayed in blood samples from a group of 46 MRS patients
, consisting of 39 patients with refractory anemia (RA), 3 with RA with exc
ess blasts (RAEB), and 4 patients with RAEB in transformation. The percenta
ge of LAP-positive cells was significantly higher in the MDS patients than
in the normal subjects and also higher in IRA than in RAEB and RAEB in tran
sformation. To investigate the cause of the elevated LAP expression, we mea
sured the serum concentrations of several cytokines. The granulocyte colony
-stimulating factor (G-CSF) level was significantly elevated in MDS patient
s, and the serum G-CSF concentration clearly correlated with the percentage
of LAP-positive cells. Thus, the LAP activity in RA is higher than in norm
al subjects, and G-CSF is thought to be one of the causes stimulating LAP e
xpression in MDS neutrophils.