Proteases and their serpin inhibitors are abundantly expressed in haemopoietic 
and peripheral blood cells. There is, however, relatively little information 
about the role played by serpins in the control of protease activity within 
these cells and in the pericellular region. The observation that mutations in 
the neutrophil elastase gene, which cause cyclic and severe congenital 
neutropenia, are associated with protease maldistribution gives some clue as to 
the potential importance of inhibitor proteins. To begin to address the role of 
protease/inhibitor balance in blood cells we used reverse transcription 
polymerase chain reaction to examine protease and serpin gene expression in 
mature peripheral blood cells, differentiating haemopoietic progenitors, 
leukaemic blasts and haemopoietic cell lines. The results demonstrate 
stage-specific expression of proteases together with widespread expression of 
intra- and extra-cellular serpins. The elastase inhibitors monocyte neutrophil 
elastase inhibitor (MNEI) and antitrypsin (AT) showed overlapping expression. 
MNEI is predominantly expressed in early haemopoietic progenitors while 
antitrypsin is mainly expressed in more mature myeloid precursors, peripheral 
blood granulocytes and mononuclear cells. Our results give an overall picture of 
serpin and protease gene expression and draws attention to the potential 
importance of elastase regulators at all stages of myelopoiesis.