Proteinase 3 (Pr3) is a serine protease that is stored primarily in azurophilic granules and
secretory vesicles in neutrophils. The high affinity Pr3 receptor, CD177, is expressed on a
subset of neutrophils (ranging from 0-100%), but usually only half of the circulating
neutrophils express CD177 in most normal individuals. As a serine proteinase, Pr3 is
controlled by a variety of inhibitors, including alpha-1-antitripsin (AAT). We investigated if
the surface expression of Pr3 and CD177 was affected by stimulation of neutrophils in the
presence or absence of serum. Methods: Neutrophil cells were isolated from healthy donors.
The cells counted and stimulated with fMLP only or cytochalasin B followed by addition of
fMLP, and compared to unstimulated controls, in the presence or absence of 100 %
autologous serum. The expression of Pr3 and CD177 was analysed by flow cytometery.
Results: The expression of membrane bound Pr3 (mPr3) by neutrophils was still detectable in
the presence of serum however, the expression was reduced on both mPr3low and mPr3high
cells in comparison to cells incubated in the absence of serum. No significant increase was
observed in mPr3 expression following stimulation with fMLP in either the presence or
absence of serum. In comparison, stimulation with cytochalasin B combined with fMLP
resulted in a 9-fold increase (P<0.0001) in the Pr3high cells compared with unstimulated cells
in the absence of serum. This increase was only 3-fold (P< 0.002) in the presence of serum.
Conclusion: Despite the ability of purified AAT to inhibit Pr3 binding to CD177, significant
surface Pr3 was still found on the surface of CD177-positive neutrophils when stimulated in
the presence of 100% autologous serum, but only following maximal stimulation of
neutrophils.
Key words: Proteinase 3, CD177, neutrophils, ANCA-associated vasculitis