The
kinin-forming pathway is activated on endothelial cells and neutrophils when
high-molecular weight kininogen (HK) is cleaved by
plasma kallikrein liberating
bradykinin, a potent mediator of
inflammation.
Kinins are released during inflammatory conditions such as
vasculitis, associated with neutrophil influx around blood vessels. Some patients with
vasculitis have elevated plasma levels of neutrophil-derived
proteinase 3 (PR3) and anti-PR3 Abs. This study investigated if neutrophil-derived PR3 could induce activation of the
kinin pathway. PR3 incubated with HK, or a synthetic
peptide derived from HK, induced breakdown and release of a novel tridecapeptide termed PR3-kinin, NH(2)-MKRPPGFSPFRSS-COOH, consisting of
bradykinin with two additional
amino acids on each terminus. The reaction was specific and inhibited by anti-PR3 and alpha(1)-antitrypsin. Recombinant wild-type PR3 incubated with HK induced HK breakdown, whereas mutated PR3, lacking enzymatic activity, did not. PR3-kinin bound to and activated human
kinin B(1) receptors, but did not bind to
B(2) receptors, expressed by transfected HEK293 cells in vitro. In human plasma PR3-kinin was further processed to the
B(2) receptor agonist
bradykinin. PR3-kinin exerted a hypotensive effect in vivo through both B(1) and
B(2) receptors as demonstrated using wild-type and B(1) overexpressing rats as well as wild-type and
B(2) receptor knockout mice. Neutrophil extracts from
vasculitis patients and healthy controls contained comparable amounts of PR3 and induced HK proteolysis, an effect that was abolished when PR3 was immunoadsorbed. Neutrophil-derived PR3 can proteolyze HK and liberate PR3-kinin, thereby initiating
kallikrein-independent activation of the
kinin pathway.