C3e: An acidic fragment of human C3 with leukocytosis-inducing activity

The authors have isolated a leukocytosis-inducing peptide of degraded human C3 that has the electrophoretic behavior of prealbumin and a m.w. of 10,000 to 12,000. The peptide has been designated C3e. It was generated by two methods: Incubation of isolated C3 with 2% (w/w) trypsin for 120 min at 37°C, or incubation of serum at 37°C for 5 days under sterile conditions. C3e preparations obtained by the two methods were virtually indistinguishable with respect to molecular size, amino acid composition, and immunochemical properties. The electrophoretic mobility of C3e at pH 8.6 is -7.5 x 10^-5 cm² V^-1 sec^-1. Anti-C3e reacts with C3e, C3, C3b, and C3c, but not with C3a or C3d. Since the β-chain of C3 is not affected by the liberation of C3e, it is concluded that C3e arises from cleavage of the portion of the α-chain that is part of the C3c fragment. Purified C3e was found to mobilize leukocyte from bone marrow upon perfusion of isolated rat femur. Intravenous injection of C3e into rabbits caused a 2- to 3-fold increase in the number of circulating leukocytes after 60 min which lasted 2 hr. Leukocytosis was preceded by mild leukopenia. Leukocytosis-inducing activity was abrogated by treating C3e preparations with solid phase anti-C3. Incubation of heparinized rabbit blood with radioiodinated C3e resulted in preferential binding of C3e to polymorphonuclear leukocytes. Intradermal injection of C3e into rabbits evoked increased vascular permeability after 15 to 30 min. The evidence indicates that C3e is a fragment of C3 with biologic activity distinct from that of other C3 fragments.