Phosphorylation of C3 by a casein kinase released from activated human platelets increases opsonization of immune complexes and binding to complementreceptor type 1
Kn. Ekdahl et N. Nilsson, Phosphorylation of C3 by a casein kinase released from activated human platelets increases opsonization of immune complexes and binding to complementreceptor type 1, EUR J IMMUN, 31(4), 2001, pp. 1047-1054
We have previously demonstrated that complement component C3 is phosphoryla
ted both in vitro and in vivo by a casein kinase released from activated hu
man platelets. In vitro, the studies have shown that cleavage of C3b by fac
tor I is decreased, and binding to various target surfaces is enhanced by a
ffecting the thiol ester. In the present study we have examined the effect
of phosphorylation on the binding of C3b to complement receptor 1 (CR1, CD3
5). Upon phosphorylation by platelet casein kinase, C3b covalently bound to
activated thiol Sepharose bound higher amounts of soluble recombinant CR1.
Similar effects were demonstrated with two ELISA systems in which microtit
er plates were coated with phosphorylated or unphosphorylated purified C3b
or with C3 activated by the alternative pathway convertase. Phosphorylated
C3b was also four times more efficient than unphosphorylated C3b in inhibit
ing the binding of complement-opsonized human aggregated gammaglobulin to e
rythrocytes. A similar increase in binding was found at low serum concentra
tions when the C3 activation occurred in C3-deficient serum reconstituted w
ith phosphorylated or unphosphorylated C3. In this serum system, using a mo
noclonal antibody specific for iC3b, we also demonstrated that the phosphor
ylated C3b was protected against cleavage to iC3b. Corresponding experiment
s using factor H showed a decrease in binding of both fluid-phase and bound
C3b to factor H. We postulate that phosphorylation of C3 by activated plat
elets amplifies the complement-mediated binding of immune complexes to CR1
by three different mechanisms: decreased cleavage of C3b to iC3b, increased
deposition of C3b to immune complexes, and increased binding of C3b to CR1
.