Gc-globulin (vitamin D-binding protein) enhances the neutrophil chemotactic activity of C5a and C5a des Arg

Several serum proteins have been shown to be important in modulating leukocyte chemotaxis and inflammation. We investigated the possibility that the multifunctional serum protein Gc-globulin (vitamin D-binding protein) may also enhance the neutrophil chemotactic activity of complement-derived peptides. Purified Gc-globulin by itself did not induce chemotaxis of human neutrophils. However, as little as 0.01 nM Gc-globulin greatly enhanced the neutrophil chemotactic activity of C5a and its derivative, C5a des Arg over a wide concentration range. The effect was most pronounced at nonchemotactic doses of C5a (0.01 nM) and C5a des Arg (1 nM). Gc-globulin was unable to augment the neutrophil chemotactic activity of FMLP and leukotriene B₄. This enhancing activity was not due to a nonspecific effect of anionic proteins since other purified serum proteins, of similar size and charge as Gc-globulin (α₁ acid glycoprotein, α₂ HS glycoprotein, α₂ histidine-rich glycoprotein), could not increase the chemotactic activity for C5a des Arg. Serum depleted of Cc-globulin by immunoaffinity chromatography totally lacked chemotactic enhanging activity for C5a des Arg. Gc-globulin-depleted serum activated with zymosan also had significantly less chemotactic activity than control- (sham-depleted) activated serum. Finally, radioiodinated C5a or C5a des Arg formed a 1:1 complex with purified Gc-globulin when analyzed by gel filtration chromatography. These results indicate that Gc-globulin is the major chemotactic enhancing factor in serum and may function as an up-regulator of the chemotactic activity of C5-derived peptides.