Co-chemotactic effect of Gc-globulin (vitamin D binding protein) for C5a transient conversion into an active co-chemotaxin by neutrophils

Gc-globulin (vitamin D binding protein) has been shown to augment significantly the leukocyte chemotactic activity of the activated C peptides C5a and C5a^desArg (i.e., the co-chemotactic effect). However, the mechanism of chemotaxis enhancement is not known. To investigate the role that the neutrophil plays in this process, cells were co-incubated with Gc-globulin for up to 45 min and washed, and their subsequent chemotactic response to a suboptimal concentration of C5a alone was measured during a 30-min assay. The generation of co-chemotactic activity during the preincubation period was time dependent, showed minimal activity for the first 10 min and a steep rise from 10 to 20 min, and was maximal and stable at 30 min. The binding of radiolabeled Gc-globulin by neutrophils at 37°C mirrored this time-dependent generation of C5a co-chemotactic activity, with stable cellular levels achieved between 30 and 45 min at 36 ± 4 fmol (2 ± 0.1 ng)/10⁶ cells. The binding of radiolabeled Gc-globulin and the generation of co-chemotactic activity were dependent upon physiologic temperatures (37°C) and levels of Ca²⁺ (1.3 mM) and Mg²⁺ (0.8 mM), and were inhibited by an Ab to Gc-globulin. Finally, the C5a co-chemotactic activity of Gc-globulin would decay rapidly if neutrophils were washed and then incubated a second time at 37°C before chemotaxis to C5a. These results demonstrate that neutrophils bind exogenous Gc-globulin and generate C5a co-chemotactic activity in a time-, temperature-, and divalent cation-dependent manner. Moreover, this activity is transient if neutrophils lack a continuous supply of Gc-globulin.