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 sign ificantly the leukocyte chemotactic activity of the activated C peptid es 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-incubate d with Gc-globulin for up to 45 min and washed, and their subsequent c hemotactic response to a suboptimal concentration of C5a alone was mea sured during a 30-min assay. The generation of co-chemotactic activity during the preincubation period was time dependent, showed minimal ac tivity for the first 10 min and a steep rise from 10 to 20 min, and wa s maximal and stable at 30 min. The binding of radiolabeled Gc-globuli n by neutrophils at 37 degrees C mirrored this time-dependent generati on of C5a co-chemotactic activity, with stable cellular levels achieve d between 30 and 45 min at 36 +/- 4 fmol (2 +/- 0.1 ng)/10(6) cells. T he binding of radiolabeled Cc-globulin and the generation of co-chemot actic activity were dependent upon physiologic temperatures (37 degree s 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 in cubated a second time at 37 degrees C before chemotaxis to C5a. These results demonstrate that neutrophils bind exogenous Gc-globulin and ge nerate C5a co-chemotactic activity in a time-, temperature-, and dival ent cation-dependent manner. Moreover, this activity is transient if n eutrophils lack a continuous supply of Gc-globulin.