Novel function of C4a anaphylatoxin. Release from monocytes of protein which inhibits monocyte chemotaxis

The complement C4-derived anaphylatoxin, C4a, possesses a strong chemotaxis inhibitory capacity to blood monocytes at concentrations as low as 10(-16) mol/L. In our study, treatment with carboxypeptidase B to convert it to C4a des Arg77 decreased the inhibitory activity to less than 1/1,000. The extraordinary inhibitory capacity of C4a suggests the presence of an amplification mechanism in this inhibition. Indeed, we found that the conditioned media of peripheral blood mononuclear cells or monocyte/macrophage lineage cell lines (U937 and THP-1 cells) preincubated with 10(-16) mol/L C4a for 5 minutes or more at 37 C possessed the inhibitory capacity 100,000-fold stronger than the original activity of C4a. The monocyte-derived chemotaxis inhibitory factor seemed monocyte-specific. This cell-derived factor was sensitive to treatment with trypsin and chymotrypsin and immunologically distinct from C4a. The apparent molecular size of the monocyte factor was estimated to be approximately 20 kd by gel filtration. These results indicate that C4a anaphylatoxin induces the release from monocytes of a protein with inhibitory activity for monocyte chemotaxis.