Potentiation of complement (C5a)-induced granulocyte aggregation by cytochalasin B

Within 4 min of exposure to C5a, normal human GR's stirring in an aggregometer undergo reversible autoaggregation, manifest by increments in both light transmission and oscillation amplitude, which are invariably preceded by pseudopod formation. The present studies show that CB causes a 10-fold increase in this aggregation response despite the fact that it prevents C5a-induced pseudopod formation. Aggregation of CB-treated GR's is irreversible and is accompanied by lysosomal exocytosis. The amplification is proportional to CB concentrations in the range 0.2 to 5.0 μg/ml, and is apparent within seconds of the addition of CB to GR's. Light transmission increases in proportion to log of the quantity of activated plasma or serum C added, and aggregating activity can be generated by incubation of ZYM with normal plasma or serum but not with heat-decomplemented or C5-deficient plasma. The aggregating activity in normal ZAP is limited to a single peak of approximately 12,000 to 30,000 daltons molecular weight, identical to that which induces aggregation of normal GR and which is chemotactic for GR's in vitro. The aggregating and chemotactic activities of this peak were heat (56° C/30 min) stable and ablated by incubation with anti-C5, but not anti-C3 antisera. The potentiation of C5a-induced GR's aggregation by CB may result from the same impairment of microfilament function which is thought to cause impaired locomotion/ingestion and facilitated lysosomal exocytosis. This model may provide a simple technique, more sensitive than currently available, for the assay of C5a.