Granulocyte-dependent extracellular cytotoxicity is enhanced by complement C3bi and independent of phagocytosis.

We have evaluated phagocytosis, C3-binding and cytotoxicity of human leukocytes simultaneously, using IgM-sensitized C3-fragment-bearing sheep erythrocytes (EAC3b, EAC3bi) as targets. In this method, 51Cr-labelled EAC3b or EAC3bi were added to human peripheral granulocytes anchored by fibronectin onto microtitre plate wells. The degrees of haemolysis, binding and ingestion of the target cells were estimated from the radioactivity released. We found that the granulocytes predominantly lysed EAC3bi but not EAC3b or EAC4b. EAC3bi lysis elicited from granulocytes was as effective as that from lymphocytes under the same assay conditions. However, one difference in the cytolysis of the two effector cells was that EAC3bi bound efficiently to all of the granulocytes similarly to EAC3b, whereas it bound to only 4% of the lymphocytes. The bound cells did not appear to be efficiently phagocytosed by the granulocytes. Blocking studies using antibodies suggested that C3bi receptors, CR3 and CR4, but not other C3-binding proteins, C3b/C4b receptor (CR1), membrane cofactor protein (MCP) or decay-accelerating factor (DAF), are involved in granulocyte-mediated haemolysis. We speculate that simultaneous stimulation of the C3bi receptors and the fibronectin receptor results in elicitation of cytotoxicity by granulocytes.