Mechanism of enhanced complement-dependent cytotoxicity of papain-treated lymphocytes: evidence for increased stability of classical pathway C3 convertase.

Abstract

The mechanisms of enhanced antibody-mediated, complement-dependent cytotoxicity caused by proteolytic treatment of cells was studied in a model system based on HLA microlymphocytotoxicity methods. In this model, papain treatment of lymphocytes resulted in a) no change in antibody binding, b) a slight decrease in initial binding of C4, c) a marked increase in stability of cell-bound C4b, resulting in d) an increase of cell-bound C3, and e) no increase in lytic efficiency of the C5b-9 membrane attack complex. We conclude that the most important step in papain enhancement of lysis of lymphocytes is an increased stability of cell-bound C4b, possibly through decreased surface binding of C4-bp. This mechanism may be relevant to the pathologically increased lysis of cells occurring in patients with hereditary erythroblastic multinuclearity with a positive acidified-serum test (HEM-PAS).