Interaction of human polymorphonuclear leukocyte elastase with human IgM. In vitro production of an Fabμ-like fragment

Abstract

Human granulocyte elatase has ample opportunity to interact with immunoglobulins at both the intracellular and extracellular level. Thus, the susceptibility of human IgM to proteolysis by this enzyme was investigated. Treatment of monoclonal IgM with elastase in the presence of 2 m M cysteine for 18 hr at 37°C resulted in the formation of a major fragment (Frag. A) and small peptides. Fragment A had an approximate mol. wt of 50,000 as determined by gel filtration. Reduction and subsequent sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of Frag. A revealed 2 bands having mol. wt of 23,000 and 30,000, respectively. Ouchterlony analysis showed Frag. A to contain light (K) chain and a portion of the heavy (μ) chain. Fragment A appeared to be antigenically identical with Fab produced by trypsin or papain digestion of IgM but completely different from Fc5μ produced by trypsin digestion of IgM. Fc5μ was digested to small peptides by the enzyme. These findings show that human PMN elastase can specifically degrade IgM in vitro, producing an Fab-like fragment and small peptides.