Rapid Degradation of the Complement Regulator, CD59, by a Novel Inhibitor

Background: Surface expression of complement inhibitors, such as the glycosylphosphatidylinositol-anchored protein CD59, prevent complement-dependent lysis of cancer cells. Results: The non-toxic domain-4 of the bacterial toxin intermedilysin (rILYd4) blocks CD59 complement inhibitory activity. Conclusion: rILYd4 induces CD59 internalization and rapid degradation in lysosomes in non-small lung carcinoma cells. Significance: CD59 serves as a model for glycosylphosphatidylinositol-anchored protein trafficking and rILYd4 shows potential for immunotherapy. There is increased interest in immune-based monoclonal antibody therapies for different malignancies because of their potential specificity and limited toxicity. The activity of some therapeutic monoclonal antibodies is partially dependent on complement-dependent cytolysis (CDC), in which the immune system surveys for invading pathogens, infected cells, and malignant cells and facilitates their destruction. CD59 is a ubiquitously expressed cell-surface glycosylphosphatidylinositol-anchored protein that protects cells from CDC. However, in certain tumors, CD59 expression is enhanced, posing a significant obstacle for treatment, by hindering effective monoclonal antibody-induced CDC. In this study, we used non-small lung carcinoma cells to characterize the mechanism of a novel CD59 inhibitor: the 114-amino acid recombinant form of the 4th domain of intermedilysin (rILYd4), a pore forming toxin secreted by Streptococcus intermedius. We compared the rates of internalization of CD59 in the presence of rILYd4 or anti-CD59 antibodies and determined that rILYd4 induces more rapid CD59 uptake at early time points. Most significantly, upon binding to rILYd4, CD59 is internalized and undergoes massive degradation in lysosomes within minutes. The remaining rILYd4CD59 complexes recycle to the PM and are shed from the cell. In comparison, upon internalization of CD59 via anti-CD59 antibody binding, the antibodyCD59 complex is recycled via early and recycling endosomes, mostly avoiding degradation. Our study supports a novel role for rILYd4 in promoting internalization and rapid degradation of the complement inhibitor CD59, and highlights the potential for improving CDC-based immunotherapy.