A novel protein that participates in nonself discrimination of malignant cells by homologous complement

The human complement (C) system protects an individual against substances of nonself origin, including xenografts and microbial pathogens. Human cells express C-regulatory proteins, CD46 and CD55, thereby circumventing attack by C3, a major effector of C. Nevertheless, certain malignant cells, particularly those undergoing apoptotic stress, can activate homologous C, overcoming the regulatory actions of CD46 and/or CD55. The molecular mechanisms whereby malignant cells are tagged by homologous C3 remain largely unknown. We identified a novel gene product that converts human cells into targets for homologous complement. Only malignant cells and cell lines exposed to Fas or X-irradiation stimuli produced this protein, designated M161Ag, which was an unglycosylated 43-kDa protein. Analysis of cloned cDNAs indicated that this molecule was a secretory protein containing five amino acids encoded by TGA codons. Its functions were unique in that once secreted from the tumor cells, it bound back to the surface of these cells and activated homologous complement (C3) via the alternative pathway, allowing for C3 deposition on the membrane. This molecule may offer new insight into innate immunity; surveillance of tumor cells by complement is a common feature in the human immune system.