The transient pore formed by homologous terminal complement complexes functions as a bidirectional route for the transport of autocrine and paracrine signals across human cell membranes

Background: We have previously shown that the membrane attack complex (MAC) of complement stimulates cell proliferation and that insertion of homologous MAC into the membranes of endothelial cells results in the release of potent mitogens, including basic fibroblast growth factor (bPGF). The mechanism of secretion of bFGF and other polypeptides devoid of signal peptides, such as interleukin 1 (IL-1) is still an open problem in cell biology. We have hypothesized that the homologous MAC pore itself could constitute a transient route for the diffusion of biologically active macromolecules in and out of the target cells. Materials and Methods: Human red blood cell ghosts and artificial lipid vesicles were loaded with labelled growth factors, cytokines and IgG, and exposed to homologous MAC. The release of the I-125-macromolecules was followed as a function of time. The incorporation of labelled polypeptides and fluorescent dextran (MW:10,000) was measured in MAC-impacted human red blood cells and human umbilical endothelial cells (HUVEC), respectively. Results: Homologous MAC insertion into HUVEC resulted in the massive uptake of 10-kD dextran and induced the release of bFGP, in the absence of any measurable lysis. Red blood cell ghosts preloaded with bFGF, IL-1 beta, and the alpha-chain of interferon-gamma(IFN-gamma) released the polypeptides upon MAC insertion, but they did not release preloaded IgG. MAC-impacted ghosts took up radioactive IFN-gamma from the extracellular medium. Vesicles loaded with IL-1 released the polypeptide when exposed to MAC. Conclusions: The homologous MAC pore in its nonlytic form allows for the export of cytosolic proteins devoid of signal peptides that are not secreted through the classical endoplasmic reticulum/Golgi exocytotic pathways. Our results suggest that the release, and perhaps the uptake, of biologically active macromolecules through the homologous MAC pore is a novel biological function of the complement system in mammals.