Fc:Fc interactions revealed by spin-labeled IgG heterosaccharides in model immune complexes

Dynamic properties of spin-labelled heterosaccharides in the Fc-region of murine monoclonal antihapten immunoglobulin G were studied in model immune complexes (IC) as a function of the IC size. Model IC dimers, trimers and oligomers were formed using bivalent photoaffinity antigens. The ESR spectrum exhibits two components. The rotational correlation time of the less-immobilized species is shorter than 10(-10) sec, and that of the more-immobilized component is in the order to 10(-9) approximately 10(-8) sec depending on the IC size. Fraction of the more-immobilized spin labels increases, and the mobility of this component decreases with increase in IC size (i.e., mobility: monomers approximately equal to dimers greater than trimers much greater than immune-complex precipitates). These data strongly suggest the existence of Fc:Fc interactions in IC, and provide the basis for a model in which such interactions underlie the initial mechanism by which the information of antigen binding to Fab region is transferred into organized Fc:Fc association structure for IgG effector activities.