STRUCTURAL AND AFFINITY STUDIES OF IGM POLYREACTIVE NATURAL AUTOANTIBODIES

Natural polyreactive autoantibodies (NAA) are an important component o f the normal B cell repertoire. One intriguing characteristic of these Abs is their binding to various dissimilar Ags. It has been generally assumed that these Abs bind the Ags with low affinity, and are encode d by germline genes. We have used surface plasmon resonance to determi ne binding of avidities, and conducted a structural analysis of five m urine monoclonal natural autoantibodies displaying a typical polyreact ive binding pattern against cytoskeleton Ags and DNA. We show that 1) all the five Abs bind the different Ags with kinetic constants similar to those observed for immune Abs; 2) they express a restricted set of V-H and V-L genes, since the same V-H gene is expressed by three out of the five, and one particular V-K gene was expressed twice. In addit ion, a single D gene segment was used by three of the five Abs; and 3) they express, in most cases, genes in a close germline configuration. Our amino acid sequence and modeling studies show that the distributi on of exposed side chains in the NAA paratopes is close to the general pattern observed in the complementarity-determining regions (CDRs) of variable domains from immune Abs. Although CDR3 regions of the heavy chain have been postulated to play a major role in determining polyrea ctivity on the basis of recombinatorial experiments, our results faile d to show any distinctive particularity of this region in terms of len gth or charge when compared with classical immune Abs.