Modified C-reactive protein selectively binds to immunoglobulins

Modified C-reactive protein (mCRP) has been reported to non-specifically bind to immunoglobulins; notwithstanding, the nature of these interactions is not clear. The aim of this study was to investigate the binding of antibodies directed against HSA and IgG to mCRP, fibrinogen (Fg), IgG, fibronectin (Fn) and C1q and its contaminants. We also studied the binding of mCRP to the antibodies directed towards receptors involved in CRP signalling (anti-CD32, anti-CD16). For the analysis of such interactions, a combination of ELISA and Western immunoblotting has been applied. The tested antibodies powerfully bound to either the contaminations of purified proteins (Fg, IgG, Fn and mCRP) or interacted directly with some of these proteins (C1q, mCRP, Fg). The effectiveness of anti-HSA binding to immobilized proteins was influenced by the antigenic specificity of the antibody, the content of various protein fractions in the contaminants of a given protein (albumin augmented the interactions), overall protein purity and a natural avidity of a given protein towards immunoglobulins. The relative binding of anti-HSA or anti-IgG to immobilized mCRP was considerably lower than that observed for plasma proteins. Furthermore, the strength of the direct interaction between immunoglobulins and mCRP varied from the lack of response (anti-HSA) or a negligible response (anti-IgG) to the relatively high signal (human IgG, anti-CD16, anti-CD32), as compared to the control. Based on these observations, we conclude that the binding of mCRP to immunoglobulins cannot be easily generalized as a kind of some universal phenomenon.