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Membrane C1q (mC1q) of macrophages (MPhi) is a precursor of the IgG-binding serum protein C1q. Thus, mC1q potentially provides one of several Fcgamma binding sites of mature MPhi and we analyzed whether simultaneous expression occurs of established receptors for IgG, FcgammaRI, II, and III, and mC1q during in vitro differentiation of MPhi. Using flow cytometry, immunoprecipitation combined with Western blotting and Northern blot analysis mC1q was hardly detected in freshly isolated blood monocytes, but increasingly in developing monocyte-derived MPhi. Laser scanning fluorescence microscopy confirmed the membrane localization of mC1q. Two-color-staining flow cytometry experiments indicated that mC1q and all three types of FcgammaRs are simultaneously expressed on mature monocyte-derived MPhi. A high correlation was found for the expression of mC1q and FcgammaRs, in particular FcgammaRII, but not mC1q and CD14, another marker of monocytes/MPhi.

The experiments in this paper provided evidence that, besides lipopolysaccharides (LPS), porins of gram-negative bacteria bind to C1q and C1. From these experiments, we concluded that the association of LPS and porins (outer membrane proteins, OMP) may potentiate the C1q and C1 binding in the absence of specific antibodies. This antibody independent binding of C1 to LPS and porins is a prerequisite for the activation of the classical pathway of complement leading to the killing of serum-sensitive bacteria.