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Objective: To characterize the functional properties of natural autoantibodies capable of preventing in vitro infection by HIV-1, present in normal human serum (NHS), and denoted as IgG-reactive antibodies., Methods: IgG-reactive antibodies were affinity purified both from normal human serum (NHS) and from a GammaBind G Sepharose Flowthrough (GBF) fraction of NHS by affinity chromatography on IgG coupled to CNBr-activated Sepharose (IgG-Sepharose)., Results: The GBF fraction was shown, by Capture ELISA relative to isotype-matched standards, to contain in addition to IgM and IgA isotypes, a low but constant level of IgG isotype. About 15% of the GBF fraction's IgG, compared to only about 0.3% of the NHS IgG, was affinity purified on IgG-Sepharose. On IgG subclass analysis, in contrast to the characteristic dominance of IgG1 in pooled NHS, the IgG-reactive antibodies obtained from NHS and from the GBF fraction each showed a dominance of IgG2. Western blot analysis confirmed the abundance of IgG2, a major IgG subclass reactive against carbohydrate antigens, and showed the presence of IgG2 dimers. The IgG-reactive antibodies separated from the GBF fraction were able to neutralize HIV-1(BaL) strain with approaching 100% and 80% effectiveness at 2 microg/ml and 0.6 microg/ml, respectively, as well as the primary isolates HIV-1(NDK) (X4-tropic isolate) and HIV-1(JR-CSF) (R5-tropic isolate) with an IC50 between 0.4 microg/ml and 1.8 microg/ml for two different preparations., Conclusion: These findings further support our previous proposal for IgG-reactive antibody preparations to be used in the treatment of HIV-1 infected individuals.