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In the sera of patients with acute bacterial infections specific autoantibodies (sIAPa) of the immunoglobulin class G (IgG) were found which bind to intestinal alkaline phosphatase (IAP) through the Fab portion. This was demonstrated using immunoaffinity (IA) isolation of sIAPa from patients' sera (particularly bacterial meningitis and ventriculitis) digestion with pepsin, purification of F(ab')2 fragments on protein A and subsequently binding on IAP coupled to CNBr (cyanogen bromide)-activated Sepharose. Immunoblots using specific anti-Fc and anti-Fab antibodies showed that the bulk of F(ab')2 fragments had bound. Additionally, binding of native IAP to the F(ab')2 fragments was observed after separation of F(ab')2 fragments using isoelectric focusing (IEF), blotting onto nitrocellulose and incubation with IAP. Moreover, we have demonstrated the occurrence of natural anti-IAP autoantibodies (nIAPa) which were isolated from sera of healthy individuals using IA chromatography. Investigation of isotype distribution revealed that IgG but not IgM or IgA were predominant even among nIAPa. The nIAPa fraction exhibited lower binding efficiencies on IEF blots than the sIAPa fraction, however, in contrast to sIAPa, cross-reactions with other autoantigens were observed for nIAPa. NIAPa and sIAPa did not show subclass restriction. As revealed by IEF the spectrotypes of sIAPa were found to be patient-specific, poly- to oligoclonal and stable during longer periods.

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