Our studies have revealed that patients with Cystic Fibrosis CF who are infected with P. aeruginosa have grossly elevated serum levels of IgG antibodies to the opsonic immunodeterminant, type-specific LPS. Second, this elevation is distributed among all four IgG subclasses, with a significant shift towards IgG3. Third, sera from colonized CF patients shows diminished opsonic capacity, although complement dependent human neutrophil phagocytosis is not notably impaired. Fourth, functional polyclonal or monoclonal antibody opsonins exhibit prozone inhibition of phagocytosis at high concentrations. Fifth, sera from uninfected CF patients have lower levels and proportions of IgG2 antibodies to P. aeruginosa LPS, and higher levels and proportions of IgG4 antibodies, than normal controls. Finally, levels of IgG4 antibodies, but not IgG1, 2, or 3, correlate inversely with opsonic capacity. We therefore make several speculations. High levels of IgG4 antibodies to opsonic immunodeterminants may inhibit normal pulmonary clearance of P. aeruginosa by alveolar macrophages in vivo. Second, high levels of opsonic antibodies may also contribute to the problem in vivo by the phenomenon of prozone inhibition. Third, reduced levels of IgG2 antibodies in uninfected CF patients raises the intriguing possibility of an wider polysaccharide antigen-related isotype-restricted immunodeficiency, with an attempted compensatory shift to IgG4 doomed to failure.
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