A comparative study of secretory immunoglobulin A and immunoglobulin G in host defense in an in vitro pneumonia model

Background An exaggerated inflammatory response to infections including nosocomial pneumonia may be detrimental to the host and contribute to morbidity and mortality. Both secretory immunoglobulin A (SIgA) and IgG contribute to the immune defense of respiratory surfaces. However, their relative ability to protect against invasive infections and the resultant host inflammatory response are unknown and were the basis for this study. Methods Calu-3 cell (a respiratory epithelial cell line) monolayers were established in a Transwell system (Costar Corp., Cambridge, MA). Escherichia coli and either polyclonal SIgA or IgG were inoculated into the apical chamber and neutrophils (polymorphonuclear neutrophils) were added to the basal chamber. PMN cytotoxic potential was indexed by %CD11b expression, superoxide anion (O2−) production, and % elastase release. Bacterial translocation into the basal chamber was quantitated after log transformation. Calu-3 monolayer integrity was indexed by permeability to dextran fluorescein isothiocyanate. Results The addition of E coli to Calu-3/polymorphonuclear cocultures led to increases in O2− generation, elastase release, and CD11b expression. These effects were diminished by the addition of SIgA but not IgG. A similar effect was noted with Calu-3 barrier function. Conclusions SIgA may function to protect against microbial invasion of respiratory surfaces and protect against tissue injury from an exaggerated inflammatory response.