Abstract
As the predominant antibody type in mucosal secretions, human colostrum, and breast milk, secretory IgA (SIgA) plays a central role in safeguarding the intestinal epithelium of newborns from invasive enteric pathogens like the Gram-negative bacterium Salmonella enterica serovar Typhimurium (STm). SIgA is a complex molecule, consisting of an assemblage of two or more IgA monomers, joining (J)-chain, and secretory component (SC), whose exact functions in neutralizing pathogens are only beginning to be elucidated. In this study, we produced and characterized a recombinant human SIgA variant of Sal4, a well-characterized monoclonal antibody (mAb) specific for the O5-antigen of STm lipopolysaccharide (LPS). We demonstrate by flow cytometry, light microscopy, and fluorescence microscopy that Sal4 SIgA promotes the formation of large, densely packed bacterial aggregates in vitro. In a mouse model, passive oral administration of Sal4 SIgA was sufficient to entrap STm within the intestinal lumen and reduce bacterial invasion into gut-associated lymphoid tissues by several orders of magnitude. Bacterial aggregates induced by Sal4 SIgA treatment in the intestinal lumen were recalcitrant to immunohistochemical staining, suggesting the bacteria were encased in a protective capsule. Indeed, a crystal violet staining assay demonstrated that STm secretes an extracellular matrix enriched in cellulose following even short exposures to Sal4 SIgA. Collectively, these results demonstrate that recombinant human SIgA recapitulates key biological activities associated with mucosal immunity and raises the prospect of oral passive immunization to combat enteric diseases.
Highlights
As the predominant antibody type in mucosal secretions, human colostrum, and breast milk, secretory IgA (SIgA) plays a central role in safeguarding the intestinal epithelium of newborns from invasive enteric pathogens like the Gramnegative bacterium Salmonella enterica serovar Typhimurium (STm)
Recent advancements in the generation of recombinant antibodies via multivector mammalian expression systems have enabled the production of SIgA monoclonal antibodies.29−31While there are examples in which passively administered mouse IgA or SIgA has been shown to afford immunity against experimental shigellosis and cholera, the use of human SIgA is only beginning to be explored.[32−35] For example, we recently showed in a mouse model of invasive Salmonella infection that Sal[4] IgA, an anti-lipopolysaccharide mouse monoclonal antibody (mAb), was sufficient at reducing the invasion of Salmonella enterica serovar Typhimurium (STm) into Peyer’s patch tissues.[36]
Triple cotransfection with a plasmid encoding human J-chain resulted in the formation of IgA products that by size-exclusion chromatography (SEC) were consistent with dimer formation, while quadruple transfection with the addition of a vector encoding human secretory component (SC) resulted in the appearance of a product with a molecular weight of >280 kDa, consistent with the formation of SIgA
Summary
We produced and characterized a recombinant human SIgA form of the monoclonal antibody, Sal[4]. In a controlled human infection model, multiple doses of hyperimmune bovine colostrum (HBC) before and after the oral ETEC challenge were necessary to prevent diarrhea.[64] In mice, preincubation of the ETEC with SIgA mAbs was required to reduce bacterial colonization.[37] Similar issues with achieving sufficient local antibody concentrations in the gut apply in the pursuit of using oral mAbs for therapeutic use for STm. While, in a mouse model, human plasma-derived secretory antibodies have been reported to promote survival after the STm challenge, the window of protection was limited to 8 h after the initial inoculation, and a number of the animals still succumbed to infection at the end of the study.[35] Despite limitations, these results emphasize the developing potential of recombinant SIgA as an alternative approach for combatting enteric pathogens.
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