Comparative innate immune interactions of human and bovine secretory IgA with pathogenic and non-pathogenic bacteria

Secretory IgA (SIgA) from milk contributes to early colonization and maintenance of 27 commensal/symbiotic bacteria in the gut, as well as providing defence against pathogens. 28 SIgA binds bacteria using specific antigenic sites or non-specifically via its glycans attached 29 to α-heavy-chain and secretory component. In our study, we tested the hypothesis that human 30 and bovine SIgA have similar innate-binding activity for bacteria. SIgAs, isolated from 31 human and bovine milk, were incubated with a selection of commensal, pathogenic and 32 probiotic bacteria. Using flow cytometry, we measured numbers of bacteria binding SIgA and 33 their level of SIgA binding. The percentage of bacteria bound by human and bovine SIgA 34 varied from 30 - 90% depending on bacterial species and strains, but was remarkably 35 consistent between human and bovine SIgA. The level of SIgA binding per bacterial cell was 36 lower for those bacteria that had a higher percentage of SIgA-bound bacteria, and higher for 37 those bacteria that had lower percentage of SIgA-bound bacteria. Overall, human and bovine 38 SIgA interacted with bacteria in a comparable way. This contributes to longer term research 39 about the potential benefits of bovine SIgA for human consumers.