Characterization of potential sources of secretory immunoglobulin A reaching the rumen and its effects on microbial growth and fermentation.

Secretory immunoglobulin A (SIgA) is a major mucosal antibody known to modulate intestinal microbial ecology in nonruminant species; however, its role in the rumen remains largely unexplored. The objectives of this study were to identify potential sources of SIgA reaching the rumen and to evaluate whether colostrum-derived SIgA can influence rumen bacterial growth and fermentation. In study 1, SIgA concentrations were quantified in colostrum and milk from 15 multiparous Holstein cows during the first 42 d of lactation and in saliva from their respective calves during the first 42 d of life. Milk SIgA concentrations peaked in colostrum (∼8,000 μg/mL) and declined postpartum, stabilizing at approximately 780 μg/mL by 14 d postpartum. In contrast, salivary SIgA concentrations in calves were low during the first days of life (∼4 μg/mL) but increased with age, reaching a plateau near 930 μg/mL at 21 d of age. In study 2, expression of genes associated with SIgA synthesis and transcytosis was evaluated in salivary glands, reticulum, rumen, and small intestine of mature cows. Salivary glands exhibited SIgA-related gene expression comparable to the small intestine and greater than that of the reticulum and rumen, supporting saliva as a major source of SIgA reaching the forestomach. In study 3, purified colostrum-derived SIgA positively affected the growth kinetics of the fibrolytic bacteria Fibrobacter succinogenes S85, Prevotella bryantii B14, and Ruminococcus albus (strains 7 and 8) in pure culture, whereas heat-inactivated IgA had no effect, indicating a "bioactive"-dependent mechanism. In study 4, inclusion of SIgA in an in vitro batch culture system increased NDF disappearance and short-chain fatty acid production. These findings indicate that colostrum and milk serve as primary sources of SIgA during early life in calves, whereas salivary gland-derived SIgA becomes the predominant source as animals mature, given the limited SIgA secretion within the rumen itself. Additionally, SIgA modulated rumen microbial growth kinetics and fermentation under in vitro conditions. Collectively, these findings identify SIgA as a previously underrecognized host-derived factor capable of modulating rumen microbial ecology and fermentation, warranting further investigation in vivo.