5-Aminolevulinic Acid Improves Immune Response in Dairy Cows In Vivo and In Vitro

5-Aminolevulinic acid (5-ALA) is a naturally occurring amino acid, which plays an important role in the biosynthesis of heme. We investigated its immunomodulatory effect as a potential bioactive feed additive by evaluating the expression of immune-related genes and oxidative status in dairy cows. Using a crossover design, six lactating Holstein cows randomized to control and treatment groups received a total mixed ration (TMR) or TMR plus 10 mg 5-ALA/kg of dry matter for 2 weeks. In vitro, peripheral blood mononuclear cells (PBMC) obtained from untreated dairy cows were incubated with 5-ALA in the presence of Escherichia coli lipopolysaccharide (LPS). Tumor necrosis factor-α (TNF-α) mRNA expression significantly decreased in PBMC of dairy cows fed 5-ALA, and interleukin-1β (IL-1β) tended to decrease. 5-ALA had no effect on the cluster of differentiation 3, interferon-γ, and Toll-like receptors in PBMC and 1,1-diphenyl-2-picrylhydrazyl free radical scavenging activity, relative superoxide dismutase activity, and thiobarbituric acid reactive substances in the blood, milk, and ruminal fluid of dairy cows. In vitro, TNF-α expression decreased in PBMC treated with 5-ALA (10 μg/mL) in the presence of LPS compared with LPS alone. Thus, 5-ALA may optimize the inflammatory response by controlling the expression of TNF-α in PBMC and eventually improving immunity in dairy cows.