Mammary serum amyloid A3 activates involution of the mammary gland in dairy cows

The dry period is a nonlactating phase in which senescent mammary cells are regenerated, which is thought to optimize milk production in the subsequent lactation. In bovines, the dry period normally coexists with pregnancy and the lactogenic hormones delay mammary gland involution and impair the activation of immune system to fight the risk of intramammary infections. Conventionally, long dry periods of up to 60 d are required to allow sufficient mammary regeneration for full milk yield in the next lactation. The aim of this study was to evaluate the potential of mammary serum amyloid A3 (M-SAA3) as an activator of the involution of the mammary gland. One milligram of recombinant M-SAA3 and the corresponding negative controls (saline solution and lipopolysaccharide) were infused into the mammary gland via the teat canal, and mammary secretion samples were taken during the first 3 d after drying off to analyze metalloproteinase activity, somatic cell count, protein, and fat contents. Primary mammary gland epithelial cell cultures and bovine dendritic cells, obtained from necropsy tissue and blood, respectively, were incubated with and without M-SAA3 and cytokine expression was quantified. Last, the protective role of the M-SAA3 against infections was evaluated after a Staphylococcus aureus challenge. Matrix metalloproteinase 9 activity, a key protein that directly participates in the onset of the involution process, was greater in quarters treated with the M-SAA3. Protein content was increased in mammary secretions compared with control quarters. M-SAA3 increased cytokines directly related to innate immunity in both epithelial and dendritic cells and reduced the infection by Staphylococcus aureus.