Colostrogenesis: Mass transfer of immunoglobulin G1 into colostrum

Bovine IgG1 is thought to be specifically transported by a process of transcytosis across the mammary epithelial cells during colostrogenesis. Mammary IgG1 appearance in cow colostrum has typically been reported as a concentration and shows IgG1 concentration to be extremely variable because of animal variation, colostrum milking time, and water dilution effects. To identify animal IgG1 transfer capacity and separate it from the other effects, our objective was to determine first colostrum IgG1 total mass. We collected 214 samples of totally milked first colostrum with recorded colostrum weights from 11 Pennsylvania dairy farms that participated in Pennsylvania Dairy Herd Improvement Association, analyzed colostrum for IgG1 by ELISA, and calculated total IgG1 mass. Median and mean concentrations of IgG1 were 29.4 mg/mL and 37.5±30.2 mg/mL, respectively, with a range of 9 to 166 mg/mL. However, total mass of IgG1 had a median of 209.1g, mean of 291.6±315.8g, and a range of 14 to 2,223g. Colostrum IgG1 concentration showed no relationship with colostrum volume, but IgG1 mass had a positive relationship with volume. Colostrum IgG1 mass was related to IgG1 concentration (R2=0.58). Using DHIA records for 196 animals, we established milk production for these animals to a 15-d equivalent. An established milk secretion relationship to mammary parenchyma tissue (secretory tissue) was calculated and showed no relationship of IgG1 mass with mammary parenchyma tissue. In addition, we show that approximately 10% of the sampled animals had IgG1 mass greater than 1 standard deviation above the mean (high mass transfer) and represented all parities tested (1–7). Whereas first-lactation animals showed less overall calculated parenchyma tissue when compared with other parities, approximately 10% of the first-lactation group animals were capable of high mass transfer, with one transporting 2,029g into first colostrum. Concentration variance of IgG1 can be attributed to water inclusion, whereas mass transfer provides a clear indication of animal IgG1 transfer capacity. The specific mechanism of bovine mammary IgG1 transfer is not clear, but secretory tissue mass does not explain the variation observed. We hypothesize that the animal variation is attributable to endocrine regulation or genetic variation of the transporter(s).