Mastitis is an inflammation of the mammary gland commonly caused by bacterial infection. The inflammatory process is a normal and necessary immunological response to invading pathogens. The purpose of host inflammatory responses is to eliminate the source of tissue injury, restore immune homeostasis, and return tissues to normal function. The inflammatory cascade results not only in the escalation of local antimicrobial factors, but also in the increased movement of leukocytes and plasma components from the blood that may cause damage to host tissues. A precarious balance between pro-inflammatory and pro-resolving mechanisms is needed to ensure optimal bacterial clearance and the prompt return to immune homeostasis. Therefore, inflammatory responses must be tightly regulated to avoid bystander damage to the milk synthesizing tissues of the mammary gland. The defense mechanisms of the mammary gland function optimally when invading bacteria are recognized promptly, the initial inflammatory response is adequate to rapidly eliminate the infection, and the mammary gland is returned to normal function quickly without any noticeable clinical symptoms. Suboptimal or dysfunctional mammary gland defenses, however, may contribute to the development of severe acute inflammation or chronic mastitis that adversely affects the quantity and quality of milk. This review will summarize critical mammary gland defense mechanisms that are necessary for immune surveillance and the rapid elimination of mastitis-causing organisms. Situations in which diminished efficiency of innate or adaptive mammary gland immune responses may contribute to disease pathogenesis will also be discussed. A better understanding of the complex interactions between mammary gland defenses and mastitis-causing pathogens should prove useful for the future control of intramammary infections.

Measures of oxidative status were examined in 14 dairy cows during the transition period. Blood samples were obtained approximately 21 d before expected calving, at calving, and again at 21 d in milk (DIM). Plasma samples were used to determine lipid hydroperoxide concentrations. Total white blood cells were used to determine the oxidative status of glutathione. Peripheral blood mononuclear cell (PBMC) lysates were used to determine the total antioxidant potential and enzymatic activities of glutathione peroxidase (GPX) and thioredoxin reductase (TrxR1). Both plasma lipid hydroperoxide concentrations and GPX activity in PBMC increased at calving and during the first 21 DIM when compared with prepartum samples. Conversely, the total antioxidant potential and TrxR activity declined in PBMC during the first 21 DIM, even though both GPX activity and the glutathione-to-GSSG ratio remained elevated during this time period. Results from this study support previous findings that report increased GPX activity when reactive oxygen metabolites, including lipid hydroperoxides, increase in transition dairy cows. The significant decrease in TrxR activity with a concomitant decrease in total antioxidant potential in PBMC during this same stage of lactation, however, would suggest that this selenoprotein is not able to rebound during periods of oxidative stress to the same extent as GPX1. This study shows for the first time that TrxR may be an important antioxidant defense mechanism in PBMC that is compromised during the periparturient period.

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This study determined the cytokine profile of CD4+ T-helper cells to elucidate the specific CD4+ T-helper phenotype during the postpartum period. Peripheral blood mononuclear cells were isolated from cows during periods of increased susceptibility (3 d postpartum, n = 7) and decreased susceptibility (mid- to late lactation, n = 6) to mastitis. Isolated mononuclear cells were magnetically separated into CD4(+)-enriched or CD4(+)-depleted populations using specific bovine monoclonal antibodies and were confirmed to be enriched or depleted by flow cytometric analysis. T-helper-1 and T-helper-2 subpopulations were distinguished by cytokine profiles, at both the molecular and protein level, by competitive quantitative reverse transcriptase-polymerase chain reaction and specific bioassays, respectively. The CD4(+)-enriched cultures isolated postpartum had enhanced interleukin-4 and interleukin-10 mRNA transcript expression; cultures isolated during the mid- to late lactating period had enhanced interleukin-2 mRNA transcripts. Depletion of CD4+ lymphocytes decreased, and enrichment of CD4+ lymphocytes increased interferon-gamma transcripts in cultures isolated from mid- to late lactation cows. Interferon-gamma and interleukin-2 bioassays revealed that cytokine secretion paralleled mRNA transcript levels. These data suggest that CD4+ lymphocytes act predominantly as T-helper-2 compared with T-helper-1 within 3 d after calving. Alterations in the T-helper-1 and T-helper-2 responses, and therefore the repertoire of cytokines produced, may be an underlying reason for diminished host immune response during the postpartum period.

Glucose is an important energy substrate, especially needed by dairy cows postpartum to support the onset of lactation. The prioritization and regulation of glucose uptake is accomplished, in part, by changes in expression of cellular glucose transport molecules (GLUT) within the mammary gland. The objectives of this study were to (1) evaluate the expression and cell-type specific localization of GLUT and hypoxia-associated genes that may regulate GLUT expression over the transition period and through lactation in bovine mammary tissue and (2) determine functionality of GLUT on primary bovine mammary endothelial cells (BMEC). Mammary tissue biopsies were taken from cows at 15 d before calving and again at 1, 15, 30, 60, 120, and 240 d post-parturition for quantitative real-time PCR analysis of GLUT and hypoxia-associated genes. Additional mammary tissue samples were used to localize GLUT within the cells of the lobulo-alveolar system via fluorescence microscopy. Cultures of primary bovine mammary endothelial cells were used to confirm the functionality of GLUT with a fluorescent glucose analog uptake assay. Significant increases in GLUT1 gene expression were observed during early lactation, whereas both GLUT3 and GLUT4 gene expression increased during late lactation. The gene expression for 2 receptors of vascular endothelial growth factor increased significantly during early lactation and remained increased throughout lactation when compared with gene expression during the transition period. All GLUT were detected on cultured BMEC and were capable of internalizing glucose through GLUT-mediated mechanisms. These data suggest mammary vascular tissues express GLUT during lactation and BMEC express functional glucose transporters. A better understanding of glucose uptake at the endothelial level may prove to be critical to improve glucose absorption from the blood for utilization by mammary epithelial cells.