Pathogenic microorganisms invading the mammary gland induce an inflammatory reaction which includes an increase of somatic cells in milk and activation of bacteriostatic enzymes and proteins in milk. During spontaneously occurring subclinical mastitis the somatic milk cells, mainly macrophages, secrete cytokines, eicosanoids, acute phase proteins and other immunomediators. In contrast, the bacteriostatic protein lactoferrin is mainly secreted by mammary epithelial tissue, while major milk proteins like α-lactalbumin and κ-casein are down-regulated already during subclinical infection.

Changes of the mRNA expression of various immunomediators in the mammary tissue of cows during 12 h after induction of mastitis via intramammary administration of lipopolysaccharide (LPS) in several studies are reported. Six healthy lactating cows were injected in one quarter with 100 μg Escherichia coli-LPS (O26[ratio ]B6) and the contralateral quarter with saline (9 g/l) serving as control. mRNA expression in mammary biopsy samples of various inflammatory factors and milk proteins at 0, 3, 6, 9 and 12 h after LPS administration was quantified by real-time reverse transcription-PCR.

In LPS-challenged quarters tumour necrosis factor α and cyclooxygenase-2 mRNA expression increased to their highest values (P<0·05) at 3 h after LPS-challenge. Expression of lactoferrin, lysozyme, inducible nitric oxide synthase, and of the apoptotic factors caspase-3, caspase-7 and FAS was elevated (P<0·05) and peaked at 6 h after challenge. No significant increase in mRNA expression of platelet-activating factor acethylhydrolase, 5-lipoxygenase, and insulin-like growth factor 1 was found. None of the parameters tested did change significantly in the control quarters. mRNA expression of major milk proteins did not change significantly in response to the LPS challenge (αS1-casein, αS2-CN, β-CN and β-lactoglobulin) except for α-lactalbumin which decreased (P<0·05) in LPS-treated and control quarters and for κ-CN which decreased in the LPS-treated quarters. In conclusion, mRNA expression of the majority albeit not all inflammatory factors changed within hours of LPS challenge. Decreased gene expression of α-lactalbumin and κ-CN may reduce milk yield and suitability for cheese production.