Coordination of the primary defense mechanisms against pathogens relies on the appropriate expression of pathogen recognition receptors (PRRs) triggering the early release of effector molecules of the innate immune system. To analyze the impact of this system on the counteraction of infections of the mammary gland (mastitis), we characterized the bovine gene encoding the key PRR Toll-like receptor 9 (TLR9) and mapped its precise position on chromosome BTA22. The sequence information was used to establish real-time PCR quantification assays to measure the mRNA abundances of TLR9, TLR2, and TLR4 together with those of β-defensin 5 (BNBD5), an early bactericidal effector molecule of the innate system, in healthy and infected mammary glands. Mastitis strongly increased (4- to 13-fold) the mRNA abundances of all of these genes except TLR9. Slight subclinical infections already caused a substantial increase in the copy numbers, though they did so the least for TLR9. Induction was not systemic, since mRNA abundance was low in uninfected control quarters of the udder but high in the severely infected quarters of the same animal. The number of TLR2 copies correlated well with those of TLR4, indicating coordinated regulation of these two PRRs during infection of the udder. Their coordinated regulation explains our unexpected observation that pureStaphylococcus aureusinfections caused a strong increase also in TLR4 mRNA abundance. In situ hybridizations revealed that BNBD5 is expressed predominantly in the mammary epithelial cells (MEC) of the infected gland. Our data therefore suggest a significant contribution of the innate immune system to counteract mastitis and attribute a prominent effector function to the MEC.

Veterinary Research, a journal on Animal Infection

Infections of the udder byEscherichia colivery often elicit acute inflammation, whileStaphylococcus aureusinfections tend to cause mild, subclinical inflammation and persistent infections. The molecular causes underlying the different disease patterns are poorly understood. We therefore profiled the kinetics and extents of global changes in the transcriptome of primary bovine mammary epithelial cells (MEC) after challenging them with heat-inactivated preparations ofE. coliorS. aureuspathogens.E. coliswiftly and strongly induced an expression of cytokines and bactericidal factors.S. aureuselicited a retarded response and failed to quickly induce an expression of bactericidal factors. Both pathogens induced similar patterns of chemokines for cell recruitment into the udder, butE. colistimulated their synthesis much faster and stronger. The genes that are exclusively and most strongly upregulated byE. colimay be clustered into a regulatory network with tumor necrosis factor alpha (TNF-α) and interleukin-1 (IL-1) in a central position. In contrast, the expression of these master cytokines is barely regulated byS. aureus. Both pathogens quickly trigger an enhanced expression of IL-6. This is still possible after completely abrogating MyD88-dependent Toll-like receptor (TLR) signaling in MEC. TheE. coli-specific strong induction of TNF-α and IL-1 expression may be causative for the severe inflammatory symptoms of animals suffering fromE. colimastitis, while the avoidance to quickly induce the synthesis of bactericidal factors may support the persistent survival ofS. aureuswithin the udder. We suggest thatS. aureussubverts the MyD88-dependent activation of immune gene expression in MEC.

Background: Coliform bacteria are the most common etiologic agents in severe mastitis of cows.Escherichia coliinfections are mostly restricted to a single udder quarter whereas neighboring quarters stay clinically inapparent, implicating the presence of a systemic defense reaction. To address its underlying mechanism, we performed a transcriptome study of mammary tissue from udder quarters inoculated withE. coli(6 h and 24 h post infection), from neighboring quarters of the same animals, and from untreated control animals.Results: After 6 h 13 probe sets of differentially expressed genes (DEG) were detected in infected quarters versus control animals. Eighteen hours later 2154 and 476 DEG were found in infected and in neighboring quarters vs. control animals. Cluster analysis revealed DEG found only in infected quarters (local response) and DEG detected in both infected and neighboring quarters (systemic response). The first group includes genes mainly involved in immune response and inflammation, while the systemic reaction comprises antigen processing and presentation, cytokines, protein degradation and apoptosis. Enhanced expression of antimicrobial genes (S100A8,S100A9,S100A12,CXCL2,GNLY), acute phase genes (LBP,SAA3,CP,BF, C6,C4BPA,IF), and indicators of oxidative stress (GPX3,MT1A,MT2A,SOD2) point to an active defense reaction in infected and neighboring healthy quarters. Its early onset is indicated by increased transcription ofNFIL3at 6 h. NFIL3 is a predicted regulator of many genes of the systemic response at 24 h. The significance of our transcriptome study was evidenced by some recent findings with candidate gene based approaches.Conclusions: The discovery and holistic analysis of an extensive systemic reaction in the mammary gland significantly expands the knowledge of host-pathogen interactions in mastitis which may be relevant for the development of novel therapies and for genetic selection towards mastitis resistance.