Mastitis is defined as an inflammation of the udder, mainly caused by an infection by various bacterial species. Amongst infectious diseases, intramammary infections (IMI) are of major importance in dairy ruminants because of their high frequency and the increased production costs that they incur (loss of milk, treatment, culling). The most prevalent etiological group causing mastitis in sheep is Staphylococcus
with 78.9% of positive cultures [1
]. Coagulase-Negative Staphylococci
(CNS) are particularly frequent and represent 74.8% of all isolates, most of which are S. epidermidis
]. CNS are considered to be minor pathogens, causing moderate inflammatory responses and often subclinical infections in dairy ruminants [1
]. On the contrary, Coagulase-Positive Staphylococci
(CPS), largely represented by S. aureus
), are major pathogens in all dairy species. They are principally associated with both chronic and clinical forms of mastitis [3
], some of which can be very severe and in the worst case lead to a high mortality rate.
Although much work has been carried out in dairy ruminants to understand the complex physiological and cellular events that occur in the mammary gland in response to pathogens [3
], the protective mechanisms are still obscure. Schematically, when pathogens enter the udder lumen via the teat canal, they are detected by both immune and non-immune cells, and this is followed by the release of chemoattractants. As a consequence, neutrophils migrate from the blood flow to the infection site [4
]. These cells can phagocyte bacteria and exert bactericidal activities by releasing potent oxidative products [6
]. This massive recruitment of neutrophils in the udder incurs a dramatic increase in the milk somatic cell count (SCC) [5
]. Accordingly, SCC has been widely advocated as an easy-to-measure tool for predicting mastitis and discriminating between chronically infected and non-infected animals [5
]. Recent advances in microarray technology, that nowadays enable the expression analysis of thousands of infection-related genes, have provided novel insights into host response to pathogens. Microarray analysis is a well-adapted technology to investigate the gene regulation mechanisms underlying immunity against pathogens. Previously, gene expression profiles for challenged mammary tissue [8
], milk cells [12
] and peripheral blood mononuclear cells [13
] have been studied using microarrays.
There is overwhelming evidence that the host's response to IMI is under genetic control, as extensively described in earlier studies [14
]. Genetic parameters have been established for milk somatic cell scores (SCS) and occurrence of clinical mastitis, thus indicating that five to twenty percent of all variability between individuals is of genetic origin. Additionally, numerous quantitative trait loci (QTL) for udder health traits have been identified [16
]. However, up to date, only one of these QTL - the forebrain embryonic zinc finger-like gene -has been fully characterised [20
]. Apart from this QTL, the genetic basis of resistance is still largely unknown.
Nevertheless, breeding programmes for mastitis resistance have been implemented throughout the world in dairy cattle [14
] and sheep [21
] using indirect predictor traits such as clinical mastitis and SCS. To assess the effect of SCS-based selection for resistance or susceptibility to IMI, two divergent lines of dairy sheep were created on the basis of their parents' breeding values for SCS [22
]. Evaluation of the frequency and duration of mastitis in the two lines demonstrated that selection for decreased SCS is associated with a decrease of IMI [22
In the present study, we performed transcriptomic analysis of milk somatic cells (MSC), collected from mastitis resistant and susceptible ewes using a generic 15K oligonucleotide chip. MSC were collected after challenge with Se and Sa during the first and second lactations respectively. Our objective was to use this animal-model of divergent-SCS-lines in order to identify some of the genes and molecular mechanisms involved in the genetic basis of the protective host response to staphylococcal IMI.