Gene expression profiling in response to experimental Staphylococcus aureus infection in mammary gland tissues of two divergent lines of goat for high and low milk somatic cell scores

Staphylococcus aureus is a frequent and costly disease for the dairy ruminants and it is responsible for chronic, subclinical, clinical and gangrenous mastitis. Disease response is a complex trait under multi-genic control, which makes it difficult to develop appropriate genetic selection strategies for improving immune response, also because we have very few information about the genetic basis of resistance and functional complexity of the host pathogen interaction during infection. The number of somatic cells in milk is correlated with intra-mammary infection and cattle breeders used somatic cell scores (SCS) in genetic selection for resistance to mastitis. For goats, the relevance of SCS as a predictor of udder health parameters and susceptibility against mastitis is still unclear. The aim of the present study was to investigate in caprine mammary gland tissue the response to an experimental infection with S. aureus between two divergent lines of goat selected for high and low milk somatic cell scores (HSCS and LSCS, respectively). For this study, the left mammary glands of six primiparous alpine goats (three from HSCS and three from LSCS, respectively) were inoculated with one S. aureus strain, while right udder halves were infused with sterile solution. Thirty hours post inoculation, tissue samples were examined by histopathological and immunohistochemical analyses and total RNA was extracted and analyzed by the bovine 90K CustomArray (Combimatrix). Normal tissues of LSCS line revealed mild interstitial lymphoplasmacytic inflammation and lesser numbers of alveolar macrophages than HSCS. Infected tissue samples showed more severe lymphoplasmacytic inflammation and intra-alveolar macrophage accumulation with maximal luminal macrophage presence in HSCS. Only infected tissues showed neutrophilic intra-alveolar inflammation; interstitial microabscesses were observed in HSCS. Moreover, differences in gene expression were found between HSCS and LSCS both in normal and in infected tissues. In normal tissues 16 genes were obtained belonging to "energy production", "molecular transport and amino acids metabolism" networks. In infected tissues 20 differentially expressed genes, encoding for host defense against microbial infection and anti-inflammatory activity, were detected. These preliminary results represent a primary approach for further studies on understanding the genetic basis of mastitis in the early stage.