Staphylococcus aureus is a major cause of bovine mastitis. Analysis of the genome sequence of the bovine isolate RF122 revealed genes encoding 11 predicted superantigens which may contribute to the persistence of infection. Toxic shock syndrome toxin-1 (TSST-1), staphylococcal enterotoxin like toxin L (SElL) and staphylococcal enterotoxin C (SECbov) are encoded on a staphylococcal pathogenicity island (SaPIbov), SEG, SEI, SElU, SElO, SElN and a truncated form of SElM are encoded by the enterotoxin gene cluster (egc) and a further 3 putative superantigen genes with homology to previously characterized superantigens were identified. Expression of all 11 predicted superantigen genes was detected in vitro, including several with growth phase dependent expression, indicating a distinct regulation.
Each of the superantigen genes have been cloned into the pALC2073 plasmid, which has an inducible promoter allowing controlled expression in a superantigen deficient strain of S. aureus, RN4220. The recent bovine genome sequencing project led to the identification of the full complement of T-cell receptor β variable (TRBV) sub-families. Lymphocyte expansion of all TRBV subfamilies in response to each superantigen in vitro is being investigated. For example, TSST-1 activates Vβ subfamilies 1, 2, 4 and 13, and SElL activates Vβ 1, 6, 10 and 16.
In order to investigate the role of superantigens in mastitis, a superantigen-deficient form of S. aureus RF122 has been constructed by sequential allele replacement, and will be used in experimental infection of dairy cows. Taken together, the results of this study will greatly enhance our understanding of the role of superantigens in disease pathogenesis.