Induction of cell-mediated immunity to Staphylococcus aureus in the mouse mammary gland by local immunization with a live attenuated mutant.

The efficacy of intramammary (Ima) immunization with a live attenuated (la) Staphylococcus aureus mutant to protect the mouse mammary gland from infection has previously been established. The present study was aimed at evaluating whether Ima immunization with la-S. aureus can induce cell-mediated immune responses to the pathogen within the mammary gland. Mice were immunized by Ima route with la-S. aureus, and regional lymph node mononuclear cells were obtained thereafter. A higher expression of the interleukin-2 receptor was found on B and T cells from immunized mice when they were compared with control mice. Immunization with la-S. aureus induced strong proliferative responses to S. aureus. Moreover, significantly increased levels of gamma interferon (IFN-gamma) were produced by CD4+ T cells when lymphocytes from immunized mice, but not from control mice, were cultured in the presence of staphylococcal antigens. Moreover, a significant increase in the percentage of IFN-gamma-producing CD4+ and CD8+ T cells was observed after S. aureus Ima challenge in immunized mice compared to challenged control mice. Our results demonstrated that Ima immunization with la-S. aureus induced primed lymphocyte populations capable of responding against staphylococcal antigens during in vitro stimulation, as well as during in vivo infection by S. aureus. CD4+ and CD8+ T cells appear to be the main lymphocyte subpopulations involved in this response. It is suggested that IFN-gamma production induced by Ima immunization may play a pivotal role in the eradication of intracellular staphylococci.