Killing of Brucella antigen-sensitized macrophages by T lymphocytes in bovine brucellosis

The present study was an investigation into the role of T lymphocytes in the killing of antigen-sensitized macrophages (MΦ) in bovine brucellosis. Following confirmation of bovine T lymphocyte cell lines derived from Brucella abortus Strain 19 vaccinated steers as antigen-specific in proliferation studies using various antigens, we adapted an apoptosis assay for evaluation of cytotoxicity by these bovine T cells against autologous monocyte-derived macrophages (MDMΦ) as target cells. Various B. abortus antigen preparations were tested including whole γ-irradiated B. abortus bacteria (γBA), a soluble cytosolic protein fraction and a membrane-associated protein fraction. Both polyclonal and cloned T lymphocyte cell lines exhibited cytotoxicity against MDMΦ targets in an antigen-specific fashion. Polyclonal and cloned T lymphocyte cell lines demonstrated cytotoxic responses to varying degrees against B. abortus antigens regardless of whether the antigen used was whole nonviable bacteria, a soluble protein extract or a membrane-associated fraction of extracted bacteria. To further develop correlation of these responses to an in vivo host defense mechanism, cytotoxicity was evaluated using target cells that had been infected with live B. abortus S19 or B. abortus Strain 2308. Cytotoxic responses were also demonstrated consistently against infected targets with either strain of B. abortus although in most cases, cytotoxicity was higher against target cells sensitized with γBA compared to those infected with live bacteria. Cloned T lymphocyte cell lines were all CD4+, CD8− cells indicating that the observed cytotoxic responses were most likely due to an inflammatory Th1 response and may represent an important host defense mechanism induced by vaccination with live attenuated strains of B. abortus in cattle.