INTRACYTOPLASMIC DELIVERY OF LISTERIOLYSIN-O BY A VACCINAL STRAIN OF BACILLUS-ANTHRACIS INDUCES CD8-MEDIATED PROTECTION AGAINST LISTERIA-MONOCYTOGENES

The facultative intracellular pathogen Listeria monocytogenes secretes a 58-kDa hemolysin, listeriolysin O (LLO), that allows bacteria to ac cess the cytoplasm and to multiply inside infected cells, LLO is also a protective Ag required for the development of specific immunity, We studied the capacity of a new bacterial vector, derived from an attenu ated strain of Bacillus anthracis, to deliver in vivo LLO and to induc e protection against L. monocytogenes infection, The hly gene encoding LLO was fused to a B. anthracis regulatory region induced in vivo and was integrated into a resident plasmid of this bacterium, This recomb inant strain secreted a functional LLO in vitro and inside phagosomes of bone marrow macrophages, This LLO production enabled the conversion of the extracellular replicating B. anthracis into an intracytoplasmi c bacterium, LLO+ B. anthracis thus mimicked the intracellular behavio r of L. monocytogenes in macrophages, Specific protection of mice agai nst lethal doses of L. monocytogenes was induced by immunization with LLO+ B. anthracis. The immunity was mediated by CD8(+) T lymphocytes a nd was associated with the activation of LLO-specific MHC class I-rest ricted CD8(+) CTL, able to recognize the immunodominant H-2(d)-restric ted epitope 91-99 of LLO. This study, therefore, suggests that intracy toplasmic delivery of LLO by, anthracis is sufficient to induce a MHC class I-restricted CD8-mediated protection against L. monocytogenes. T he LLO+ B. anthracis recombinant strain represents a potential vector for delivering foreign Ags involved in CD8-mediated protective respons es.