Live virulent Rhodococcus equi, rather than killed or avirulent, elicits protective immunity to R. equi infection in mice

Mice inoculated intravenously with a sublethal dose of live virulent Rhodoc occus equi ATCC 33701 that contained an 85-kb virulence plasmid were immune to a lethal intravenous challenge of ATCC 33701. This immunity depended up on the dose of immunization and developed rapidly: mice primed with 10(5) l ive ATCC 33701 eliminated the challenged bacteria more rapidly than mice pr imed with doses ranging from 10(2) to 10(4) bacteria, and mice given 10(5) live ATCC 33701 intravenously withstood the lethal challenge as early as 5 days after the initial inoculation. However, this protective immunity did n ot develop in mice immunized with doses of heat-killed ATCC 33701 ranging f rom 10(6) to 10(8), or in mice immunized with doses of live ATCC 33701P(-), a plasmid-cured derivative (avirulent), in doses ranging from 10(5) to 10( 7). These mice had positive antibody titers against R. equi at the challeng e (14 days after priming). Adoptive transfer of resistance to virulent R. e qui was obtained with spleen cells from mice immunized with live ATCC 33701 , but not monoclonal antibody to 15- to 17-kDa virulence-associated antigen s. These results revealed that live ATCC 33701P-, a plasmid-cured derivativ e of virulent R. equi, could not elicit protective immunity, and are consis tent with previous observations that protective immunity was induced by liv e virulent, but not killed organisms. (C) 1999 Federation of European Micro biological Societies. Published by Elsevier Science B.V. All rights reserve d.