Minimal requirements for murine resistance to infection with Francisella tularensis LVS.

Intraperitoneal or intravenous infection of mice with Francisella tularensis LVS is lethal, with an intraperitoneal 50% lethal dose (LD50) approaching a single bacterium. Intradermal (i.d.) LVS infection has a much higher LD50, about 10(6) bacteria in BALB/cByJ mice, and survival of i.d. infection leads to solid generation of immunity against lethal challenge. To define the minimal requirements for both initial and long-term survival of i.d. infection, we characterized the nature of i.d. LVS infection in lymphocyte-deficient BALB/cByJ.scid (scid) mice. scid mice infected i.d. with strain LVS survived for about 20 days and then died from overwhelming disseminated infection. However, scid mice treated with monoclonal antibodies to gamma interferon, tumor necrosis factor alpha, or neutrophils-granulocytes all died within 1 week of infection, indicating that these were essential for early control of infection. Studies using GKO (gamma interferon knockout) mice emphasized that gamma interferon is absolutely required for initial survival of i.d. LVS infection. scid mice could be reconstituted for long-term survival of i.d. LVS infection and clearance of bacteria by intravenous transfer of splenic lymphocytes or purified B220-/T+ lymphocytes but not nu/nu lymphocytes. T cells are therefore required for long-term clearance and survival of i.d. LVS infection; efforts to determine whether CD4+ T cells, CD8+ T cells, or both are involved are ongoing.