In vivo modulation of the murine immune response to Francisella tularensis LVS by administration of anticytokine antibodies

The role(s) of gamma interferon (IFN-γ), tumor necrosis factor alpha (TNF-α), and interleukin-4 (IL-4) in establishment and maintenance of protective immunity to Francisella tularensis LVS in mice (C3H/HeN) was examined by selective removal of these cytokines in vivo with neutralizing antibodies. The 50% lethal dose (LD₅₀) for mice infected intradermally with F. tularensis alone was 136,000 CFU; treatment of mice with anti-IFN-γ or anti-TNF-α at the time of infection significantly reduced (P << 0.05) the LD₅₀ to 2 and 5 CFU, respectively. Abrogation of protective immunity, however, was effective only when anti-IFN-γ or anti-TNF-α was administered prior to day 3 postinfection. In contrast, the LD₅₀ for mice treated with anti-IL-4 was repeatedly higher (555,000 CFU) than for controls; this difference, however, was not significant (P > 0.05). Thus, IL-4 may be detrimental, while IFN-γ and TNF-α were clearly crucial to the establishment of protective immunity to F. tularensis during a primary infection. The importance of IFN-γ and TNF-α during a secondary immune response to F. tularensis was also investigated. Spleen cells from immune mice passively transfer protective immunity to recipient mice in the absence of confounding antibody-mediated immunity. This passive transfer of immunity, however, was abrogated by treatment of recipient mice with anti-IFN-γ or anti-TNF-α at the time of challenge infection. That anticytokines effectively abrogate protective immunity very early in the course of infection with F. tularensis suggests that T-cell-dependent activation of macrophages for microbicidal activity is unlikely. These T-cell-independent events early in the course of infection may suppress bacterial replication until a T-cell-dependent response ultimately clears the bacteria.