An essential role for gamma interferon in innate resistance to Shigella flexneri infection

Shigella spp. are the major cause of bacillary dysentery worldwide. To identify immune effectors associated with protection of the naive host during infection, the susceptibility to pulmonary Shigella infection of each of various mouse strains that have a targeted deletion in a specific aspect of the immune system was evaluated. Our results demonstrate that mice deficient in gamma interferon are 5 orders of magnitude more susceptible to Shigella than are wild-type mice, whereas mice deficient in B and T lymphocytes or in T lymphocytes alone exhibit no difference in susceptibility. Significantly lower numbers of shigellae were recovered from immunocompetent compared with gamma-interferon-deficient mice after infection. While immunocompetent mice were able to clear a sublethal Shigella inoculum by day 5 postinfection, progressively increasing numbers of shigellae were cultured from the lungs of gamma interferon-deficient mice over the same period. Histopathology of the lungs from immunocompetent mice infected with a sublethal Shigella inoculum showed mild inflammatory changes, whereas the lungs from gamma interferon-deficient mice demonstrated progressively worsening acute bronchiolitis with ulceration. Further, the time to death in gamma interferon-deficient mice correlates inversely with the size of the Shigella inoculum. To identify the cellular source of gamma interferon, we infected SCID mice, T-cell-receptor-deficient mice, beige mice (a mouse strain deficient in natural killer [NK] cell activity), and mice depleted of NK cells using anti-asialo-GM1. Each NK cell-deficient mouse strain exhibited a 10-fold-greater susceptibility to Shigella infection than immunocompetent mice. To test the protective effects of gamma interferon in vitro, survival of intracellular Shigella was examined in primary macrophages from wild-type mice, primary macrophages from gamma interferon-deficient mice, a macrophage cell line, and a fibroblast cell line. Following activation with gamma interferon, each cell type eradicated intracellular Shigella, while nonactivated macrophages fostered Shigella replication and nonactivated fibroblast cells fostered both Shigella replication and intercellular spread. Taken together, these data establish that NK cell-mediated gamma interferon is essential to resistance following primary Shigella infection.