Serum cytokines. The levels of proinflammatory cytokines (A, tumor necrosis factor α [TNF-α]; B, interleukin-6 [IL-6]; C, interferon-gamma [IFN-γ]; D, IL-10) in the sera were measured (N = 6, duplicate) after primary infection with NK19_mouse (a representative of the mouse-derived strains, white circles) or NK19_soil (a representative of the soil-derived strains, gray circles). No data were collected after 2 d of infection with NK19_mouse because all the mice died. Fourteen days after infection with NK19_soil (arrows), the infected mice were reinfected with a mouse-derived strain (NK19_mouse, white triangles; NKcd_mouse, white squares) or a soil-derived strain (NK19_soil, gray triangles; NKcd_soil, gray squares). The results from another primary infection experiment (NKcd_mouse versus NKcd_soil) are not shown, as the pattern of altered cytokine expression in each reinfected condition was similar to the results shown (primary infection with NK19_mouse versus NK19_soil). The symbol (*) signifies P < 0.05.