Interferon-gamma (IFN-γ) is closely associated with the generation of cell-mediated immunity and resistance to intracellular parasites. Interleukin-18 (IL-18) was known to strongly induce IFN-γ production by T cells and natural killer (NK) cells. In order to determine whether injection with DNA encoding IL-18 can stimulate the resistance to Mycobacterium avium complex (MAC) infection, the mature IL-18 cDNA with κ leader sequence was cloned under control of the cytomegalovirus (CMV) promoter (TcCMVIL-18) and its effect on MAC infection was investigated in genetically susceptible BALB/c mice. Injection with the TcCMVIL-18 DNA during intranasal infection with MAC resulted in a significant decrease in bacterial load of lung during the entire 8-week observation period, while injection with the TcCMV control DNA did not. Lung cells in mice injected with the TcCMVIL-18 DNA showed persistent production of IFN-γ throughout the 8-week period. Furthermore, immunization with the TcCMVIL-18 DNA induced and maintained significantly higher levels of cytotoxic activity and nitric oxide production by lung cells than immunization with the TcCMV control vector. This work suggests that IL-18 DNA vaccination may be useful in the immunotherapeutic or immunoprotection approaches of infections by intracellular parasites such as mycobacteria.
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