Response of scid mice to establishment of Leishmania major infection.

The initiation of Leishmania major infection in susceptible BALB/c mice is regulated by interferon-gamma (IFN-gamma). To examine further the mechanisms of IFN-gamma-dependent regulation of the establishment of L. major, we studied the characteristics of the infection in severe combined immunodeficient (scid) mice. In the first 2 weeks of infection, we observed a delay in the development of the lesions in the footpads and lower numbers of parasites in scid compared with BALB/c mice. By week 5 after infection, the size of the leishmanial lesion was similar in both strains of mice, but the number of parasites in scid mice was 100-fold higher than in BALB/c. Treatment with anti-IFN-gamma during the establishment of L. major did not alter the course of infection in scid mice, while it exacerbated lesion development in BALB/c mice. Macrophages from scid mice were unable to kill L. major when stimulated with IFN-gamma in vitro, and produced lower levels of nitric oxide compared with macrophages from susceptible BALB/c or the resistant C57Bl/6 mice. We examined whether delayed lesion development in scid mice was due to their inability to mount appropriate inflammatory responses. While significantly fewer nucleated cells were present in the footpads of scid mice compared with BALB/c, 2 and 3 weeks after infection, no difference in inflammatory response between scid and BALB/c mice was observed in response to L. major antigen in the footpads. In contrast, there was a dramatic increase in the number of cells in the popliteal lymph nodes of BALB/c mice. Decreased inflammatory responses of scid mice in the footpad (at the site of infection) may contribute to slower development of leishmanial lesions during the first 2 weeks of infection.