Mice were fed ad libitum with a normal diet (25% protein) or low-protein diets (0–12.5% protein) for a wk and then infected with a nonlethal or lethal strain of Plasmodium yoelii, that is, blood stage infection. The same diet was continued until recovery. Mice fed with a normal diet showed severe parasitemia during nonlethal infection, but survived the infection. They died within 2 wk in the case of lethal infection. However, all mice fed with low-protein diets survived without apparent parasitemia (there were small peaks of parasitemia) in cases of both nonlethal and lethal strains. These surviving mice were found to have acquired potent innate immunity, showing the expansion of NK1.1!TCRint cells and the production of autoantibodies during malarial infection. Severe combined immunodeficiency (scid) mice, which lack TCRint cells as well as TCRhigh cells, did not survive after malarial infection of lethal strain of P. yoelii, even when low-protein diets were given. These results suggest that low-protein diets enhanced innate immunity and inversely decreased conventional immunity, and that these immunological deviations rendered mice resistant against malaria. The present outcome also reminds us of our experience in the field study of malaria, in which some inhabitants eventually avoided contracting malaria even after apparent malarial infection

Mice were fed ad libitum with a normal diet (25% protein) or low-protein diets (0–12.5% protein) for a wk and then infected with a nonlethal or lethal strain of Plasmodium yoelii, that is, blood stage infection. The same diet was continued until recovery. Mice fed with a normal diet showed severe parasitemia during nonlethal infection, but survived the infection. They died within 2 wk in the case of lethal infection. However, all mice fed with low-protein diets survived without apparent parasitemia (there were small peaks of parasitemia) in cases of both nonlethal and lethal strains. These surviving mice were found to have acquired potent innate immunity, showing the expansion of NK1.1!TCRint cells and the production of autoantibodies during malarial infection. Severe combined immunodeficiency (scid) mice, which lack TCRint cells as well as TCRhigh cells, did not survive after malarial infection of lethal strain of P. yoelii, even when low-protein diets were given. These results suggest that low-protein diets enhanced innate immunity and inversely decreased conventional immunity, and that these immunological deviations rendered mice resistant against malaria. The present outcome also reminds us of our experience in the field study of malaria, in which some inhabitants eventually avoided contracting malaria even after apparent malarial infection

Balance between inflammatory and anti-inflammatory cytokines may be important in malaria presentation and outcome. To clarify cytokine interactions that produce pathology of malaria and control infection, C57BL/6 mice were infected with 10(4) parasitized RBCs from a non-lethal strain of Plasmodium yoelii. Kinetics was monitored showing the course of parasitemia, and cytokines were determined by RT-PCR from liver and spleen tissues. Inflammatory cytokines such as interferon-γ (IFNγ), interleukin (IL)-12, IL-6, tumor necrosis factor-α (TNFα) and anti-inflammatory cytokines, including IL-4 and IL-10, were investigated as key molecules that interact with immune cells in the activation of the immune responses. The production of IFNγ mRNA was found to be higher on day 7 than on day 21 after infection, and IL-12 and IL-6 showed higher expression in the liver than in the spleen. Though TNFα was highly expressed on day 14 after infection and on day 21 in the liver, such expression was decreased on day 21 in the spleen. Anti-inflammatory cytokines showed high expression in both the liver and spleen. The results suggest that a relative balance between inflammatory and anti-inflammatory cytokines is crucial and that the increase of inflammatory cytokine levels during the acute phase of malaria may reflect an early and effective immune response.The counteraction effect of anti-inflammatory cytokines is thought to play a role in limiting progression from uncomplicated malaria to severe life-threatening complications.

Athymic nude mice carry neither conventional T cells nor NKT cells of thymic origin. However, NK1.1
TCRint cells are present in the liver and other immune organs of athymic mice, because these lymphocyte subsets are truly of extrathymic origin. In this study, we examined whether extrathymic T cells had the capability to protect mice from malarial infection. Although B6-nu/nu mice were more sensitive to malaria than control B6 mice, these athymic mice were able to survive malaria when a reduced number of parasitized erythrocytes (5 103 per mouse) were injected. At the fulminant stage, lymphocytosis occurred in the liver and the major expanding lymphocytes were NK1.1
TCRint cells (IL-2RTCR). Unconventional CD8 NKT cells (V14
) also appeared. Similar to the case of B6 mice, autoantibodies (IgM type) against denatured DNA appeared during malarial infection. Immune lymphocytes isolated from the liver of athymic mice which had recovered from malaria were capable of protecting irradiated euthymic and athymic mice from malaria when cell transfer experiments were conducted. In conjunction with the previous results in euthymic mice, the present results in athymic mice suggest that the major lymphocyte subsets associated with protection against malaria might be extrathymic T cells.