Plasmodium berghei XAT is an irradiation-induced attenuated variant derived from the lethal strain P. berghei NK65, and its blood-stage parasites are spontaneously cleared in immune competent mice. In the present study, we studied the mechanism of host resistance to blood-stage malaria infection using P. berghei XAT. Infection enhanced Ab-dependent phagocytosis of PRBC by splenic macrophages in wild-type C57BL/6 mice. In contrast, FcR γ-chain knockout (FcRγ−/−) mice, which lack the ability to mediate Ab-dependent phagocytosis and Ab-dependent cell-mediated cytotoxicity through FcγRI, FcγRII, and FcγRIII, could not induce Ab-dependent phagocytic activity. These FcRγ−/− mice showed increased susceptibility to the P. berghei XAT infection, with eventually fatal results, although they produced comparable amounts of IFN-γ by spleen cells and anti-XAT Abs in serum. In addition, passive transfer of anti-XAT IgG obtained from wild-type mice that had recovered from infection into FcRγ−/− mice could not suppress the increase in parasitemia, and almost all of these mice died after marked parasitemia. In contrast, passive transfer of anti-XAT IgG into control wild-type mice inhibited the increase in parasitemia. IFN-γ−/− mice, which were highly susceptible to the P. berghei XAT infection, failed to induce Ab-dependent phagocytic activity and also showed reduced production of serum anti-XAT IgG2a isotype compared with control wild-type mice. These results suggest that FcR-mediated Ab-dependent phagocytosis, which is located downstream of IFN-γ production, is important as an effector mechanism to eliminate PRBC in blood-stage P. berghei XAT infection.

The mechanism of development of host resistance to blood-stage malarial infection was studied by use of an irradiation-induced attenuated variant, Plasmodium berghei XAT, obtained from a lethal strain, P. berghei NK65. The infection enhanced mRNA expression of interleukin (IL)-12 p40 and also of interferon (IFN)-gamma, IL-4, IL-10, and cytokine-inducible nitric oxide synthase (iNOS) in spleen. Treatment of these mice with anti-IL-12 or anti-IFN-gamma led to the progression of parasitemia and fatal outcome. Anti-IL-12 treatment significantly reduced the secretion and mRNA expression of IFN-gamma and greatly diminished the augmentation of iNOS mRNA expression. In addition, recombinant IL-12 administration delayed the onset of parasitemia because of the enhanced IFN-gamma production. These results suggest that blood-stage P. berghei XAT infection induces IL-12 production, which is important for the development of host resistance via IFN-gamma production.

We have examined the roles of gamma interferon (IFN-γ), nitric oxide (NO), and natural killer (NK) cells in the host resistance to infection with the blood-stage malarial parasite Plasmodium berghei XAT, an irradiation-induced attenuated variant of the lethal strain P. berghei NK65. Although the infection withP. berghei XAT enhanced NK cell lytic activity of splenocytes, depletion of NK1.1+ cells caused by the treatment of mice with anti-NK1.1 antibody affected neither parasitemia nor IFN-γ production by their splenocytes. The P. bergheiXAT infection induced a large amount of NO production by splenocytes during the first peak of parasitemia, while P. berghei NK65 infection induced a small amount. Unexpectedly, however, mice deficient in inducible nitric oxide synthase (iNOS−/−) clearedP. berghei XAT after two peaks of parasitemia were observed, as occurred for wild-type control mice. Although the infected iNOS−/− mouse splenocytes did not produce a detectable level of NO, they produced an amount of IFN-γ comparable to that produced by wild-type control mouse splenocytes, and treatment of these mice with neutralizing anti-IFN-γ antibody led to the progression of parasitemia and fatal outcome. CD4−/− mice infected withP. berghei XAT could not clear the parasite, and all these mice died with apparently reduced IFN-γ production. Furthermore, treatment with carrageenan increased the susceptibility of mice toP. berghei XAT infection. These results suggest that neither NO production nor NK cell activation is critical for the resistance to P. berghei XAT infection and that IFN-γ plays an important role in the elimination of malarial parasites, possibly by the enhancement of phagocytic activity of macrophages.

Interleukin (IL)-16 is a chemoattractant cytokine for CD4+ leukocytes. Because delayed-type hypersensitivity (DTH) reaction is mediated by T helper 1 (Th1) cells and CD4+ T cells can be chemoattracted by IL-16, we have investigated the involvement of IL-16 in the DTH reaction. Immunohistochemical analysis revealed the IL-16 expression in infiltrating cells and epithelial cells in the DTH footpads. The IL-16 expression was also detected intracellularly in the infiltrating cells. In addition, markedly increased production of IL-16 was detected in the DTH footpad extracts, but not in the control footpad extracts, by an enzyme-linked immunosorbent assay and also by Western blot analysis. The DTH footpad extracts exhibited a strong chemoattractant activity toward splenic T cells, which was significantly inhibited by the inclusion of neutralizing monoclonal antibody (mAb) against IL-16 in the migration assay. Furthermore, treatment of sensitized mice in vivo with the anti-IL-16 neutralizing mAb significantly suppressed the footpad swelling induced by an antigen challenge, together with decreased infiltration of leukocytes including not only CD4+ T cells but also CD8+ T cells and macrophages into the DTH footpads. Decreased production of macrophage inflammatory protein 1 was also observed in the DTH footpad extracts by the mAb treatment. These results suggest that IL-16 plays an important role in the recruitment of leukocytes—presumably including antigen-specific Th1 cells, which secrete cytokines and chemokines mediating the following hypersensitivity reaction after activation by the interaction with Langerhans cells carrying the antigen—for the elicitation of DTH response.