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Intestinal helminthes induce immunosuppressive responses as well as type 2 immunity. Their suppressive properties are intended to regulate inflammatory diseases such as allergies and autoimmune diseases. This study evaluated whether helminthic infections suppress obesity, a chronic inflammatory state, using an intestinal nematode, Heligmosomoides polygyrus (Hp). Infection with Hp at the same time as feeding a high‐fat diet (HFD) prevented weight gain, dyslipidaemia and glucose intolerance observed in uninfected obese mice. Immunologically, Hp infection skewed M1 macrophages to M2 macrophages and induced type 2 innate lymphoid cells in adipose tissues. The expression of interleukin (IL)‐33, a potent initiator of type 2 responses, was also increased in association with uncoupled protein 1 (UCP1). To further investigate the anti‐obesity effects of IL‐33 in mice infected with Hp, IL‐33‐deficient mice were fed the HFD and infected with Hp. These mutant mice rapidly gained weight compared with wild‐type mice, indicating the anti‐obesity effect of IL‐33. In the absence of IL‐33, the rapid increase in weight was not prevented, and type 2 responses and UCP1 expression were not observed even during Hp infection. These results suggested that the suppression of obesity by Hp is dependent on IL‐33. [ABSTRACT FROM AUTHOR]

Our previous studies of protective immunity and pathology against blood stage malaria parasites have shown that not only CD4+ T cells, but also CD8+ T cells and macrophages, are important for host defense against blood stage malaria infection. Furthermore, we found that Plasmodium yoelii 17XNL (PyNL) parasitizes erythroblasts, the red blood cell (RBC) precursor cells, which then express MHC class I molecules. In the present study, we analyzed spleen cytokine production. In CD8+ T cell-depleted mice, IL-10 production in early stage infection was increased over two-fold relative to infected control animals and IL-10+ CD3− cells were increased, whereas IFN-γ production in the late stage of infection was decreased. At day 16 after PyNL infection, CD8+ T cells produced more IFN-γ than CD4+ T cells. We evaluated the involvement of the immunoproteasome in induction of immune CD8+ T cells, and the role of Fas in protection against PyNL both of which are downstream of IFN-γ. In cell transfer experiments, at least the single molecules LMP7, LMP2, and PA28 are not essential for CD8+ T cell induction. The Fas mutant LPR mouse was weaker in resistance to PyNL infection than WT mice, and 20% of the animals died. LPR-derived parasitized erythroid cells exhibited less externalization of phosphatidylserine (PS), and phagocytosis by macrophages was impaired. Furthermore, we tried to identify the cause of death in malaria infection. Blood lactate concentration was increased in the CD8+ T cell-depleted PyNL-infected group at day 19 (around peak parasitemia) to similar levels as day 7 after infection with a lethal strain of Py. When we injected mice with lactate at day 4 and 6 of PyNL infection, all mice died at day 8 despite demonstrating low parasitemia, suggesting that hyperlactatemia is one of the causes of death in CD8+ T cell-depleted PyNL-infected mice. We conclude that CD8+ T cells might control cytokine production to some extent and regulate hyperparasitemia and hyperlactatemia in protection against blood stage malaria parasites. [ABSTRACT FROM AUTHOR]

Amebiasis, a global intestinal parasitic disease, is due to Entamoeba histolytica. This parasite, which feeds on bacteria in the large intestine of its human host, can trigger a strong inflammatory response upon invasion of the colonic mucosa. Whereas information about the mechanisms which are used by the parasite to cope with oxidative and nitrosative stresses during infection is available, knowledge about the contribution of bacteria to these mechanisms is lacking. In a recent study, we demonstrated that enteropathogenic Escherichia coli O55 protects E. histolytica against oxidative stress. Resin-assisted capture (RAC) of oxidized (OX) proteins coupled to mass spectrometry (OX-RAC) was used to investigate the oxidation status of cysteine residues in proteins present in E. histolytica trophozoites incubated with live or heat-killed E. coli O55 and then exposed to H2O2-mediated oxidative stress. We found that the redox proteome of E. histolytica exposed to heat-killed E. coli O55 is enriched with proteins involved in redox homeostasis, lipid metabolism, small molecule metabolism, carbohydrate derivative metabolism, and organonitrogen compound biosynthesis. In contrast, we found that proteins associated with redox homeostasis were the only OX-proteins that were enriched in E. histolytica trophozoites which were incubated with live E. coli O55. These data indicate that E. coli has a profound impact on the redox proteome of E. histolytica. Unexpectedly, some E. coli proteins were also co-identified with E. histolytica proteins by OX-RAC. We demonstrated that one of these proteins, E. coli malate dehydrogenase (EcMDH) and its product, oxaloacetate, are key elements of E. coli-mediated resistance of E. histolytica to oxidative stress and that oxaloacetate helps the parasite survive in the large intestine. We also provide evidence that the protective effect of oxaloacetate against oxidative stress extends to Caenorhabditis elegans. [ABSTRACT FROM AUTHOR]

Although IL-12 is believed to contribute to protective immune responses, the role played by IL-23 (a member of the IL-12 family) in malaria is elusive. Here, we show that IL-23 is produced during infection with Plasmodium berghei NK65. Mice deficient in IL-23 (p19 KO) had higher parasitemia and died earlier than wild-type ( WT) controls. Interestingly, p19 KO mice had lower numbers of IL-17-producing splenic cells than their WT counterparts. Furthermore, mice deficient in IL-17 (17 KO) suffered higher parasitemia than the WT controls, indicating that IL-23-mediated protection is dependent on induction of IL-17 during infection. We found that macrophages were responsible for IL-17 production in response to IL-23. We observed a striking reduction in splenic macrophages in the p19 KO and 17 KO mice, both of which became highly susceptible to infection. Thus, IL-17 appears to be crucial for maintenance of splenic macrophages. Adoptive transfer of macrophages into macrophage-depleted mice confirmed that macrophage-derived IL-17 is required for macrophage accumulation and parasite eradication in the recipient mice. We also found that IL-17 induces CCL2/7, which recruit macrophages. Our findings reveal a novel protective mechanism whereby IL-23, IL-17, and macrophages reduce the severity of infection with blood-stage malaria parasites. [ABSTRACT FROM AUTHOR]

General cellular functions of proteasomes occur through protein degradation, whereas the specific function of immunoproteasomes is the optimization of antigen processing associated with MHC class I. We and others previously reported that deficiency in subunits of immunoproteasomes impaired the activation of antigen-specific CD8+ T cells, resulting in higher susceptibility to tumor and infections. We demonstrated that CD8+ T cells contributed to protection against malaria parasites. In this study, we evaluated the role of immunoproteasomes in the course of infection with rodent malaria parasites. Unexpectedly, Plasmodium yoelii infection of mice deficient in LMP7, a catalytic subunit of immunoproteasomes, showed lower parasite growth in the early phase of infection and lower lethality compared with control mice. The protective characteristics of LMP7-deficient mice were not associated with enhanced immune responses, as the mutant mice showed comparable or diminished activation of innate and acquired immunity. The remarkable difference was observed in erythrocytes instead of immune responses. Parasitized red blood cells (pRBCs) purified from LMP7-deficient mice were more susceptible to phagocytosis by macrophages compared with those from wild-type mice. The susceptibility of pRBC to phagocytosis appeared to correlate with deformity of the membrane structures that were only observed after infection. Our results suggest that RBCs of LMP7-deficient mice were more likely to deform in response to infection with malaria parasites, presumably resulting in higher susceptibility to phagocytosis and in the partial resistance to malaria. [ABSTRACT FROM AUTHOR]

When developing malaria vaccines, the most crucial step is to elucidate the mechanisms involved in protective immunity against the parasites. We found that CD8 T cells contribute to protective immunity against infection with blood-stage parasites of Plasmodium yoelii. Infection of C57BL/6 mice with P. yoelii 17XL was lethal, while all mice infected with a low-virulence strain of the parasite 17XNL acquired complete resistance against re-infection with P. yoelii 17XL. However, the host mice transferred with CD8 T cells from mice primed only with P. yoelii 17XNL failed to acquire protective immunity. On the other hand, the irradiated host mice were completely resistant to P. yoelii 17XL infection, showing no grade of parasitemia when adoptively transferred with CD8 T cells from immune mice that survived infection with both P. yoelii XNL and, subsequently, P. yoelii 17XL. These protective CD8 T cells from immune WT mice had the potential to generate IFN-γ, perforin (PFN) and granzyme B. When mice deficient in IFN-γ were used as donor mice for CD8 T cells, protective immunity in the host mice was fully abrogated, and the immunity was profoundly attenuated in PFN-deficient mice. Thus, CD8 T cells producing IFN-γ and PFN appear to be involved in protective immunity against infection with blood-stage malaria. [ABSTRACT FROM AUTHOR]

Proteasome-mediated proteolysis is responsible for the generation of immunogenic epitopes presented by MHC class I molecules, which activate antigen-specific CD8 T cells. Immunoproteasomes, defined by the presence of the three catalytic subunits LMP2, MECL-1, and LMP7, have been hypothesized to optimize MHC class I antigen processing. In this study, we demonstrate that the infection of mice with a protozoan parasite, Toxoplasma gondii, induced the expression of LMP7 mRNA in APC and increased the capacity of APC to induce the production of IFN-γ by antigen-specific CD8 T cells. In vitro infection of a DC cell line with T. gondii also induced the expression of LMP7 and resulted in enhanced proteasome proteolytic activity. Finally, mice lacking LMP7 were highly susceptible to infection with T. gondii and showed a reduced number of functional CD8 T cells. These results demonstrate that proteasomes containing LMP7 play an indispensable role in the survival of mice infected with T. gondii, presumably due to the efficient generation of CTL epitopes required for the functional development of CD8 T cells. [ABSTRACT FROM AUTHOR]

Human malaria vaccine trials have revealed vaccine efficacy but improvement is still needed. In this study, we aimed to re-evaluate vaccination with blood-stage naturally attenuated parasites, as a whole-organism vaccine model against cross-strain and cross-species malaria, to establish a better vaccination strategy. C57BL/6 mice controlled blood-stage Plasmodium yoelii 17XNL (PyNL) within 1 month of infection, while mice with a variety of immunodeficiencies demonstrated different susceptibilities to PyNL, including succumbing to hyperparasitemia. However, after recovery, survivors had complete protection against a challenge with the lethal strain PyL. Unlike cross-strain protection, PyNL-recovered mice failed to induce sterile immunity against Plasmodium berghei ANKA, although prolonged survival was observed in some vaccinated mice. Splenomegaly is a typical characteristic of malaria; the splenic structure became reorganized to prioritize extra-medullary hematopoiesis and to eliminate parasites. We also found that the peritoneal lymph node was enlarged, containing activated/memory phenotype cells that did not confer protection against PyL challenge. Hemozoins remained in the spleen several months after PyNL infection. Generation of an attenuated human blood-stage parasite expressing proteins from multiple species of malaria would greatly improve anti-malaria vaccination. [ABSTRACT FROM AUTHOR]

Summary Mast cells are important for eradication of intestinal nematodes; however, their precise mechanisms of action have remained elusive, especially in the early phase of infection. We found that Spi-B-deficient mice had increased numbers of mast cells and rapidly expelled the Heligmosomoides polygyrus (Hp) nematode. This was accompanied by induction of interleukin-13 (IL-13)-producing group 2 innate lymphoid cells (ILC2) and goblet cell hyperplasia. Immediately after Hp infection, mast cells were rapidly activated to produce IL-33 in response to ATP released from apoptotic intestinal epithelial cells. In vivo inhibition of the P2X7 ATP receptor rendered the Spi-B-deficient mice susceptible to Hp, concomitant with elimination of mast cell activation and IL-13-producing ILC2 induction. These results uncover a previously unknown role for mast cells in innate immunity in that activation of mast cells by ATP orchestrates the development of a protective type 2 immune response, in part by producing IL-33, which contributes to ILC2 activation. [ABSTRACT FROM AUTHOR]

Gastrointestinal symptoms, such as abdominal pain and diarrhea, are frequently observed in patients with Plasmodium falciparum malaria. However, the correlation between malaria intestinal pathology and intestinal microbiota has not been investigated. In the present study, infection of C57BL/6 mice with P. berghei ANKA (PbA) caused intestinal pathological changes, such as detachment of epithelia in the small intestines and increased intestinal permeability, which correlated with development with experimental cerebral malaria (ECM). Notably, an apparent dysbiosis occurred, characterized by a reduction of Firmicutes and an increase in Proteobacteria. Furthermore, some genera of microbiota correlated with parasite growth and/or ECM development. By contrast, BALB/c mice are resistant to ECM and exhibit milder intestinal pathology and dysbiosis. These results indicate that the severity of cerebral and intestinal pathology coincides with the degree of alteration in microbiota. This is the first report demonstrating that malaria affects intestinal microbiota and causes dysbiosis. [ABSTRACT FROM AUTHOR]

Entamoeba histolytica, the parasitic amoeba responsible for amoebiasis, causes approximately 100,000 deaths every year. There is currently no vaccine against this parasite. We have previously shown that intracecal inoculation of E. histolytica trophozoites leads to chronic and non-healing cecitis in mice. Entamoeba moshkovskii, a closely related amoeba, also causes diarrhea and other intestinal disorders in this model. Here, we investigated the effect of infection followed by drug-cure of these species on the induction of immunity against homologous or heterologous species challenge. Mice were infected with E. histolytica or E. moshkovskii and treated with metronidazole 14 days later. Re-challenge with E. histolytica or E. moshkovskii was conducted seven or 28 days following confirmation of the clearance of amoebae, and the degree of protection compared to non-exposed control mice was evaluated. We show that primary infection with these amoebae induces a species-specific immune response which protects against challenge with the homologous, but not a heterologous species. These findings pave the way, therefore, for the identification of novel amoebae antigens that may become the targets of vaccines and provide a useful platform to investigate host protective immunity to Entamoeba infections. [ABSTRACT FROM AUTHOR]

Abstract: Cerebral malaria (CM) is the most severe complication of Plasmodium infection. Although inappropriate immune responses to Plasmodium falciparum are reported as the major causes of CM, the precise mechanisms for development remain unclear. IL-23 and IL-17 have critical roles in the onset of autoimmunity and inflammatory diseases triggered by microbial infections. Thus, we investigated the influence of IL-23 and IL-17 on experimental CM (ECM) using Plasmodium berghei ANKA infection of C57BL/6 mice. Both IL-23 deficient mice and wild-type (WT) mice developed ECM. IL-17 deficient mice also developed ECM, while IL-17 producing cells other than CD4+ T cells (Th17) were increased in WT mice that developed ECM. In conclusion, this study showed that IL-23 and IL-17 are not involved in ECM development. [ABSTRACT FROM AUTHOR]

The consumption of dietary acrylamide (ACR), a carcinogen, results in the dysfunction of various organs and the immune system. However, the impact of ACR exposure on the progression of infectious diseases is unknown. This study investigated the effect of ACR on the progression of malaria infection using a mouse model of malaria. C57BL/6 mice were continuously treated with ACR at a dose of 20 mg/kg bodyweight/day for six weeks (long-term exposure) or phosphate-buffered saline (PBS). Next, the mice were infected with the rodent malaria parasite, Plasmodium berghei NK65 (PbNK). Parasitemia and survival rate were analyzed in the different treatment groups. Magnetic resonance imaging (MRI) and histopathological analyses were performed to evaluate the effect of ACR exposure on the morphology of various organs. Long-term ACR exposure exacerbated PbNK-induced multiorgan dysfunction. MRI and histopathological analysis revealed signs of encephalomeningitis and acute respiratory distress syndrome in the PbNK-infected long-term ACR exposure mice, which decreased the survival rate of mice, but not in the PbNK-infected long-term PBS exposure group. These findings enhance our understanding of the impact of ACR on the progression of infectious diseases, such as malaria. [Display omitted] • Long-term ACR exposure caused earlier death in mice with malaria. • Long-term ACR exposure induced brain damage in mice with malaria. • Signs of meningitis appeared in long-term ACR-exposed mice with malaria. • Long-term ACR exposure exacerbates malaria-associated acute respiratory distress syndrome. [ABSTRACT FROM AUTHOR]

Queuosine is a naturally occurring modified ribonucleoside found in the first position of the anticodon of the transfer RNAs for Asp, Asn, His, and Tyr. Eukaryotes lack pathways to synthesize queuine, the nucleobase precursor to queuosine, and must obtain it from diet or gut microbiota. Here, we describe the effects of queuine on the physiology of the eukaryotic parasite Entamoeba histolytica , the causative agent of amebic dysentery. Queuine is efficiently incorporated into E. histolytica tRNAs by a tRNA-guanine transglycosylase (EhTGT) and this incorporation stimulates the methylation of C38 in [Formula: see text] Queuine protects the parasite against oxidative stress (OS) and antagonizes the negative effect that oxidation has on translation by inducing the expression of genes involved in the OS response, such as heat shock protein 70 (Hsp70), antioxidant enzymes, and enzymes involved in DNA repair. On the other hand, queuine impairs E. histolytica virulence by downregulating the expression of genes previously associated with virulence, including cysteine proteases, cytoskeletal proteins, and small GTPases. Silencing of EhTGT prevents incorporation of queuine into tRNAs and strongly impairs methylation of C38 in [Formula: see text], parasite growth, resistance to OS, and cytopathic activity. Overall, our data reveal that queuine plays a dual role in promoting OS resistance and reducing parasite virulence. IMPORTANCE Entamoeba histolytica is a unicellular parasite that causes amebiasis. The parasite resides in the colon and feeds on the colonic microbiota. The gut flora is implicated in the onset of symptomatic amebiasis due to alterations in the composition of bacteria. These bacteria modulate the physiology of the parasite and affect the virulence of the parasite through unknown mechanisms. Queuine, a modified nucleobase of queuosine, is exclusively produced by the gut bacteria and leads to tRNA modification at the anticodon loops of specific tRNAs. We found that queuine induces mild oxidative stress resistance in the parasite and attenuates its virulence. Our study highlights the importance of bacterially derived products in shaping the physiology of the parasite. The fact that queuine inhibits the virulence of E. histolytica may lead to new strategies for preventing and/or treating amebiasis by providing to the host queuine directly or via probiotics., (Copyright © 2021 Nagaraja et al.)

Obesity is increasingly causing lifestyle diseases in developed countries where helminthic infections are rarely seen. Here, we investigated whether an intestinal nematode, Heligmosomoides polygyrus , has a suppressive role in diet-induced obesity in mice. Infection with H. polygyrus suppressed weight gain in obese mice, which was associated with increased uncoupling protein 1 (UCP1) expression in adipocytes and a higher serum norepinephrine (NE) concentration. Blocking interactions of NE with its receptor on adipocytes resulted in the failure to prevent weight gain and to enhance UCP1 expression in obese mice infected with H. polygyrus , indicating that NE is responsible for the protective effects of H. polygyrus on obesity. In addition to sympathetic nerve-derived NE, the intestinal microbiota was involved in the increase in NE. Infection with H. polygyrus altered the composition of intestinal bacteria, and antibiotic treatment to reduce intestinal bacteria reversed the higher NE concentration, UCP1 expression, and prevention of the weight gain observed after H. polygyrus infection. Our data indicate that H. polygyrus exerts suppressive roles on obesity through modulation of microbiota that produce NE., (Copyright © 2019 Shimokawa et al.)

Intestinal amebiasis is a major cause of diarrhea. However, research on host-amebae interactions has been hampered owing to a lack of appropriate animal models. Recently, a mouse model of intestinal amebiasis was established, and using it, we reported that Entamoeba moshkovskii colonized the intestine in a manner similar to that of the pathogenic Entamoeba histolytica In this study, we evaluated the protective mechanisms present against amebae using this model. CBA/J mice infected with E. histolytica had a persistent infection without apparent symptoms. In contrast, E. moshkovskii- infected mice rapidly expelled the ameba, which was associated with weight loss, diarrhea, and intestinal damage characterized by apoptosis of intestinal epithelial cells (IECs). Expression of NKG2D on intestinal intraepithelial lymphocytes (IELs) and IFN-γ-producing cells in Peyer's patches were significantly induced after infection with E. moshkovskii but not with E. histolytica IFN-γ-deficient mice infected with E. moshkovskii showed no obvious symptoms. Notably, none of these mice expelled E. moshkovskii , indicating that IFN-γ is responsible not only for intestinal symptoms but also for the expulsion of amebae. Furthermore, apoptosis of IECs and expression of NKG2D on IELs observed in E. moshkovskii- infected mice did not occur in the absence of IFN-γ. In vivo blocking of NKG2D in mice infected with E. moshkovskii enabled ameba to survive longer and remarkably reduced apoptotic IECs. Our results clearly demonstrate a novel protective mechanism exerted by IFN-γ against intestinal amebae, including induction of cytotoxicity of IELs toward IECs., (Copyright © 2018 by The American Association of Immunologists, Inc.)

IFN-γ plays both pathological and protective roles during blood-stage malaria. One of its pathological roles is its contribution to anemia by suppressing erythropoiesis. Here, to evaluate the effects of IFN-γ-mediated alterations in erythropoiesis on the course of malaria infection, mice deficient in IFN-γ (GKO) were infected with two strains of the rodent malaria parasite Plasmodium yoelii, 17XL (PyL) and 17XNL (PyNL), whose host cell ranges differ. Regardless of genotype, all mice infected with PyL, which can invade any erythrocyte, developed high parasitemia and died quickly. Although PyNL caused a transient non-lethal infection in wild-type (WT) mice, some GKO mice were unable to control the infection and died. However, GKO mice were resistant to the early phase of infection, showing an impaired increase in parasitemia compared with WT mice. This resistance in the GKO mice was associated with having significantly fewer reticulocytes, which are the preferred host cells for PyNL parasites, than the WT mice. Compared with the amount of reticulocytes in GKO mice during the early stages of infection, there was a significant increase in the amount of these cells at later stages, which coincided with the inability of these mice to control the infection. We found that the growth of PyNL parasites correlated with the amount of reticulocytes. Thus, the reduced number of reticulocytes in mice lacking IFN-γ appears to be responsible for the limited parasite growth. Notably, these differences in GKO mice were at least partially reversed when the mice were injected with exogenous IFN-γ. Additionally, an artificial induction of hemolytic anemia and an increase in reticulocytes by phenylhydrazine treatment in GKO mice completely abolished the lower parasitemia and resistance during early phase infection. These results suggest that IFN-γ may contribute to the early growth of PyNL parasites by increasing the amount of reticulocytes, presumably by enhancing erythropoiesis.

The protective immunity afforded by CD8(+) T cells against blood-stage malaria remains controversial because no MHC class I molecules are displayed on parasite-infected human erythrocytes. We recently reported that rodent malaria parasites infect erythroblasts that express major histocompatibility complex (MHC) class I antigens, which are recognized by CD8(+) T cells. In this study, we demonstrate that the cytotoxic activity of CD8(+) T cells contributes to the protection of mice against blood-stage malaria in a Fas ligand (FasL)-dependent manner. Erythroblasts infected with malarial parasites express the death receptor Fas. CD8(+) T cells induce the externalization of phosphatidylserine (PS) on the infected erythroblasts in a cell-to-cell contact-dependent manner. PS enhances the engulfment of the infected erythroid cells by phagocytes. As a PS receptor, T-cell immunoglobulin-domain and mucin-domain-containing molecule 4 (Tim-4) contributes to the phagocytosis of malaria-parasite-infected cells. Our findings provide insight into the molecular mechanisms underlying the protective immunity exerted by CD8(+) T cells in collaboration with phagocytes.

Entamoeba histolytica, the parasitic amoeba responsible for amoebiasis, causes approximately 100,000 deaths every year. There is currently no vaccine against this parasite. We have previously shown that intracecal inoculation of E. histolytica trophozoites leads to chronic and non-healing cecitis in mice. Entamoeba moshkovskii, a closely related amoeba, also causes diarrhea and other intestinal disorders in this model. Here, we investigated the effect of infection followed by drug-cure of these species on the induction of immunity against homologous or heterologous species challenge. Mice were infected with E. histolytica or E. moshkovskii and treated with metronidazole 14 days later. Re-challenge with E. histolytica or E. moshkovskii was conducted seven or 28 days following confirmation of the clearance of amoebae, and the degree of protection compared to non-exposed control mice was evaluated. We show that primary infection with these amoebae induces a species-specific immune response which protects against challenge with the homologous, but not a heterologous species. These findings pave the way, therefore, for the identification of novel amoebae antigens that may become the targets of vaccines and provide a useful platform to investigate host protective immunity to Entamoeba infections.

When developing malaria vaccines, the most crucial step is to elucidate the mechanisms involved in protective immunity against the parasites. We found that CD8(+) T cells contribute to protective immunity against infection with blood-stage parasites of Plasmodium yoelii. Infection of C57BL/6 mice with P. yoelii 17XL was lethal, while all mice infected with a low-virulence strain of the parasite 17XNL acquired complete resistance against re-infection with P. yoelii 17XL. However, the host mice transferred with CD8(+) T cells from mice primed only with P. yoelii 17XNL failed to acquire protective immunity. On the other hand, the irradiated host mice were completely resistant to P. yoelii 17XL infection, showing no grade of parasitemia when adoptively transferred with CD8(+) T cells from immune mice that survived infection with both P. yoelii XNL and, subsequently, P. yoelii 17XL. These protective CD8(+) T cells from immune WT mice had the potential to generate IFN-gamma, perforin (PFN) and granzyme B. When mice deficient in IFN-gamma were used as donor mice for CD8(+) T cells, protective immunity in the host mice was fully abrogated, and the immunity was profoundly attenuated in PFN-deficient mice. Thus, CD8(+) T cells producing IFN-gamma and PFN appear to be involved in protective immunity against infection with blood-stage malaria.