Plasmodium Chabaudi AS Infection Research Articles

Plasmodium chabaudi AS Infection Induces CD4+ Th1 Cells and Foxp3+T-bet+ Regulatory T Cells That Express CXCR3 and Migrate to CXCR3 Ligands.

Control and elimination of blood-stage Plasmodium chabaudi AS infection requires CD4+ Th1 cells that secrete IFN-γ and T follicular help (Tfh) cells together with B cell production of antibody. Foxp3+ regulatory T cells (Tregs) are also crucial to protect the host from immunopathology and severe disease, but these cells can suppress protective immune responses to malaria. The chemokine receptor CXCR3 expressed by activated T cells is important for trafficking of CD4+ Th1 cells to sites of inflammation and infection. Previous studies demonstrated CXCR3 is expressed on CD4+ T cells in the spleen during malaria, but the phenotype was not defined. We identified the phenotype of CD4+ T cells that expressed CXCR3 in C57BL/6 (B6) mice during acute P. chabaudi AS infection by analyzing expression of the transcription factors T-bet and Foxp3. We also investigated if CXCR3 contributes to control of parasite replication and survival. The frequency and number of CD4+CXCR3+ T cells increased dramatically in the spleen of infected B6 mice coincident with increased CD4+IFN-γ+ T cells. CXCR3 was up-regulated on effector CD4+Foxp3− T cells as well as Foxp3+ Tregs. Consistent with our previous observations, CD4+T-bet+Foxp3− T cells increased in B6 mice during acute infection. T-bet+Foxp3+ Tregs also increased significantly and a high frequency of these cells expressed CXCR3 supporting the notion that these cells may be Th1-like Tregs. Despite this, the percentage of CD4+Foxp3+ Tregs from infected B6 mice that migrated in vitro to the CXCR3 ligands CXCL9 and CXCL10 was significantly less than naïve mice. To investigate the in vivo contribution of CXCR3 to control of acute blood-stage malaria, we compared the course and outcome of P. chabaudi AS infection in wild-type (WT) B6 and CXCR3-deficient mice. Parasitemia levels were significantly higher around the time of peak parasitemia in CXCR3−/− compared to WT mice but survival was similar suggesting a role for CXCR3 in controlling parasite replication during acute P. chabaudi AS infection. Together, our findings indicate Th1-like CD4+T-bet+Foxp3+ Tregs that express CXCR3 are induced during acute blood-stage malaria and suggest CXCR3 expression on CD4+ Th1 cells may contribute to their migration to the spleen.

Increased Plasmodium chabaudi malaria mortality in mice with nutritional iron deficiency can be reduced by short-term adjunctive iron supplementation

BackgroundIron deficiency is the most widespread nutrient deficiency and an important cause of developmental impairment in children. However, some studies have indicated that iron deficiency can also protect against malaria, which is a leading cause of childhood morbidity and mortality in large parts of the world. This has rendered interventions against iron deficiency in malaria-endemic areas controversial.MethodsThe effect of nutritional iron deficiency on the clinical outcome of Plasmodium chabaudi AS infection in A/J mice and the impact of intravenous iron supplementation with ferric carboxymaltose were studied before and after parasite infection. Plasma levels of the iron status markers hepcidin and fibroblast growth factor 23 were measured in animals surviving and succumbing to malaria, and accompanying tissue pathology in the liver and the spleen was assessed.ResultsNutritional iron deficiency was associated with increased mortality from P. chabaudi malaria. This increased mortality could be partially offset by carefully timed, short-duration adjunctive iron supplementation. Moribund animals were characterized by low levels of hepcidin and high levels of fibroblast growth factor 23. All infected mice had extramedullary splenic haematopoiesis, and iron-supplemented mice had visually detectable intracellular iron stores.ConclusionsBlood transfusions are the only currently available means to correct severe anaemia in children with malaria. The potential of carefully timed, short-duration adjunctive iron supplementation as a safe alternative should be considered.

The Impact of Genetic Susceptibility to Systemic Lupus Erythematosus on Placental Malaria in Mice

Severe malaria, including cerebral malaria (CM) and placental malaria (PM), have been recognized to have many of the features of uncontrolled inflammation. We recently showed that in mice genetic susceptibility to the lethal inflammatory autoimmune disease, systemic lupus erythematosus (SLE), conferred resistance to CM. Protection appeared to be mediated by immune mechanisms that allowed SLE-prone mice, prior to the onset of overt SLE symptoms, to better control their inflammatory response to Plasmodium infection. Here we extend these findings to ask does SLE susceptibility have 1) a cost to reproductive fitness and/or 2) an effect on PM in mice? The rates of conception for WT and SLE susceptible (SLEs) mice were similar as were the number and viability of fetuses in pregnant WT and SLEs mice indicating that SLE susceptibility does not have a reproductive cost. We found that Plasmodium chabaudi AS (Pc) infection disrupted early stages of pregnancy before the placenta was completely formed resulting in massive decidual necrosis 8 days after conception. Pc-infected pregnant SLEs mice had significantly more fetuses (∼1.8 fold) but SLE did not significantly affect fetal viability in infected animals. This was despite the fact that Pc-infected pregnant SLEs mice had more severe symptoms of malaria as compared to Pc-infected pregnant WT mice. Thus, although SLE susceptibility was not protective in PM in mice it also did not have a negative impact on reproductive fitness.

The role of P2X7 receptor in hepatic necrosis caused by Plasmodium chabaudi AS infection

The role of P2X7 receptor in hepatic necrosis caused by Plasmodium chabaudi AS infection

Plasmodium chabaudi AS induces pregnancy loss in association with systemic pro‐inflammatory immune responses in A/J and C57BL/6 mice

The molecular mechanisms that underlie poor birth outcomes in malaria during pregnancy remain poorly defined. To assess the role of host immune responses, mice known to respond differentially to Plasmodium chabaudi AS infection were studied. Following infection at day 0 of pregnancy, A/J mice developed significantly higher parasitemia than C57BL/6 (B6) mice and succumbed to infection. Both strains had evidence of parasite accumulation in the placenta at mid-gestation and aborted, with significantly higher embryo loss in infected A/J mice on day 9. While infection-induced systemic tumour necrosis factor (TNF) and interleukin (IL)-1β in the latter were significantly higher at day 11, day 10 IL-10 levels were higher in B6 mice. No differences in the levels of splenic lymphocyte subsets, neutrophils or monocytes between infected pregnant A/J and B6 mice were observed, with most cell types expanding in response to infection regardless of pregnancy. Antibody ablation of TNF exacerbated infection in A/J mice and did not ameliorate pregnancy outcome. Thus, malaria induces poor pregnancy outcome in both the mouse strains in the context of quantitatively different systemic inflammatory responses. Further evaluation of the roles of soluble and cellular immune components, particularly at the uteroplacental level, will be required to define the most critical pregnancy-compromising mechanisms.

IL-2 Contributes to Maintaining a Balance between CD4+Foxp3+ Regulatory T Cells and Effector CD4+ T Cells Required for Immune Control of Blood-Stage Malaria Infection

To investigate the role of CD4(+)CD25(+)Foxp3(+) regulatory T (Treg) cells in blood-stage malaria, we compared Plasmodium chabaudi AS infection in wild-type (WT) C57BL/6 and transgenic mice overexpressing the transcription factor Foxp3 (Foxp3Tg) and observed that Foxp3Tg mice experienced lethal infection and deficient malaria-specific immune responses. Adoptive transfer of total CD4(+) T cells from Foxp3Tg mice or CD4(+)CD25(+) T cells from WT mice to naive WT recipients confirmed that high numbers of Treg cells compromised immune control of malaria. Transfer of GFP(+)CD4(+)CD25(+) T cells to naive WT recipients together with immunohistochemical staining of spleens from infected WT mice demonstrated that Foxp3(+) Treg cells localized in the T cell area of the spleen. Determination of CD4(+)Foxp3(+) Treg cell responses in the spleen of infected WT mice revealed a significant but transient increase in CD4(+)Foxp3(+) Treg cells early in infection. This was followed by a significant and sustained decrease due to reduced proliferation and apoptosis of CD4(+)Foxp3(+) Treg cells. Importantly, the kinetics of IL-2 secretion by effector CD4(+)Foxp3(-) T cells coincided with changes in CD4(+)Foxp3(+) cells and the differentiation of CD4(+)T-bet(+)IFN-γ(+) cells required for immune control of infection. Administration of the IL-2/anti-IL-2 mAb (clone JES6-1) complex to infected WT mice increased the severity of P. chabaudi AS infection and promoted expansion of Foxp3(+) Treg cells. Collectively, these data demonstrate that the ability to control and eliminate P. chabaudi AS infection is due to a tight balance between natural Treg cells and effector CD4(+) Th1 cells, a balance regulated in part by IL-2.

Blood Transfusion Alters the Course and Outcome of Plasmodium chabaudi AS Infection in Mice

The importance of severe anemia in the mortality of susceptible A/J mice during blood-stage Plasmodium chabaudi AS infection was assessed. Blood transfusion during and 2 to 3 days after peak parasitemia rescued 90% of susceptible mice from severe anemia and death and allowed these mice to clear the infection and acquire immunity to reinfection. However, blood transfusion prolonged the patency of the infection for up to 5 days after peak parasitemia. Blood transfusions in resistant C57BL/6 mice produced an identical effect, that is, prolongation of the patency of parasitemia. In addition, blood transfusion increased the numbers of gametocytes in both mouse strains. In both strains of mice, the rapid reduction in parasitemia, which occurs during crisis, was associated with the development of moderate levels of anemia. The possible mechanisms for the modulation of parasitemia by blood transfusion and the implications of the present observations for our understanding of the events which occur during crisis are discussed. It is proposed that parasitologic crisis is induced and/or maintained by physiological alterations associated with anemia.

Investigation of memory responses following Plasmodium chabaudi AS infection in mice distinct in susceptibility to clinical malaria

Plasmodium chabaudi chabaudi AS blood stage infection results in various degrees of clinical symptoms to malaria depending on the mouse strain. This study aimed to investigate the development of memory responses in susceptible A/J and resistant C57BL/6 mice which differ in the degree of susceptibility to clinical malaria following P. chabaudi AS infection. A rapid increase of activated cells (CD25(+), CD44(high), and CD62L(low)) and production of both interferon-gamma and interleukin-4 was found in spleens of both malaria-infected mouse strains. After the parasitemia had been cleared, these activated cells were converted to their resting phenotypes. Although exhibiting susceptible phenotype and having lower magnitude of cellular changes during primary infection, susceptible A/J mice that had been exposed to malaria parasites and drug-cured were able to generate protective immunity capable of control parasite growth following reinfection to the same level as C57BL/6 mice. This may be due to the capability of susceptible mice to produce parasite-specific antibodies, in particular of the IgG2a and IgG3 isotypes. The results from this study may provide more insights useful for the development of vaccines against malaria.

Association of Malaria-Induced Murine Pregnancy Failure with Robust Peripheral and Placental Cytokine Responses

Malarial infection in nonimmune pregnant women is a major risk factor for pregnancy failure. The biological mechanisms that underlie malaria-associated fetal loss, however, are poorly understood. Plasmodium chabaudi AS infection during early pregnancy results in midgestational embryonic loss in naive C57BL/6 mice. To define the immunopathogenesis of this malaria-induced pregnancy compromise, cytokine production in plasma, spleen, and placenta cell culture supernatants during the first 11 days of infection and gestation was studied. In infected pregnant mice, systemic interleukin-1beta and both systemic and splenic gamma interferon levels were elevated relative to those in uninfected pregnant mice, and gamma interferon was also robustly produced within the placenta 1 to 2 days before malaria-induced fetal loss. Although circulating tumor necrosis factor production was not affected by pregnancy or infection, circulating soluble tumor necrosis factor receptor II was highest in infected pregnant mice, particularly those undergoing abortion, but decreased at the placental level preceding abortion. Systemic levels of interleukin-10 were also high in infected mice at this time point, but this cytokine was not detected at the placental level. Histological examination revealed that trophoblast giant cells of aborting mice phagocytosed infected red blood cells and hemozoin. Furthermore, in vitro-cultured trophoblast cells isolated from embryos on day 7 of gestation phagocytosed P. chabaudi AS-infected red blood cells and secreted tumor necrosis factor. These results suggest that systemic and placenta-level proinflammatory antimalarial immune responses, in the absence of adequate and sustained counterregulatory mechanisms, contribute to pregnancy loss in this model.

Differential induction of helper T cell subsets during blood-stage Plasmodium chabaudi AS infection in resistant and susceptible mice.

The induction of T helper cell subsets during the course of non-lethal or lethal blood-stage Plasmodium chabaudi AS infection was investigated using inbred strains of mice which differ in the level of resistance to this intraerythrocytic parasite. Resistant C57Bl/6 mice experience a non-lethal course of infection characterized by moderate levels of both parasitaemia and anaemia and resolution of primary acute infection by 4 weeks, while susceptible A/J mice experience lethal infection with fulminant parasitaemia and severe anaemia. T helper subset function was assessed during infection by determining the kinetics of spleen cell production in vitro of the Th1-derived cytokine, interferon-gamma (IFN-gamma), and of the Th2-derived cytokine, IL-5, using sandwich ELISAs. Spleen cells from resistant C57Bl/6 mice were found to produce high levels of IFN-gamma within 1 week of infection in response to both the mitogen concanavalin A (Con A) and malaria antigen. Furthermore, CD4+ T cells were found to be the source of IFN-gamma while both CD4+ and CD8+ T cells were found to produce IL-5. Decreased IFN-gamma production after day 10 was concomitant with significant production of IL-5 between 2 and 3 weeks post infection. In contrast, spleen cells from susceptible A/J mice produced high levels of IL-5 within the first week of infection. In addition, these animals were found to have high serum levels of IL-5. These results, thus, confirm previous observations that resolution of primary blood-stage P. chabaudi infection occurs by sequential activation of Th1 CD4+ T cells followed by activation of the Th2 subset, and in addition, suggest that induction of a strong Th2 response early in infection may lead to a severe and lethal course of malaria.

Regulatory T cells (Tregs) contribute to suppression of anti-malarial immunity by concurrent nematode infection (51.1)

Abstract Here, we investigated the mechanisms involved in modulation of anti-malarial immunity by concurrent nematode infection. Both WT C57BL/6 (B6) and STAT-6 deficient (KO) mice co-infected with the gastrointestinal nematode Heligmosomoides polygyrus (Hp) 2 wk prior to blood-stage Plasmodium chabaudi AS (Pc) infection had significantly higher parasitemias and significantly lower serum IFN-γ levels compared to their respective controls infected with Pc alone. Hp-infected STAT-6 KO compared to WT mice produced significantly lower levels of Th2 cytokines (IL-4, IL-13) but Tregs (CD4+CD25+Foxp3− and CD4+CD25−Foxp3+) were significantly and similarly increased within 2 wk after Hp infection in both strains. Transfer of CD4+CD25+ Tregs from uninfected and Hp-infected B6 mice significantly increased malaria parasitemia in recipient mice while depletion of CD25+ cells suppressed parasitemia in Pc-infected mice but not in co-infected mice. FACS analysis revealed that anti-CD25 mAb (PC61) treatment reduced both CD4+CD25+ Tregs and CD4+Foxp3+ Tregs in Pc-infected but not in co-infected mice. Co-infection with Hp significantly enhanced Pc-induced TGF-β1 and IL-10 production. In vivo neutralization of TGF-β1 and blocking IL-10R with mAbs significantly reduced parasitemia in both Pc-infected and co-infected B6 mice; anti-IL-10R mAb treatment resulted in severe mortality. These results suggest that Tregs, and possibly immunoregulatory but not Th2 cytokines, contribute to suppression of anti-malarial immunity in nematode and malaria co-infected mice.

Modulation of malaria-induced immunopathology by concurrent nematode infection (46.8)

Abstract Gastrointestinal nematode infections, highly prevalent in malaria endemic areas, are known to modulate host immune responses to unrelated pathogens. However, the complex relationship between worms and malaria and the impact on malarial incidence vs. severity are unclear. Previously, we observed that C57BL/6 (B6) mice infected with the gastrointestinal nematode Heligosomoides polygyrus (Hp) for 2 wks before blood-stage Plasmodium chabaudi AS (Pc) infection developed significantly higher parasitemia compared to mice infected with Pc alone. Despite this, malaria-induced body weight loss, anemia and hepatosplenic damage were not exacerbated in co-infected mice. Indeed, there was less severe hypothermia and hypoglycemia and more robust reticulocytosis in co-infected mice. Co-infection with Hp suppressed type 1 immune responses induced by P. chabaudi infection with significantly reduced plasma levels of IFN-γ, TNF-α and nitric oxide, but increased IL-6 and IL-10 levels. Co-infected STAT-6−/− mice, which have disrupted Th2 responses, experienced only slightly more severe hypothermia, hypoglycemia, and anemia than co-infected WT B6 mice while in vivo administration of anti-IL-10R (1B1.2) mAb resulted in higher mortality in co-infected WT mice. Although concurrent nematode infection resulted in increased parasitemia due to impaired anti-malarial type 1 immunity, our data suggest that malaria-associated pathology is alleviated by a complex immunoregulatory network, possibly involving IL-10, and, to a lesser extent, Th2 cytokines.

Parasite‐specific IgM plays a significant role in the protective immune response to asexual erythrocytic stage Plasmodium chabaudi AS infection*

A comparison of Plasmodium chabaudi AS infection in BALB/c and BALB/c IgM-deficient mice demonstrated a protective role for IgM during infection. IgM-/- mice, unlike microMT mice, display competent B cell humoral immune responses. Increased susceptibility of IgM-/- mice was demonstrated by increased mortality, an advanced ascending infection and higher peak parasitaemia, as well as enhanced anaemia and weight loss compared with wild-type mice. The recrudescent parasitaemias were also higher in the IgM-/- mice. Early specific IgM production in P. chabaudi-infected wild-type mice was followed by IgG1 and IgG2a production, while IgG1 and IgG2a production in IgM-/- mice was preceded by specific IgD production. No protective role for natural IgM against P. chabaudi AS infection was detected as passive transfer of naïve WT serum into IgM-/- mice did not alter the disease outcome or reduce parasite numbers. Passive transfer of WT antiserum, containing predominantly specific IgM, into IgM-/- mice delayed the ascending parasitaemia and reduced mortality. Similarly, coating parasitized red blood cells with WT antiserum, but not IgM-/- antisera, prior to infection also slightly delayed the ascending acute parasitaemia. Specific IgM therefore plays an important role in the limitation of parasite replication during asexual erythrocytic P. chabaudi AS infection.

Opsonin-independent phagocytosis: an effector mechanism against acute blood-stage Plasmodium chabaudi AS infection.

Opsonin-independent macrophage phagocytosis was investigated as a possible mechanism of controlling early blood-stage Plasmodium chabaudi AS infection. Early during infection, peritoneal macrophages from resistant C57BL/6 (B6) mice exhibited increased phagocytosis of parasitized red blood cells (pRBCs) and free merozoites, which was absent in mice with deficient interferon (IFN)-gamma production during infection, including susceptible A/J, interleukin (IL)-12 p40, and IFN-gamma gene knockout mice. IFN-gamma treatment of macrophages collected from B6 and A/J mice early during infection enhanced phagocytosis of pRBCs, but IL-10 treatment inhibited this function. In vitro and in vivo studies in which type I and II class A scavenger receptor-deficient mice and inhibitors of scavenger and mannose receptors were used revealed that scavenger receptors other than class A type I and II and mannose receptors may play a role in malaria parasite uptake. These results indicate that opsonin-independent phagocytosis contributes to the IFN-gamma-dependent control of acute blood-stage malaria infection.

Central role of endogenous gamma interferon in protective immunity against blood-stage Plasmodium chabaudi AS infection.

The role of endogenous gamma interferon (IFN-gamma) in protective immunity against blood-stage Plasmodium chabaudi AS malaria was studied using IFN-gamma gene knockout (GKO) and wild-type (WT) C57BL/6 mice. Following infection with 10(6) parasitized erythrocytes, GKO mice developed significantly higher parasitemia during acute infection than WT mice and had severe mortality. In infected GKO mice, production of interleukin 12 (IL-12) p70 and tumor necrosis factor alpha in vivo and IL-12 p70 in vitro by splenic macrophages was significantly reduced compared to that in WT mice and the enhanced nitric oxide (NO) production observed in infected WT mice was completely absent. WT and GKO mice had comparable numbers of total nucleated spleen cells and B220(+) and Mac-1(+) spleen cells both before and after infection. Infected WT mice, however, had significantly more F4/80(+), NK1.1(+), and F4/80(+)Ia(+) spleen cells than infected GKO mice; male WT had more CD3(+) cells than male GKO mice. In comparison with those from WT mice, splenocytes from infected GKO mice had significantly higher proliferation in vitro in response to parasite antigen or concanavalin A stimulation and produced significantly higher levels of IL-10 in response to parasite antigen. Infected WT mice produced more parasite-specific immunoglobulin M (IgM), IgG2a, and IgG3 and less IgG1 than GKO mice. Significant gender differences in both GKO and WT mice in peak parasitemia levels, mortality, phenotypes of spleen cells, and proliferation of and cytokine production by splenocytes in vitro were apparent during infection. These results thus provide unequivocal evidence for the central role of endogenous IFN-gamma in the development of protective immunity against blood-stage P. chabaudi AS.

Natural killer cell cytokine production, not cytotoxicity, contributes to resistance against blood-stage Plasmodium chabaudi AS infection.

Our recent study showed that IL-12 treatment of susceptible A/J mice induces Th1-mediated, protective immunity against lethal blood-stage Plasmodium chabaudi AS infection. To further understand the mechanism of this protection, we examined NK cell cytotoxic (NKCC) and cytokine secretory functions in untreated and IL-12-treated A/J mice, along with resistant C57BL/6 (B6) mice. Normal A/J mice receiving six daily doses of 0.1 microg IL-12 exhibited significant increases in NKCC in total spleen cell populations. Defective NKCC evident in vitro in enriched NK cells from infected A/J mice was corrected by addition of 10 ng/ml IL-12 and was comparable with that seen in B6 mice. In vivo and in vitro analyses revealed that enriched NK cells from day 6 infected A/J mice were defective not only in NKCC, but also in IFN-gamma, and to a certain extent, TNF-alpha secretion, which could also be corrected by IL-12 treatment. Depletion of NK cells from resistant B6 mice resulted in a more severe course of infection, while NK cell-depleted, IL-12-treated A/J mice had significantly higher parasitemia, as well as 100% mortality, suggesting the importance of NK cells in IL-12-mediated protection. NKCC-defective bg/bg mice produced optimum IFN-gamma and TNF-alpha and recovered from infection similar to bg/+ controls; in vivo depletion of these cytokines resulted in significantly higher parasitemia early in infection. Based on these results, we conclude that IFN-gamma, and possibly TNF-alpha, secretion by NK cells during early infection plays a major role in protective immunity to blood-stage malaria.

Role of macrophage-derived nitric oxide in suppression of lymphocyte proliferation during blood-stage malaria.

Examination of the proliferative responses in vitro to mitogens (concanavalin A, phytohemagglutinin, lipopolysaccharide) of spleen cells recovered from C57BL/6 mice during blood-stage Plasmodium chabaudi AS infection revealed that the most severe suppression occurred during the first 14 days post infection, that is, during the acute phase of infection. Coincidently, inducible nitric oxide synthase gene expression was found to be up-regulated in the spleens of infected mice, and both splenic and peritoneal macrophages produced high levels of NO in vitro in response to stimulation with lipopolysaccharide (LPS). The roles of NO, a molecule recently found to mediate immunosuppression during parasitic infections, and of the well-recognized immunosuppressive molecule prostaglandin were, therefore, investigated in the suppression of proliferation to mitogens and specific antigen of spleen cells from 7- and 14-day P. chabaudi AS-infected mice. Addition of either 0.5 mM NG-monomethyl-L-arginine (L-NMMA) or 0.5 mM aminoguanidine (AG), inhibitors of NO synthase, or 10 micrograms/ml indomethacin (INDO), a prostaglandin inhibitor, partially but significantly abrogated the suppression in response to concanavalin A (Con A) and phytohemagglutinin (PHA). Only the addition of INDO significantly increased the responses to LPS. Addition of L-NMMA or AG in combination with INDO partially but significantly abrogated the suppression in response to Con A and completely abrogated the suppression in response to PHA. The addition of L-NMMA or AG also significantly increased proliferation in response to parasite antigen. The contribution of NO to suppression of lymphoproliferation was confirmed by adding 3-morpholino-sydnonimine-hydrochloride (SIN-1), a chemical generator of NO, to mitogen-stimulated splenocyte cultures prepared from normal mice. The mechanism of NO-mediated suppression was investigated in coculture experiments using spleen cells from normal mice and peritoneal macrophages from either normal or day 7 infected mice. The addition of 5-10 x 10(4) peritoneal macrophages from infected mice significantly and consistently suppressed Con A- or PHA-stimulated proliferation of normal splenocytes. Moreover, suppression correlated with production of NO and could be reversed by the addition of L-NMMA or AG. These results suggest that, in addition to prostaglandin, increased NO production by macrophages within the first 2 weeks after infection with P. chabaudi AS contributes to immunosuppression associated with blood-stage malaria.

Blood transfusion alters the course and outcome of Plasmodium chabaudi AS infection in mice

The importance of severe anemia in the mortality of susceptible A/J mice during blood-stage Plasmodium chabaudi AS infection was assessed. Blood transfusion during and 2 to 3 days after peak parasitemia rescued 90% of susceptible mice from severe anemia and death and allowed these mice to clear the infection and acquire immunity to reinfection. However, blood transfusion prolonged the patency of the infection for up to 5 days after peak parasitemia. Blood transfusions in resistant C57BL/6 mice produced an identical effect, that is, prolongation of the patency of parasitemia. In addition, blood transfusion increased the numbers of gametocytes in both mouse strains. In both strains of mice, the rapid reduction in parasitemia, which occurs during crisis, was associated with the development of moderate levels of anemia. The possible mechanisms for the modulation of parasitemia by blood transfusion and the implications of the present observations for our understanding of the events which occur during crisis are discussed. It is proposed that parasitologic crisis is induced and/or maintained by physiological alterations associated with anemia.

Plasmodium chabaudi AS: Erythropoietic responses during infection in resistant and susceptible mice

The course of anemia and the erythropoietic response in the bone marrow, spleen, and blood were studied during Plasmodium chabaudi AS infection in resistant C57BL/6 (B6) and susceptible A J (A) mice. Infections in B6 mice were characterized by moderate levels of both parasitemia and anemia and survival. In contrast, A mice experienced high parasitemia, severe anemia, and high mortality rates. During the period of anemia, erythropoiesis, as measured by in vivo 59Fe incorporation, was significantly more depressed in bone marrow and more increased in the spleen in resistant B6 mice. The increase in splenic 59Fe incorporation was a function of the size of the spleen. Bone marrow CFU-E were decreased to 50% of control in both strains, while splenic CFU-E were increased twofold greater in B6 mice compared to those in A mice. However, the absolute numbers of CFU-E per spleen in the two strains were not significantly different during peak parasitemia. Bone marrow BFU-E were transiently increased before peak parasitemia whereas splenic BFU-E peaked during peak parasitemia. A mice had significantly lower numbers of BFU-E per spleen on all days except at peak parasitemia. The frequency of blood-borne BFU-E and plasma erythropoietin titers was increased earlier and to a greater extent in A mice. These results suggest that an impaired amplification of late-stage splenic erythropoiesis may be an important determinant in the severity of anemia and lethality of infection with P. chabaudi AS in A mice. Moreover, these results demonstrate that the defective amplification of splenic erythropoiesis in A mice is neither caused by a defect in the mobilization of BFU-E from the bone marrow to the spleen nor caused by a defect in erythropoietin production.

Macrophage activation during Plasmodium chabaudi AS infection in resistant C57BL/6 and susceptible A/J mice.

Macrophage activation was examined in resistant C57BL/6 and susceptible A/J mice during the course of blood-stage infection with Plasmodium chabaudi AS. Three parameters of macrophage activation (lipopolysaccharide [LPS]- and malaria antigen-induced tumor necrosis factor [TNF] production in vitro, phorbol myristate acetate [PMA]-induced production of oxygen metabolites in vitro, and Ia antigen expression) were assessed during infection in populations of peritoneal and splenic macrophages recovered from infected mice of the two strains. The peak level of LPS-induced TNF production in vitro by splenic macrophages from both infected C57BL/6 and infected A/J mice occurred on day 7, which was 3 days before the peak of parasitemia. Although the kinetics of TNF production in vitro in response to either LPS, soluble malaria antigen, or intact parasitized erythrocytes varied in some of the other macrophage populations during infection, there was no significant difference in the peak level of production. Peritoneal and splenic macrophages from infected C57BL/6 mice exhibited significantly increased PMA-induced production of H2O2 in vitro on day 7. Peritoneal macrophages from infected A/J mice also exhibited significant PMA-induced H2O2 production on day 7, while production by splenic macrophages from these hosts was not increased in comparison with production by cells from normal animals. Only peritoneal macrophages from infected C57BL/6 mice produced significantly increased levels of O2-, and this occurred on day 7 postinfection. Ia antigen expression by both peritoneal and splenic macrophages from resistant C57BL/6 and susceptible A/J mice was significantly increased during P. chabaudi AS infection. However, the percentage of Ia+ peritoneal macrophages on days 8 and 10 postinfection and Ia+ splenic macrophages on day 3 postinfection was significantly higher in C57BL/6 than in A/J mice. Thus, these results demonstrate that macrophages from P. chabaudi AS-infected A/J mice exhibit defects in oxygen metabolism and Ia antigen expression which may contribute to the susceptibility of these hosts to this intraerythrocytic parasite. The cause-and-effect relationship between these defects and the susceptibility of A/J mice to P. chabaudi AS is unknown.