Malaria associated apoptosis is not significantly correlated with either parasitemia or the number of previous malaria attacks

Unraveling Cell Death Pathways during Malaria Infection: What Do We Know So Far?

Malaria is a parasitic disease (caused by different Plasmodium species) that affects millions of people worldwide. The lack of effective malaria drugs and a vaccine contributes to this disease, continuing to cause major public health and socioeconomic problems, especially in low-income countries. Cell death is implicated in malaria immune responses by eliminating infected cells, but it can also provoke an intense inflammatory response and lead to severe malaria outcomes. The study of the pathophysiological role of cell death in malaria in mammalians is key to understanding the parasite-host interactions and design prophylactic and therapeutic strategies for malaria. In this work, we review malaria-triggered cell death pathways (apoptosis, autophagy, necrosis, pyroptosis, NETosis, and ferroptosis) and we discuss their potential role in the development of new approaches for human malaria therapies.

Analysis of the lymphocyte cell population during malaria caused by Plasmodium vivax and its correlation with parasitaemia and thrombocytopaenia

BACKGROUND

The mechanisms of activation and regulation of T lymphocytes and their cytokines in malaria caused by Plasmodium vivax are complex and poorly understood. Previous data suggest that T cells balance protective immune responses with immune mediated pathology in malaria. This study investigates the lymphocytic profile of patients infected with P. vivax by identifying and quantifying the specific sub-populations of Th1, Th2, Th17 and Treg cells and observing the correlation between parasitaemia and the number of platelets.

METHODS

A cross-sectional study was carried out in an endemic area of the state of Acre, Brazil. In order to obtain identification and quantification of lymphocyte sub-populations through flow cytometry, blood samples were collected from 50 individuals infected with P. vivax and 20 non-infected controls. To differentiate Th1 from Th2, the presence of cytokines IL-4 and TNF was examined by enzyme-linked immunosorbent assay. Utilizing the Mann-Whitney and Spearman coefficient tests, comparison and correlation analysis were rendered to test the parasitaemia and the number of platelets relationship.

RESULTS

The data indicate that individuals infected with P. vivax present a significant reduction in Th1, Th2 and Th17 cell sub-populations when compared to the non-infected control group. A negative correlation exists between parasitaemia and platelet counts in individuals infected with P. vivax. There is no correlation of parasitaemia or thrombocytopaenia with any sub-population of T lymphocytes analysed. Interestingly, patients with serum Th1 cytokine profile present inversely proportional parasitaemia to the increase in the number of Th1, Th2, Th17 and Treg cells while patients with serum Th2 cytokine profile present directly proportional parasitaemia to the increase in number of Th1 and Th2 cells. Regarding the number of platelets, patients with serum Th1 cytokine profile show a correlation directly proportional to the Th17 sub-population. In contrast, platelet counts are directly proportional only to Treg and activated Treg cells in patients with serum Th2 cytokine profile.

CONCLUSIONS

During the P. vivax infection patients with serum Th1 versus Th2 cytokine profile present different biological mechanisms for activating the immune system against parasite load.

A novel in vitro model reveals distinctive modulatory roles of Plasmodium falciparum and Plasmodium vivax on naïve cell-mediated immunity

BACKGROUND

To date, human peripheral blood mononuclear cells (PBMCs) have been used mainly in immune stimulation assays and the interpretation of data can be influenced by the previous immunological history of donors and cross reactivity with other infectious agents. Resolving these limitations requires an alternative in vitro model to uncover the primary response profiles.

METHODS

A novel in vitro model of mononuclear cells (MNCs) generated from haematopoietic stem cells (HSCs) was developed and these cells were then co-cultured with various antigens from Plasmodium falciparum and Plasmodium vivax to investigate the response of naïve immune cells to malaria antigens by flow cytometry.

RESULTS

In vitro stimulation of naïve lymphocytes showed that CD4+ and CD8+ T lymphocytes were significantly reduced (P < 0.01) by exposure to lysates of infected erythrocytes or intact erythrocytes infected with P. falciparum. The depletion was associated with the expression of CD95 (Fas receptor) on the surface of T lymphocytes. Maturation of T lymphocytes was affected differently, showing elevated CD3+CD4+CD8+ and CD3+CD4-CD8- T lymphocytes after stimulation with cell lysates of P. falciparum and P. vivax, respectively. In addition, antigen presenting monocytes and dendritic cells derived from haematopoietic stem cells showed impaired HLA-DR expression as a consequence of exposure to different species of malaria parasites.

CONCLUSION

These results suggest that naïve mononuclear cells differentiated in vitro from HSCs could provide a valid model for the assessment of immunity. P. falciparum and P. vivax malaria parasites could modulate various populations of immune cells starting from newly differentiated mononuclear cells.

Immune response pattern in recurrent Plasmodium vivax malaria

Background

Plasmodium vivax is the causative agent of human malaria of large geographic distribution, with 35 million cases annually. In Brazil, it is the most prevalent species, being responsible by around 70 % of the malaria cases.

Methods

A cross-sectional study was performed in Manaus (Amazonas, Brazil), including 36 adult patients with primary malaria, 19 with recurrent malaria, and 20 endemic controls. The ex vivo phenotypic features of circulating leukocyte subsets (CD4⁺ T-cells, CD8⁺ T-cells, NK, NKT, B, B1 and Treg cells) as well as the plasmatic cytokine profile (IL-2, IL-4, IL-6, IL-10, TNF and IFN-γ) were assessed, aiming at establishing patterns of immune response characteristic of primary malaria vs recurrent malaria as compared to endemic controls.

Results

The proportion of subjects with high levels of WBC was reduced in malaria patients as compared to the endemic control. Monocytes were diminished particularly in patients with primary malaria. The proportion of subjects with high levels of all lymphocyte subsets was decreased in all malaria groups, regardless their clinical status. Decreased proportion of subjects with high levels of CD4⁺ and CD8⁺ T-cells was found especially in the group of patients with recurrent malaria. Data analysis indicated significant increase in the proportion of the subjects with high plasmatic cytokine levels in both malaria groups, characterizing a typical cytokine storm. Recurrent malaria patients displayed the highest plasmatic IL-10 levels, that correlated directly with the CD4⁺/CD8⁺ T-cells ratio and the number of malaria episodes.

Conclusion

The findings confirm that the infection by the P. vivax causes a decrease in peripheral blood lymphocyte subsets, which is intensified in the cases of “recurrent malaria”. The unbalanced CD4⁺/CD8⁺ T-cells ratio, as well as increased IL-10 levels were correlated with the number of recurrent malaria episodes. These results suggest that the gradual remodelling of the immune response is dependent on the repeated exposure to the parasite, which involves a strict control of the immune response mediated by the CD4⁺/CD8⁺ T-cell unbalance and exacerbated IL-10 secretion.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-016-1501-5) contains supplementary material, which is available to authorized users.

Phenotypic profiling of CD8(+) T cells during Plasmodium vivax blood-stage infection

Background

For a long time, the role of CD8⁺ T cells in blood-stage malaria was not considered important because erythrocytes do not express major histocompatibility complex (MHC) class I proteins. While recent evidences suggest that CD8⁺ T cells may play an important role during the erythrocytic phase of infection by eliminating parasites, CD8⁺ T cells might also contribute to modulate the host response through production of regulatory cytokines. Thus, the role of CD8⁺ T cells during blood-stage malaria is unclear. Here, we report the phenotypic profiling of CD8⁺ T cells subsets from patients with uncomplicated symptomatic P. vivax malaria.

Methods

Blood samples were collected from 20 Plasmodium vivax-infected individuals and 12 healthy individuals. Immunophenotyping was conducted by flow cytometry. Plasma levels of IFN-γ, TNF-α and IL-10 were determined by ELISA/CBA. Unpaired t-test or Mann–Whitney test was used depending on the data distribution.

Results

P. vivax-infected subjects had lower percentages and absolute numbers of CD8⁺CD45RA⁺ and CD8⁺CD45RO⁺ T cells when compared to uninfected individuals (p ≤ 0.0002). A significantly lower absolute number of circulating CD8⁺CD45⁺CCR7⁺ cells (p = 0.002) was observed in P. vivax-infected individuals indicating that infection reduces the number of central memory T cells. Cytokine expression was significantly reduced in the naïve T cells from infected individuals compared with negative controls, as shown by lower numbers of IFN-γ⁺ (p = 0.001), TNF-α⁺ (p < 0.0001) and IL-10⁺ (p < 0.0001) CD8⁺ T cells. Despite the reduction in the number of CD8⁺ memory T cells producing IFN-γ (p < 0.0001), P. vivax-infected individuals demonstrated a significant increase in memory CD8⁺TNF-α⁺ (p = 0.016) and CD8⁺IL-10⁺ (p = 0.004) cells. Positive correlations were observed between absolute numbers of CD8⁺IL-10⁺ and numbers of CD8⁺IFN-γ⁺ (p < 0.001) and CD8⁺TNF-α⁺ T cells (p ≤ 0.0001). Finally, an increase in the plasma levels of TNF-α (p = 0.017) and IL-10 (p = 0.006) and a decrease in the IFN-γ plasma level (p <0.0001) were observed in the P. vivax-infected individuals.

Conclusions

P. vivax infection reduces the numbers of different subsets of CD8⁺ T cells, particularly the memory cells, during blood-stage of infection and enhances the number of CD8⁺ memory T cells expressing IL-10, which positively correlates with the number of cells expressing TNF-α and IFN-γ.

Electronic supplementary material

The online version of this article (doi:10.1186/s12879-015-0762-x) contains supplementary material, which is available to authorized users.

CD4+ T cells apoptosis in Plasmodium vivax infection is mediated by activation of both intrinsic and extrinsic pathways

BACKGROUND

Reduction in the number of circulating blood lymphocytes (lymphocytopaenia) has been reported during clinical episodes of malaria and is normalized after treatment with anti-malaria drugs. While this phenomenon is well established in malaria infection, the underlying mechanisms are still not fully elucidated. In the present study, the occurrence of apoptosis and its pathways in CD4+ T cells was investigated in naturally Plasmodium vivax-infected individuals from a Brazilian endemic area (Porto Velho - RO).

METHODS

Blood samples were collected from P. vivax-infected individuals and healthy donors. The apoptosis was characterized by cell staining with Annexin V/FITC and propidium iodide and the apoptosis-associated gene expression profile was carried out using RT2 Profiler PCR Array-Human Apoptosis. The plasma TNF level was determined by ELISA. The unpaired t-test or Mann-Whitney test was applied according to the data distribution.

RESULTS

Plasmodium vivax-infected individuals present low number of leukocytes and lymphocytes with a higher percentage of CD4+ T cells in early and/or late apoptosis. Increased gene expression was observed for TNFRSF1B and Bid, associated with a reduction of Bcl-2, in individuals with P. vivax malaria. Furthermore, these individuals showed increased plasma levels of TNF compared to malaria-naive donors.

CONCLUSIONS

The results of the present study suggest that P. vivax infection induces apoptosis of CD4+ T cells mediated by two types of signaling: by activation of the TNFR1 death receptor (extrinsic pathway), which is amplified by Bid, and by decreased expression of the anti-apoptotic protein Bcl-2 (intrinsic pathway). The T lymphocytes apoptosis could reflect a strategy of immune evasion triggered by the parasite, enabling their persistence but also limiting the occurrence of immunopathology.

Liver changes in severe Plasmodium falciparum malaria: histopathology, apoptosis and nuclear factor kappa B expression

Background

Liver involvement in severe Plasmodium falciparum infection is commonly a significant cause of morbidity and mortality among humans. The clinical presentation of jaundice often reflects a certain degree of liver damage. This study investigated the liver pathology of severe P. falciparum malaria as well as the regulation and occurrence of apoptosis in cellular components of formalin-fixed, paraffin-embedded liver tissues.

Methods

The liver tissues used in the study came from patients who died from P. falciparum malaria with hyperbilirubinaemia (total bilirubin (TB) ≥ 51.3 μmol/L or 3 mg/dl) (12 cases), P. falciparum malaria without hyperbilirubinaemia (TB < 51.3 μmol/L) (10 cases); and patients who died due to accidents, whose liver histology was normal (the control group) (10 cases). The histopathology of the liver tissue was studied by routine histology method. Caspase-3 and nuclear factor kappa B (NF-κB) p65 expressions were determined using immunohistochemistry.

Results

The severity of liver histopathology, occurrence of apoptosis and NF-κB p65 activation in P. falciparum malaria were associated with higher TB level. Significant correlations were found between NF-κB p65 expression and apoptosis in Kupffer cells and lymphocytes in the portal tracts.

Conclusions

Hyperplastic Kupffer cells and portal tract inflammation are two main features found in the liver tissues of severe P. falciparum malaria cases. In addition, NF-κB is associated with Kupffer cells and lymphocyte apoptosis in severe P. falciparum malaria.

Analysis of lymphocytes in patients with Plasmodium vivax malaria and its relation to the annexin-A1 and IL-10

BACKGROUND

Malaria is the most prevalent parasitic disease in the world. In Brazil, the largest number of malaria cases (98%) is within the Legal Amazon region, where Plasmodium vivax is responsible for over 80% of diagnosed cases. The aim of this study was to investigate the annexin-A1 expression in CD4+, CD8+ T cells, regulatory T cells (Treg) and cytokine IL-10 quantification in plasma from patients with malaria caused by P. vivax.

METHODS

The quantification of the cytokine IL-10 of patients infected with P. vivax and healthy controls were evaluated by enzyme-linked immunosorbent assay (ELISA). The determination of the expression of annexin-A1 in lymphocytes from patients and healthy controls was determined by immunofluorescence staining. All results were correlated with the parasitaemia and the number of previous episodes of malaria.

RESULTS

The cytokine IL-10 plasma levels showed a significant increase in both patients with low (650.4 ± 59.3 pg/mL) and high (2870 ± 185.3 pg/mL) parasitaemia compared to the control (326.1 ± 40.1 pg/mL). In addition, there was an increase of this cytokine in an episode dependent manner (individuals with no previous episodes of malaria--primoinfected: 363.9 ± 31.1 pg/mL; individuals with prior exposure: 659.9 ± 49.4 pg/mL). The quantification of annexin-A1 expression indicated a decrease in CD4+ and CD8+ T cells and an increase in Treg in comparison with the control group. When annexin-A1 expression was compared according to the number of previous episodes of malaria, patients who have been exposed more than once to the parasite was found to have higher levels of CD4+ T cells (96.0 ± 2.5 A.U) compared to primoinfected (50.3 ± 1.7). However, this endogenous protein had higher levels in CD8+ (108.5 ± 3.1) and Treg (87.5 ± 2.5) from patients primoinfected.

CONCLUSION

This study demonstrates that in the patients infected with P. vivax the release of immunoregulatory molecules can be influenced by the parasitaemia level and the number of previous episodes of malaria. annexin-A1 is expressed differently in lymphocyte sub-populations and may have a role in cell proliferation. Furthermore, annexin-A1 may be contributing to IL-10 release in plasma of patients with vivax malaria.

Longevity and composition of cellular immune responses following experimental Plasmodium falciparum malaria infection in humans

Cellular responses to Plasmodium falciparum parasites, in particular interferon-gamma (IFNγ) production, play an important role in anti-malarial immunity. However, clinical immunity to malaria develops slowly amongst naturally exposed populations, the dynamics of cellular responses in relation to exposure are difficult to study and data about the persistence of such responses are controversial. Here we assess the longevity and composition of cellular immune responses following experimental malaria infection in human volunteers. We conducted a longitudinal study of cellular immunological responses to sporozoites (PfSpz) and asexual blood-stage (PfRBC) malaria parasites in naïve human volunteers undergoing single (n = 5) or multiple (n = 10) experimental P. falciparum infections under highly controlled conditions. IFNγ and interleukin-2 (IL-2) responses following in vitro re-stimulation were measured by flow-cytometry prior to, during and more than one year post infection. We show that cellular responses to both PfSpz and PfRBC are induced and remain almost undiminished up to 14 months after even a single malaria episode. Remarkably, not only ‘adaptive’ but also ‘innate’ lymphocyte subsets contribute to the increased IFNγ response, including αβT cells, γδT cells and NK cells. Furthermore, results from depletion and autologous recombination experiments of lymphocyte subsets suggest that immunological memory for PfRBC is carried within both the αβT cells and γδT compartments. Indeed, the majority of cytokine producing T lymphocytes express an CD45RO⁺ CD62L^- effector memory (EM) phenotype both early and late post infection. Finally, we demonstrate that malaria infection induces and maintains polyfunctional (IFNγ⁺IL-2⁺) EM responses against both PfRBC and PfSpz, previously found to be associated with protection. These data demonstrate that cellular responses can be readily induced and are long-lived following infection with P. falciparum, with a persisting contribution by not only adaptive but also (semi-)innate lymphocyte subsets. The implications hereof are positive for malaria vaccine development, but focus attention on those factors potentially inhibiting such responses in the field.

A decade into the 21st century, malaria remains responsible for an intolerable global health burden and an effective vaccine is sorely needed. Compounding the many technical hurdles in developing such a vaccine, (naturally-acquired) immunity to malaria is generally perceived to be short-lived, although direct evidence from field studies is conflicting. To overcome this issue, we measured the development of immune responses against the malaria parasite Plasmodium falciparum in human volunteers undergoing experimental malaria infections for the first time, allowing a uniquely detailed analysis thereof. We found that cellular immune responses against two clinically-relevant life-stages of the parasite are not only rapidly acquired following even a single malaria infection, but also remain virtually undiminished over a year later – an unprecedented measurement. These findings refute conclusively the notion that an intrinsic defect exists in either the development or persistence of cellular immune responses against malaria. This realization, in conjunction with a growing recognition that such responses are indeed associated with clinical protection against malaria, markedly enhances the prospect of one day developing a successful vaccine. Simultaneously, however, these results re-focus attention on the question of why the development of long-lived immune responses is often inhibited under conditions of natural exposure.

Identification of inflammatory biomarkers for pediatric malarial anemia severity using novel statistical methods

Areas where Plasmodium falciparum transmission is holoendemic are characterized by high rates of pediatric severe malarial anemia (SMA) and associated mortality. Although the etiology of SMA is complex and multifactorial, perturbations in inflammatory mediator production play an important role in the pathogenic process. As such, the current study focused on identification of inflammatory biomarkers in children with malarial anemia. Febrile children (3 to 30 months of age) presenting at Siaya District Hospital in western Kenya underwent a complete clinical and hematological evaluation. Children with falciparum malaria and no additional identifiable anemia-promoting coinfections were stratified into three groups: uncomplicated malaria (hemoglobin [Hb] levels of ≥11.0 g/dl; n = 31), non-SMA (Hb levels of 6.0 to 10.9 g/dl; n = 37), and SMA (Hb levels of <6.0 g/dl; n = 80). A Luminex hu25-plex array was used to determine potential biomarkers (i.e., interleukin 1β [IL-1β], IL-1 receptor antagonist [IL-1Ra], IL-2, IL-2R, IL-4, IL-5, IL-6, IL-7, IL-8, IL-10, IL-12p70, IL-13, IL-15, IL-17, tumor necrosis factor alpha [TNF-α], alpha interferon [IFN-α], IFN-γ, granulocyte-macrophage colony-stimulating factor [GM-CSF], macrophage inflammatory protein 1 alpha [MIP-1α], MIP-1β, IFN-inducible protein of 10 kDa [IP-10], monokine induced by IFN-γ [MIG], eotaxin, RANTES, and monocyte chemoattractant protein 1 [MCP-1]) in samples obtained prior to any treatment interventions. To determine the strongest biomarkers of anemia, a parsimonious set of predictor variables for Hb was generated by least-angle regression (LAR) analysis, controlling for the confounding effects of age, gender, glucose-6-phosphate dehydrogenase (G6PD) deficiency, and sickle cell trait, followed by multiple linear regression analyses. IL-12p70 and IFN-γ emerged as positive predictors of Hb, while IL-2R, IL-13, and eotaxin were negatively associated with Hb. The results presented here demonstrate that the IL-12p70/IFN-γ pathway represents a set of biomarkers that predicts elevated Hb levels in children with falciparum malaria, while activation of the IL-13/eotaxin pathway favors more profound anemia.

Apoptosis of non-parasitized red blood cells in malaria: a putative mechanism involved in the pathogenesis of anaemia

Background

Severe anaemia is a common complication of Plasmodium falciparum malaria in hyperendemic regions. Premature elimination of non-parasitized red blood cells (nRBC) has been considered as one mechanism involved in the genesis of severe malaria anaemia. It has been reported that apoptosis can occur in RBC and, consequently, this cell death process could contribute to anaemia. This study was performed to evaluate the susceptibility of nRBC to apoptosis in a malaria anaemia murine model.

Methods

Balb/c mice were intraperitonially inoculated with 1 × 10⁶ P. yoelii 17XL parasitized RBC (pRBC) and, then, parasitaemia and anaemia were monitored. Apoptosis in both pRBC and nRBC was assessed during early and late phases of infection by flow cytometry using Syto 16 and annexin V-PE double staining and forward scatter measurement.

Results

As expected, experimental infection of Balb/c mice with Plasmodium yoelii 17XL parasites was characterized by progressive increase of parasitaemia and acute anaemia, leading to death. Flow cytometry analysis showed that a number of pRBC was in the apoptotic process. It was noteworthy that the increase of nRBC apoptosis levels occurred in the late phase of infection, when anaemia degree was notably accentuated, while no significant alteration was observed in the early phase.

Conclusion

The increased levels of nRBC apoptosis herein firstly reported, in malaria infection could represent a putative mechanism worsening the severity of malarial anaemia.

Blood stage malaria antigens induce different activation-induced cell death programs in splenic CD4+T cells

CD4(+) T cells respond to antigen immunization through a process of activation, clonal expansion to generate activated effector T cells followed by activation-induced clonal deletion of the responding T cells. While loss of responding T cells in post-activation death by apoptosis is a major factor regulating immune homeostasis, the precise pathways involved in downsizing of Plasmodium falciparum antigen-induced T cell expansions are not well characterized. We report in this study that splenic CD4(+) T cells from mice immunized with nonreplicating immunogens like OVA or recombinant blood stage P. falciparum antigens, PfMSP-3 and PfMSP-1(19) or crude parasite antigen (PfAg) undergo sequential T cell activation, proliferation followed by activation-induced cell death (AICD) in a dose- and time-dependent manner after Ag restimulation. While PfMSP-3 and OVA-induced AICD was mediated through a death receptor-dependent apoptotic program, PfMSP-1(19) and PfAg-induced AICD was via a mechanism dependent on the activation of mitochondria apoptosis signalling pathway through Bax activation. These results provide insights into the mechanism through which two blood stage merozoite antigens trigger different apoptotic programs of AICD in splenic CD4(+) T cells.

Germinal center architecture disturbance during Plasmodium berghei ANKA infection in CBA mice

Background

Immune responses to malaria blood stage infection are in general defective, with the need for long-term exposure to the parasite to achieve immunity, and with the development of immunopathology states such as cerebral malaria in many cases. One of the potential reasons for the difficulty in developing protective immunity is the poor development of memory responses. In this paper, the potential association of cellular reactivity in lymphoid organs (spleen, lymph nodes and Peyer's patches) with immunity and pathology was evaluated during Plasmodium berghei ANKA infection in CBA mice.

Methods

CBA mice were infected with 1 × 10⁶ P. berghei ANKA-parasitized erythrocytes and killed on days 3, 6–8 and 10 of infection. The spleen, lymph nodes and Peyer's patches were collected, fixed in Carson's formalin, cut in 5 μm sections, mounted in glass slides, stained with Lennert's Giemsa and haematoxylin-eosin and analysed with bright-field microscopy.

Results

Early (day 3) strong activation of T cells in secondary lymphoid organs was observed and, on days 6–8 of infection, there was overwhelming activation of B cells, with loss of conventional germinal center architecture, intense centroblast activation, proliferation and apoptosis but little differentiation to centrocytes. In the spleen, the marginal zone disappeared and the limits between the disorganized germinal center and the red pulp were blurred. Intense plasmacytogenesis was observed in the T cell zone.

Conclusion

The observed alterations, especially the germinal center architecture disturbance (GCAD) with poor centrocyte differentiation, suggest that B cell responses during P. berghei ANKA infection in mice are defective, with potential impact on B cell memory responses.

Plasmodium falciparum glycosylphosphatidylinositol induces limited apoptosis in liver and spleen mouse tissue

Plasmodium falciparum malaria affects about 500 million people worldwide and is responsible for approximately 2.5 million deaths per year. Glycosylphosphatidylinositol (GPI) is the major anchor for membrane-associated proteins of P. falciparum and GPI plays a major role as a toxin in the pathology of malaria. Therefore, we tested the hypothesis that GPI, like LPS, induces apoptosis in vitro and in vital organs of mice. Our data does not provide evidence for direct cardiomyocyte apoptosis induced by GPI in vitro. However, in vivo injection of GPI induced limited apoptosis in mouse liver and spleen tissue. Apoptosis may be due to a direct GPI apoptotic effect or to an indirect effect via the induction of TNFalpha and nitric oxide production.

Does malaria suffer from lack of memory?

It is widely perceived that immunity to malaria is, to an extent, defective and that one component of this defective immune response is the inability to induce or maintain long-term memory responses. If true, this is likely to pose problems for development of an effective vaccine against malaria. In this article, we critically review and challenge this interpretation of the epidemiological and experimental evidence. While evasion and modulation of host immune responses clearly occurs and naturally acquired immunity is far from optimal, mechanisms to control blood-stage parasites are acquired and maintained by individuals living in endemic areas, allowing parasite density to be kept below the threshold for induction of acute disease. Furthermore, protective immunity to severe pathology is achieved relatively rapidly and is maintained in the absence of boosting by re-infection. Nevertheless, there are significant challenges to overcome. The need for multiple infections to acquire immunity means that young children remain at risk of infection for far too long. Persistent or frequent exposure to antigen seems to be required to maintain anti-parasite immunity (premunition). Lastly, pre-erythrocytic and sexual stages of the life cycle are poorly immunogenic, and there is little evidence of effective pre-erythrocytic or transmission-blocking immunity at the population level. While these problems might theoretically be due to defective immunological memory, we suggest alternative explanations. Moreover, we question the extent to which these problems are malaria-specific rather than generic (i.e. result from inherent limitations of the vertebrate immune system).