For a few parasites more: Inoculum size, vector control and strain-specific immunity to malaria

What precipitates malaria illness in an area of intense transmission? Greenwood, Marsh and Snow(1) and Marsh(2) have discussed the hypothesis that the severity of malaria depends on the size of the inoculum, that is, on the number of sporozoites inoculated at one time(3). According to these authors, this is suggested by the results of vector control trials in which parasite prevalence remained the same but episodes of disease were reduced. This is clearly an important observation, but Jo Lines and Jo Armstrong interpret it differently. They see the primary implication of these results as being that natural immunity to malaria must be largely strain specific, and only if there is strain specificity can inoculum size be important. Here they present arguments to support this view,and point out that, if there is strain specific immunity, then short-term evaluation o f vector control is likely to overestimate its long-term benefit.     

Impact of placental Plasmodium falciparum malaria on pregnancy and perinatal outcome in sub-Saharan Africa: II: effects of placental malaria on perinatal outcome; malaria and HIV

Placental malaria is recognized as a common complication of malaria in pregnancy in areas of stable transmission, and, as a consequence, serious health problems arise for the mother and especially her baby [1]. Although malaria in pregnancy is a major factor associated with adverse perinatal outcome, the link between malaria and perinatal morbidity/mortality is less clear in areas with stable endemic malaria where pregnant women have acquired immunity [2]. Histological examination of the placenta is a predictor of fetal morbidity, as well as being the most sensitive detector of maternal infection [3]. Adverse perinatal outcome has been described as an important indicator of poor quality of obstetric care and social development [4]. A variety of adverse perinatal outcomes associated with placental malaria have been described, including low birth weight, preterm delivery, intrauterine growth retardation, fetal anemia, congenital malaria, and fetal mortality. The most common clinical features in 80 percent of perinatal cases are fever, anemia, and splenomegaly [5]. Other signs and symptoms include hepatomegaly, jaundice, regurgitation, loose stools, poor feeding, and, occasionally, drowsiness, restlessness, and cyanosis also can be seen [5,6].A review of studies that investigated these poor fetal outcomes associated with placental malaria in sub-Saharan Africa is presented here.     

Plasmodium falciparum variant surface antigen expression varies between isolates causing severe and nonsevere malaria and is modified by acquired immunity

In areas of endemic parasite transmission, protective immunity to Plasmodium falciparum malaria is acquired over several years with numerous disease episodes. Acquisition of Abs to parasite-encoded variant surface Ags (VSA) on the infected erythrocyte membrane is important in the development of immunity, as disease-causing parasites appear to be those not controlled by preexisting VSA-specific Abs. In this work we report that VSA expressed by parasites from young Ghanaian children with P. falciparum malaria were commonly and strongly recognized by plasma Abs from healthy children in the same area, whereas recognition of VSA expressed by parasites from older children was weaker and less frequent. Independent of this, parasites isolated from children with severe malaria (cerebral malaria and severe anemia) were better recognized by VSA-specific plasma Abs than parasites obtained from children with nonsevere disease. This was not due to a higher infection multiplicity in younger patients or in patients with severe disease. Our data suggest that acquisition of VSA-specific Ab responses gradually restricts the VSA repertoire that is compatible with parasite survival in the semi-immune host. This appears to limit the risk of severe disease by discriminating against the expression of VSA likely to cause life-threatening complications, such as cerebral malaria and severe anemia. Such VSA seem to be preferred by parasites infecting a nonimmune host, suggesting that VSA expression and switching are not random, and that the VSA expression pattern is modulated by immunity. This opens the possibility of developing morbidity-reducing vaccines targeting a limited subset of common and particularly virulent VSA.     

The Dynamics of Naturally Acquired Immunity to Plasmodium falciparum Infection

Severe malaria occurs predominantly in young children and immunity to clinical disease is associated with cumulative exposure in holoendemic settings. The relative contribution of immunity against various stages of the parasite life cycle that results in controlling infection and limiting disease is not well understood. Here we analyse the dynamics of Plasmodium falciparum malaria infection after treatment in a cohort of 197 healthy study participants of different ages in order to model naturally acquired immunity. We find that both delayed time-to-infection and reductions in asymptomatic parasitaemias in older age groups can be explained by immunity that reduces the growth of blood stage as opposed to liver stage parasites. We found that this mechanism would require at least two components – a rapidly acting strain-specific component, as well as a slowly acquired cross-reactive or general immunity to all strains. Analysis and modelling of malaria infection dynamics and naturally acquired immunity with age provides important insights into what mechanisms of immune control may be harnessed by malaria vaccine strategists.     

Malarial toxins and the regulation of parasite density

For over a century it has been recognized that many of the clinical symptoms of malaria are caused by toxins released by rupturing schizonts, but it is only in the past few years that the underlying mechanisms have begun to be understood. Dominic Kwiatkowski here focuses on the toxins that cause malaria fever by stimulating host cells to produce tumour necrosis factor a (TNF) and other pyrogenic cytokines. Both TNF and fever have antiparasite properties, and it is proposed that the release of these toxins plays an important role in the regulation of parasite density within the host. Cerebral malaria is related to excessive TNF production. Recent data indicate that this can be the consequence of genetic variation in the host's propensity to produce TNF.     

Systems immunology of human malaria

Plasmodium falciparum malaria remains a global public health threat. Optimism that a highly effective malaria vaccine can be developed stems in part from the observation that humans can acquire immunity to malaria through experimental and natural P. falciparum infection. Recent advances in systems immunology could accelerate efforts to unravel the mechanisms of acquired immunity to malaria. Here, we review the tools of systems immunology, their current limitations in the context of human malaria research, and the human 'models' of malaria immunity to which these tools can be applied.     

Transmission dynamics of Plasmodium falciparum

Recent models of malaria have been developed by Gupta and her co-workers. A frequent assumption used to illustrate these models is that levels of malaria are controlled by lifelong strain-specific immunity. In this article, Allan Saul examines the predictions this model makes about the equilibrium values of parasite prevalence and the dynamics of an epidemic following the introduction of a new strain. He reaches the conclusion that the stability of malaria makes long-term strain-specific immunity highly improbable, thus rendering models requiring lifelong strain-specific immunity unlikely to be of relevance in most epidemiological contexts.     

Innate immunity against malaria parasites in Anopheles gambiae

Malaria continues to exert a huge toll in the world today, causing approximately 400 million cases and killing between 1-2 million people annually. Most of the malaria burden is borne by countries in Africa. For this reason, the major vector for malaria in this continent, Anopheles gambiae, is under intense study. With the completion of the draft sequence of this important vector, efforts are underway to develop novel control strategies. One promising area is to harness the power of the innate immunity of this mosquito species to block the transmission of the malaria parasites. Recent studies have demonstrated that Toll and Imd signaling pathways and other immunity-related genes （encoding proteins possibly function in recognition or as effector molecules） play significant roles in two different arms of innate immunity： level of infection intensity and melanization of Plasmodium oocysts. The challenges in the future are to understand how the functions of these different genes are coordinated in defense against malaria parasites, and if different arms of innate immunity are cross-regulated or coordinated.     

The species specificity of immunity generated by live whole organism immunisation with erythrocytic and pre-erythrocytic stages of rodent malaria parasites and implications for vaccine development

A promising strategy for the development of a malaria vaccine involves the use of attenuated whole parasites, as these present a greater repertoire of antigens to the immune system than subunit vaccines. The complexity of the malaria parasite's life cycle offers multiple stages on which to base an attenuated whole organism vaccine. An important consideration in the design and employment of such vaccines is the diversity of the parasites that are infective to humans. The most valuable vaccine would be one that was effective against multiple species/strains of malaria parasite. Here we compare the species specificity of pre-erythrocytic and erythrocytic whole organism vaccination using live parasites with anti-malarial drug attenuation. The cross-stage protection afforded by each vaccination strategy, and the possibility that immunity against one stage may be abrogated by exposure to other stages of both homologous and heterologous parasites was also assessed. The rodent malaria parasites Plasmodium yoelii yoelii and Plasmodium vinckei lentum are to address these questions, as they offer the widest possible genetic distance between sub-species of malaria parasites infectious to rodents. It was found that both erythrocytic and pre-erythrocytic stage immunity generated by live, attenuated parasite vaccination have species-specific components, with pre-erythrocytic stage immunity offering a much broader pan-species protection. We show that the protection achieved following sporozoite inoculation with concurrent mefloquine treatment is almost entirely dependent of CD8(+) T-cells. Evidence is presented for cross-stage protection between erythrocytic and pre-erythrocytic stage vaccination. Finally, it is shown that, with these species, an erythrocytic stage infection of either a homologous or heterologous species following immunisation with pre-erythrocytic stages does not abrogate this immunity. This is the first direct comparison of the specificity and efficacy of erythrocytic and pre-erythrocytic stage whole organism vaccination strategies utilising the same parasite species pair.     

MHC-I Affects Infection Intensity but Not Infection Status with a Frequent Avian Malaria Parasite in Blue Tits

Host resistance against parasites depends on three aspects: the ability to prevent, control and clear infections. In vertebrates the immune system consists of innate and adaptive immunity. Innate immunity is particularly important for preventing infection and eradicating established infections at an early stage while adaptive immunity is slow, but powerful, and essential for controlling infection intensities and eventually clearing infections. Major Histocompatibility Complex (MHC) molecules are central in adaptive immunity, and studies on parasite resistance and MHC in wild animals have found effects on both infection intensity (parasite load) and infection status (infected or not). It seems MHC can affect both the ability to control infection intensities and the ability to clear infections. However, these two aspects have rarely been considered simultaneously, and their relative importance in natural populations is therefore unclear. Here we investigate if MHC class I genotype affects infection intensity and infection status with a frequent avian malaria infection Haemoproteus majoris in a natural population of blue tits Cyanistes caeruleus. We found a significant negative association between a single MHC allele and infection intensity but no association with infection status. Blue tits that carry a specific MHC allele seem able to suppress H. majoris infection intensity, while we have no evidence that this allele also has an effect on clearance of the H. majoris infection, a result that is in contrast with some previous studies of MHC and avian malaria. A likely explanation could be that the clearance rate of avian malaria parasites differs between avian malaria lineages and/or between avian hosts.     

Inferring malaria parasite population structure from serological networks

The malaria parasite Plasmodium falciparum is characterized by high levels of genetic diversity at antigenic loci involved in virulence and immune evasion. Knowledge of the population structure and dynamics of these genes is important for designing control programmes and understanding the acquisition of immunity to malaria; however, high rates of homologous and non-homologous recombination as well as complex patterns of expression within hosts have hindered attempts to elucidate these structures experimentally. Here, we analyse serological data from Kenya using a novel network technique to deconstruct the relationships between patients' immune responses to different parasite isolates. We show that particular population structures and expression patterns produce distinctive signatures within serological networks of parasite recognition, which can be used to discriminate between competing hypotheses regarding the organization of these genes. Our analysis suggests that different levels of immune selection occur within different groups of the same multigene family leading to mixed population structures.     

TNF family members and malaria: Old observations, new insights and future directions

Tumor necrosis factor (TNF) has long been recognized to promote malaria parasite killing, but also to contribute to the development of severe malaria disease. The precise molecular mechanisms that influence these different outcomes in malaria patients are not well understood, but the virulence and drug-resistance phenotype of malaria parasites and the genetic background and age of patients are likely to be important determinants. In the past few years, important roles for other TNF family members in host immune responses to malaria parasites and the induction of disease pathology have been discovered. In this review, we will summarize these more recent findings and highlight major gaps in our current knowledge. We will also discuss future research strategies that may allow us to better understand the sometimes subtle and intricate effects of TNF family molecules during malaria infection.     

Recent insights into humoral and cellular immune responses against malaria

Effective immunity to malaria has been clearly demonstrated among individuals naturally exposed to malaria, and can be induced by experimental infections in animals and humans. The large number of malaria antigens has presented a major challenge to identifying protective responses and their targets, and it is likely that robust immunity is mediated by responses to multiple antigens. These include merozoite surface antigens and invasion ligands, variant antigens on the surface of parasitized red blood cells, in addition to sporozoite and liver-stage antigens. Immunity seems to require humoral and cellular immune components, probably in co-operation, although the relative importance of each remains unclear. This review summarizes recent progress towards understanding the targets and mechanisms that are important for mediating immunity to malaria.     

Expression and function of erythrocyte-associated surface antigens in malaria

As the malaria parasite develops within the erythrocyte, a series of molecules are produced, which find their way first across the parasitophorous vacuole membrane, then through the system of membranous clefts in the cytoplasm of the infected cell, to end up associated with the erythrocyte membrane. The domains of the erythrocyte-associated malaria antigens which are exposed at the cell surface are readily recognised by the host's immune system and represent important targets in the early stages of acquired immunity to malaria. The malaria parasite, in turn, appears to have developed some very effective mechanisms of escaping this immune response, including sequestration and antigenic variation. This paper reviews recent findings in the field of erythrocyte-associated malarial antigens and discusses these findings in the context of disease severity and malaria immunity.     

How malaria has affected the human genome and what human genetics can teach us about malaria

Malaria is a major killer of children worldwide and the strongest known force for evolutionary selection in the recent history of the human genome. The past decade has seen growing evidence of ethnic differences in susceptibility to malaria and of the diverse genetic adaptations to malaria that have arisen in different populations: epidemiological confirmation of the hypotheses that G6PD deficiency, alpha+ thalassemia, and hemoglobin C protect against malaria mortality; the application of novel haplotype-based techniques demonstrating that malaria-protective genes have been subject to recent positive selection; the first genetic linkage maps of resistance to malaria in experimental murine models; and a growing number of reported associations with resistance and susceptibility to human malaria, particularly in genes involved in immunity, inflammation, and cell adhesion. The challenge for the next decade is to build the global epidemiological infrastructure required for statistically robust genomewide association analysis, as a way of discovering novel mechanisms of protective immunity that can be used in the development of an effective malaria vaccine.     

Natural Regulatory T Cells in Malaria: Host or Parasite Allies?

Plasmodium falciparum malaria causes 500 million clinical cases with approximately one million deaths each year. After many years of exposure, individuals living in endemic areas develop a form of clinical immunity to disease known as premunition, which is characterised by low parasite burdens rather than sterilising immunity. The reason why malaria parasites persist under a state of premunition is unknown but it has been suggested that suppression of protective immunity might be a mechanism leading to parasite persistence. Although acquired immunity limits the clinical impact of infection and provides protection against parasite replication, experimental evidence indicates that cell-mediated immune responses also result in detrimental inflammation and contribute to the aetiology of severe disease. Thus, an appropriate regulatory balance between protective immune responses and immune-mediated pathology is required for a favourable outcome of infection. As natural regulatory T (T_reg) cells are identified as an immunosuppressive lineage able to modulate the magnitude of effector responses, several studies have investigated whether this cell population plays a role in balancing protective immunity and pathogenesis during malaria. The main findings to date are summarised in this review and the implication for the induction of pathogenesis and immunity to malaria is discussed.     

Host age as a determinant of naturally acquired immunity to Plasmodium falciparum

The usual course of infection by Plasmodium falciparum among adults who lack a history of exposure to endemic malaria is fulminant. The infection in adults living with hyper- to holoendemic malaria is chronic and benign. Naturally acquired immunity to falciparum malaria is the basis of this difference. Confusion surrounds an essential question regarding this process: What is its rate of onset? Opinions vary because of disagreement over the relationships between exposure to infection, antigenic polymorphism and naturally acquired immunity. In this review, Kevin Baird discusses these relationships against a backdrop of host age as a determinant of naturally acquired immunity to falciparum malaria.     

Vaccine-induced immunity to malaria parasites and the need for novel strategies

History shows that vaccines are most easily developed for those organisms that induce natural immunity after a single infection. For malaria, partial antiparasite immunity develops only after several years of endemic exposure. Evidence suggests that this inefficient induction of immunity is partly a result of antigenic polymorphism, poor immunogenicity of individual antigens, the ability of the parasite to interfere with the development of immune responses and to cause apoptosis of effector and memory T and B cells, and the interaction of maternal and neonatal immunity. Vaccine strategies that are likely to be ultimately successful are those that combine many antigens to induce a maximal response to protective determinants that might not be normally recognized following normal infection of naive individuals. Whole organismal approaches and the use of ultra-low doses of antigens have shown success in human and animal studies by inducing enhanced immune responses to multiple antigens. These, and related hypervalent subunit approaches, could lead to a viable vaccine.     

Phagosomal Acidification Prevents Macrophage Inflammatory Cytokine Production to Malaria,and Dendritic Cells Are the Major Source at the Early Stages of Infection: IMPLICATION FOR MALARIA PROTECTIVE IMMUNITY DEVELOPMENT*

Inflammatory cytokines produced at the early stages of malaria infection contribute to shaping protective immunity and pathophysiology. To gain mechanistic insight into these processes, it is important to understand the cellular origin of cytokines because both cytokine input and cytokine-producing cells play key roles. Here, we determined cytokine responses by monocytes, macrophages, and dendritic cells (DCs) to purified Plasmodium falciparum and Plasmodium berghei ANKA, and by spleen macrophages and DCs from Plasmodium yoelii 17NXL-infected and P. berghei ANKA-infected mice. The results demonstrate that monocytes and macrophages do not produce inflammatory cytokines to malaria parasites and that DCs are the primary source early in infection, and DC subsets differentially produce cytokines. Importantly, blocking of phagosomal acidification by inhibiting vacuolar-type H⁺-ATPase enabled macrophages to elicit cytokine responses. Because cytokine responses to malaria parasites are mediated primarily through endosomal Toll-like receptors, our data indicate that the inability of macrophages to produce cytokines is due to the phagosomal acidification that disrupts endosomal ligand-receptor engagement. Macrophages efficiently produced cytokines to LPS upon simultaneously internalizing parasites and to heat-killed Escherichia coli, demonstrating that phagosomal acidification affects endosomal receptor-mediated, but not cell surface receptor-mediated, recognition of Toll-like receptor agonists. Enabling monocytes/macrophages to elicit immune responses to parasites by blocking endosomal acidification can be a novel strategy for the effective development of protective immunity to malaria. The results have important implications for enhancing the efficacy of a whole parasite-based malaria vaccine and for designing strategies for the development of protective immunity to pathogens that induce immune responses primarily through endosomal receptors.