Plasmodium knowlesi: functional immunity and antimerozoite antibodies in rhesus monkeys after repeated infection

The role of Plasmodium knowlesi in the history of malaria research

In recent years, a malaria infection of humans in South East Asia, originally diagnosed as a known human-infecting species, Plasmodium malariae, has been identified as a simian parasite, Plasmodium knowlesi. This species had been subject to considerable investigation in monkeys since the 1930s. With the development of continuous culture of the erythrocytic stages of the human malarial parasite, Plasmodium falciparum in 1976, the emphasis in research shifted away from knowlesi. However, its importance as a human pathogen has provoked a renewed interest in P. knowlesi, not least because it too can be maintained in continuous culture and thus provides an experimental model. In fact, this parasite species has a long history in malaria research, and the purpose of this chapter is to outline approximately the first 50 years of this history.

Human malarial disease: a consequence of inflammatory cytokine release

Malaria causes an acute systemic human disease that bears many similarities, both clinically and mechanistically, to those caused by bacteria, rickettsia, and viruses. Over the past few decades, a literature has emerged that argues for most of the pathology seen in all of these infectious diseases being explained by activation of the inflammatory system, with the balance between the pro and anti-inflammatory cytokines being tipped towards the onset of systemic inflammation. Although not often expressed in energy terms, there is, when reduced to biochemical essentials, wide agreement that infection with falciparum malaria is often fatal because mitochondria are unable to generate enough ATP to maintain normal cellular function. Most, however, would contend that this largely occurs because sequestered parasitized red cells prevent sufficient oxygen getting to where it is needed. This review considers the evidence that an equally or more important way ATP deficiency arises in malaria, as well as these other infectious diseases, is an inability of mitochondria, through the effects of inflammatory cytokines on their function, to utilise available oxygen. This activity of these cytokines, plus their capacity to control the pathways through which oxygen supply to mitochondria are restricted (particularly through directing sequestration and driving anaemia), combine to make falciparum malaria primarily an inflammatory cytokine-driven disease.

Expression and serologic activity of a soluble recombinant Plasmodium vivax Duffy binding protein

Plasmodium vivax Duffy binding protein (DBP) is a conserved functionally important protein. P. vivax DBP is an asexual blood-stage malaria vaccine candidate because adhesion of P. vivax DBP to its erythrocyte receptor is essential for the parasite to continue development in human blood. We developed a soluble recombinant protein of P. vivax DBP (rDBP) and examined serologic activity to it in residents of a region of high endemicity. This soluble rDBP product contained the cysteine-rich ligand domain and most of the contiguous proline-rich hydrophilic region. rDBP was expressed as a glutathione S-transferase (GST) fusion protein and was isolated from GST by thrombin treatment of the purified fusion protein bound on glutathione agarose beads. P. vivax rDBP was immunogenic in rabbits and induced antibodies that reacted with P. vivax and Plasmodium knowlesi merozoites. Human sera from adult residents of a region of Papua New Guinea where malaria is highly endemic or P. vivax-infected North American residents reacted with rDBP in an immunoblot and an enzyme-linked immunosorbent assay. The reactivity to reduced, denatured P. vivax rDBP and the cross-reactivity with P. knowlesi indicated the presence of immunogenic conserved linear B-cell epitopes. A more extensive serologic survey of Papua New Guinea residents showed that antibody response to P. vivax DBP is common and increases with age, suggesting a possible boosting of the antibody response in some by repeated exposure to P. vivax. A positive humoral response to P. vivax DBP correlated with a significantly higher response to P. vivax MSP-1(19). The natural immunogenicity of this DBP should strengthen its usefulness as a vaccine.

A malaria vaccine strategy based on the induction of cellular immunity

In this article, Michael Good offers an alternative to the current emphasis on developing subunit, humoral-response-inducing malaria vaccines. He develops a three-pronged proposal comprising (1) a greater emphasis on cellular immunity, (2) analysis of the role of the spleen in protection, and (3) the development of an attenuated vaccine.

Inability of Plasmodium vinckei-immune spleen cells to transfer protection to recipient mice exposed to vaccine 'vectors' or heterologous species of plasmodium

Mice can be immunized to Plasmodium vinckei by repeated infections followed by cure. Such immunity is dependent on CD4 T cells and an architecturally modified spleen, but has little requirement for antibody. Thus, athymic mice can be exposed to P. vinckei and cured, but do not develop immunity. They are resistant to challenge with parasites, however, if they are then given spleen cells from euthymic immunized animals. Such immune spleen cells, however, cannot transfer resistance to normal mice which have been exposed to BCG, Salmonella typhimurium, or vaccinia virus, and are only partially effective in transferring resistance to mice which have been previously immunized with heterologous plasmodia, P. yoelii, P. chabaudi and P. berghei. Mice exposed to varying numbers of irradiated P. vinckei-pRBC do not develop immunity and nor are such animals protected following adoptive transfer of immune spleen cells. Cellular immunity to malaria may not only be dependent on a population of immune CD4 T cells, but may require a specifically architecturally modified spleen which may not occur following either exposure to candidate vaccine vectors, heterologous plasmodia or non-viable homologous plasmodia.

The implications for malaria vaccine programs if memory T cells from non-exposed humans can respond to malaria antigens

Although the goal of current candidate vaccines is to expand a population of malaria antigen-specific lymphocytes, accumulating evidence suggests that peripheral blood of adult humans contains significant numbers of malaria-specific T cells prior to any exposure to vaccine or actual infection. The reason why such naive humans are susceptible to malaria infection may thus relate not to inadequate T-cell surveillance but to some other factor--possibly lack of suitable splenic modification. It is possible that current vaccine programs are misdirected because these other factors are not being addressed. The possibility of an attenuated vaccine should be re-examined.

The Duffy receptor family of Plasmodium knowlesi is located within the micronemes of invasive malaria merozoites

Plasmodium vivax and Plasmodium knowlesi merozoites invade human erythrocytes that express Duffy blood group surface determinants. A soluble parasite protein of 135 kd binds specifically to a human Duffy antigen. Using antisera affinity purified on the 135 kd protein, we cloned a gene that encodes a member of a P. knowlesi family of erythrocyte binding proteins. The gene is a member of a family that includes three homologous genes located on separate chromosomes. Two genes are expressed as major membrane-bound products that give rise to soluble erythrocyte binding proteins: the 135 kd Duffy binding protein and a 138 kd protein that binds only rhesus erythrocytes. These different erythrocyte binding specificities may result from sequence divergence of the homologous genes. The Duffy receptor family is localized in micronemes, an organelle found in all organisms of the phylum Apicomplexa.

Release of merozoite dense granules during erythrocyte invasion by Plasmodium knowlesi

We used immunoelectron microscopy to study the fate of dense granules during the invasion of erythrocytes by Plasmodium knowlesi merozoites. When merozoites entered host cells, dense granules moved to the pellicle, released their contents into the parasitophorous vacuole space, and then moved into fingerlike channels of the vacuole membrane. This is the first report showing that the content of dense granules of P. knowlesi is different from the contents of rhoptries and micronemes and is associated with the formation of channels from the parasitophorous vacuole.

Involvement of T cells in malaria immunity: implications for vaccine development

T cells are critical for immunity to malaria, not only because they function as helper cells for an antibody response, but also because they serve as effector cells. Such cellular immunity is directly implicated in protection from sporozoites and plays an important role in protection from blood-stage parasites. It also can block transmission of malaria from mammalian host to the mosquito. Both CD8 and CD4 effector cells have important roles. The parasite's defence from immune attack, however, is designed to minimize activation of T cells. Thus, there appears to be limitation of the number of T sites within many malaria proteins and variation within these limited sites. Homology to host proteins and resultant immune-escape due to tolerance may be another mechanism. These parasite defence mechanisms highlight both the importance of T-cell immunity in malaria and the challenge of designing effective vaccines to stimulate T cells.

Cell-mediated immunity to Plasmodium falciparum infection: evidence against the involvement of cytotoxic lymphocytes

Blood mononuclear cells (PBMC) recognizing soluble malaria antigens (SPag) are present in the peripheral blood of individuals clinically immune to malaria, and they proliferate after exposure to such antigens. To test whether these cells have effector activity against Plasmodium falciparum, we stimulated PBMC from malaria-immune donors by SPag and purified protein derivative (PPD) in culture for 7 days. The PBMC were then co-incubated with P. falciparum for 48 h, and parasitaemia was determined by microscopy. Parasite growth was only significantly impaired after incubation with PBMC stimulated by either SPag or PPD in the presence of immune serum. Studies on subpopulations of PBMC indicated that the inhibitory cells resided among the adherent cell fraction. Furthermore we tested PBMC for cytotoxic activity against P. falciparum-infected autologous or heterologous erythrocytes. Experiments were done both in the absence and the presence of immune serum. Neither fresh PBMC nor PBMC activated by SPag or PPD for 7 days prior to assay were cytotoxic, indicating that cytotoxic T cells, natural killer (NK) cells, and K cells did not possess cytotoxic activity directed against parasitized erythrocytes. These data support the hypothesis that activated monocytes are the most important effector cells in the peripheral blood of malaria immune individuals.

Cellular mechanisms involved in recovery from acute malaria in Gambian children

This paper reports the results of in vitro experiments which attempt to elucidate the mechanisms whereby Gambian children control acute infections of Plasmodium falciparum. It was shown initially that mononuclear cells from children with acute malaria, in the presence of specific antibody, caused a marked reduction in in vitro parasite growth. IgM antibodies appeared to be considerably more effective than IgG. T or B lymphocytes were ineffective in the system; adherent cells alone had some effect, but much less than the unfractionated cell population. Adherent cells were however fully effective after exposure to supernatants from T cells activated either non-specifically by phytohaemagglutinin (PHA), or specifically by P. falciparum antigens. Depression of parasite growth was also observed, independent of anti-malarial antibody. This was achieved when adherent cells from healthy Europeans, as well as those from infected children, were exposed to the supernatants from previously stimulated T cells before adding to the culture. Furthermore, intra-erythrocytic parasite death occurred after a short exposure to the supernatants of 'activated' adherent cells from both infected children and Europeans.

Proliferation induced by Plasmodium falciparum antigen and interleukin-2 production by lymphocytes isolated from malaria-immune individuals

Affinity-purified Plasmodium falciparum soluble antigens (SPAg) isolated from in vitro cultures of the parasite were shown to be relatively free of nonspecific polyclonal activators. To determine the presence of lymphocytes with specificity against SPAg in the peripheral blood of malaria-immune individuals, the proliferative response and the interleukin-2 (IL-2) production of SPAg-activated mononuclear cells (MNCs) from individuals unexposed, sensitized, and immune to malaria were measured. It was found that MNC isolated from malaria-immune individuals proliferated in response to SPAg and that this activation resulted in measurable IL-2 production in 5 of 10 MNC cultures. MNC isolates from most unexposed individuals did not respond to SPAg. To establish which cells responded to SPAg, different subpopulations of MNCs were tested. Only T helper cells were found to respond, and they responded only when cocultured with monocytes. The finding of parasite-specific T helper cells in the blood of malaria-immune individuals and the fact that some of these cells were able to produce IL-2 in vitro support the hypothesis that in malaria the cellular part of the protective immune response is initiated by immune T cells. These cells may activate nonspecific effector cells (i.e., macrophages) that eliminate the parasite.

Cellular and humoral inhibition of Plasmodium falciparum growth in vitro and recovery from acute malaria

Both mononuclear cell cytotoxicity and serum inhibition of Plasmodium falciparum growth in vitro were found to vary according to the stage of infection in Gambian children with clinical malaria. Cytotoxicity was displayed by mononuclear cells and serum from children with acute malaria but this form of parasite killing was more effective in children with low grade P. falciparum infections of at least 10 days duration. Parasite inhibitory antibody was not evident in sera from acutely infected children but was found in sera from children recovering from malaria and reached a peak in convalescent children when P. falciparum growth was inhibited by at least 50%. The humoral response in convalescent children was strain related, being more effective against the most recent infecting parasite strain than against other 'wild' P. falciparum isolates. In contrast, mononuclear cell cytotoxicity was not strain related; when effective, multiplication of all parasite isolates tested was retarded to the same degree. The discussion considers the role of mononuclear cell cytotoxicity in the development of protective immunity and suggests that it may be a 'front line' defense mechanism during each malaria attack.

Serotypic variation of culture-adapted isolates of P. falciparum

Culture adapted isolates of P. falciparum had different growth patterns (P less than 0.01) when cultured in the presence of sera obtained from 31 healthy Papua New Guinea residents. Both enhancement and suppression of growth was observed. Less variation was observed using an indirect immunofluorescent antibody method directed against schizont antigens, and no correlation between the two assays was found. This bioassay is assessed as a basis for serotyping P. falciparum strains.

2-Mercapto-ethanol enhancement of agglutination reaction--a possible in vitro serological correlate for assessment of functional immunity in simian malaria

Conventional indirect haemagglutination test was performed in rhesus monkey sera (collected from Plasmodium knowlesi infected animals) with and without prior treatment of sera with 2-mercapto-ethanol (2-ME). Surprisingly, many sera samples showed significant enhancement of final titre with 2-ME. The 2-ME enhancement effect was more pronounced in the sera of hyperimmune monkeys on further injection of antigen or parasites. It was also noticeable in the sera during primary drug-suppressed P. knowlesi infection and appeared to have a bearing on the immune status of the animals to rechallenge. The use of a soluble antigen prepared from P. knowlesi infected erythrocytes was found to be essential in IHA test to demonstrate the 2-ME enhancement effect. Antigen prepared from freed parasites (commonly used) failed to show a similar effect in IHA. The possible role of certain T-lymphocyte products - antigen binding, non-agglutinating, 2-ME sensitive molecules - in malarial immunology has been proposed.

Opsonic activity of ascitic fluids from Plasmodium falciparum-infected Saimiri monkey: positive correlation with protection in passive transfer assay

An investigation into the protective activity of ascitic fluids from Saimiri monkeys infected with Plasmodium falciparum and the role played by opsonins in that activity was undertaken. P. falciparum-parasitized blood was collected from splenectomized Saimiri (when parasitaemia reached at least 20% or more) and used in an in vitro phagocytic assay including ascitic fluid and cultures of peripheral blood monocytes from normal Saimiri. Under the conditions of this in vitro assay, we found that ascitic fluid phagocytosis-promoting factors were opsonic rather than cytophilic. The opsonic activity was highly specific for parasitized red blood cells and was effective against all stages of development of the parasite. A highly positive relationship between in vitro opsonizing activity and in vivo protective capacity of immune ascitic fluid was found.

Antibody-dependent ingestion of P. falciparum merozoites by human blood monocytes

The binding of malarial antibodies to peripheral blood monocytes and the ability of these armed monocytes to attach to and ingest P. falciparum merozoites and schizont infected erythrocytes was evaluated by an in vitro assay. Monocytes from normal unsensitized subjects were preincubated with sera from individuals with various states of immunity to malaria and sera from normal controls. A marked difference in the level of merozoite phagocytosis was observed depending on immune status of the individuals whose sera were tested, but not on the antibody levels measured by fluorescence or precipitation tests. By protein-A-sepharose fractionation of these sera it appeared that the merozoite phagocytosis was mediated by immunoglobulin of the IgG class. Immunoglobulins eluted from these preincubated monocytes were able to bind to the parasites as detected by indirect fluorescent test. Similar assays performed with different strains and antibodies from various geographical areas indicated that the merozoite recognition and ingestion was not strain specific. The monocyte-immunoglobulin co-operation was effective in the phagocytosis of merozoites but not of schizont infected erythrocytes or normal erythrocytes. However, some degree of adhesion to the schizonts was recorded. These data seem to indicate that human blood monocytes can be specifically armed in vitro by cytophilic IgG with antimalarial specificity, and that such an effect is able to enhance markedly the clearance of free parasites but not of intact schizonts.

Induction of crisis forms in cultured Plasmodium falciparum with human immune serum from Sudan

Serums from 90 individuals from three areas in Sudan were tested for inhibitory activity against cultures of Plasmodium falciparum. In addition to inhibitory activity against merozoite invasion, all of the serums demonstrated, in varying degrees, the ability to retard intraerythrocyte development, leading to crisis forms and parasite deterioration. These retardation factors could be removed by absorption of immune serum with parasite-infected erythrocytes and were demonstrable in purified immunoglobulin fractions. Serum from donors in hypoendemic Khartoum did not retard parasite development.

Immunoprecipitation of biosynthetically-labelled proteins from different Papua New Guinea Plasmodium falciparum isolates by sera from individuals in the endemic area

The human serum antibody response to Plasmodium falciparum infection in Papua New Guinea has been studied by electrophoretic analysis of immunoprecipitated biosynthetically-labelled malaria proteins from three different isolates maintained in long-term in vitro culture. Differences in protein antigenic composition in different lines have been described and simplified by examination of antigens recognized only by hyperimmune serum. An in vitro assay has been used to screen various human sera containing antimalarial antibody for their ability to inhibit parasite growth and the immunoprecipitation profiles of non-inhibitory sera have been compared with those of a hyperimmune serum pool. In the discussion, emphasis is placed on the value of immunoprecipitation analyses using clinically-defined sera with known in vitro function in the identification of antigens which may be responsible for the induction of host-protective immunity.

Rhesus monkeys protected against Plasmodium knowlesi malaria produce antibodies against a 65,000-MrP. knowlesi glycoprotein at the surface of infected erythrocytes

Sera from 27 rhesus monkeys immunized in various ways against the H strain of Plasmodium knowlesi were analyzed by quantitative crossed immunoelectrophoresis. The reaction of the sera was compared with a reference immune serum only reactive with P. knowlesi-specific 65,000-Mr glycoprotein-immune component 13 (gp65/ic13) in membranes of infected rhesus monkey erythrocytes. Triton X-100-solubilized, 125I-labeled membranes of schizont-infected erythrocytes were used as an antigen. Sera from 9 or 10 monkeys immunized by repeated infections with P. knowlesi reacted with gp65/ic13. In 6 of 10 sera, anti-gp65/ic13 was the only antibody reacting with host cell membrane proteins. In contrast, vaccination of 15 monkeys with predominantly sexual stages or trophozoites of P. knowlesi in Freund complete adjuvant resulted in protection against blood challenges in 7 monkeys, only 2 of which contained precipitating antibody against gp65/ic13. None of the sera from monkeys not protected by infections or vaccinations contained detectable levels of precipitating antibodies against gp65/ic13. Our data indicate that gp65/ic13 acts as a prominent immunogen in vivo during natural p. knowlesi infections of rhesus monkeys. There is a positive correlation suggested between anti-gp65/ic13 antibody and protection in the monkeys analyzed. This correlation does not apply to monkeys protected against P. knowlesi malaria by vaccination, pointing to other effective immune defense mechanisms.

Immunity to malaria

Malaria remains prevalent throughout tropical and subtropical regions and almost a third of the World's population is exposed to the risk of infection. There is currently a serious resurgence of the disease in Asia and Central America. The failure of global eradication measures based upon the use of insecticides and chemotherapy has resulted from difficulties of practical implementation compounded by the spread of insecticide and drug resistance. Repeated natural infection does not produce detectable resistance to the exo-erythrocytic cycle of malaria in man. Irradiated sporzoite vaccines do, however, induce stage specific immunity in murine malaria and in a proportion of human subjects. Vaccinated individuals remain susceptible to blood stage infection which causes clinical malaria. In addition the vaccine is unstable and must be administered by intravenous inoculation. Since neither sporogonic nor exo-erythrocytic parasite development is cyclical in human malarias, there is little prospect for vaccine production through cultivation of these stages. The inhabitants of hyperendaemic areas become increasingly resistant to malaria during childhood and adolescence, through the slow development of specific, acquired immunity to asexual blood stage parasites. Immunity is mediated by antibody, which blocks merozoite invasion of red cells, as well as by cell mediated mechanisms and non-specific cytotoxic agents. Vaccination with merozoites induces long lasting immunity of broad serological specificity active against the blood-stage of the parasite. Merozoite vaccines can be preserved by freeze drying and harvested from continuous cultures of blood stage parasites. The major problem in development of a human merozoite vaccine concerns the requirement for Freund's complete adjuvant which is not acceptable for man. The effective immunity induced by vaccination contrasts with the slow development of incomplete resistance which follows repeated natural infection. The latter is associated with the generation of immune suppressor cells, lymphoid cell mitogens and soluble antigens, and in some species by the occurrence of antigenic variation--all of which may favour parasite survival. It is probable that vaccination with non-viable antigen of appropriate composition, induces immune effector processes without activating mechanisms which allow parasites to escape the consequences of immunity. Many effective vaccines such as those against measles, poliomyelitis, tetanus and rabies are commercially available but barely used in the developing world. The affected nations cannot afford their purchase, nor do the means exist for their distribution. It follows that if a safe and effective malaria vaccine were to be developed, its bulk manufacture and administration would require massive international support and cooperation.