Immunity to malaria. 3. Possible occurrence of a cell-mediated immunity to Plasmodium knowlesi in chronically infected and Freund's complete adjuvant-sensitized monkeys

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.

Mechanisms of immunity to malaria and the possibilities of a blood-stage vaccine: a critical appraisal

Resistance developed by the immune system in response to blood-stage malaria is complex in nature, involving humoral and non-antibody effector mechanisms. Different species of malarial parasites may vary in their ability to elicit, or their susceptibility to, those immune effectors. This complexity is enhanced by the different results obtained in vaccinated as opposed to drug-controlled infections. It is therefore important that some attempt be made to unravel these interactions. This is particularly so when we have to decide on methods for assessing the potentiality of antigens to induce protective immunity. In this review the limitations of some in vitro assays of immunity, as well as those of various host-parasite models, are discussed. The relative importance of cell-mediated and humoral immunity in laboratory models and natural infections is also considered in the context of vaccine development.

Protective immunity to malaria: studies with cloned lines of Plasmodium chabaudi chabaudi in CBA/Ca mice. III. Protective and suppressive responses induced by immunization with purified antigens

The protective effect of affinity purified antigen has been investigated in an experimental model for malaria which shows a well marked recrudescence of parasitaemia, a feature of the disease in man. A monoclonal antibody (MoAb) recognizing an epitope common to two genetically distinct cloned lines of Plasmodium chabaudi (AS and CB), was used to purify a Mr250,000 polymorphic schizont antigen (PSA) from these parasites. The purified preparations were then examined for the presence of specific and cross-reactive epitopes by immunoprecipitation with a panel of MoAb raised against P. chabaudi AS. When tested previously on smears of parasitized blood by immunofluorescence, or against lysates of parasitized erythrocytes by immunoprecipitation, most of these MoAb had been found to be AS specific. When either AS or CB affinity purified Mr250,000 PSA was used as the target, these same MoAb immunoprecipitated both antigens, and in some cases, a number of associated polypeptides (AP) which copurify with the Mr250,000 PSA. Subsequently, mice were immunized with either the purified AS or CB antigens in Freund's complete adjuvant (FCA). Prechallenge sera were compared by indirect immunofluorescence and immunoprecipitation. Sera from mice immunized with AS antigen reacted strongly with AS and cross-reacted with CB parasite preparations. Pre-challenge serum from CB antigen immunized mice reacted well with CB, but only faintly with AS preparations. In mice immunized with the AS antigen and then challenged with either AS or CB parasites, the initial parasitaemias were delayed in appearance and the height of the peak parasitaemia reduced, an effect which was most pronounced after challenge with homologous parasites. Only homologous challenge of the mice immunized with CB antigen produced statistically significant modification of the initial parasitaemia. In the immunized mice challenged with homologous parasites, the delayed appearance and slightly reduced peak of the primary parasitaemia was associated with delayed resolution of the patent parasitaemia and significant enhancement of the recrudescence.

Deficiency of Con A-induced suppressor cell activity in peripheral blood mononuclear cells from Thai adults naturally infected with Plasmodium falciparum and Plasmodium vivax

Con A-pretreated mononuclear (MNC) cells from Thai adults with naturally acquired P. falciparum or P. vivax malaria were significantly less effective in suppressing the responsiveness of autologous or normal allogeneic responder cells to mitogenic lectins or allogenic stimulator cells than pretreated cells from healthy donors. Serial studies of three patients demonstrated that reduced suppressor cell activity was present early in malaria infection but returned to normal soon after treatment. These studies demonstrate that the loss of T cells previously observed in patients with malaria, in part may functionally represent a loss of suppressor T cells.

Killing of the malarial parasite Plasmodium yoelii in vitro by cells of myeloid origin

Cytotoxic effects of mouse cells on Plasmodium yoelii were sought directly by incubating parasitized red cells with cells of various kinds for 16 h and then determining the percentage parasite survival in vivo, in terms of infectivity for the mouse. Cell populations rich in lymphocytes, e.g. lymph node and spleen, were less active than peritoneal cells and blood. Parasite killing by peritoneal cells was associated with macrophages: treatment with anti-macrophage serum (AMS) or depletion by adherence or centrifugation on Ficoll decreased activity. Polymorphonuclear leucocytes (PMN) in induced exudates may have contributed to killing, although not as actively cell for cell, and an effect of eosinophils in worm-induced exudates was not excluded. White blood cells were most active of all and fractionation on Ficoll confirmed that lymphocytes were relatively ineffective. The effector cell was phagocytic but it was insensitive to AMS. Tests on populations wih high or low proportions of PMN showed that parasite killing was independent of PMN number. It is concluded that the effector cell belongs to the monocyte-macrophage series and has acquired the ability to kill the parasite before becoming fully differentiated into a macrophage.

Cell-mediated immunity in the liver of mice vaccinated against malaria

Mice can be protected against several species of lethal malaria infection by vaccination, and their recovery correlates well with increased anti-malarial antibody levels, particularly IgG (ref.2). However, there is also a good correlation between protection by vaccines and priming for delayed-type hypersensitivity in the skin, although there is no obvious explanation for this effect. We now report an apparent relationship between protection and a cell-mediated immune response involving the migration of various types of cell capable of killing malaria parasites in vitro to the liver. We suggest that the effect of vaccination is to bring together parasites, specific antibody and nonspecific cytotoxic cells, and that the liver may be a major site for their interaction.

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.

Mechanisms of malarial immunity

Immunity to malaria in many species, including man, is acquired only after long exposure to infection and is associated with chronic low-grade parasitaemia. Vaccination of Rhesus monkeys with P. knowlesi merozoites in FCA induces sterilizing immunity which is species specific. Merozoite-blocking (inhibitory) antibody usually correlates with clinical immunity and protection can be passively transferred with immune sera. However, vaccination using adjuvants other than FCA may induce inhibitory antibody without clinical protection. In addition, vaccinated animals usually become susceptible to challenge 4 to 5 weeks after splenectomy, although inhibitory antibody levels are not reduced. These observations indicate that immunity induced by merozoite vaccination involves merozoite blocking (inhibitory) antibody and also specific antibody or immune complexes acting synergistically with cytotoxic splenic cells stimulated by FCA. During natural infection on the other hand soluble circulating antigens, partly derived from the merozoite coat during red cell penetration, are produced and these may block immune effector mechanisms and promote parasite survival.

Specific lymphocyte transformation in murine malaria

Lymphoblast transformation tests were performed on convalescent rats and mice, after infection with Plasmodium berghei had been reduced to latency. Spleen cells from immune animals reacted in vitro to specific plasmodial antigen and not to control antigen produced from non infected RBC. The response in vitro was dependent on the concentration of the antigen and the time of exposure to it. A correlation was observed between the parasitaemia of the convalescent animal during its acute infection and the reaction in vitro: the higher the parasitaemia the higher the in vitro simulation.

Delayed dermal hypersensitivity in rhesus monkeys (Macaca mulatta) immunized against Plasmodium knowlesi

Eighteen Rhesus monkeys were immunized with a fresh or lyophilized antigen preparation obtained from erythrocytes infected with Plasmodium knowlesi. Sixteen of these monkeys showed a positive delayed hypersensitivity response when subsequently skin tested with the immunizing antigen. Ten of the 16 positive reactors survived a P, knowlesi challenge, the remaining 6 showed several indications of protection. None of the 18 control monkeys gave a positive skin reaction or survived the challenging inoculum. Macro- and microscopical evidence suggests that the dermal reaction is of a delayed type. Preliminary biochemical analysis of the antigen used in this experiment is included and its significance is discussed. Hematological data is also presented.