Plasmodium berghei: passive transfer of immunity by antisera and cells

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.

Protective immunity against malaria after vaccination

A good understanding of the immunological correlates of protective immunity is an important requirement for the development of effective vaccines against malaria. However, this concern has received little attention even in the face of two decades of intensive vaccine research. Here, we review the immune response to blood-stage malaria, with a particular focus on the type of vaccine most likely to induce the kind of response required to give strong protection against infection.

Immunoregulation of malarial infection: balancing the vices and virtues

Malaria continues to extract an incalculable cost on human morbidity and mortality throughout tropical and subtropical regions of the world, and effective control measures are urgently needed. Despite considerable efforts in recent years to develop subunit vaccines targeted at various stages of the Plasmodium life-cycle, the commercial availability of a vaccine is still a distant prospect. One of the underlying difficulties hindering successful vaccine design is our incomplete knowledge of the precise type(s) of immune response to aim for, and then how to achieve it. A greater appreciation of the mechanisms of protective immunity, on the one hand, and of immunopathology, on the other, should provide critical clues on how manipulation of the immune system may best be achieved. Ten years have passed since the identification of the Th1/Th2 paradigm for distinguishing CD4+ T cells according to cytokine secretion patterns which determine their function. This review summarises our progress towards understanding the broad spectrum of immune responsiveness to the blood stages of the malaria parasite during experimental infections in mice and highlights the way in which examination of rodent malarias provides a powerful tool to dissect the interaction of Th1 and Th2 cells during an immune response to an infectious disease agent. It is proposed that the pliability of rodent systems for investigating immunoregulation provides valuable insight into the balance between protection and pathology in human malaria and throws light on the factors involved in the modulation of vaccine-potentiated immunity.

Remarkable activation of polyamine biosynthesis in hematopoiesis and hyperplasia of spleen in mice with hemolytic anemia caused by infection with Plasmodium berghei

Ornithine decarboxylase (ODC) activity was markedly induced in the spleen of mice infected with Plasmodium berghei, showing maximal activity at 8 days after the infection. The increase of spleen weight, on the other hand, reached its peak after 14 days of infection. In the blood of P. berghei-infected mice, no increase of ODC activity was observed. This indicated that ODC was induced in the spleen cells, but not in the parasites themselves which existed in the blood. Polyamines (putrescine, spermadine and spermine) were also elevated in the spleen following induction of the ODC activity. On the other hand, increases of ODC activity and spleen weight were observed in the spleen of mice with hemolytic anemia induced by acetylphenylhydrazine, but the extent of these increases were smaller than those in the spleen of mice infected with P. berghei. The present results suggest that increases in ODC activity and polyamine levels in the spleen of P. berghei-infected mice are related to hyperplasia of the spleen (splenomegaly) where the formation of leukocytes and erythrocytes (hematopoiesis) was dramatically stimulated by the infection.

Comparative evaluation of the protective effect of immune spleen cells and immune RNA against Plasmodium berghei

Earlier studies from this laboratory indicated that passive transfer of viable or frozen-thawed cells from spleens and lymph nodes of immune mice resulted in a significant protective immunity against Plasmodium berghei in syngeneic recipients. To assess whether immune RNA played a role in conferring such protection, experiments were designed wherein immune RNA was isolated from immune monkeys, rats and mice and transferred to normal mice. The effect of transfer was assessed by challenging RNA-primed animals with Plasmodium berghei. Results indicated that immune RNA failed to confer resistance against P. berghei both in syngeneic and in heterologous systems.

Manipulation of the immune response in parasitic infections

The following brief survey considers various manoeuvres which can be applied to manipulate the immune response to parasitic infectionsin vivo. The examples quoted largely concern malaria, babesiosis, schistosomiasis and leishmaniasis, predominantly in inbred mouse strains. Since my own relevant research experience has been restricted to leishmaniasis, this will receive undue emphasis, although it does illustrate particularly well points I wish to stress. The types of intervention described do not all provide the precision of interpretation with which they are sometimes credited. Thus, effects of immunosuppression or T-cell depletion alone can usually only implicate the specific immune response (in its broad sense) in shaping the natural history and outcome of an infection or in underlying the effect of prophylactic immunization. Nevertheless, more precise delineation of lymphocyte subset involvement can be obtained by cell replacement studies in some of these models or by exclusion of antibody. The outcomes of these approaches have been (or are) predictable in most cases. More fascinating, however, are the various instances which will be stressed where totally unpredicted and contrary observations have been made which led (or may lead) to fresh insight into the disease. These serendipitous findings illustrate at the same time the value of applying the manoeuvres, even though they imply that the logical immunologist cannot yet always outsmart the parasite by design.

Entamoeba histolytica: antibody responses and lymphoreticular changes in gerbils (Meriones unguiculatus) in response to experimental liver abscess and amebic extract infection

Antibody responses and histological changes in hepatic lymph nodes and spleen of gerbils (Meriones unguiculatus) during the course of experimental hepatic amebiasis (5-60 days), or in those injected with extracts of Entamoeba histolytica, are described. Lymph node and spleen responses in infected animals paralleled the proliferation of the amebic liver abscess. However, spleen follicle responses were similar in animals that received low or high doses of the amebic extract and differed histologically from those with amebic liver abscess. Liver abscesses, up to 30 days postinfection (pi), doubled in weight between 10 and 15 and between 20 and 30 days pi. Early changes (10 days pi) in the lymphoreticular tissues were characterized by increased size and weight of the organs, hyperplastic follicles, and blastogenesis in the T-dependent areas. At 20 and 30 days pi, the size of spleen follicles increased and there was depletion of lymphocytes from the periarterial area (PAA), as well as gross extension of the red pulp, accompanied by extramedullary erythropoiesis and megakaryocytosis. The paracortical areas (PCA) of lymph nodes were depleted of lymphocytes and histiocytosis throughout the organ, and there was intense plasma cell activity in the medulla. At 60 days pi, lymphocyte repopulation was noted in the PCA and PAA; germinal centers were depleted of blast cells and the spleen red pulp had contracted. Antiamebic antibody titers were low throughout the infection. Changes in the cellularity of the lymphoid organs are discussed in relation to the proliferation of the amebic liver abscesses in infected animals and in those which were injected with the amebic extract.

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.

Effect of splenectomy on the size of amoebic liver abscesses and metastatic foci in hamsters

The role of the spleen in hepatic amoebiasis in hamsters was studied. In hamsters receiving an intrahepatic inoculation of 10(5) trophozoites of axenic Entamoeba histolytica at 7 or 14 days postsplenectomy, the mean weight of metastatic foci increased significantly when compared with sham-splenectomized or intact controls. In contrast, when both splenectomy and intrahepatic inoculation with amoebae were carried out at the same time, there was not only a significant increase in the mean weight of metastatic foci but also in the liver abscess. It is suggested that the spleen is important for host defense against E. histolytica infection, especially in the reduction in the degree of metastatic spread from the primary site.

Recent advances in applied malaria immunology

Our present knowledge of cellular and humoral factors which are involved in immunity to plasmodial infections are discussed. Immunization against plasmodial infection has been achieved in birds, rodents, simians, and humans. Avian hosts have been immunized against gametocytes which resulted in inhibition of gametocytes within the mosquito vector. Immunization of humans against plasmodial gametocytes would indirectly protect them against malaria by blocking mosquito transmission to other susceptible individuals. Immunization by sporozoites provides short-lived protection against sporozoite challenge, but gives no protection against erythrocytic forms. Some success has been obtained in immunizing avian and mammalian hosts with exoerythrocytic forms obtained from cultured avian cells. The most significant advances have occurred in immunizing simian hosts against simian or human malaria by vaccinating with fresh erythrocytic merozoites or a nonviable lyophilized antigen obtained from intraerythrocytic forms. The development of an antigen preparation suitable for use as a human malaria vaccine is dependent upon prior development of an in vitro system which would provide adequate amounts of parasite material. Efforts to cultivate the sporogonic, exoerythrocytic, and erythrocytic, and erythrocytic phases of plasmodia as well as the feasibility of using these forms for vaccination are discussed.

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.

Cell mediated and humoral immunity in experimental Plasmodium berghei infection

Adoptive passive transfer of immunity to Plasmodium berghei infection has been investigated in an inbred strain of Swiss mice. The mice were made hyperimmune by repeated passage of 10(3) parasites and subsequent therapy with an antimalarial drug. Immune sera and cells obtained from thymus, spleen and peritoneal exudate were transferred to normal animals which were subsequently challenged with standard doses of P. berghei. It was observed that: (a) immune serum in high doses (0.5 ml/mouse) enhanced parasitaemia; when used in smaller doses (0.1 ml/mouse), it afforded a considerable degree of protection; (b) viable immune lymphocytes obtained from thymus and lymph node afforded protection; (c) the mixed population of cells obtained from spleens of immunized mice, as well as peritoneal exudate, protected mice against challenge inoculum; (d) glutaraldehyde-treated spleen cells and material obtained after freezing and thawing the same number of spleen cells, macrophages and lymph node also afforded protection. These findings confirm that, under these experimental conditions, immunity against P. berghei is mediated through (i) specific antibody which is dose-dependent, (ii) cell-mediated immunity and (iii) effective response to processed antigen.

T cells and protective immunity to Plasmodium berghei in rats

Experiments were carried out in which unfractionated spleen cells, and T lymphocyte subpopulations characterized by certain experimental criteria, were isolated at various times from rats infected with Plasmodium berghei. By adoptive transfer it was shown that unfractionated spleen cells, and T cells alone, could transfer protection to syngenic recipients as early as 11 days after infection of the cell donors. The protection conferred by T cells increased with the duration of the infection in the donors, at least up to 100 days. The additional presence of B cells in transferred lymphocyte populations enhanced their protective capacity over that shown by T cells alone. The role of T cells in protective immunity to malaria is discussed.

Adoptive transfer of immunity to Plasmodium berghei with immune T and B lymphocytes

Immunity to malarial infection may be transferred with immune lymphocytes. This study was designed to determine which lymphocyte type is responsible for the adoptive transfer of immunity to malarial infection. In one set of experiments, the ability of immune T and B lymphocytes, separated by passage through nylon-wool columns, to transfer immunity to infection was determined. In another experiment, the effect of killing T lymphocytes with anti-theta serum on the transfer of immunity was determined. The effect on the ability of immune lymphocyte suspensions to transfer immunity after B lymphocytes were removed from such suspensions by centrifugation on Ficoll-Hypaque gradients, after they had formed rosettes with sensitized, complement-coated sheep erythrocytes, was also determined. The ability of lymphocyte suspensions to adoptively transfer resistance to malarial infection was greatly impaired by the removal from the suspensions of differentiated B-type lymphocytes. Our results indicate that it is the differentiated B cell, most probably an antibody-producing cell, which lacks both theta antigen and the complement receptor that is responsible for conferring immunity to malaria.

The nature of age immunity to Plasmodium berghei in the rat

The intensity of Plasmodium berghei infections decreases as the age of the rat host increases. The nature of this 'age immunity' was investigated. No experimental support was found for innate resistance involving either serum non-antibody factors or changes in the erythrocytes that inhibit parasites in older rats. A cross reacting immune response active against P. berghei was not found. Evidence is presented which shows that rats less than 7 weeks old lack at least part of the functional immunological apparatus by which older rats produce a protective immune response. It is suggested that the defect might involve T lymphocytes.

Immunity to Plasmodium berghei in rats: passive serum transfer and role of the spleen

Pools of rat antiserum to Plasmodium berghei had different levels of protective activity as assessed by a passive transfer test. Preincubation of parasite inocula with an effective pool before injection did not significantly enhance protective activity. Removal of the antiserum from preincubated parasite inocula abolished the bulk of protective activity. Similarly, antiserum effective in intact animals was largely ineffective in splenectomized recipients. These experiments suggest a minimal role for antibody acting directly on P. berghei-parasitized cells and reemphasize a significant role for the spleen in immunity to this plasmodium.

Immunity to Plasmodium berghei in rats: maximum levels of protective antibody activity are associated with eradication of the infection

Maximum levels of protective antibody activity were reached in the circulation of rats given a primary infection or one or two re-infections of Plasmodium berghei at the time the infection was finally eradicated. This association of maximum protective antibody activity with complete clearance of the parasite was also found in recipients of lymphoid cells from immune donor rats and suggests that a high level of protective antibody activity is one essential factor in complete elimination of the parasite.

Excellent technical assistance was given at different times by Mrs J. Page, Mr A. J. Edwards and Miss D. Leader.