Gamma interferon, CD8+ T cells and antibodies required for immunity to malaria sporozoites

Circulating Epstein-Barr virus-carrying B cells in acute malaria

Epstein-Barr virus (EBV) infection and Plasmodium falciparum malaria are two known cofactors in the aetiology of endemic Burkitt's lymphoma. To assess the relation between these factors, limiting dilution analysis was used to assess the number of EBV-carrying B cells in the circulation of Gambian children during and after acute malaria. Numbers of virus-carrying cells were five times higher in acute malaria patients and in UK patients with infectious mononucleosis than in convalescent malaria patients and in healthy control adults from the UK. Spontaneous outgrowth in limiting dilution cultures from acute malaria samples was inhibited by acyclovir, a viral DNA polymerase inhibitor. The mechanism of outgrowth, therefore, was virus release from the in-vivo infected cell, which led to infection and immortalisation of co-cultured normal B cells. The findings provide evidence that acute malaria is associated with an increase in the number of EBV-carrying B cells in the circulation. Because of this increase, there is a greater chance of a cytogenetic abnormality occurring in such a cell, with consequent evolution of Burkitt's lymphoma.

Identification of epitopes within the circumsporozoite protein of Plasmodium vivax recognized by murine T lymphocytes

The murine cellular immune response to the circumsporozoite (CS) protein of Plasmodium vivax was characterized using five synthetic peptides, some of which we identified as corresponding to T cell epitopes. The peptides P308-320, P344-355 and P353-364 were immunogenic, inducing a genetically restricted proliferative response, due to the activation of CD4+ T cells. The peptide P308-320 was recognized only by the lymphocytes of B10 (H-2b) mice. The other two peptides were recognized by primed lymphocytes of H-2a and H-2k mice. Of interest was the finding that one of these peptides, P353-364, induced a proliferative response of a large percentage of immune outbred Swiss mice. Our data provide evidence that, at least in mice, there is recognition of multiple T cell epitopes within the major surface antigen of P. vivax sporozoites.

Cytokines kill malaria parasites during infection crisis: extracellular complementary factors are essential

Malaria infection crisis, at which the parasitemia drops precipitously and the parasite loses infectivity to the mosquito vector, occurs in many natural malaria systems, and has not been explained. We demonstrate that in a simian malaria parasite (Plasmodium cynomolgi in its natural host, the toque monkey), the loss of infectivity during crisis is due to the death of circulating intraerythrocytic gametocytes mediated by crisis serum. These parasite-killing effects in crisis serum are due to the presence in the serum of cytokines tumor necrosis factor and interferon gamma, which are produced by the host as a result of the malaria infection. The killing activity of each cytokine is absolutely dependent upon the presence of additional, as yet unidentified factor(s) in the crisis serum.

Molecular parasitology: progress towards the development of vaccines for malaria, filariasis, and schistosomiasis

Advances in molecular biology have allowed for the identification of potential vaccine candidates against several parasitic diseases. Antigens from various life stages of Plasmodium and Schistosoma species and filarial worms have been cloned, sequenced and tested as vaccines. Results to date in animal models have been promising. Modest levels of protection against experimental human malaria have been obtained using both sporozoite and blood-stage antigens. However, a greater understanding of the mechanisms which lead to immunity against parasites is required before effective vaccines can be developed.

Gamma interferon-mediated inhibition of Eimeria vermiformis growth in cultured fibroblasts and epithelial cells

The growth of Eimeria vermiformis within cultured murine fibroblastlike (L-929) or rat epithelial-like (RATEC) cells was inhibited by treatment of the cells with the appropriate recombinant gamma interferon. The effect was apparent as a reduction in both the initial numbers of intracellular sporozoites and, to a much greater extent, the numbers of subsequent developmental stages. Pretreatment of the host cells was more effective than treatment in the early postinvasive period, and recombinant gamma interferon had no effect on the development of the parasite if added 24 h or later after the inoculation of sporozoites. Incubation of sporozoites in medium containing recombinant gamma interferon in no way affected their ability to invade or to grow within host cells. These findings indicate that the inhibitory effects of recombinant gamma interferon on the growth of E. vermiformis are mediated via the host cell and are directed mainly against the transforming sporozoite, although the ability of the sporozoite to invade the host cell was also reduced to some extent. The later developmental stages were refractory to the effects of this lymphokine.

CD4+ and CD8+ T lymphocytes both contribute to acquired immunity to blood-stage Plasmodium chabaudi AS

In the present study, the contribution of CD4+ and CD8+ T lymphocytes to acquired immunity to blood-stage infection with the murine malaria species Plasmodium chabaudi AS was investigated. C57BL/6 mice, which are genetically resistant to infection with this hemoprotozoan parasite and exhibit a transient course of infection, were treated intraperitoneally with monoclonal antibodies to T-cell epitopes, either anti-Thy-1, anti-CD4, or anti-CD8. After intraperitoneal infection with 10(6) parasitized erythrocytes, control C57BL/6 mice exhibited a peak parasitemia on day 9 of approximately 35% parasitized erythrocytes and eliminated the infection within 4 weeks. Mice depleted of Thy-1+ or CD4+ T cells had significantly higher parasitemias on day 7 as well as significantly higher peak parasitemias. These mice were unable to control the infection and developed a persistent, high parasitemia that fluctuated between 40 and 60% until the experiment was terminated on day 56 postinfection. Depletion of CD8+ T lymphocytes was found to have no effect on the early course of parasitemia or on the level of peak parasitemia. However, mice depleted of CD8+ T cells experienced two recurrent bouts of parasitemia during the later stage of the infection and required more than 5 weeks to eliminate the parasites. After the peak parasitemia, which occurred in control and experimental animals on day 9, there was a sharp drop in parasitemia coinciding with a wave of reticulocytosis. Therefore, the contribution of the influx of reticulocytes, which are not the preferred host cell of this hemoprotozoan parasite, to limiting the parasitemia was also examined by determining the course of reticulocytosis during infection in control and T cell-depleted animals. Early in infection, there was a marked and comparable reticulocytosis in the peripheral blood of control and T cell-depleted mice; the reticulocytosis peaked on day 12 and coincided with the dramatic and sudden reduction in parasitemia occurring in all groups. In both control and CD8-depleted mice the percentage of reticulocytes decreased as the infection was resolved, whereas in CD4-depleted mice marked reticulocytosis correlated with high, persistent parasitemia. These results thus demonstrate that both CD4+ and CD8+ T cells are involved in acquired immunity to blood-stage P. chabaudi AS and that the influx of reticulocytes into the blood that occurs just after the peak parasitemia may contribute temporarily to limiting the parasitemia.

Inflammatory status and preerythrocytic stages of malaria: role of the C-reactive protein

In the acquisition of protection against malaria, the role played by nonspecific factors, some being part of the cascade effect of cytokines, has to be considered. The C-reactive protein, a major acute phase reactant secreted by interleukin-1 stimulated hepatocytes, has an effect on the hepatic development of Plasmodia, both by preventing penetration of the sporozoite into the hepatocyte and by blocking parasite division through an antibody-like effect. This latter effect confirms the potential interest of targeting the uninuclear form of the parasite. Nevertheless, C-reactive Protein alone does not account for all the effects of the inflammatory response, other reactants from both serum and hepatocytes are also involved.

L-arginine-dependent destruction of intrahepatic malaria parasites in response to tumor necrosis factor and/or interleukin 6 stimulation

There is growing evidence that cytokines (interleukin [IL] 1, IL 6, interferon-gamma, tumor necrosis factor [TNF]) directly or indirectly interfere with the intrahepatic development of malaria parasites. Recent work in our laboratory clearly showed that TNF can affect the hepatic development of parasites via IL 6 secreted by liver nonparenchymal cells. The possible participation of an L-arginine-dependent effector mechanism has been studied to explain the TNF/IL 6-induced inhibition. We thus investigated if NGmonomethyl-L-arginine and N omega-nitro-L-arginine, two specific inhibitors of inorganic nitrogen oxide synthesis from L-arginine, were able to affect the inhibitory effect of TNF and/or IL 6 in co-cultures. At 0.1 and 0.5 mM both L-arginine analogues reversed the inhibitory effect of these cytokines. An interesting observation is that L-arginine analogues enhance schizont development in the absence of prior cytokine contact. This result indicates an hepatic basal L-arginine-dependent anti-parasitic activity which might explain the existence of self-degenerating hepatic forms as previously reported.

Immunity to the liver stage of malaria

The search for subunit vaccines against malaria has concentrated on asexual and sexual blood stage and sporozoite antigens. In recent years the search for the basis of the protection against sporozoite challenge obtained in mice immunized with irradiated sporozoites has focused attention on the liver or exoerythrocytic (EE) stage of the malaria life cycle. Here, Andreas Suhrbier looks at the various immune responses that appear to be active against this stage, which was once thought to be immunologically insignificant. The liver stage of malaria has thus emerged as a legitimate target for vaccine development.

Generation of a cytotoxic T-lymphocyte response using a Salmonella antigen-delivery system

We have constructed a general-use vector for the cloning and stable expression of foreign genes in the chromosome of attenuated Salmonella typhimurium. Using this chromosomal expression vector (CEV), we expressed the circumsporozoite (CS) gene of the mouse malaria Plasmodium yoelii in an aroA S. typhimurium strain. Mice immunized with CS-expressing Salmonella recombinants mount a CS-specific cytotoxic T-lymphocyte (CTL) response. This is the first demonstration that attenuated Salmonella can elicit a specific CTL response to a foreign protein in mice. The ability to easily and stably express foreign genes from the Salmonella chromosome and the generation of specific CTL greatly expands the potential of Salmonella as an antigen-delivery system.

Role of gamma interferon during infection with Plasmodium chabaudi chabaudi

A role has been proposed for inflammatory mediators such as gamma interferon (IFN-gamma) and reactive oxygen intermediates in the control of the blood stages of Plasmodium organisms. It was previously shown that IFN-gamma can be detected in the plasma of mice with a primary infection by Plasmodium chabaudi chabaudi (AS). We found that susceptible and other resistant mouse strains produced IFN-gamma, suggesting that susceptibility is not due to a defect in IFN-gamma production. Administration of IFN-gamma to intact C57BL/6 mice slightly decreased and partially delayed parasitemia, whereas in vivo depletion of IFN-gamma through injection of a "cocktail" of monoclonal antibodies against IFN-gamma exacerbated infection. Since CD4+ T cells are essential for the development of a protective immune response to P. chabaudi chabaudi, we tested whether CD4+ T cells are responsible for IFN-gamma production in vivo and whether exogenous IFN-gamma can replace the protective function of the CD4+ T cells. Mice depleted of CD4+ T cells were unable to produce IFN-gamma, but factors in addition to IFN-gamma may be important in parasite clearance.

CD4+ cytolytic T cell clone confers protection against murine malaria

A CD4+ T cell clone (A1.6) was derived from spleen cells of mice immunized with irradiated sporozoites. This T cell clone recognizes an antigen that is shared by sporozoites and blood forms of Plasmodium berghei and differs from the circumsporozoite protein. Clone A1.6 displays cytotoxic activity, produces IFN-gamma and IL-2 in vitro, and recognizes the plasmodial antigen in the context of the class II I-Ed molecule. Passive transfer of this CD4+ clone into naive mice resulted in a high degree of protection against sporozoite challenge.

Eimeria vermiformis: differences in the course of primary infection can be correlated with lymphocyte responsiveness in the BALB/c and C57BL/6 mouse, Mus musculus

BALB/c and C57BL/6 mice are high- and low-responders, respectively, to infection with Eimeria vermiformis, this genetically determined difference being immunologically mediated. In order to identify the level at which response phenotype is determined, the proliferation of mesenteric lymph node cells and their ability to transfer immunity adoptively were investigated in each strain; the development of circulating serum antibodies to E. vermiformis was also determined. In all respects BALB/c mice responded earlier than the C57BL/6 but peak values were similar in both strains. The relationship between the temporal differences noted and the characteristic, differing course of the primary infection in the two strains is discussed.

Oral Salmonella: malaria circumsporozoite recombinants induce specific CD8+ cytotoxic T cells

Oral immunization with an attenuated Salmonella typhimurium recombinant containing the full-length Plasmodium berghei circumsporozoite (CS) gene induces protective immunity against P. berghei sporozoite challenge in the absence of antibody. We found that this immunity was mediated through the induction of specific CD8+ T cells since in vivo elimination of CD8+ cells abrogated protection. In vitro studies revealed that this Salmonella-P. berghei CS recombinant induced class I-restricted CD8+ cytotoxic T cells that are directed against the P. berghei CS peptide epitope spanning amino acids 242-253. This is the same peptide that previously was identified as the target of cytotoxic T lymphocytes (CTL) induced by sporozoite immunization. Salmonella-P. falciparum CS recombinants were constructed that contained either the full-length CS gene or a repeatless gene consisting of CS flanking sequences. Both of these vaccines were able to induce CD8+ CTL directed against P. falciparum CS peptide 371-390, which is identical to the target of CTL induced by sporozoites and vaccinia CS recombinants. These results directly demonstrate the ability of an intracellular bacteria such as Salmonella to induce class I-restricted CD8+ CTL and illustrate the importance of CD8+ CTL in immunity to malaria.

Number of cells from Plasmodium falciparum-immune donors that produce gamma interferon in vitro in response to Pf155/RESA, a malaria vaccine candidate antigen

Secretion of gamma interferon (IFN-gamma) in response to stimulation of Plasmodium falciparum-primed T cells by specific antigens may be a useful indicator of cellular immunity to malaria. An enzyme-linked immunospot (ELISPOT) assay designed to detect IFN-gamma at the single-cell level was used to study IFN-gamma-producing cells from P. falciparum-primed donors from The Gambia after in vitro stimulation with various malarial antigens. IFN-gamma secreted into the culture supernatant was measured by conventional enzyme-linked immunosorbent assay (ELISA). There was a good correlation in individual donors between the level of IFN-gamma secreted into the culture supernatant and the number of IFN-gamma-secreting cells. However, the ELISPOT assay was apparently more sensitive in demonstrating low levels of IFN-gamma production than the ELISA was. Thus after stimulation with crude P. falciparum antigen from infected erythrocytes, 72% of the primed donors responded positively in the ELISPOT assay but only 55% responded positively in the ELISA. When stimulated with synthetic peptides representing immunodominant epitopes of the malarial antigen Pf155/RESA, a vaccine candidate, 31 to 55% responded in the ELISPOT assay and 21 to 36% responded in the ELISA. Unprimed Europeans did not respond positively to these antigens in either of the assays, and background in antigen-free controls was generally low. These results indicate that measurement of IFN-gamma by the ELISPOT assay or ELISA should have wide applications in large-scale epidemiological studies of malaria immunity. In addition, the ELISPOT assay makes it possible to analyze the T cells responding to malarial antigens in terms of both numbers and functional heterogeneity.

Survival and antigenic profile of irradiated malarial sporozoites in infected liver cells

Exoerythrocytic (EE) stages of Plasmodium berghei derived from irradiated sporozoites were cultured in vitro in HepG2 cells. They synthesized several antigens, predominantly but not exclusively those expressed by normal early erythrocytic schizonts. After invasion, over half the intracellular sporozoites, both normal and irradiated, appeared to die. After 24 h, in marked contrast to the normal parasites, EE parasites derived from irradiated sporozoites continued to break open, shedding their antigens into the cytoplasm of the infected host cells. Increasing radiation dosage, which has previously been shown to reduce the ability of irradiated sporozoites to protect animals, correlated with reduced de novo antigen synthesis by EE parasites derived from irradiated sporozoites.

An antigen specific to the liver stage of rodent malaria recognized by a monoclonal antibody

Vaccines currently being evaluated against malaria are based on proteins derived from the blood, sporozoite and sexual stages. Antigens from the liver stage, which is now recognized as the major target of protective sporozoite induced immunity, have received comparatively little attention. This paper describes the generation of a monoclonal antibody (MoAb), which recognizes an antigen specific to the liver stage of the rodent malaria Plasmodium berghei. The antigen is expressed throughout liver stage development and appears to be localized to the parasitophorous vacuole membrane. The MoAb did not affect the growth of liver stages cultured in vitro nor could protection be demonstrated in vivo following passive transfer of the antibody.

Recognition of dominant T cell-stimulating epitopes from the circumsporozoite protein of Plasmodium falciparum and relationship to malaria morbidity in Gambian children

Cellular immune responses to the Plasmodium falciparum circumsporozoite (CS) protein were measured by proliferation and interferon-gamma production in a cohort of children aged 3 to 8 years, living in The Gambia. Anti-CS antibody titres, malariometric indices and sickle cell status were also determined. Malaria morbidity in the ensuing malaria transmission season was monitored by weekly health questionnaire, axillary temperature measurements and examination of blood films. Exposure to malaria was inferred from entomological data collected during the transmission season. Immunological and parasitological measurements were repeated at the end of the rainy season. Immunological findings were compared between children who experienced clinical malaria or asymptomatic infection and children who had no evidence of infection. No association was found between cellular immune responses to the CS protein at the beginning of the transmission season and subsequent susceptibility to infection except among children with high titres of antibody to (NANP)40. Seropositive children who did not become infected had a higher mean proliferative response to the Th3R epitope than seropositive children who did become infected. High titres of anti-(NANP)40 antibodies alone were not protective. Responses to the Th2R epitope were significantly higher at the end of the rainy season than at the beginning in children who experienced an asymptomatic infection. Responses to variant sequences of the 2 epitopes were highly correlated at an individual level but there was no correlation between proliferative and interferon responses to a particular epitope.

Peptide analysis of the T cell response to the malaria circumsporozoite (CS) protein

The T cell response to the circumsporozoite (CS) protein is still not well understood. There is still not agreement on the degree of immunological non-responsiveness or even on the basic question of whether the response to native CS protein requires T cells at all. Recombinant proteins and synthetic peptides are tools that are helping us learn the basics of the immune response to this protein. Here, the human and murine responses to the protein are probed using these valuable research tools. Then, the possibility of using peptide vaccines is discussed.

Host-parasite interactions and immunity to irradiated sporozoites

We compare and contrast the results of immunizing mice with irradiated sporozoites of Plasmodium berghei and Plasmodium yoelii. Host genetic control of protective immunity is different in the two rodent malarias. Few mouse strains are strongly protected by P. yoelii sporozoites, while all are protected by P. berghei sporozoite immunization. The role of CD8+ T cells in the protective immune response to each of these malarias varies with the strain of mouse. Moreover, a single strain will use a CD8+ T cell-dependent mechanism against one malaria, and a CD8+ independent mechanism against the other. Thus, each host-parasite pairing in these rodent malarias engenders a unique set of immune responses. Such variety should be expected in the immune response to the human malarias, and may complicate the development of universally applicable vaccines.

Non-CS pre-erythrocytic protective antigens

Three novel non-CS antigens have been identified on P. falciparum and P. berghei sporozoites and exoerythrocytic parasites. CSP-2 is a sporozoite surface protein common to P. falciparum and P. berghei that elicits antibody-mediated protection, and is also found within P. berghei EE parasites. LSA is a P. falciparum EE-specific antigen localized within the parasitophorous vacuole. LSA-2 is a P. berghei EE-specific antigen, localized on the parasitophorous vacuole membrane, that protected mice to P. berghei sporozoite challenge, and elicited cytotoxic T cells that killed P. berghei EE parasites in vitro.

Hepatic phase of malaria is the target of cellular mechanisms induced by the previous and the subsequent stages. A crucial role for liver nonparenchymal cells

Both the sporozoites and the erythrocytic stages can modulate the hepatic phase by cytokines, notably IFN-gamma, TNF and IL-6, either directly or as a result of a cascade of events, and by MHC-restricted and antibody-dependent cell-mediated cytotoxicity. The role played by CD8+ T cells in inducing protective immunity against pre-erythrocytic stages is clearly established. The potential interest of triggering peptide-primed CD4+ T cells has to be considered regarding protection. Indeed, CD4+ T cells induced by the non-repetitive part of the CS protein of Plasmodium yoelii are protective, by eliminating malaria from hepatocytes. The crucial role of the liver NPC has to be emphasized, their participation in TNF schizonticidal effect and in ADCC mechanisms being strongly supported by our data.

Peptide-primed CD4+ cells and malaria sporozoites

We have mapped a T cell epitope in the circumsporozoite (CS) protein of the murine malaria parasite, Plasmodium yoelii. A 21-mer synthetic peptide corresponding to the amino acid positions 59-79 (referred to as Py1), induced specific proliferation in BALB/c and C57BL/6 mice, and provided help for the production of antibodies to peptides from the repetitive region, (QGPGAP)n, of the same CS protein, when mice were immunized with the Py1 peptide conjugated to the repetitive peptide. Long-term CD3+CD4+CD8-TCR alpha beta+ T cell lines and clones were derived from both strains of mice. These lines and clones, that proliferated in an MHC-restricted fashion, did not recognize peptides from the homologous region of another murine malaria parasite, P. berghei. About 50% of these clones produced detectable amounts of IFN-gamma and IL-2, whereas the remaining produced IL-4, IL-5, and IL-6. In preliminary experiments, some of these clones specifically inhibited P. yoelii sporozoite development in vitro and conferred protection in vivo in passive transfer experiments. These findings show that heterogenous T cell populations are activated in mice upon immunization with a short peptide from the P. yoelii CS protein and that some of these cells could be active in the effector arm of the immune response against malaria sporozoites.

Anti-parasite effects of cytokines in malaria

Cytokines induced during natural malaria infections, e.g., at crisis of a blood infection of Plasmodium cynomolgi, and during clinical paroxysms in human Plasmodium vivax infections, mediate killing of intra-erythrocytic blood stage malaria parasites. These cytokines, TNF and IFN-gamma, require additional, yet unidentified complementary factors that are present in "crisis" and "paroxysm" serum to kill intra-erythrocytic blood stage parasites. In contrast, cytokines, (mainly IFN-gamma) are able to effect killing of intra-hepatic stages of the parasite by themselves independent of serum complementary factors, suggesting that the mechanisms of killing may be different with respect to the two parasite stages. Cytokines also appear to be critical intermediates in mechanisms of clinical disease in malaria. Serum cytokine (TNF) levels and killing effects on blood stage malaria parasites were lower in patients who were exposed to endemic P. vivax malaria who had partial clinical immunity, than in non-immune patients. Evidence suggest that individuals acquire natural immunity to the disease by avoiding the induction of high levels of cytokines and complementary factors.

Isolation and characterization of protective cytolytic T cells in a rodent malaria model system

Protective immunity against malaria is induced by immunization with irradiation-attenuated sporozoites. Here we report the isolation of cytolytic T-cell (CTL) clones from BALB/c (H-2d) mice immunized with either Plasmodium berghei or Plasmodium yoelii sporozoites. The epitopes recognized by these CTL can be mimicked by synthetic peptides corresponding to a homologous region in the CS proteins of both rodent malaria species. Both peptides are recognized by the CTL in the context of the same MHC class I molecule, H-2 Kd. In vivo adoptive transfer of the CTL clones into non-immune syngeneic mice protected them from a lethal challenge of infectious sporozoites.

Measuring cellular immune responses to malaria antigens in endemic populations: epidemiological, parasitological and physiological factors which influence in vitro assays

Measurements of in vitro cellular immune responses to malaria antigens are influenced by a variety of external factors. The physiological status of the donor, which is affected by, for example, malaria infection, intercurrent illness and pregnancy, can influence the lymphoproliferative response to specific antigens. Prior exposure to malaria antigens, determined by malaria endemicity, seasonal variations in transmission and the degree of polymorphism of the particular antigen, will also affect the prevalence and intensity of responses. Malaria-related immunosuppression may be both generalised and antigen specific. Although in vitro responses to malaria antigens are profoundly suppressed in acutely infected individuals, there is evidence that lymphocyte activation does occur in vivo. We conclude that longitudinal studies, correlating specific immune responses with subsequent malaria morbidity are required, to identify potentially protective antigens and appropriate effector mechanisms.

Irradiated sporozoite vaccine induces cytotoxic T lymphocytes that recognize malaria antigens on the surface of infected hepatocytes

The observation that protective immunity induced by immunization with radiation attenuated Plasmodium berghei and Plasmodium yoelii sporozoites is dependent on CD8+ T lymphocytes in some strains of mice led us to speculate that immunization with sporozoites induces cytotoxic T lymphocytes (CTL) that recognize malaria antigens on the surface of malaria-infected hepatocytes. In this report we summarize a series of experiments that confirm this hypothesis. We first showed that when immune mice are challenged with live sporozoites they develop malaria-specific, CD8+ T cell-dependent infiltrates in their livers. Next we demonstrated that spleen cells from immune mice eliminate malaria infected hepatocytes from in vitro culture in an antigen specific and genetically restricted manner, indicating that these immune cells recognize malaria antigens on the surface of infected hepatocytes. Finally we defined a CTL epitope of the P. yoelii CS protein, and demonstrated that CTL against this 16-amino-acid peptide (PYCTL1) eliminate infected hepatocytes from culture in an antigenic specific, and MHC restricted manner, indicating that this 16-amino-acid peptide from the CS protein is present on the surface of the infected hepatocytes. We are currently working on constructing vaccines that induce protective CTL against PYCTL1, and identifying additional pre-erythrocytic stage targets of CTL mediated protective immunity.

Production by activated human T cells of interleukin 4 but not interferon-gamma is associated with elevated levels of serum antibodies to activating malaria antigens

T cells play a crucial role in antibody-mediated and antibody-independent immunity against Plasmodium falciparum malaria. Therefore, a vaccine immunogen should include parasite-derived B- and T-cell epitopes capable of giving rise to protective responses in both systems. The P. falciparum antigen Pf155/ring-infected erythrocyte surface antigen (RESA), a vaccine candidate, contains immunodominant T- and B-cell epitopes located in the central (5') and C-terminal (3') invariant repeat regions of the molecule. To relate Pf155/RESA-peptide-specific responses of T cells to function, T cells from P. falciparum immune donors were activated with peptides corresponding to these immunodominant regions. Activation was measured as induction of interferon-gamma secretion, T-cell proliferation (DNA synthesis), or transcription and translation of interleukin 4 (IL-4) mRNA. Peptides from both regions were shown to induce interferon-gamma, IL-4, proliferation, or any combination. In individual donors, there was no correlation between these different activities. Rather, they were negatively correlated, demonstrating the importance of examining multiple parameters of T-cell activation when estimating the proportion of individuals responding to a given epitope. However, IL-4 mRNA and intracellular IL-4 could be induced in T cells of donors who had elevated concentrations of serum antibodies to the same peptide that was used for T-cell activation. These results suggest that a causal relationship exists between the activation of IL-4-producing T-cell subsets and production of the anti-Pf155/RESA-specific antibodies in individuals in which immunity has been induced by natural infection. This finding has implications that should be considered for the selection of immunogens to be included in a future P. falciparum subunit vaccine and for vaccine development in general.

Human T cell proliferative responses to Plasmodium vivax antigens: evidence of immunosuppression following prolonged exposure to endemic malaria

Human T cell proliferative responses, of 33 adult Sri Lankans convalescing from Plasmodium vivax infections, to several P. vivax antigens (i.e. a soluble extract of asexual erythrocytic stage parasites and two cloned antigens that are potential vaccine candidates PV200 and GAM-1) were assessed. The peripheral blood mononuclear cell proliferative responses to the soluble extract of P. vivax, as assessed by studying both the proportion of responders and the degree of the response, were significantly lower in a group of individuals resident in a malaria endemic area in Sri Lanka than in another group that did not have a life-long exposure to malaria but had acquired the disease on a visit to an endemic region. Individuals of both groups responded equally well to mitogen. The responses to a non-malarial antigen such as purified protein derivative of tuberculin were only marginally lower in residents of the malaria-endemic region. These findings suggest that exposure to endemic P. vivax malaria leads to a specific immunosuppression to P. vivax antigens. Immunosuppression of a much lower degree was evident to a non-malarial antigen.

Immune responses to defined epitopes of the circumsporozoite protein of the murine malaria parasite, Plasmodium yoelii

We have investigated the immunogenicity of defined sequences of the circumsporozoite (CS) protein of the murine malaria parasite, Plasmodium yoelii. A 21-ner synthetic peptide from the nonrepetitive region of the CS protein (position 59-79, referred to as Py1) induced T cell proliferative responses in H-2d and, to a lesser extent, in H-2b mice. Conversely, a synthetic peptide (referred to as Py4) consisting of four (QGPGAP) repeats of the P. yoelii CS protein, induced an antibody response only in H-2b mice. No antibody response was observed when the Py3 peptide, consisting of three (QGPGAP) repeats, was used as an immunogen. When cross-linked to the Py4 repetitive peptide, the Py1 sequence behaved as a T helper epitope allowing the production of anti-Py4 antibodies in H-2d mice. Several long-term T cell lines and clones specific for the nonrepetitive Py1 peptide were originated in vitro from both H-2d and H-2b mice. These lines and clones were CD4+ and proliferated in a major histocompatibility complex-restricted fashion. Furthermore, Py1-specific T cell lines and clones did not proliferate in the presence of synthetic peptides from an analogous region of another rodent malaria parasite, P. berghei, despite the high degree of homology existing in this sequence of the two CS proteins. Finally, supernatants from 7 out of 13 clones (from BALB/c mice) produced detectable amounts of interleukin 2 and interferon-gamma; whereas supernatants from the 4 clones from C57BL/6 and 2 from BALB/c mice contained detectable amounts of interleukin 5. These results show that functionally heterogenous CD4+ T cell populations, belonging to either TH1 or TH2 subset, are activated upon immunization of mice with the P. yoelii Py1 synthetic peptide. It is not yet known what differential role these CD4+ subsets play during the malaria infection or after immunization with different malaria T cell epitopes. This knowledge may have a particular impact in the design of effective subunit vaccines against malaria.

Presence of gamma interferon in human acute and congenital toxoplasmosis

The production of gamma interferon in acute acquired and congenital toxoplasmosis was studied. Gamma interferon was produced at significant titers (P less than 0.001) in the course of both congenital toxoplasmosis and acquired toxoplasmosis at an early stage of infection, when Toxoplasma gondii was multiplying. Its presence in fetal blood was correlated with the positive inoculation of fetal blood or amniotic fluid into mice (95%). The data suggest that the fetus is able to synthesize gamma interferon as early as week 21 of pregnancy. This test, easily and rapidly performed, could be included among those useful for diagnosing fetal toxoplasmic disease.

Involvement of L3T4+, LYT2.2+ T cell subsets and non-T cells in the resistance of mice against Trypanosoma cruzi infection

Cellular populations involved in resistance against T. cruzi infection were characterized from mice chronically infected with the parasite. Mice transfused with spleen cells (SC), nylon-wool-non-adherent spleen cells (NWNA) or sera from mice chronically infected with T. cruzi, showed an enhanced resistance against challenge with the parasite. The protective activity of NWNA but not of SC was completely abrogated by treatment with anti-Thy1.2 monoclonal antibodies (mAb) and complement (C). Pretreatment of NWNA cells from chronically infected mice with either anti-L3T4 or anti-Lyt 2.2 mAb partially reduced the transfer of resistance. When both L3T4+ and Lyt2.2+ cells were depleted from NWNA populations, transfer of resistance was abolished. These results appear to indicate that L3T4+, Lyt2.2+ T cell subsets and non-T cells are involved in the immunity to T. cruzi.

Use of a tuberculin purified protein derivative--Asn-Ala-Asn-Pro conjugate in bacillus Calmette-Guérin primed mice overcomes H-2 restriction of the antibody response and avoids the need for adjuvants

Because of its immunodominancy, and because it is conserved in different geographical isolates of Plasmodium falciparum, the repetitive sequence of the circumsporozoite protein, (Asn-Ala-Asn-Pro)n [(NANP)n], has been envisaged for the development of an anti-falciparum malaria subunit vaccine. However, the murine immune response to (NANP)n peptides, either carrier-free or coupled to carrier proteins, was shown to be inducible only by using strong (e.g., Freund's) adjuvants. Furthermore, response to the carrier-free peptide, administered in adjuvant, is genetically restricted to I-Ab mice. In the present paper, we report that high titers of antibodies against the NANP repetitive epitope were obtained in responder C57BL/6 (H-2b) mice when they were primed with live BCG (bacillus Calmette-Guérin Mycobacterium tuberculosis var. bovis) and immunized once with the synthetic peptide (NANP)40 coupled to tuberculin purified protein derivative (PPD) without the use of any adjuvant. This approach also led to the production of high titers of anti-NANP antibodies in ASW (H-2s), B10.RIII (H-2r), BALB/c (H-2d), C3H/He (H-2k), and DBA/1 (H-2q) nonresponder mice after two injections of the conjugate. In both cases, BCG priming was obligatory for the induction of antibodies reacting with the synthetic peptide. The levels of anti-NANP antibodies in nonresponder BALB/c mice were demonstrated to be comparable to the levels induced after PPD-(NANP)40 immunization in Freund's complete or incomplete adjuvant. The antibodies induced were also capable of recognizing P. falciparum sporozoites in immunofluorescence assays and, furthermore, these antibodies inhibited the penetration of live sporozoites into human hepatocytes in vitro. This system functioned independently of the subjects' resistance or susceptibility to BCG infection. Given the widespread natural exposure to mycobacterial antigens and the extensive use of BCG and PPD in the human population, this approach might be envisaged for vaccination with malaria peptides.

Cytotoxic T cells recognize a peptide from the circumsporozoite protein on malaria-infected hepatocytes

Irradiated malaria sporozoites can induce CD8+ T cells that are required for protection against infection. However, the parasite antigens targeted by this immune response are unknown. We have discovered a 16-amino acid epitope from the Plasmodium yoelii circumsporozoite (CS) protein that is recognized by cytotoxic T cells from immune mice. Lymphocytes stimulated with this peptide can kill P. yoelii liver stage parasites in vitro in an MHC-restricted, antigen-specific manner. Thus, epitopes from the CS protein are presented on the surface of infected hepatocytes and can be targets for T cells, even though intact CS protein has not been detected on the surface of the infected hepatocyte. A vaccine that induced CTL to parasite antigens might protect humans against malaria by eliminating liver stage parasites.

Transforming growth factor beta decreases the immunogenicity of rat islet xenografts (rat to mouse) and prevents rejection in association with treatment of the recipient with a monoclonal antibody to interferon gamma

Culture of rat islets of Langerhans for 1 week at 37 degrees C with recombinant transforming growth factor beta prolonged the survival of islet xenografts transplanted into diabetic mouse recipients. Treatment of diabetic recipients with a neutralizing monoclonal antibody to murine interferon gamma did not affect the survival of islet xenografts cultured 7 days in control medium. However, treatment of donor islets with transforming growth factor beta in combination with treatment of diabetic recipients with interferon gamma antibody produced a 75% survival of the islet xenografts at 100 days. Fifty percent of the recipients who had accepted their graft for more than 100 days were immune unresponsive to a transplant of freshly isolated islets from the same donor strain.

Plasmodium berghei subunit vaccine: repeat synthetic peptide of circumsporozoite protein comprising T- and B-cell epitopes fails to confer immunity

In the murine malaria model induced by Plasmodium berghei, we studied the immunogenicity of the repeat region of the circumsporozoite (CS) protein, which is the main target of the antibody response in infected animals. We immunized several strains with a synthetic peptide--Y(DPPPPNPN)3--corresponding to one of the two P. berghei repeat sequences in complete Freund's adjuvant. Only C57BL/6 immune sera reacted with the synthetic peptide in ELISA and with the native CS protein on P. berghei sporozoites, as detected by immunofluorescence. From lymph node cells of immunized C57BL/6 we isolated two repeat-specific T-cell lines which proliferated in the presence of the synthetic peptide or the recombinant CS protein. We analysed the protective role of this repeat-specific response by injecting infectious sporozoites into mice immunized with irradiated sporozoites or with the repeat peptide. The percentage of mice developing parasitaemia was 80-90% in the peptide-immunized group and only 10-20% in the group immunized with irradiated sporozoites. Anti-repeat antibody titres were comparable in the two groups. On the basis of these results, we can conclude that the T- and B-cell response to the CS repeat obtained with this synthetic peptide immunization is not sufficient for a protective immunity.

The relationship between splenomegaly and antibody to the circumsporozoite protein of Plasmodium falciparum in two groups of women with high and low enlarged spleen rates in Madang, Papua New Guinea

The objective of this study was to determine the prevalence of antibodies recognizing the circumsporozoite (CS) protein of Plasmodium by an enzyme-linked immunosorbent assay, in 2 subpopulations of women with significantly different enlarged spleen rates but similar exposure to malaria, on the north coast of Papua New Guinea. Antibody levels of immunoglobulin G (IgG) antibody to CS protein in the high and low spleen rate groups were similar (56.2% and 55.1%) but there was a significant difference in IgM (29.6% and 16.7%). In neither group did antibodies increase with parity (age). In both groups a high level of either IgG or IgM antibody to CS protein was associated with a high spleen rate and women with hyper-reactive malarious splenomegaly were more likely to be positive for both. Lower parasite rates were associated only with increased IgM antibody titres. High levels of antibody to blood-stage parasites were also present in the high spleen rate group, suggesting that antibodies to the CS protein were not protective. It is considered that cell-mediated immunity may be deficient in women with persistent splenomegaly.

Incorporation of T and B epitopes of the circumsporozoite protein in a chemically defined synthetic vaccine against malaria

We show here an effective and novel approach to engineer peptide-based vaccines using a chemically defined system, known as multiple peptide antigen systems (MAPs), to protect an inbred mouse strain from infection against rodent malaria. 10 mono- and di-epitope MAP models containing different arrangements and stoichiometry of functional B and/or T helper cell epitopes from the circumsporozoite protein of Plasmodium berghei were used to immunize A/J mice. While these mice did not respond to the mono-epitope MAP bearing only the B or T epitope, very high titers of antibody and protective immunity against sporozoite challenge were elicited by di-epitope MAPs, particularly those with the B and T epitopes in tandem and present in equimolar amounts. These results, obtained in a well-defined rodent malaria model, indicate that MAPs may overcome some of the difficulties in the development of synthetic vaccines, not only for malaria but also for other infectious diseases.

T lymphocyte interferon-gamma production induced by Plasmodium falciparum antigen is high in recently infected non-immune and low in immune subjects

Interferon (IFN) alpha and gamma were measured by radio-immunoassays in supernatants from cultures of peripheral blood mononuclear cells (PBMC) or purified T cell subsets incubated with either Plasmodium falciparum schizont-enriched malaria antigen (mAg), uninfected red blood cells (RBC) or pokeweed mitogen (PWM). Cell donors were 24 clinically immune, healthy African adult native residents of a P. falciparum-endemic region, Haut-Ogooué, Gabon, and seven non-immune, European temporary residents with a history of a single to a few malaria infections during the previous 1 to 9 months. When PBMC were cultured in medium alone or with RBC antigen no or low titres of IFN-gamma were detected. PBMC proliferation and IFN-gamma production observed in the presence of mAg were dose dependent and significantly correlated. When cultured with mAg, PBMC from non-immune Europeans produced significantly higher levels of IFN-gamma than did PBMC from clinically immune Africans. No such difference was found when PBMC were cultured with PWM. The mAg-induced IFN-gamma production was due mainly to CD4+ T cells and was not enhanced by CD8+ T cell depletion. No IFN-alpha was detected in culture supernatants. Thus, P. falciparum antigens are able to induce in vitro production of IFN-gamma by CD4+ T cells; however, in this sample, individuals considered to be clinically resistant to malaria were low producers of IFN-gamma.

Conserved repetitive epitope recognized by CD4+ clones from a malaria-immunized volunteer

T cell clones obtained from a human volunteer immunized with Plasmodium falciparum sporozoites specifically recognized the native circumsporozoite (CS) antigen expressed on P. falciparum sporozoites, as well as bacteria- and yeast-derived recombinant falciparum CS proteins. The response of these CD4+ CD8- cells was species-specific, since the clones did not proliferate or secrete gamma interferon when challenged with sporozoites or recombinant CS proteins of other human, simian, or rodent malarias. The epitope recognized by the sporozoite-specific human T cell clones mapped to the 5' repeat region of the CS protein and was contained in the NANPNVDPNANP sequence.

Molecular basis of host-parasite relationship: towards the definition of protective antigens

In spite of some remarkable progress in our understanding of the immune response to parasites and in the molecular cloning of dozens of genes encoding for potentially protective proteins, no definitive step has yet been made towards operational vaccines against major human parasitic diseases. The reasons for our present failures are no longer attributable to the lack of appropriate tools but rather to our rather primitive knowledge of the basic mechanisms governing host-parasite relationship. Mainly on the basics of our personal observations, we have attempted to review and discuss some of the prominent factors in host-parasite interactions, such as molecular mimicry, phyletic convergence, molecular mechanisms of infectivity and lures of cell communication. In many respects, the development of efficient vaccines applicable to humans appears closely dependent on a better understanding of the basic biological mechanisms underlying the natural history of parasitic diseases. In this context, the development of new concepts regarding the definition of potentially protective antigens based on the study of non-surface molecules, cross-reactive stage antigens and antibody isotype selection might represent promising alternatives.

The response of CD4+ T cells to Plasmodium chabaudi chabaudi

We have studied the role of CD4+ T cells in the immune response to Plasmodium chabaudi chabaudi. From in vivo experiments in which the different subsets of T cells were depleted, it is clear that CD4+ T cells are essential for the generation of protective immunity. Our limiting dilution analysis show that the CD4 T-cell response to P. chabaudi antigens is heterogeneous, in that distinct functions can be performed by different responding T cells, and these responses change during infection. During the first phase of the infection the predominant response is that of a TH1-type cell, producing IL-2 and IFN-gamma. This correlates with the appearance of IFN-gamma in the serum of infected animals. After the clearance of the acute parasitemia, i.e. in the second phase of the infection, the specific response is characterised by TH2 cells, which are effective helper cells for antibody production and presumably are necessary for the switch of IgM to IgG. CD4+ T cells are effector cells are not necessary in the second phase of the infection; mice which have been depleted of CD4+ T cells at this time are able to control their infection in a manner similar to untreated mice. This ability to control parasitemia coincides with the production of specific IgG but not IgM antibodies and the predominance of TH2 type helper cells. Therefore, our data suggest that malaria-specific IgG antibodies are important effectors in the second phase of an infection with P. chabaudi chabaudi.

Wild isolates of Plasmodium falciparum show extensive polymorphism in T cell epitopes of the circumsporozoite protein

Variation in the immunodominant T cell epitopes Th2R and Th3R of the Plasmodium falciparum circumsporozoite protein has been analysed from Gambian clinical isolates using the polymerase chain reaction. The degree of polymorphism in these epitopes is more extensive than that found in several geographically diverse laboratory isolates. These findings strongly suggest that it will not be feasible to include all variants in a polyvalent subunit sporozoite vaccine.

Interactions of CD4+ and CD8+ human T lymphocytes from malaria-unprimed donors with Plasmodium falciparum schizont stage

During Plasmodium falciparum malaria, a wide spectrum of parasite-encoded blood-stage proteins is presented to the immune system of the host. To explore their multiple interactions with T cells from donors who have had no previous exposure to the parasite, whole schizont extract was used in vitro. Both CD4+ and CD8+ lymphocytes from all individuals tested were stimulated to proliferate. The responses were dependent on the presence of accessory cells and were only partially replaced by recombinant interleukin-1. Responses were inhibited by monoclonal antibodies to CD3, the alpha beta-chain T-cell receptor, or CD4 molecules but not to CD2. P. falciparum schizont extract-specific T-cell clones were generated and maintained by using sole stimulation by P. falciparum extract with autologous accessory cells or recombinant interleukin-2. Monoclonal antibodies to CD3 (or the alpha beta-chain T-cell receptor) blocked cloned T-cell responses to the schizont extract, and although the responses of the majority of the CD4+ or CD8+ T-cell clones were restricted by autologous accessory cells and inhibited by monoclonal antibodies to either CD4 or CD8, other clones responded to P. falciparum in the absence of accessory cells and were not regulated by the same monoclonal antibodies. The last category of clones consisted of autoreactive T cells. These data suggest that at the first contact with P. falciparum, requirements are met for significant T-cell stimulation.

Immunological alterations in uncomplicated Plasmodium falciparum malaria. Relationship between parasitaemia and indicators of macrophage activation

Numerical alterations of circulating lymphocytes were investigated in 37 Brazilian patients with uncomplicated Plasmodium falciparum malaria and in a group of 15 healthy controls. The number of CD4+ T helper/inducer cells was significantly lower in patients than controls, whereas absolute numbers of CD8+ suppressor/cytotoxic T cells did not differ between the groups. TNF and neopterin levels were markedly increased in the plasma of patients and remained slightly elevated after chemotherapy with clindamycin. Neopterin, but not TNF levels, were significantly correlated with parasitaemia. TNF was inversely related to monocyte counts. Interferon gamma could not be detected in the plasma of control subjects and was observed in only one patient. We conclude that in uncomplicated falciparum malaria the distribution of phenotypes of circulating lymphocytes are altered slightly and that the high plasma levels of TNF and neopterin indicate excessive release of these molecules by activated macrophages and the activation of cellular immune mechanisms during the infection.

Contrasts in antigen expression in the erythrocytic and exoerythrocytic stages of rodent malaria

The time and site of expression of five antigens, recognized by monoclonal antibodies raised against blood-stage parasites, were studied in the exoerythrocytic stage of Plasmodium berghei using indirect immunofluorescent antibody staining. Two monoclonal antibodies (W 3.5, I 2.6), which stain the cytoplasm of infected erythrocytes, did not stain the cytoplasm of the infected liver cell but stained the parasite itself suggesting a difference in the antigenic architecture of the erythrocytic and exoerythrocytic parasites. Another antibody (17.6.1) revealed a further difference in the antigenic composition of the blood and liver-stage parasites being expressed almost exclusively in the former. Two others (C139 and 17.3.9) showed broadly similar patterns of expression in these two stages of the malarial life-cycle.