James M. Burns

Host-parasite Interactions Revealed by Plasmodium falciparum Metabolomics

Intracellular pathogens have devised mechanisms to exploit their host cells to ensure their survival and replication. The malaria parasite Plasmodium falciparum relies on an exchange of metabolites with the host for proliferation. Here we describe a mass spectrometry-based metabolomic analysis of the parasite throughout its 48 hr intraerythrocytic developmental cycle. Our results reveal a general modulation of metabolite levels by the parasite, with numerous metabolites varying in phase with the developmental cycle. Others differed from uninfected cells irrespective of the developmental stage. Among these was extracellular arginine, which was specifically converted to ornithine by the parasite. To identify the biochemical basis for this effect, we disrupted the plasmodium arginase gene in the rodent malaria model P. berghei. These parasites were viable but did not convert arginine to ornithine. Our results suggest that systemic arginine depletion by the parasite may be a factor in human malarial hypoargininemia associated with cerebral malaria pathogenesis.

Protection against Plasmodium chabaudi Malaria Induced by Immunization with Apical Membrane Antigen 1 and Merozoite Surface Protein 1 in the Absence of Gamma Interferon or Interleukin-4

ABSTRACT Strategies to optimize formulations of multisubunit malaria vaccines require a basic knowledge of underlying protective immune mechanisms induced by each vaccine component. In the present study, we evaluated the contribution of antibody-mediated and cell-mediated immune mechanisms to the protection induced by immunization with two blood-stage malaria vaccine candidate antigens, apical membrane antigen 1 (AMA-1) and merozoite surface protein 1 (MSP-1). Immunologically intact or selected immunologic knockout mice were immunized with purified recombinant Plasmodium chabaudi AMA-1 (PcAMA-1) and/or the 42-kDa C-terminal processing fragment of P. chabaudi MSP-1 (MSP-142). The efficacy of immunization in each animal model was measured as protection against blood-stage P. chabaudi malaria. Immunization of B-cell-deficient JH−/− mice indicated that PcAMA-1 vaccine-induced immunity is largely antibody dependent. In contrast, JH−/− mice immunized with PcMSP-142 were partially protected against P. chabaudi malaria, indicating a role for protective antibody-dependent and antibody-independent mechanisms of immunity. The involvement of γδ T cells in vaccine-induced PcAMA-1 and/or PcMSP-142 protection was minor. Analysis of the isotypic profile of antigen-specific antibodies induced by immunization of immunologically intact mice revealed a dominant IgG1 response. However, neither interleukin-4 and the production of IgG1 antibodies nor gamma interferon and the production of IgG2a/c antibodies were essential for PcAMA-1 and/or PcMSP-142 vaccine-induced protection. Therefore, for protective antibody-mediated immunity, vaccine adjuvants and delivery systems for AMA-1- and MSP-1-based vaccines can be selected for their ability to maximize responses irrespective of IgG isotype or any Th1 versus Th2 bias in the CD4+-T-cell response.

Enhanced Protection against Malaria by a Chimeric Merozoite Surface Protein Vaccine

ABSTRACT The 42-kDa processed fragment of Plasmodium falciparum merozoite surface protein 1 (MSP-142) is a prime candidate for a blood-stage malaria vaccine. Merozoite surface protein 8 contains two C-terminal epidermal growth factor (EGF)-like domains that may function similarly to those of MSP-142. Immunization with either MSP-1 or MSP-8 induces protection that is mediated primarily by antibodies against conformation-dependent epitopes. In a series of comparative immunogenicity and efficacy studies using the Plasmodium yoelii rodent model, we tested the ability of recombinant P. yoelii MSP-8 (rPyMSP-8) to complement rPyMSP-1-based vaccines. Unlike MSP-1, PyMSP-8-dependent protection required immunization with the full-length protein and was not induced with recombinant antigens that contained only the C-terminal EGF-like domains. Unlike PyMSP-8, the immunogenicity of the PyMSP-1 EGF-like domains was low when present as part of the rPyMSP-142 antigen. Immunization with a mixture of rPyMSP-142 and rPyMSP-8 further inhibited the antibody response to protective epitopes of rPyMSP-142 and did not improve vaccine efficacy. To improve PyMSP-1 immunogenicity, we produced a chimeric antigen containing the EGF-like domains of PyMSP-1 fused to the N terminus of PyMSP-8. Immunization with the chimeric rPyMSP-1/8 antigen induced high and comparable antibody responses against the EGF-like domains of both PyMSP-1 and PyMSP-8. This enhanced MSP-1-specific antibody response and the concurrent targeting of MSP-1 and MSP-8 resulted in improved, nearly complete protection against lethal P. yoelii 17XL malaria. Unexpectedly, immunization with rPyMSP-1/8 failed to protect against challenge infection with reticulocyte-restricted P. yoelii 17X parasites. Overall, these data establish an effective strategy to improve the efficacy of P. falciparum MSP-based vaccines.

A Protective Glycosylphosphatidylinositol-Anchored Membrane Protein of Plasmodium yoelii Trophozoites and Merozoites Contains Two Epidermal Growth Factor-Like Domains

ABSTRACT Using sera from mice immunized and protected againstPlasmodium yoelii malaria, we identified a novel blood-stage antigen gene, pypag-2. The 2.1-kbpypag-2 cDNA contains a single open reading frame that encodes a 409-amino-acid protein with a predicted molecular mass of 46.8 kDa. Unlike many characterized plasmodial antigens, blocks of tandemly repeated amino acids are lacking in the pypAg-2 protein sequence. Recombinant pypAg-2, comprising the full-length protein minus the predicted N-terminal signal and C-terminal anchor sequences, was produced and used to raise a high-titer polyclonal rabbit antiserum. This antiserum was used to identify and characterize the native protein through immunoblotting, immunoprecipitation and immunofluorescence assays. Consistent with the presence of a glycosylphosphatidylinositol anchor, pypAg-2 fractionated with the detergent phase of Triton X-114-solubilized proteins and could be metabolically labeled with [3H]palmitic acid. By immunofluorescence, pypAg-2 expression was localized to both the trophozoite and merozoite membranes. Similar to Plasmodium falciparum merozoite surface protein 1, pypAg-2 contains two C-terminal epidermal growth factor (EGF)-like domains. Most importantly, immunization with recombinant pypAg-2 protected mice against lethal P. yoeliimalaria. Thus, pypAg-2 is a target of protective immune responses and represents a novel addition to the family of merozoite surface proteins that contain one or more EGF-like domains.

Immunization against Plasmodium chabaudi malaria using combined formulations of apical membrane antigen-1 and merozoite surface protein-1

The control of Plasmodium falciparum malaria by vaccination will require immunization with multiple parasite antigens effectively formulated in combination. In this regard, proteins expressed on the surface of blood-stage merozoites are attractive as vaccine targets given their functional importance in the invasion of erythrocytes and accessibility to serum antibodies. We have utilized a Plasmodium chabaudi vaccine model to begin to evaluate the efficacy of immunization with combined formulations of apical membrane antigen-1 (AMA-1) and merozoite surface protein-1 (MSP-1). Using a pET/T7 RNA polymerase bacterial expression system, we have expressed, purified and refolded recombinant antigens representing the 54 kDa ectodomain of Pc AMA-1 and the 42 kDa C-terminus of Pc MSP-1. Immunization with recombinant Pc AMA-1+Pc MSP-1(42) induced a high level of protection against P. chabaudi malaria with protective efficacy varying with antigen dose, choice of adjuvant, and immunization protocol. Based on the reduction of P. chabaudi parasitemia, Alum proved effective for use with the combination of Pc AMA-1 and Pc MSP-1(42). The use of Quil A was similarly effective with single or combined antigen immunizations, particularly with low antigen dose. In general, serological analysis of prechallenge sera indicated a dominant IgG1 response. For a given formulation, immunization with the combination of Pc AMA-1 and Pc MSP-1(42) elicited IgG responses comparable to those observed following immunization with each antigen alone. However, prechallenge antibody titers alone were not predictive of protective efficacy. While Pc AMA-1 and Pc MSP-1(42) can be effectively formulated in combination, further study is needed to define measurable parameters of protective T cell and B cell responses induced by Pc AMA-1+Pc MSP-1(42) that are predictive of vaccine efficacy.

γδ T Cells but Not NK Cells Are Essential for Cell-Mediated Immunity against Plasmodium chabaudi Malaria

ABSTRACT Blood-stage Plasmodium chabaudi infections are suppressed by antibody-mediated immunity and/or cell-mediated immunity (CMI). To determine the contributions of NK cells and γδ T cells to protective immunity, C57BL/6 (wild-type [WT]) mice and B-cell-deficient (JH−/−) mice were infected with P. chabaudi and depleted of NK cells or γδ T cells with monoclonal antibody. The time courses of parasitemia in NK-cell-depleted WT mice and JH−/− mice were similar to those of control mice, indicating that deficiencies in NK cells, NKT cells, or CD8+ T cells had little effect on parasitemia. In contrast, high levels of noncuring parasitemia occurred in JH−/− mice depleted of γδ T cells. Depletion of γδ T cells during chronic parasitemia in B-cell-deficient JH−/− mice resulted in an immediate and marked exacerbation of parasitemia, suggesting that γδ T cells have a direct killing effect in vivo on blood-stage parasites. Cytokine analyses revealed that levels of interleukin-10, gamma interferon (IFN-γ), and macrophage chemoattractant protein 1 (MCP-1) in the sera of γδ T-cell-depleted mice were significantly (P < 0.05) decreased compared to hamster immunoglobulin-injected controls, but these cytokine levels were similar in NK-cell-depleted mice and their controls. The time courses of parasitemia in CCR2−/− and JH−/− × CCR2−/− mice and in their controls were nearly identical, indicating that MCP-1 is not required for the control of parasitemia. Collectively, these data indicate that the suppression of acute P. chabaudi infection by CMI is γδ T cell dependent, is independent of NK cells, and may be attributed to the deficient IFN-γ response seen early in γδ T-cell-depleted mice.

Members of the Merozoite Surface Protein 7 Family with Similar Expression Patterns Differ in Ability To Protect against Plasmodium yoelii Malaria

ABSTRACT Previously, we described the isolation of the Plasmodium yoelii sequence-related molecules P. yoelii MSP-7 (merozoite surface protein 7) and P. yoelii MSRP-2 (MSP-7-related protein 2) by their ability to interact with the amino-terminal end of P. yoelii MSP-1 in a yeast two-hybrid system. One of these molecules was the homologue of Plasmodium falciparum MSP-7, which was biochemically isolated as part of the shed MSP-1 complex. In the present study, with antibodies directed against recombinant proteins, immunoprecipitation analyses of the rodent system demonstrated that both P. yoelii MSP-7 and P. yoelii MSRP-2 could be isolated from parasite lysates and from parasite culture supernatants. Immunofluorescence studies colocalized P. yoelii MSP-7 and P. yoelii MSRP-2 with the amino-terminal portion of MSP-1 and with each other on the surface of schizonts. Immunization with P. yoelii MSRP-2 but not P. yoelii MSP-7 protected mice against a lethal infection with P. yoelii strain 17XL. These results establish that both P. yoelii MSP-7 and P. yoelii MSRP-2 are expressed on the surface of merozoites and released from the parasite and that P. yoelii MSRP-2 may be the target of a protective immune response.

Alteration in Host Cell Tropism Limits the Efficacy of Immunization with a Surface Protein of Malaria Merozoites

ABSTRACT Immunization with Plasmodium yoelii merozoite surface protein-8 (PyMSP-8) has been shown to protect mice against lethal P. yoelii 17XL malaria. Here we demonstrate that PyMSP-8-specific antibodies preferentially suppress P. yoelii 17XL growth in mature erythrocytes compared to growth in reticulocytes and do not suppress the growth of nonlethal P. yoelii 17X, a parasite that primarily replicates in reticulocytes. The protection against normocyte-associated P. yoelii malaria parasites is mediated by antibodies that recognize conformational epitopes of PyMSP-8 that are nonpolymorphic. We examined changes in gene expression in reticulocyte-restricted P. yoelii 17XL parasites that escaped neutralization by PyMSP-8-specific antibodies using P. yoelii DNA microarrays. Of interest, Pymsp-8 gene expression decreased, while the expression of msp-1, msp-7, and several rhoptry protein genes increased. Breakthrough parasites also exhibited increases in the expression of a subset of yir and Pyst-a genes that are predicted to encode polymorphic antigens expressed on the surface of infected erythrocytes. These data suggest that changes in the expression of parasite proteins expressed on the merozoite surface, as well as the surface of infected erythrocytes, may alter host cell tropism and contribute to the ability of malaria parasites to evade merozoite-specific, neutralizing antibodies.

Suppression of Lethal Plasmodium yoelii Malaria following Protective Immunization Requires Antibody-, IL-4-, and IFN-γ-Dependent Responses Induced by Vaccination and/or Challenge Infection

Immunization with Plasmodium yoelii merozoite surface protein (PyMSP)-8 protects mice from lethal malaria but does not prevent infection. Using this merozoite surface protein-based vaccine model, we investigated vaccine- and infection-induced immune responses that contribute to protection. Analysis of prechallenge sera from rPyMSP-8-immunized C57BL/6 and BALB/c mice revealed high and comparable levels of Ag-specific IgG, but differences in isotype profile and specificity for conformational epitopes were noted. As both strains of mice were similarly protected against P. yoelii, we could not correlate vaccine-induced responses with protection. However, passive immunization studies suggested that protection resulted from differing immune responses. Studies with cytokine-deficient mice showed that protection was induced by immunization of C57BL/6 mice only when IL-4 and IFN-γ were both present. In BALB/c mice, the absence of either IL-4 or IFN-γ led to predictable shifts in the IgG isotype profile but did not reduce the magnitude of the Ab response induced by rPyMSP-8 immunization. Immunized IL-4−/− BALB/c mice were solidly protected against P. yoelii. To our surprise, immunized IFN-γ−/− BALB/c mice initially controlled parasite growth but eventually succumbed to infection. Analysis of cytokine production revealed that P. yoelii infection induced two distinct peaks of IFN-γ that correlated with periods of controlled parasite growth in intact, rPyMSP-8-immunized BALB/c mice. Maximal parasite growth occurred during a period of sustained TGF-β production. Combined, the data indicate that induction of protective responses by merozoite surface protein-based vaccines depends on IL-4 and IFN-γ-dependent pathways and that vaccine efficacy is significantly influenced by host responses elicited upon infection.

Analysis of antigen‐specific antibodies and their isotypes in experimental malaria

Measuring antibody production in response to antigen exposure or vaccination is key to disease prevention and treatment. Our understanding of the mechanisms involved in the antibody response is limited by a lack of sensitive analysis methods. We address this limitation using multiplexed microsphere arrays for the semi ‐quantitative analysis of antibody production in response to malaria infection.