Plasmodium Berghei Circumsporozoite Research Articles

Malaria vaccine: Immunization of mice with a synthetic T cell helper epitope alone leads to protective immunity

The immunogenicity of the non-repetitive sequences of the Plasmodium berghei circumsporozoite (CS) protein was studied using synthetic peptides. Two CS sequences (residues 20-39 and 57-70) exhibiting T cell helper activity were identified. Immunization of BALB/c mice with a branched peptide containing either the 20-39 or the 57-70 sequence and two repeats (B epitope) in a linear sequence induced high titers of anti-repeat and anti-sporozoite antibodies. Mice immunized with the T-B construct (high antibody titers) or with the 57-70 epitope alone (no serum anti-repeat or anti-peptide antibodies) were protected to a similar degree after challenge with infective sporozoites. No protection was obtained in mice immunized with the 20-39 epitope. These results indicate that BALB/c mice can be protected either by effector T cells or by high levels of anti-repeat antibodies. Thus, in the same strain, a double mechanism of protection can be obtained by a synthetic peptide vaccine.

MHC class I H-2Kd-restricted antigenic peptides: additional constraints for the binding motif.

The previously defined binding motif of MHC class I H-2Kd-restricted antigenic peptides consists of a Y residue in position P2 and a hydrophobic residue with a large aliphatic side chain (L, I, or V) in position P9/P10 of optimal 9- or 10-mer peptides. We show now that the presence of a charged or a F residue in position P5 reduces the Kd-restricted competitor activity of several cytotoxic T lymphocyte (CTL) epitopes and model peptides, at a degree comparable to A substitutions for the P2 or the P9/P10 anchor residues. Various charged, polar, aromatic, and aliphatic amino acids were substituted for S256 in the CTL epitope Plasmodium berghei circumsporozoite (CS) 253-260 8-mer and in its CS 252-260 9-mer form, whereas a more restricted panel of substitutions was tested in the CTL epitopes influenza nucleoprotein 147-155 9-mer and HLA-CW3 170-179 10-mer. Analysis of all the Kd-restricted epitopes so far defined also revealed an uncharged residue at this position. These additional structural constraints present in the Kd binding motif may thus improve the prediction of new epitopes recognized by T cells in the context of this MHC molecule.

Highly diverse T cell recognition of a single Plasmodium berghei peptide presented by a series of mutant H-2Kd molecules.

We have tested 21 independent CTL clones for recognition of a single peptide derived from the Plasmodium berghei circumsporozoite protein in the context of 13 mutants of the murine MHC class I molecule H-2Kd. In this series of Kd mutants, amino acid residues located on the upper surface of the alpha-helices were individually substituted by alanine. Remarkably, most clones displayed individual recognition patterns on the Kd mutants. We had previously found that this series of CTL clones was likewise highly diverse in terms of both TCR primary structure and peptide fine specificity. Our data thus reinforce the concept that multiple T cell epitopes are available on the surface of a single peptide-MHC class I complex for recognition by specific TCR.

Purification and ligand binding of a soluble class I major histocompatibility complex molecule consisting of the first three domains of H-2Kd fused to beta 2-microglobulin expressed in the baculovirus-insect cell system.

A recombinant baculovirus encoding a single-chain murine major histocompatibility complex class I molecule in which the first three domains of H-2Kd are fused to beta 2-microglobulin (beta 2-m) via a 15-amino acid linker has been isolated and used to infect lepidopteran cells. A soluble, 391-amino acid single-chain H-2Kd (SC-Kd) molecule of 48 kDa was synthesized and glycosylated in insect cells and could be purified in the absence of detergents by affinity chromatography using the anti-H-2Kd monoclonal antibody SF1.1.1.1. We tested the ability of SC-Kd to bind antigenic peptides using a direct binding assay based on photoaffinity labeling. The photoreactive derivative was prepared from the H-2Kd-restricted Plasmodium berghei circumsporozoite protein (P.b. CS) peptide 253-260 (YIPSAEKI), a probe that we had previously shown to be unable to bind to the H-2Kd heavy chain in infected cells in the absence of co-expressed beta 2-microglobulin. SC-Kd expressed in insect cells did not require additional mouse beta 2-m to bind the photoprobe, indicating that the covalently attached beta 2-m could substitute for the free molecule. Similarly, binding of the P.b. CS photoaffinity probe to the purified SC-Kd molecule was unaffected by the addition of exogenous beta 2-m. This is in contrast to H-2KdQ10, a soluble H-2Kd molecule in which beta 2-m is noncovalently bound to the soluble heavy chain, whose ability to bind the photoaffinity probe is greatly enhanced in the presence of an excess of exogenous beta 2-m. The binding of the probe to SC-Kd was allele-specific, since labeling was selectively inhibited only by antigenic peptides known to be presented by the H-2Kd molecule.

A T cell clone directed at the circumsporozoite protein which protects mice against both Plasmodium yoelii and Plasmodium berghei.

Clone B is a cytotoxic T cell clone induced by immunization with Plasmodium yoelii sporozoites which recognizes an epitope on both the P. yoelii and Plasmodium berghei circumsporozoite proteins. It is CD8, uses the V beta 8.1 TCR, and is Kd restricted. When adoptively transferred, it protects mice against infection by both species of malaria sporozoites, and this protection is dependent on IFN-gamma. Clone B cells are more broadly reactive and protective than previously described murine T cell clones against malaria. Clone B may be an important model for immune protection against the spectrum of variant parasites in nature.

H-2-restricted cytolytic T lymphocytes specific for HLA display T cell receptors of limited diversity.

We previously showed that H-2Kd-restricted cytotoxic T lymphocyte (CTL) clones specific for a single nonapeptide derived from the Plasmodium berghei circumsporozoite (PbCS) protein displayed T cell receptors (TCRs) of highly diverse primary structure. We have now analyzed the TCR repertoire of CTLs that recognize a peptide derived from the human class I major histocompatibility complex (MHC) molecule HLA-Cw3 in association with the same murine class I MHC molecule H-2Kd. We first sequenced the TCR alpha and beta genes of the CTL clone Cw3/1.1 and, based on this genomic analysis, the TCR alpha and beta cDNA junctional regions of 23 independent H-2Kd-restricted CTL clones specific for HLA-Cw3. The results show that the TCR chains display very limited heterogeneity, both in terms of V alpha, J alpha, V beta, and J beta segments, and in terms of length and sequence of the CDR3 alpha and beta loops. The TCR repertoire used in vivo was then analyzed by harvesting CTL populations from the peritoneal cavity of immune mice. The peritoneal exudate lymphocytes (PELs) displayed HLA-Cw3-specific cytolytic activity in the absence of any stimulation in vitro. Remarkably, most of these freshly isolated PELs expressed TCRs that shared the same structural features as those from HLA-Cw3-reactive CTL clones. Thus, our results show that a peptide from HLA-Cw3 presented by H-2Kd selects CTLs that bear TCRs of very limited diversity in vivo. When taken together with the high diversity of the TCRs specific for the PbCS peptide, these findings suggest that natural tolerance to self peptides presented by class I MHC molecules may substantially reduce the size of the TCR repertoire of CTLs specific for antigenic peptides homologous to self.

Distinct T cell specificities are induced with the authentic versus recombinant Plasmodium berghei circumsporozoite protein.

Plasmodium berghei sporozoite (SPZ)-immune lymph node (LN) cells obtained from mice of different H-2 haplotypes were analyzed for the presence of circumsporozoite (CS) protein-reactive T cells in proliferative assays. Although lymphocytes from each strain responded in vitro to the priming Ag and to the soluble rCS protein, they did not respond to CS protein synthetic peptides. Parallel analysis of rCS protein-primed LN cells revealed that the two Ag are unequal in generating T cell specificities: although SPZ priming did not induce CS protein peptide-reactive T cells, priming with rCS protein did. Not being privy to the processing and presentation of SPZ Ag, we postulated that a different order of processing of the authentic, i.e., SPZ-associated CS protein vs soluble rCS protein might be responsible for the generation of different T cell specificities. Accordingly, authentic CS protein might not be processed by APC, or the processed fragments might obscure the recognition of smaller peptide fragments. Therefore, we subjected the SPZ to three cycles of a freeze/thaw procedure and used the denatured SPZ preparation for priming. We observed that contrary to priming with the authentic SPZ, denatured SPZ generated T cells reactive to some of the CS protein synthetic peptides. The hypothesis that each form of the SPZ Ag is subject to a unique Ag processing was also confirmed in experiments demonstrating a lack of recognition of the authentic CS protein by rCS protein-primed LN cells. Hence, the evidence presented in this work that complex protozoan Ag, such as Plasmodia, might present different requirements for Ag-specific T cell induction/activation not only enhances the basic understanding of the immune system, but is essential for the development of antimalaria vaccine(s). In addition, these observations support the hypothesis that the molecular context of the priming Ag influences the outcome of T cell specificities, by providing evidence that the authentic CS protein induces a T cell repertoire that is distinct from that induced by the rCS protein.

Interaction of antigenic peptides with MHC class I molecules on living cells studied by photoaffinity labeling.

Using a direct binding assay based on photoaffinity labeling, we have studied the interaction of antigenic peptides with murine MHC class I molecules on living cells. Photoreactive derivatives were prepared by N-terminal amidation with iodo, 4-azido salicylic acid of the Kd restricted Plasmodium berghei circumsporozoite (P.b. CS) peptide 253-260 (YIPSAEKI) and the Db-restricted Adenovirus 5 early region 1A (Ad5 E1A) peptide 234-243 (SGPSNTPPEI). As assessed in functional competition experiments, both peptide derivatives retained the specific binding activity of the parental peptides for Kd or Dd, respectively. The P.b. CS photoprobe specifically labeled Kd molecules on P815 (H-2d) cells, but failed to label RMA (H-2b) cells. Conversely, the Ad5 E1A photoprobe specifically labeled Db molecules on RMA cells, but failed to label P815 cells. When the two photoprobes were tested on a panel of Con A-activated spleen cells expressing 10 different H-2 haplotypes, significant photoaffinity labeling was observed only on H-2d cells with the P.b. CS photoprobe and on H-2b cells with the Ad5 E1A photoprobe. Labeling of cell-associated Kd or Db molecules with the photoprobes was specifically inhibited by antigenic peptides known to be presented by the same class I molecule. Photoaffinity labeling of Kd with the P.b. CS photoprobe was used to study the dynamics of peptide binding on living P815 cells. Binding increased steadily with the incubation period (up to 8 h) at 37 degrees C and at ambient temperature, but was greatly reduced (greater than 95%) at 0 to 4 degrees C or in the presence of ATP synthesis inhibitors. The magnitude of the labeling was twofold higher at room temperature than at 37 degrees C. In contrast, binding to isolated Kd molecules in solution rapidly reached maximal binding, particularly at 37 degrees C. Dissociation of the photoprobe from either cell-associated or soluble Kd molecules was similar, with a half time of approximately 1 h at 37 degrees C, whereas the complexes were long-lived at 4 degrees C in both instances.

T cell receptor genes in a series of class I major histocompatibility complex-restricted cytotoxic T lymphocyte clones specific for a Plasmodium berghei nonapeptide: implications for T cell allelic exclusion and antigen-specific repertoire.

We report here the first extensive study of a T cell repertoire for a class I major histocompatibility complex (MHC)-restricted cytotoxic T lymphocyte (CTL) response. We have found that the T cell receptors (TCRs) carried by 28 H-2Kd-restricted CTL clones specific for a single Plasmodium berghei circumsporozoite nonapeptide are highly diverse in terms of V alpha, J alpha, and J beta segments and aminoacid composition of the junctional regions. However, despite this extensive diversity, a high proportion of the TCRs contain the same V beta segment. These results are in contrast to most previously reported T cell responses towards class II MHC-peptide complexes, where the TCR repertoires appeared to be much more limited. In our study, the finding of a dominant V beta in the midst of otherwise highly diverse TCRs suggests the importance of the V beta segment in shaping the T cell repertoire specific for a given MHC-peptide complex. As an additional finding, we observed that nearly all clones have rearranged both TCR alpha loci. Moreover, as many as one-third of the CTL clones that we analyzed apparently display two productive alpha rearrangements. This argues against a regulated model of sequential recombination at the alpha locus and consequently raises the question of whether allelic exclusion of the TCR alpha chain is achieved at all.

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.

Cloned cytotoxic T cells recognize an epitope in the circumsporozoite protein and protect against malaria

Protective immunity against malaria is induced by vaccination of hosts with irradiation-attenuated sporozoites. This immunity is mediated in part by neutralizing antibodies that are directed mainly against the repeat domain of the circumsporozoite protein. Early experiments showed, however, that B-cell-depleted mice that are immunized with sporozoites can resist challenge, indicating that T-cell effector mechanisms may also have a role in protection. This idea was supported by the recent observation that protective immunity also requires T-cells expressing the CD8 antigen (CD8+ T cells) whose target is probably the developing liver-stage parasites. Moreover, an oral Salmonella vaccine that expresses the circumsporozoite protein is able to protect against murine malaria in the absence of antibodies. Here we report the identification of an epitope contained within amino acids 249-260 of the Plasmodium berghei circumsporozoite protein that is recognized by H-2Kd-restricted cytotoxic T cells. Passive transfer into mice of cytotoxic-T-cell clones that recognize this epitope conferred a high degree of protection against challenge. These results provide the first direct evidence that CD8+ T cells that are specific for a defined epitope can confer protection against a parasitic infection.

Immune response gene regulation of immunity to Plasmodium berghei sporozoites and circumsporozoite protein vaccines. Overcoming genetic restriction with whole organism and subunit vaccines.

We conducted a series of experiments to define Ir gene regulation of the immune response to Plasmodium berghei sporozoites and circumsporozoite (CS) protein-derived subunit vaccines. The studies demonstrated that there is no apparent genetic restriction of the capacity to develop protective immunity against a large sporozoite challenge after immunization with irradiation-attenuated P. berghei sporozoites; that the Th response to (Asp-Pro-Ala-Pro-Pro-Asn-Ala-Asn)n, the predominant protective B epitope on the P. berghei CS protein, is genetically restricted and regulated by Class II genes (I-Ab) and by genes in the Class I region (H-2Dk) or telomeric to this region; and that this restriction can be overcome by immunization with a r protein including the entire P. berghei CS protein. The results support the development of full length human CS protein vaccines to take advantage of all potential T epitopes on this protein.

Oral Salmonella typhimurium vaccine expressing circumsporozoite protein protects against malaria.

Immunization with radiation-attenuated malaria sporozoites induces potent cellular immune responses, but the target antigens are unknown and have not previously been elicited by subunit vaccines prepared from the circumsporozoite (CS) protein. A method is described here for inducing protective cell-mediated immunity to sporozoites by immunization with attenuated Salmonella typhimurium transformed with the Plasmodium berghei CS gene. These transformants constitutively express CS antigens and, when used to immunize mice orally, colonize the liver, induce antigen-specific cell-mediated immunity, and protect mice against sporozoite challenge in the absence of antisporozoite antibodies. These data indicate that the CS protein contains T cell epitopes capable of inducing protective cell-mediated immunity, and emphasize the importance of proper antigen presentation in generating this response. Analogous, orally administered vaccines against human malaria might be feasible.

Synthetic peptide vaccine confers protection against murine malaria.

A synthetic peptide, (DPPPPNPN)2D, representing a subunit of the repeat domain of the Plasmodium berghei circumsporozoite protein, was conjugated to tetanus toxoid using bisdiazobenzidine. Immunization of mice and rats with the conjugate induced high serum titers of antibodies to the parasite, and most of the animals were completely protected from malaria infection when challenged with sporozoites.

Plasmodium berghei: Cloning of the circumsporozoite protein gene

A DNA fragment encoding the carboxy terminal 80% of the Plasmodium berghei circumsporozoite protein was selected from a genomic DNA expression library. Sequencing revealed that the P. berghei circumsporozoite protein was similar in overall structure to circumsporozoite proteins from other malaria species, although the central repeat region was unique in comprising two different blocks of tandem peptide repeats: 11 eight amino acid repeats with predominant sequence DPAPPNAN were followed by 16 two amino repeats, predominantly PQ. The P. berghei circumsporozoite protein exhibited limited, but about equal amino acid homology to circumsporozoite proteins from P. knowlesi, P. vivax, and P. falciparum, indicating that P. berghei is not closely related to any of these other malaria species. Cloning of the P. berghei circumsporozoite protein gene will allow direct testing of sporozoite vaccines in mice.

Efficacy of Murine Malaria Sporozoite Vaccines: Implications for Human Vaccine Development

As part of a study of potential vaccines against malaria, the protective efficacy of sporozoite subunit vaccines was determined by using the Plasmodium berghei murine malaria model. Mice were immunized with recombinant DNA-produced or synthetic peptide-carrier subunit vaccines derived from the repetitive epitopes of the Plasmodium berghei circumsporozoite gene, or with radiation-attenuated sporozoites. Immunization with subunit vaccines elicited humoral responses that were equivalent to or greater than those elicited by irradiated sporozoites, yet the protection against sporozoite challenge induced by either of the subunit vaccines was far less than that achieved by immunization with attenuated sporozoites. Passive and adoptive transfer studies demonstrated that subunit vaccines elicited predominantly antibody-mediated protection that was easily overcome whereas irradiated sporozoites induced potent cell-mediated immunity that protected against high challenge doses of sporozoites. These studies indicate that new strategies designed to induce cellular immunity will be required for efficacious sporozoite vaccines.