Conservation and antigenicity of N-terminal sequences of GP185 from different Plasmodium falciparum isolates

Proof of intragenic recombination in Plasmodium falciparum

Intragenic recombination in the merozoite surface protein MSP-1 of Plasmodium falciparum has been demonstrated in a cross between two cloned lines (3D7 and HB3) of this species. Following passage of a mixture of the clones through mosquitoes, uncloned progeny were examined by PCR for molecules containing sequences of both parent MSP-1 alleles. A recombinant molecule possessing both 3D7 and HB3 sequences has been obtained. Such molecules were not obtained from artificial mixtures of the blood forms of each clone. It is concluded that the novel allele was formed by a recombination event during meiosis of a hybrid 3D7/HB3 zygote.

Analysis of sequence diversity in the Plasmodium falciparum merozoite surface protein-1 (MSP-1)

Immunization with the first identified Plasmodium falciparum merozoite surface protein (MSP-1) protected monkeys from an otherwise fatal infection. The question of whether the high degree of diversity in MSP-1 among parasite clones will be an impediment to its development as a vaccine candidate needs to be resolved. We have aligned all published sequences, identifying errors, resequencing a portion of one parasite clone, and identifying probable duplicate sequences of four pairs of parasite clones. The sequences are displayed in a fashion that facilitates the study of variation and its potentially diverse origins. The original dimorphic sequences described by Tanabe et al. have been modified to include only common sequences throughout the entire gene. The extension of the dimorphic region to the 5' end of block 3 brings into question the involvement of intragenic crossover as the major mechanism generating allelic diversity. Additional diversity developed from point mutations and recombination in certain regions of the gene. The regions of variability and conservation should serve as a data base for planning vaccine trials.

Plasmodium falciparum: the repetitive MSA-1 surface protein of the RO-71 isolate is recognized by mouse antibody against the nonrepetitive repeat block of RO-33

We have expressed in Escherichia coli the nonrepetitive repeat zone of the MSA-1 surface protein of the RO-33 isolate of Plasmodium falciparum. The recombinant protein was used to immunize mice and the resulting RO-33 monospecific serum was used to screen our P. falciparum strain collection in order to recover additional alleles lacking tripeptide repeats in block 2 of MSA-1. Only 1 (RO-71) out of 30 isolates tested reacted strongly with the serum by indirect immunofluorescence assay. Surprisingly, block 2 of the RO-71 MSA-1 allele contains tripeptide repeats resembling those of the K1 isolate of P. falciparum. Additional sequence analysis of the entire DNA coding for the 80-kDa MSA-1-derived surface component did not reveal any amino acid stretches similar to block 2 of RO-33 which could rationalize the immunological cross-reactivity. We eliminated the possibility that the RO-71 culture was contaminated with RO-33 type alleles of MSA-1 by Southern blotting and PCR analysis. The RO-33-specific mouse serum used for the initial selection of RO-71 did not react with the antigen in the denatured state (Western blot). This and the sequence analysis suggest that the cross-reactive epitope in the MSA-1 protein of RO-71 is conformational. The possibility that a truncated frame-shift protein encoded by mutated MSA-1 mRNA is recognized by the serum is discussed.

Sequence variation in the tripeptide repeats and T cell epitopes in P190 (MSA-1) of Plasmodium falciparum from field isolates

The N-terminal part of p190, the precursor to the major merozoite surface antigens of Plasmodium falciparum, contains the T and B cell epitopes and tripeptide repeats. The p190 gene exhibits allelic dimorphism, but the tripeptide repeat-encoding region is the only exception to the dimorphic variations of the gene. To date, sequences available to document variations in the epitopes are very limited. Thus, in this study, the extent of the variation in these regions was analyzed using the polymerase chain reaction to amplify the DNA fragment encompassing these regions, followed by sequencing. Twenty-five gene clones were obtained from 19 isolates from the Mae Sod district in Thailand and their sequences were compared with those reported elsewhere. Results reveal 3 sequence types in the tripeptide-encoding region, and each contains a novel repetitive consensus nucleotide sequence. On the other hand, almost all nucleotide substitutions in block III are dimorphic. Identification of linkages of the dimorphic substitutions has led us to postulate 6 potential crossover sites, where intragenic recombinations of p190 alleles could occur. Sequences of T and B cell epitopes are highly conserved among these wild isolates and those from geographically diverse culture-adapted parasites.

Ability of recombinant or native proteins to protect monkeys against heterologous challenge with Plasmodium falciparum

To circumvent problems associated with polymorphic vaccine candidates for Plasmodium falciparum malaria, we evaluated recombinant proteins representing sequences from relatively high conserved regions of the precursor to the major merozoite surface proteins, gp190, for their ability to protect Saimiri monkeys against malaria challenge. Recombinant proteins represented amino acid residues 147 to 321 (p190-1) or 147 to 321 and 1060 to 1195 (p190-3), and their efficacy was compared with that of native gp190 and its processed products. All antigens were derived from P. falciparum K1, a Thai isolate, while the challenge strain was Palo Alto (from Uganda, Africa), which contains, with the exception of the N-terminal 375 amino acids, which are almost identical to the K1 sequence, essentially the MAD-20 allelic form of gp190. By 12 days following challenge, each control monkey required drug treatment. Three monkeys injected with p190-3 required therapy, while one cleared the parasites without therapy. Two monkeys injected with p190-1 received therapy on day 14, while the remaining two cleared the parasites without therapy. Of four animals injected with native gp190, because of health reasons unrelated to malaria, one was not challenged with parasites and one was removed from the study 8 days after challenge when its parasitemia was 1.1% (parasitemias in control animals ranged from 4.3 to 9%); the remaining two cleared the parasites after maximum parasitemias of 0.45 and 0.53%. The highest levels of antiparasite antibody were produced by animals immunized with native gp190. There was a significant correlation between monkeys which did not require drug treatment and antiparasite antibody. These results may suggest that native gp190 and/or its processed products can provide excellent protection against heterologous challenge and that antibody is important for protection. The challenge for vaccine development is to identify the protective sequence(s).

Structural diversity of Plasmodium falciparum gp200 is detected by T cells

T lymphocyte clones (TLC) specific for P. falciparum gp200 (a glycoprotein precursor of the main merozoite surface component) were obtained from two individuals with past exposure to malaria. The 25 established TLC carried the CD4 antigen and proliferated in the presence of immunopurified gp200, crude lysate of the parasite and intact infected red blood cells. They were further tested in proliferation assays for their capacity to recognize the structural diversity displayed by gp200. The stimulating antigen used in these assays was either sonicated or viable preparations of schizonts from five P. falciparum isolates differing in their gp200. The majority of the TLC proliferated similarly in the presence of each of the isolates. One third of the TLC proliferated to a different extent depending on the isolate used for stimulation, while two clones gave isolate-specific responses. These results indicate that the majority of human TLC raised in vitro against gp200, is directed against common determinants. This also suggests that immunization with full length gp200 will not lead predominantly to T cell help restricted to isolate-specific determinant.

Cross-reactive asparagine-rich determinants shared between several blood-stage antigens of Plasmodium falciparum and the circumsporozoite protein

From a Plasmodium falciparum cDNA expression library derived from mRNA of the asexual blood stages, we isolated and sequenced five different cDNA clones whose predicted protein products were unusually rich in asparagine (Asn). Two of the clones, R5 and G5, contain tandem imperfectly repeated sequences based on Asn-Asn-Thr (NNT) and Asn-Asn-Met (NNM) respectively. The other three, E4, C5 and R13, as well as G5, contain stretches of polyasparagine varying in length from 2 to 26 residues. Results of DNA blotting experiments with the individual cDNA sequences as probes suggest that each of the five clones corresponds to a different P. falciparum gene. The fragments of P. falciparum proteins expressed by the cDNA clones shared cross-reactive antigenic determinants which were present on multiple P. falciparum proteins. In immunoblotting experiments, owl monkey antibodies selected for binding to the polypeptide expressed by clone E4, C5 or G5 reacted with the expressed proteins from all 5 clones, and with at least 10 proteins from schizont infected erythrocytes. The cross-reactive epitopes could be modeled by two Asn-rich peptide structures: (1) (NNT)8, whose sequence was based on the R5 repeat; and (2) (NPNA)6, whose sequence was based on the Asn-rich repeat of the P. falciparum circumsporozoite protein (CSP). Antibodies that bound to each peptide were selected from sera of immune monkeys that had never been exposed to sporozoites. The selected antibodies bound all 5 expressed proteins in immunoblotting assays and also bound to several proteins from parasitized erythrocytes. Such cross reactivity between the CSP repeating unit and several blood-stage antigens has not been previously reported.

Characterization of a high-molecular-weight phosphoprotein synthesized by the human malarial parasite Plasmodium falciparum

During its intra-erythrocytic cycle, Plasmodium falciparum synthesizes a protein of apparent Mr 250,000-300,000. Its precise size is dependent on the P. falciparum isolate examined. This protein contains phosphate covalently bound to one or more serine residues and hence is termed PP300. Monoclonal antibody, McAb4-1F, binds to PP300 on immunoblots of protein extracts from all parasite isolates tested, both those exhibiting and those lacking the knob phenotype. Using McAb4-1F, the polypeptide was shown to be physically associated with the plasma membrane in a membrane-isolation procedure. However, in an indirect immunofluorescence assay the McAb appeared to bind to antigen associated with the erythrocyte plasma membrane in parasitized cells. However, it reacted only to fixed, not unfixed, parasitized erythrocytes indicating that the epitope is not normally exposed to extracellular antibodies. Clone 29-2 was isolated by a McAb4-1F immunoscreen of a P. falciparum complementary DNA (cDNA) expression library created in pUC8. Rat anti-clone serum which was raised to the purified protein encoded by the lacZ-29-2 fusion in pUC8 reacted with PP300 in immunoblots of parasite antigen. In Southern-blot analyses of parasite DNA digested with EcoRI, HindIII, or EcoRV, the 29-2 DNA insert hybridized to more than one fragment even though the insert lacked internal sites for these enzymes. In addition, hybridization studies were conducted using two oligodeoxy-nucleotides which were constructed based on the sequence of a cDNA clone which encoded part of a similar high-molecular-weight P. falciparum protein [Coppel et al., Mol. Biochem. Parasitol. 20 (1986) 265-277]. Analysis of these results indicates that the two cDNA sequences are parts of the same gene or a family of related genes.