Outer membrane proteins of Pseudomonas aeruginosa as vaccine candidates

We tested the ability of recombinant outer membrane proteins of Pseudomonas aeruginosa to serve as a protective vaccine against this gram negative pathogen under two main pathophysiological events leading to P. aeruginosa sepsis. i) systemic infection during immunosuppression, and ii) bacterial translocation. A hybrid vaccine was cloned combining protective epitopes of outer membrane protein F (OprF) and outer membrane protein I (OprI). This vaccine proved to be highly protective against an intraperitoneal challenge with P. aeruginosa in immunosuppressed mice. Oral immunization of mice, with recombinant Salmonella dublin expressing OprI induced s-IgA antibodies in the gut mucosa against OprI and provided protection against translocation of P. aeruginosa in an immunosuppressed mouse model. To test whether OprI is safe for use in humans, recombinant OprI was purified and used for immunization of volunteers. Vaccination was well tolerated and no major side effects were observed. The induction of serum antibodies against OprI was found to be dose-dependent and was observed in total in 65% of the volunteers.

Serine-stretch protein (SERP) of Plasmodium falciparum corresponds to the exoantigen Ag2, a target of antibodies associated with protection against malaria

A mixture of Plasmodium falciparum exoantigens inducing lymphocyte activation and cytokine production was shown to contain the malaria vaccine candidate, the serine-stretch protein. This protein was shown serologically to correspond to Ag2, an exoantigen recognized by antibodies linked with protection against malaria. The glycophorin-binding protein, the histidine-rich protein II, the S-antigen, the heat shock protein 70, the ring-infected erythrocyte surface antigen, and the apical membrane antigen-1 were also shown serologically to be present in the mixture of exoantigens.

Conservation of antigen components from two recombinant hybrid proteins protective against malaria

Recently, we have shown that two hybrid proteins carrying partial sequences of the blood-stage antigens SERP, HRPII, and MSAI from Plasmodium falciparum confer protective immunity on Aotus monkeys against an experimental parasite infection (B. Knapp, E. Hundt, B. Enders, and H. A. Küpper, Infect. Immun. 60:2397-2401, 1992). The malarial components of the hybrid proteins consist of amino acid residues 630 to 892 of SERP, amino acid residues 146 to 260 of MSAI, and the 189 C-terminal residues of HRPII. We have studied the diversity of these protein regions in field isolates of P. falciparum. Genomic DNA was extracted from the blood of six donors from two different areas where malaria is endemic. The gene regions of SERP and MSAI coding for the corresponding sequences of the protective hybrid proteins and the exon II region of the HRPII gene were amplified by polymerase chain reaction and sequenced. All three regions were found to be highly conserved. In the 262-amino-acid fragment of SERP, one single conservative amino acid substitution was found. The exon II region of HRPII showed only a slight variability in number and arrangement of the repeat units. The 115-amino-acid fragment of MSAI which is located within an N-terminal region known to be conserved among different parasite strains was shown to be the most variable among the vaccine components: amino acid substitutions were found in 14 different positions of this MSAI region when both laboratory strains and field isolates were compared.

Protection of Aotus monkeys from malaria infection by immunization with recombinant hybrid proteins

On the basis of investigations of the malarial blood-stage antigens SERP, HRPII, and MSAI from Plasmodium falciparum, we chose two Escherichia coli-expressed hybrid proteins containing selected partial sequences of these antigens. Antibodies raised against both hybrid proteins in rabbits and Aotus monkeys recognize the corresponding P. falciparum polypeptides. In two independent trials with 13 animals, immunization of Aotus monkeys with either of the two hybrid proteins administered in a well-tolerated oil-based formulation protected the animals from an experimental P. falciparum infection.

Strategies for the development of an antimalarial vaccine

Susceptible Aotus monkeys were immunized with Escherichia coli-derived fusion proteins containing partial sequences of the proteins MSAI, SERP, HRPII and with a group of three recombinant antigens isolated by screening with an antiserum raised against the protective 41 kDa protein band. HRPII, the combination of the fusion proteins of the 41 kDa group and a mixture of two sequences of SERP conferred significant protection against a challenge infection with Plasmodium falciparum blood stages. Based on the protective capacity of these recombinant antigens we have expressed two hybrid proteins (MS2/SERP/HRPII and SERP/MSAI/HRPII) in E. coli containing selected partial sequences. In two independent immunization trials it was shown that immunization of Aotus monkeys with either of the two hybrid proteins can protect the animals from an experimental P. falciparum infection.

A recombinant hybrid protein as antigen for an anti-blood stage malaria vaccine: a study on the conservation of a protective component

Recently we have shown that two hybrid proteins expressed in Escherichia coli confer protective immunity to Aotus monkeys against an experimental Plasmodium falciparum infection (Knapp et al., 1992). Both hybrid proteins carry a sequence containing amino acids 631 to 764 of the serine stretch protein SERP (Knapp et al., 1989b). We have studied the diversity of this SERP region in field isolates of P. falciparum. Genomic DNA was extracted from the blood of six donors from different endemic areas of Brazil and West Africa. The SERP region encoding amino acids 630 to 781 was amplified by polymerase chain reaction (PCR) and sequenced. Only conserved amino acid substitutions in maximally two positions of the analyzed SERP fragment could be detected which supports the suitability of this SERP region as a component of an anti-blood stage malaria vaccine.

Protection of aouts monkeys after immunization with recombinant antigens of Plasmodium falciparum

The genus Aotus spp. (owl monkey) is one of the WHO recommended experimental models for Plasmodium falciparum blood stage infection, especially relevant for vaccination studies with asexual blood stage antigens of this parasite. For several immunization trials with purified recombinant merozoite/schizont antigens, the susceptible Aotus karyotypes II, III, IV and VI were immunized with Escherichia coli derived fusion proteins containing partial sequences of the proteins MSAI (merozoite surface antigen I), SERP (serine-stretch protein) and HRPII (histidine alanine rich protein II) as well as with a group of recombinant antigens obtained by an antiserum raised against a protective 41 kD protein band. The subcutaneous application (3x) of the antigen preparations was carried out in intact animals followed by splenectomy prior to challenge, in order to increase the susceptibility of the experimental hosts to the parasite. A partial sequence of HRPII, the combination of three different fusion proteins of the 41 kD group and a mixture of two sequences of SERP in the presence of a modified Al(OH)3 adjuvant conferred significant protection against a challenge infection with P. falciparum blood stages (2-5 x 10(6)) i. RBC). Monkeys immunized with the MS2-fusion protein carrying the N-terminal part of the 195 kD precursor of the major merozoite surface antigens induced only marginal protection showing some correlation between antibody titer and degree of parasitaemia. Based on the protective capacity of these recombinant antigens we have expressed two hybrid proteins (MS2/SERP/HRPII and SERP/MSAI/HRPII) in E. coli containing selected partial sequences of SERP, HRPII and MSAI. Antibodies raised against both hybrid proteins in rabbits and Aotus monkeys recognize the corresponding schizont polypeptides. In two independent immunization trials using 13 animals (age 7 months to 3 years) we could show that immunization of Aotus monkeys with either of the two hybrid proteins administered in an oil-based well tolerated formulation protected the animals from a severe experimental P. falciparum (strain Palo Alto) infection.

A Plasmodium falciparum blood stage antigen highly homologous to the glycophorin binding protein GBP

We have isolated a gene coding for a protein highly homologous to an antigen known as the glycophorin binding protein (GBP) which was therefore called GBPH. The gene consists of 2 exons interrupted by an intron located at a position corresponding to that of the GBP gene. The deduced amino acid sequence of GBPH comprises 427 residues and is characterized by a signal sequence and by an extended repeat region consisting of 8 units of 40 amino acid residues. The comparison of the amino acid sequences of GBPH and GBP reveals an identity of 69%. Antisera raised against a GBPH fragment that carries part of the repetitive region cross-react with GBP (105 kDa) and additionally detect some bands between 40 and 70 kDa, one of which may correspond to GBPH. The genes coding for GBP and GBPH are located on chromosomes 10 and 14, respectively. The GBP gene is transcribed as a highly abundant 6.5 kb mRNA in the blood-stage form, whereas Northern blot analysis using a GBPH specific probe detects 2 less abundant mRNAs of 2.3 kb and 2.7 kb. Southern blot analysis of P. falciparum DNA identifies a third member of the GBP gene family.

In vitro translation of Plasmodium falciparum aldolase is not initiated at an unusual site

It has been proposed recently that translation of fructose-1,6-diphosphate aldolase of the malaria parasite Plasmodium falciparum is initiated at a UAG codon, both in the parasite and in a rabbit reticulocyte cell-free translation system. We have introduced mutations around that UAG codon and find that cell-free expression of a construct encoding an AUG in this position results in a slightly larger translation product. The translation product of the construct encoding the UAG codon is of the same apparent molecular weight as the products obtained from two other constructs; one in which the UAG is replaced by AAG, and one in which nucleotides upstream from a second AUG codon are deleted. Thus we show that translation is not initiated at the UAG and conclude that synthesis of aldolase in the parasite starts at an AUG, provided after splicing of pre-mRNA.

Surface expression of malarial antigens in Salmonella typhimurium: induction of serum antibody response upon oral vaccination of mice

The Escherichia coli OmpA protein can serve as a carrier for the expression of foreign antigens on the surface of gram-negative bacteria. Employing OmpA vectors, immunogenic moieties of the Plasmodium falciparum blood stage antigens SERP and HRPII have been expressed in the attenuated Salmonella typhimurium SR-11 strain. Upon induction, the malaria specific sequences of 189 (HRPII) and 451 (SERP) amino acids, fused into the OmpA protein, have been expressed. By indirect immunofluorescence studies, live bacteria expressing the fusion proteins react anti-SERP and anti-HRPII sera, respectively, indicating that the hybrid OmpA proteins become integrated into the bacterial outer membrane and expose the malarial antigens at the exterior surface. Mice that were immunized orally with S. typhimurium cells expressing HRPII and SERP on their surface show a humoral immune response as determined by the anti-SERP and anti-HRPII IgG and IgM titres. From these experiments it can be concluded that the OmpA surface expression system in combination with established Salmonella vaccine strains can be used to efficiently deliver large antigens to the mucosal immune system.

Demonstration of alternative splicing of a pre-mRNA expressed in the blood stage form of Plasmodium falciparum

By screening of a lambda gt11 library from Plasmodium falciparum genomic DNA with an antiserum raised against a 41-kDa protein band, which was shown to confer protective immunity to monkeys, the phage clone 41-3 was identified. The entire 41-3 gene was isolated, and its coding regions were determined by amplification and sequencing of 41-3 specific mRNA fragments. The 41-3 gene has a complex structure consisting of nine exons, encoding 375 amino acids in total with a calculated molecular weight of 43,400. Provided that the N-terminal hydrophobic residues function as signal sequence which is cleaved off, the molecular weight of the 41-3 protein decreases to 41,200 and could therefore be considered to be a component of the protective Mr = 41,000 protein band. Indeed, a 41-kDa protein could be detected by Western blot analysis using antisera raised against different recombinant expression products of the 41-3 gene. We furthermore demonstrate an alternative splice process for the mRNA precursor transcribed from the 41-3 gene to yield at least three distinct mRNAs. The major splice product carries all exons E1 to E9, whereas at least two minor 41-3 mRNA species can be identified which show deletions in the region between exons E5 and E7. The possible role of this differential splice process for the parasite is discussed.

A recombinant hybrid protein as antigen for an anti-blood stage malaria vaccine

Based on investigations on several blood stage antigens from Plasmodium falciparum we have expressed a hybrid protein in E. coli containing 262 amino acids of the serine-stretch protein SERP and 189 amino acids of the histidine alanine rich protein HRPII. Antibodies raised against the hybrid protein by immunization of rabbits and Aotus monkeys react with both corresponding schizont polypeptides. Two Aotus monkeys immunized with the SERP/HRPII hybrid protein showed only low parasitemias after challenge infection with P. falciparum, compared to the control group. The result suggests that hybrid proteins of this type may be the basis for the development of a malaria vaccine.

Structure and possible function of Plasmodium falciparum proteins exported to the erythrocyte membrane

On infecting the red blood cell, the malaria parasite Plasmodium falciparum induces alterations in the erythrocyte membrane. The parasite appears to synthesize proteins that are exported across the parasitophorous vacuole membrane and through a system of membraneous structures within the cytoplasm of the host cell to the surface membrane of the erythrocyte. There, these proteins are either released or remain associated with the membrane. In this review we describe the structure and discuss the possible functions of some of the exported proteins.

A new blood stage antigen of Plasmodium falciparum highly homologous to the serine-stretch protein SERP

We have isolated the gene coding for a new protein (SERP H), highly homologous to the 113-kDa serine-stretch protein SERP which confers protective immunity to monkeys. The gene consists of four exons interrupted by three short introns located at positions corresponding to those of the SERP gene. Both genes were shown to be linked on chromosome 2 of Plasmodium falciparum suggesting that both originate from a common ancestral gene. Both genes are transcribed in the blood-stage form as 3.8-kb mRNAs with high yield. The deduced amino acid sequence of SERP H is highly homologous to SERP, although it does not contain a serine stretch. A highly hydrophilic region specific for the protein which was shown to be identical among different P. falciparum isolates was expressed in Escherichia coli for preparation of SERP H specific antisera. A schizont polypeptide of 130 kDa within the parasitophorous vacuole was detected by Western blot analysis and immunoelectron microscopy. Like SERP, the 130-kDa protein exhibits a region homologous to cysteine proteinases, suggesting that these proteins, or their processing products, may play a role as proteinases at the time of merozoite release from the infected erythrocyte.

Responses of T cells from sensitized donors to recombinant and synthetic peptides corresponding to sequences of the Plasmodium falciparum SERP antigen

In the present work, we intend to determine the capacity of human lymphocytes to recognize subfragments of the serine-stretch protein SERP, a blood-stage antigen from Plasmodium falciparum. Individuals sensitized by a previous P. falciparum infection were studied. Some recombinant proteins (RP) including RP7 and RP10 (amino acids 631-684 and 631-892 of SERP, respectively), were recognized in proliferation assays by lymphocytes from 28 sensitized individuals and not by lymphocytes from control, non-sensitized, donors. Synthetic peptides covering predefined zones of particular interest were tested and appeared to induce proliferative responses of lymphocytes from sensitized donors, allowing identification of putative T cell epitopes.

In vitro biosynthesis and membrane translocation of the serine rich protein of Plasmodium falciparum

The serine-rich protein (SERP) of Plasmodium falciparum is found within the parasitophorous vacuole. Exons 1 and 2 of the SERP gene were combined to a continuous open reading frame and expressed in a cell free translation/translocation system to study translocation of the protein across membranes. The protein was found to be translocated co-translationally across canine pancreatic microsomes. This process required the presence of the signal recognition particle, and it was accompanied by cleavage of a signal peptide. We conclude that the authentic SERP is exported from the parasite cell via the endoplasmic reticulum.

Plasmodium falciparum aldolase: gene structure and localization

A genomic clone was isolated which codes for the fructose bisphosphate aldolase of Plasmodium falciparum. The aldolase gene is interrupted by one intron which divides the coding region into two exons. The first one codes for one amino acid only, the initiation methionine, while the second one encodes the residual 368 amino acids of the protein. The gene, which is represented only once in the genome, is transcribed at high rates as a 2.4-kb mRNA in the P. falciparum blood stage. The aldolase gene encodes a protein of 40,105 Da, which is 61-68% homologous to known eukaryotic aldolases. The protein was expressed in Escherichia coli cells in an unfused and enzymatically active form. Antisera raised against amino acids 9-96 recognize a 41-kDa protein band previously shown to protect monkeys against a P. falciparum infection. These antisera cross-react with aldolases of different species, which confirms the strong conservation of this enzyme during evolution. The aldolase could be localized in the cytoplasm of the parasite as an active and soluble form. An inactive form was found to be associated with the membrane fraction. Digestion data with phospholipase C suggest a membrane association of this polypeptide via a glycosylphosphatidylinositol anchor.

A new blood stage antigen of Plasmodium falciparum transported to the erythrocyte surface

On screening a lambda gt11 library from Plasmodium falciparum genomic DNA with an antiserum against the 41-kDa protein band, which confers protective immunity to monkeys, two strongly reacting clones were isolated. One of the clones codes for parts of the P. falciparum 41-kDa aldolase, while the other (41-2) codes for parts of an unknown antigen; this was analyzed further. The 41-2 insert was used to identify two genomic fragments which carry the entire gene. The 41-2 gene codes for 184 amino acids specifying a 29-kDa schizont protein as estimated by Western blot analysis. The protein contains no repetitive sequences, and is characterized by a signal sequence and by two additional hydrophobic segments which could function as membrane anchor sequences. Computer analysis of the protein sequence predicted a dominant epitope in the N-terminal part which was confirmed by immunoreactivity data. The 41-2 protein could be localized in the schizont membrane, associated with membranous structures in the erythrocytic cytoplasm and with the erythrocyte membrane.

Molecular cloning, genomic structure and localization in a blood stage antigen of Plasmodium falciparum characterized by a serine stretch

Two short DNA segments were isolated by screening of a lambda gt11 library from Plasmodium falciparum schizont cDNA with an antiserum against the 140 kDa protein, which confers protective immunity to monkeys. The segments were used to identify a genomic fragment which carries the entire coding sequence for a protein of 113 kDa characterized by a stretch of serine residues (SERP I). We present the complete nucleotide and deduced amino acid sequence as well as the structure of the SERP I gene. The gene consists of four exons interrupted by three short introns located at the amino-terminal half. Exon 1 and the first part of exon 2 code for hydrophobic amino acids of a putative signal sequence. Exon 2 contains two repetitive segments, the first encoding six glycine rich octapeptides and a second region coding for 37 consecutive serine residues. Southern blot analysis demonstrated the conservation of the SERP I gene in four different parasite strains. SERP I could be localized in the parasitophorous vacuole and in the surrounding membranes. We discuss the relationship of this protein to the recently described P126 polypeptide and the possible role of this antigen as a vaccine candidate.

A histidin alanine rich recombinant antigen protects Aotus monkeys from P. falciparum infection

We have isolated a cDNA clone coding for 165 amino acids of a histidine alanine rich protein. An extended repeat region of this clone codes for the tripeptide Ala-His-His, which is extremely conserved, and for the tripeptide Ala-Ala-Asp, which shows only slight variability among the repeat units. This antigen exhibits high homology to the HRPII antigen described by Wellems and Howard (1986). The coding region was expressed in E. coli as a MS2-polymerase fusion protein, which was purified and used for immunization of Aotus monkeys. The animals immunized with this fusion protein showed only low parasitemias (less than 2%) after infection with P. falciparum, while animals from the control group or animals immunized with a MS2-fusion protein carrying other malaria specific sequences were not protected. The result suggests that this antigen is a good candidate for a malaria vaccine.

An unusual type-II 70-kilodalton keratin protein of mouse epidermis exhibiting postnatal body-site specificity and sensitivity to hyperproliferation

Keratin extracts from the epidermis of adult mouse ears, footpads, and tail contain large amounts of a 70-kilodalton (kDa) protein which has not been detected in any other body site of the adult mouse or in the epidermis of neonatal mice. Two-dimensional immunoblotting using an antiserum which recognizes both type-I and type-II murine keratins revealed that the 70-kDa protein is indeed a keratin belonging to the type-II subfamily. Its postnatal induction occurs during the first 2 weeks after birth, being first observed in tail epidermis, then in footpad epidermis, and only rather late in ear epidermis. Although in vitro translation experiments with polyA+-RNA from adult tail and footpad epidermis consistently failed to reveal the 70-kDa protein among the translation products, we obtained evidence using a specific cDNA clone that, in vivo, the protein is encoded by a discrete mRNA. This clone, termed pke70, was isolated from a cDNA library of footpad epidermal mRNA. Homology comparisons with a variety of known keratin cDNAs indicated that pke70 contains sequence information for a type-II keratin that is substantially larger than the mouse 67-kDa keratin protein. Northern-blot analysis with a specific 3'-fragment of pke70 demonstrated a single 2.8 +/- 0.1 kb mRNA species exclusively in adult ear, footpad, and tail epidermis. In situ hybridization with the same fragment revealed the presence of the pke70-hybridizing mRNA in both basal and suprabasal cells of ear and footpad epidermis as well as in the orthokeratinizing parts of the tail epidermis; however in the epidermis covering the balls of the feet, labeling was restricted to suprabasal cells at the base of these nodular elevations. Continuous treatment of adult tail or ear epidermis with hyperplasiogenic agents, e.g., vitamin A acid and the tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA), leads to a gradual disappearance of the 70-kDa protein. We obtained evidence using in situ hybridization that the loss of the 70-kDa keratin is preceded by a specific suppression of the transcription of its putative mRNA in basal cells, whereas initially suprabasal cells are apparently still able to complete their original commitment. The particular properties of the 70-kDa keratin protein, i.e., its topological restriction, its postnatal and time-dependent acquisition, and its pronounced sensitivity to hyperplasiogenic stimuli, make this keratin subunit an especially suitable candidate for studies concerning the regulation of keratin expression and morphogenesis in general, as well as for studies of the factors that control its expression so specifically.

Three cDNA sequences of mouse type I keratins. Cellular localization of the mRNAs in normal and hyperproliferative tissues

We have constructed cDNA libraries with poly(A)+ RNA from normal mouse footpad epidermis and from a squamous cell carcinoma of mouse back skin. Both libraries were screened for type I keratin clones. We present sequence data of three keratin cDNA clones which selected mRNAs coding for two 52-kDa proteins (clones pke 52 and pkSCC 52) as well as for a 50-kDa protein (clone pkSCC50). According to their carboxyl-terminal sequences, the two 52-kDa keratin proteins belong to a group of keratins with serine-rich subdomains adjacent to the alpha-helix, whereas the short carboxyl-terminus of the 50-kDa protein lacks a distinct substructure. Sequentially the two 52-kDa keratins are more closely related to each other than to any other mouse type I keratin. However, in situ hybridization with specific subclones reveals a distinctly different pattern of expression in mouse epithelia. Clone pkSCC 52 contains sequence information for a 52-kDa keratin present in basal cells of epidermis and other stratified epithelia, whereas the pke 52 cDNA encodes a keratin which is predominantly expressed in suprabasal cells of nonepidermal tissues. In terms of nucleotide sequence identities, it cannot precisely be decided which of the two mouse 52-kDa proteins is the equivalent of the human epidermal 50-kDa keratin protein (Hanukoglu, I., and Fuchs, E. (1982) Cell 31, 243-252). In the case of the bovine keratin VII, however (Jorcano, J.L., Rieger, M., Franz, J.K., Schiller, D.L., Moll, R., and Franke, W.W. (1984) J. Mol. Biol. 179, 257-281) the sequence identity values speak for an equivalence with the mouse ke 52 keratin. Obviously, in situ hybridization experiments would best be suited to unravel the precise interspecies relationship between the four highly similar keratins. The discriminatory efficacy of this technique is further emphasized by the demonstration that the mRNA for a 50-kDa keratin is present not only in hyperproliferative epithelia, but also in normal cells of hair follicles.