Expression of biologically active human chorionic gonadotropin and its subunits by recombinant vaccinia virus

Synthesis and chromatographic purification of recombinant human pituitary hormones

Recombinant DNA-derived proteins and, in particular, human pituitary hormones, are increasingly used for research, diagnostic and therapeutic purposes. This trend has demanded new synthetic approaches and improved purification techniques. The type and sequence of the purification steps have to be selected in accordance with the cloning and protein expression strategy, the host organism and cellular localization of the protein of interest, with a view to producing the desired product at a required purity, biological activity and acceptable cost. This review article describes and analyzes the main synthetic and purification strategies that have been used for the production of recombinant human growth hormone, prolactin, thyrotropin, luteinizing hormone and follicle-stimulating hormone, giving special consideration to the few published downstream processes utilized by the biotechnology industry. Practically all types of prokaryotic and eukaryotic organisms utilized for this purpose are also reviewed.

Recombinant adenovirus synthesizing cell surface-anchored beta hCG induces bioneutralizing antibodies in rats

A recombinant adenovirus (re-Ad) has been constructed that synthesizes a cell surface-anchored form of the beta-subunit of human chorionic gonadotropin (beta hCG). This was achieved by in-frame fusion of beta hCG cDNA at its C terminus with the gene sequences coding for the vesicular stomatitis virus glycoprotein (VSVg) transmembrane domain. The fusion protein gene was placed under the control of human cytomegalovirus (hCMV) immediate early promoter and this expression cassette was inserted into the E1 region of Ad type 5 by homologous recombination. In vitro experiments using re-Ad-infected 293 cells showed that beta hCG fusion protein was made as early as 6 h post infection and the protein was anchored to the cell membrane as seen by immunofluorescence staining. The re-Ad induced bioneutralizing antibodies (Ab) to hCG when inoculated in rats through intraperitoneal or intramuscular routes. The Ab were made in a dose-dependent manner. However, orally delivered re-Ad failed to generate any significant immune response.

The HSD-hCG vaccine prevents pregnancy in women: feasibility study of a reversible safe contraceptive vaccine

PROBLEM

To develop a vaccine for reversible control of fertility in women. MATERIALS AND PROTOCOLS: Purified beta subunit of hCG annealed to purified alpha subunit of ovine LH linked chemically to tetanus toxoid (TT) and diphtheria (DT); vaccine employed at 300 micrograms gonadotropin equivalent per injection adsorbed on alhydrogel with 1 mg SPLPS added in the first injection; Phase I safety trials in 47 women with elective tubal ligation; Phase II efficacy studies in 148 proven fertile women (2 children), sexually active, desirous of family planning using IUD; IUD removed when anti-hCG titres exceed 50 ng/ml hCG bioneutralization capacity; boosters given to maintain above threshold antibody levels; post coital tests conducted in 8 volunteers; sera of protected women analysed for immuno-determinants recognized by competitive enzyme immunoassays employing a panel of monoclonal antibodies and by direct binding to synthetic peptides; recombinant vaccines expressing beta hCG as a secreted product or as a fused protein anchored on membrane.

RESULTS

Immunization was well tolerated with no significant changes in endocrine, metabolic and hematological indices. Normal ovulatory cycles were maintained as indicated by menstrual regulation. The vaccine was highly effective in preventing pregnancy (1 pregnancy in 1224 cycles ) at and above antibody titres of 50 ng/ml. Antibodies declined in course of time in absence of boosters, with conceptions occurring below 35 ng/ml titres indicating regain of fertility. Ability of antibodies to prevent pregnancy was confirmed by post coital tests. High avidity (10(10) M-1) and other characteristics of antibodies generated by the vaccine are described and compared with those induced by two other hCG vaccines having undergone Phase I trials. The antibody response of the HSD vaccine in humans is characterized predominantly to an epitope recognized by the monoclonals 206 and P3W80. The antibodies had low or no reactivity with the carboxy terminal peptide and 38-57 region peptide. Live recombinant vaccines expressing beta hCG as a membrane anchored peptide generated antibody response to hCG in all animals following a single injection.

CONCLUSIONS

Reversible fertility control is feasible with the HSD-hCG vaccine without impairment of ovulation or disturbance of menstrual regularity. Suggestions have been made for further optimization of the vaccine, which include replacement of TT and DT by a panel of T non B determinants communicating with the entire MHC spectrum and development of recombinant vaccine expressing beta hCG along with membrane anchored carrier.

Production of the beta-subunit of human chorionic gonadotropin in Escherichia coli and its export mediated by the heat-labile enterotoxin chain-B signal sequence

The PCR-amplified beta-subunit of the human chorionic gonadotropin structural gene (betahCG) was cloned under the control of the tac promoter and the heat-labile enterotoxin chain B (LTB) signal sequence (LTBss). BetahCG was successfully produced, processed and exported to the periplasmic space in Escherichia coli. Expression of betahCG was confirmed by immunoblot analysis using an anti-betahCG polyclonal antibody. The processing of the protein was very efficient, as only the processed band could be detected at all time points during the course of induction. Expression was evident soon after the addition of the lactose analogue, IPTG. These results demonstrate that E. coli cells can synthesize, process and export betahCG using the LTBss.

Targeting vaccinia virus-expressed secretory beta subunit of human chorionic gonadotropin to the cell surface induces antibodies

We carried out experiments designed to study the effect of a protein's localization on its immunogenicity. A novel cell-surface protein was generated from a small, glycosylated secretory protein. The DNA sequence encoding the entire precursor of the human chorionic gonadotropin beta (beta hCG) subunit was fused in the correct reading frame to the DNA sequence encoding the transmembrane and cytoplasmic domains of vesicular stomatitis virus glycoprotein. This chimeric gene was introduced into the vaccinia virus genome to generate a recombinant virus. The recombinant virus, when used to infect animal cells, expressed a 135-amino-acid beta hCG subunit anchored in cellular membranes by the 48 carboxy-terminal amino acids of vesicular stomatitis virus glycoprotein. The immunogenicity of this recombinant virus with respect to its ability to generate anti-hCG antibodies was compared with that of a second recombinant vaccinia virus expressing the native secretory form of beta hCG. All animals immunized with the vaccinia virus expressing beta hCG on the cell surface elicited high titers of anti-hCG antibodies. Even after a single immunization with the recombinant vaccinia virus, the anti-hCG antibody titers persisted for a long period of time (more than 6 months). None of the animals immunized with vaccinia virus expressing the native secretory form of beta hCG showed any hCG-specific antibody response.

Physiological factors of growth and susceptibility to virus regulating Vero cells for optimum yield of vaccinia and cloned gene product (beta-hCG)

Replication of vaccinia virus carrying beta-hCG gene at the 'tk' locus was studied in microcarrier culture of Vero cell. The yields of recombinant vaccinia virus and its expressed product (beta-hCG) were influenced by the physiological state of the host cell, at which it was infected. Cells infected with the virus between the 3rd and 6th day gave maximum amount of virus and beta-hCG in a parallel manner, the yields of which were 2.3 x 10(9) pfu per 10(6) cells and 450 ng per 10(6) cells, respectively. The yields were reduced by 33-36% in the virus-infected stationary culture of the 8th day. The physiological state of the host cell has been quantified by the cellular activity parameter 'P' (mu(t)/mu max) in the scale of 1 to 0. The value of 'P' was highest in the exponentially growing culture, which reduced with decline in growth rate. The cellular energy generation has been related with the parameter 'P'. ATP generation was reduced to 55% in the stationary culture on the 8th day (P = 0). Also, the adsorption of virus was reduced by nearly 55% in extremely slow growing or stationary culture. Host cells at moderate growth rate (P > or = 0.15) were found to be optimum for maximizing the yield of viral products.

A birth control vaccine is on the horizon for family planning

Vaccines for control of fertility are likely to have an important impact on family planning methods. They are designed to act by mobilization of an internal physiological process and do not require external medication on a continuous basis. A number of birth control vaccines are at different stages of development, the most advanced being a vaccine inducing antibodies against human chorionic gonadotrophin (hCG). This vaccine consists of a heterospecies dimer (HSD, beta hCG associated with alpha-subunit of ovine luteinizing hormone, beta hCG:alpha oLH) linked to tetanus toxoid (TT) or diphtheria toxoid (DT) as carriers. The vaccine has recently passed an important milestone; it has completed the first leg of phase II efficacy trials. Women of proven fertility leading active sexual life were protected from becoming pregnant at antibody titres > or = 50 ng of hCG bioneutralization capacity per ml. This vaccine has previously been demonstrated to be reversible in its effect. It is free from any notable side-effects on endocrine, cardiovascular and other body functions. Ovulation was not disturbed and menstrual regularity was maintained. A logistic disadvantage of the present vaccine is the requirement for multiple injections. This is expected to be overcome by encapsulation of the requisite doses of the vaccine in biodegradable microspheres, which could be given at a single contact point for sustained antibody titres lasting over a year. A live recombinant vaccine has also been made that elicits high anti-hCG titres in monkeys for nearly 2 years following primary immunization and a booster at 8-9 months.

Anti-hCG vaccines are in clinical trials

Two vaccines inducing antibodies against human chorionic gonadotropin (hCG) have completed Phase I clinical trials, indicating the reversibility and safety of these vaccines. One is currently in Phase II efficacy trials in women in three major centres in India. The available data suggest that the vaccine prevents pregnancy above antibody titres of 50 ng/ml hCG bioneutralization capacity.

Expression of two forms of carp gonadotropin alpha subunit in insect cells by recombinant baculovirus

There are two types of cDNA clones (designated alpha 1 and alpha 2) encoding the alpha subunit of carp gonadotropin. These two cDNAs are derived from different genes and encode proteins that differ by seven amino acid residues (three in the signal peptide and four in the mature polypeptide). Expression of these two cDNAs in insect cells by recombinant baculovirus revealed that the alpha 1 subunit, after noncovalent association with the beta subunit, has the same potency as the native alpha subunit purified from the pituitary. In contrast, the alpha 2 subunit can associate with the beta subunit, but only to form an inactive gonadotropin. Competition of the alpha 2 subunit with the alpha 1 subunit for association with the beta subunit decreases the gonadotropin activity of the alpha/beta complex. In addition, both alpha 1 and alpha 2 subunits are secreted into the culture medium by insect cells and have an apparent molecular mass approximately 5 kDa higher than that of the native alpha subunit. These results indicate that the insect cell-derived alpha 1 subunit is biologically active and that those four amino acid changes in the mature of alpha 2 protein affect the biological activity and thus provide valuable clues for the study of the structure-function relationship of the alpha subunit of glycoprotein hormones.

The alpha subunit of human chorionic gonadotropin hormone synthesized in insect cells using a baculovirus vector is biologically active

A recombinant baculovirus, vAc alpha hCG, having a replacement of the viral polyhedrin gene with the cDNA encoding the alpha subunit of hCG was used to express alpha hCG, an extensively glycosylated hormone, in insect cells. Virus-infected cells, 72 h pi, secreted approximately 11.3 micrograms alpha hCG/2 x 10(6) cells/ml which was identical to the native hormonal peptide in terms of electrophoretic mobility, immunoreactivity and bioactivity on association with beta subunit, as evident by its binding to rat testicular cells and induction of steroidogenesis in a mouse Leydig cell bioassay system. The alpha hCG secreted into the medium represented approximately 20-30% of the total hCG synthesized by vAc alpha CG infected insect cells. The implications of using a very late promoter, in a baculovirus expression system, for directing the transcription of a gene whose gene product requires extensive post-translational modifications are discussed.