Vaccination against respiratory syncytial virus: problems and progress

Generation of atypical pulmonary inflammatory responses in BALB/c mice after immunization with the native attachment (G) glycoprotein of respiratory syncytial virus

The feasibility of using the highly purified native attachment (G) protein in a subunit vaccine against respiratory syncytial virus (RSV) was examined in a murine model with or without the fusion (F) protein of RSV and the adjuvant QS-21. The studies established that QS-21 was more potent than AIOH as an adjuvant for both F and G glycoproteins. Augmented antigen-dependent killer cell activity and complement-assisted serum neutralizing and anti-F and G protein immunoglobulin G2a antibody titers were observed. Immunization with G/QS-21 generated immune responses that were characterized by low levels of antigen-dependent killer cell activity, elevated levels of interleukin-5 (IL-5) and percentages of eosinophils in the bronchoalveolar lavage fluids after challenge, and splenic immunocytes that secreted IL-5 but not gamma interferon (IFN-gamma) after in vitro stimulation with purified whole virus antigens. The pulmonary eosinophilia was similar to that induced by a facsimile of a formalin-inactivated vaccine used in previous clinical trials and was prevented by prior in vivo treatment with anti-IL-5 but not with control immunoglobulin G or anti-IFN-gamma neutralizing monoclonal antibodies. Thus the data implied that vaccination with G/QS-21 generated helper T-cell immune responses that were type 2 in nature. Alternatively, the data suggested that the helper T-cell immune responses elicited by F/QS-21 were more type 1 in character. Neither eosinophilia nor elevated levels of IL-5 were observed in the lungs of mice after challenge. Noteworthy levels of antigen-dependent killer cell activity was observed, and splenic immunocytes secreted copious quantities of IFN-gamma. Immunization with a combination vaccine composed of highly purified native F and G proteins plus QS-21 (F+G/QS-21) resulted in augmented complement-assisted serum neutralizing antibody titers compared with vaccination with either F/QS-21 or G/QS-21 alone. However, following vaccination with F+G/QS-21, the bronchoalveolar lavage fluids contained significant increases in IL-5 and percentages of eosinophils after challenge, the spleen cells appeared to secrete less IFN-gamma after in vitro stimulation, and there was no evidence of increased numbers of antigen-dependent killer cell precursors. Taken together, the data imply that native G protein influences the nature of the immune responses elicited by F/QS-21. The results therefore suggest that G, not F, protein has more potential to bias the host for atypical pulmonary inflammatory responses.

Surface display on staphylococci: a comparative study

Two different host-vector expression systems, designed for cell surface display of heterologous receptors on Staphylococcus xylosus and Staphylococcus carnosus, respectively, were compared for the surface display of four variants of a 101 amino acid region derived from the G glycoprotein of human respiratory syncytial virus (RSV). Surface localization of the different chimeric receptors was evaluated by a colorimetric assay and by fluorescence-activated cell sorting. It was concluded that the S. carnosus system was better both in the ability to translocate inefficiently secreted peptides and in the number of exposed hybrid receptors. The potential use of the described staphylococci as live bacterial vaccine vehicles or alternatives to filamentous phages for surface display of protein libraries is discussed.

Hydrophobicity engineering to facilitate surface display of heterologous gene products on Staphylococcus xylosus

Protein engineering has been employed to investigate the effect of specific amino acid changes on the targeting of heterologous proteins to the outer cell surface of the Gram-positive bacterium Staphylococcus xylosus. Three different variants, corresponding to a 101 amino acid region of the major glycoprotein (G protein) of human respiratory syncytial virus (RSV), were generated in which multiple hydrophobic phenylalanine residues were either substituted or deleted. The different G protein fragments were expressed as one part of recombinant receptors designed for surface display on S. xylosus cells. The engineered variants of the RSV G protein hybrid receptors were, in contrast to a non-engineered fragment, efficiently targeted to the outer cell surface of recombinant S. xylosus cells as determined by different methods, including fluorescence-activated cell sorting. In addition, immunization of mice with live recombinant S. xylosus demonstrated that surface exposure was required to generate receptor-specific antibodies. The present strategy of hydrophobic engineering should be of general interest in surface-display applications and for secretion of proteins otherwise difficult to translocate through host cell membranes.

Hydrophobicity engineering to increase solubility and stability of a recombinant protein from respiratory syncytial virus

Site-directed mutagenesis has been employed to engineer the hydrophobic properties of a 101-amino-acid fragment from the human respiratory syncytial virus (RSV) major glycoprotein (G protein). When this protein was produced in Escherichia coli, more than 70% of the gene product was found as inclusion bodies, and the product recovered from the soluble fraction was severely degraded. Substitution of two cysteine residues for serine residues, did not significantly change the solubility or stability of the gene product. In contrast, a dramatic increase in both solubility and stability was achieved by multiple engineering of hydrophobic phenylalanine residues. As compared to the non-engineered protein, the fraction of soluble protein in vivo could be increased from 27% to 75%. Surprisingly, this effect was accompanied by a remarkable increase in stability. The in vitro solubility of the purified gene products was similarly increased approximately fivefold. Structural studies using circular dichroism suggest that the two engineered fragments have a distribution of secondary-structure elements similar to the non-engineered fragment. In addition, the two engineered G-protein variants were demonstrated to be at least in part antigenically authentic to the non-engineered gene product. These results demonstrate that engineering of hydrophobic residues can be used as a tool to increase the solubility and proteolytic stability of poorly soluble and labile proteins.

Formulation of the purified fusion protein of respiratory syncytial virus with the saponin QS-21 induces protective immune responses in Balb/c mice that are similar to those generated by experimental infection

The feasibility of employing a vaccine composed of the purified fraction 21 of Quillaja saponaria (QS-21) and the fusion (F) protein of respiratory syncytial virus (RSV) to induce protective immune responses in the lower respiratory tract of Balb/c mice was examined. Our goal was to compare local and systemic immune responses with those induced following immunization with the protein adsorbed to aluminium hydroxide (F/ALOH) adjuvant or by experimental infection. Sera from mice vaccinated with the QS-21 formulation (F/QS-21) contained elevated anti-F protein IgG antibody titres that were dependent on the dose of QS-21 employed. Similar to the immune responses generated by experimental infection, the sera from mice vaccinated with F/QS-21 possessed greater capacity to neutralize virus infectivity that was associated with the generation of heightened complement-fixing IgG2a antibody titres. In contrast, vaccination with F/ALOH elicited systemic immune responses that were characterized by a predominance of protein-specific antibodies of the IgG1 subclass and lower neutralizing antibody titres. The capacity of F/QS-21 to facilitate local pulmonary immune responses was also examined and found to be similar to those induced by experimental infection. After virus challenge, a 90-fold increase in the number of F protein-specific antibody-secreting cells was observed and associated with the clearance of virus from the infected lungs. Moreover, elevated levels of antigen-dependent killer cell activity were detected and appeared to be mediated by class I major histocompatibility complex restricted CD8+ T cells. Additional characterization of the pulmonary immune response was performed on the cellular infiltrates obtained after bronchoalveolar lavage and on formalin-fixed lung tissue. The local protective immune responses induced after challenge of the groups immunized with F/QS-21 or infectious virus were significantly different from those elicited in naive control mice injected with adjuvant alone, or in mice immunized with F/ALOH. The cellularity of the lavage fluids from the former groups was characterized by a significantly greater percentage of lymphocytes and less neutrophils. In similar fashion histological evaluation of the lungs from mice immunized with F/QS-21 or infectious virus revealed significantly elevated local immune responses after challenge. In conclusion, the results suggest that formulation with F/QS-21 alters the qualitative and quantitative nature of the immune response to the F glycoprotein when compared with the traditional aluminium-based adjuvants.

The pulmonary immune response of Balb/c mice vaccinated with the fusion protein of respiratory syncytial virus

We have investigated the efficacy of vaccination with the purified fusion (F) protein of respiratory syncytial virus (RSV) on aluminium hydroxide adjuvant in Balb/c mice. The purpose of the study was to define the role of the local pulmonary mononuclear cell (PMC) infiltrate in the clearance of virus from the lower respiratory tract. Balb/c mice immunized with F protein were able to inhibit the replication of virus in the lungs as early as 4 days after intranasal challenge. In contrast, unimmunized mice required 8 days. Examination of humoral immune mechanisms demonstrated that vaccination with the purified protein induced moderate titres of serum neutralizing antibody. In addition, immunization induced low to moderate levels of antigen-dependent killer cell activity. To examine the immunological events responsible for virus clearance in vivo, PMC infiltrates were isolated after virus challenge and tested directly for protective capacity. After virus challenge, the F protein-immune mice were able to recall the cytolytic cells to the pulmonary tissues. The results further suggested that the local antigen-dependent killer activity was mediated by cytolytic T cells of the CD8 phenotype. Adoptive transfer studies were also conducted to identify further the role the PMC infiltrate had in protective immunity. Adoptive transfer of F protein-educated PMC into naive syngeneic recipients suggested that the pulmonary infiltrates contained the cellular constituents necessary for protective immunity. Both humoral and cellular immune elements were present.(ABSTRACT TRUNCATED AT 250 WORDS)